Emission Spectrometric Determination of Arsenic F. E. Lichte and R. K. Skogerboe Department of Chemistry, Colorado State University, Fort Collins, Colo. 80521 AN EXAMINATION OF THE LITERATURE indicates a paucity of analytical methods capable of determining minute amounts of arsenic. The detection capabilities of the preferred colorimetric methods ( I , 2) closely approximate those reported (3-5) for the more widely used atomic absorption methods at approximately 0.1 pg/ml. Holak (6), however, extended the atomic absorption capability through the use of an arsine generator; collection of the arsine in a cold trap; and sweeping the arsine into a flame to obtain a detection limit of 0.04 pg. Subsequently, at least one instrument manufacturer has adapted this separation-preconcentration approach to permit the direct introduction of the arsine into an absorption flame, In this adaptation, detection limits at the parts per billion level are reported. While these developments have extended the general analytical capabilities for arsenic analyses, a need still exists to develop other means for determining trace to ultratrace amounts of arsenic, particularly where the sample size available is limited (7). Previous reports from this laboratory have indicated the general potential of microwave-induced plasma excitation for the determination of minute amounts of several elements (8-11). These reports have also indicated that introduction of large amounts of material (greater than gram/minute) into the plasma results in the development of instability and/or inability to maintain the plasma. Consequently, means for introduction of samples in the gas phase while, at the same time, separating the analytical species from the sample matrix are particularly useful for analyses based on microwave excitation (11). The present report deals with one such approach which has been developed based on the generator system mentioned above (6). In brief, an arsine generator has been coupled directly to the excitation plasma, and the generator system has been optimized to permit the rapid, quantitative removal of arsenic from an aqueous sample. The system offers unusually high sensitivity; the detection capability, which is primarily limited by the reagent blank, is approximately five nanograms. Thus, less than 5 parts per billion of arsenic can be determined when a 1-ml sample is used. The sensitivity that can be obtained, coupled with the relative simplicity of the instrumentation required, suggests that this method should find widespread application in problems requiring the determination of trace amounts of arsenic. (1) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” Interscience, New York, N.Y., 1950, Chap. VII. (2) Environmental Protection Agency, Methods for Chemical Analysis of Water and Waste, U.S. Dept. of the Interior, Nov. (1969). (3) H. L. Kahn and J. E. Schallis, At. Absorption Newslett., 1, 5 (1968). (4) 0. Menis and T. C. Rains, ANAL.CHEM., 41, 952 (1969). (5) A. Ando, M. Suzuki, K. Fuwa, and B. L. Vallee, ibid., p 1974. (6) W. Holak, ibid., p 1712. (7) Conservation News, 36 (€9,3 (1971). (8) J. H. Runnels and J. H. Gibson, ANAL.CHEM., 39, 1398, (1967). (9) H. E. Taylor, J. H. Gibson, and R. K. Skogerboe, ihid., 42, 876 (1970). (10) Ibid., p 1569. (11) F. E. Lichte and R. K. Skogerboe, ANAL. CHEM.,44, 1321
(1972). 1480
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
/WLE
INJECTION
PORT (G.C. SEPTUM)
Figure 1. Diagram of the arsine generator EXPERIMENTAL
Instrumentation. The basic facilities used in this investigation have been previously described (11). Briefly, a 2450 MHz microwave generator (Scintillonics, Model HV-15) was used with an Evenson quarter wave cavity (12) to induce a plasma in argon at atmospheric pressure. The plasma was contained in a 1-mm i.d. quartz tube. Wavelength dispersion was with a half meter monochrometer (Jaco, Model 8200) equipped with an S-19 response photomultiplier and the wavelength scanning device described by Snellman et al. (13). The phase sensitive lock-in amplifier (Ithaco Model 353B) was coupled to a strip chart recorder (HP 7101B). The arsine generator system is described in Figure 1. Reagents. All reagents used were A. R. grade or better. Granular (20 mesh) zinc was used in the arsine generator. Procedure. SAMPLEAND STANDARD PREPARATION. All samples and standards were made 1.5N in HzS04or HCl and the arsenic reduced to the As(II1) state with stannous chloride. The stannous chloride in the final solution was 4 wjv. Nonaqueous samples ranging in size from 0.05 to 0.5 gram dry weight were digested in concentrated H N 0 3 with a few drops of HzOzfollowed by evaporation to near dryness. The solutions were transferred to volumetric flasks using a few drops of 1 M NaOH followed by several rinses with 1 : 1 HC1. The stannous chloride was added at this point and the samples were diluted to volumns of 10 to 25 ml with 1.5M HCl. Water samples were treated such that the final concentrations of hydrogen ion and stannous chloride were identical with the standard solutions. ANALYSIS.As indicated below, the argon flow rate was adjusted to 600 ccjmin and the plasma was operated at a microwave power input of 100 watts. The system was preconditioned for analysis by injection of three successive 0.5-ml (12) F. C. Fehsenfeld K. M. Evenson and H. P. Broida Rev. Sci. Instrum., 36, 294 (1965). The cavity referred to is No. 5 in this
reference.
(13) W. Snellman, T. C. Rains, K. W. Yee, H. D. Cook, and 0. Menis, ANAL.CHEM.,42, 394 (1970).
Table I. Comparison of Microwave Emission and Colorimetric Analysis Results Parts per million As Microwave Colorimetry Sample description emission (1) 1.32 1.34 Water A B 0.22 0.22 C ... 0.15 D E F Blood A (50 ppm added) Blood B
Hair Plant leaves
0.14 0.09 0.02 52
0.22 0.23 9.5
As, Pg/ml ( I ml sample) 0.02 0.03 0.04
0
0.01
0
IO
0.05
...
0.11
... 62
... ... 9.5
aliquots of 1N HC1 through the sample entrance port (Figure 1). This reduces the blank to a constant level and must be repeated each time the zinc or drying agent is replaced. Magnesium perchlorate is placed between the plasma and the generator to remove excess water vapor. Aliquots of the samples and standards, varying in size from 0.1 to 2 ml depending on the arsenic content, are subsequently injected for calibration and analysis. The system was flushed by injection of 0.5 ml of 1M HCI after each sample. EGission intensity measurements were made using the As 2350 A line. For the system described herein, it was necessary to replace the zinc and the drying agent on a regular basis-Le., after approximately 50 sample injections. Comparative analysis results were obtained using the heteropoly molybdenum blue colorimetric procedure outlined by Sandell ( I ) . Sample aliquots of 5 to 10 ml, prepared as described above, were used for duplicate analyses via this procedure.
20
40
30
NANOGRAMS
50
As
Figure 2. Example analytical curve
RESULTS AND DISCUSSION
Although a minimum time of 30 minutes is frequently recommended as necessary for the complete conversion of arsenic to arsine ( I , 6), the system described herein is designed to produce complete conversion and removal to the excitation plasma in a nominal time of 30 seconds depending on the arsenic concentration. As a consequence, 20-30 samples per hour can be analyzed once the dissolution step is complete. In addition to preservation of the arsenic containing solution during storage, a primary purpose of the acid is to serve as a hydrogen source for the arsine evolution reaction. While hydrogen ion concentrations as low as 0.5Mare usually experimentally adequate for complete arsenic removal, the 1.5M level was chosen to guarantee completeness of reaction. Examination of the effects of microwave power on the recorded intensity indicated a linear increase in the signal with increasing power up to 100 watts and levelling off thereafter. Consequently, 100 watts was chosen as the operating level. The argon flow rate affects the arsine delivery rate to the plasma and, hence, the intensity of the recorded signal (11). An optimum level of 600 cc/min was selected on the basis of maximum analytical signal per microgram arsenic. Figure 2 presents an example of an analytical curve which indicates the sensitivity of the technique and illustrates a problem. The peak plotted at the zero concentration position is characteristic of the reagent blank--i.e., approximately 2.5 nanograms of arsenic for this case. The acids used are a primary source of the blank since our analyses indicate concentrated HC1, for example, to contain 10 to 50 ppb As, varying from batch to batch. Distilling in quartz generally lowers this by a factor of two. The zinc and reducing agents used
0.063 y g h l : s , , = 2 . 3 1
0.125 j g / m l : s,.,=2.9%
Figure 3. Replicate analyses on two samples (0.1 ml-samples) are secondary sources of blank. As a consequence, the data presented in Figure 2 and elsewhere in this report were accumulated at reduced amplifier sensitivity because of the magnitude of the blank which effectively limits the ultimate sensitivity (detection capability) of the method. For the conditions and reagents used herein, the blank limited determination limit is estimated to be five nanograms of As. The reproducibility of the blank measurement as estimated by the relative standard deviation is nominally i10% or better, making the five nanogram limit fairly conservative. The availability of reagents of higher purity would allow this to be reduced. Similarly, the use of larger samples to increase the absolute amount of arsenic introduced would permit determinations at lower relative concentrations. An indication of the measurement precision of the technique can be obtained from Figure 3 where successive analyses were carried out on two different samples. Typically, relative standard deviation values of better than i5 % are observed for replicate analysis when the signal-to-noise ratio is 10 to 1or greater. The potential applicability and accuracy of the technique can be inferred from the data in Table I which presents comparative analysis results. For the samples analyzed via the two methods, the results agree within experimental error suggesting that the microwave system provides reliable results. ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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It should be noted that considerably larger sample sizes were required for the colorimetric analysis than were necessary for comparable emission measurements. For example, duplicate emission analysis of water sample E required 0.2 ml to obtain a measurement 14 times the blank, whereas the colorimetric procedure required 10 ml to obtain a measurement 8 times the blank. The water sample data included in Table I offer a point of interest. Sample A is from the runoff of Morning Glory Pool (a high arsenic source) in Yellowstone National Park while the remaining samples were taken at succeeding points down the drainage system from the same pool. Consequently, the
decreasing As concentrations reflect dilution and/or removal effects in the drainage field. In view of the results summarized herein, one may conclude that the microwave emission technique offers a convenient, sensitive, and reliable method for the determination of arsenic in a wide variety of sample types. Because the method is based on the separation of arsenic as arsine, it may be generally anticipated that the method will not be subject to deleterious chemical or physical interference problems. RECEIVED for review January 3, 1972. Accepted March 7, 1972. Research supported by NSF Grant No. GP-21306.
Determination of MetalIic-Element Impurities in Uranium Hexafluoride by Spark Source Mass Spectrometry Olin H. Howard Oak Ridge Gaseous Diffusion Plant, Union Carbide Corp., Nuclear Dioision, Oak Ridge, Tenn. 37830
URANIUM HEXAFLUORIDE (UFs) delivered to the Atomic Energy Commission (A.E.C.) for enrichment in zr5U must be of a specified purity (I). Specified maximum concentrations of some metallic-element impurities are shown in Table I. These elements are usually determined by several different optical spectrometric procedures (2), some of which necessitate preliminary separation of the elements from the U. The capability of spark source mass spectrometry (SSMS) for determining many elements in a single analysis, with high sensitivity and without separation from the matrix, prompted an investigation of its suitability for determining impurities in UFs. A procedure was developed by which nearly all of approximately 35 elements of interest can be determined to 1 ppm or less, with an average relative standard deviation of =t33 %, and an analysis time of 5 hours. Several papers on the determination of elements in nonconductive powders by SSMS have been written by other authors (3-7). Usually the sample is mixed with a conductive powder, e.g., graphite or silver, and pressed into solid form for sparking in the mass spectrometer. The procedure described herein is similar since the UF6 is converted to uranium oxide (U3Os) and compressed with silver powder for the analysis. EXPERIMENTAL
Procedure. The UF6 is sampled as a liquid (2),hydrolyzed, and diluted to a known uranium concentration of approximately 0.1 gram per gram of solution. To a weight of the (1) “Uranium Hexafluoride: Base Charges, Use Charges, Special Charges, Table of Enriching Services, Specifications, and Packaging,” Fed. Register, No. 32, 16289 (1967). (2) J. C. Barton, C. W. Weber, L. A. Smith, and W. D. Hedge, At. Energy Camm. Rept. K-L-6140,Oak Ridge, Tenn., April 1967. (3) G. D. Nicholls, A. L. Graham, Elizabeth Williams, and Margaret Wood, ANAL.CHEM.,39,584 (1967). (4) R. M. Jones, W. F. Kuhn, and Charles Varsel, ibid., 40, 10 (1968). (5) Riley D. Carver and Paul G. Johnson, Appl. Specrrosc., 22, 431 (1968). (6) G. H. Morrison and A. T. Kashuba, ANAL.CHEM.,41, 1842 (1969). (7) R. Brown and P. G. T. Vossen, ibid., 42,1820 (1970). 1482
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
Table I. A.E.C. Specifications for Metallic-Element Impurities in UF6 Elements Ti Nb Ru Sb Ta Elements forming nonvolatile fluorides V Mo W 233U
Maximum concn, ppm 1 1 1 1 1
300(Total) 200 200 200 500
Basis U U U U U
U
solution containing 0.5 gram of uranium is added 5 pg of yttrium (10 ppm, U basis) as internal standard. (Yttrium was chosen because it was not expected to be in the UF6, was readily available, and did not interfere with the analysis.) Sulfuric acid, 0.25 ml, is added. The solution is evaporated, and the residue, U02S04,is ignited at 850 “C for 10 min to U308. (The HzS04enables removal of the fluorine by evaporation of HF, preventing loss of volatile metal fluorides.) The U308is mixed with 0.5 gram of high-purity (Cominco American) silver powder (weight ratio AgiU = 1). Two electrodes, 1/~6-in.diameter X in. long, are pressed with a molding die using a polyethylene mold (AEI Scientific Apparatus). Preliminary pressing of partial electrodes cleans the mold, particularly of fragments of polyethylene which contribute hydrocarbon background to the spectrum. A thin strip of tetrafluoroethylene placed between the sample and die acts as a lubricant, preventing distortion or breakage of electrodes. The electrodes are mounted in the mass spectrometer (AEI MS702) with Norbeck type, silver electrode holders (Vacumetrics, Inc.) This spring-clamp holder is preferred over the standard screw-clamp, tantalum holder for this application because it is more convenient, causes less electrode breakage, and is not a potential source of tantalum contamination. TO
