REALIZATION OF LOW FLOW TORCHES
REALIZATION OF LOW FLOW TORCHES
FOR INDUCTIVELY COUPLED PLASMA
SPECTROMETRY
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus, prof. dr. J . M . Dirken, in het openbaar te verdedigen ten overstaan van een commissie aangewezen door het College van Dekanen
op dinsdag 28 april 1987 te 16.00 uur door
Petrus Stephanus Cornells van der Plas
doctorandus in de wis- en natuurwetenschappen, geboren te Roelofarendsveen
Dit proefschrift is goedgekeurd door de promotor prof. dr. L. de Galan
STELLINGEN
Ten onrechte verwaarlozen WEINGHRTNER en HERTZ het vectoriële karakter van het dipoolmoment in hun berekening van de componenten van de electrische veldgradiönttensor.
H. WeingHrtner en H.G. Hertz, Ber. Bunsenges. Physik. Chem. 81_, 1204 (1977).
De identificatie van gechloreerde 2-oxo-3-penteenzuren in gebleekte houtpulp door LINDSTROM en OSTERBERG is hoogst twijfelachtig. K. Lindstrom en F. Osterberg, Environ. Sci. Technol. 20, 133 (1986).
De bewering dat uit fosforzuuroplossingen met een concentratie groter dan 45 v% P2Cv onder atmosferische druk geen CdS kan precipiteren is onjuist.
Duitse octrooiaanvrage DE 2422902, Chemische Fabrik Kalk GMBH.
Het gecompliceerde gedrag van de ion-lens instellingen in ICP-MS noodzaakt tot een nauwkeuriger beschrijving dan door LONG en BROWN wordt gegeven.
S.E. Long en R.M. Brown, Analyst 111. 901 (1986)
De geldigheid van het door DITTRICH et al. opgestelde verband tussen de verdampingsenthalpie van organische oplosmiddelen en de optimale verstuiver voordruk in ICP-AES is sterk afhankelijk van het gebruikte type vernevelaar.
K.D. Dittrich, K. Niebergall en S. Rothe, Wiss. Z. Karl Marx Univ. Leipzig, Math-Naturwiss. R. 35, 70 (1986)
De criteria die worden toegepast bij de optimalisering van
chromatograflsche scheidingen zijn doorgaans niet geschikt voor multi-dimensionale benaderingen zoals mogelijk gemaakt door diode-array detectoren.
Een objectieve vergelijking van optimaliseringsstrategieën voor de mobile fase in de reversed-phase chromatografie is slechts dan
mogelijk wanneer niet alleen componenten en stationaire fase, maar ook de kolomafmetingen worden vastgelegd.
D.A. Doornbos en A.K. Smilde, Computer-based events in analytical chemistry, Graz, Austria, 15-19 sept. 1986.
De invoering van het vierde boek van het Wetboek van Burgerlijke Rechtsvordering, Arbitrage (artikel 1020-1076) maakt
geschillenbeslechting middels het onzuiver bindend advies overbodig.
Het aantal ongelede woorden van het Nederlands ligt in de orde van grootte van 15000.
Het formuleren van stellingen met het uitsluitende oogmerk van publicatie in NRC Handelsblad dient te worden ontmoedigd.
Samenleven verhoudt zich tot trouwen als mengsel tot verbinding.
Het hoge tempo waarin de maatschappij verandert maakt het steeds moeilijker om uit de macht der gewoonte te blijven handelen.
Teveel stellingen zijn negatief van instelling.
Daar het gebruik van grenen schrootjes in de Nederlandse architectuur in hoge mate is ingeburgerd, dient het gebruik in senaatskamers van Nederlandse Universiteiten tot het uiterste beperkt te blijven.
opgedragen aan mijn ouders, aan Petra
Contents
Chapter 1 General introduction. 1 1. Introduction. 1 2. Applications of the ICP. 2
2.1. ICP-AES. 2 2.2. ICP-MS. 2 3. Instrumentation. 2
3.1. The sample introduction unit. 3
3.2. The torch. ' 3 3.3. The optical spectrometer. 4
3.4. The mass spectrometer. 5
3.5. The computer. 6 4. The low flow plasma. 6
Chapter 2 Analytical evaluation of an air cooled
1 1/min argon ICP. 11 1. Introduction. 11 2. Experimental. 12
2.1. Design of the air cooling. 12
2.2. Torch design. 13 2.3. Silica as a torch material. 14
2.4. Physical appearance of the
low flow plasma. 15 2.5. Operation of the low flow plasma. 16
2.6. Instrumentation. 17 3. Results and Discussion. 18
3.1. Detection limits in aqueous
solutions. 18 3.2. Detection in organic solvents and
with hydride generation. 20 3.3. Reference materials. 23 3.4. Analysis of samples with high salt
concentrations. 23
Chapter 3 A radiatively cooled torch for ICP-AES
using 1 1/min of argon. 27
1. Introduction. 27
2. Theory of radiative power release. 28
2.1. Power balance. 28
2.2. Torch materials. 30
3. Experimental. 32
4. Results and discussion. 34
4.1. Operation of radiatively cooled
torches. 34
4.2. Temperature of the torch. 36
4.3. Analytical performance. 38
5. Conclusions. 40
Chapter 4 An evaluation of ceramic materials for
use in non cooled low flow ICP torches. 43
1. Introduction. 43
2. Ceramic materials. 44
2.1. Crystallographic and
micro-structure. 44
2.2. Properties. 46
2.2.1. Mechanical properties. 46
2.2.2. Thermal properties. 49
2.2.3. Thermal shock resistance. 49
2.2.4. Electrical properties. 51
2.2.5. Chemical inertness. 51
3. Experimental. 52
4. Results and discussion. 55
5. Conclusions. 61
Chapter 5 Use of a water cooled low flow torch in
ICP-MS. 65
1. Introduction. 65
2. Experimental. 66
Chapter 6 An algorithm for automated background correction in ICP-AES using a
photo-diode array detector. 77
1. Introduction. 77 2. Theoretical. 78
2.1. Spectral interferences. 78 2.2. Photodiode arrays. 80
3. Experimental. 82 4. Results and discussion. 89
5. Conclusions. 96
Appendix Low power ICP - physical principles and
analytical performance. 99
1. Introduction. 99 2. Power considerations. 100
2.1. Power balances. 100 2.2. Minimum power requirement. 100
2.3. To lower power. 101 2.4. Realizations of low power torches. 101
3. Analytical performance. 102 3.1. General remarks. 102 3.2. Operating conditions. 102 3.3. Detection limits. 103 3.4. Interferences. 104 3.5. Other aspects. 104 4. Conclusions. 105 Summary 107 Samenvatting 109 Nawoord 113
CHAPTER 1
GENERAL INTRODUCTION
1. Introduction.
It is only in the last decades that instrumental methods have replaced the classical "wet" methods of chemical analysis. Although the basics of most techniques were known for a long time, their proliferation has been made possible by two relatively recent developments: at the supply side by the development of electronics and computers and at the demand side by the increasing demand for more sensitive, faster and less expensive chemical analyses.
An example is inductively coupled plasma (ICP). PLUCKER [1] in 1858 and HITTORF [2] in 1884 have laid the fundaments with their work on gas discharges. THOMSON [3] developed in 1927 a theory for the elec tromagnetic field in the electrodeless ring discharge. Some twenty years later, BABAT [U] made a ring discharge which he could maintain
up to atmospheric pressure, after starting it at reduced pressure. In the early sixties REED [5] solved the problem of the attack of melting of the walls by using an argon stream to protect the torch, whereafter two groups independently recognized the analytical possibilities of the plasma as an excitation source in atomic emission spectrometry (AES). GREENFIELD et al [6] and FASSEL and coworkers [7-8] optimized the torch parameters and developed an ICP-AES system that could be used in practice. Commercial ICP-AES instruments based on this design appeared around 1970.
2. Applications of the ICP.
2.1 ICP-AES.
ICP-AES is the oldest application of the ICP for analytical purposes. The elements to be analyzed are thermally excited in the plasma dis charge and after a short period return to their ground-state. The energy absorbed during excitation is released through emission of electromagnetic radiation of discrete wavelengths. These wavelengths are characteristic for the elements and the intensity of the radiation at each wavelength is proportional the concentration of the element. The ICP is a powerful excitation source compared to older excitation sources (flame, arc, spark) used for AES. Because of its high tempera ture the sensitivity of most elements is very good, which results in typical detection limits in the ng/ml range. Another advantage of ICP-AES is the absence of chemical matrix effects due to the formation of molecular fragments. The confinement of the sample in a thin channel diminishes self-absorption at high concentrations and provides a linear dynamic range of 5 decades. Therefore, ICP-AES initiated a renaissance of atomic emission spectrometry, which had been partially overshadowed by other analytical techniques.
2.2. ICP-MS.
Instead of using the photons emitted by the atoms and ions, the ions can also be sampled directly with a mass spectrometer. The output of the instrument is a spectrum in the mass-domain.
ICP-MS is a relatively new technique that has several analytical advantages over ICP-AES, of which a typical increase in sensitivity by at least a factor of 10 is the most interesting. The main problems of current ICP-MS are the occurence of chemical interferences due to the formation of oxides and the occurence of spectral interferences caused by mass overlap of argon-species with the isotopes of several light elements.
3. Instrumentation.
An ICP source consists basically of a sample-introduction unit (pump, nebulizer and nebulizer chamber), the torch, and the high-frequency
components. An optical spectrometer or a mass spectrometer completes the instrument in case of ICP-AES or ICP-MS, respectively.
3.1 The sample introduction unit.
The first step of the analysis is to introduce the sample into the torch.
Liquid samples are converted by a nebulizer to an aerosol with partic les of varying diameter. Spray droplets with diameters up to a few micrometers only are selected by the spray chamber to be carried to the plasma by the carrier gas flow, while larger droplets are removed through the drain. Solid samples on the other hand can be vaporized by means of a laser, a spark source or a glow discharge.
In both cases aerosol and vapour are transported to the plasma by a carrier gas flow.
3.2 The torch.
The plasma source itself is a torch consisting of three concentric silica tubes through which argon is fed. Around the torch an rf-coil is mounted that supplies a strong rf-field with a frequency of 27 - 50 MHz and a power of 1 - 2 kW. The rf-field is inductively coupled to the plasma which means that the plasma behaves as the secondary coil of a transformer of which the rf-coil is the primary coil. A ring-shaped (torroidal) plasma with a gas temperature of 5000 K is genera ted and maintained.
The main argon flow is the plasma gas flow (12 - 15 1/min), which has the dual function of supplying the argon that maintains the plasma and to provide a heat shield in order to protect the outer silica tube of the torch against the high temperatures of the plasma.
An intermediate argon gas flow ( 0 - 1 1/min) may be used to adjust the vertical position of the plasma.
The sample is introduced in the center of the plasma by means of the carrier gas flow (1 1/min). Due to the torroidal shape of the plasma, the center is relatively cool, which facilitates introduction of the sample. The linear velocity of the carrier gas is a compromise between a minimal value necessary to penetrate the plasma and a maximal value that allows sufficient residence time in the plasma. For instance,
exit diameter (1 mm) and assuming room temperature gas density and an effective plasma height of 10 mm, the linear velocity and sample residence time are 20 m/s and 0.5 msec, respectively.
3.3 The optical spectrometer.
The purpose of the optical system is to select the wavelengths of interest from the total light signal emitted by the plasma (Figure 1). By means of lenses and mirrors an image of the plasma is projected on the entrance slit of the optical system. A diffraction grating is used to disperse the signal and to project the lines of interest onto one or more exit-slits. Behind each exit-slit a photomultiplier is mounted which converts the light signal passing through the slit into a pro portional electrical signal.
.mirror computer controlled monochromator/ plasma RF generator 27.2 MHz >\ kW peristaltic pump nduction coil _$orch nebulizer chamber rain nebulizer .1 sample solution
Figure 1. An ICP-AES instrument consists of a sample introduction unit (pump, nebulizer and nebulizer chamber), the ICP (torch and rf-generator), an optical unit (monochromator or polychromator and photomultiplier) and computer (connected to photomultiplier, not shown).
Two types of optical spectrometers a r e in u s e . A monochromator has
only one e x i t - s l i t , and the wavelength to be measured i s selected by
changing the angular position of the g r a t i n g r e l a t i v e to the e x i t
-s l i t . A polychromator ha-s multiple e x i t - -s l i t -s , the grating i -s fixed
and the position of every e x i t - s l i t i s pre-determined to correspond to
the wavelength of an element of i n t e r e s t .
The advantage of a polychromator i s i t s speed: a l l elements of i n t e
rest can be measured simultaneously, which allows shorter
measurement-times and improves precision. On the other hand, the wavelengths of a
polychromator are fixed and the advantage of the monochromator i s that
any wavelength of i n t e r e s t can be selected. Generally, polychromators
are used for r o u t i n e - a n a l y s e s whereas monochromators are used for
applications where more f l e x i b i l i t y i s desired.
3.4 The mass spectrometer.
In ICP-MS the ions have to be sampled from the plasma a t a p o s i t i o n
where the density of ions i s greatest (Figure 2). Sampling from cooler
parts decreases the s e n s i t i v i t y and increases the level of oxides. The
sampling interface between the mass spectrometer and the ICP has been
and s t i l l i s a major problem in the design of ICP-MS instruments.
Figure 2. An ICP-MS instrument consists of the sample introduction unit, the ICP (torch mounted horizontally) and mass spectrometer (interface, lenses, electron
It must resist the high temperature of the plasma and aid to attain a -4 pressure decrease from atomospheric pressure in the plasma to 10 torr in the mass spectrometer.
The mass spectrometer itself is a quadrupole type of medium resolution and a high scanning speed. As a result, a complete spectrum can be recorded in a fraction of a second.
The spectrometer is focussed and adjusted by means of a set of lenses of which the optimal position depends on both the mass spectrometer, the sample and the plasma-parameters.
3.5 The computer.
The computer is used for two separate tasks: to control the instrument and to process the data. Both ICP-AES and ICP-MS are complicated techniques with many instrumental variables that must be optimized before an analysis. Unfortunately, the processes in the nebulizer and the plasma source are not very well understood and the response of instrument performance on most variables cannot be predicted theoreti cally. Consequently, only empirical knowledge can be used and as the variables are often interdependent, instrument operation is generally reserved to skilled operators. Only recently software-routines have become available that are able to replace these human experts (see Chapter 6 ) .
4. The low flow plasma.
The main disadvantage of the ICP from an economical point of view is its high consumption of argon. In the torches used in commercial instruments, the total argon flow is 12 - 15 1/min, which amounts to running costs of DF1 30,000 per year. Therefore, soon after its intro duction the ICP has invoked research to reduce the running costs without sacrificing analytical capabilities.
At a first glance, the most obvious approach might seem to recirculate the argon used in the plasma. This implies that the used argon has to be cooled and the water, matrix and analyte have to be removed. Es pecially the removal of analyte makes this approach unattractive both for economical as well as practical reasons.
A second possibility is to use a less expensive gas, such as nitrogen. However, the temperature and consequently the analytical properties of plasmas generated with molecular gases are inferior compared to an argon plasma since the dissociation of these gases absorbs too much power (see Appendix).
The most succesful approach has proved to be to maintain argon, but to reduce the total gas requirements of the torch. Three approaches can be distinguished in the realization of low flow argon torches.
In the first category the physical dimensions of the three torch tubes are reduced to diminish the overall argon flow. These so-called "mini-torches" use an outer diameter of 9 mm or 13 mm, rather than the conventional 22 mm [10-12], Although a reduction in the argon require ment is realised, the penetration of the high-frequency field prevents further miniaturization and is generally held responsible for the moderate analytical performance of these torches.
In the second category of low flow argon torches, the torch dimensions are optimized to achieve enhanced cooling efficiency of the coolant argon. In an extensive study of various torch designs, ALLEMAND and BARNES [13] were the first to demonstrate the advantages of a tulip-shaped intermediate tube. This suggestion was subsequenly perfected in a cooperation of REZAAIYAAN and HIEFTJE with ANDERSON, KAISER and MEDDINGS [14], The overall size of the torch is maintained, but the separation between the two outer silica tubes is reduced to 0.5 - 0.3 mm. When forced through the narrow annular gap, the necessary protec tive argon-shield can already be obtained with a flow rate of 5 1/min. Although some additional argon is needed to introduce the sample and to sustain the plasma, a total consumption of 7 1/min is found to be adequate.
In the third type of low flow torches, the outer silica tube is cooled externally. The underlying idea is that the cooling action of the internal argon flow can be obviated when the silica tube is protected from softening by external cooling. Water, used by KORNBLUM et al [15], is an obvious first choice and, indeed, KAWAGUCHI et al [16,17] obtained acceptable performance with a total argon flow of 4.8 1/min
reduced further to 2 1/min and its coupling to a mass-spectrometer is described by GORDON, VAN DER PLAS and DE GALAN [18] (Chapter 5 ) . RIPSON et al [19,20] claimed beter results when the water cooling is replaced by air cooling. An air cooled torch was constructed where pressurized air is blown perpendicularly against the outside of the torch. An analysis of the power-balance of the torch revealed that although water is a more efficient coolant medium than air, it effec tively acts as a heat-sink and drains too much power from the plasma, as shown by RIPSON and DE GALAN [21] and DE GALAN and VAN DER PLAS
[22,23] (see appendix). An improved design of the air cooled torch is presented by VAN DER PLAS and DE GALAN [24], Chapter 2, and the analy tical capabilities are shown to be equivalent to a conventional ICP [24].
The last and ultimate approach in the development of reduced argon flow ICP torches is to use neither internal cooling by argon, nor external cooling by a cooling-device. Protective cooling of the ICP torch by either internal argon or external devices is necessary when silica is used as the outer tube material. Such aids are no longer needed when the tube is made from an alternative ceramic that can be heated to substantially higher temperatures [25]. The feasibility of this approach is demonstrated in Chapter 3. A plasma can be sustained at 1 1/min argon and 600 W power in a non-cooled torch with an outer tube containing a central piece of boron nitride or silicon nitride. The ceramic is placed at the hottest part of the tube, around the plasma in the rf-coil. Selection of a suitable ceramic is discussed in Chapter 4.
References
[I] J. Pluecker, Pogg. Ann. .103, 88 (1858). [2] W. Hittorf, Ann. Phys. 21., 90 (1884).
[3] J.J. Thomson, Phil. Mat. Ser. 74, 1128 (1927).
[4] G.I. Babat, J. Inst. Elec. Engrs. (Lond.) 94, 27 (1947). [5] T.B. Reed, J. Appl. Phys. 32, 821 (1961).
[6] S. Greenfield, I.L. Jones and C.T. Berry, Analyst 89, 713 (1969). [7] R.A. Wendt and V.A. Fassel, Anal. Chem. 37, 920 (1965).
[8] R.H. Wendt and V.A. Fassel, Anal. Chem. 38, 337 (1966).
[9] C D . Allemand, R.M. Barnes and C.C. Wohlers, Anal. Chem. 51., 2392 (1979). [10] R.N. Savage and G.M. Hieftje, Anal. Chem. 52, 1267 (1980).
[II] A.D. Weiss, R.N. Savage and G.M. Hieftje, Analyt. Chim. Acta .123, 245 (1981).
[12] R.N. Savage and G.M. Hieftje, Analyt. Chim. Acta 123, 319 (1981). [13] C D . Allemand and R.M. Barnes, Appl. Spectrosc. 3J_, 434 (1977). [14] R. Rezaaiyaan, G.M. Hieftje, H. Anderson, H. Kaiser
and B. Meddings, Appl. Spectrosc. 36, 627 (1982).
[15] G.R. Kornblum, W. van der Waa and L. de Galan, Anal. Chem. 5J_, 2440 (1980). [16] H. Kawaguchi, T. Ito, S. Rubi and A. Mizuike, Anal. Chem. 52, 2440 (1980). [17] H. Kawaguchi, T. Tanaka, S. Miura, J. Xu and A. Mizuike,
Spectrochim. Acta 38B, 1319 (1983).
[18] J.S. Gordon, P.S.C, van der Plas and L. de Galan, submitted to Anal. Chem. [19] P.A.M. Ripson, L. de Galan and J.W. de Ruiter,
Spectrochim. Acta 36B, 71 (1981).
[20] P.A.M. Ripson, E.B.M. Jansen and L. de Galan, Anal. Chem. 56, 2329 (1984). [21] P.A.M. Ripson and L. de Galan, Spectrochim. Acta 38B, 707 (1983).
[22] L. de Galan and P.S.C, van der Plas, Fresenius Z. Anal. Chem. 3124, 472 (1986).
[23] L. de Galan and P.S.C, van der Plas, in "Inductively Coupled plasmas in analytical atomic spectrometry". A. Montaser and D.W. Golightly (eds.) 1987 VCH Publishers, New York USA.
[24] P.S.C, van der Plas and L. de Galan, Spectrochim. Acta 40B, 1457 (1985). [25] P.S.C, van der Plas and L. de Galan, Spectrochim. Acta 39B, 1161 (1984).
CHAPTER 2
ANALYTICAL EVALUATION OF AN AIR COOLED 1 L/MIN ICP
1. Introduction.
The development of low flow ICP torches is gaining attention from both users and manufacturers of ICP instruments. The interest is caused by various considerations. First, in many countries outside the USA and western Europe, operation of an ICP torch is often impeded by a struc tural shortage of argon gas. Second, a low flow ICP torch is an essen tial part of a low cost sequential ICP system, which should be attrac tive for users of more sophisticated AAS systems. Third, a truck- or ship-board ICP will be attractive for specialized applications [1]. The approaches to the development of low flow torches have been reviewed by HIEFTJE [2], Miniature torches [3-8], high-efficiency torches [8-16] and externally cooled torches [17-25] have been inves tigated. However, the acceptance of any one of these innovations is not based on argon flow reduction, but on the analytical performance. The general opinion is that no loss in analytical performance compared to the conventional ICP may result from the argon reduction. There fore, widespread introduction can only be expected when a low flow torch has proven its analytical equivalence to the conventional plas ma. We believe that the air cooled torch developed in our laboratory has reached this stage. Its stable operation allows use by relatively inexperienced personel; it shows excellent detection power; it can be used with various solvents and can be applied to practical analysis.
2. Experimental.
2.1 Design of the a i r cooling.
The principle of the a i r cooled torch and i t s power-balance have been
given by RIPSON and de GALAN [23]. To prevent the torch from overheat
ing, the a i r cooling must dissipate a minimum amount of power t r a n s
ported from the plasma through the torch. RIPSON showed t h a t to keep
the torch temperature below 1000 K with a 1 1/min argon plasma a power
d i s s i p a t i o n of 100 - 200 W through the torch wall i s necessary. In
RIPSON's design 50 1/min of a i r was blown perpendicular to the t o r c h
through five tubes placed within two t u r n s of a f l a t - p l a t e d copper
c o i l . I t allowed a power transfer of just 100 W by forced convection
and the torch melted a t generator powers over 600 W. As a r e s u l t ,
torch operation was rather c r i t i c a l and required constant supervision.
Observation of damaged t o r c h e s indicated that they often melted b e
tween the i n l e t points of the a i r i n s i d e the work c o i l , due to i n
-homogeneous cooling of the t u b e . The improved design i s shown in
Figure 1. The same a i r flow of 50 1/min i s now introduced through 14
tubes t i l t e d at an angle of 10 . The assembly consists of two p a r t s
held together by two screws and f i t s t i g h t l y around a c o n v e n t i o n a l
Figure 1. Design of the new cooling device used in t h i s study. Air i s introduced by means of 14 a i r i n l e t s , placed in an helical arrangement and t i l t e d upwards at an angle of 10 .
helical work coil. The screws and the air tubes are made of polyethy lene, the device itself of fibered PTFE. The device offers significant benefits over the earlier design. The use of a conventional rf-coil allows the use of an air cooled ICP in combination with virtually any type of rf-generator. Its more rugged design improved the reliability of cooling and permits unattended full-day operation. Most important, the improved cooling permits significantly higher powers to be used. Up to 1200 W generator power can be used momentarily without torch melting and under normal operating powers of 600 - 800 W the lifetime of the torch is 2 - 3 weeks.
2.2 Torch design.
As described by RIPSON and de GALAN [24], a two tube torch is normally used (Figure 2a) for the analysis of aqueous solutions because of an enhanced stability at flows of about 1 1/min. However, a conventional three-tube torch may be used if the power level is somewhat increased.
0 7
o
ol
40 ;o0
O 0 i i1'
Ir
Figure 2. Design of the torch. Two tube set-up, used with aqueous solvents ( a ) ; three tube design used for organic solvents (b); torch with sidetube and observation window made of optical quartz, used in either set-up ( c ) . In a three-tube set-up, the plasma gas i s introduced between the outer and the intermediate tube. Dimensions in ram.
Conversely, for organic analyses a three tube set-up has the best stability (Figure 2b). In either case the outer silica tube extends 60 mm above the torch to protect the plasma from the turbulences of the air cooling. The effect of a torch extension on the background spec trum of a low flow torch has been described by SAVAGE and HIEFTJE [5]: the intensity of the continuum increases, while the intensity of the molecular bands in the spectrum decrease. As a consequence of the extension, the plasma is observed through the torch. If the tube becomes clouded, e.g. by devitrification or carbon deposit, the use of a side-tube with a silica window (Figure 2c) is advantageous. More over, a side tube improves the signal to noise ratio for lines below 250 nm, unless the entire torch is made of optical quality quartz. No influence of the side tube on the stability of the plasma is observed. The sample introduction system consisting of a V-groove nebulizer and a 35 ml nebulizer chamber has been described before [25].
2.3 Silica as a torch material.
The demands on an ICP torch material have been defined before [25]: maximum temperature of use above the operating temperature; mechani cally stable to withstand thermal shock, transparant to rf-fields and chemically inert at the operating temperature. Fused silica is a nearly ideal material, except for a moderate upper temperature of about 1300 K. With the present cooling device this temperature is not reached up to 1200 W generator power. However, at normal power levels other phenomena occur which can be attributed to the high operating temperature. When silica is heated above 600 K, it vaporizes at a temperature dependent, rapidly increasing rate. The amount of silica which vaporizes does not significantly diminish the wall thickness, but the deposit of vapour on the torch extension slowly diminishes its transparancy in the observation region. Torch attack also results from reactions of the fused silica with some sample constituents, espe cially acids. This phenomenon is also observed in the conventional ICP, but it is enhanced by the higher temperature of the air-cooled torch. In either case, the torch is easily cleaned by treating it with HF. Alternatively, observation of the plasma through a side-tube is advisable (Figure 2c) when solutions with a high acid content are analyzed. The main problem is however that after prolonged heating
above 1000 K fused silica recrystallizes to the structurally more stable form of high-cristobalite. This process is called devitrifica tion or recrystallisation. When the temperature of the devitrified silica is decreased below 275 C, the layer of cristobalite cracks and becomes translucent [26], The cracks often propagate into the unaf fected fused silica. The higher the operating temperature, the more rapidly the devitrification occurs. Its rate is considerably influen ced by impurities and can vary by a large factor. The purity of clear fused silica is very high and to keep the surface clean, a newly installed torch is cleaned with alcohol and dried with a tissue. This works well for most sample solutions. However, any sodium present in the analyte above concentrations of 100 ppm enhances the devitrifica tion process and significantly detoriates the lifetime of the tube. Thus, although fused silica satisfies our demands relatively well, for enhanced reliability it should be replaced with another material. An attractive possibility is to combine the principle of the air cooled plasma with the segmented torch used with the radiatively cooled plasma [25]. In this way, we expect to combine the advantages of both approaches.
2.4. Physical appearance of the low flow plasma.
Several reports on high-efficiency torches show that plasmas can be sustained on an argon flow down to 4 l/min [10 - 14], Below this flow, the plasma becomes unstable and eventually extinguishes. Indeed, some authors use this "quenching point" as a means to estimate the stabili ty of the plasma when developing high-efficiency torches [10,14]. However, the torches used in these studies had a small extension only. When we added a 60 mm torch extension (Figure 2a) and air cooling to a high-efficiency MAK torch, we were unable to note quenching of the plasma. Starting from 12 l/min the coolant gas flow was slowly dimi nished to 0.5 l/min. During the process, the generator power was kept at 800 W and the reflected power is continuously adjusted to zero. The presence or absence of 0.3 l/min carrier gas flow does not signifi cantly affect these experiments. At 12 l/min the plasma has a very long tail, obviously due to the torch extension. When the gas flow is
l/min, the plasma becomes somewhat unstable, but it is not extin guished. When the flow rate is 2 1/min the plasma becomes suddenly very white and is stable again. The plasma now has a spherical shape, rather than the conventional tail-flame shape. At 2 1/min, the plasma touches the wall of the torch, which is then glowing softly inside. A large hole is present in the centre of the plasma with a diameter of 5 .mm. When the gas flow is reduced further, the plasma shrinks somewhat in size, until a gap of 1 - 1.5 mm exists between the plasma and the torch, and the torch wall stops glowing. The hole in the plasma be comes smaller. At flows less than 1 1/min, the hole becomes just a darker spot in the plasma and a gap of 2 mm exists between the torch and the plasma. Below 0.5 1/min, the plasma shrinks somewhat further in size, but the brightness remains the same. No hole can be observed anymore. When the gas flow is turned off, the plasma gradually turns purple due to oxygen diffusion and extinguishes after one minute. Conversely, when the gas flow is raised from 0.5 1/min to 2 - 3 1/min, the spherical plasma becomes unstable. The conventional type of plasma does not appear spontaneously until an argon flow of 6 - 8 1/min. The transition from the conventional tail-flame plasma to the spherical plasma indicates that an observed quenching of a plasma below flows of 4 1/min is due to an enhanced diffusion of air, and not to an inherent instability of the plasma.
2.5 Operation of the low flow torch.
The small size of our 1 1/min plasma compels us to confine observa tions to a small region above the coil. Our observation region is between 2 - 7 mm. Since the observation area is smaller than in the conventional plasma, the observation height is a less important para meter. In the air-cooled plasma only two gas flows are applied. In deed, for many analyses, no change of observation height is necessary. Ignition of the plasma is straightforward. With aqueous solvents, the plasma gas flow is set to 2 1/min, the carrier gas flow to normal analytical conditions (150 - 200 ml/min), and the plasma is ignited at 700 W. Normal analytical conditions then are 500 - 800 W generator power, 0.6 - 1.0 1/min plasma gas flow, 150 - 200 ml/min carrier gas flow and a solvent delivery rate of 2 ml/min. Organic solvents may be used in a conventional ICP whith a typical increase in rf-power of
some 200 W, mainly because the organics take more energy to dissociate than water. This may cause problems in the low power torches since the power used for organic solvent dissociation will be a significant part of the total rf-power, making the plasma more vulnerable to instabili ties in the solvent introduction. In contrast to aqueous solutions, a three-tube torch offers better stability. When our plasma is operated at 800 W, introduction of MIBK and xylene is possible with the normal carrier gas flow and sample flow rates. The plasma remains stable, its color changes from bluish-white to an intense green, with a very small orange spot in the center. However, after only a few minutes, the torch extension is clouded by a steady deposit of carbon, which makes a side-tube necessary. The carbon deposit can be prevented by the addition of 10 - 15 ml/min oxygen to the plasma gas, which is applied between the intermediate and the outer tube. The exact amount of oxygen needed can be derived from a sudden change in color: if the amount of oxygen is enough to prevent carbon deposit, the color of the plasma changes from dark green to white. End-on observation of the plasma reveals a still green sample introduction hole, although the plasma is completely white. This indicates that in the center of the plasma a reducing atomosphere still prevails, which is advantageous for the analysis of refractories. If the plasma is used with the hydride technique, the introduction of hydrogen causes no problems, when the generator power is increased to 700 - 800 W. To ease the change-over between normal and hydride sample generation, the conven tional sample introduction system is maintained. One peristaltic pump and a reaction-chamber are added to the system; the nebulizing chamber acts as the liquid-gas separator. To enhance the signal to noise ratio below 220 nm, the side-tube torch proves to be advantageous.
2.6 Instrumentation.
Measurements are carried out with a nebulizing system which has been described earlier [25]. The relevant details of the two air cooled ICP instruments used in this study are given in Table 1. All reagents used are of analytical grade. Sample pretreatment of the solid reference samples is based on conventional procedures, and consists of dissolu
Table la. Instrumentation B RF-Generator Torch Nebulizing system Monochromator Photomultiplier Amplifier Philips PV 8490, 50 KHz, free running 2.0 kW, 2 turn coil Plasmatherm HFS 1000G, 40 MHz, crystal controlled 1.2 kW, 3 turn coil See Figure 2
Babington type, 100 um hole, integrated liquid feed 35 ml nebulizing chamber McPherson 2501 lm Czerny-Turner grating 1200 grooves/n blaze 250 nm slits 25 um EMI 9865 QA PAR 128A lock-in
Jarrel Ash 82-001 0.50 m Ebert g r a t i n g 1180 grooves/mm, b l a z e 300 nm s l i t s 25 um Hamamatsu R 106 UH PAR 120 l o c k - i n T a b l e l b . O p e r a t i n g c o n d i t i o n s Generator power Plasma argon flow
Aqi
Nebulizer argon flow Sample uptake rate Observation height Oxygen flow NaBH, uptake NaBH, concentr. HC1 concentration jeous solvents 600 - 900 W 0.7 - 1.0 1/m 150 ml/min 1 ml/min 2 - 5 mm
-_
Organic solvents 850 W I 1/min 150 ml/min 1 ml/min 2 - 5 mm 10 ml/min -Hydride introduction 850 W 1 1/min 0.3 1/min 5 ml/min 5 mm -5 ml/min 0.25 % 4 M3 sigma c r i t e r i o n with a time c o n s t a n t of 15 s e e s , t o f a c i l i t a t e com p a r i s o n with l i t e r a t u r e v a l u e s . E x c i t a t i o n t e m p e r a t u r e s a r e c a l c u l a t e d from t h e r a t i o of t h e Ti I I l i n e s a t 322.28 and 3 2 2 . 4 2 nm, a c c o r d i n g to MERMET [27]
3 . R e s u l t s and D i s c u s s i o n .
3.1 Detection l i m i t s i n aqueous s o l u t i o n s .
The f i r s t d e t e c t i o n l i m i t s r e p o r t e d for t h e e x t e r n a l l y cooled p l a s m a s w e r e i n f e r i o r t o t h e c o n v e n t i o n a l plasma [ 1 9 , 2 1 ] , I n a l a t e r study [ 2 4 ] , the d e t e c t i o n l i m i t s could be improved, but e s p e c i a l l y t h e l i n e s
below 250 nm (mostly "hard" lines [28,29] ) remained worse than in the conventional plasma. Recently, REZAAIYAAN and HIEFTJE [7] made the same observation for an high-efficiency torch, for which an excitation temperature of 4036 K was reported. This observation is confirmed by the measurements of MICHAUD-POUSSEL and MERMET with their high-ef ficiency' torch [14]. This could indicate that the problem is not connected to any particular type of low flow, low power plasma. Figure 3 shows the optimal power level for some hard and soft lines. It appears that although low flow plasma s can be operated on powers as low as 400 W, a generator power of 900 W is necessary to improve the detection limits for the hard lines. This observation is supported by the excitation temperature presented in Figure 4. At low power, the
S/p
108
6
4
2
-Nal 589.0 Zn I 213.9 Cr I 267.7 Co I 238.9 Cd I 228.9600 800 1000
Generator Power ,W
Figure 3. Dependence of the Signal to Background Ratio of various elements on the generator power. Other operating conditions as stated in Table lb. Concentrations introduced are 0.1 mg/1 (Ca); 1 mg/l (Cu, Na, Zn, Co); 2 mg/ml
Texc -K 6000 -5000 U000 3000 600 700 800 900 1000 1100
Generator Power. W
Figure 4. Variation of the excitation temperature with the applied generator power, measured with Ti ( I I ) lines in a two-tube torch.
Other operating conditions as stated in Table l b .
excitation temperature i s e q u i v a l e n t to the low value r e p o r t e d by
REZAAIYAAN and HIEFTJE. I t i n c r e a s e s with power, and a t 800 W i t
reaches a value comparable to that observed in conventional ICP. This
may i n d i c a t e t h a t the concepts used in the development of low flow
plasmas ( e . g . power balances, quenching p o i n t s ) may give a good i n
dication of the s t a b i l i t y of the discharge, but do not give an i n d i c a
tion of i t s analytical p o t e n t i a l . As a r e s u l t , the detection l i m i t s of
the l i n e s below 250 nm are s i g n i f i c a n t l y improved if the generator
power i s increased to 900 W (Table 2 ) . These detection l i m i t s a r e now
completely e q u i v a l e n t to the r e f e r e n c e values for a conventional
plasma.
3.2 Detection in organic solvents and with hydride generation.
The detection l i m i t s for some r e p r e s e n t a t i v e elements in xylene a r e
presented in Table 3 . Generally, they are similar to those shown in
Table 2 for aqueous solutions. The i n t e r a c t i o n of oxygen to p r e v e n t
carbon deposit has no adverse effects on the detection limits for such
refractory elements as Mo and Ti, as long as the g e n e r a t o r power i s
kept above 800 W. At lower wavelength (Fe, Pb, Sn, Zn) the detection
limits are degraded in comparison to aqueous s o l u t i o n s , caused by an
enhanced background n o i s e . The d e t e c t i o n l i m i t s with the hydride
technique are shown in Table 4 and c o n s t i t u t e an improvement over
conventional sample introduction by two orders of magnitude. Elements
with multiple o x i d a t i o n . s t a t e s are more sensitive in the lower-valence
s t a t e , which i s important since the destruction techniques yield the
elements in the higher s t a t e [ 3 2 ] . To improve the d e t e r m i n a t i o n of
wavelengths below 210 nm a s i d e - t u b e with a quartz window i s used
(Figure 2c), because the s i l i c a used for the torch has a very poor
t r a n s m i t t a n c e below 220 nm. Upon the introduction of organic solvents
Table 2. Detection limits of the air-cooled plasma under optimal operating power. Element [30] Al I B I Ba II Ca II Cd II Co II Cr II Cu I Fe II Ho II La II Mg II Mn II Na I Ni II Pb II Th II Ti II V II Ï II Zn I Wavelength (nm) 396.152 249.773 455.403 393.366 214.438 238.892 267.716 324.754 238.204 345.600 408.672 279.553 257.610 588.995 221.647 220.353 283.730 334.941 309.311 371.030 213.856 This study 14 2.9 0.4 0.06 2.1* 6.2* 4.0* 1.9 2.8* 4.3 9.0 0.11 1.0 1.0 6.0* 40* 49* 1.3 11 2.9 2.3* RIPS0N et a 40 10
-0. 16 44 10 5 -10 0. 2 7 26 230 -8 -12 13 3 1 [24] WINGE 28 4.8 1.3 0.19 2.5 6.0 7.1 5.4 4.6 5.7 10 0.15 1.4 29 10 42 65 3.8 5.0 3.5 1.8*:Measured at 900 W generator power, otherwise 600 W generator power i s used. Other operating conditions as stated in Table l b .
Table 3. Detection limits of some elements in xylene. Element Wavelength (nm) Al I Ca II Cu I Fe II Mg II Mo II Pb II Sn II Ti II Zn I 396.152 393.366 324.754 238.204 279.553 202.030 220.353 189.989 334.941 213.850 Detection limit (ng/ml)
This study literature literature high-efficiency conventional [34] 10 2 3 14 0.8 221 15 air-coo 20 0.2 2.0 12 0.15 8.5 500 50 1.4 5.0 led high-e: [15] 17 4.8 4.0 6.8 1.0
-21 -3.0Operating conditions as stated in Table l b .
Table 4. Detection l i m i t s of some elements with hydride generation.
Element Wavelength (nm) As I (3+) 193.696 As I (5+) 193.696 Sb I (3+) 206.833 Sb I (5+) 206.833 Se I (4+) 196.026 Bi I (3+) 223.061 Hg 11(2+) 194.227 This 0.6 0.9 1.0 3.0 1.1 0.6 1.0 Detection study limit (ng/ml) THOMPSON et al [32] 0.8 1.1 1.0 8.0 0.8 0.8 "
—
Operating conditions as s t a t e d in Table l b .or hydrogen the impedance of the plasma changes significantly. Conse
quently, variations in the sample introduction r a t e negatively a f f e c t
the plasma s t a b i l i t y , t h e more so s i n c e the Plasma-Therm generator
features a semi-automatic matching-network only. This phenomenon may
well offer an important explanation for the need for more power.
3.3 Reference materials.
Generally, the analyses on reference materials show results which on the whole are in good agreement with the certificate values (see Table 5 ) . Our 95% confidence intervals are somewhat larger than commonly reported in the literature, which should be attributed to our ex perimental set-up. With hydride generation, a long-term variation of the background occurs. We believe that this effect might be caused by an imperfect drain. The interferences encountered seem to be quite equivalent to the conventional plasma, as reported earlier [24]. However, a full scale investigation 'of the interferences of the air-cooled plasma will be reported in a future article. With all deter minations the method of standard additions is used to overcome the interferences of the matrix.
3.4 Analysis of samples with high salt concentrations.
The introduction of solutions with high concentrations of solids is notoriously troublesome in the ICP. Clogging of the nebulizer or the the carrier gas flow tip causes signal instabilities or even extin guishes the plasma. Often the system must be cleaned with acid after each sample or operated under non-optimal conditions. Recently, BAGIN-SKI and MEINHARD [33] showed that most problems are caused by clogging of the carrier gas tip, rather than by clogging of the nebulizer. Introduction of 41% aluminiumsulfate extinguished their conventional plasma after 3 to 20 minutes due to clogging of the carrier gas tip. Somewhat surprisingly, the air cooled plasma shows none of these problems when highly salted aqueous solutions are introduced. For instance, when a saturated (65 % w/w) aluminiumsulfate solution is introduced under normal gas and sample flow conditions, the plasma runs stable for an entire day and no salt particles are found in either the nebulizer or the carrier gas tip. Figure 5 shows the signal of 1 mg/1 Mg (279.5 nm) in 65 % aluminumsulfate, compared to the signal of the same concentration in water. Although with aluminiumsul fate the noise is enhanced, the long-term stability is excellent. It is to be noted that the signal enhancement due to a matrix of this concentration is only about 10 %.
Table 5. Analysis of reference samples. A. Fly-ash Element Wavelength Al I 396.152 Ca II 393.366 Fe II 238.204 Found (%) 22.7 5.01 8.14 reference (%) 22.7 4.6 8.6
B. Bovine liver NBS/SRM 1577A
Element Wavelength Found (mg/kg) Mg II 280.270 588 + 50 Ca II 393.366 120 + 12 Cu I 327.396 157 + 14 Fe II 259.940 209 + 13 Mn II 257.610 12.1 + 1.3 Sr II 407.771 0.10 + 0.04 Zn I 334.502 150 + 40 certified (mg/kg) 600 + 15 120 + 7 158 + 7 194 + 20 9.9 + 0.8 0.138 + 0.003 123 + 8 C. Citrus Leaves NBS/SRM Element Wavelength Mg II 280.270 Ca II 393.366 Cu I 327.396 Fe II 259.940 Mn II 257.610 Sr II 407.771 Zn I 334.502 Found (mg/kg) 0.63 + 0.04% 3.2 + 0.2 19 + 41 98 + 15 28 + 12 100 + 20 30 + 5 certified (mg/kg) 0.58 + 0.03 % 3.15 + 0.1 6.5 + 1 90 + 10 23 + 2 1 0 0 + 2 29 + 2
D. Arsene in Steel with hydride technique
Sample Found (%) certified (%) NBS S5C 0.069 + 0.023 0.007 BAM 187-1 0.058 + 0.012 0.048 BAM 9 (079-1) 0.046 + 0.016 0.051 BCS 260/3 0.029 + 0.017 0.026
E. Wear metals in Oil NBS/SRM 1572 Element Al I Cu I Fe II Mg II Mo II Pb II Ti II Wavelength 396.152 324.754 238.204 279.553 202.030 220.353 334.941 Found (mg/kg) 100 + 12 9 5 + 8 93 + 11 101 + 3 93 + 12 111 + 20 9 4 + 8 certified (mg/kg) 98 + 2 9 8 + 4 100 + 3 98 + 4 97 + 5 101 + 4 99 + 5
Signal intensity 1ug/ml Mg 1201 1201 0 — -100 90-1 r//i 1 0 1 2 5 6 Time , hours
Figure 5. Long-terra variation of the signal of 1 mg/1 Mg (279.553 nm) in 65 % aluminium sulfate ( ) and in water ( ).
4. Conclusions.
The air cooled ICP operated at 1 1/min argon and 600 - 900 W generator power, has been evaluated for analytical purposes. Detection limits in aqueous, organic and hydride solutions indicate that the air cooled plasma can be as sensitive as a conventional plasma. In order to increase the sensitivity of the "hard" lines to the level of a conven tional plasma, the generator power must be raised to 900 W. Literature data indicate that this observation might also apply to other types of low flow, low power plasmas [7,14]. Although a full scale study of interferences will be referred to a future study, the results obtained for reference materials show that the air cooled plasma can be applied to real samples.
•t x
References
[1] C D . Allemand, ICP information Newslett. 2_, 1 (1976). [2] G.M. Hieftje, Spectrochira. Acta 38B, 1465 (1983). [3] R.N. Savage and G.M. Hieftje, Anal. Chem. 5J_, 408 (1979). [4] R.N. Savage and G.M. Hieftje, Anal. Chem. 5_2_, 1267 (1980). [5] R.N. Savage and G.M. Hieftje, Anal. Chim. Acta 123, 319 (1981).
[6] A.D. Weiss, R.N. Savage and G.M. Hieftje, Anal. Chim. Acta 1_24_, 245 (1981). [7] R. Rezaaiyaan and G.M. Hieftje, Anal. Chem. 57, 412 (1985).
[8] C D . Allemand, R.M. Barnes and C.C. Wohlers, Anal. Chem. 51_, 2392 (1979). [9] C D . Allemand and R.M. Barnes, Appl. Spectrosc. 3j_, 434 (1977).
[10] R. Rezaaiyaan, G.M. Hieftje, H. Anderson, H. Kaiser and B. Meddings, Appl. Spectrosc. 36, 627 (1982).
J.L. Genna, R.M. Barnes and C D . Allemand, Anal. Chem. 49_, 1450 (1977). A. Montaser, G.R. Huse, R.A. Wax, S. Chan, D.W. Golightly, J.S. Kane, A.F. Dorrzapf, Anal. Chem. 56_, 283 (1984).
G. Angleys and J.M. Mermet, Appl. Spectrosc. ^ 8 , 647 (1984) E. Michaud-Poussel and J.M. Mermet, in press.
R.C. Ng, H. Kaiser and B. Meddings, Spectrochim. Acta 40B, 73 (1985). R. Rezaaiyaan, J.W. Olesik and G.M. Hieftje,
Spectrochim. Acta 40B, 73 (1985).
H. Kawaguchi, T. Ito, S. Rubi and A. Mizuike, Anal. Chem. 5_2_, 2440 (1980). H. Kawaguchi, T. Tanaka, S. Miura and A. Mizuike,
Spectrochim. Acta 38B, 1319 (1983).
G.R. Kornblum, W. van der Waa and L. de Galan, Anal. Chem. 5J_, 2378 (1979). G.R. Kornblum, Proc. Winter Conf. on Plasma Chemistry,
San Juan, 1980, ed. R.M. Barnes, Heyden, London (1981).
P.A.M. Ripson and L. de Galan, Spectrochim. Acta 36B, 71 (1981). P.A.M. Ripson, L. de Galan and J.W. de Ruiter,
Spectrochim. Acta 37B, 733 (1982).
P.A.M. Ripson and L. de Galan, Spectrochim. Acta 38B, 707 (1983). P.A.M. Ripson, E.B.M. Jansen and L. de Galan, Anal. Chem. 56, 2329 (1984). P.S.C van der Plas and L. de Galan, Spectrochim. Acta 39B, 1161 (1984). Bulletin Q-A 1/112-2, Heraeus Quarz Schmelze GMBH, Hanau, W. Germany. J.M. Mermet, Thesis. Universite Claude Bernard, Lyon (1974).
M.W. Blades and G. Horlick, Spectrochim. Acta 36B, 861 (1981). P.W.J.M. Boumans and F.J. de Boer, Spechtrochim. Acta ^2B, 365 (1977). R.K. Winge, V.J. Peterson and V.A. Fassel, Appl. Spectrosc. J33, 206 (1979).
A.W. Boom and R.F. Browner, Anal. Chem. 54_, 1402 (1984). M. Thompson, B. Pahlavanpour, S.J. Walton and G.F. Kirkbright, Analyst 103, 568 (1978).
B.R. Baginski and J.E. Meinhard, Appl. Spectrosc. 38, 568 (1984). A.F. Ward and L. Marciello, Jarrel Ash Plasma Newsl. 1_, 10 (1978). M.W. Blades and B.C. Caughlin, Spectrochim. Acta 40B, 579 (1985).
CHAPTER 3
A RADIATIVELY COOLED TORCH FOR ICP-AES USING 1 L/MIN OF ARGON
1. Introduction.
A significant obstacle to the more rapid proliferation of inductively coupled plasma (ICP) spectrometry among smaller laboratories is the high cost of acquiring and running the instrument. This might explain the increasing interest in more economical ICP torches which require substantially less argon gas and rf-power than the current commercial ly available torches. In his revieuw on this subject, HIEFTJE [1] distinguishes three different approaches to reach this goal: (i) miniaturized torches that are about twice as narrow as conventional torches; (ii) high-efficiency torches of conventional dimensions, but with carefully optimized torch geometry; (iii) externally cooled torches that use either water or air as external coolant. Although the performance of the different approaches, both in economical and in analytical aspect, varies considerably, they have as a common feature that the flow of argon is reduced. The presence of the high argon flow in conventional ICP torches stems from the need to protect the outer silica torch tube against overheating. Indeed, if the argon flow is to be reduced drastically, and fused silica is maintained as the torch material, special precautions -specifically, external cooling- must be taken. However, as was first suggested by KORNBLUM [2], such precau tions are not required, if a torch material is used that can withstand a higher temperature. HIEFTJE [1] has suggested that a tube heated to 1400 K will release sufficient power by means of heat radiaton to sustain an ICP. In this paper the feasibility of this approach will be
runs on 600 W and a total argon flow of 1 1/min without external cooling. This is achieved by using a segmented outer torch tube with the central part made of boron nitride.
2. Theory of radiative power release.
2.1. Power balance.
The following analysis of a radiatively cooled ICP torch will be based on the extensive power balances formulated by RIPSON and DE GALAN [3]. As these authors have shown, a well tuned fixed frequency rf-generator succeeds in delivering about 70 - 80% of the generator power (Pgen) in the torch. The remaining 20 - 30% is dissipated as heat in the match ing circuit and in the work coil. The power dissipated in the torch (Ptorch) is only partly used to sustain the plasma (Pplasma) and to heat and atomize the sample aerosol (Psample). Depending on the torch construction, major portions may be lost by heating an excess argon flow (Pconv) and/or by dissipation through the outer torch tube (Pwall). The general power balance of an ICP torch can thus be expres sed as:
0.75 Pgen = Ptorch = Pplasma + Psample + Pconv + Pwall {1}
In a power-efficient torch the latter two terms, Pconv and Pwall, would be negligible in comparison to the former two, Pplasma and Psample. For example, if we assume that at least 1 1/min of argon is needed to carry 0.5 mg/s of sample aerosol into the plasma and to sustain the plasma, the minimum power required in this torch can be calculated [3] to be the sum of 65 W to heat the plasma argon to an average gas kinetic temperature of 3500 K, 25 W to account for conti nuum radiation of the plasma and 25 W to heat and dissociate the aqueous aerosol. For organic solvents the latter contribution would increase to about 200 W. We conclude therefore, that this hypothetical ICP would require no more than 100 - 300 W torch power or 150 - 400 W generator power. Obviously however, such a plasma would be rather sensitive to variations in the sample introduction, especially when using organic solvents. The additional power used in a real ICP may well act as a buffer against such fluctuations. With a conventional
torch an outer argon flow of 15 1/min requires another 1000 W (Pconv). Even when the argon flow is reduced to 5 1/min, as is realized in miniaturized and high-efficiency torches [1], an additional 350 W is still required. On the other hand, in the conventional and the high-efficiency torch the outer tube is so well shielded that virtually no power is transfered from the plasma to the wall (Pwall - 0 ) . Conver sely, when the excess argon flow is completely abandoned (Pconv = 0 ) , the tube cannot be shielded that effectively and power is lost by conduction through the outer tube and subsequent release to the envi ronment. According to RIPSON and DE GALAN [3], the latter process requires external cooling, if the temperature of the fused silica tube is to be maintained below its softening point. Indeed, with water cooling the torch tube can easily be kept at room temperature, but a substantial amount of power is lost through the wall (Pwall ^ 800 W)
[4]. However, with the less efficient air cooling, the tube tempera ture rose to about 1000 K, in which case a minor but significant part of the power conducted through the torch tube (Pwall -300 W) was released by heat radiaton. Therefore, if we want to abandon external cooling, the power lost from the plasma through the torch tube can be expressed as:
Pwall = Pcond = Prad
Pwall = A.Q.AT/d=A.e.a.T4 (2)
w
where A is the heated tube area, Q is the thermal conductivity of the tube material, AT is the temperature drop over the tube thickness d, £ is the emissivity of the tube, a is the Stefan-Boltzman constant, and T is the outer temperature of the tube. In this expression the power dissipated by free convection and the influence of heat radia tion reflected back to the torch by the coil or the enclosure are neglected. The consequences of the expressions in Equation (2) are presented in Figure 1. The fourth power dependence of the radiative power transfer on the outer tube temperature makes this contribution to power release relatively insignificant for tube temperatures below
2
2 0 0 0 -P,W 1000 5 0 0 2 0 0 - 100-1000 1500 2 0 0 0 T.K
Figure l . Radiative power release by a heated surface. Upper and lower curve 2
have been calculated after Equation 2 for an area of 20 cm and an emissivity of 1 and 0.5, respectively; For the case of an emissivity of 1, the curve Ti n gives the corresponding inner temperature, calculated for a wall thickness of 1 mm and a thermal conductivity of 2.8 W m K
only 0.5 the outer temperature of the tube wall must be i n c r e a s e d to
1670 K to release t h i s amount of power. Simultaneously, the inner wall
temperature i s higher and related to the outer temperature through the
necessary conduction through the wall. Since for obvious reasons the
thickness of the wall cannot be chosen much smaller than 1 mm, the
temperature drop across the tube i s mainly determined by i t s thermal
conductivity. However, as has been indicated in Figure 1, even for a
r e l a t i v e l y poor thermal conductor as fused s i l i c a (Q=2.8 W m K at
1000 K) the temperature difference amounts to only 80 K for a power
transfer of 440 W.
2.2. Torch materials.
From the curves in Figure 1 the f i r s t requirements for m a t e r i a l s
suitable for a radiatively cooled ICP torch can be readily derived. To
keep the inner temperature of the outer tube as low as possible, the
emissivity should be close to unity and the thermal conductivity must
be sufficiently high. To dissipate between 500 and 1000 W the material
must withstand temperatures ranging from 1500 to 1800 K. With lower
values for the emissivity and the thermal conductivity, the correspon
ding tube temperature would r i s e to values well over 2000 K. However,
t h e s e a r e not the only r e q u i r e m e n t s . To prevent damage to the tube
when the plasma i s ignited, the material should have a high thermal
shock r e s i s t a n c e , which property i s proportional to the thermal con
ductivity and the t e n s i l e strength and inversely p r o p o r t i o n a l to the
c o e f f i c i e n t of thermal expansion and Young's modulus. Also, the
m a t e r i a l should have a low e l e c t r i c a l c o n d u c t i v i t y t o p r e v e n t
i n t e r a c t i o n with the r f - f i e l d . These requirements v i r t u a l l y r e s t r i c t
the choice to ceramic materials. Relevant data for some possible mate
r i a l s a r e c o l l e c t e d in Table 1, although i t must be emphasized that
these can only serve as rough estimates. The actual values a r e highly
dependent upon the manufacturing process and the exact composition. If
we take fused s i l i c a as a reference material, i t i s c l e a r t h a t t h e r e
a r e s e v e r a l m a t e r i a l s t h a t allow a higher tube temperature, even in
the presence of oxygen. The thermal conductivity poses no problem, but
the thermal shock r e s i s t a n c e shows a wide v a r i a t i o n . For example,
alumina allows by far the highest temperature, but i t s high expansion
coefficient and high Young's modulus lead to a poor resistance against
thermal shock. The e l e c t r i c a l conductivity of the ceramics i s s u f f i
c i e n t l y low except for s i l i c o n c a r b i d e a t e l e v a t e d temperatures.
Finally, i t must be remarked that with the exception of fused s i l i c a ,
none of t h e s e m a t e r i a l s i s transparent to optical radiation. Conse
quently, spectrometric o b s e r v a t i o n s w i l l r e q u i r e a d d i t i o n a l design
considerations.
Table 1 . Physical p r o p e r t i e s of high-temperature ceramics.
A12CL SiC Si3N4 BeO 2200 1650 1700 1700 10 50 17 40 15 20 20 10 7 4 3 6 s i l i c a 1350 2 . 8 1650 10 fused BN Tmax, K ( o x i d i z i n g atmosphere). Thermal conductivity a t 1000 K, W.of1.*"1 7 -2 Tensile strength, 10 N.m 5 3 Coefficient of thermal expansion, 10 .K 0.6 2
3. Experimental.
The feasibility of the principle of the radiatively cooled plasma was
tested on fused silica and four other ceramics. Alumina (99.2% Al^O-;
Corning, O h i o ) ; translucent alumina (A1„0.,; P h i l i p s , Eindhoven);
zirconia (ZrCL; Corning, Ohio); silicon carbide (SiC; Corning, Ohio)
and boron nitride (BN; grades HP and M, Carborundum, Niagara Falls)
were machined to tubes with an inner diameter of 16 mm and outer
diameter of 18 mm. Of these materials, boron nitride could be machined
on a lathe, the other three materials required special glass-polishing
tools. The tube was mounted in a dismountable torch holder in which
the torch tubes could be aligned independently. Initially, an arrange
ment without an intermediate tube was used, as described by RIPSON et
al. [5], For the experiments with boron nitride a special, segmented
outer tube has been designed (Figure 2 ) . This tube is made of two
pieces of fused silica with a central part of boron nitride, which is
located in the coil. The three pieces are machined to close tolerances
and simply connected without any adhesive. The design has the obvious
advantage that it permits observation of the plasma through the torch.
A conventional three-tube design was completed by adding two
additio-silica
BN
/A\
-15-Figure 2. Three-tube torch with a segmented outer tube made from boron nitride (shaded central part) and fused silica (open); dimensions in mm.
nal tubes with dimensions available in the laboratory. The interme diate tube and the segmented tube are separated by 0.4 mm, while the carrier tip diameter was chosen to be 0.5 mm. The torch is mounted in a standard three-turn coil.
The nebulizing system consisted of a V-groove nebulizer and a spray chamber described by RIPSON and DE GALAN [6]. To improve the resistan ce against agressive sample solutions, the nebulizer is now made of PCTFE (Kel-F) rather than stainless steel. Additionally, a sample channel was added to the design to enable easy and reproducible admi nistration of the sample without the need for a second entrance in the nebulizer chamber (Figure 3). The gas nozzle diameter was kept at 0.1 mm and the sample channel diameter is 1.0 mm wide. Further instrumen tation and operating conditions are listed in Table 2. Detection limits were calculated according to the 3 sigma criterion. The genera tor power is read directly from the rf-power meter available on the generator. Since the reflected power could always be adjusted to zero, the power dissipated to the torch can be estimated from this reading as 0.75 Pgen. The temperature of the outer torch tube has been measu red with an optical pyrometer (LN 8627; Leeds and Northrup, Pennsyl vania) . 10 nebulizing chamber sample
t
i\
J"KWA argonFigure 3. V-groove nebulizer from PCTFE (Kel-F), modified after [ 6 ] ; dimensions i n mm.
Table 2. Instrumentation and operating conditions for the radiatively cooled ICP R.F. generator Plasmatherm HFS 1000 G; 40 MHz, 1.2 kW, 3 turn coil. Torch See F i g u r e 2 . N e b u l i z e r system See F i g u r e 3 . Monochromator Mc Pherson 2051; 1 m Czerny T u r n e r ; g r a t i n g 10 x 10 cm; 1200 grooves/mm; b l a z e 250 nm; s l i t s 25 um. P h o t o m u l t i p l i e r EMI 9865 QA O p e r a t i n g c o n d i t i o n s rf-power 600 W o b s e r v a t i o n h e i g h t 2 . 0 mm plasma g a s flow 800 ml/min c a r r i e r g a s flow 200 ml/min s o l v e n t d e l i v e r y r a t e 2 . 0 ml/min.
4. Results and discussion.
4 . 1 . Operation of radiatively cooled torches.
As expected, a torch e n t i r e l y made of fused s i l i c a s t a r t e d to glow
r a p i d l y when the plasma was ignited and maintained at 600 W generator
power and 0.5 1/min argon flow only. After about 30 seconds t h e t o r c h
extension (the part of the torch above the c o i l ) started to t i l t as a
result of the softening of the s i l i c a in the c o i l . If the p o s i t i o n of
the t o r c h extension was continuously corrected with a glass rod, t h e
plasma could be maintained for about 15 minutes. This proves t h a t t h e
temperature of the outer tube i s only a few 100 K above the softening
point of fused s i l i c a . When the outer tube was replaced by one made of
s i l i c o n c a r b i d e , i t was not p o s s i b l e t o i g n i t e the plasma because
spark bridges appeared between the torch and the c o i l , piercing h o l e s
through the s i l i c o n carbide tube. Attempts to ignite the plasma with
an iron rod inserted in the coil f a i l e d . Obviously, t h e s p a r k s are a
r e s u l t of the r e l a t i v e l y low ohmic resistance of silicon carbide a t
increased temperatures, i m p l i c a t i n g t h a t s i l i c o n c a r b i d e i s not a
suitable material. No sparking problems occurred with outer tubes made
of alumina and zirconia. Indeed, with both materials a plasma could be
i g n i t e d and maintained for a w h i l e . However, upon i g n i t i o n small
cracks appeared in the portion of the outer tube enclosed by the c o i l .
Upon repeated extinguishing and re-ignition of plasma, the number of cracks increased rapidly until the torch extension was severed inside the rf-coil. Apparently, the thermal shock resistance of alumina and zirconia is not sufficient to withstand the temperature shock. Much better results were obtained with an outer tube made of boron nitride. The plasma could be easily ignited and maintained for hours. Repeated ignition did not destroy the torch and no sparking holes were obser ved. However, plasmas run on less than 1 1/min of total argon required an extended outer tube to improve the stability of the plasma and to minimize air entrainment. Because the opaque boron nitride does not permit side-on observation, the segmented outer tube shown in Figure 2 was designed. After initial experiments with a two-tube torch after RIPSON and DE GALAN [5], it was observed that a more stable plasma was obtained with a conventional three-tube arrangement as shown in Figure 2. The following observations refer to this torch.
The plasma is ignited with an incident power of 800 W and 2.0 1/min plasma gas applied between the outer tube and the intermediate tube; no argon gas is applied between the intermediate and the sample intro duction tube. The argon carrier gas flow (0.2 1/min) can be either on or off.
Although with other conditions a plasma can be ignited, persistent capacitive discharges appear downwards to the torchholder. The plasma usually ignites readily and the desired operating conditions can then be set freely with a generator power between 500 and 1000 W and a total argon flow between 0.2 and 2.0 1/min. The plasma is very stable and insensitive to gas or power fluctuations. For instance, the plasma gas may be changed from 0.2 to 1.0 1/min or vice versa within a second without the plasma showing any instability. However, if the generator power is increased above 800 W in the presence of sample solution, the boron nitride segment is rapidly oxidized, leading to the evaporation of boric acid which precipitates onto the silica torch extension and the coil. To improve the resistance against oxidation of the outer tube, grade HP boron nitride (93% boron nitride) was exchanged for grade M (40% boron nitride, 60% Si0„). Indeed, this material is much better resistant against oxidation, although some evaporation of boric
The boron nitride segment extends only 1 mm above the coil. This implies that the bottom of the upper fused silica extension is close to the plasma and thus susceptible to heating near or above its softe ning temperature. As a result, the silica extension becomes fused to the boron nitride segment after a few hours of operation. However, the torch remained stable and no other disadvantages were observed. The plasma resides completely in the boron nitride segment with a tail of about 10 mm extending above it. Background emission measurements indicate a point of maximum emission about 5 mm above the coil. The plasma is white, with a green hue from boron oxide bands that is particularly obvious at the edges of the tail. If the plasma is viewed end-on, a clearly visible dark hole can be observed in the center of the plasma. The following data have been collected during 75 hours of operation, with the plasma being ignited some 20 times. Throughout the experiments the plasma remained very stable.
4.2. Temperature of the torch.
The power transfer through the torch tube (Pwall) may be estimated from the outer tube temperature provided that the emissivity of the tube material is known. This quantity appears as a parameter in the measurement of the tube temperature with the optical pyrometer and also in the calculation of the radiative power release through Equa tion (2).
•-1800 Temperature along BN segment
T, K -1600 -1400 -1200 -1000 RF-coil
© © ©
10 20 30 height,mmFigure 4. Temperature profile of the boron nitride section of the outer tube, measured with an optical pyrometer, for a generator power of 700 W,
