Anal. Chem. 2002, 74, 1165-1170
Technical Notes
Development of Microwave-Assisted Drying Methods for Sample Preparation for Dried Spot Micro-X-ray Fluorescence Analysis Dirk D. Link,*,† H. M. “Skip” Kingston,‡ George J. Havrilla,§ and Lisa P. Colletti§
Department of Energy, National Energy Technology Laboratory, MS 94-215, P.O. Box 10940, Pittsburgh, Pennsylvania 15236-0940, Department of Chemistry and Biochemistry, 308 Mellon Hall, Duquesne University, Pittsburgh, Pennsylvania 15282-1503, and Los Alamos National Laboratory, MS G740, Los Alamos, New Mexico, 87545
Although dried spot micro X-ray fluorescence (MXRF) is an effective analytical technique for trace elemental analysis, the sample preparation procedures currently used suffer from a number of drawbacks. These drawbacks include relatively long preparation times, lack of control of the sample preparation environment, and possibility of loss of volatile analytes during the drying process. Microwave-assisted drying offers several advantages for dried spot preparation, including control of the environment and minimized volatility because of the differences between microwave heating and conventional heating. A microwave-assisted drying technique has been evaluated for use in preparing dried spots for trace analysis. Two apparatus designs for microwave drying were constructed and tested using multielement standard solutions, a standard reference material, and a “real-world” semiconductor cleaning solution. Following microwave-assisted drying of these aqueous samples, the residues were redissolved and analyzed by ICPMS. Effective recovery was obtained using the microwave drying methods, demonstrating that the microwave drying apparatus and methods described here may be more efficient alternatives for dried spot sample preparation. Dried spot micro-X-ray fluorescence (MXRF) is a novel tool for trace elemental analysis. The technique has many advantages over traditional X-ray fluorescence, allowing the technique to approach the detection capabilities of inductively coupled plasma mass spectrometry (ICPMS). Dried spot MXRF involves the transfer of a small amount of a liquid sample onto a thin film substrate, where it is subsequently dried into a small solid residue. The residue specimen is then excited by a collimated beam of X-rays, and the X-rays emitted by the sample are detected. Use of polycarbonate film supports combined with collimated beams and preconcentration of microliter volumes of samples has allowed detection limits for dried spot MXRF analysis to be reduced to the low parts-per-billion range.1 †
National Energy Technology Laboratory. Duquesne University. § Los Alamos National Laboratory. ‡
10.1021/ac010726t CCC: $22.00 Published on Web 01/15/2002
© 2002 American Chemical Society
One advantage of the dried spot technique is that it produces a specimen within the thin film regime, which minimizes matrix effects. Further, by coupling the dried spot technique with an instrument that is capable of adjusting the diameter of the X-ray beam to match the diameter of the sample spot, the background arising from the sample substrate is further reduced.2 By transforming the liquid samples into a dried spot, the analytes are concentrated prior to analysis. Another advantage of drying the samples is that this procedure removes differences in matrix chemistry from different solutions. For example, solutions of many different compositions are used in the semiconductor industry for cleaning of silicon wafers, as shown in Table 1.3 Each of these solutions is used to remove a specific target class of impurities from silicon wafers. Because each of these solutions presents a different matrix, the use of a single analytical technique may not be universally applicable. However, transforming each sample into a dried solid residue minimizes the chemistry differences from solvents of different chemical compositions. This allows the dried spot MXRF technique, as well as other trace analysis techniques such as ICPMS, to be more applicable to a wide variety of solution matrixes. Much of the improvement in sensitivity for the dried spot method has evolved from reduction of the background contribution from the thin film substrate.4,5 The method was even commercialized.6 More recent work has demonstrated the utility and sensitivity of both a homemade mini XRF system7 and a commercial MXRF instrument.8 Although all of this work has focused on the sensitivity of the method, one of the major drawbacks that has limited the commercial viability of this method (1) Meltzer, C.; King, B. S. Advances in X-ray Analysis: Proceedings of the Annual Conference on Application of X-ray Analysis 1991, 34, 41-55. (2) Colletti, L. P.; Havrilla, G. J. Adv. X-ray Anal. 1997, 42, 64-73. (3) Kern, W. Handbook of Semiconductor Wafer Cleaning Technology: Science, Technology and Applications; Noyes Publications: Westwood, NJ, 1993. (4) Nielson, A. J.; Turner, D. C.; Wilson, A.; Wherry, D. C.; Wong, R. Adv. X-ray Anal. 1995, 39, 799-804. (5) Wilson, A.; Turner, D. C.; Robbins, A. A. Adv. X-ray Anal. 1997, 41, 301307. (6) Spectrace Instruments, Inc. Product literature for Microsample X-ray Analysis, 1998. (7) Cheburkin, A. K.; Shotyk, W. X-ray Spectrom. 1999, 28, 379-383. (8) Sugihara, K.; Tamura, K.; Sato, M.; Ohno, K. X-ray Spectrom. 1999, 28, 446-450.
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Table 1. Typical Solutions and Reagents Used in Silicon Wafer Cleaning Procedures3 soln name
composition
formulation
purpose
ultrapure water “UPW”
H 2O
100%
primary dilution reagent
SC-1 RCA-1 “Huang 1”
NH4OH:H2O2:H2O
1:1:5
organic removal metal ions’ complexing
SC-2 RCA-2 “Huang 2”
HCl:H2O2:H2O
1:1:6
alkali ions’ removal metal hydroxide dissolution residual trace metal removal
SPM “piranha”
H2SO4:H2O2
2:1
organic removal
dilute HF
HF:H2O
1:10-100
native oxide removal
is a lack of rapid specimen preparation and reproducibility. This work addresses some of the issues relevant to rapid and reproducible specimen preparation. The ideal sample preparation methodology should produce uniform dried spots having a small spot diameter in order to minimize background contributions from an uneven sample spot. In addition, the dried spot preparation should be a rapid process, while at the same time minimizing contamination and loss of analyte during the procedure.2 Current drying methods include the use of heat lamps, vacuum ovens, and airdrying, with preparation times ranging from 30 min to over 2 h. These methods have produced spots with varying sizes and shapes, and the drying conditions are difficult to control and reproduce.2 In addition, drying methods that use conventional heating have demonstrated losses of certain analytes during the drying procedure.9-12 Microwave-assisted evaporation has demonstrated many advantages for analyte preconcentration and matrix removal while also demonstrating the capability of retaining volatile analytes at ultratrace levels.9,13 Microwave heating involves a fundamentally different mechanism that allows for controlled heating of small sample volumes in which the sample actually becomes cooler as energy is supplied and evaporation, or drying, is taking place. This concept has been validated in a study in which environmental sample digestates were evaporated using microwave energy and a custom-designed evaporation vessel.9 It was shown that even for notoriously volatile inorganic analytes, like As, Sb, and Hg, microwave evaporation demonstrated quantitative recoveries, whereas hot plate evaporation demonstrated losses of volatile analytes of up to 40%. Microwave heating is also the method of choice for minimizing contamination during the sample preparation procedure, because it allows for a level of control of the environment that is unattainable using conventional heating methods.14 This control during the evaporation process has been demonstrated for analysis of semiconductor materials in which (9) Link, D. D.; Kingston, H. M. Anal. Chem. 2000, 72, 2908-2913. (10) Walter, P. J.; Chalk, S. J.; Kingston, H. M.; Link, D. D. http://www. sampleprep.duq.edu/sampleprep, 1996-2000. (11) Sulcek, Z.; Povondra, P. Methods of Decomposition in Inorganic Analysis; CRC Press: Boca Raton, FL, 1989. (12) Bock, J. S. A Handbook of Decomposition Methods in Analytical Chemistry, 1st ed.; John Wiley and Sons: New York, 1979. (13) Han, Y.; Link, D. D.; Richter, R. C.; Kingston, H. M. Paper no. 1249, Pittsburgh Conference, New Orleans, LA, 2000. (14) Kingston, H. M.; Walter, P. J.; Chalk, S. J.; Lorentzen, E.; Link, D. In Microwave-Enhanced Chemistry: Fundamentals, Sample Preparation, and Applications; Kingston, H. M., Haswell, S. J., Eds.; American Chemical Society: Washington, DC, 1997; pp 223-349.
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trace impurities in silicon material were determined at the picograms-per-gram level using an optimized microwave evaporation procedure.15 For these reasons, it was projected that microwave energy could be used to prepare dried spots for analysis by dried spot MXRF. The goal of this study was to determine whether a microwave-assisted evaporation method could be developed and adapted for use in preparation of dried spot samples for MXRF analysis. Several aspects for the use of the microwave-assisted technique were studied, including the compatibility of the materials with microwave energy, the capability of these “microsamples” (samples of volumes on the order of 100 µL or smaller) to efficiently absorb microwave energy, and the recovery of analytes using the microwave-assisted technique. The study also involved the development and refinement of novel apparatus designs for using microwave energy to prepare the dried spot samples. EXPERIMENTAL SECTION Novel Apparatus Designs. To use microwave-assisted drying for dried spot sample preparation, an apparatus was designed to have the capability to provide closed, controlled environments during the drying process, the capability to evacuate the system during drying, and the capability to incorporate external heating via convection and conduction to aid in the drying process, if necessary. A schematic representation of the original apparatus that was designed is shown in Figure 1. The original film support apparatus consisted of a hollow (1/8 in.) PFA Teflon ferrule fitting (Johnson Equipment, McDonald, PA). A small section of polypropylene film (Prolene) was stretched tightly across the bottom of the fitting and secured in place using a glue stick. The sample solution was deposited directly onto the film using a microliter pipet in the area above the hollow portion in the center of the ferrule. This film support device was placed into a specially machined holder. This holder was created by hollowing out the center section of a segment of polypropylene rod, then widening the upper hollow portion to accommodate the film support device. The section of rod below was hollowed further so that a disk constructed out of Weflon (Milestone USA, Monroe, CT), which is a microwave-absorbing solid material, could be accommodated by placing the holder over the disk. The entire apparatus was placed inside a Teflon liner equipped with a custom evaporation vessel closure and placed inside the microwave cavity for drying. The capabilities of this custom evaporation vessel (15) Han, Y.; Kingston, H. M.; Richter, R. C.; Pirola, C. Anal. Chem. 2001, 73, 1106-1111.
Figure 1. Schematic representation of the original apparatus designed for microwave-assisted dried spot preparation.
closure have been described elsewhere.9 Upon interaction with the microwave energy, the Weflon disk heated, and the thermal energy from the disk was transferred upward through the hollow center sections of the polypropylene holder and the film support. Drying of the sample solutions was conducted using a combination of the transfer of thermal energy and reduced pressure inside the evaporation vessel. A popular thin film support currently being used in dried spot MXRF consists of a 35-mm slide mount with film stretched across the mount and glued in place;2 however, these supports are too large to be used with the microwave evaporation liner and closures used previously. Following testing of the original apparatus, a modified apparatus was designed and tested so that these 35-mm slide mounts could be used. An apparatus that could accommodate current materials and supports would avoid alteration of the MXRF analysis procedure and the costs and difficulties associated with altering the analytical technique. A schematic representation of the modified apparatus design is shown in Figure 2. For this apparatus, the thin polypropylene film is stretched across a 35-mm plastic slide mount and glued in place. A Teflon basket-type apparatus (Savillex Corp., Minnetonka, MN) was used to hold a Weflon disk and support the film. The Weflon disk is placed into the basket area, and the film support is placed on top of the basket area. The distance between the film and the disk is ∼1 cm. Figure 2 shows the vessel in which the basket and film is placed. It is a 100-mL threaded Teflon vessel with a threaded cap (Savillex Corp., Minnetonka, MN). The cap has two threaded ports that accommodate fittings for 1/4-in. tubing. For drying, one port was fitted with a section of 1/4-in. Teflon tubing through a ferrule fitting, and the other port was plugged using a plug ferrule. The tubing was connected to a central fume scrubber/vacuum pump (FAM-40, Milestone, Inc., Monroe, CT) to reduce the pressure inside the vessels. The vessels were secured to a rotor plate and rotated 180° through the microwave cavity during evaporation.
Incorporation of Thermal Heat for Drying. It has been shown that as the sample volume decreases, the efficiency of the interaction of the sample with microwave energy also decreases.9,16,17 The sample volumes discussed in these studies were much larger than the microsample volumes used for the dried spot MXRF technique, and it follows that samples on the order of 0.1 g or less would interact even less efficiently. Therefore, it was determined that incorporation of a microwave-absorbing substance was needed to transfer thermal energy to the samples to perform the drying of the solvent. A problem with the use of thermal energy for evaporation or drying, namely the potential to lose volatile analytes, has been shown.9,13 This problem stems from the overheating of the solid residue by the heat source at dryness, a phenomenon that is difficult to control using thermal heating methods.9 Therefore, a large degree of control over the process of heating the samples by the transfer of thermal energy was needed. A series of heating studies were performed on the Weflon disks to determine the rate at which the disks were heated and cooled using different microwave powers and different heating and cooling times. Fiber optic probes (Luxtron 750, Santa Clara, CA) were attached directly to the Weflon disks, and the temperature of the solid surface was monitored during various heating and cooling programs. The heating and cooling profiles of the Weflon disks using three different power settings is shown in Figure 3. Using a combination of these heating profiles, a heating program was developed to provide efficient drying of the sample solutions without creating a “microwave hot-plate” situation in which the transfer of thermal energy would go out of control and lead to overheating and analyte losses. Drying Procedures. The performance of each microwaveassisted drying apparatus was evaluated using different sets of sample solutions. One set of standard solutions was prepared by diluting a multielement standard solution in matrixes of either 4% HNO3 or 25% HNO3/25% HF, but the other set of test solutions were “real-world” solutions of different matrixes, including SRM 1643d and SC-1 semiconductor cleaning solution. For testing of the original dried spot apparatus (Figure 1), 50-µL volumes of the sample solution were delivered onto the thin film. The apparatus was placed inside a Teflon liner equipped with the custom microwave-assisted evaporation cap, and the solutions were dried. The heating program used an initial heating of 15 min at 350 W followed by 10 min at 0 W, followed by 4 cycles of a program of 5 min at 350 W and 5 min at 0 W, yielding a total drying time of 65 min. For testing of the modified apparatus (Figure 2), 50-µL aliquots of the sample solutions (SRM 1643d trace elements in water, and SC-1, a silicon wafer cleaning solution matrix containing NH4OH, H2O2, and H2O) were delivered onto the film. The initial 50-µL aliquots were dried using the following heating program: 5 min at 500 W, 5 min at 0 W, 3 min at 500 W, 2 min at 0 W, and 3 min at 250 W, for a total of 18 min. Following drying of the first aliquot, the apparatus was opened, an additional 50-µL aliquot of the sample was delivered onto the film, and the drying process (16) Kingston, H. M.; Jassie, L. B. In Introduction to Microwave Sample Preparation: Theory and Practice; Kingston, H. M., Jassie, L. B., Eds.; American Chemical Society: Washington, DC, 1988; pp 93-154. (17) Neas, E. D.; Collins, M. J. In Introduction to Microwave Sample Preparation: Theory and Practice; Kingston, H. M., Jassie, L. B., Eds.; American Chemical Society: Washington, DC, 1988; pp 7-32.
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Figure 2. Schematic representation of the modified apparatus used for microwave-assisted dried spot preparation.
Figure 3. Temperature profiles for the heating of Weflon disks using different power settings and different heating and cooling times.
was repeated. The heating program used for this aliquot was as follows: 5 min at 350 W, 4 min at 0 W, 5 min at 250 W, 4 min at 0 W, and 2 min at 250 W, for a total program time of 20 min. Following drying of the second aliquot, a third aliquot of 50 µL was delivered onto the films, and the drying process was repeated according to the 20-minute heating program used for the second aliquot, giving a final sample volume of 150 µL. Sample Processing and Analysis. Following drying of the solutions into a dried spot, the spots were redissolved using subboiled HNO3 and 18 MΩ cm water (duoPur, Milestone, Inc., Monroe, CT) and rinsed into clean dilution vials. Samples were diluted to appropriate volumes and analyzed by ICPMS (HP 4500, Agilent Technologies) in clean environments of class 100 or class 10 conditions. RESULTS AND DISCUSSION Evaluation of Original Drying Apparatus. The performance of the original microwave-assisted drying apparatus for dried spot preparation was evaluated using 50-µL samples of multielement standard solutions. Figure 4 shows the recovery of analytes following the drying of a multielement standard in a matrix of 4% 1168
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HNO3. Although this matrix is not typically considered to be difficult in terms of analyte volatility, complete recovery of analytes suggests that the technique does not lose analytes, either during the drying step or during the tedious post-drying transfer of the dried residues to the dilution vials. Figure 5 shows the recovery of analytes following the drying of a standard solution in a matrix of 25% HNO3 and 25% HF. This matrix presents more difficulty, because some analytes could be expected to become volatile from the fluoride-containing matrix. However, the data demonstrates that drying of microsamples of this more difficult matrix using the original apparatus design gave essentially quantitative recovery within the specified uncertainty for the target analytes. Evaluation of Modified Drying Apparatus. The performance of the apparatus that was modified to incorporate the 35-mm slide mount supports, currently used as thin film substrates in dried spot MXRF, was evaluated. The procedure involved the drying of a total volume of 150 µL of the sample solution in three drying steps using 50-µL aliquots in each step. Recovery data for the drying of two different solutions using the modified apparatus is shown in Figures 6 and 7.
Figure 4. Recovery of analytes after microwave-assisted thermally aided drying of 50-µL aliquots of a 10 ppm multielement standard solution in a matrix of 4% HNO3. Uncertainties are expressed as (standard deviation, with n g 3.
Figure 5. Recovery of analytes after microwave-assisted thermally aided drying of 50-µL aliquots of a 10 ppm multielement standard solution in a matrix of 25% HNO3 and 25% HF. Uncertainties are expressed as (standard deviation, with n g 3.
Figure 6. Results of analysis for the drying of 150 µL of SRM 1643d, as compared to certified values. Uncertainties expressed as the 95% confidence interval, with n ) g 3.
For the drying of SRM 1643d, the analyzed concentration of most analytes agrees with the certified value at the 95% confidence interval. Good agreement between analyzed and certified values was achieved for analytes such as Mn, Sr, and Pb, as well as potentially volatile As, Se, and V. For the SC-1 cleaning solution, there is also generally good agreement between the dried sample and the undried original solution. For both drying tests, however, some differences between the analyte concentration in the dried residues and the expected concentrations, as well as increased
uncertainties, were experienced. These differences are proposed to result from a number of different sources. First, the procedure of drying using multiple aliquots has demonstrated increased variation in results. Because three aliquots were delivered and dried, increased uncertainty was expected. In addition, the process of redissolution of the dried residue and transfer of the microliter sample solution was tedious and may have led to increased variation in results, as well as some physical losses of analytes. Another source of potential differences was the use and level of Analytical Chemistry, Vol. 74, No. 5, March 1, 2002
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Figure 7. Results of analysis for the drying of 150 µL of CSA cleaning solution, as compared to results of analysis for the original solution. Uncertainties expressed as the 95% confidence interval, with n ) g 3.
control of thermal energy. Although care was taken to prevent overheating of the samples by the Weflon disks, some films had been damaged as a result of overheating during the drying procedure. Film damage may have led to either the physical loss of portions of the sample, or volatilization losses due to overheating of the dried spot residue.
step in evaluating the drying procedure is evaluating its use for analysis by the MXRF technique; however, although further optimization of the apparatus and procedures is necessary, this study showed that microwave-assisted drying is a viable method for preparing dried spot samples for trace elemental analysis.
CONCLUSIONS This study demonstrated the use of microwave-assisted drying for the preparation of dried spot samples. The microwave preparation procedure successfully addressed several of the requirements of an optimum dried spot preparation method: sample preparation times were reduced, the samples were prepared in a closed environment, minimizing contamination, and effective recovery of target analytes from a variety of matrixes was achieved. Each apparatus design demonstrated good performance, although some variation in results was experienced. These variations can be attributed to several aspects of the drying procedure, such as multiple aliquotting, overheating, and sample transfer. The next
ACKNOWLEDGMENT
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The authors thank Milestone, Inc., Agilent Technologies, and Los Alamos National Laboratory for equipment support. The authors also thank Los Alamos National Laboratory for funding portions of this research. Portions of this research are patented or have patents pending.
Received for review June 29, 2001. Accepted October 22, 2001. AC010726T
