Characteristics of Picoliter Droplet Dried Residues as Standards for

The characteristics of dried residues of picodroplets of single-, two-, and three-element aqueous solutions, which qualify these as reference material...
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Anal. Chem. 2008, 80, 1967-1977

Characteristics of Picoliter Droplet Dried Residues as Standards for Direct Analysis Techniques Ursula E. A. Fittschen,* Nicolas H. Bings, Stephan Hauschild, Stephan Fo1 rster, Arne F. Kiera, Ezer Karavani,† Andreas Fro 1 msdorf, and Julian Thiele

Chemistry Department, University of Hamburg, Martin-Luther-King Platz 6, 20146 Hamburg, Germany Gerald Falkenberg

Hamburger Synchrotron-Strahlungslabor at Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22603 Hamburg, Germany

The characteristics of dried residues of picodroplets of single-, two-, and three-element aqueous solutions, which qualify these as reference materials in the direct analysis of single particles, single cells, and other microscopic objects using, e.g., laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOFMS) and micro-X-ray fluorescence (MXRF), were evaluated. Different single-, two-, and three-element solutions (0.01-1 g/L) were prepared in picoliter volume (around 130 pL) with a thermal inkjet printing technique. An achievable dosing precision of 4-15% was calculated by total reflection X-ray fluorescence (TXRF) determination of the transferred elemental mass of an array of 100 droplets. The size of the dried residues was determined by optical microscopy to be 5-20 µm in diameter depending on the concentration and the surface material. The elemental distribution of the dried residues was determined with synchrotron micro-X-ray fluorescence (SR-MXRF) analyses. The MXRF results show high uniformity for element deposition of every single droplet with an RSTD of 4-6% depending on the concentration of spotted solution. The shape and height profile of dried residues from picoliter droplets were studied using atomic force microscopy (AFM). It was found that these dry to give symmetrical spherical segments with maximum heights of 1.7 µm. The potential of this technique for direct LA-ICP-TOF-MS analysis is shown. In recent years the direct analysis of solid samples gained importance to solve scientific questions where sample amounts are limited. This applies for analysis of biological samples like tissue sections or proteins and environmental studies on aerosols. Analytical techniques using micro- and even nanobeams like micro-proton-induced X-ray emission (mPIXE),1 electron micro* To whom correspondence should be addressed. E-mail: Ursula.fittschen@ chemie.uni-hamburg.de. Fax: +49-40-42838-4381. † Current address: Department of Biotechnology Engineering, Ben Gurion University, Beer Sheva, Israel. (1) Johanssons, S. A. E.; Campbell, J. L. PIXE: A Novel Technique for Elemental Analysis; Wiley: New York, 1988. 10.1021/ac702005x CCC: $40.75 Published on Web 02/12/2008

© 2008 American Chemical Society

probe analysis (EMPA),2,3 micro-X-ray fluorescence (MXRF),4,5 and laser-related methods like laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS)6 have been applied to extract information on elemental composition and amounts from very small sample volumes (cubic micrometers). These microscopic techniques have become powerful tools for two- or even three-dimensional elemental mapping,7,8 high-throughput screenings,9 and elemental determination in minute amount of samples like single particles10-13 or single cells.14 Although numerous successful applications of these techniques have been reported, a suitable reference material that covers the different demands on elemental composition and concentrations is often lacking. For elemental mapping homogeneous references like foils or plates covered with one or more elements have been applied successfully in MXRF analysis.11,15 In these studies the concentration of the reference was related to that of the sample. If the absolute amounts of elements in minute amounts of samples like single cells or single particles have to be addressed, a small reference well(2) Worobiec, A.; Kaplinski, A.; Van Grieken, R. X-Ray Spectrom. 2005, 34, 245-252. (3) Osan, J.; Torok, S.; Beckhoff, B.; Ulm, G.; Hwang, H.; Ro, C.-U.; Abete, C.; Fuoco, R. Atmos. Environ. 2006, 40, 4691-4702. (4) Snigireva, I.; Snigirev, A. J. Environ. Monit. 2006, 8 (1), 33-42. (5) Bjeoumikhov, A.; Langhoff, N.; Bjeoumikhova, S.; Wedell, R. Rev. Sci. Instrum. 2005, 76 (6), 063115 1-7. (6) Gu ¨ nther, D.; Audetat, A.; Frischknecht, R.; Heinrich, C. A. J. Anal. At. Spectrom. 1998, 13, 263-270. (7) Fredrick, P.; de Ryck, I.; Janssens, K.; Schryvers, D.; Petit, J.-P.; Doecking, H. X-Ray Spectrom. 2004, 33 (5), 326-333. (8) Zoeger, N.; Wobrauschek, P.; Streli, C.; Pepponi, G.; Roschger, P.; Falkenberg, G.; Osterode, W. X-Ray Spectrom. 2005, 34 (2), 140-143. (9) Miller, T.; Mann, G.; Havrilla, G.; Wells, C.; Warner, B.; Baker, R. J. Comb. Chem. 2003, 5, 245-252. (10) Miller, T. C.; Langley DeWitt, H.; Havrilla, G. J. Spectrochim. Acta, Part B 2005, 60, 1458-1467. (11) Vincze, L.; Somogyi, A.; Osan, J.; Vekemans, B.; Toeroek, S.; Janssens, K.; Adams, F. Anal. Chem. 2002, 74 (5), 1128-1135. (12) Ranebo, Y.; Eriksson, M.; Taborini, G.; Niagolova, N.; Bildstein, O.; Betti, M. Microsc. Microanal. 2007, 13, 179-190. (13) Berendes, A.; Neimke, D.; Schumacher, R.; Barth, M. J. Forensic Sci. 2006, 51, 1085-1090. (14) Kemner, K. M.; Kelly, S. D.; Lai, B.; Maser, J.; O’Loughlin, E. J.; SholtoDouglas, D.; Cai, D.; Schneegurt, M. A.; Kulpa, C. F.; Nealson, K. H. Science 2004, 306 (5696), 686-687. (15) Osterode, W.; Falkenberg, G.; Ho¨ftberger, R.; Wrba, F. Spectrochim. Acta, Part B 2007, 62, 682-688.

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defined in size and shape and having an elemental composition that can be custom-made for the analytical question would be favorable. In this case the absolute elemental amount delivered by the reference is related to that of the sample. For this calibration strategy the examination of the whole sample and the whole reference is of utmost importance, which has been shown for dried residues of ink droplets (117 µm diameter) in mPIXE analysis.16 Bulk particulate reference materials like the NIST SRM 1577a, trace elements in bovine liver, or NIST SRM 1645, trace elements in urban dust, e.g., are not very suitable to serve as such references as amounts are certified only if at least 250 mg are analyzed. In this study we introduce dried residues of picodroplets generated by inkjet printers as a reference for microbeam analysis techniques. The picodroplet characteristics such as the reproducibility in elemental content of dried residues of picodroplets for both single- and several element standard solutions, the size, and the shape were studied. As stated before for the use of picodroplet residues as a calibration strategy the whole sample and the whole reference droplet must be analyzed completely. Therefore, a careful evaluation of this calibration procedure for the technique in question has to be done. Additionally, to demonstrate the suitability of dried residues from picodroplets as a reference material for microscopic analysis techniques a calibration strategy using LA-ICPMS was evaluated. Preparation and drying of small amounts of liquids in the microliter range has been studied intensively for the optimization of total reflection X-ray fluorescence (TXRF) applications17,18 because homogeneous thin sample structures are of utmost importance for TXRF analysis. Recently improvements in liquid sample preparation have been made by spotting amounts in the nanoliter range using automated biopipet systems.19 Procedures using this system have been developed for sample preparation in MXRF20 and analysis of silicon wafers using TXRF.21,22 Nanoliter droplets of ink have been applied as reference in mPIXE but without any additional characterization besides the PIXE analysis.16 Spotting of even smaller amounts of liquids in the picoliter range has been reported by us using modified Hewlett-Packard (HP) inkjet printers.23 A calibration strategy which benefits from the small volumes of standard solutions for elemental determination in aerosols using TXRF was described. Automated printer systems offer several advantages for the preparation of reference materials. In the studies cited before, different technologies to deliver well-defined volumes as automated nanopipetting systems equipped with microsolenoid valves

driven by pressure variation have been applied. To generate picodroplets, two competitive so-called drop-on-demand inkjet technologies have been applied for scientific purposes. Widespread and commercially available from different manufacturers are pipetting systems using piezo crystals for droplet formation.24,25 Thermal inkjet technology that was invented by HP has been used by several working groups from the “nanoscience” field using slightly modified inkjet printers for a variety of applications.26-28 In comparison to the piezo method the thermal inkjet system is said to be less sensitive to air accidentally entered the system and therefore more favorable for the purpose of this study where the standard solutions inside the cartridges were exchanged frequently.29 To evaluate the applicability of “picodroplets” for analytical purpose a thorough characterization was performed. Dosing. TXRF was used to quantify the total amount of material deposited by the printing of standard solutions over a large area using a selected number of droplets. The number of droplets was used to determine the amount of material deposited by each individual droplet. The precision of the described dosing procedure was determined through the relative standard deviation (RSD) from the analysis of individual residues of dried droplets studied by MXRF. Size. The diameters of the dried residues were studied using a light microscope. Since a large number (e.g., 10 × 10) of droplets can be delivered by the printer in less than 1 s, a representative number (100) of dried residues from different standard solutions could be easily generated. Shape. The absorption effects in MXRF and the volume that has to be volatilized by the laser were estimated from the shape of the dried residues. The shape was studied using atomic force microscopy (AFM). As AFM studies on nanoliter droplets revealed that these droplets dried mostly to give ringlike residues,22 it was important to check if that would still apply if the delivered volume is reduced by 1 order of magnitude. Applicability for LA-ICPMS. Laser ablation in combination with plasma source mass spectrometry has evolved to a mature tool in the field of elemental analysis of solid samples. Its advantages are manifold, since complex sample preparation steps are seldom necessary, which can result in high sample throughput even in the case of refractory materials.30,31 Additionally, depending on the number of laser pulses used for the ablation and due to the micrometer-sized laser spot diameter, only a very small sample volume is ablated, allowing the analysis of minute amounts of sample material,32-35 which is important in the field of archeometry

(16) Bohlen, v. A.; Ro ¨hrs, S.; Salomon, J. Anal. Bioanal. Chem. 2007, 387, 781790. (17) Fabry, L.; Palke, S.; Kotz, L. Fresenius’ J. Anal. Chem. 1996, 345, 266270. (18) Pahlke, S.; Fabry, L.; Mantler, C.; Ehemann, T. Spectrochim. Acta, Part B 2001, 56, 2261-2274. (19) Miller, T. C.; Havrilla, G. J. X-Ray Spectrom. 2004, 33, 101-106. (20) Miller, T. C.; Hastings, E. P.; Havrilla, G. J. X-Ray Spectrom. 2006, 35 (2), 131-136. (21) Miller, T. C.; Sparks, C. M.; Havrilla, G. J.; Beebe, M. R. Spectrochim. Acta, Part B 2004, 59, 1117-1124. (22) Sparks, C. M.; Gondran, C. H.; Havrilla, G. J.; Hastings, E. P. Spectrochim. Acta, Part B 2006, 61, 1091-1097. (23) Fittschen, U. E. A.; Hauschild, S.; Amberger, M. A.; Lammel, G.; Streli, C.; Fo ¨rster, S.; Wobrauschek, P.; Jokubonis, C.; Pepponi, G.; Falkenberg, G.; Broekaert, J. A. C. Spectrochim. Acta, Part B 2006, 61, 1098-1104.

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in case of precious samples.36,37 Also it enables three-dimensional visualization of elemental distributions in solid samples with high spatial resolution.38,39 Nonetheless, accurate calibration in laser ablation is often difficult, since it is hampered by elemental fractionation and the lack of available standard reference materials.40,41 The fractionation effect is caused by boiling point-correlated sequential vaporization of the sample components during the ablation process and might be followed by sequential or incomplete evaporation of the generated aerosol particles within the ICP.42,43 This effect can be minimized to some extent by reducing the laser wavelength and/ or the duration of the laser pulse used for the ablation of the sample.44-49 The homogeneity of the calibration standards is of additional importance,50 which gains significance with decreasing amount of sampled analyte mass, e.g., when performing singleshot LA,51-53 for the analysis of precious samples or the characterization of individual aerosol particles. Matrix-matched calibration techniques, i.e., the combination of continuous nebulization of standard solutions with LA of the solid sample material, are well-known in LA-ICPMS54-57 and are especially helpful when solid reference material is lacking. However, if the absolute mass of sample material ablated by individual laser pulses has to be quantified, benefits of this strategy are limited, for example, in the case of quantitative elemental analysis of minute amounts of solid sample, such as individual particles. An improved strategy is based on the correlation of the transient signal in LA-ICPMS originating from the sample material, with such a signal resulting from the complete ablation of a known (33) Bings, N. H. J. Anal. At. Spectrom. 2002, 17, 759-767. (34) Elish, E.; Karpas, Z.; Lorber, A. J. Anal. At. Spectrom. 2007, 22, 540-546. (35) Gu ¨ nther, D.; Hattendorf, B.; Audetat, A. J. Anal. At. Spectrom. 2001, 16, 1085-1090. (36) Devos, W.; Senn-Luder, M.; Moor, C.; Salter, C. Fresenius’ J. Anal. Chem. 2000, 366, 873-880. (37) Devos, W.; Moor, C.; Lienemann, P. J. Anal. At. Spectrom. 1999, 14, 621626. (38) Zoriy, M.; Matusch, A.; Spruss, T.; Becker, J. S. Int. J. Mass Spectrom. 2007, 260, 102-106. (39) Hoffmann, E.; Stephanowitz, H.; Ullrich, E.; Skole, J.; Lu ¨ dke, C.; Hoffmann, B. J. Anal. At. Spectrom. 2000, 15, 663-667. (40) Kuhn, H. R.; Pearson, N. J.; Jackson, S. E. J. Anal. At. Spectrom. 2007, 22, 547-552. (41) Kuhn, H. R.; Gu ¨ nther, D. Anal. Chem. 2003, 75, 747-753. (42) Kroslakova, I.; Gu ¨ nther, D. J. Anal. At. Spectrom. 2007, 22, 51-62. (43) Bleiner, D.; Gu ¨ nther, D. J. Anal. At. Spectrom. 2001, 16, 449-456. (44) Koch, J.; Gu ¨ nther, D. Anal. Bioanal. Chem. 2007, 387, 149-153. (45) Koch, J.; von Bohlen, A.; Hergenro ¨der, R.; Niemax, K. J. Anal. At. Spectrom. 2004, 19, 267-272. (46) Guillong, M.; Horn, I.; Gu ¨ nther, D. J. Anal. At. Spectrom. 2003, 18, 12241230. (47) Becker, J. S.; Tenzler, D. Fresenius’ J. Anal. Chem. 2001, 370, 637-640. (48) Russo, R. E.; Mao, X. L.; Borisov, O. V.; Liu, H. C. J. Anal. At. Spectrom. 2000, 15, 1115-1120. (49) Gu ¨ nther, D.; Heinrich, C. A. J. Anal. At. Spectrom. 1999, 14, 1369-1374. (50) Kempenaers, L.; Bings, N. H.; Jeffries, T. E.; Vekemans, B.; Janssens, K. J. Anal. At. Spectrom. 2001, 16, 1006-1011. (51) Leach, A. M.; Hieftje, G. M. Appl. Spectrosc. 2002, 56, 62-69. (52) Leach, A. M.; Hieftje, G. M. Anal. Chem. 2001, 73, 2959-2967. (53) Liu, H. C.; Mao, X. G.; Russo, R. E. J. Anal. At. Spectrom. 2001, 16, 11151120. (54) Halicz, L.; Gu ¨ nther, D. J. Anal. At. Spectrom. 2004, 19, 1539-1545. (55) Chan, G. C. Y.; Chan, W. T.; Mao, X. L.; Russo, R. E. Spectrochim. Acta, Part B 2001, 56, 1375-1386. (56) Pickhardt, C.; Becker, J. S.; Dietze, H. J. Fresenius’ J. Anal. Chem. 2000, 368, 173-181. (57) Gu ¨nther, D.; Cousin, H.; Magyar, B.; Leopold, I. J. Anal. At. Spectrom. 1997, 12, 165-170.

amount of standard material. The latter can be performed through the ablation of dried residues from small droplets with known volume of standard solutions. While this special approach was not investigated so far, only a little work was done using dried droplets as a general sampling technique for liquid samples. In 1988 Odom et al.58 first described a quantitative method for the determination of trace elements in aqueous samples by the analysis of its dried residues of nanoliter droplets through secondary ion mass spectrometry (SIMS), whereas Yang et al.59,60 suggested this strategy for the determination of selenomethionine in yeast by off-line coupling HPLC to LA for the evaporation of dried microliter droplets of chromatographic fractions. In neither of the reported cases the dried residues were explicitly used as calibration standards for mass spectrometric analysis of solid samples, but for high-efficiency sampling of minute amounts of previously dissolved sample material. The obtained measurement precision remained unsatisfying, related to limitations caused by detector counting statistics due to the limited analyte mass, as well as to poor sampling precision achieved by manual pipetting of microliter and nanoliter volumes of sample solution. The aim of this work was to introduce and to evaluate a novel calibration strategy for LA-based analysis techniques, using residues of dried elemental standard solution-based picoliter droplets, which were “printed” by a modified inkjet printer onto the sample carrier. Once the absolute amount of transferred material is accurately determined, the dried residues of such droplets qualify as calibration standards in various fields of sample surface analysis, e.g., LA-ICPMS, provided that the residue is completely ablated. The measured signal can then be reliably correlated with the absolute amount of “printed” standard material, principally allowing quantitative LA-ICPMS analysis of ultralow sample mass, such as single particles or individual cells. Therefore, the complete evaporation of the sample material is necessary, to gain information on the absolute analyte mass within the respective particle. Additionally, due to the integration of the complete transient signal generated by total ablation of the sample material in the picogram range, contributions to elemental fractionation from LA should be eliminated. As a prerequisite for simultaneous multielemental analysis of such low sample mass provided through picoliter droplets, a fast and sensitive detector has to be selected. This eliminates scanning detectors, and therefore, sequential elemental mass spectrometers were selected for the detection of transient signals. Simultaneous ion extraction and high spectral generation rates in the range of up to 20 kHz make plasma source time-of-flight mass spectrometry (ICP-TOF-MS) nearly ideal for fast and sensitive simultaneous multielemental detection of short transient signals. EXPERIMENTAL SECTION Chemicals. Several standard solutions containing 1 g/L of cobalt, gallium, nickel, scandium, titanium, and barium (ICP standard CertiPUR Merck, Darmstadt, Germany), sub-boiled (58) Odom, R. W.; Lux, G.; Fleming, R. H.; Chu, P. K.; Niemeyer, I. C.; Blattner, R. J. Anal. Chem. 1988, 60, 2070-2075. (59) Yang, L.; Sturgeon, R. E.; Mester, Z. Anal. Chem. 2005, 77, 2971-2977. (60) Yang, L.; Sturgeon, R. E.; Mester, Z. J. Anal. At. Spectrom. 2005, 20, 431435.

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Table 1. Optimum LA-ICP-TOF-MS Operating Parameters LA system wavelength

New Wave Research UP-213 213 nm, frequency quintupled Nd:YAG pulse length