Table V. Total Tin (pg/g) in Beef Tissue after Feeding with Me,SnCl,
Organ Liver Liver Fat Kidney Kidney Brain
AAS graphite furnace (Av. of 3 injects) 0,156 0.056
Chemical
0.016
0.016 0.236
0,099
0.106 0.027
0.016 0.222 0.094
AAS standard addition 0.157
0.024
anal.
pyrocatechol violet 0.156 0.066
0.025
Application to Tissue Digests. The standards for analysis of tissue digests in Soluene-350 are prepared from the particular organic compound used in the feeding study, as described in the reagent section. Table V shows typical data obtained from organ digests of beef cattle fed dimethyltin dichloride. As interference effects of the inter-element type have been reported in flameless atomic absorption studies, some verification using the standard method of additions has been made, as well as comparisons with standard wet chemical colorimetric procedures. There is insufficient data for statistical evaluation at this time, but work looks very promising. Further studies for specific interferences in the graphite furnace analysis of tin are planned as we expand the method to other systems.
ACKNOWLEDGMENT The authors thank Kenneth Buxton, Bionomics, for his valuable discussions and J. Pekola and H. Corbin of the M&T Central Analytical Department for providing the colorimetric tin determinations.
LITERATURE CITED H. B. Corbin, Anal. Chem., 45, 534-537 (1973). E. J. Newman and P. D. Jones, Analyst (London), 91, 406 (1966). H. B. Corbin, J. Assoc. Off. Anal. Chem., 53, 140-146 (1970). M. Farnsworth and J. Pekola, Anal. Chem., 26, 735-737 (1954). I.M. Kolthoff and P. J. Elving, “Treatise on Analytical Chem.”. Part 11, Vol. 3, Interscience, New York, 1961, p 363. J. E. Schallis and H. L. Kahn, At. Absorpt. Newsl., 7, 84 (1968). H. L. Kahn and J. E. Schallis, At. Absorpt. Newsl., 7, 5 (1968). T. Nakahara, M. Munemori, and S. Musla, Anal. Chim. Acta, 62, 267 (1972). J. D. Mensik and H. J. Seideman, Jr., At. Absorpt. Newsl.. 13, 8 (1974). A. Engberg, Analyst (London), 98, 137 (1973). G. Everett and T. West, Anal. Chim. Acta, 70, 296 (1974). J. C. Meranger, J . Assoc. Off. Anal. Chem., 58, 1143 (1975). Food and Drug Act and Regulations (1973), Information Canada, Ottawa, Ontario, Sec. B 23.003. Food, Drug and Cosmetic Law Reports, Food Additive Regulations, 21 CFR, Section 121,2602, paragraph 56,952. Food,Drug and Cosmetic Law Reports, Food Additive Regulations, Section 121.2602, Fed. Regist., 40, No. 11, January 16, 1975. Technical Bulletin, Packard Instrument Co., Ino., 2200 Warrenville Rd., Downers Grove, Ill., 60515. P. J. Barlow and A. K. Khera, At. Absorpt. Newsl., 14 (6) 149 (1975). Association of Official Analytical Chemists, “Official Methods”, 12th ed., pp 386-390.
RECEIVED for review November 29,1976. Accepted April 27, 1977.
Determination of Sub-microgram per Liter Quantities of Arsenic in Water by Arsine Generation Followed by Graphite Furnace Atomic Absorption Spectrometry AH U. Shaikh and Dennis E. Tallman” Department of Chemistry, North Dakota State University, Fargo, North Dakota 58 102
The arsenic in a large volume (50 mL) of water sample is subjected to NaBH4 reduction and the liberated arslne is trapped in a small volume of a chloroform/ephedrine solution of silver diethyldithiocarbamate(SDDC). The arsenlc in the SDDC solutlon Is then determined by graphlte furnace atomic absorption spectrometry. The relative standard deviation for ten replicate determlnatlons4s less than 3 % at the 5 ppb level. A detection limit of 10 ng is obtained, corresponding to 0.2 ppb for a 50-mL water sample. The detection llmit can be extended to below 0.05 ppb by employing a larger volume of water sample and by evaporation of solvent from the SDDC solution as a means of further preconcentratlon. The relatlve accuracy of the method at the 5 ppb level is better than 5 % as determined from the analysls of EPA reference samples.
The determination of arsenic in trace amount in the environment continues to be of considerable interest, due in large part to the known toxicity of this substance. A variety of methods have been developed for the trace determination of arsenic (I-3), often involving generation of arsine (AsH3)from As(ITI)/As(V) followed by atomic absorption spectrometry (AAS) in which the arsine is atomized by introduction directly into either a flame (2) or an electrical furnace ( 3 ) . Recent
reports ( 4 , 5 )describe an automated technique in which arsine is generated on a hot reaction bed a t high pressure followed by AAS detection. Using A1 and HC1 for reduction ( 4 ) , 40 specimens per hour can be analyzed with a detection limit of 0.1 ppb, whereas NaBH, and HC1 reduction ( 5 ) permits the analysis of 70 specimens per hour with a detection limit of 0.011 ppb. The automated approach, however, requires a somewhat elaborate experimental setup and thus may not be worth pursuing unless many arsenic samples are being determined routinely. It appears that a relatively simple and rapid method for arsenic determination with high sensitivity and precision employing readily available instrumentation is still in need. Such a method is described in this paper. It has been reported (6) that AsH3 forms a stable complex with silver diethyldithiocarbamate (SDDC). A standard method for the determination of As involves the dissolution of generated AsH3 in a pyridine solution of SDDC followed by colorimetric determination ( I ) . More recently, Kopp (7) has shown that 1-ephedrine in chloroform can be used as a solvent for SDDC in the colorimetric method (thus avoiding the disagreeable odor of pyridine) without loss of sensitivity, accuracy, or precision. We have found in our laboratory that AsH3 is very conveniently trapped in a chloroform/ephedrine solution of SDDC which can then be analyzed for As by Graphite-Furnace ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1093
Rubber
S
- SDDC
Solutlon
- Frit
Figure 1. Arsine generation and trapping apparatus
Atomic Absorption Spectrometry. Since arsine may be generated from a relatively large volume of sample (50 mL or larger) and trapped in a smaller volume of SDDC solution (5 mL or less), a preconcentration is effected. If desired, further concentration of the As may be achieved by evaporation of the solvent from the As-SDDC solution. This simple method of preconcentration, coupled with excellent reproducibility, provides a sensitive, accurate method for As determination. Using this technique, a detection limit of less than 0.2 ppb has been obtained. At the 5-ppb level, ten replicate determinations yield a relative standard deviation (RSD) of 2.8%. An average relative error of less than 5% is typical as observed by testing the method against EPA reference samples.
EXPERIMENTAL Apparatus. The apparatus used for the generation of arsine and its subsequent trapping in SDDC is illustrated in Figure 1. I t consists of four pieces: 1)a 100-mL round bottom flask with ground-glass joint and a side tube fitted with a rubber septum for introduction of the NaBH4 reducing solution; 2) a ground-glass stopper fitted with a purge gas inlet tube extending to the bottom of the flask and a side-on outlet tube; 3) a 90" glass elbow with a frit at one end and a ball and socket joint a t the other for connection to the gas outlet tube; and 4) a 10-mL graduated cylinder containing the SDDC solution into which the frit is immersed. This arrangement provides for easy dismantling of the apparatus for cleaning. The frit diameter is slightly less than the inside diameter of the graduated cylinder, providing for efficient contact between the arsine gas and the SDDC solution. The inner space of the ground glass stopper is filled with anhydrous CaClz supported by a plug of cotton. Atomic absorbance measurements were carried out on a Perkin-Elmer Model 603 AA Spectrometer equipped with a deuterium background corrector and a Model HGA-2100 graphite tube furnace. Although the Model 603 has an on-board microcomputer capable of performing nonlinear least squares analysis of standard addition data, we elected to record the absorbance data from standard addition experiments and perform the least squares analysis on a PDP-11/40 computer equipped with a CRT graphics terminal. This approach permitted us to interactively compute the best fit curves and display them on the CRT terminal. It also enabled us to employ a larger number of standard additions than is possible when the analysis is done by the on-board microcomputer (limited to two standard additions). The peak height mode of absorbance measurement gave excellent precision, sensitivity and linearity. No significant improvement was observed when absorbance measurements were made in the peak area mode and, thus, the peak height mode was used throughout this study. A simple relay circuit was added to the graphite furnace controller and was used to automatically initiate the read cycle of the Model 603 at the onset of the atomization cycle of the HGA-2100. A Perkin-Elmer electrodeless discharge lamp (EDL) was used with a Perkin-Elmer EDL power supply at the recommended power setting of 8 W. Absorbance measurements were made at the most sensitive arsenic resonance line of 193.7 nm. Background correction was found necessary and was used throughout this study. Argon was used for purging the furnace with an internal 1094
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
gas flow of 10 divisions on the controller. The furnace drying temperature and time were 90 "C and 30 s unless stated otherwise. The charring temperature and time as well as the atomizing temperature were varied to obtain the optimum conditions (vide infra). A charring cycle of 600 "C for 60 s and an atomizing temperature of 2300 "C were used throughout this work. A relatively long atomizing time of 10 s was arbitrarily selected to ensure complete destruction of the sample before proceeding to the next determination. Reagents. All regeants were of analytical grade and were used without further purification. Distilled, doubly-deionized water (Millipore, Inc.) was used for preparing solutions. Sodium borohydride reductant was obtained from Alfa Chemicals and was used as a 2% solution in 0.1 M NaOH. A standard 1000-ppm solution of As was prepared by dissolving the appropriate amount of anhydrous Asz03 in 0.1 M NaOH. All other As working solutions were prepared by appropriate dilutions of this standard. Solutions having concentrations less than 1 ppm were prepared the same day the experiment was performed. Three metal reference samples were obtained from EPA (Cincinnati, Ohio) and diluted according to the instructions. Since the As in these samples is inorganic in form, no digestion was necessary. The chloroform solution of SDDC and ephedrine was prepared as follows. Ephedrine, when exposed to air, absorbs a considerable amount of C 0 2 (8). I t was, therefore, produced in situ from its hydrochloride. One gram of ephedrine hydrochloride was dissolved in 100 mL water, basified with 1M NaOH to pH greater than 11and shaken vigorously with 130 mL CHC13. The CHCIB layer was collected in a flask, a few pieces of anhydrous CaClz were added to take up any water and the solution was then filtered. Analysis of a 20-mL aliquot of the filtered CHC13 solution for ephedrine by hydrochloride precipitation indicated essentially quantitative extraction of the ephedrine from the aqueous phase. To 100 mL of the CHC13/ephedrine solution was added 100 mL of CHC13and 0.5 g of SDDC. The solution was shaken vigorously, filtered, and diluted to a final volume of 400 mL with CHC13. Procedure. Arsenic samples were prepared for measurement by placing in the flask (Figure 1)an aliquot of the water sample, typically 5.0 to 50.0 mL, depending upon the arsenic level, adding 10 mL concd HCl and bringing the total volume in the flask to approximately 60 mL with distilled demineralized water. The apparatus was then assembled (Figure 1)and a constant flow of purging gas (either argon or air) was maintained. Five mL of the SDDC trapping solution was placed in the 10-mL graduated cylinder and the frit was immersed in the solution so as to just touch the bottom of the cylinder, allowing maximum contact time between the SDDC solution and the arsine carried along with the purging gas. With purge gas passing through the system at a rate of 150 cm3/min,2 mL of 2% NaBH4 solution was quickly drawn into a syringe and injected into the sample solution through the rubber septum. Arsine generation and trapping were complete in 6 min (vide infra) whereupon the frit was removed from the cylinder and the SDDC solution was diluted to the 5.0-mL mark by adding chloroform (to supplement the loss of chloroform by evaporation during purging). A 25-fiLaliquot of this solution was used for the AA measurement, Standard additions were performed prior to the arsine generation step by spiking the sample solution in the arsine generation flask with the appropriate amounts of standard As solution. Four standard additions were normally used for each analysis.
RESULTS AND DISCUSSION The arsenic content of most potable waters seldom exceeds 20 ppb. For this reason, a 50-mL aliquot of 5.0 ppb As solution was used to optimize the experimental method. The charring and atomization temperatures were varied so as to obtain the maximum background-corrected analyte absorbance. T h e variation of the arsenic atomization absorbance as a function of charring temperature and duration is shown in Figure 2. Maximum absorbance is obtained with a charring temperature of 600 "C. Charring temperatures above 600 "C result in a decrease in absorbance, most likely due t o the loss of As resulting from premature vaporization during the charring cycle. Below 600 "C the decrease in signal may be due t o
I
1
I
I
I
I
I
l
l
4 400
600
800
I000
TEMPERATURE
1200
140020
("C)
40
60
80
I00
I20
TIME (Seconds)
Figure 2. Optimization of charring cycle parameters for the analysis of the As-SDDC solution. (a) Variation of temperature for a cycle duration of 60 s. (b) Variation of cycle duration for a charring temperature of 600 "C. Arsenic absorbance was measured during a 2300 "C atomization cycle 0
i
m
I
1500
1700
1900
2100
2300
TEMPESATURE
5
15
10
25
20
I
T I M E (Tinutes)
Figure 4. Arsenic absorbance vs. duration of purge as a function of purging gas flow rate: (0)50 cm3/min;(A) 150 cm3/min.; (0)250 cm /min. Each point corresponds to a separate analysis of a 50.0-mL aliquot of a 5.0-ppb A s sample. The curves for the 50 cm3/min. and 150 cm3/min.flow rates merge after a purge time of 45 min
2500
("C1
Figure 3. Dependence of arsenic absorbance on atomization temperature for analysis of the As-SDDC solution. Charring cycle: 60 s at 600 "C incomplete atomization of As as a result of incomplete decomposition of the SDDC complex. The absorbance signal is not particularly sensitive to the duration of the charring cycle over the range of 20 to 60 s. Figure 3 illustrates the dependence of the measured absorbance on the atomization temperature. From these optimization experiments, a 600 "C charring cycle of 60-s duration and an atomization temperature of 2300 "C were selected for this work. These conditions appear to be optimum for sample aliquots containing up to 2.5 wg of As. Sample aliquots containing more than 2.5 wg As appear to require a somewhat higher atomizing temperature to ensure complete atomization (for example, 2500 "C for a 5-pg sample). The rapidity with which an analysis can be performed is for the most part limited by the time required for complete arsine generation and trapping. Figure 4 illustrates that by controlling the purging gas flow rate, complete generation of arsine and its subsequent trapping in SDDC can be achieved in 6 min. This time, of course, will be a function of the size and the geometry of the arsine generation and trapping apparatus employed. For our apparatus, a flow rate of 150 cm3/min. seems to be nearly optimum. Lower flow rates serve only to lengthen the analysis time. Higher flow rates result in loss of As due to incomplete trapping by the SDDC solution. We find no detectable difference between results obtained with air as the purging gas and results obtained with argon as the purging gas. This is not too surprising since the purging gas serves only to transport the arsine from the generation flask to the SDDC trapping solution and is not involved in the atomic absorption measurement. Standard calibration curves covering the concentration range 1 to 10 ppb and 10 to 60 ppb are shown in Figure 5. Excellent linearity is observed with a slope corresponding to a sensitivity of 1 milliabsorbance unit per 7 ng of As contained in the arsine generation flask. Table I reflects the reproducibility of 10 replicate As determinations a t the 5.0-ppb level, each determination involving arsine generation and
60
0
Z 40 Q
m
30
1
0
2 0
4 0
I
6 0
I
I
I
80
100
120
C O N C A s (ppb) Figure 5. Standard calibration curves for 50-mL samples covering the range 1 to 10 ppb and 10 to 60 ppb As ___--
Table I. Reproducibility of Replicate Arsenic Determinations at the 5.0-ppb Level Absorbance Ten individual 0.034, 0.035, 0.035, 0.034 determinations' 0.034, 0.036, 0.036 0 036 0.036, 0.035b Mean 0.035 i 0.001 Re1 std dev 2.8% '
a
Each determination involved generation and trapping
of arsine from 50.0 mL of a 5.0-ppb As standard. b Each of these ten absorbance values is the mean of five or more measurements on the same As-SDDC solution with a RSD of a 2.6%. All absorbance values are blank corrected.
-
trapping. Note that the precision reported in Table I(2.8% RSD) largely reflects the uncertainty associated with the arsine generation and trapping steps. The uncertainty arising from the absorbance measurement itself was minimized by averaging five or more such measurements (footnote b, Table I). A detection limit of 10 ng As, corresponding to arsine generation from 50.0 mL of a 0.2-ppb sample, produces an ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1095
Table 11. Analysis of EPA Reference Samples by Standard Additions Concentration, ppb
Dilution Re1 Av Sample factor Obsd Accepted error, % recovery, % EPA 1 1:lO 2.43 -6.5 90.7 i 3.3 2.60 EPA 2 3:50 -3.0 93.9 i: 7.2 4.25 4.38 EPA 3 1:50 5.75 5.56 +3.4 97.4 i 3.3 increase in absorbance which is twice as large as the uncertainty associated with a blank analyzed in identical fashion. This 0.2-ppb detection limit can be reduced even further by using a larger arsine generator which can accommodate larger volumes of water sample. Alternatively, the SDDC solution after arsine trapping can be further concentrated by controlled evaporation of the solvent, the As absorbance increase being proportional to the volume contraction. Attempts to reduce the volume of the As-SDDC solution by more than a factor of 5 resulted in a loss of As due to precipitation of SDDC and/or complex. Using this latter approach, we have extended the detection limit to below 0.05 ppb for a 50-mL sample. Absorbance measurements employing pyrolytically coated graphite furnace tubes (9) did not differ significantly from those utilizing uncoated tubes. T o test the accuracy of our method, three EPA reference samples were analyzed by standard additions. The results of these analyses are summarized in Table 11. The reference samples were diluted so as to bring them into the range of arsenic concentration generally observed in natural waters. The average relative error of the method is typically less than *5%. Using the sensitivity computed from the data of Figure 5, the average percent recovery of the arsenic added to each of the EPA samples during the standard addition analysis can be calculated, and these results are also summarized in Table 11. The somewhat less than quantitative recovery may be due to a slight suppression of arsine generation by heavy metals present in the EPA reference samples (10,11). In each case, better than 90% recovery was obtained. Selected river water samples from various parts of North Dakota were analyzed by the method described in this report. In every case the standard addition curves exhibited good
linearity with As recoveries comparable to those achieved with the EPA samples. To illustrate a typical result, a n analysis of a sample taken from Cedar Creek, North Dakota, yielded an As content of 8.9 ppb for a n undigested aliquot and a n As content of 17.8 ppb for an aliquot digested in HzSOd/HN03 prior to analysis. These results indicate that a substantial portion of the total arsenic in this particular sample exists in one or more organic forms not detected by our method without prior digestion of the sample. Under the acidic conditions of our arsine generation reaction, many of the organo-arsenic forms found in natural waters should be reduced to the corresponding organo-substituted arsines (12). Certain of these organic forms, however, may not be trapped by the SDDC solution, possibly because of weak complex formation with SDDC or low vapor pressure of the higher molecular weight species. In any event, to obtain the total organic plus inorganic arsenic by this method, digestion of the sample is essential. We are currently exploring the nature and extent of interaction of various arsenic forms with SDDC as well as a method involving injection of the various arsenic hydrides directly into the graphite tube furnace. The results from these investigations will be reported a t a later date.
LITERATURE CITED (1) “Standard Methods for the Examination of Water and Waste Water,” Michael J. Taras, Ed., American Public Health Association, Washington, D.C., 1971. (2) F. J. Fernandez, At. Absorp. News/., 12, 93 (1973). (3) R. C. Chu, G. P. Barron, and P. A. W. Baumberger, Anal. Chem., 44, 1476 (1972). (4) P. D. Goulden and P. Brooksbank, Anal. Chem., 46, 1431 (1974). (5) F. D. Pierce, T. C. Lamoreux, H. R. Brown, and R. S. Fraser, Appl. Spectrosc., 30, 38 (1976). (6) P. F. Wyatt, Analyst (London),80, 368 (1955). (7) J. F. Kopp, Anal. Chem., 45, 1786 (1973). (8) G.C. Clarke in “Isolation and Identification of Drugs”, Pharmaceutical Press, London, 1969, p 327. (9) R. E. Sturgeon and C. L. Chakrabarti, Anal. Chem., 4g, 90 (1977). ,(lo) F. D. Pierce and H. R. Brown, Anal. Chem., 48, 693 (1976). (11) A. E. Smith, Analyst(London), 100, 300 (1975). (12) D. L. Johnson and R. S. Braman. Deep Sea Res., 22, 503 (1975).
RECEIVED for review January 24, 1977. Accepted April 15, 1,977. Financial support from the U.S. Department of the Interior, Office of Water Resources Research (C-6307) and from the Environmental Protection Agency (R803727-01-1) is gratefully acknowledged.
Comparisons of Methods of Hydride Generation Atomic Absorption Spectrometric Arsenic and Selenium Determination Darryl D. Siemer” and Prabhakaran Koteel Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233
Experiments performed with simple apparatus to optimize hydride generator atomlc absorption arsenic and selenium determinations are described. Absolute sensitivities of 5.5 X lo7 and 1.5 X lo7 absorbance unlts/g are obtained by freezing out arsenic and selenlum hydrides in a “U” tube prlor to introductlon into a quartz tube burner. These sensitlvities are compared both with published values obtalned with other hydride-AA systems and wlth graphite furnace sensitivites to obtain relative system efficlencies. Many hydrlde-AA techniques in use are not very efficient In generating analytical signal per gram of analyte metal taken.
Atomic absorption (AA) has an advantage over other atomic 1096
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
spectroscopic techniques in that the signal measured (absorbance) is a direct indication of the number of atoms present per unit atomizer cross section per unit time which is to a large degree independent of the stability and intrinsic brightness of the light source. It is therefore possible to directly compare the efficiencies of various atomizers used in atomic absorption by looking a t values of published sensitivities. The term “Sensitivity” will be used throughout this paper to mean “the absorbance signal observed per gram of metal”. This is not the traditional AA definition of the term but it is in line with those used by practitioners of other disciplines who publish in this journal. Stone ( 1 ) has made this comparison for a variety of nonflame graphite furnaces and has shown that atomizers, which are large enough to contain all of the sample
