The analysis of ambient air using atmospheric pressure chemical ionization mass spectrometry

TUE AUALYSIS OF AMBIEUT AIR

USIUG ATt[)SPHERIC PRESSURE CUEMICAL IOtlIZATION

t

·

1ASS

SPECTROMETRY

by

U. M. Reid,

J.

B. French and

J.

A. Buckley

TECHN\SCHE

HOGESCHOOL nEl:

.

_r

• I ' " ~ nE~H •.

LUCHT ,,: ... - •

. - DELFT

KIL'{~~;" ';J

THE ANALYSIS OF AMBIENT AIR

USING ATIDSPHERIC PRESSURE CHEMICAL IONIZATION MASS SPECTROMErRY

by

N. M. Reid, J. B. French a.nd J. A. Buckley

Submitted January, 1978

p

Acknowledgement

The authors thank Sc1ex Inc. for access to a TAGA(TMJ 2000

system on which the

bulk

of this work was carr1ed out.

J>.

Abstract

The Trace Atmospheric Gas AnaJ.yzer (TAGA) technique is based on the use of mass spectrometry to identify and quantify ultra trace species in gases at atmospheric or near atmospheric pressure. To make this practicaJ. for parts ratios in the range of interest between 1 in 10

6

to 1 in 1014 in ambient air it is necessary to produce a massive degree of pre-separation or pre-selection of the trace gas molecules from air molecules before their

. introduction into the mass spectrometer. This is done by an extremely rapid preferentiaJ. ionization at atmospheric pressure of the trace gas molecules using ions produced in a primary ionization process from air and water

contained in the air (reactant ions). This generaJ. process of using reactant ions fr om a carrier to ionize traces of interest in the carrier has been termed ChemicaJ. ionization (Cl). If this process is carried out at atmos-pheric pressure it is termed atmosatmos-pheric pressure chemicaJ. ionization.

This report gives a brief description of the technique, a general review. of Cl and some specific results of the anaJ.ysis of ambient air using this technique.

1. 2.

3·

4.

CONTENrS Acknow1edgement Abstract INrRODUCTION

OVERALL DESCRIPTION OF TEE TAGA 2000 SYSTEM CHEMICAL IONIZ.ATION MASS SPECTROMErRY

ATMOSPHERIC CHEMICAL IONIZATION MASS SPECTROMErRY IN AMBIENT AIR

REFERENCES FIGURES

APPENDIX A - RECOMMENDED BACKGROUND READING MATERIAL ON MASS SPECTROMErRY AND ION MOLECULE CHEMISTRY

i i i i i 1 1

4

9

15

APPENDIX B - LIST OF SOME OF THE COMPOUNDS DEfECTED BY ATIDSPHERIC CHEMICAL IONIZ.ATION IN TEE POSITIVE MODE

APPENDIX C - LIST OF SOME OF THE COMPOUNDS DEfECTED BY ATIDSPHERIC CHEMICAL IONIZ.ATION IN THE NEGATIVE MODE

1. INTRODUcrION

The Trace Atmospheric Gas Analyzer (TAGA) technique is based on the use of mass spectrometry to identify and quantify ultra trace species in gases at atmospheric or near atmospheric pressure . To make this practical for parts ratios in the range of interest between 1 in 10

6

to 1 in 1014 in ambient air it is necessary to produce a massive degree of pre-separation or pre-selection of the trace gas molecules from air molecules beforetheir introduction into the mass spectrometer. This is done by an extremely rapid preferential ionization at atmospheric pressure of the trace gas molecules using ions produced in a primary ionization process from air and water

contained in the air (reactant ions). This gener al process of using reactant ions from a carrier to ionize traces,of interest in the carrier has been

termed chemical ionization (Cl). Due to the fact that this ionization process occurs at atmospheric pressure in the TAGA instrument, the number of collisions that occur between the ionizing species and the trace is enormously increased compared to low pressure electron impact' ion sources, conventionally used in mass spe ct romet ry , and is also large in comparisOn to conventional Cl sources operating at ,...., 1 torr. Ionization efficiencies are extremely high which explains, in part, the potentially high sensitivities that can be obtained by this technique. However, it is also equally necessary to maximize all the sequences of transfer coeffic1.ents that exist between 'the original 'signal' trace molecules in air and their final arrival as mass-analyzed ions per

second at the detector. This has been done in the TAGA instrument and accounts for the very high sensitiv'ities that have been obtained. (Note: Much of which is described here also applies, to, the analysis of ultra trace

species in gases other than air.) ,

2 . OVERALL DESCRIPrION OF THE TAGA 2000 SYSTEM

In this section a brief overall description of the TAGA 2000*is given. The TAGA system has been designedto meet many applications and has therefore been designed in modular form. The following description applies to the TAGA 2000 configuration for ambient air analysis usirig certain air-intáke subsystems which were specifically designed for this task. Other intake modules have been designed to cover other tasks. _{, '}

Figure 1 is a block diagram of the TAGA 2000 systemand 'Fig. 2 is a photograph, of the system. A high capacity air pump draws ambient air through the inlet duct and into the atmospheric pressure ionization (API)

region at flow rates, 'which are variable up to 100 litre!min. Large, flow

rates,ensure mi~imtimsample memoryeffects on the duct walls. Th~ dimensions ,

of the' itllet duet are chosen so that adequate mixing o,f any calibrant gas added at the entrance of the inlet is achieved before being ionized in the API source. Calibrant gas at known flow rates can be added at the duct inlet to allow quantitative calibration of the system orto characterize the system' s response to a specific compound. It is preferable in any sampling situation, where feasible (e.g. through the wallof a vehicle in a mobile installation),

" to use a short inlet duct. This minimizes memoryeffects in the system, thus

, rapid changes' in air composition can be analyzed (e. g. in aircraft flight

'thi'ough a plume, downward of an industrial installation). However, the air pump has the capability of aspirating large sample flows through a considerable length of sampling tube which can be substituted for the existing air inlet where necessary; in this event a potential for sample tube memoryeffects exists as' in all conventional sampling systems.

*Developed initially at illIAS and now marketed by Sciex, Inc.,

55

Glencameron Rd., Thornhill, Ont ari 0 , L3T lP2.

The atmospheric pressure ionization (API) source causes very fast ion formation from traces within the central stream of the flowing sample. This process is described in more detail in the next two sections. This

source utilizes a central needle to produce reactant ions by corona dis charge . Stable currents of either positive or negative ions controllable between 10-7 and 10-5 amps can be produced. Electric fields are applied in the API source to focus the signal ions onto the aperture to the air-vacuum. interface and ultimately to the quadrupole mass spectrometer. The air stream is drawn away into the discharge plenum. by theair pump.

The atmospheric pressure vacuum interface subsystem provides several. key functions in atmospheric air sampling applications.

Direct analysis of air by samplingthrough an orifice into a mass spectrometer is not satisfactory because:

(a) Particulate matter clogging the origice nor.mally presents a practical. problem and necessitates frequent cleaning. This is particularly true under dusty conditions or other adverse situations such as combustion product sampling.

(b) The expansion of ion-air mixtures into vacuum. usually is accompanied by signal loss or unwanted mass spectral alteration due to clustering or condensation effects around the ions, particularly associated with the water vapour'in air.

These problems have been overcome in this system. The atmospheric pressure vacuum interface separates the functions of trace ion creation, ion extraction and ion injection into the mass spectrometer in a manner which precludes the s~ultaneous injection of particulate matter in ambient air into the mass spectrometer.

The atmospheric pressure vacuum interface also contains a very thin, specially designed gate valve to close the interface apert~e isolating the vacuum. system from the atmosphere, permitting aJJnost instantaneous shut down,

or start up if the cryogenic pump in the vacuum. system has been left in a stand-by mode.

The cryopump system was designed to be completely automatic in operation. The pumpdown sequence is initiated with a single push button and the rest of the operation is automatic and fail-sa.:fed throughout. The system is rough pumped wi th an adsorption trapped, direct drive mechanical pump for starting; sub sequently , it is isolated fram this source and is then pumped by a cryopump array cooled by a high performance refrigerator. The cryopump was specifically developed to provide an optimum cryopumping conf'iguration for the tasks of pumping the free jet expansion and the mass spectrometer

chamber separately. The gas ion mixture is admitted at one end and expands in a free jet. The gas ions follow their expansion trajectories until

collected by the ion lenses and then are injected into the mass spectrometer for analysis according to their mass to charge ratio.

From a completely shut-down condition the vacuum system takes about 3-1/2 hours to reach its operational condition. However, it can be left overnight in a stand-by mode unattended, with the isolation valve closed. The cryopump can accept gas deposition for many days operation of

continuous sampling before the necessity of recycling, which is automatically accomplished overnight by a simple pushbutton. A conical baffle in front of the mass spectrometer defines a second stage vacuum region which includes the mass spectrometer and ion detector. The effective pump speed for this second stage vacuum region is hundr~dS of litres/sec and the typical operating

pressures are in the mid 10- .torr range, as indicated by the gauge supplied. The operational characteristics of the specially modified quadrupole mass spectrometer

(Q,MS)

are given below:

Mass Spectrometer 'Specifications Quadrupole Analys er Mass Range Mass Position Stability Mass Lineari ty Mass Readout start Mass Scan Width Zero Sweep Time Constant Resölution Multiplier

Total Ion Monitor

1.59 cm x 20 cm r~ds with alumina ceramic collars , designed for high resolution and high transmission. 2 to 560

AMU

in one continuous linear range.

Better than

±

0.1

AMU

over

8

hour period.

Better than

±

0.2 .AMU over a total mass range. Digital display on CRT 'to

o.i

AMU.

(a) CRT display as well as hard copydisplay is automatically mass mark.ed to ± 0.3 AM(].'

. ,

(b) Analoguè strip chart' event marker issued with pulses at every 10 and 100

AMU

linked to mass readout, accurate to

±

,

O.3

'

AMU;

allowing direct readings for speèific mass identification.

2 to 560 .AMU variable by nlariual control . 1 to 560

AMU

variable by manual control. Function available for single ion monitoring. Signal and CRT display have a time constant which is automatically selected wi th the scan rate. Adjusted by keyboard or manual control for near constant 3M over entire mass range. Maximum of 5M to 560

AMU.

Channeltron-type, gain 10

8

at

4

kV. Power supply switched from positive to negative ion detection by manual control.

Total ion current can be monitored. Readout is from 102 to 10

7

ions per second fsd with variable time constant.

Signal Handling A unique digital, pulse counting signal handling system was specifically developed for positive and negative ion detection and also extremely wide dynamic range response.

The complete system was designed for swi tching, within seconds, between positive and negative ions. The entrance ion optics were configured

to provide a maximum intens i ty paraJ.lel bundle of ions, having a controllable low energy and very low energy spread, in order to maximize the achievable transmission andresolution of the quadrupole mass spectrometer.

3 .

CHEMICAL IONIZATION MMS SPECTROMmRY*

Chemical ionization mass spectrometry both at 1 torr and atmos-pheric pressure has recently been reviewed by Horning et al (Ref. 1) and a modified extract of this review is included here to give a general over-'

, view of this new but rapidly developing technique.'

"Until very recently, mass spectrometry was a field of interest only to chemists and physicists. The design of, mass spectrometers was dictated largely by their analyticaJ. uses in hydrocarbon (pétroleum) chemistry or by speciaJ. requirements of physicists and physical chemists (who often bUilt their own instrum:mts from available components). This situation changed when Ryhage (Ref.' 2) developed the 'molecule separator' and a combined gas chromatograph-mass spectrometer (LKB-9000) was designed for use in biologicaJ. and meQical research studies. Since that time, the requirements for increased sensitivity of'detection, for rapid or repetitive scanning, and for operation as part of an analyticaJ. systeminvolving flowing gases have increasingly led to considerations of redesign of mass spectrometers. One of the issues under study, . . • is the way in which ions are formed in the ion source of a mass spectrometer.

The electron impact ionization process, usèd in most mass spectro-meters, involves the bombardment of vaporized compounds in the source with

electrons generated from a heated filament. The efficiency of ionization is low, but the energy transfer when ionization occurs is high (the usual

electron energy range is 20 to 70 eV). The ini.tiaJ. step is formation of a positive ion (~) from the molecule, but the energy transfer is usually so great that fragmentation occurs, forming many ions. Cleavages a t ' weak' bonds occur to yield ions of expected structure , but rearrangements and

eliminations of unsuspected nature of ten occur,.: The intensive study of fragment at ion processes has provided much insight into the relationship between molecular structure and fragment ions, and mass spectrometry with

electron impact (EI) conditions is highly useful in structural studies. When the technique of mass spectrometry is used for quantitative purposes in GC-M3 (gas chromatograph-mass spectrometer) and GC-M3-COM (gas

chromatograph-mass spectrometer-computer) system, the formation of many ions * For further background information on mass spectrometry and ion molecule

from a single compound is almost always disadvantageous. The ionization process should be as efficient as possible (to increase sensitivity of

detection (Note: This is also true for the TAGA system.) and should yield

.only a few instead of many fragment ions (to avoid interference in selective

ion detection procedures). This is usually possible when chemical ionization (Cl) processes are used for ion formation. The general term "chemical ioniza-tion" is used to describe ionization which occurs by an ion-molecule reaction. It is necessary, of course, to establish a condition which results in a

primary ionization step. Since gas-phase. ion-molecule reactions occur with

great speed, a complex series of reactions may occur (depending on the

con-ditions in the ion source)yielding' one or a few ions fram the neutral

mole-cules under study. Ion-molecule reactions usually involve comparatively low

energies.

Two kinds of chemical ionization reaction conditions are of interest for their applications in this area. Most mass spectrometers can be modified for chemical ionization wi th gases under pressures of 1 to 2 torr without

changing the source location or 'sample inlet ~rangements, although

modifica-tion of the vacuum system may he neede<L It is also possible to carry out

ion-molecule :reactions at atmospheric pressure in an"external source; a

novel mass spectrometer with very hig~ sensitivity of detection has been

devised utilizing this design. The following sections describe a ll'lmber of chemical ionization methods and their applications.

Basic Principles

An ion and a neutral moiecul'e, present together in the usual

internal or in an external source, will react if an exothermic reaction can

occur. This may be expressed in terms 'of ionization potentials, or in the

usual form of achemical reaction. Virtually no activation energy is required. The colli sion inay result in charge transfer alone, but addition of an ion is also a common process. If the resulting structure is stabIe, no further

reaction occurs. If not,· asimple cleavage will occur. This is the route

followed in proton transfer reactions and in proton abstraction. When ions formed by charge transfer or pboton transfer are not sufficiently stabIe,

functional group elimination may occur. When this happens , the mass spectrum

usually shows M"" or :MH+ and typical ions due to loss of neutral fragments .

Theformation of many ion radicals is usually due to high energy processes which are not usually seen under Cl conditions.

In ordinary mass spectrometers, the Cl source is usually operated with methane, isobutane, or nitrogen-ammonia; other reagents that have been used include tetramethylsilane and ni tric oxide. The pressure in the source is usually approximately 1 torr (sometimes referred to as a high-pressure condition in contrast to the usual EI condition where the source pressure is only slightly abofe the pressure in the mass analyzer) , and the primary ionization step takes place through electron impact (the electrons are generated from a filament, as usual) with the reagent gas. Most of the original work was with methane, and later with isobutane; the background

A. Proton Transfer Reactions + M + CH 5 -+ MH+ + CH

4

(1) M+ C

₄

H+

9

-+ MH+ + C

4

H

S

(2) M+ C

₆

H

₆

+ -+ MH + + C

6

H5 (3) + M + NH

4

-+ MH + + NH3

(4)

+ M + H 30 -+ + MH + H 20 (5)

Proton transfer is perhaps the most widely used chemical ionization reaction. I1' M is a strong base in the gas phase (a basic drug, for example), MH+ will be formed by reaction with proton donors derived from water or ammonia, from ion radicals such as those derived from benzene, or from reaction with a very powerful proton donor CH5+ from methane . If M is a weak base (weaker than ammonia or water, for example), reaction with ions from methane or isobutane may be required to form MH+. The most powerful proton donor known in gas phase reactions is CH5+.

If the ion MEI+ is stable, the mass spectrum may show only this ion (in addi tion to the reagent ions). In some instances, however, additional ions may be present because of fragment at ion , or because stable addition products are formed.

B • Addi tion Reactions

+ M + NH4 -+ MHNH+ 3 (6) + _{MHH 0+}

(7)

M + H 30 -+ 2 + M + C 2H5 -+ + ~2H5

(S)

+ M + C3~ -+

~3~

( 9)

M + MEI + -+ MHM+ (10) M + M+ -+ MM+ ₍₁₁₎ ~O + H + + (12) 30 -+ H2OHH2O

In many instances a polar molecule, M, will react with a posi ti vely charged ion to yield an addi tion product. For example, a basic drug may show ions MH+ (reaction 4) and

MNH4

+ (reaction 6) when ammonia is used as a re agent gas. The ion ratio will depend upon the structure of Mand the reac·tion

conditions (temperature , pressure , concentration). Reactions 8 and 9 are usually observed whenever methane is used as a reagent gas, since ethyl and isopropyl ions are present at the time. The Cl mass spectrum of a barbiturate in methane, for example, will contain MH+ (from reaction 1), MC2H5+ (from reaction 8), and MC~H7+ (from reaction 9). Water is rarely used for Cl purposes in convent~onaJ. mass spectrometers, but the formation of cluster ions from the reagent (reaction 12, continuing to form larger ions) is a familiar occurrence in reactions carried Olrt at atmospheric pressure . . (Note: This is certain1y the case in the TAGA souree operating wi th ambient air.) A kind of' dimerization reaction, in which M may react with ei ther

W

or MH+, may aJ..so occur. An example of reaction 11 is ~he formation of C12H12 + from C6H6+ when benzene is a reagent at atmospheric pressure . The possible forma-tion of

MH:W-

(reaction 10) shouid be considered whenever MH+ is the major reaction product; the occurrence of this reaction depends both on the struc-ture of M and on the reaction conditions .

C. Proton Transfer wi th Elimination or Cleavage

(13) + + M + CH 5 -4 (M-~O) + H20 + CH4 (14) M + CH; -4 (MH-90)+ + (CH3)3SiOH + CH 4 (15) The ion CH5+is sufficiently strong proton donor to react with hydrocarbons. If M is an alkane, for example, reaction 13 will occur; the initial adduct is not stable and the reaction is one of proton abstraction. A similar reaction may occur for aJ..icyclic compounds; steroids, for example, of ten show (M-l)+ or related ions when methane is the re agent gas. This reaction does not occur wi th ammonia, water, or benzene. An initiaJ..ly formed MH+ willoften undergo cleavage when a powerful proton donor is used. For example, if M is a monofunctionaJ.. aJ..cohol and methane is the reagent gas, reaction 14 will occur. If M contains a trimethylsilyl ether group, the usual reaction is protonation followed by loss of trimethylsilanol (reaction 15) • If' t wo or three trimethylsilyl ether groups are pres ent, the i ons whi eh are formed are usuaJ.ly MH+, (MH-90)+, (MH-180)+, and additional ions formed by repetition of the elimination process ... .

D. Charge Transfer with or without Fragmentation

(16) (17)

Mest Cl werk has been carried eut wi th methane , isebutane, er

ammonia, with emphasi!' in drug studies en iens ef the MH+ type. Tt is alse

pessible te ienize compeunds by charge transfer, if an apprepriate reagent gas ef higher ienizatien potential is used. Fer example, trimethylsilyl

ethers have lew ienizatien potentials . Stereid trimethylsilyl ethers can

be ienized in the presence ef benzene (reactien 16), but this is fellewed by less ef trimethylsilanel (reactien 17). The relative propertiens ef iens which are fermed depend upen the structure ef the parent compeund.

E. Preten Abstractien with Negative Ien Fermatien

M + Cl- ~ (M-H)- + HCl

Acidic sub stances , in the presence ef iens ef basic character, will lese a preten te ferm (M-H)- (reactien 18). Reactiens ef this kind

(18)

are net ebserved in mass spectremeters unless previsien has been made fer eperatien in a negative ien mede. The reactien prebably takes place threugh

an intermediate

M:a-.

(The TAGA instrument can eperate in beth the pesitive

and negative medes and can be switched from ene te anether in secends .) other reactien types that we have discevered te be impertant in negative chemical ienizatien are:

Charge Transfer (18a) and Disseciative Attachment MX + 02 ~

x-

+ M + 02

(lSb)

(Very cemmen if X

=

I, Cl er Br.)

F. Primary er Initial Ienizatien Reactiens

CH

₄

(19)

(20) When methane is ienized under EI cenditiens, the preductiens are largely these expected frem a high-energy, lew-pressure ienizatien cenditien

(reactien 19). If the cencentratien ef methane is increased te 0.5 te 1 terr, a series ef secendary reactiens eccurs and the ien preducts are these shewn in reactien 20. The ien fermed in highest yield (47%) is CH5 +, but

ethyl and iseprepyl iens are alse present (41% and

6%,

respectively). Since

pelar melecules will add ethyl and isepropyl iens as well as pretens, the methane Cl spectra ef bases will nermally shew MH+, (M+29)+ and (M+ln)-. A

similar but less marked effect 0CCurS when isebutane is used as the reagent gas; the maj er ien preduct is MH+, but iens at (M+57) + and (M+39)+ may be seen. Fer this reasen , and because the energy transfer is lewer, isebutane is preferred as a reagent gas in many applicatiens invelving bases.

Spectra

Figure

3

shows the EI (70 eV) mass spectrum of the N.N-dimethy1 derivative of phenobarbital. When a Cl spectrum of a barbiturate derivative is obtained in methane , the chief ion isMH+, and ions corresponding to

(M+29)+ and (M+41)+ are a1so present (Fig.

4).

C1eavage occurs to a very sma11 degree, in contrast to the effect of the high-energy fragmentation process i11ustrated in Fig.

3.

Wi th isobutane as the carrier, and reagent gas, c1eavage does not occur and the ion product is MH+ (Fig.

5),

together with some

Mi"

formed by charge transfer. Amines are protonated. Figures

6

and 7 show Cl (methane) mass sp,ectra for nicotine and methadone. The chief ion product in each instance 'is MH+; ions at (M+29)+ and (M+41)+ are also present as minor reaction products. Results of this kind were obtained in all ear1y studies of methane chemical ionization spectra. Fales and his associates' (Milne, F,a1es and Axe1rod, Ref. 4) emphasized the simp1ici ty of the mass spectra, and the formation of MH+ asthe principal ion from many

drugs (see Fig.

8).

(Note: We consider this simplicity to be an extreme1y important factor in the analysis of complex mixtures wi thout the require-ment of -complex deconvo1ution techniques.) When ammonia is used as a reagent, basic drugs are protonated. Ions corresponding to' (M+NH4)+ are sometimes observed, but c1eavage produets are rare1y seen. (Note: We fe el this abi1ity to se1ective1y ionize a compound c1ass has enormous potential for the analysis of complex mixtures.)

Tt should be emphasized that high-energy (EI) processes lead to fragmentation products which inc1ude odd electron (radical) ions, as well as even electron ions. C1eavages which occur under el conditions do not usually yie1d radical ions; the functional group cleavage reactions which have been observed correspond to e1imination of water, methanol, trimettwlsi1ano1, or other molecular enti ties. "

4.

ATMOSPHERIC CHEMICAL IONIZATION MASS SPECTROMErRY IN AMBIENT AIR What has been described so far has been mainly conventional chemical ionization carried out at around 1 torr, using well controlled carrier and reagent gases. App1ying chemical ionization processes to ambient air at atmospheric pressure changes the picture significant1y.

The effect of raising the pressure of these processes from 1 torr to 760 torr has the obvious effect of increasing the number of collisions which occur both during the formation of reactant ions and the formation from product ions. Col1isional stabi1ization is thus even more in effect and the end result of this is th at equilibrium conditions are essentially reached within fractions of a mi11isecond (and within a very short distance .of the primary ions produced near the corona discharge needle tip of a TAGA 2000 ionization source) due to the extreme1y high reaction rates of ion molecule reactions in general.

Thus chemical ionization at atmospheric pressure (API) is an

equilibrium process and it is equilibrium constants rather than rate constants that control the final distribution of reactant and trace product ions. Al-though the equilibrium distribution of ions is 1ike1y to differ between Cl at 1 torr and API at 760 torr, the effec't of increased pressure onthe ion-molecule reaction types has been shown to be slight. (Dzidic et al, Ref. 5)

One result of'this ·is that adctitiOn or solvation reactions (class B f'rom

previous extract) of the yy.pe M + H30+ ~ MH30+ are common at atmospheric

pressure . The previo~ ' c8'imnents on reaction processes appear, therefore,

to apply.'equally well to

br

as evidep.ced. f'rom the results obtained in our

laboratory wi th the 'TAÖ:~ 2000 instrument, the work of' Dzidic et al, and

the results f'rom plasmachromat'ography (Ref'.

6),

the plasma chromatograph

being a mobility spectroÎn.eter employing chemical ionization at 760 torr using dry nitrogen as a carrier.

The second major ef'f'ect of operating at atmospheric pressure with ambient air is that conditions are much less controlled than those normally used in conventional chemical ionization. The main consequence is that the choice of reagent gas is out of' the hands of' the experimenter to some extent, unless reagent gases are added to alter the reactant ion composition.

The f'act that ambient air is a complex mixture, and that it is from this complex: carrier that the reactant ions are f'ormed, which in turn react with traces in the carrier to form trace signal ions, would appear to produce an alm.ost intractable problem of' interpreting spectra produced f'rom a complex trace ion mixture f'ormed from a complex of reactant ions. However, as mentioned previously, the reaction ion f'ormation processes are so rapid that equilibrium conditions are reäched within fractions of a millisecond

wi thin the ion source region . This results in the appearance, as reactant

ions, of' only a f'ew species which are formed from the components of air wi th the highest acti vi ty .

In the pos i tive ion mode, species with the highest gas phase

basicity are the most reactive, while in the negative ion mode, species with the highest gas phase acidity are the most reactive. Proton affinity and electron affini ty respecti vely are the experimentally determined quant i ties most cloSely related to acidi ty and basici ty .

. In ambient air the mass spectrum of' potential reactant ions is

surprisingly simple both in the positive and negative mode and correspondingly

trace sample ions f'ormed from them are qui·te easily identified. Much of the

work of' determining the initial reaction sequences resulting from gases con-taining water vapour, oxygen and carbon dioxide (as does ambient air and expired air) bèing subjected to ionizing conditions has been carried out at

pressures no higher than 10 torr and wi th pure gases. Nevertheless, the

reaction sequences determined appear to be similar in nature to those th at

lead to the final reactant ions f'ound in a TAGA ion source. Reac·tant ions

appear also to be the same, whether the ionization is initiated by a beta

source or a corona discharge. For example, in the positive mode, Good et

al studies the complex series of' reactions that resulted when nitrogen containing a small amount of water is subj ected to ionizing conditions in a 63Ni reaction cello The initial reaction of' the thermalized electrans f'rom the radioactive source is wi th nitrogen and the products are f'ormed through change transfer and proton transfer leading finally to the formation of an equilibrium distribution of' ion clusters of proton hydrates as shown below:

+ + N 4 + H20 ~ H20 + 2N2 H 0+ + 2 + ~O ~ H30 + OH + ~ + H 30 + H20 + N2 ~ ,H30 (H20) + N2 + ~ + H 30 (H2O)n + H20 + N2 ~. H30 (H20)n+l + N2

A similar sequence was found for oxygen leading to a similar equilibrium distribution of proton hydrates in air. At atmospheric pressure in ambient air wi ththe TAGA system we find a very similar picture. Figure 9( a) shows a typical outside ambient air positive ion mass spectrum taken at low sensi-tivity at 23°C and air flow rate of 1500 cm3 sec- l and a mass scan rate of

1 PJvJIJ/sec. The equilibrium distribution of proton hydrates H30+(H20)n can

be clearly seen at 19, 37, 55, 73 and 91 PJvJIJ; peaks at 18, 36 and 54 PJvJIJ

are also seen and have been identified as the

NH4

+ (H20)n series formed

from proton transfer from the hydrated proton clusters to estimated 10-9

mole parts of ammonia in the air.

Ammonia is known to have' a high proton affini ty. We have also

shown further th at the re act an t 'ions from room air are still dominat ed by the

, proton hydrate clusters as shown in Fig. 9( a), but that the typical annnonia

levels' are somewhat higher. (This has been found to be due to the presence

of human aC'tivity in the room; humans appear to give off NH3 from their

bodies as well as intheir breath.) ,

Therefore, for ,ambient outside and room air tne positive reactant

ions are niainly proton hydrate clusters with a small fraction of, hydrated

ammonium ion clusters.

These reactant ions are therefore available f,or the ionization of certain traces in the air by proce.sses such as:

Addition as previously described.

The ease of detection of these traces will depend then on their basicity or proton affinity to that of the major reactant ions. Fortunately manytraces of interest have proton affinities larger than these ions. As mentioned previously it is possible to modify the reactant ions relative

'acidities' bythe addition of chemical reagent gases, or by raising the temperature ofthe reaction region. This has the effect of moving the equilibrium distribution of reactant hydrated proton clusters to lower values of n. This is important as i t has been shown by Kebarle that the proton affinity of proton hydrates decreases with cluster size, thus essen-tially increasing the 'acidity' of the reactant ion distribution, and thus

their reactivity to certain classes of compounds. However the use of

temperature and selective chemical ionization reagents to modify the ion-molecule chemistry will be described in more detail in a later application note.

Figure 9 (b) is the same posi ti ve i on spectrum at ten times higher sensitivity to show more clearly the many other peaks that are associated

wi th trace species in the air. At 18, 36, 54 are seen the peaks from 10-9

mole parts ammonia as previously described. Other species that have been

clearly or tentatively identified are rnarked on the figure . These include

propylene H+ and (propylene H+)H20 at 43 and 61, Ü2H+ at 33, N02+ at 46,

(NO)2+ from NO at 60, acetone H+ and acetone H+(H20) at 59 and

77,

and many

others which have yet to be identified. It should be noted here that the ol,l.tdoor air being sampled is from a light industrial environment. Also

shown in Fig·. 9 are 'blow':'ups' of higher mass regions of the spectrum by

1000 (9c) and (ge) and by 10,000 (9d) and (9f) to demonstrate the high signal to noise characteristics of the TAGA 2000 system, and the wide dynamic range characteristics of atmospheric pressure ionization produced signals.

Spectra such as these clearly demonstrate the power of this tech-nique for the continuous real-time monitoring of traces in a complex mixture.

(To give an idea of. the sensitivity of this technique, the ion signal at mass

18 from ammGnia is equivalent to ~ 50 ionsl second. Recognizing th at the

total system noise at mass 18, both electrical and from other sources can be

less than 5 ions/sec gives an equivalent detectability limit of 10-13 mole

parts at

siN

=

10 for continuous real-time monitoring ~ )

In the negative mode the reactant ion spectrum from ambient outside air is quite different,as might be expected. Less appears to be understood about negative ion reaction sequences that are expected to occur in air under

ionizing conditions . Our work with the TAGA 2000 together with some work by

Dzidic et al at thé Baylor College of Medicine in Texas, comprises most of what is experimentally known about negative ions formed in air at atmospheric

pr.essure. Figure

]D(

a) shows a typical ambient outsid.e air negati ve ion

spectrum taken at low sensitivity and under the same conditions as the

positive ion spectrum described previously. The major reactant ion sequence is not as clear as in the positive mode but at least 2 cluster sequences

appear. These are: 0-(02)n(H20)m and 02-(02)n(H20)m. These sequences are

seen more clearly in Fig. 10(b) which is run at ten times more sensitivity. These reactant ions are thus available for the detection of certain traces in the air by processes such as

Proton Abstraction M+O

-

-+ (M-H) + OH

Charge Exchange M + 0; -+ M + O₂

Dissociative Attachment MX + 0; -+ X +M+ O₂

as previously described, or by

-

-Associative Attachment M+O -+ MO

which appears to be occurring in our source for many of the major traces in air such as N02 and C02.

p - - - --- -- -

_._--Figure 10(b) shows many peaks which have been clearly or tentatively identified. These include OH- at 17, 02H- at 33,

cr

at 35 and 37, N02- at 46, 03- at 48, C03- at 60, N03- at 62, and many others which have yet to be identified. Figures lOC c), (a), (e) and (f) are shown to demonstrate the high signal to noise characteristics of the TAGA 2000 system in the regular mode.

To demonstrate the power of this technique in the negative mode, the detectability limit to trinitrotoluene at the M'" ion is 1 x 10-13 mole parts at SiN = 10 for continuous real time monitoring. (For more details on the application of the TAGA technique to explosive vapour monitoring, see

UTIAS

Technical Note No. 213.)

No mention has yet been made of the general response characteristics of the TAGA technique as the concentration of the trace changes. The first response characteristic is the time response to a step function change in concentration. The rise time of the instrument appears to be in the fraction of a second range and, even more dramatic, the clear-out time is also in the same range. This latter point is of major importance in many real-time studies. This extremely fast clear-out time stems from the relatively high sample flow rates used so that at room temperature the effect of adsorption of traces on the inlet walls to the ionizer is minimal. The other response characteri stic of importance is the relationship between trace ion signal to trace ion con-centration. A preliminary assessment of the quantitative response of the TAGA shows that there is a direct correspondence between trace concentration andtrace ion signal. The functional relationship between concentration and response should ideally be linear for quantitative accuracy aver a wide range of concentration. Results on many compounds suggest that over three or more decades of trace gas concentration (tested from ppm to ppb), a (Beers Law ty'pe) power law relationship applies. Concentrations above ppm appear to produce a saturated response. A saturated response implies that at high concentrations of the trace all of the reactant ions are replaced by product ions. For n-propylamine, for example, saturation is experimentally observed at about 60 ppm. Dilution of sample, which contains the trace gas, by addi-tional pure reactant gas (air) could restore quantitative response if required, also increasing the reactant ion concentration wouldhave the same effect. Tt has been found that below ppb levels the response becomes linear.

The applicable rate equation describing the processes taking place in the ion source are as follows, where"B represents the trace molecule and A+ the reactant ion.

(1)

There is no need to add terms for loss of C+ by either wall reactions or by recombination with ions of the opposite sign becau~e (a) the ion source is a

'wall-less' reaction and (b) because the process takes place under astrong electric field and only uni-polar ions are present in the reaction region. Then

and and since then [A+]

=

e-k[B]t [A+]o

(4)

(5)

The bracketed quantities denote concentration. [A+]o is the initial reactant

ion concentration, [C+] is the trace or product ion concentration, [B] is the

trace molecule concentration. Equation 5 describes a Beers Law like response

to trace molecule concentration which becomes linear at low values of k[B]t, i.e. ,

(6)

Now typical values forthese quantities in the TAGA ion source are as follows:

[A+]O =

5

x 10

9

molecules/cm3

k

=

1 x 10-9 cm3 molecule-l

t

=

10

-4

seconds

-1 sec

It can therefore be seen that as [B] the concentration of the trace approaches

the ppb level, i . e. approximately 1010 molecules/ cm3 then 1

»

k[B]t' holds and

1. Horning, E. C. Horning, M. G. Carroll, D. I. Dzidic, I . 2 . Ryhage, R. 3. Field, F. H. 4. Milne, G.W.A. Fales, H. M. Axenrod, T.

5.

Dzidic et al 6. Karasek, W. REFERENCES

Chemical Ionization Mass Spectrometry . Adv. in

Biochemical Psychphar.macology 7, 15, 1973.

Use of' a Mass Spectrometer as a Detector and Analyzer f'or Ef'f'luents Emerging f'rom High

Tempera-ture Gas-Liquid Chromatography Columns. Analytical

Chemistry 36, 1964, 759-762.

Chemical Ionization Mass Spectrometry. Accounts of' Chemical Research, 1:42-49, 1968.

Identif'ication of' Dangerous Drugs by Isobutane Chemical Ionization Mass Spectrometry. Analytical Chemistry, 43, 1971, 1815-1820.

Private Cammunication.

Plasma Chromatography. Anal. Chem. 46, No. 8, 1974, 710A.

~

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L-Inlet System, Choice of

keyboard control hard copy & graphic display

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System----~, ion lenses

I

I

16k word processor quadrupole mass spectrometer interface Dual floppy disc ion detection & signa I processing

]

(2)

Keyboard Control

o

Interactive Graphics Display

o

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8

Q M S & Detector

o

Atmospheric Pressure Vacuum Interface

G

Atmospher;c Pressure lonization Source

EI

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[70eVJ SPECTRUM OFTHE N,N-DIMETHYL

OERIVATIVE OF PHENOBARBITAL(1J

100

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118

(M-28) 232

146

(232-57)

175

188

(M)

260

150

200

250

300

Fig.

:3

ELECTRON IMPACT

( EI- 70 eV) MASS

SPECTRUM OF THE N, N- DIMETHYL DERIVATIVE

OF PHENOBARBITAL.

>-

I

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Z

w

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c.-O

-METHANE Cl SPECTRUM OF THE

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C1J

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M-28

M+29

M+41

I

I

I

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

mie

50

100

150

200

250

300

Fig. 4

METHANE CHEMICAL IONIZATION ( Cl) SPECTRUM

OF THE N,N-DIMETHYL DERIVATIVE OF

PHENO-BARBITAL.

THE PRINCIPAL ION IS MH+, AND MC

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H

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- - - --- --~

•

ISOBUTANE SPECTRUM OF THE N.I'J-DIMETl-1YL

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Fig.

5

100

M+1

M

50

100

150

200

250

300

350

ISOBUTANE CHEMICAL IONIZATION ( Cl) MA SS

SPECTRUM OF THE N,N-DIMETHYL DERIVATIVE

OF PHENOBARBITAL.

THE PRINCIPAL ION IS MH+ ,ALONG WITH M+

FORMED BY CHARGE TRANSFER .

NICOTINE

100

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70 90 110 130 150 170 190 210 , ,

Fig. 6

METHANE CHEMICAL IONIZATION (Cl) MASS

SPEC-TRUM OF NICOTINE.

THE PRINCIPAL ION IS MH+; IONS AT M+29+ AND

M+41+ ARE ALSO PRESENT, TOGETHER WITH A FEW

FRAGMENT IONS.

•

100

->-

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en

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W

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I

II

I1

o

III

I

MIE

250

I

310

I

Fig. 7 METHANE CHEMICAL IONIZATION (Cl) MASS

SPEC-TRUM OF METHADONE.

THE PRINCIPAL ION IS MH+; IONS AT M+29+ AND

M+41+ ARE ALSO PRESENT, TOGETHER WITH A

FEW FRAGMENT IONS.

>-

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APPENDIX A

RECOMMENDED BACKGROUND READING MATERIAL

ON MASS SPECTROMErRY AND ION MOLECULE CHEMISTRY

(Not ·in order of priority)

Reference

1. "Mass Spectrometry" , ed. by C. A.

Mc.Dowell, McGraw-Hill Book Co. ,. New

York,

1963,

Chaps.

1-4, 7, 9, 12,

13

.

2. "Mass Spectrometry and lts

Applica-tions to Organic Chemistry" , J. R ..

Beynon Elsevier, ~terdam,

1960.

3.

"Introduc'tion to MassSpectrometry

-Instrum.entation and Techniques", J.

Roboz, John Wiley & Sons,

1968,

Chaps.

1, 2, 3, 4, 5, 9, 10, 14.

4.

"Ionization Sources· in Mass

Spectro-metry", E. M. Chant, Anal. Chem.,

44,

1973, 77A.

5.

"Techniques of Combined Gas

Chroma-tography (G.C.)/Mass Spectrometry,

W. McFadden, J. Wiley & Sons, New

York,

1973.

6.

"Principle of Mass Spectrometry and

Negative Ions", C. E. Melton, Marcel

Dekker Inc., New York,

1970.

7.

"Ion-Molecule Reactions", Vol. I, ed.

by

J.

L. Franklin, Plenum Press,

1972.

8.

"Ion-Molecule Reactions", E. W.

McDaniel et al, Wiley Interscience,

New York,

1970.

Subject Matter and Comments

Gene~al background in mass spectrome·try, ion sources, chemi cal ana;Lysis by M. S . ion-molecule reactions. Although out of date i.n many areas, still an excellent source reference on mass spectrometry. At one time the most comprehensive

book on the application of M.S. Although

out of date in some areas, is a source reference th at can be "skimmed" for relevant material.

MOre up to date general reference source on M.S. techniques and instrumentation.

Very good source r~ference. Ras excellent

Ohap.

14

on information, bibliographic

and' data sources which can be used to

extend this selected list on the subject of M.S.

Recent general review on ionization sources for M.S.

Recent and comprehensive review of the

combination of gas .chromatography and

mass spectrometry (GC/MS). Excellent

source reference for this technique can be 'skimmed' for years of useful

information. All chapters except .

probably Chap.

8

relevant in sane way.

Excellent review of M.S. and negative

ion M.S.· .

Detailed review of ion-molecule reactions.

Chaps. 1, 2,

5, 6.

Excellent review of ion molecule