F . P . Ca r e y, G. Bl o d g e t t, a n d H . S. Sa t t e r l e e, 802 L exin gton A ve., N e w York, N . Y .
I
N T H E course of a clinical investigation of persons observed to be habitually excreting arsenic in their urine, especially with respect to certain cases exhibiting disturbances suggestive of chronic poisoning through the action of arsenic in the tissues of the nervous system, it became im
po rtan t to stu d y the possible existence of sources from which arsenic m ight gain direct access to the central nervous system by way of the nasal tract. This stu d y led to the examination of m any specimens of the finely particulate and air-suspensible materials to which most city dwellers are directly exposed in the course of their daily lives. Such substances include sedimentary house dusts, as represented by th e powdery portion of the material ordinarily collected b y a household vacuum cleaner, and those finer particles of dust and soot of the inside and outside atmosphere which are obtainable by methods of filtration from the air.
A report on this study will be published elsewhere (16), b u t we are here concerned only with analytical methods.
The quest which led to the development of the method to be described was n o t directed towards the estimation of arsenic in food materials, b u t primarily towards the better detection of m inute quantities of arsenic in biological specimens as well as in house and atmospheric dusts. For these purposes it became highly desirable to devise an analytical method which would be capable of relatively rapid, simple, and in
expensive performance, and which would give reliable re
sults in th e detection of organically bound arsenic in any form. T he examination of other materials such as foodstuffs has been included in the series here reported in order to show th e general scope of usefulness of the method.
M ethods of acid digestion with heat, as usually employed in preparing organic substances for the determination of arsenic in the M arsh or G utzeit apparatus, are not only slow, laborious, and productive of corrosive fumes, b u t the results have frequently been questioned (6-13) on the basis of the possible volatilization of a t least a portion of the arsenical content in th e form of higher molecular arsines.
This same objection holds with the well-known chlorate m ethod of Fresenius and Babo, the perchloric acid method of Noyes and B ray (11), or the MgO-Mg(NOs)n oxidation of Kohn-Abrest (8), all of which employ digestion in open receptacles a t boiling tem perature, as well as with air-com- bustion methods in ignition tubes (12).
Numerous references (5, 7, 10) m ay be cited, questioning the validity of findings on the ground of contamination of the specimens by reagents employed during preparation or testing, or by exposure to possible contamination from atmospheric dust or laboratory apparatus. I t is therefore desirable to employ reagents in which freedom from arsenic is not only generally conceded b u t readily demonstrable, and still further to avoid criticism on this ground by using relatively small am ounts of a few reagents. The proportion, by weight, of reagents to samples ranges high in all acid- digestion methods, especially when supplemented by re
peated adsorptions of residues by means of precipitated adsorbents, as ferric ammonium sulfate (5). In such proc
esses lengthy exposure in open vessels of residues and of
adsorbents to atmospheric dust contam ination is an im
portant source of error.
The method of bomb combustion using pure oxygen was first devised by Berthelot (1) in 1899. In 1903 B ertrand (2) reported its successful use in the determ ination of small am ounts of arsenic in organic m atter. Both of these chemists employed a platinum bomb and ignited the charge b y means of a small fuse of gun cotton; they took no cognizance, how
ever, of the loss of arsenic in the form of finely divided oxide dispersed in the combustion gases, and these gases were allowed to escape from the bomb untreated.
E arly in their investigation the authors became convinced th a t the complete extraction of suspended particles of arsenious oxide from combustion gases was an essential requirem ent in developing an adequate m ethod. Pre
liminary experiments indicated th a t the loss assignable to this cause varies widely with the nature of the substance burned and the details of the technic employed in its com
bustion.
T a b l e I. V a r i a t i o n i n A r s e n i c Loss
Ar s e n i c Co n t e n t o r Sa m p l e
L a b . N o. Ash only All products of com bustion
P . p. m. P . p. m.
117 10 20
99 3 12
102 22 30
120 16 17
467 28 28 .0
The arsenical content of the smoke resulting from the explosion of some of the substances examined w'as found to be present in such a finely divided state th a t its recovery by ordinary methods was impossible except with a long series of Liebig or Geissler bulbs. The opinion was expressed to one of the authors (H. S. S.) by an authority on electrostatic precipitation1 th a t a cloud precipitation would probably be more easily applicable to the problem and quite as effec
tive as an electrostatic precipitation, such as th a t provided by the apparatus of D rinker and Thomson (3). This was found to be true.
The preparation, by desiccation, of the m aterial to be burned in oxygen under pressure is not difficult, b u t certain precautions against volatilization losses are necessary. The means of ignition, as provided in oxygen bombs of approved type, involve no difficulty. In the selection of an oxygen bomb it is obviously im portant to have the surfaces which are exposed to the flame and to the gases of combustion composed of m aterial which is absolutely resistant to the products of combustion. In this respect lined bombs, either of m etal or porcelain, have been found less satisfactory than those of solid construction. I t is also desirable to have the valve stem and the passages through which the gases of combustion are exhausted to th e washing train, not only of noncorrodible m aterial, b u t of simple design and readily accessible to washing. To m eet these qualifications a bomb co%'er was designed, the details of which are shown in Figure 1, A . Although pressures are safely controlled by a reduction valve on the oxygen line, it is also advisable to place the
1 The authors are indebted to F. G. Cottrell for his opinion on this subject.
327
328 A N A L Y T I C A L E D I T I O N Vol. 6, No. 5 switch which closes the ignition circuit some distance from
the bomb, thus eliminating any possible risk to the operator.
Although the ordinary fusible nichrome wire of 34 B & S gage has been usually employed by the authors, it was found practicable to use a nonfusing platinum wire to ignite the charge. (While the work here reported has been done with electrical ignition in a rather capacious bomb, it is hoped th a t an oxygen bomb of smaller capacity can be developed which will be adapted to ignition of the charge by simple conduction of heat.)
biological specimens requires special precautions which are not discussed here.
Where the heat of combustion is small, it has been found advantageous to line the small combustion crucible with arsenic- free cotton gauze. The weighed sample is placed in the bomb and the ignition wires are connected in the usual manner; the bomb is then closed and oxygen is admitted to a pressure be
tween 20 and 30 atmospheres through a pressure-reducing valve connected with the oxygen supply. The charge is then ignited by closing a sw-itch which is preferably situated a t some distance from the bomb.
Bomb B. C loud-precipitation cham ber and nebulizer C, D. K oeninck bulbs C, D, E . W ashing tra in
The bomb combustion method finally evolved combines a specially designed nebulizing and cloud-precipitating cham
ber, B, with two or more Koeninck gas-w'ashing tubes in series, C, D. In this nebulizer the combustion gases enter as a fine jet which sprays hydrochloric acid, producing an am
monium chloride cloud within the chamber by reaction with ammonia. This cloud of ammonium chloride vapor acts as a precipitating agent upon the finely particulate combustion products and facilitates extraction by the washing train.
W ith this arrangement the reagents employed are such as m ay be easily obtained in a pure state and are few in number— namely, nichrome (or platinum) wire, hydro
chloric acid, ammonium hydroxide, sulfuric acid, oxygen, and water, and the quantities of the first four are very small.
The sample is heated only while confined in the bomb, so th a t losses by volatilization during the oxidizing process are most surely prevented. The quantity of acid introduced into the Gutzeit or M arsh apparatus is only th a t used in the gas-extraction train and is therefore under close control.
This avoids the error affecting quantitative determ inations ascribed by Lawson and Scott (9) to differences in the rate of evolution of the arsine caused by variations in acid concen
tration.
Me t h o d
The sole prim ary requisite is th a t the sample be readily combustible. Specimens of high moisture content should be dehydrated, preferably a t low temperatures, and for a bomb capacity of 380 ml. as here illustrated, from 0.5 to 1 gram of th e dried m aterial is taken. The preparation of certain
The combustion gases are passed into the nebulizing chamber, B, by opening first the cut-off valve, 7, and then the needle valve, 6. The passage of the combustion gases through jet 10 produces a sprav of hydrochloric acid (1 to 1), which is controlled by operation of the needle valve. This spray causes a simul
taneous precipitation of an ammonium chloride cloud by re
action with the ammonia vapor derived from the ammonium hydroxide reservoir, 15, in the tubulature of the chamber. The resulting vaporous mixture is carried through the train of Koe
ninck bulbs containing a measured amount of sulfuric acid (1 to 3). The residual gases are then washed out of the bomb into the train by first closing the cut-off valve, 7, on the bomb and repeatedly introducing 2 to 4 atmospheres of oxygen and washing through.
The bomb is next opened, the valve passages and bomb are washed with water, and the ash, washings, and solutions from the absorption train are introduced into the Gutzeit generator.
Re s u l t s
Over six hundred samples have been examined by this method, b u t m any were not appropriate for use as check samples, principally because of difficulties in obtaining true duplicates. Table I I shows th e results obtained on a wide variety of specimens by the nitric-sulfuric acid digestion method of G autier as modified by Sanger and Black (14), in comparison w ith the m ethod of the authors.
I t seems evident th a t no paralleled series for comparison by recovery of known am ounts of arsenious oxide added to organic materials, as used by Sanger and others (5,9), can be a criterion as to the adequacy of different m ethods, since the discrepancies between the methods m ay be attrib u ted to loss of the organically linked arsenical content.
By the m ethod of comparison which the authors have
ad op ted , em p lo y in g id e n tica l sam p les o f unknow n arsenical
° Sam ples of atm ospheric d u st were collected and their weights determ ined by a tta ch in g a dried and ta re d piece (50 sq, cm.) of specially tested filter cloth to an o rd in ary dom estic air filter and reweighing after exposure and a second drying. F ilter and contents were consumed in the analysis.
t> Samples of v acu u m cleaner d u s t were collected b y the ordinary household any other methods of determination, as with the Marsh-Berzelius generator or with certain electrolytic modifications of the Marsh or Gutzeit method as recommended by Thorpe (17) and Lawson and Scott (9). But where the electrolytic evolution of the arsine involves the use of porous cells, the method cannot be con
sidered reliable because of objections taken by Fink (4), which are based on possible retention of arsenic by the porous cell.
A predetermined constant strength of the acid solutions and combined washings has been found to provide a uniformity of action in the Gutzeit generator which greatly facilitates accurate quantitative estimations in comparison with a standard scale covering a range of low values, and it is desirable to select a suitable aliquot from the combined washings to fall within the optimum range of from 0.5 to 10.0 micrograms.
Where too dense an ammonium chloride cloud is formed, difficulty is experienced in its complete absorption by the wash
ing train, C, D, E. I t has been demonstrated, however, th at a complete absorption of the ammonium chloride is not essential to the recovery of the arsenical smoke, probably because the particles of arsenious oxide act as nuclei in the cloud formation and are washed out in its coarser particles. Control of the cloud density is, however, desirable, and is effected by adjusting the concentration of the acid in the nebulizer and more particularly by adjusting the surface area of ammonium hydroxide in reservoir
1 5 , by rotation of arm 1 3 in stopper 1 2 , and by application of gentle heat to this reservoir.
.Co n c l u s io n s
In the opinion of the authors the disparity in the results shown in Table II, especially w ith those samples where the arsenic present was presumably in organic linkage, is a ttrib u t
able to loss by volatilization in the acid-digestion treatm ent.
Should the arsenical organic be resistant to oxidation by tiie mixed acids, or, should the arsenic linkage be of a fugacious type in m inute concentration, even an almost inappreciable vapor pressure a t the tem perature of boiling acid would account for a considerable loss of arsenic due to the relatively large am ount of acid boiled off during the digestion. Experi
mental support for this opinion was given by an apparent decrease in the arsenical content of certain specimens either during desiccation a t relatively high tem peratures or on long standing a t room tem perature.
Negative results have repeatedly been obtained throughout the entire series of experiments, and w ith other samples the
Insurance is provided against losses of organically linked arsenic which may be volatilized by the older methods.
Because th e process of oxidation is practically instantaneous and within an air-tight vessel and the extractions are carried on within closed vessels, there is no exposure to possible contam ination of th e sample or reagents by arsenical dust as with oxidations involving lengthy digestions, filtrations, and adsorptions. Simplicity is attain ed through the use of
330 A N A L Y T I C A L E D I T I O N Vol. 6, No. 5 (10) I.ockem ann, G ., Chem.-Ztg., 50, 7 0 1 -2 (1926).
(11) N oyes, A. A ., an d B ray , W . C ., “ Q u a lita tiv e A nalysis of th e R a re r E lo m en ts,” p. 285, M acm illan Co., N . Y ., 1927.
(12) R e m in g to n , R . E . , ./. A m . Chem. Soc., 49, 1410 (1927).
(13) Sanger, C. F „ an d Black, O. F ., J . Soc. Chem. In d .. 26, 153-67 (1907).
(14) Ib id ., 26, 1118-23 (1907).
(15) S anger, C. F ., a n d B lack, O. F ., Proc. A m . Acad. A rts Sci., 43, 327-40 (1907); J . Soc. Chem. In d ., 26, 1123-7 (1907).
(16) S atterle e, H . S., “ T h e A rsenical C o n te n t of C ity D u s t, w ith Som e O b serv atio n s on th e Biological Significance of A rsenic,"
n o t y e t published.
(17) T h o rp e, T . E ., J . Chem. Soc., Proc., 19, 183 (1903).
(IS) U. S. P h a rm a c o p e ia X , p. 428 (1925).
Re c e i v e d J a n u a ry 3 1 , 19 3 4 . T h e expenses of this research have been in p a rt defrayed by a p riv a te g ra n t “ In m em ory of G. P. C .," for which grateful acknow ledgm ent is hereby made.