Losses of Arsenic during the Low Temperature Ashing of Atmospheric Particulate Samples Paul R. Walsh" and James L. Fasching Department of Chemistry, University of Rhode Island, Kingston, R.I. 0288 1
Robert A. Duce Graduate School of Oceanography, University of Rhode Island, Kingston, R.I. 0288 1
Neutron activation and atomic absorption procedures have been used to study arsenic losses during low temperature ashing at power levels between 50 and 125 watts (RF). Losses of arsenic from ambient atmospheric particulate matter and various synthetic sea salt matrices containing known quantities of arsenic was observed. in general, the magnitude of arsenic losses by this treatment will depend on applied power levels and the physical and chemical properties of the arsenic sample matrix.
The decomposition of solid sample matrices prior to arsenic analysis has always required caution to prevent the loss of this volatile element. Dry ashing a t 500 O C leads to the loss of a considerable percentage of the arsenic present ( I ) . The technique of dry ashing in the presence of magnesium nitrate ( 2 )has had only moderate success in preventing this loss (3, 4 ) . Certain wet ashing methods give complete arsenic recovery, providing specific precautions are followed. Recent reviews (5, 6) have discussed their application. Gleit and Holland (7) introduced the basic procedure of low temperature ashing in the presence of electronically excited oxygen. At a power level of 100 watts (RF), they obtained complete recovery of hydrated arsenic trioxide (HAs02) that had been added to whole blood. Ashing temperatures were said to have been less than 100 "C. Complete recovery of sample arsenic has also been reported in the low temperature ashing of plants and animal tissue (8).In a later low temperature ashing study ( 9 ) ,arsenic in a synthetic cellulose, agaragar matrix was fully recovered a t an ashing power of 35 watts (RF). At higher power, losses became significant and increased proportionally to applied power levels. Arsenic recoveries in low temperature ashing procedures appear, therefore, to be dependent on the nature of the sample matrix, R F power level, and the chemical form of the element. Recovery studies utilizing inorganic laboratory solutions as standard additions to a sample matrix are inferior to investigations which work with the natural sample components. For this latter approach, methods must be available for determining the natural analyte content both before and after ashing. Neutron activation analysis is most suitable in this regard. The low temperature ashing recovery of arsenic from samples of atmospheric particulate matter has been studied using neutron activation and flameless atomic absorption procedures. This matrix is much different from the biological matrices previously studied by other workers. Because of the complex nature of atmospheric particulate matter, recoveries of arsenic from this matrix may be highly variable. Ashing recoveries may, in fact, depend on the atmospheric sampling location since this will determine the major constituents of the matrix. The majority of our atmospheric particulate samples are collected in the marine environment and our efforts in this work were, therefore, concentrated on marine 1012
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
atmospheric samples. Ashing recoveries of arsenic from solid synthetic samples made from sea water constituents were also studied to determine any specific conditions under which arsenic is more consistently lost or more favorably recovered. In some studies with synthetic samples, the nonvolatile element copper was used as an internal standard to demonstrate that complete recovery of the ashing residue was routinely obtained.
EXPERIMENTAL Procedures. Eight atmospheric particulate samples were collected at the Graduate School of Oceanography, University of Rhode Island, by drawing air through Whatman 41 filters a t a flow rate of -40 ft3/min for 24 h. One sample was similarly collected a t a remote site in northern Canada. Replicate discs were cut from these filters using a 13ile-inchOsborne Arch Punch (C. S. Osborne Co., Harrison, N.J.) wrapped in polyethylene. Some of the cut discs were directly packed in polyethylene vials and sealed for neutron activation to determine total arsenic. Other discs from each sample were carefully folded and placed in Teflon beakers for low temperature ashing at 50 watts (RF). After ashing, the residue was taken up in 1 ml of 4 N "03. For neutron activation of the residue, 20 fil of this solution was spotted on a 25-mm Whatman 42 filter disc. The dried disc was then folded and placed in a polyethylene vial and sealed for neutron activation. For atomic absorption, 100 fi1 of the 4 N "03 solution was diluted with 500 fil of 500 pg/ml Mg in 0.1 N H N 0 3 according to developed procedures (IO). Standards were also prepared in 500 pg/ml Mg solutions. Synthetic samples were prepared by spotting a 50-fil aliquot of a matrix (Table I) which closely resembled the major element content of marine atmospheric particulate samples on a 47-mm Whatman 41 filter disc. After drying, these discs were carefully folded and placed in Teflon beakers for low temperature ashing a t either 50,75, or 125 watts (RF). Samples were ashed a t powers greater than 50 watts (RF) to determine any dependency of loss on applied power levels. The 50 watt (RF) level was the minimum power level that would remain stable for extended periods of time. Ashing residues of the synthetic samples were prepared for analysis according to the same procedures used for actual atmospheric particulate samples. Arsenic was determined in all samples. In addition, copper was analyzed in the residues from matrices D and E (Table I). Reagents. Standard arsenic, copper, and magnesium solutions were obtained from Fisher Scientific, Pittsburgh, Pa. Dimethyl arsenic acid was obtained from the J. T. Baker Chemical Co., Phillipsburg, N.J. Reagent nitric acid, also from Fisher Scientific Co., was redistilled in quartz prior to use. Standard Copenhagen Sea Water (C1= 19.3755 %o) was obtained from stock at the Graduate School of Oceanography, University of Rhode Island. Aliquots of this standard sea water were treated with nitric acid, boiled, and reconstituted with distilleddeionized water to remove chloride. Dilutions were made as specified in the text. Low Temperature Ashing. All samples were ashed at specified power levels in a Tracerlab Model 505 Low Temperature Asher, LFE Corporation, Waltham, Mass. Ashing times at 50 watts (RF) were approximately 3 to 4 h. Atomic Absorption. A Perkin-Elmer Model 503 atomic absorption spectrophotometer with the Model 2100 Heated Graphite Atomizer (HGA), deuterium background corrector, and strip chart recorder were used. The line source was an arsenic electrodeless discharge lamp. All analyses were performed using the 193.7-nm arsenic resonance line. Sample injection volumes were 10 or 20 1 1 depending on concentration levels. The temperature program for the HGA was as
Table I. Synthetic Sample Matrices for Low Temperature Ashing Studies Solution A B
C D E
Content 5 pg/ml As(V) in 0.1 N HN03 5 pg/ml As(V), 2693 pg/ml Na as Copenhagen Sea Water with chloride in 0.1 N HN03 5 pg/ml As(V), 2693 ,ug/ml Na as Copenhagen Sea Water without chloride in 0.1 N HN03 5 pg/ml As(V), 5 wg/ml Cu, 2693 pg/ml Na as Copenhagen Sea Water without chloride in 0.1 N "03 1775 pg/ml As (as dimethyl arsenic acid), 40 pg/ml Cu,
430 pg/ml Na as Copenhagen Sea Water with chloride in 0.1 N "03
Table 11. Recoveries of Arsenic in Ashed Atmospheric Particulate Samples Sample No.
Table 111. Recoveries of Arsenic in Ashed Synthetic Matrices
Total pg As 5.1 f 2.0 1.2 & 0.2 0.67 f 0.09 1.3 & 0.2 0.40 f 0.11 1.3 & 0.2 0.78 & 0.10 0.14 & 0.01 9.9 f 0.5
Recovered Pg As
Recovery, %
41 f 16 74 f 12 54 & 8 55 f 6 & 0.04 f 0.03 63 f 17 & 0.06 63 f 10 & 0.05 73 f 11 57 f 7 & 0.01 f 1.2 86 & 12 Mean and propagated std dev 63 f 38 2.1 0.89 0.37 0.72 0.25 0.81 0.57 0.08 8.5
f 0.1 f 0.03 f 0.05
follows: drying, 125 "C for 25 s; charring, 600 O C for 20 s; atomizing, 2300 "C for 8 s. For copper, a hollow cathode source was used at the 324.7-nm resonance line. Five p1 injection volumes were used. The temperature program was as follows: drying, 125 "C for 10 s; charring, 1000 "C for 10 s; atomizing, 2500 O C for 10 s. Neutron Activation Analysis. Atmospheric particulate samples were irradiated for 7 h at a neutron flux of 4 X 10l2neutrons/cm*/s in the Rhode Island Nuclear Science Center's 2-megawatt swimming pool reactor. After irradiation, samples were allowed to decay for 1 day and then counted each for 4000 s on an Ortec 40 cm3 Ge(Li) coaxial detector (resolution of 2.3 keV for the 1332 keV gamma of 6oCo) coupled to a Nuclear Data Model 2200 4096 channel analyzer. The 559 keV gamma of 76As( t l l z = 26.4 h) was monitored for all samples. Counting data were automatically transferred t o magnetic tape for storage and eventual processing by computer programs.
RESULTS AND DISCUSSION The recoveries of arsenic in atmospheric particulate samples are presented in Table 11. Total arsenic in the unashed filter sections was determined by neutron activation. Arsenic in the residues of ashed filters was determined by both neutron activation and atomic absorption procedures. At least two total filters and two ashed discs were analyzed for each sample. The standard deviations of the recoveries and that of the mean of the recoveries have been propagated from individual uncertainties (11).Losses of arsenic are evident in each case. The magnitude of the standard deviation of the mean of the recoveries emphasizes that these losses are highly variable. This may be due to the complex and mixed character of these environmental samples. Arsenic in crustal material, for example, may be extensively bonded in the rock lattice and may, therefore, be nonvolatile in the ashing process. In contrast, volatile oxides are most likely present from smelting operations and the burning of fossil fuels. In addition, methylated arsenicals are also likely present in the atmosphere (12,13).
Solution A B B C
D D E
No. of determinations 4 10 5 5 5 5 4
Power
(RF
Arsenic recovery,
Copper recovery,
Watts)
%
%
50 50 75 50 50 125 50
64 f 16 69 f 2 72 f 5 70 f 3 55 f 3 29 f 9 54 3
*
... ... 101 & 1 100 & 6 103 & 2
In the samples studied, all except No. 9 were collected at a point very near the ocean but not remote from pollution sources. Sample 9 was taken a t a remote continental site where a smelter emitted large quantities of arsenic oxides and where the quantity of atmospheric crustal material was rather high. I t is interesting to note that the recovery of arsenic in sample 9 was greater than in the others and, therefore, reflects the greater loading of crustal particulates that have inhibited losses of the volatile oxides by matrix entrapment. Samples 1through 8 do not have this crustal loading and their As losses may, therefore, be related to arsenic species derived from the ocean or pollution sources. Ashing recoveries of arsenic in the synthetic matrices described in Table I were studied to determine whether the losses in the marine atmospheric particulate samples could be attributed to a specific matrix component or arsenic species. The recoveries of arsenic from these synthetic matrices are in Table 111. Recoveries of 10Wh for copper indicate the complete solution uptake of the sample residue. Arsenic losses are again very significant in the various samples a t ashing powers of 50,75, and 125 watts (RF). No difference is apparent among the salt, no-salt, and organoarsenic samples, although the arsenic speciation is undoubtedly different in each case. There is no observable effect due to the presence of chloride, even though arsenic trichloride is extremely volatile. This may indicate that arsenic preferentially exists as a pentavalent salt in the sea salt matrices. The dimethyl arsenic acid species has been proposed to exist in the ambient atmosphere (12, 13). The refractory nature of this specific compound (14) would suggest that it retains its structure in the acid and salt matrix. Whether or not the compound is broken down during low temperature ashing is not known. In any case, these results indicate that inorganic arsenic salts and organic arsenic are lost from the tested matrices during the ashing procedures. As found previously ( 9 ) ,,these losses appear to increase with applied power.
CONCLUSION The results indicate that substantial quantities of arsenic will be lost during the low temperature ashing of certain solid samples. Losses could not be attributed to any specific arsenic species. The magnitude of the losses are probably dependent on sample matrix, ashing power, and duration. The sample matrix is critical because its physical as well as chemical structure may influence the release of arsenic components. From these observations, one can conclude that low temperature ashing should not be used in connection with the determination of arsenic because significant losses occur even a t an R F power of 5 0 watts.
LITERATURE CITED (1) T. T. Gorsuch, Analyst(London), 84, 135 (1959). (2) R. J. Evans and S.L. Bandemer, Anal. Chem., 26, 595 (1954). (3) J. E. Portmann and J. P. Riley. Anal. Chim. Acta, 31, 509 (1964).
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
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(4) W. F. Carey, J. Assoc. Anal. Chem.. 51, 1300 (1968). (5) W. R. Penrose, Crit. Rev.Environ. Control, 5, 465-482 (1974). (6) Y. Talmi and C. Feldman, "The Determination of Traces of Arsenic: A Review", in "Arsenical Pesticides", ACS Symposium Series No. 7, E. Woolson. Ed., American Chemical Society, Washington, D.C., 1975, pp 13-34. (7) C. E. Gleit and W. D. Holland, Anal. Chem., 34, 1454 (1962). (8) H. A. Schroeder and J. J. Balassa, J. Chronic Dis., 19, 85 (1966). (9) C. E. Mulford, At. Absorp. Newsl., 5, 135 (1966). (10) P. R. Walsh, J. L. Fasching, and R . A. Duce, Anal. Chem., 48,1014 (1976). (1 1) P. R. Bevington, "Data Reduction and Error Analysis for the Physical Sci-
ences", McGraw-Hill. New York, 1969. (12) J. M. Wood, Science, 183, 1049 (1974). (13) G. R. Sandberg and I. K. Allen, "A Proposed Arsenic Cycle in an Agronomic Ecosystem", in "Arsenical Pesticides", E. A. Wooison. Ed.. ACS Symposium Series No. 7, American Chemical Society, Washington,D.C., 1975, pp 124-147. (14) G. 0. Doak and L. D. Freedman, "Organometallic Compounds of Arsenic, Antimony, and Bismuth", Interscience, New York, 1970.
for review October 2of
Accepted February
20, 1976.
Matrix Effects and Their Control during the Flameless Atomic Absorption Determination of Arsenic Paul R. Walsh" and James L. Fasching Department of Chemistry, University of Rhode Island, Kingston, R.I. 0288 1
Robert A. Duce Graduate School of Oceanography, University of Rhode lsland, Kingston, R.1. 0288 1
Various matrices have been found to affect the flameless atomic absorption analysis of arsenic with the Heated Graphite Atomizer. Sodium in the presence of sulfate presentsthe most serious interference. The use of magnesium in these solutions is described to enhance the sensitivity and control the interference effects. Environmental matrices have been analyzed for arsenic by this method. Results are consistent with those obtained by standard addition and neutron activation techniques.
In flameless atomic absorption determinations of arsenic, certain matrices may cause chemical interferences in which reactions of concomitant materials with arsenic may affect the production rate and population of gaseous elemental arsenic from which absorbance measurements are obtained. Ion exchange ( I ) , tantalum lined graphite tubes (2), and solvent extraction ( 3 )have been used to eliminate chemical, as well as the spectral ( 4 ) interferences related to the flameless determination of arsenic. Recent studies on the flameless atomic absorption determination of arsenic in steel and alloys ( 5 , 6 )has shown, however, that arsenic absorbances are highly dependent on certain solution components and that matrix modification techniques, in which a specific reagent is added to.remove or control an interference, should be applied (7). Ammonium nitrate, for example, has been used as an agent for the removal of well documented sodium chloride interferences (7,8).The addition of a protective agent enhances the absorbance signal by forming compounds that atomize more efficiently than the unprotected analyte-interferent matrix (9). For arsenic sample matrices, a protective agent may be used to reduce volatility during drying and ashing cycles or to promote more efficient volatization of arsenic in the atomization cycle. This report presents a study of arsenic absorbances in various mineral acids and metal salt matrices. In particular, the matrix effects of certain environmental constituents which many analysts encounter is discussed in conjunction with a protective agent that may be used in their presence. Neutron activation analysis is used to verify arsenic determinations in natural aerosol matrices by flameless atomic absorption. 1014
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
EXPERIMENTAL Apparatus.Atomic Absorption. A Perkin-Elmer Model 503 atomic absorption spectrophotometer equipped with the Model 2100 Heated Graphite Atomizer (HGA), deuterium background corrector, and chart recorder was used with an arsenic electrodeless discharge line source. The 193.7-nm arsenic resonance line was used a t all times. Instrumental settings were in accordance with manufacturer's operation procedures. The optimum time-temperature program of the Model 2100 HGA was experimentally determined to be: dry, 125 "C for times ranging from 10 to 40 s for sample volumes of 5 to 20 fil; ash, 600 "C for 20 s; atomize, 2300 "C for 8 s. The normal nitrogen purge cycle (at 30 units) was used for all measurements. Eppendorf and Oxford pipets were used for sample injections as well as for sample preparation. Sample injection volumes were typically 20 11. Calibration curves for arsenic were linear over wide concentration ranges. For 20-fil volumes, a detection limit of 5 &g/l. As a t a S/N ratio of 2 was obtained. Neutron Actiuation. Samples were irradiated in the Rhode Island Nuclear Science Center 2 Megawatt swimming pool reactor a t a thermal flux of 4 X 1OI2 neutrons/cm2/s. After a 24-h decay period, the 559-keV gamma ray of the '6As ( t l l z = 26.4 h) isotope was monitored by counting on an Ortec 40 cm3 Ge(Li) coaxial detector (resolution of 2.3 keV for the 1332-keV gamma ray of 6oCo) coupled to a Nuclear Data (Model 2200) 4096 channel analyzer. Data were stored on magnetic tape for processing by computer programs. Reagents. All standard solutions were made from 1000 pg/ml stock solutions obtained from Fisher Scientific Co., Pittsburgh, Pa. Suprapur acids from E. M. Laboratories, Inc., Elmsford, N.Y. were used where acids are specified. Standard solutions and test solutions were made on a daily basis, although no variation in arsenic solutions was noticed over a three-week period a t concentrations ranging from 0.02 to 10 fig/ml.
RESULTS AND DISCUSSION The absorbance of 0.5 to 1Fglml arsenic concentrations in various mineral acids indicated that maximum values are obtained from solutions of dilute (0.01-1N) nitric acid. As found previously by other workers (5, IO),hydrochloric and hydrofluoric acids gave very reduced signals and these anions should be avoided for this application. Arsenic absorbances in 0.01 to 0.1 N sulfuric acid are slightly reduced relative to nitric acid solutions. Nitric acid is, therefore, used as the solvent for subsequent sample preparation unless otherwise noted. Acid concentrations are kept below 2 N because of erratic absorbance readings and enhanced graphite tube dete-
