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ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
Sampling and Atomic Absorption Spectrometric Determination of Arsine at the 2 pglm3 Level R. B. Denyszyn, P. M. Grohse," and D. E. Wagoner Research Triangle Institute, Post Office Box 12 194, Research Triangle Park, North Carolina 27709
A method has been developed and reflned for the sampllng and analytical measurement of arsine at the 2 ,ug/m3 level with a collectlon perlod of 15 min. Arsine standard samples were collected on charcoal, recovered by acld desorptlon, and analyzed by a comblned chemlcal modification-flameless atomlc absorption technique utillzlng a heated graphite atomlrer (HGA). Mean percent recovery and relatlve standard devlation were 89.1% and f0.10, respectlvely, for 105 samples over the sampling and analysis procedure. The chemical modification prevents sample loss during analysis and results In Improvements In sensltlvlty and llnearlty over previously employed methods. A callbration system employlng a twofold dilutlon was fabrlcated to generate arsine standard samples In the 1 to 10 pg/m3 range.
In its 1975 Arsenic Criteria Document, NIOSH recommended that no worker be exposed to inorganic arsenic in a concentration in excess of 2 pg/m3. Prior to the release of that document, an effort was made by NIOSH t o develop a sampling and analytical method that could trap and analyze 2 pg/m3 arsine in air during a 15-min sampling period (1). The sampling procedure involved trapping arsine on charcoal tubes, followed by desorption into dilute nitric acid. The dilute acid solution was then analyzed by flameless atomic absorption spectrometry, using a heated graphite atomizer. This procedure met with only limited success. In this work, it was noted that the percent recovery of arsine a t loadings of less than 0.09 pg was less than 75% (if sampled over a 15-min period). I t was also noted that there was a large variation in results for the analytical method (heated graphite furnaceatomic absorption) when different preparation procedures for calibration standards were used. Different response curves were obtained, with the addition of 0,50, and 100 mg charcoal to the calibration standards. The 100-mg loading simulates an actual sampling matrix. Several investigators have studied arsenic loss during low-temperature ashing (2,3). Molecular absorption is known to present a major problem in arsenic determinations. Investigations into the necessity of background correction for arsenic determinations have been conducted by Fitchett e t al. ( 4 ) and Robinson e t al. ( 5 ) . Improved arsenic stability during the charring step in flameless atomic absorption has been obtained by employing a chemical pretreatment (6-9). In this work, a sampling and analytical system has been developed to measure arsine gas a t the 1 to 10 pg/m3 concentration level. The method consists of collection of the gas on charcoal in glass tubes followed by electrothermal atomic absorption analysis utilizing the heated graphite furnace. EXPERIMENTAL Apparatus. A Perkin-Elmer Model 603 Atomic Absorption Spectrophotometer with the Model 2100 Heated Graphite Atomizer, deuterium background corrector, and strip-chart recorder were used. The line source was an arsenic electrodeless discharge lamp set at 193.7 mm. Instrumental settings were as specified in the manufacturer's operation procedures. The temperature program used was: dry 40 s at 100 "C; char 15 s at 1200 "C and atomize 7 s at 2540 "C using a 44-mL/min normal 0003-2700/78/0350-1094$01 .OO/O
nitrogen purge. Typical injection volume was 50 p L using an MLA pipet. Reagents. Certified reagent As203,1000 pg/mL arsenic stock solution, and lo00 pg/mL nickel stock solution were obtained from Fisher Scientific. Otherwise, ACS grade reagents were used throughout. SKC No. 106 activated coconut charcoal in glass tubes was purchased from SKC, Inc., Pittsburgh, Pa. Generation and Collection of 1-10 wg/m3 Arsine. A two-stage dilution system for generation of arsine gas at the desired concentration levels (1to 10 pg/m3) was designed and fabricated and is illustrated in Figure 1. Arsine at a known concentration was delivered through a twofold dilution system to a spherical glass manifold to obtain arsine concentration levels between 1 and 10 pg/m3. The spherical bulb was designed to accommodate up t o 10 sampling tubes; six ports were utilized. Simultaneous samples of arsine gas were collected with six charcoal tubes attached to critical orifices, which were in turn mounted on a common vacuum manifold (see Figure 1). Sampling consisted of collection of the gas mixture on charcoal in glass tubes for 15 min at a sample flow rate of approximately 1L/rnin. The vacuum manifold could be quickly disconnected from the pump to obtain precise sampling duration. Critical orifices were calibrated (with charcoal tubes attached) via a wet test meter, which was calibrated against a bubble flowmeter. The arsine cylinder, dilution air cylinder, and MB41 metal bellows pump were turned on 3 h prior to generation of the standard samples. After equilibration of the system for 30 min, arsine flow rates and the flow rate from the MB41 pump were set and measured with a soap bubble flowmeter. The two dilution , air flow rates were measured with a 1-L/revolution wet test meter. Each of these parameters was remeasured at each concentration level to verify constant flow. Thirty minutes were allowed for the system to equilibrate at each concentration level. In one 8-h day, approximately four sets of six tubes each could be generated. Procedure. Sample Desorption. Following sampling, the charcoal contents of each of the two sections are transferred to individual 10-mL-capacity, glass-stoppered centrifuge tubes and treated separately. The 50-mg downstream portion is used to check for possible arsine breakthrough. One milliliter of 0.01 M HN03 is pipetted into each centrifuge tube. The tube and contents are then agitated in an ultrasonic bath for 30 min. After centrifuging,the supernatant liquid is analyzed as described below. Standardization. To each centrifuge tube designated as a calibration standard, add 100 mg SKC No. 106 activated charcoal. Add 1 mL of the appropriate standard solution to each tube, stopper, and agitate in an ultrasonic bath for 30 min as with samples. Repeat this procedure for each new sample batch. Atomic Absorption Measurement. Instrumental parameters are adjusted according to the above described settings. A 50-pL aliquot of sample is injected into the furnace followed by a 50-pL aliquot of lOO-pg/mL Ni as Ni (NO,),. The absorbance is then measured. Sample loadings greater than 500 ng are diluted to a lower range. RESULTS A N D DISCUSSION Calibration Procedure. Verification of the arsine tank concentration was accomplished by the silver diethyldithiocarbamate colorimetric procedure (10). During the colorimetric determination of the tank concentration, it was observed that, a t delivery flow rates less than ZO-mLlmin, lower tank concentration values were obtained. This phenomenon could not be explained by any data a t hand, but various experiments were carried out to check whether collection efficiency below certain flow rates may have been 0 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
1095
10 PORT
DILUTION AIR
Gu ARSINE 111 AIR
CYLINDER
CRITICAL ORlFlCES
I OLA€S MANIFOLD
Z I 4
MB 151 METAL BELLOWS
CRITICAL ORIFICE
Figure 1. Twofold dilution arsine calibration system
0
"1
B 20
l
02 "dmi
04 AS
Figure 3. Arsenic HGA-atomic absorption calibration curve at 310 "C char without nickel modification
-1 O
-
1 ARSiNE FLOW RATE mLiMlN
Figure 2. Arsine tank concentration as a function of delivery rate
reduced because of poor bubble patterns in the absorbing solution. In order to answer this question, it was desired to: (1) determine the flow rate giving optimum bubbling efficiency while (2) maintaining a high enough flow rate through the stainless steel regulator to minimize any possible wall interaction that might occur. This was achieved by splitting the flow exiting the regulator using one branch as an approximate 30-mL/min bleed. The other branch could then be adjusted to obtain maximum collection efficiency. I t was noted that this maximum was achieved at 5 mL/min, as can be seen in Figure 2. Using these flow rates, the tank concentration was then determined to be 32 ppm. After repeated measurements of the arsine tank concentration, the calibration system was fabricated. The design, which incorporated a metal bellows pump for the second dilytion, raised several questions. 1. How constant was the flow rate from the critical orifice? 2. How much degradation of arsine would occur inside the pump? 3. How much conditioning would the pump and associated calibration system require? The flow rate from the pump was checked during the entire program and was shown to fluctuate by no more than &5%. Both the arsine concentrations from the initial dilution stage and from the output of the pump were monitored and produced an experimental concentration value within 10% of the theoretical concentration. Prior to the initial run, no arsine could be detected in the spherical bulb. The total system was conditioned for 30 min with gas taken directly from the arsine tank prior to the initial experiment run. This initial conditioning period was not repeated or required during the remainder of the program. Calculations. The theoretical arsine concentration was calculated as follows:
2 = (3.225Y)(F,)(T)
(2)
where Y = arsine concentration generated, ppb; X = arsine tank concentration, ppm; F1 = dilution air flowrate, 1st stage, L/min; F 1 A = arsine flow rate from tank, L/min; F2 = dilution air flow rate, second stage, L/min; F 2 A = flow rate from the MB41 pump, L/min; F3 = critical orifice flow rate on charcoal tube, L/min; 3.225 yg/m3 = 1 ppb ASH,; 2 = total ng arsine loaded; and T = sampling time = 15 min. Analysis Procedures. The analytical approach in the desorption step was unchanged from the NIOSH procedure ( I ) . Standards containing varying amounts of charcoal were prepared as in the NIOSH work. The same discrepancies noted by NIOSH appeared again in this work, as shown in Figure 3. Analysis of calibration standards without the nickel matrix modification generally gave nonlinear results. The 50-mg charcoal loadings invariably gave higher absorbance than the 100-mg loading. The standard solutions without added charcoal deviated from Beer's law, especially at concentrations above 0.2 pg/m3. During these studies and throughout the later developed procedure, blanks were run with every series of standards (i.e., desorptions on unloaded charcoal) and no arsenic was detected. The original analytical procedure has been improved by incorporating a nickel modification procedure and raising the char temperature from 310 to 1200 "C. The nickel apparently acts as a stabilizing agent during the char step in the temperature cycle by reacting to form a nickel arsenide. The increased stability of the nickel arsenide allows the temperature during charring to be increased to 1200 "C without loss of arsenic. Charring at the higher temperature minimizes any subsequent matrix effects and permits a more efficient atomization step. This modification accomplished the following: (1)it increased the method's sensitivity approximately 35%; (2) it produced calibration curves which were essentially linear from 0.01 to 0.2 pg As; and (3) it caused the slopes of the 0-, 50-, and 100-mg charcoal loading calibration curves to coincide. The nickel modification was employed throughout the concentration range and with standards containing a 100-mg charcoal loading duplicating sample matrix. The recoveries a t all levels averaged from 85 to 90%, thereby eliminating the need for two separate standard solution preparation procedures (see Table I). A 0.01-yg As standard corresponded to an approximately 0.8 pg/m3 concentration in air sampled for a 15-min period. The relative standard deviation over all levels was *0.095. The detection limit (twice the noise level) was determined to be 0.3 pg/m3 over 15 min with a sample volume of 13.15 L (flow rate: 875 mL/min). This corresponds to a total loading of 3.3 ng AsH3. Mean slopes from each day's analysis were utilized to check for deterioration in arsenic working standards, graphite tube life, and total analytical system performance. Using the nickel modification, mean percent recoveries for arsine loadings for all levels from 0.01 to 0.2 pg As were within
1096
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
Table I. Mean Percent Recovery and Relative Standard Deviation as a Function of Arsine Concentration Level
I
LEVELpglm3 (ng) 6.1-10.6 (80-1401
I
2.7-4.6 (35.60)
1
I 1
NO. OF
SAMPLES 28 28
I 1
MEAN % RECOVERY 89.6 88.3
I 1
5.14 6.48
1.5-2.3 (20.30)
21
87.6
8.65
0.8-1.2 (10.15)
28
85.2
9.46
I
1
-~
5 70indicating: (1)That the efficiency of collection-desorption process did not vary significantly, and (2) That the same calibration procedure (standard preparation procedure) was applicable a t all levels of arsine loading. In examining the variability encountered previously in the NIOSH work when no matrix modification was used, it was speculated that a matrix modification similar to the above procedure may be taking place due to contribution from some species in the unloaded charcoal itself. In light of the above speculation, a trace metal analysis was performed on 10 mL of 0.01 M H N 0 3 solution after desorption of 1000 mg charcoal (equivalent to 100 mg in 1 mL 0.01 M HNO, solution used during a typical analysis). Species of interest were those, such as nickel, known to enhance the arsenic signal and linearize the analytical range. These include Ni, Mg, Cr, Cu, and Cd. Of these, only Mg was detectable and then only a t the 50-ppb level, which is unlikely to produced an enhancement or linearizing effect similar to the above developed procedure. The mechanism involving absorption of AsH3 on charcoal is not entirely understood, but the indications are that AsH3
is converted to metallic arsenic on the charcoal surface. An experiment was conducted, which attempted AsH3 desorption from charcoal containing a known arsenic (ASH,) concentration. Silver diethyldithiocarbamate dissolved in pyridine was used as the desorbing media. The media is selective for arsenic only as the hydride. Following ultrasonic desorption, no arsenic signal was detected by the HGA modified silver diethyldithiocarbamate procedure. Since the arsine was loaded on the charcoal in a reducing atmosphere (nitrogen) without air dilution, it apparently does not exist on the charcoal as the oxide. Preliminary evidence, therefore, points to a mechanism involving conversion of arsine to elemental arsenic. This is supported by the fact that addition of chloroform does not achieve desorption of the arsine (soluble in chloroform), whereas addition of dilute acid gives a significant signal. LITERATURE CITED NIOSH Summary Report on the Development of a Sampling and Analytical Method for Arsine in Air (unpublished work, 1976). J. W. Robinson, R. Garcia, G. Hindman, and P. Slevin, Anal. Chim. Acta, 69, 203 (1974). P. R. Walsh, J. L. Fasching, and R. A. Duce, Anal. Chem.,48, 1012 (1976). A. W. Fitchett, E. Hunter Daughtrey, Jr., and Paul Mushak, Anal. Chim. Acta, 79, 93 (1975). J. W. Robinson, G. D. Hindman, and P. J. Slevin, Anal. Chim. Acta, 66, 165 (1973). P. R. Walsh, J. L. Fasching, and R. A. Duce, Anal. Chem.,48, 1014 (1976). R. Ediger, Atomic Absorption Application Study, No. 584, Perkin-Elmer Corporation, Norwalk, Conn. F. J. Fernandez, R. D. Ediger, and J. D. Kerber, Atomic Absorption Application Study, No. 588, Perkin-Elmer Corporation, Norwalk, Conn. R. Ediger, At. Absorp. News/., 14 (5),127 (1975). “Standard Methods for the Examination of Water and Wastewater”, 13th ed., 1971, American Public Health Association, 1015 Eighteenth Street, N.W., Washington, D.C. 20036.
RECEIVED for review May 11, 1977. Accepted April 3, 1978. Work supported by the National Institute for Occupational Safety and Health, Contract No. 210-76-0142 (T.O. No. 2).
