of polonium-210 in the tellurium precipitation. Because polonium-210 emits alpha particles having energies only 20 keV lower than those of uranium-232, any polonium-210 not removed will add to the uranium-232 peak and make the recovery of tracer appear to be higher than it really is, giving low results. For example, the solutions can be centrifuged rather than filtered if desired, but about 1%of the polonium-tellurium precipitate will be decanted with the supernate because of a scum at the surface that is not pulled down by centrifugation. A considerably larger error, and one that is not readily apparent, is produced in the presence of hydrochloric acid. If 5 mL of concentrated hydrochloric acid is used instead of the 2 mL of concentrated sulfuric acid specified, the reduction of the tellurous acid and subsequent agglomeration of the elemental tellurium takes place so much more rapidly that much of the polonium will not have been precipitated and/or included in the tellurium carrier by the time the available surface of the carrier has been reduced to negligible proportions. The results will be 5 to 10%low. The accuracy of the procedure for determination of the three main thorium isotopes in thorium ores was verified by comparing the results obtained for thorium-232 by the present alpha spectrometric procedure with those obtained for natural thorium by a fluorometric procedure of previously demonstrated accuracy (B), and by direct measurement of the lead212 daughter by gamma spectrometry. On 24 measurements of 8 different solid aliquots of a standard monazite ore, the fluorometric procedure gave a mean of 1.404 f 0.017 (f0.004)% thorium, where the first uncertainty is that of an individual about the mean and the one in parentheses is that of the mean itself. Using a value of 2428 f 21 dpm/g of thorium-232 per percent natural thorium, the percentage values convert to 3409 f 51 ( f 3 1 ) dpm/g of thorium-232. On 20 measurements on the same 8 aliquots of the sample by the present alpha spectrometric procedure, a mean of 3421 f 74 (f16) dpm/g was obtained, in excellent statistical agreement
with the chemical values. Four measurements by direct gamma spectrometry gave a mean of 1.444 f 0.018 (f0.009)% thorium, also in acceptable agreement. In addition, the mean ratio of thorium-228 to thorium-232 for the 20 measurements was 0.996 f 0.024 (f0.005),showing that the measurement of thorium-228 is no less exact and that the system is in equilibrium so that the gamma spectrometric measurements can be related unequivocally to the thorium-232 concentration. Although no independent method was available to check the thorium-230 result in thorium ores, there is no reason to believe that its determination will be any less accurate than the other two isotopes.
ACKNOWLEDGMENT The author wishes to express his appreciation for the assistance of his associates, particularly to F. D. Hindman for the thorium and uranium separations, to R. L. Williams for the electrodepositions and alpha spectrometry, and to J. S. Morton for the gamma spectrometry.
LITERATURE CITED (1)J. N. Rosholt, A. P. Butler, E. L. Garner, and W. R. Shlelds, €con. Geol., 60, 199 (1965). (2)J. N.Rosholt, E.N. Harshman, W. R. Shlelds, and E.L. Garner, €con. GeoL, 59, 570 (1964). (3)J. Kronfeld, Nucl. Sci. Abstr., 26, 2982 (1969),Abstract No. 31018. (4)C.W. Sill, "Simultaneous Determlnation of U-238.U-234,Th-230, Ra-226 and Pb-210 in Uranium Ores, Dusts and Mill Tailings, U. S. Energy Research and Development Administration, Health Services Laboratory, Idaho Falls, Idaho. (5) C. W. Sill, K. W. Puphal, and F. D. Hlndman, Anal. Chem., 46, 1725
(1974). (6)C.W. Sill, Anal. Chem., 46, 1426 (1974). (7)C.W. Sill and R. L. Williams, Anal. Chem., 41, 1624 (1969). (8)C.W. Slll and C. P. Willis, Anal. Chem., 34,954 (1962). (9)C. W. Sill and F. D. Hlndman, Anal. Chem., 46, 113 (1974). (10)D. R. Perclval and D. 8. Martin, Anal. Chem., 46, 1742 (1974). (11)C.W. Silland C. P. Willis, Anal. Chem., 37, 1661 (1965).
RECEIVED for review October 29,1976. Accepted January 14, 1977.
Separation and Determination of Nanogram Amounts of Inorganic Arsenic and Methylarsenic Compounds Robert S. Braman,* David L. Johnson,' Craig C. Foreback,2 James M. Ammons, and Joseph L. Bricker Department of Chemistry, University of South Florida, Tampa, Fla. 33620
Arsenate and arsenite ions, methylarsonlcacid, and dimethylarslnic acid in aqueous solutions are reduced to arslne and the corresponding methylarslnes respectlvely at pH 1-2 by sodlum borohydrlde In a reaction chamber. Entrained by He carrier gas the arsines are frozen out In a liquid nltrogen cooled U-trap. Their separation Is accompllshed by volatlilzatlon upon warming the U-tube trap. Arslnes carried out of the trap by the carrler gas are passed through a direct current electrlcai dlscharge. Arsenic atomic emlsslon lines produced in the discharge are detected by a recording, scannlng monochromator system. Llmlts of detectionfor arsenic are approximately 1 ng for each of the arsines.
Present address, State University of New York, College of Environmental Science and Forestry. Syracuse, N.Y. 13210. * Present address, Chemistry Division, Henry Ford Hospital, Detroit, Mich. 48202.
Considerable interest in the environmental chemistry of arsenic stems from the toxicity of its compounds and their use as silvicides or pesticides. An extensive annotated bibliography on arsenic in the environment has been prepared (1). Nearly all of this referenced work used analytical methods for total arsenic with no differentiation of arsenic by chemical form except for thin-layer and paper chromatography separation methods reported by Sachs, Anastasia, and Wells ( 2 ) . Methylarsenic acids are important compounds of arsenic produced by biomethylation and could play an important role in its environmental chemistry. Concentrations of arsenic found in the environment are generally small, several parts per billion in sea water to less than 1ppb in many fresh waters and parts per million in soil. Prior to the work of Braman and Foreback (3) little had been available for the determination of the arsenic forms in environmental samples. Johnson and Pilson ( 4 ) developed a spectrophotometric method for differentiation of arsenic(II1) ANALYTICAL CHEMISTRY, VOL. 49,
NO.
4, APRIL 1977
621
Table I. Reduction of Arsenic Compounds agb at
Molecular form
PKa
PH 1
(YO a t PH 4
Reduction PH
As(III), Arsenous acid, HAsOz As(V), Arsenic acid, H3As04 Methylarsonic acid, CH3AsO(OH)2 Dimethylarsenic acid, (CH3)2AsO(OH) Trimethylamine," (CH3)3As Phenylarsonic acid, C6HsAsO(OH)z
9.23 2.25 (pK1) 2.60 6.19
1.00 0.948 0.975 1.00
1.00 0.017 0.038 1.00
3.59
1.00
0.280
4 1-2 1-2 1-2 1-4 1-2
...
Or its oxidized form presumed t o be (CH3)sAsO.
cy0
...
bP
ASH^ ASH^
-55
CH3AsH2 (CW2AsH (CH~)~AS CsH&Hz
2 35.6 70 148
-55
is the fraction of the acid in the undissociated form.
1
ElDiSC'
Figure 1. Sample reaction chamber, U-trap, and detector arrangement
and arsenic(V) anions. Peoples, Lakso, and Lais (5) reported a method to some extent capable of distinguishing between inorganic arsenic and organic arsenic acids in aqueous samples containing more than 2 pg of arsenic. Gas chromatography of methyliodide-treated dimethylarsinic acid has been reported (6)but this technique has not been applied to the other arsenic compounds. Braman, Justin, and Foreback (7)have reported a method for inorganic arsenic based upon continuous detection of volatilized arsine. The low limits of detection served as a basis of the method reported here. T a l m i and Bostick (8) have recently reported based upon gas chromatographic separation of sodium borohydride reduced arsenic compounds with detection by a microwave discharge emission type detector. Talmi and Norvell(9) have reported a method for inorganic As and Sb based upon gas chromatographic separation after preparation of triphenyl derivatives. E d m o n d s and Francesconi (10) have recently reported a separation similar to the one reported here; t h e y used a flame type atomic absorption spectrometer as t h e detection unit. This paper reports i n detail on the analytical method of B r a m a n and Foreback (3) including improvements since its first use in their environmental studies. The method has been adapted to air analysis i n environmental studies ( 3 , l l ) .
EXPERIMENTAL Reduction Chemistry of Arsenic Compounds. Development of this method proceeded from our finding that the several arsenic acid compounds may be reduced to arsine or corresponding methylarsines in aqueous solution by sodium borohydride. Further, the reduction reaction was found to be pH dependent, and related to the pK, of the arsenic acids, a fact which aids the identification of arsenic compounds found in samples. Table I shows the reduction conditions now used 622
...
Reduction product
ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
for analysis and gives the pKa data and (YO, fraction of arsenic acids in the undissociated form present at analysis pH conditions. Arsenous acid or arsenic(II1) ions, dimethylarsinic acid, and oxidized forms of trimethylarsine are partially reduced above pH 4, even though, considering the reduction potential for sodium borohydride (-1.24 V), one might predict that inorganic arsenic(V) ions and methylarsonic acid would also be reduced. A pH 3.5-4.0 condition was selected for determination of arsenic(II1) ions in the analysis procedure because arsenic(V) was not reduced in this pH range and because reduction of arsenic(II1) was more rapid than a t higher pH values. The reduction of arsenic(V) noted in earlier work (7) was found to be due to a partial reduction observed when strongly acidic solutions of arsenic(V) were injected into sodium borohydride solutions or vice versa. Therefore, it is necessary to buffer samples to the appropriate pH prior to addition of NaBH4 when arsenic(II1) is to be determined. Reduction of arsenic(V) by sodium cyanoborohydride in a prereduction step prior to analysis proved less convenient than using several treatments with 2% aqueous NaBH4 a t pH 1-1.5. The cyanoborohydride ion decomposes and produces hydrogen cyanide which obscures the methylarsine and the dimethylarsine peaks, thus preventing their analysis in the same sample and at the same time as total inorganic arsenic. Sodium cyanoborohydride was also unsuitable as a prereducing agent for seawater analyses. Although sodium borohydride rapidly hydrolyzes at pH 1-1.5, it apparently has sufficient life to effect reduction of arsenic compounds. Completeness of arsenic(V) reduction was studied. Results indicated that over 95% of arsenic(V) was reduced after 4 to 5 additions of 2 mL each of 2% NaBH4 in both seawater and distilled water. Reduction of all other arsenic compounds was over 99% complete by the analysis procedure. Oxalic acid was found to be the best acid for pH control a t pH 1-1.5. The use of sulfuric acid to produce pH 1or lower was tried but found to cause approximately a 2-5% disproportionation in analysis of dimethylarsinic acid. This type of effect was not observed with the procedures reported here. The analysis process by itself did not cause the methylation of inorganic arsenic either in standard mixes or in natural water samples to which inorganic arsenic has been added. Methylarsenic compounds were not demethylated. Zinc and hydrochloric acid were also found to reduce methylarsonic acid and dimethylarsinic acid but were not as rapid or as convenient to use as sodium borohydride. Apparatus. The apparatus arrangement used and characteristics of the detector and the scanning monochromator system have been reported previously (12,13). Arsines are generated in a reaction chamber, trapped out in the U-trap, and carried through the detector by helium carrier gas in the apparatus shown in Figure 1.All glassware was constructed locally. Quartz tubing, 6-mm 0.d. was used to construct the detector chamber. Polytetrafluoroethylene (PTFE) tubing, 0.25 inch 0.d. was used to connect parts of the glassware system. The tubing connectors fit tightly over 6-mm 0.d. glass tubing, yet could be easily disconnected to facilitate disassembly of the apparatus. The detector was attached to the monochromator by means of a linen-filled Bakelite mounting board. The reaction chamber was constructed from a ground glass joint size 34/35 and could be used with samples up to approximately 70-mL volume. A side arm injection port was constructed with a 0.25-inch Swagelok nut fitting sealed to the 6-mm 0.d. tube by epoxy cement. Medium or coarse frits were used. Heavy hooks were used to secure the top and bottom parts of the chamber by means of rubber bands. Ball joints and O-ring joints have also been successfully used in similar reaction chamber designs. The U-trap, 8 inches long each arm, was half-filled with 60-80 mesh glass beads.
Table 11. Reproducibility of Volatilization Timesa
As(II1) (sodium arsenite)
Standards in H2O Methylarsine Dimethylarsine
9.2 s f 1.2 s.d. 23.2 s f 2.0 s.d.
N =5 N =7
13.0 s f 0.8 s.d. 24.1 s f 0.9 s.d. 34.2 s f 1.3 s.d.
y (cm2) = 0.172 f 0.010 ng (as As) n=5
As(V) (sodium arsenate)
Lake water analyses Methylarsine Dimethylarsine Trimethylamine
T a b l e 111. Calibration C u r v e s f o r Arsenic Compounds
N =3 N =3 N =3
Appearance of peak maximum after starting t o heat t h e Utrap. 0
The addition of a carbon dioxide absorber with a bypass, a 2- to 3-inch long column packed with small sodium hydroxide beads, was a major improvement removing carbon dioxide interference with arsine detection. These beads used were not the usual larger pellets but were approximately 2-mm 0.d. obtained from Fisher Scientific Co., NaOH No. S612. The carbon dioxide absorber occasionally needs replacement and must be protected from water vapor. The bypass directs methylamines around the sodium hydroxide trap so as to avoid small losses of the methylamines. Characteristics of the Separation Method. Differences in boiling points of the arsines suggested separation by a gas chromatographic-like procedure. Water vapor, arsines, carbon dioxide, and any other vapors from the sample or generated in the reduction process are frozen out in the U-trap. After removal of the liquid nitrogen flask and upon warming, all trapped compounds are evolved from the U-trap generally in order of their boiling points. Reasonably good separation of the arsines is obtained. The carrier gas is then passed through the detector cell where an arsenic atomic emission line intensity is observed and recorded to provide a higher degree of selectivity. The separation is carried out under non-isothermic conditions. The liquid nitrogen is removed from the U-tube which is at first allowed to warm up without heating. Any low boiling gases volatilize out of the trap in 10-30 s. Arsine volatilizes out just prior to carbon dioxide which is always present, at least in small amounts, as an impurity in sodium borohydride. After the carbon dioxide has been vaporized out of the U-tube, the wire heater is turned on with the heating rate controlled by a laboratory autotransformer. A slow heating rate is optimum; U-tubes wound with approximately 48 inches of B & S gauge 24 Chrome1 A wire, approximately 1.65 R per foot, were heated by an impressed voltage of 12-20 V from the autotransformer. The optimum heater voltage for each U-tube is determined by experiment with standards. Generally, the more rapid the heating rate, the more rapidly do the methylamines volatilize out of the U-tube, but the separation can become poorer a t high heating rates. The response of the arsines was found to be uninfluenced by heating rate with the exception of dimethylarsine. At high heating rates, a 2040% decrease in signal was noted for dimethylarsine attributable to thermal decomposition. After removal of the arsines, water finally starts to volatilize out of the trap, quenches the discharge, and ends the analysis. The U-trap volatilization separation could be considered to be a programmed temperature gas chromatographic effect on a solid phase with a rapid heating rate. The heating rate is greater than 100 "C per minute since the U-trap is heated from -195 to approximately 60 "C in 1-2 min. The half-packed U-trap is an interesting feature. A fully-packed U-trap did not function well nor did an empty open U-tube. Water vapor condenses and freezes in the up-stream end of the U-tube while the lower boiling arsines and other gases apparently freeze out further down stream. During the volatilization and analysis step, the lighter gases pass through or out of the packed section before water. Water vapor apparently is kept from volatilizing out of the U-tube at the same times as the arsines by the still-cold glass beads. Nevertheless, the gas velocity through a fully packed column may be sufficient to distribute water vapor almost throughout the entire column. Water vapor then emerges in time to quench the dimethylarsine and trimethylarsine peaks. The effect of carrier gas flow rate on separation of peaks was found to be important. A flow rate of 300 mL per min was optimum for analysis. Considerably lower values gave poorer separation of the alkyl arsines. Table I1 gives typical retention time reproducibility data under optimum operating conditions and with the same liquid ni-
y (cm2) = 0.164 f 0.005 ng (as As) n=7
- 0.25 f 0.16 - 0.23 f 0.11
Sodium methylarsonate y (cm2) = 0.172 f 0.0017 ng (as As) n=6
- 0.26 f 0.051
Dimethylarsinic acid y (cm2) = 0.165 i 0.0033 ng (as As) n = 5
- 0.40 f 0.11
trogen level. Retention time reproducibility is not critical to the analysis. Reagents. Powdered sodium borohydride and sodium methylarsonate were obtained from Ventron Corp., Beverly, Mass. Dimethylarsinic acid was obtained from Research Organic/Inorganic Chemical Corp., Sun Valley, Calif. Oxalic acid and other reagent grade chemicals used were obtained from Matheson, Coleman and Bell Corp., East Rutherford, N.J. Sodium arsenate and sodium arsenite were Baker Analyzed Reagents grade. All non-arsenic reagents must be analyzed for the presence of traces of the various arsenic compounds. Methylarsenic acid compounds were analyzed for purity by acidimetric titration. Airco laboratory grade helium has been used with no purification. Sodium borohydride used for reductions should be low in carbonate ion content. Commercial aqueous solutions of sodium borohydride have been found to contain much larger amounts of carbonate ion than the powders. Although the carbon dioxide trap prevents interference in arsine detection, large amounts of carbon dioxide can interfere in methylarsine detection. Prepared solutions of 2% sodium borohydride in distilled water were kept in a closed plastic dispenser bottle to avoid absorption of carbon dioxide from air. Standards. Stock solutions of from 500 to 1000 ppm of the several arsenic compounds were prepared in distilled water. These were reasonably stable for several days to approximately one week. Serial dilutions, usually 1 : l O O of the stock solutions were made. Diluted solutions had to be prepared fresh each time a calibration sample was needed; their arsenic content decreased perceptibly in 15-30 min. Microliter range syringes (1-10) were used to sample the diluted standard solutions. Instrument Parameters. The 228.81 nm or 234.98 nm atomic emission lines for arsenic are the most sensitive (7) but the former was used in most work. Each has approximately the same sensitivity if a 1P28 photomultiplier (PM) tube is used. Slit widths used were from 100 to 200 pm. Photomultiplier voltages used ranged from 900 to 1050 V. The slit width and P M tube voltage may be adjusted to whatever voltage range is desired to meet sensitivity. At these settings and with a 40-mA discharge current, output currents of the photomultiplier A range. A chart speed of 4 module were in the 1 X 10-8-1 X inchedmin was used. Procedure for Inorganic Arsenic(II1). Samples containing from 0 to 100 ng of arsenic in up to 70 mL of solution are neutralized, if necessary, and placed in a clean bubbler. To this is added 1-3 mL of 5% potassium biphthalate. The sample should now be approximately pH 3.5-4. Carrier gas is then allowed to pass through the system at 300-350 mL/min for 1-2 min to flush out air. The U-trap is then connected and cooled by the liquid nitrogen trap. Two milliliters of 2% sodium borohydride is then added and the system is allowed to stand with the carrier gas passing through it for 5 min. The discharge is turned on 1 min prior to analysis and allowed to warm up. The liquid nitrogen flask is removed and arsine passes through the carbon dioxide trap into the detector. A small fraction of dimethylarsine and trimethylamine may also be detected. The system is then prepared for the next analysis. The U-trap is disconnected from the system and heated while passing carrier gas through it to remove water prior to reuse. Procedure for Arsenic(V). A sample just analyzed for arsenic(II1) according to the previous procedure may be further analyzed for arsenic(V). The sample is buffered to pH 1-1.5 by adding 5 mL of a saturated solution (approximately 10% wt/vol) of oxalic acid in water. After assembling the trapping system, four additions of 2 mL of 2% ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
623
Table IV.Reproducibility of Water Analyses Golf Course Pond (10-mL samples) Total found, as As 64 f 4.2 (s.d.) ng, 6.5% rel, N = 4 74 f 4.4 (s.d.) ng, 5.9% rel, N = 4 trace not detected
Arsenic compound As(II1 V) Methylarsenic acid Dimethylarsenic acid Trimethylarsine
+
+
As(I1 V) Methylarsonic acid Dimethylarsenic acid Trimethylarsine compounds
Lake Carroll (25-mL samples) 50 f 5.5 (s.d.) ng, 11%rel, N = 3 4.39 f 0.23 (sad.)ng, 5.2%rel, N = 3 6.31 f 0.38 (s.d.) ng, 5% rel, N = 4 5.31 f 0.44 (sod.)ng, 8.4%rel, N = 3
DMAA Standard in distilled water, replicates, (25-mL Samples) 20.0 f 0.49 ng (s.d.), 2.5% rel, N = 5 sodium borohydride each are made at 30-8intervals. Carrier gas is passed through the system for 6 min. Arsine in the U-trap is measured as in the prior procedure. The balance of dimethylarsine and trimethylamine and all of the methylamine produced will also be detected. Procedure for Total Inorganic Arsenic and Alkylarsenic Compounds. It is most convenient to analyze samples for total inorganic arsenic compounds and alkylarsenic compounds in a single procedure. Samples are neutralized, if necessary, and are buffered by addition of 5 mL of oxalic acid reagent. Four 2-mL additions of 2% sodium borohydride are made 30 s apart. After collectionof the arsines for 6-8 min, the liquid nitrogen trap is removed. Arsine is passed through the carbon dioxide trap. The methylarsines bypassed around it while heating the U-trap.
RESULTS AND DISCUSSION Response a n d Limits of Detection. Response curves for the four arsenic compounds studied were found to be linear from the limit of detection near 1 ng to the upper limit of sample sizes studied, 100 ng (as As). Table I11 gives typical calibration curves for four arsenic compounds. The four arsenic compounds give approximately the same calibration curve based upon response as arsenic. The uncertainty in detecting small amounts of the arsenic compounds in the 1-10 ng range as measured by the noise level of the discharge response was found to be: 0.2 ng for arsenic(II1) and (V), 0.2 ng for methylarsonic acid, and 1 ng for dimethylarsinic acid and trimethylamine. Dimethylarsinic acid has a poorer limit of detection, likely because it is evolved from the cold trap more slowly than the other arsines and background noise is integrated over a larger time period. Concentration limits of detection calculated using 50-mL samples range from 0.004 to 0.02 ppb (as arsenic). Interferences. Any volatile compounds which are scrubbed out of the reaction chamber and which are evolved out of the U-trap prior to water can be an interference if present in sufficient amounts to increase the background emission, or if they quench arsenic emission. I t is possible by wavelength scanning to determine if detected peaks are arsenic line emission or background or band emission. Fortunately, few compounds encountered in environmental samples have the requisite volatility. None have been observed in any application of the method. A number of metal ions were tested for interference in the method for inorganic arsenic. The following ions were not an interference a t 20 ppm: A13+, Cd2+, C r ~ 0 7 ~ Fe3+, -, Mn2+, Ni2+,Pb2+,Zn2+,NO3-, Br-, and HP0d2-. The ions Ag+ and Cu2+ at 20 ppm partially inhibit arsine evolution, but had no effect at 2 ppm. Seawater and a number of fresh waters had no effect on evolution of the arsines. 624
ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
Silver and copper in interferences were removable by passing sample solutions through a cation exchange column in the ammonium ion form. The Amberlite IR124 (Rohm and Haas) exchange resin had no arsenic blank background. Samples which contain large amounts of surface active materials may foam excessively. T o prevent this, 1-3 mL of a 0.1% (wt/vol) solution of Anti-Foam B (Technicon product) may be added to the sample. Antimony does not interfere in the arsenic analysis in a ratio of up to at least 1001, despite the fact that it is reduced to stibine under the same conditions. If comparatively large samples of arsenic, over 1 pg, are run through the detector, small amounts may deposit in the chamber. This arsenic is removed by the stibine vapor and can lead to a positive arsenic signal. Deposition of arsenic in the discharge can also decrease the signal response by blocking the light path. Removal of deposited arsenic was done by injection of air or ammonia vapors into the discharge while it was operating. Samples containing up to a t least 100 ng of arsenic did not deposit arsenic in the detector chamber a t an appreciable rate. Identification of Arsenic Compounds. Comparison of retention times for detected compounds to that of methylarsenic standards is useful for identification. The disappearance of arsenic peaks (by analysis of samples a few angstroms off of the arsenic emission line) indicates the presence of arsenic compounds. The pH effect on reduction of arsenic compounds may also be used. Analysis at pH 4-5 will eliminate signals of inorganic arsenic(V) ions, methylarsonic acid and most of the dimethylarsinic acid present. Thus, disappearance of these peaks by pH effect is also positive evidence of the presence of arsenic acids. GC-mass spectroscopy has also been used to positively identify the methyl arsines. Applications. The analytical procedures have been used for the analysis of a variety of samples in environmental chemistry studies. Lakes and rivers in the Tampa Bay, Florida area, seawater, human urine, and various types of biological materials (3, 14, 15) have been analyzed. Analysis of air particulate and of filtered air for the arsines (11, 16) has been successfully carried out. Air particulate on glass wool is analyzed after tearing apart the filter pad in a small amount of 0.05 N NaOH. Inorganic arsenic appears to be completely trapped by the Gelman Instrument Co. type A glass fiber filters used. No apparent volatility of AS203 was observed in the air analysis work reported (11)as none was ever observed on the silvered bead column used under each filter pad to pick up alkyl arsines. Reproducibility of the procedures for the replicate analyses of water samples is given in Table IV. The standard error of the technique which is approximately f10% relative is likely satisfactory for most trace analysis applications.
LITERATURE CITED (1) "Arsenic in the Environment-An Annotated Bibliography", Oak Ridge National Laboratory, Oak Ridge, Tenn., ORN-2-EIS-73-16 (1973). (2) R. M. Sachs, F. 8. Anastasia, and W. A. Wells, Proc. Northeast. Weed Control Conf., 24, 316 (1970). (3) R. S. Braman and C. C. Foreback, Science, 182, 1247 (1973). (4) D. L. Johnson and M. E. Q. Pilson, Anal. Chim. Acta, 58, 289 (1972). (5) S. A. Peoples, J. Lakso, and T. Lais, Proc. West. Pharmacol. Soc.,14, 178 (1971). (6) C. J. Sonderquist, D. G. Crosby, and J. B. Bowers, Anal. Chem., 46, 155 (1974). (7) R. S. Braman. L. L. Justen, and C. C. Foreback, Anal. Chem., 44, 2195 (1972). (8) Y. Talmi hnd D. T. Bostick, Anal. Chem., 47, 2145 (1975). (9) Y. Talmi and V . Norvell, Anal. Chem., 47, 1510 (1975). (10) J. S.Edmonds and K. A. Francesconi,Anal. Chem., 48, 2019 (1976).
D. L. Johnson and R. S.Braman, Chemosphere, 6, 333 (1975). R. S. Braman and A. Dynako, Anal. Chem., 40, 95 (1968). R. S.Braman, Anal. Chem., 43, 1462 (1971). D. L. Johnson and R. S.Braman, Deep-sea Res., 22,503 (1975). C. C. Foreback, Ph.D. Thesis, University of South Florida, Tampa, Fla., 1973. (16) R. S. Braman, "Arsenic in the Environment" in "Arsenical Pesticides", E. A. Woolson, Ed., American Chemical Society, Washington, D.C., 1975.
(1 1) (12) (13) (14) (15)
RECEIVEDfor review October 1, 1976. Accepted December 23,1976. The support of this research by the National Science Foundation, RANN program, under grants No. GI-43753, and AEN 74-14598 A01 is gratefully acknowledged.
Band Broadening Studies Using Parameters for an Exponentially Modified Gaussian R. E. Pads1 and L. B. Rogers* Department of Chemistry, University of Georgia, Athens, Ga. 30602
The effects on chromatographic peak shape of dead volume and flow rate have been examined using the standard devlatlon of the Gaussian component of the peak and the exponential decay constant. For a nonretalned solute, addltlon of dead volume led to an increase in the standard devlation that was Independent of flow rate while the decay constant was lnversely proportional to the flow. Smaller changes were observed for a retalned species.
A gas chromatographic column is normally assumed to act as a Gaussian operator, broadening the 6 input into a Gaussian distribution as it passes through the column. However, pure Gaussian peaks are not found experimentally. This is because noncolumn factors such as dead volume, detector time-constants, and injection profile convolute the Gaussian distribution. Schmauch ( I ) , as well as Johnson and Stross ( 2 ) ,have shown that detector dead-volume will exponentially modify a chromatographic peak. McWilliam and Bolton (3) have also shown that time constants of detector-amplifier systems will exponentially convolute a Gaussian input profile. For these reasons, an exponentially modified Gaussian has been widely used as a model for chromatographic peak shapes. Several workers (4-6) have applied an exponentially modified Gaussian as a model in the least-square fitting and deconvolution of chromatographic peaks. An exponentially modified Gaussian is generated by the following integral: N
f(t) = --
r u v z
where N is the peak amplitude, t~ is the center of gravity of the Gaussian component, u is the standard deviation of the Gaussian, r is the time constant of the exponential decay and t' is a dummy variable of integration. The width of an exponentially modified Gaussian has two components: u, a symmetrical component due to the original Gaussian distribution Present address, Amoco Research Center, P.O. Box 400, Naperville, Ill. 60540.
and, r, a nonsymmetrical contribution due to the exponential decay. These two width terms are additive to give the second moment or peak variance.
Mz = u2 + r2
(2)
Sternberg (7)has published a comprehensive review of extra-column broadening and discussed the contributions to the peak variance of input profile, connecting tubing, and detector time Constants. He distinguished contributions from mixing chambers, diffusion chambers, and tubing-diameter expansions. Several workers ( I , 2, 8, 9) have published on extra-column factors, especially on the effects on column efficiency of dead volume, such as connecting tubing, fittings, and detector volume. Perhaps the most extensive experimental study on the effects of dead volume on column efficiency has been that by Maynard and Grushka (IO).In that work they showed that pre-column dead volume degraded column efficiency more than post-column dead volume. They also showed that dead-volume effects were much larger for nonretained species, and that expansions in tubing diameter seriously affected column performance even if the expansion occurred after the column. The purpose of this study was to briefly examine the effect of pre-column dead volume on the values of c and r for a chromatographic peak. Post-column effects were not examined because they were reported to be small (10).
EXPERIMENTAL Reagents. Helium (Selox, Inc.) was used as the carrier gas after it had been purified by passage over a molecular sieve. Methane and n-pentane (spectrophotometric grade, Aldrich Corp., East Rutherford, N.J.) were used as solutes. Chromosorb W, 100/120 mesh, and SE-30 silicone stationary phase were obtained from Alltech Associates (Arlington Heights, Ill.). Apparatus. A large chromatographic oven described earlier (11) was used in this study. The temperature in the oven was controlled by a Thermatrol proportional controller (Hallikainen Instrument Co., Richmond, Calif.). The oven temperature was measured using a platinum resistance thermometer (Omega Engineering Inc., Stamford, Conn.) in conjunction with a digital multimeter (Keithley Instruments, Cleveland, Ohio). Carrier gas was fed to the sampling valve through 1.5 m of capillary tubing so that back-diffusion of the sample into the gas supply would be negligible. Carrier gas flow was controlled by a Millaflow pressure regulator (Veriflow Corp., Richmond, Calif.) and a manual flow ANALYTICAL CHEMISTRY, VOL. 49,
NO. 4,
APRIL 1977
625
