44
Anal. Chem. 1980, 52, 44-49
Determination of Nickel, Copper, Selenium, Cadmium, Thallium, and Lead in Coal Gasification Products by Isotope Dilution Spark Source Mass Spectrometry D. W. Koppenaal,”’ R. G. Lett, and F. R. Brown Pittsburgh Energy Technology Center,
U.S.Department of
Energy, 4800 Forbes A venue, Pittsburgh, Pennsylvania 752 73
S. E. Manahan Department of Chemistry, University of Missouri-
Columbia, Columbia, Missouri 6520 1
A method for the simultaneous preparation and determination of Ni, Cu, Se, Cd, TI, and Pb in coal and coal gasification products is described. The procedure is based on the acid pressure decomposition of samples, electrochemical preconcentration of analytes, and isotope dilution spark source mass spectrometric analysis. The method is particularly advantageous in that it minimizes sample manipulation during preparation, is relatively immune to contamination, and possesses multisample, multielement Capabilities. Analyses of Standard Reference Materials illustrate the procedure’s applicability. Precision, primarily affected by the heterogeneous nature of coal and coal-derived materials, is on the order of 7 % (relative). Multisample processing permits the analyses of up to 20 samples per week.
T h e efficient conversion of coal into environmentally acceptable, easily transportable liquid and gaseous fuels is currently a subject of intense scientific, economic, and political interest. Examples of proposed conversion technologies include coal gasification and direct and indirect coal liquefaction. These conversion processes will hopefully provide an important (albeit partial) solution to our increasing reliance upon foreign petroleum and diminishing supplies of domestic natural gas. T h e required development and evaluation of these processes encompasses a wide range of chemical, technical, and environmental considerations. The latter consideration is especially important when the diverse and complex nature of the raw fuel is taken into account. Coal is not only a major source of NO, and SO, pollutants, but is also a geological repository for many inorganic elements (1-3). T h e subsequent redistribution of these elements during the utilization of coal may have profound environmental ramifications. T o assess the distribution patterns of trace elements in coal conversion process streams, adequate methods of analysis need t o be developed or applied. Since coal and coal-derived materials present difficult and relatively unfamiliar matrix and solubilization problems, new sample preparation procedures may also be required. We have developed a convenient method for the preparation of coal and coal-derived materials for trace element analysis by isotope dilution spark source mass spectrometry (IDSSMS). The method involves sample decomposition under the combined action of pressure, temperature, and mineral acids using commercially available “acid bombs” ( 4 , 5 ) . Acid pressure decomposition is now a well documented alternative to conventional wet digestion procedures (6-15). Merits of Present address, Institute for Mining and Minerals Research. University of Kentucky, P.O. Box 13015, Lexington, Ky. 40583. 0003-2700/80/0352-0044$01. O O / O
this type of dissolution include complete retention of volatile elements, freedom from continuous supervision, and shorter decomposition times (6-8). T h e analytical technique of isotope dilution spark source mass spectrometry is precise, accurate, and theoretically applicable to all multi-isotopic elements. The general procedure involves the chemical alteration of the natural isotopic composition of the analyte element by spiking with a highly purified, enriched isotope. Following the spiking of the sample with the enriched isotope, the analyte element and spiked isotope must be intimately mixed to ensure chemical equilibrium. This is usually accomplished by solubilizing the sample and spike through conventional wet digestion procedures. T h e applications of ID-SSMS techniques to metallurgical (16-ZO), biological @ I ) , geological (22),and coal (21, 23) materials have been reported. General reviews of the ID-SSMS technique are available (22-24). The method described herein utilizes the novel combination of acid pressure decomposition with ID-SSMS analysis for the determination of several elements in coal and related materials. The method also utilizes, in part, an electrochemical preconcentration technique similar t o t h a t described by Paulsen and co-workers (17,21). Advantages of the overall procedure include minimal sample manipulation, relative freedom from contamination, and multisample, multielement capabilities. T h e method is demonstrated for the determination of Ni, Cu, Se, Cd, T1, and P b in various process streams of a prototype coal gasification unit.
EXPERIMENTAL Sample Collection. Samples were obtained during coal gasification runs using the 40 lb/h SYNTHANE Gasifier Process Development Unit at the Pittsburgh Energy Technology Center of the United States Department of Energy. A detailed description of this fluidized-bed gasification unit can be found in the literature (25-27). Samples collected included the feed coal, gasification char, filtered particulate matter, and condensate water. Feed coal samples (6-8 kg) were thieved as the coal was loaded into the gasification hopper. The gasifier char (partially reacted coal that had f d e n through the gasifier) was collected in its entirety (16-18 kg) after termination of the run. Particulate matter (fines that had been filtered from the product gas stream as it left the gasification reactor) was also collected in its entirety (-0.5 kg). Considerable attention was given to the comminution of these bulk samples to analytical size (100-200 g) samples. Further details concerning the collection, comminution, and preparation procedures are available (28, 29). Apparatus. An Associated Electrical Industries (now Kratos, Inc.) MS 702R Spark Source Mass Spectrometer was utilized for determining the altered isotopic ratios. Ilford Q2 photoplates were used throughout this investigation, and were examined using a Grant Industries Microdensitometer. Mass spectra were recorded with a Bell & Howell Ultraviolet Recorder. An LFE Corporation Model 504 Low Temperature Asher was employed when ashing ‘C 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
Vent
~
\ il
~t u b l n fg /
G r a p h i t e plug'
11 l
connection
4
~ ,Electrode ~
reloinel assembly
Figure 1. Schematic diagram of electrodeposition assembly
samples. Parr 4745 General Purpose Acid Digestion Bombs were utilized for the dissolution and equilibration procedures. Several spare sets of the Teflon digestion vessels (Parr zA236AC) were also kept available. Microliter Pipettes (Finnpipette, Markson Science, Inc.) were used for the addition of isotope spikes. A Cole-Parmer Magne-4 Magnetic Stirrer, capable of simultaneously stirring up to four different samples, was used for sample agitation during electrodeposition. A Keithley Instruments Model 225 Constant Current Source was employed for the electrolyses. High purity electrodes were fabricated from National Bureau of Standards SRM No. 685-W (High Purity Gold Wire). The sixnines grade wire was cut into pieces approximately 1 cm long. Reagents, Ultrex grade (J. T. Baker Chemical Co.) nitric, hydrochloric, and hydrofluoric acids were utilized. Reagent grade ammonium hydroxide was found to be sufficient for the isopiestic neutralization. Enriched stable isotopes were obtained from Oak Ridge National Laboratory. Enriched isotopes (typically >96 7'~ enriched) used during the investigation include 62Ni,"Cu, %e, l13Cd, 203T1,and 206Pb.The isotopic purity and composition were taken as supplied by Oak Ridge National Laboratory. Electrodeposition Assembly. The electrodeposition assembly is shown schematically in Figure 1. The assembly consists of two basic parts, one being the Teflon digestion cup of the acid bomb apparatus, the other being a specially constructed electrodeposition cap which holds both electrodes and provides for a contamination free atmosphere over the sample. The cap itself is machined from translucent Lucite so that it contacts the digestion cup in the same close fitting manner as the usual Teflon digestion lid. The cathode consists of a 0.25-in. (0.64-cm) spectrographic quality graphite rod which extends to within a few millimeters of the solution surface. The rod is vertically adjustable so that different volumes of sample solution can be accommodated. The 1-cm long gold cathode (which extends into the solution) is fitted into a 1.4-mm hole that is drilled vertically into the unill fit snugly, derside of the graphite support rod. New electrodes w but for previously used electrodes a retainer assembly is necessary to ensure electrical contact. The retainer assembly consists of a Teflon plug and a retaining ring. The Teflon plug fits into a predrilled hole which intersects the gold cathode at a 90" angle. The retainer ring, a section of 0.25-inch (0.64-cm) i.d. Tygon tubing, then slips over the plug, holding the gold electrode in place. The anode consists of a section of heat shrink Teflon tubing, the end of which has a high-purity graphite plug sealed into it. During electrodeposition, the anode compartment is filled with dilute nitric acid. Electrical contact is established by inserting a length of blackened platinum wire into the dilute acid electrolyte. The anode is also vertically adjustable. Optional vent holes may be drilled through the cap to facilitate purging of the electrodeposition cell. Procedure . Prior to analyses, the Teflon digestion vessels are cleaned by heating in concentrated nitric acid at 80 "C for several hours and rinsed with deionized, distilled water of at least
45
17 MR quality. The high purity gold cathodes are cleaned by etching with hot ultrapure aqua regia. This procedure removes surface contamination from previous samples and exposes fresh, clean surfaces for the next deposition. The cathodes are therefore reusable until the etching-sparking cycle reduces the diameter of the electrode to less than 0.5 mm. Isotope spikes are prepared by weighing (to *0.001 mg) appropriate quantities of the enriched isotope element or compound. The spike compounds or elements are then quantitatively dissolved using an appropriate mineral acid and brought to volume. Weighed quantities and dilution volumes are adjusted to yield spike stock solutions of approximately 100 ppm. After dilution to volume, the solutions are transferred to polypropylene containers and stored under refrigeration to minimize concentration changes due to adsorption and/or evaporation. Spike solutions prepared and stored in this manner were found to be stable for several months. Isotope spike concentrations were verified through analysis of Standard Reference Materials and other samples previously analyzed by independent methods. The more accurate calibration technique of "reverse spiking", recommended by Paulsen and co-workers (27, 28) can also be used. Approximately 100-mg samples are transferred to the Teflon cup. Whole coal, char, and filtered particulate matter samples can be decomposed, but to remove especially resistant organic constituents and to further concentrate analyte elements, lowtemperature ashing of these samples is recommended. When low-temperature ashing (LTA) of samples is performed, O2flow rates of 100 cm3/min and R F power settings of 25 W/chamber are recommended. Estimated maximum ashing temperatures are < 150 "C under these conditions (15). Condensate water samples (-10 mL) are transferred to the bomb, acidified with 1 mL of concentrated nitric acid, and evaporated to near dryness under an infrared lamp. Nitric, hydrochloric, and hydrofluoric acids are then added (1.5, 1.5, and 0.5 mL, respectively). Isotope spikes are added in amounts calculated so that an approximate 1:l ratio of spike isotope to major isotope will result. This procedure minimizes photoplate calibration errors (17, 28). The determination of the amount of spike to add depends on a prior knowledge of the approximate concentration of the element in the sample. In most cases, these samples had previously been analyzed by conventional SSMS analysis, so approximate concentration data were available. Spikes are added by pipetting microliter volumes of the stock solutions directly into the Teflon cup (which contains the sample and acids). Although spike volumes are generally small ( 5 2 0 0 pL), the pipets used were calibrated and found to deliver volumes accurate to within 1-2% with a precision of better than 1%. The Teflon digestion vessel is then covered with the Teflon bomb lid and sealed inside the stainless steel jacket. The bomb assembly is placed in a conventional laboratory oven set at 11&150 "C for several hours (alternatively, they can be left in the oven overnight). After heating, the bomb is removed from the oven and air or water cooled. The bomb is disassembled and the Teflon digestion vessel opened. The inside of the vessel is rinsed down with conductivity grade water. Since the high acidity inhibits the electrodeposition of some analyte elements, partial neutralization is necessary. This is readily accomplished using the isopiestic distillation of ammonia vapor (30,32)into the highly acidic sample digestate. Approximately 150-200 mL of concentrated (58%) ",OH is placed in the bottom of an empty glass desiccator. The digested sample (in the uncovered Teflon digestion vessel) is then placed on the desiccator shelf, the desiccator lid is replaced, and contamination-free neutralization occurs. Neutralization times using this procedure are typically 30-60 min. The optimum deposition pH was determined to be 4.0-4.5. If neutralized solutions were of higher pH, ultrapure nitric acid was used to readjust the pH to the optimum range The electrode caps (with clean gold cathodes inserted) are then placed on the digestion vessel. A micro stir-bar is placed in the vessel and the entire assembly transferred to a magnetic stirrer. Electrical contact to the current source is made using 18-gauge platinum coated patch cord wire. The cathode lead connects to the graphite cathode holder by insertion of the wire tip into a slightly undersize hole drilled in the graphite rod. The anode connection is made by contact with the platinum anode. The current source is then set to a current of 1-2 m.4 (voltage com-
46
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
Figure 3. Scanning electron photomicrographs of electrode surface: (a) prior to electrodeposition, (b) after electrodeposition
Table I. Electrodeposition Efficiencies
Flgure 2. Photograph of multisample electrodeposition arrangement
pliance of 5-10 V). If desired, a VOM can be used to establish and monitor the current flow through the system. Since constant current electrolysis is used, it is a trivial modification t o electrodeposit several samples simultaneously. This is especially convenient provided that several acid digestion bombs are available. Multiple digestion vessels fitted with electrodeposition caps are simply connected in series (anode to cathode). Such an arrangement, using four acid bombs and a multiple unit magnetic stirrer, is illustrated in Figure 2. After electrodeposition (3-4 hour deposition times are sufficient), the gold cathodes are rinsed with high purity water (with cell potential still applied). The cathode is then carefully placed in a covered electrode stand. Caution should be observed so as to not abrade the electrodeposited material. The cathode is mounted in the mass spectrograph along with a clean gold counter electrode. The sample compartment of the instrument is then evacuated t o a pressure of 1 X lo4 Torr. The electrodes are sparked using pulse repetition rates and pulse lengths of 10-300 cycles/s and 25-200 s, respectively. Photoplate exposures are taken from 1 x to 1 X lo2 nC, attempting t o spark as much of the electrodeposited material as possible. After analysis, the photoplate is developed using the procedure recommended by Franzen, Maurer, and Schuy (32). Photoplate calibration and line intensity quantitation is performed using the Hull equation (33, 34).
RESULTS AND DISCUSSION Decomposition. Most samples were easily decomposed with the described procedure. If dissolution did appear incomplete, additional acids were added and the heating process was repeated. Occasionally, some undissolved material was still present after digestion. This material was accumulated and a conventional semiquantitative spark source analysis was performed on this material. The analysis showed the major constituents to be Ca, Ba, and F, with minor amounts of Mg, Si, and Al. I t was thus presumed that this material was residual siliceous matter and insoluble fluorides. This has also been observed previously (35). Complete extraction of the analyte elements was achieved, however. Electrodeposition. Successful electrodeposition was evidenced by a visible gray-black deposit on the gold cathode. Scanning electron microscopy was utilized to observe the microstructural characteristics of the deposit. Photomicrographs of the electrode surface before and after deposition are shown in Figure 3. The photomicrographs shown the electrodeposit to be concentrated on the upraised areas of the electrode surface. The deposit appears to be in a reasonably adherent form, as also evidenced by frequent manipulation of the electrodes with little apparent loss of the deposit. Microscopic observation of acid etched, cleaned electrodes showed complete removal of residual electrodeposited material.
deposition deposition element efficiency, % element efficiency, 7% Ni 91 Cd 89 cu 76 TI 73 Se 67 Pb 99 Although quantitative electrodeposition of analyte elements is not necessary when performing the analysis by isotope dilution, the extent of deposition is of interest since the sensitivity of the measurement will increase with increasing deposition efficiency. For this reason, the electrodeposition efficiency was studied as follows. A sample of known composition was decomposed and electrodeposited using the basic procedure described above with the exception that the isotope spikes were not added. After this decomposition and electrodeposition, the distribution of each analyte element can be described by the following relation:
where XT is the initial amount of analyte in solution (known from the sample composition and weight), XDis the amount of analyte deposited on the electrode, and Xs is the amount of analyte left in solution after deposition. The deposition efficiency, E, can then be calculated using the following equation:
E , = XD/XT
X
100
The parameter XD can be easily determined using the IDSSMS procedure. The nonspiked electrodeposit obtained as just described is redissolved in 1 mL of warm aqua regia. Isotope spikes are then added to this solution assuming that 100% deposition of the analyte elements from the sample had occurred. This solution is then heated and stirred to bring about equilibration, and is electrochemically redeposited on another clean electrode. This electrode is then sparked and analyzed by the usual ID-SSMS procedure. The values of XD are then used with Equation 2 to calculate electrodeposition efficiencies. Table I illustrates data for a typical determination of deposition efficiencies. Near-quantitative deposition occurs for most elements. It should be emphasized, however, that the individual deposition efficiencies for various samples will probably differ markedly, as factors such as pH, matrix constituents, stirring rate, electrode area, solution volume, current density, and deposition time all affect the completeness of deposition (36). Another factor that could affect the deposition efficiency results is the rinsing of the deposit. One investigation reports that as much as 20% of submicrogram quantities of trace elements deposited on electrodes can be lost through dissolution during electrode rinsing (37). If losses of this nature occurred during the rinsing of the unspiked deposit (i.e., that resulting from first deposition), E, values obtained from the spiked deposit (Le., the second de-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
47
Table 11. ID-SSMS Results on Gasifier Samples run number 294
293 sample
element
feed coal
Ni
cu Se Cd TI Pb gasifier char
Ni
cu
Se Cd TI Pb
Ni cu Se Cd TI Pb
filter fines
-
I
rsd
2.83 7.50 0.56 0.12 0.05 5.14
0.09 0.04 0.03 0.09
0.02 0.07
11.6 25.5
0.10 0.02 0.10
0.14 0.10 0.11
0.14 0.06 0.05
21.2 10.3 105 0.65 2.16
0.05 0.08
0.05 0.06 0.03 0.05
0.09
13.7
x=
a average of four determinations, ppm by weight. minations.
2.56 7.52 0.54 0.12 0.04 4.80
0.11
Y .06
0.04 0.06
0.03 0.07
2.39 7.24 0.54 0.14 0.04 4.77
0.10 0.03 0.03 0.15 0.12 0.07
11.6 27.8 0.19 0.13 0.15 19.3
0.14 0.12 0.07 0.10
0.13 0.06 0.06 0.06
1o.oc
0.07
131 0.74 1.81
0.04 0.08
0.10
17.1
11.5 150 0.94 2.88 0.11 13.5
0.09 0.05
cu Se Cd TI Pb
15.5 16.8 2.51 0.23 0.61 27.9
0.07 0.06 0.05 0.09
0.06 0.09
15+1 18i 2 2.9 i 0.3 0.19 t 0.03 0.59 i: .03 30 ir 9
All results in ppm by weight, results for four determinations. only.
posit) would be erroneously low. Calculations. All concentrations were computed using the equation formulated by Paulsen and co-workers ( 1 4 2 1 ) . A computer program was written to assist in the computations. The following isotope ratios were usually measured: Ni 62/58, Cu 65/63, Se 78/80, Cd 113/114, TI 2031205, and P b 206/208, where the first listed isotope denotes the enriched isotope spike. Occasionally, other ratios were also measured (Le., Ni 62/60, Cd 113/112). When possible, both singly-charged and multiply-charged (2+, 3+) isotope ratios were used in the calculations. T h e use of multiply-charged isotope ratios can be particularly valuable since molecular interferences a t nonintegral m / e ratios (Le., %Pb3+ = 68.66) are minimal (21). Results. Three gasification runs were made to provide a reliable set of samples. The results for determinations of six elements in the three solid process streams (feed coal, gasifier char, and filtered particular matter) are presented in Table 11. Quadruplicate analyses were performed. Precision (relative standard deviation) of the ID-SMS measurements ranged from 0.02 to 0.15 with an average precision of 0.07. Evaluation of the individual variances of each procedural step (e.g., weighing, pipetting, photoplate measurements, etc.) and application of the additivity of variances leads to an overall variance value of -5%. Variances in excess of this value were assumed to be indicative of sample inhomogeneity. Elements lowest in concentration (Cd, T1) exhibited somewhat poorer precision. Blank contributions were negligible (
