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Simultaneous Mass Bias and Fractionation Corrections Utilizing Isotopic Solid Standards and Laser Ablation ICPMS W. Clay Davis,*,† S. J. Christopher,† and Gregory C. Turk‡
Hollings Marine Laboratory, National Institute of Standards and Technology (NIST), 331 Fort Johnson Road, Charleston, South Carolina 29412, and Analytical Chemistry Division, National Institute of Standards and Technology (NIST), 100 Bureau Drive Stop 8392, Gaithersburg, Maryland 20899
Homogeneous incorporation of analytes of known isotopic abundance into sol-gel-derived standards that mimic important mineral systems has the potential to contribute significantly to providing solid standard benchmarks for a range of applications. This preliminary study reports on the synthesis of solid glass standards produced via the sol-gel method and their doping with Standard Reference Material (SRM) 981 Common Lead Isotopic Standard and SRM 982 Equal-Atom Lead Isotopic Standard. Custom isotopic materials were also prepared using mixtures of the two isotopic SRMs. Particles from these solid samples were then introduced into an inductively coupled plasma mass spectrometer via laser ablation to determine whether materials of suitable homogeneity could be developed as isotopic reference materials. Preliminary results for Pb isotope ratios show that these solid isotopic reference standards are capable of correcting for instrumental mass bias and laser ablation-induced bias due to fractionation simultaneously. Correction factors generated from the quotient of the certified and measured Pb isotopic ratios in sol-gel disks spiked with SRMs 981 and 982 were successfully applied to produce accurate isotope ratios using comparative control/unknown checks. These correction factors were also used to assign Pb isotopic ratios in NIST SRM 612 Trace Elements in Glass that were in excellent agreement with published measurements, suggesting that tunable matrix sol-gel disks can serve as adequate control matrixes for evaluation of isotope ratios in glass samples. Rapid advancement of analytical techniques capable of performing in situ elemental and isotopic analyses on the micrometer scale dictates that suitable matrix-matched, homogeneous certified reference materials be put forward for a range of analytical systems. Major limitations of employing sensitive, solid sampling techniques such as laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) for quantitative analysis and isotopic measurements include the following: (1) lack of availability of * To whom correspondence should be addressed. Phone: (843) 762-8995. Fax: (843) 762-8742. E-mail:
[email protected]. † Hollings Marine Laboratory, NIST. ‡ Analytical Chemistry Division, NIST. 10.1021/ac050872p Not subject to U.S. Copyright. Publ. 2005 Am. Chem. Soc.
Published on Web 09/07/2005
suitable standards for calibration and validation, (2) absence of a true analytical blank, (3) instrument fractionation of isotopes and poor control of elemental (including isotopic) fractionation during the sample ablation process, and (4) dynamic range and abundance sensitivity limitations in quadrupole mass spectrometry and sensitivity limitations in magnetic sector mass spectrometry when utilizing high-resolution modes of operation. It would be beneficial to produce solid sample standards in a manner more akin to solution sample preparation that allows for the production of analytical blanks and matrix-matched standards through gravimetric control of analytes and homogeneous incorporation of matrix particles into the solid support. By utilizing LA-ICPMS, the solid sample is ablated inside a simple chamber and the resultant aerosol of particles is transported by a carrier gas to the ICPMS. The composition of the ablated aerosol should ideally be representative of the solid sample to ensure accuracy, and the aerosol transport efficiency should be maximized in order to enhance sensitivity. There have been numerous reports indicating that recorded instrument signals often do not represent a bulk sample’s chemical or isotopic composition on the micro- or macroscale.1-8 This phenomenon is referred to as elemental fractionation and occurs due to a number of factors including preferential vaporization of elements (or isotopes) from the sample, failure of large particulates to be transported to the ICP, and incomplete vaporization of large particulates that manage to reach the ICP source.5 The combination of laser ablation with ICP, MS, or atomic emission spectrometry has been utilized for direct solid sample chemical analysis due to its many advantages including the ability to analyze any dry sample material, minimal sample preparation, high spatial resolution (µm), small sample quantities (µg or less), and minimal analyst exposure to toxic samples.1,4,5,9 Qualitative (1) Russo, R. E.; Mao, X. L.; Liu, C.; Gonzalez, J. J. Anal. At. Spectrom. 2004, 19, 1084-1089. (2) Kuhn, H. R.; Gunther, D. J. Anal. At. Spectrom. 2004, 19, 1158-1164. (3) Kuhn, H. R.; Gunther, D. Anal. Chem. 2003, 75, 747-753. (4) Hattendorf, B.; Latkoczy, C.; Gunther, D. Anal. Chem. 2003, 75, 341A347A. (5) Russo, R. E.; Mao, X. L.; Liu, H. C.; Gonzalez, J.; Mao, S. S. Talanta 2002, 57, 425-451. (6) Koch, J.; Feldmann, I.; Jakubowski, N.; Niemax, K. Spectrochim. Acta, Part B 2002, 57, 975-985. (7) Jackson, S. E.; Gunther, D. Geochim. Cosmochim. Acta 2002, 66, A359A359. (8) Guillong, M.; Gunther, D. J. Anal. At. Spectrom. 2002, 17, 831-837.
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and semiquantitative analyses have become routine; however, calibration remains a challenge for quantitative analysis. There is no universal method of calibration that can be applied to all solid sample types. Typical calibration strategies include the use of commercially available certified reference materials (CRMs) for certain types of solids matrixes (glass, ceramic, cement, metals), external matrix-matched samples,10,11 and simultaneous introduction of a solution reference standard with the ablated aerosol. 12-14 Calibration strategies employing isotopic reference standards have remained elusive because although high-purity isotopic reference materials are available, they cannot be readily incorporated into types of matrixes sought after for laser ablation work. Significant research efforts have focused on understanding and eliminating elemental fractionation. One strategy to overcome elemental fractionation involves the production of CRMs certified for a range of concentrations and isotopic compositions, in a variety of matrixes. There are practical metrological limitations to this type of approach, as the number of unique samples will always be larger in scope relative to available CRMs, which are typically only produced for sectors where the economic, environmental, or health variables justify production. Due to these realities, a technology-transfer strategy may alternatively be considered where readily synthesizable, tailorable, isotopic solid reference standards that are homogeneous and traceable to a CRM are prepared by end users. Thus, custom traceable standards could be used for instrument tuning and establishment of instrument-specific correction factors for elemental fractionation induced by either the laser ablation process or the mass spectrometer. One could envision that the created matrix-matched standards could be analyzed concurrently with analytical samples of interest under conditions optimized for sensitivity, and the resultant isotopic responses of the standards could be used to correct for elemental fractionation occurring at the laser ablation and ion focusing steps (mass bias) on a very specific instrument platform. Isotopic compositions in similar-matrix unknown materials could then be measured with a good degree of confidence. Similar strategies could be implemented to develop solid CRMs for determination of amount of substance. This would be of great benefit for the analytical community and would result in overcoming one of the major limiting factors (availability of matrix-matched, solid sample CRMs) in employing one of the most sensitive solid sampling techniques available for routine isotopic and quantitative analysis. Our preliminary investigations in this study test the feasibility of implementing these strategies through synthesis and characterization of solid sol-gel glasses doped with Pb from NIST isotopic Standard Reference Materials. It is yet to be determined whether NIST will pursue development of these and similar types of LA-ICPMS standards through large batch production of standard reference materials or rely upon and further develop novel modes of technology transfer to end users that maintain traceability to a primary NIST material. (9) Russo, R. E.; Mao, X. L.; Mao, S. S. Anal. Chem. 2002, 74, 70A-77A. (10) Borisov, O. V.; Mao, X. L.; Fernandez, A.; Caetano, M.; Russo, R. E. Spectrochim. Acta, Part B 1999, 54, 1351-1365. (11) Gunther, D.; Cousin, H.; Magyar, B.; Leopold, I. J. Anal. At. Spectrom. 1997, 12, 165-170. (12) Leach, J. J.; Allen, L. A.; Aeschliman, D. B.; Houk, R. S. Anal. Chem. 1999, 71, 440-445. (13) Mao, X. L.; Russo, R. E. J. Anal. At. Spectrom. 1997, 12, 177-182.
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One application of LA-ICPMS, which may be aided in the development of solid standards, which are certified for isotopic composition, is U/Pb geochronology, which is used as a principal dating tool in the earth sciences where ages are calculated by measuring 206Pb/238U, 207Pb/235U, and 207Pb/206Pb ratios.5,15 Some of these studies only show 207Pb/206Pb data and only a few include the 206Pb/238U ratio, because of fractionation.16-18 Without the 206Pb/238U ratio, the analysis cannot be extended to “young” (
