Anal. Chem. 1997, 69, 758-766
Determination of Environmental Lead Using Continuous-Flow Microwave Digestion Isotope Dilution Inductively Coupled Plasma Mass Spectrometry Ellyn S. Beary,*,† Paul J. Paulsen,† Lois B. Jassie,‡ and J. D. Fassett†
Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and CEM Corporation, 3100 Smith Farm Road, Matthews, North Carolina 28106
A commercial flow-through microwave system was successfully applied as an alternative sample preparation procedure for lead determinations using isotope dilution inductively coupled plasma quadrupole mass spectrometry. Sample is introduced as a slurry and then injected into a carrier stream which is continually flowing through the system. As configured, the sample dissolution is sequential, taking about 10 min/sample. This closed system is easy to use and produces low blanks, thus providing a viable alternative to the direct introduction of untreated samples, which can degrade analytical results. Leaves, air filters, urine, sludge, dust, and paint standard reference materials previously certified for lead using isotope dilution mass spectrometry (IDMS) were used to evaluate the accuracy of this automated sample preparation device. Lead concentrations in the nanograms to micrograms per gram range were accurately determined when compared with the certified value and previous IDMS results. The commercial success of inductively coupled plasma mass spectrometry (ICPMS) in analytical instrumentation has had a significant impact on chemical analyses worldwide.1-6 However, sample preparation can be a limiting factor for accurate analyses, since it is often labor intensive and has the potential to introduce errors from contamination, incomplete digestion, or losses of volatile analytes. The recent availability of automated, computercontrolled flow injection devices such as flow-through microwave systems7,8 and on-line matrix separations systems9 may significantly enhance the already powerful capabilities of ICPMS. Stable †
NIST. CEM Corp. (1) Evans, E. H; Gigilio, J. J. J. Anal. At. Spectrom. 1992, 8, 1-17. (2) Moens, L.; Vanhoe, H.; Vanhaeche, F.; Goossens, J.; Campbell, M.; Dams, R. J. Anal. At. Spectrom. 1994, 9, 187-191. (3) Kane, J. S.; Beary, E. S.; Murphy, K. E.; Paulsen, P. J. Analyst 1995, 120, 1505-1511. (4) Lam, J. W.; McLaren, J. W. J. Anal. At. Spectrom. 1990, 5, 419-424. (5) Wiederin, D. R.; Smith F. G.; Houk, R. S. Anal. Chem. 1991, 63, 219-225. (6) Jarvis, K. E.; Gray, A. L.; Houk, R. S. Handbook of Inductively Coupled Plasma Mass Spectrometry; Chapman and Hall: New York, 1992. (7) King, E. E.; Barclay, D.; Ferguson, D.; Jassie, L. B. Proceedings of the 10th Annual Waste Testing and Quality Assurance Symposium, Arlington, VA, July 11, 1994; No. 41, pp 294-298. (8) Moshiri, B.; Leikin, S. V.; Poling J. Eastern Analytical Symposium, Somerset, NJ, Nov 14, 1995; No. 167. (9) Wiederin, D.; Gjerde, D. T.; Smith, F. G. 1995 European Winter Conference, Cambridge, U.K., Jan 9, 1995; No. M9. ‡
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isotope tracer measurements of lead using isotope dilution (ID) ICPMS were used to assess the flow-through microwave process, with specific emphasis on the chemistry of slurry sampling of environmental solids. The parameters studied included the following: sample size; size and source of blank, and isotopic crosstalk; analyte recovery and on-line chemical leaching; acid/ sample ratio; sample loop size; flow rate; and residence time in the microwave coil. The accuracy and precision of a slurry sampling technique, central to the continuous-flow process, were evaluated by comparing lead measurements with previous lead certification activities conducted at NIST. Measuring toxic metals in the environment must be accomplished with precision and accuracy since action levels are legally defined. Lead is ubiquitous, and its toxic effects are well known.10,11 In the 1970s, U.S. Government legislation was introduced restricting lead in paint, water, gasoline, and solders used for welding tin cans, faucets, and water-bearing pipes. Over the past 20 years, the action level for blood lead in children was reduced from 60 µg/dL to a current level of 10 µg/dL,10,11 straining current routine analytical capabilities. This paper describes analytical results obtained from using a commercially available continuous-flow microwave digestion system as a sample dissolution procedure for ICPMS isotope dilution. Although these experiments were conducted off-line, the use of autosamplers with similar flow rates and sample volumes suggests the compatibility of these two instruments for on-line analyses. This continuous-flow microwave system utilizes slurry sampling and requires the use of internal standards for accurate analyses of powdered solid samples.7 An enriched isotope of the analyte serves as the internal standard in this work. Microwave decomposition of the sample eliminates instrumental fouling related to the introduction of undigested liquid or slurried biological materials. The results indicate that systematic errors due to sampling are minimal when using this microwave device. The continuousflow microwave system can provide laboratories with a costeffective alternative to direct sample introduction without decreasing sample throughput. The goals of good analytical accuracy and reliability with an uncertainty of less than 1% are achieved using the methods described here. Such an automated, highly (10) National Research Council Committee. Measuring Lead Exposure in Infants, Children and Other Sensitive Populations; National Academy Press: Washington, DC, 1993. (11) Roper, W. L. Preventing Lead Poisoning in Young childrensa Statement by the Centers for Disease Control; USHHS: Atlanta, GA, 1991; Chapters 2 and 3. S0003-2700(96)00692-0 CCC: $14.00
© 1997 American Chemical Society
Table 1. Sample Type and Analytical Sample Sizes analytical sample size effective material
recommended
experimental
10 mL loop
SRM 1547, peach leaves SRM 2676d, toxic metals on air filters SRM 2670, toxic metals in urine SRM 2781, domestic sludge SRM 2583, household dust SRM 2582, powdered paint
150 mg 1 filter 10 g, reconstituted 250 mg 250 mg 100 mg
422-510 mg 1 filter (∼40 mg) 8.13-10.32 g 459-621 mg 515-620 mg 550 mg
84-102 mg partial filter (∼8 mg) 1.62-2.06 g 92-122 mg 103-124 mg 22 mg
reproducible process could make isotope dilution more accessible to laboratories now using plasma source mass spectrometry. EXPERIMENTAL SECTION Materials, Reagents, and Methods. All high-purity mineral acids used for digestion were prepared and analyzed at NIST.12 Sample preparation was performed in a Class 10-100 clean laboratory, a multipurpose laboratory designed primarily for trace transition metal determinations. Surfactant-free distilled, deionized water was used as the carrier stream for the flow-through microwave system. It was stored in bulk in a cleaned polyethylene carboy so that the Pb blank in the carrier stream could be easily monitored. Plastic 50-mL centrifuge tubes were cleaned by soaking in 3 mol/L HNO3, air-dried under Class 100 conditions, and stored in clean linear polyethylene bags. The mixed acid solution was prepared in bulk containing 0.15 mol/L HClO4, 1 mol/L HF, and 3 mol/L HNO3 in high-purity water. The enriched 206Pb isotopic spike (206Pb/Pb208 ) 3500) was obtained from Oak Ridge National Laboratory as PbO2. It was dissolved in HNO3 and then diluted and stored as a master solution containing ∼200 µg/g total Pb in 0.5 mol/L HNO3. The working spike solutions were calibrated using a “reverse” isotope dilution process which has been previously described.13 In addition, the isotopic composition of all natural lead was determined experimentally using a common lead isotopic standard (SRM 981), since its composition varies in nature due to the radiogenic decay of U and Th to 206Pb, 207Pb, and 208Pb. Sample Selection. SRM 2670 toxic metals in urine, SRM 2781 domestic sludge, SRM 2676d toxic metals on air filters, SRM 2583 household dust, SRM 2582 powdered paint, and SRM 1547 peach leaves were analyzed. The six materials represented typical environmental matrices and were selected on the basis of relevance to environmental lead testing programs. The minimum recommended sample size for each powdered SRM analyzed is listed in Table 1, and is based on homogeneity studies conducted during the certification process. IDMS Methods. IDMS was one of the certification methods for lead in all the SRMs selected, using either thermal ionization mass spectrometry (TIMS) or ICPMS. Each IDMS sample (including those in this study) was spiked with 206Pb prior to its respective dissolution procedure. Regardless of the mass spectrometric procedure, the 206Pb/208Pb ratio was measured, and concentrations were calculated using isotope dilution equations (12) Paulsen, P. J.; Beary, E. S.; Bushee, D. S.; Moody, J. R. Anal. Chem. 1988, 60, 971-975. (13) De Bievre, P. Anal. Proc. 1993, 30, 328-333.
2 mL loop
18-24 mg 21-25 mg
previously described.13-15 The amount of spike added for isotope dilution quantification was based on the usual IDMS considerations.14-16 These samples were spiked so that the altered 206Pb/208Pb ratio was approximately 2.5, resulting in an “error propagation factor” around 1.2.16,17 Wet acid digestion, typically using a HNO3 and HClO4 mixture, was used to decompose organic matrices, and HF was used to volatilize residual siliceous material as SiF4. This digestion also promotes analyte oxidation and isotope mixing/equilibration. (CAUTION: Hydrofluoric acid is particularly corrosive and a contact and inhalation hazard.18 In addition, perchlorates can form explosive mixtures in combination with carbonaceous material.19) ID ICPMS certification analyses, as well the work described here, were performed without chemical separations with no degradation of analytical results. Sample Preparation Instrumentation. Dissolution/decomposition of samples in these experiments was accomplished using a SpectroPrep (CEM Corp., Matthews, NC) on-line microwave digestion system based on a flow injection design of Haswell and Barclay.20,21 The computer-controlled commercial system has been modified to include a waveguide cavity for digestion and filters for removing particles. The microwave digestion coil is ∼30 m of narrow-bore (about 0.7 mm i.d.) reinforced Teflon tubing which is wound onto a bobbin, allowing simultaneous irradiation of nearly the entire sample slug. As configured for this work, the system was equipped with two autosamplers, one for uptake (sampling) and one for collection. A flow diagram of the digestion system, shown in Figure 1, illustrates the manner in which slurried samples are aspirated to fill an injection loop and then are incorporated into a carrier stream of high-purity water. The end of the sampling probe is fitted with a paddle, which stirs the solid sample and reagents to form a slurry. Time, speed, and position of the stirrer are computer controlled. For these experiments, the sample was stirred for 25 s, and the probe position, which was predetermined for optimum slurry sampling, was based on the manufacturers recommendation.7 (14) Heumann, K. G. Int. J. Mass Spectrom. Ion Processes 1992, 118/119, 575591. (15) Heumann, K. G. Int. J. Mass Spectrom. Ion. Phys. 1982, 45, 87-110. (16) Fassett, J. D.; Paulsen, P. J. Anal. Chem. 1989, 61, 643A-649A. (17) Adriaens, A. G.; Kelly, W. K.; Adams, F. C. Anal. Chem. 1993, 65, 660663. (18) Sax, N. I. Dangerous Properties of Industrial Materials; Van Nostrand Reinhold Co.: New York, 1979. (19) Schilt, A. A. Perchloric Acid and Perchlorates, The G. Frederick Smith Chemical Co.: Columbus, OH, 1979. (20) Haswell, S.; Barclay D. Analyst 1992, 117, 117-120. (21) Williams, K. E.; Haswell, S. J.; Preston, G. Analyst 1993, 118, 245-248.
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Figure 1. Schematic diagram of continuous-flow microwave device.
The system is equipped with two dedicated peristaltic pumps. One pulls the sample into the sample loop and washes the sample loop after the sample is injected into the microwave coil. The second pump supplies fresh, ultrapure water to the wash stations located at each autosampler station. It is also plumbed to clean the diamond dust filters, which are designed to remove any residual particles that are greater than 75 µm immediately prior to sample collection. The loss of pressure that occurs during the filtering and cooling process is moderated using predetermined lengths of narrow-bore Teflon tubing. The carrier stream of high-purity water is continually pumped through the instrument using a standard single-stroke, highpressure liquid chromatography pump (from Scientific Systems, Inc.). The pump speed (flow rate) controls the sample residence time in the microwave coil and can be altered. Pump speed was set at 60%, resulting in a 10-11-min residence time in the device and about 2.5 min in the microwave digestion coil. The presence of sample is signaled using in-line sensors (located after the high-pressure pump), which detect changes in conductivity between the water and sample stream and activate valves which direct the sample appropriately through the system. 760
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A thermocouple measures the temperature as the sample exits the microwave coil. For these experiments, the temperature ranged from 100 to 150 °C, with organic matrix samples having the higher associated temperature. An in-line interactive (feedback) pressure sensor can temporarily shut off the microwave power and the high-pressure pump if pressure spikes are detected. A threshold pressure of
