J. Mandel, "The Statistical Analysis of
Experimental Data", Interscience, New York, 1964, p 253. F. S. Acton, "Analysis of Straight-Line Data". J. Wlley, New York, 1959, p 38. (10) C. Liteanu, I. C. Popescu, and E. Hopbtean, Anal. Chem., 48, 2010 (1976). (1 1) C. Liteanu, E. Hophean, and I. R b , in press. (12) E. Pungor, K. Toth, and J. Havas, Acta Chim. Acad. Sci. Hung., 48, 17 (1966). (13) C. J. Moody and J. D. R . Thomas, "Selective Ion Sensitive Electrodes", Merrow, Watford, 1971, p 77. (14) W. E. Morf, G. Kahr, and W. Simon, Anal. Chem., 46, 1538 (1974). (15) C. J. Moody and J. D. R. Thomas, "Selective Ion Sensitive Electrodes", Merrow, Watford, 1971, D 5.
(16) J. H. Woodson and H. H. Liebhafsky. Anal. Chem.,41, 1894 (1969). (17) M. S. Frant, Cronache di Chimica, No. 44, 3 (1974). (18) IUPAC, Information Bulletln No. 43 (1975), p 2, "Recommendations for Nomenclature of Ion-selective Electroges." (19) C. Liteanu, I. C. Popescu, and V. Ciovirnache, Stud. Univ. BabepBolyai, 1s (I), 53 (1973). (20) J. Buffle,N. Parthasarathy, and W. Haerdi, Anal. Chim. Acta, 68, 253 (1974). (21) W. J. Blaedel and D. E. Dinwiddie, Anal. Chem., 46, 873 (1974).
RECEIVEDfor review September 18,1975. Accepted July 21, 1976.
Estimation of Methylated Arsenicals by Vapor Generation Atomic Absorption Spectrometry. Sir: Most methods of estimating arsenic involve evolution of arsine after digestion of the sample either in oxidizing acids or by dry ashing (1).The arsine is generated from the digest by the addition of a reducing agent such as sodium borohydride ( 2 ) or zinc and hydrochloric acid ( 3 ) . A variety of methods have been used for estimating the arsine, the traditional procedures being colorimetric ( 3 ) .Arsine can be determined by flame atomic absorption spectrometry (usually hydrogen-nitrogen entrained air flame) provided it is injected into the flame with sufficient speed (4-7). We wish to report that the alkylated arsenicals methylarsonic acid and dimethylarsinic acid may be estimated directly by vapor generation atomic absorption spectrometry without prior digestion. Sodium borohydride treatment produces methylamine, and dimethylarsine, respectively, which are swept directly into a hydrogen-nitrogen entrained air flame by the excess hydrogen generated by hydrolysis of the sodium borohydride. These methylated arsines are estimated in a manner identical to the arsine produced following acid digestion or dry ashing. The calibration curves and instrument response are directly comparable and are dependent only on the quantity of arsenic entering the flame. This observation is of particular relevance when it is noted that these methylated forms of arsenic appear universally in the environment (8). Investigations such as those carried out by Braman and co-workers ( 4 9 ) into speciation of environmental arsenic now become possible without the specialized electrical discharge apparatus used by them (IO). We have successfully analyzed standard mixtures of arsenate, methylarsonic acid, and dimethylarsinic acid for each component by a procedure analogous to that of Braman (8).
EXPERIMENTAL A Varian Model 1 000 atomic absorption spectrometer fitted with a Varian Model 64 arsenic determination apparatus was employed with the following instrumental parameters: Wavelength, 193.7 nm; spectral band width, 1.0 nm; lamp current, 7 mA. Gas flow rates were: hydrogen, 20 1. min-l; nitrogen 20 1. m i x 1 . Estimations were carried out as described by Duncan and Parker ( 2 )with similar calibration curves and instrumental response. Detection limits were 500 ng for inorganic arsenic and 1l g for monomethylarsonic acid and dimethylarsinic acid. Aqueous solutions of methylarsonic acid and dimethylarsinic acid were adjusted to 2% in hydrochloric acid before addition of the sodium borohydride solution.
Standard mixtures of sodium arsenate, methylarsonic acid, and dimethylarsinic acid were treated with sodium borohydride and the mixed arsines generated trapped in a glass bead packed tube (200 mm X 25 mm) a t -180 "C. The cooling agent was removed and the trap allowed to warm slowly in the laboratory atmosphere. A valve assembly kept the trap sealed and was released periodically with a simultaneous flow of nitrogen through the trap into the flame. The valve was opened for 5 s each min. The instrument response was recorded on a chart moving a t 2 mm min-' and resembled, in outline, a gas chromatographic trace with arsine peaking a t 3 min, methylamine at 8 min, and dimethylarsine a t 13 min.
DISCUSSION A time consuming and possible error inducing digestion stage can be omitted from atomic absorption spectrophotometric procedures for estimating some alkylated arsenicals. This is of particular value when it is considered that not only are methylated arsenicals widely distributed in the environment but methylarsonic and dimethylarsinic acids are used as herbicides, and methods for their estimation have been sought (I1,12). A possible explanation for the discrepancy between the results of Uthe (13)and Penrose (14) is now apparent. These authors each compared the efficiency of wet ashing (oxidizing acid) and dry ashing (magnesium oxide/magnesium nitrate). Whereas Uthe found wet ashing gave a slightly higher recovery than dry ashing (both approach loOo/o),Penrose obtained good recovery for dry ashing but less than 5% for wet ashing. The difference lies in their methods of estimating the arsine generated after the reductive step. Penrose used a colorimetric method which is much more responsive to arsine itself than to the methylated arsines whereas Uthe used vapor generation atomic absorption spectrometry which does not distinguish between methylated arsines and unsubstituted arsine. Therefore around 100%recovery would have been achieved if no oxidation had occurred under the digestion conditions. It is interesting that in the method described here dimethylarsine, boiling at 55 O C , enters the flame with sufficient ease to produce no loss of signal. This implies that a considerable range of substituted arsines could be estimated directly in this manner.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
2019
LITERATURE CITED (1) T. T. Gorsuch, Analys?(London), 84, 135 (1959). (2) L. Duncan and C. R. Parker, Varian Techtron, Palo Alto, Calif., “Technical Topics”, 1974. (3) “Standard Methods for the Examinatlon of Water and Wastewater”, 13th ed. American Public Health Association, Washington, 1971, pp 62-66. (4) W. Holak, Anal. Chem., 41, 1712 (1969). (5) R. E. Madsen, At. Absorp. News/., IO, 57 (1971). (6) E. F. Dalton and A. J. Malanoski, At. Absorp. Newsl., 10, 92 (1971). ( 7 ) F. J. Fernandez and D. J. Mannlng, A?. Absorp. Newsl., 10, 85 (1971). (8) R. S. Braman and C. C. Foreback, Science, 182, 1247 (1973). (9) D. L. Johnson and R. S. Braman, Deep-sea Res., 22,503 (1975). (10) R. S. Braman, L. L. Justen, and C. C. Foreback, Anal. Chem., 44, 2195 (1972). (11) C. J. Soderquist, D. G. Crosby, and J. B. Bowers, Anal. Chem., 46, 155 11974) .I. ~
(12) R. M. Sachs, J. L. Michael, F. B. Anastasia, and W. A. Wells, WeedSci., 19, 412 (1971).
(13) J. F. Uthe, H.C. Freeman, J. R. Johnston, and P. Michalik, J. Assoc. Offic. Anal. Chem., 57, 1363 (1974). (14) W. R. Penrose, R. Black, and M. J. Hayward, J. Fish. Res. BdCan., 32, 1275 (1975).
John S. Edmonds* Kevin A. Francesconi Western Australian Marine Research Laboratories P.O. Box 20 North Beach, Western Australia, 6020 Australia
RECEIVEDfor review April 2, 1976. Accepted August 13, 1976.
Comments on Use of Exponential Dilution Flask in Calibration of Gas Analyzers Sir: The exponential dilution flask (EDF) has previously been used for calibration of gas analyzers (e.g., 1,2).Recently, an improved version of the EDF was used in the calibration of a flame ionization detector (FID) for 3-50 ppm propane in air ( 3 ) .The FID response is known to be very rapid (4)in comparison to usual EDF dilution rates so detector response time characteristics should exert a minimal influence on the time variation of EDF/FID signals. This led us to consider use of the EDF with other gas analyzers. In particular we have examined the possible effects of detector response kinetics on the signalltime relationship for an ideal EDF in combination with detectors which respond more slowly than the FID. MATHEMATICAL DERIVATIONS For an analyzer to be useful, the steady state signal,S,, must be a recognizable function of gas concentration, C, such as S, = BCn
(1)
where B and n are empirical constants. When a detector is subjected to a stimulus AC, it responds by altering its output signal in the direction of steady state as defined by the S,/C relationship. As with other dynamic systems (5), the rate of change of signal with time is a function of the difference between the actual signal, S, and that which would obtain a t steady state, S,, for the instantaneous concentration, C, of gas in the detector. For example, dS/dt = &X(S - S,)”
(2)
where dS/dt < 0 for S > S , and vice versa. X is the mth order response coefficient. The empirical parameters X and m are determined as follows. At t < 0, the analyzer is at steady state sampling zero concentration and the signal S = 0. At t = 0 the analyzer is suddenly subjected (6) to a sample gas of constant concentration, CO,for which S, = BCZ. Integration of Equation 3 then gives m # 1:
m = 1: ln(1 - S/BCg) = -At (3) (1- S/BC$)l-m = 1 (m - l)X(BCE)m-l t (4)
+
Alternatively at t = 0, the analyzer is a t steady state sampling gas of concentration CO with signal BCE. Then zero concentration is suddenly introduced. The ultimate S , = 0. Integration of Equation 2 for signal decay gives m = 1: m # 1: 2020
ln(S/BCt) = - A t
+
Complications arise in that values of X and/or m on signal rise may be different from values for signal decay. This seems to be the case with H2S analyzers (7,8).Moreover, we have observed (8,9) that the speed of response for an electrochemical H2S analyzer increases with increasing H2S concentration and then decreases somewhat as the length of time between samples increases. Despite differing response rates at different sampling intervals, the steady state HzS signal always returned (9) to the same value (&l%).Ritter and Adams ( 3 ) noted possible differences in FID behavior at constant high propane concentrations as compared to transient levels of propane from the EDF. After characterization of the detector response by Equations 3-6, or by other expressions, the formalism of the EDFIdetector system can be developed. We consider only the ideal EDF for which,
(5)
(S/BCg)l-m = 1 (m - l)X(BCg)m-l t (6)
C = COexp(-at)
(7)
where a = G/Vedf (G is sample flowrate). Substitution of Equations 1 and 7 into 2 gives dS/dt = &(S- BCg exp(-nat))m
(8)
Equation 8 requires a numerical solution for m # 1 (cf. Ref. 9 for sinusoidal variation of H2S concentration and m = 2). The particular solution for m = 1is as follows for two sets of initial conditions. I. t 5 0, S = 0; t 2 0, C = COexp(-cut). Example: Pure calibrant injected into ideal EDF a t t = 0.
-
S = [xBC$/(X - na)] [exp(-nat) - exp(-At)]
(9)
11. t IO,S=BC:;t < O , C = C o ; t 1 0 , C = C o e x p ( - a t ) . Example: Standard reference mixture with constant concentration passing through EDFldetector with dilution commencing only after initial steady state signal has been attained.
-
S = [BCt/(X - na)] [A exp(-nat)
- n a exp(-At)]
(10)
The true area, At, under the ideal “EDF curve” ( 3 )is
At = BCg
Jm
exp(-nat)dt = BCg/na
(11)
Similar integration of Equation 9 for any X yields the same area. However the area under Equation 10 exceeds At by the factor (1 ncuh).
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
+
