Surface Sampling by Spray-Desorption Followed by Collection for

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Anal. Chem. 2010, 82, 1674–1679

Surface Sampling by Spray-Desorption Followed by Collection for Chemical Analysis Andre R. Venter,* Afrand Kamali, Shashank Jain, and Semere Bairu Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008 Desorption electrospray ionization (DESI) directly analyzes soluble chemical components present on surfaces when a pneumatically assisted electrospray is directed at the sample. Here we demonstrate that the same spray desorption mechanism that operates in DESI can be used as a general technique to collect soluble materials present on surfaces. After desorption analytes are collected on a suitable collection surface, large areas can be scanned and collected onto a small collected area, which allows for preconcentration of low abundance material before analysis. This collection surface can then subsequently be analyzed by DESI but also by many other techniques such as gas chromatography-mass spectrometry or UV-vis spectroscopy. In addition this technique can be used to study desorption mechanisms in DESI independently from ionization mechanisms. Preliminary results indicate that the optimized conditions in DESI are a compromise between those conditions that are optimum for desorption and conditions that lead to efficient ionization. Few options are available for in situ sample collection from surfaces. Both swabbing and rinsing or washing with solvents distributes sample material over large areas or volumes. Swabbing also suffers from incomplete extraction of analyte from the sample surface and potential difficulty in recovery of analytes from swabbing material.1 Furthermore, with these techniques preconcentration by large area sampling varies from cumbersome to impossible. Great care needs to be exerted during sampling to ensure that a representative surface is analyzed. This can easily lead to the accumulation of a large number of collected samples for subsequent analysis. Solid phase microextraction2 and other absorbent traps have proven indispensable for preconcentration of low levels of analyte in liquid or gaseous samples for analysis by liquid or gas chromatography.3-6 A similar sampling and preconcentration technique does not yet exist for collection from the surface of solid phase samples. * Author to whom correspondence should be addressed. [email protected]. (1) Liu, L.; Pack, B. W. J. Pharm. Biomed. Anal. 2007, 43, 1206–1212. (2) Risticevic, S.; Niri, V. H.; Vuckovic, D.; Pawliszyn, J. Anal. Bioanal. Chem. 2009, 393, 781–795. (3) Michulec, M.; Wardencki, W.; Partyka, M.; Namiesnik, J. Crit. Rev. Anal. Chem. 2005, 35, 117–133. (4) Helmig, D. J. Chromatogr., A 1999, 843, 129–146. (5) Hildebrandt, A.; Lacorte, S.; Barcelo, D. Anal. Bioanal. Chem. 2006, 386, 1075–1088. (6) Rawa-Adkonis, M.; Wolska, L.; Namiesnik, J. Crit. Rev. Anal. Chem. 2003, 33, 199–248.

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Recently a new paradigm of mass spectrometric analysis has become available with the development of the so-called direct ambient ionization methods.7 As described below, these methods bypass sample collection and preparation steps and allow for direct analysis of chemicals from surfaces. Ambient ionization methods have great potential for in situ analysis; however these are yet hard to couple to portable mass spectrometers. Ambient ionization methods use atmospheric pressure ionization interfaces, which places a high demand on the vacuum systems of the mass spectrometer. While such systems are under development,8 none are currently commercially available. Hence, at the moment, ambient ionization mass spectrometry can be used in situ only when the sample is in the laboratory or if it can easily be moved into the laboratory. With ambient desorption ionization mass spectrometry analyte desorption usually accompanies the ionization step in concerted, multistep processes. Ambient desorption ionization methods typically require little to no sample preparation, offer a much simplified work flow, and deliver unprecedented ease of use to mass spectrometric analyses. Since the introduction of desorption electrospray ionization (DESI)9 in 2004 and the Direct Analysis in Real Time (DART)10 in 2005, this new field of mass spectrometry has developed rapidly. Numerous permutations of the various options for analyte desorption and ionization have been demonstrated. Desorption steps such as momentum transfer, dissolution into ricocheting droplets, and thermal desorption have been combined with ionization steps including electrospray ionization, atmospheric pressure chemical ionization, and photoionization.7,11 Spray desorption and momentum transfer are especially suitable for the desorption-collection technique described here, though other desorption methods could conceivably also be used. In DESI9,12 surface bound analytes are desorbed from the surface by the impact of fast moving micrometer-sized aqueous droplets.13-15 Current understanding of the process, known as the droplet pickup mechanism, proposes that the surface is prewetted by initial droplets into which surface analytes dissolve (7) Venter, A.; Nefliu, M.; Graham Cooks, R. TrAC Trends Anal. Chem. 2008, 27, 284–290. (8) Gao, L.; Cooks, R. G.; Ouyang, Z. Anal. Chem. 2008, 80, 4026–4032. (9) Takats, Z.; Wiseman, J. M.; Gologan, B.; Cooks, R. G. Science 2004, 306, 471–473. (10) Cody, R. B.; Laramee, J. A.; Durst, H. D. Anal. Chem. 2005, 77, 2297– 2302. (11) Gary J. Van Berkel, S. P. P. O. O. J. Mass Spectrom. 2008, 43, 1161–1180. (12) Cooks, R. G.; Ouyang, Z.; Takats, Z.; Wiseman, J. M. Science 2006, 311, 1566–1570. (13) Venter, A.; Sojka, P. E.; Cooks, R. G. Anal. Chem. 2006, 78, 8549–8555. (14) Costa, A. B.; Cooks, R. G. Chem. Commun. 2007, 3915–3917. (15) Costa, A. B.; Cooks, R. G. Chem. Phys. Lett. 2008, 464, 1–8. 10.1021/ac902013x  2010 American Chemical Society Published on Web 02/11/2010

Figure 1. Spray desorption collection followed by DESI analysis of the collected sample.

and that later-arriving droplets impact this surface solvent-layer and break it up, creating numerous off-spring droplets containing a large percentage of the material originating from the solvent layer including the dissolved analytes. Thus, analyte desorption occurs by momentum transfer in the form of charged submicrometer droplets that are then ionized by electrospray ionization mechanisms.13–15 Although charge build-up on the surface has been shown to affect the charge transfer and therefore the ion currents in DESI,16 simulations of the DESI process14,15 show that hydrodynamic forces play the major role in analyte desorption from surfaces. DeSSI is a version of DESI where no voltage is applied to the spray emitter, therefore producing solvent droplets of lower charge density.17 Further investigation is needed to elucidate the charging effects in the desorption process. Here we describe a new surface sampling technique that decouples desorption from analysis by directing a pneumatically assisted electrospray or sonicspray ionization source at the primary sample surface. However, unlike the case for desorption electrospray ionization, where the spray is analyzed by mass spectrometry directly after leaving the surface, the spray is collected onto a suitable surface. This well-defined and potentially analytically active secondary surface can then be analyzed by diverse analytical techniques including, as demonstrated here, direct ambient ionization mass spectrometry, gas chromatography (GC-MS), and ultraviolet absorption spectroscopy (UV-vis) and many others. METHODS A simple spray-desorption-collection (SDC) experiment is demonstrated in Figure 1. Here a regular home-built electrosonic spray ionization source18 is used as if for a typical DESI analysis. Briefly, the source is constructed from a 1/16” Swagelok t-piece, a fused silica solvent line (O.D. 190 µm, I.D. 75 µm), and a coaxial stainless steel nebulizing gas capillary (O.D. 1/16” I.D. 250 µm). This configuration produces a sample surface coverage of about 0.2-1 mm2, depending on spray conditions and distances. A collection surface is placed close to the impact site of the sprayer on the sample surface. Typically a distance of 1 mm (16) Volny, M.; Venter, A.; Smith, S. A.; Pazzi, M.; Cooks, R. G. Analyst 2007, xxx. (17) Haddad, R.; Sparrapan, R.; Eberlin, M. N. Rapid Commun. Mass Spectrom. 2006, 20, 2901–2905. (18) Takats, Z.; Wiseman, J. M.; Gologan, B.; Cooks, R. G. Anal. Chem. 2004, 76, 4050–4058.

was used. Porous PTFE sheets (1/16” thick, 25 µm pore, part number PPTS-06030, Small Parts Inc.), office paper, glass microscope slides, and silica gel coated-thin layer chromatography plates (Silica Gel IB2-F, part number 4449-00, Mallinckrodt Baker Inc.) were used as collection surfaces. Analytes, once collected on the surfaces from the spray, were analyzed by various techniques including: a) Mass spectrometry, using a prototype moving stage mounted DESI source (Purdue University, West Lafayette, IN) and a ThermoScientific LTQ linear ion trap mass spectrometer. DESI conditions were similar to optimized conditions published in the literature.19 b) Gas chromatography-mass spectrometry was obtained with a Agilent 5972 GC-MSD with a 30 m 0.32 mm I.D. SE-30 column. For GC-MS analysis, surface material was collected by placing the mouth of an autosampler vial (2 mL Clear Glass, National Scientific, C4000-192) directly in the path of the spray leaving the surface. Soluble chemical compounds collected onto the inside wall of the vial and were washed down by adding 250 µL solvent. A 10 µL injection with a 1:50 split ratio was used for a temperature programmed analysis from 70 to 250 °C at 10 °C/min using helium as carrier gas. c) UV-vis spectroscopy was performed using a Shimadzu UV 2101 Pc UV-vis spectrophotometer. Here, ≈0.5 mg of TiO2 nanoparticles was spotted onto an acrylic surface and collected by SDC into a second acrylic cuvette followed by dilution of the collected material with 3 mL of a 50% methanol-water solution. The absorbance of this was compared to the absorbance obtained by dissolving ≈0.5 mg of nanoparticles in 3 mL of solvent followed directly by UV measurement. RESULTS AND DISCUSSION a) Morphology of Collected Spots. Previously it was demonstrated that during a DESI analysis droplets leave the sample surface after collision in a trajectory very close to the sample surface.13–15 This has the effect that desorbed material collects on the collection surface in a narrow band very close to the sample surface, as can be seen in Figure 2. A moderate distribution of sample material in the equatorial plane is also observed. This distribution on the collection surface is a function of the distance of collector to impact site. As for light projection, the collected image increases in size with distance; therefore, during collection, we attempted to keep this distance at a minimum. A collection distance of 1 mm was typical. b) Critical Parameters and Collection Efficiency. Conditions were optimized with two basic aims in mind: to achieve efficient recovery and to have the analyte collected in a welldefined spot on the collection surface. Optimization parameters included electrospray voltage, sprayer to sample distance, solvent impact site to collector distance, and gas and solvent flow rates and are particular for the standard electrosonic sprayer used. The optimization results are summarized in Figure 3a-c. To obtain these results, porous PTFE was impregnated with a concentrated solution of rhodamine 6G. Methanol was used as spray solvent, and the analyte was collected on silica gel TLC plates. After collection the TLC plate was analyzed with DESI-MS by scanning (19) Green, F. M.; Stokes, P.; Hopley, C.; Seah, M. P.; Gilmore, I. S.; O’Connor, G. Anal. Chem. 2009, 81, 2286–2293.

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Figure 2. Collection of rhodamine 6G from a saturated porous PTFE sample surface to a silica gel TLC plate. Note the thin line in which the desorbed sample is collected.

through each collected band lengthwise, using standard DESI conditions and methanol as spray solvent to quantify the collected material. Areas obtained after integration of the chronogram obtained with DESI-MS are reported on the ordinates of Figure 3a-c. Where error bars are indicated each data set was repeated three times by going through the full range of settings before returning to the starting point. Key observations include that the collection efficiency is independent from the applied electrospray voltage (Figure 3a). Therefore a high voltage power supply is not required during collection, which will greatly simplify the future design of a portable spray-desorption-collection (SDC) device. As indicated in Figure 3b, when the solvent flow rate is increased, the amount of collected material also increases. During a typical ESI or DESI experiment desolvated ions are produced by the evaporation of solvent through heat and vacuum and ion suppression and chemical noise is observed at higher solvent flow rates. With SDC removal of this large amount of solvent occurs before and independent from the subsequent MS analysis and so, unlike during a direct DESI analysis, has no negative analytical effect. Also from Figure 3b it can be seen that there is a minimum volumetric flow rate of nebulizing gas below which collection efficiency suffers. This is due to inefficient nebulization of the solvent and the production of large, slow moving droplets incapable of efficient desorption, and analytes are not collected in a well-defined collection band on the collection surface. In these experiments changes in flow rate were obtained by controlling the pressure at the nitrogen supply from 50 to 200 psi. This minimum gas flow rate for which efficient nebulization is observed is dependent on the solvent flow rate. For the lowest solvent flow rate, the effect is not yet visible at the lowest gas pressure studied (50 psi, 0.765 L/min), while it is very prominent at the higher solvent flow rate of 15 µL/min. At high nebulizinggas flow rates the collection efficiency is again reduced. This is probably due to fast evaporation of the solvent at high gas flow 1676

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Figure 3. a. Voltage optimization. The applied spray voltage does not have a significant influence on the peak area obtained by DESI of the collected analyte. b. The influence of nebulizing gas flow rate on peak areas obtained by DESI at three different solvent flow rates is compared. c. Peak areas obtained by DESI after collection while the vertical and horizontal distances were changed equally, with the sprayer at a 45° angle. A maximum in collected material was observed when the sprayer to surface distance was at 3-4 mm.

rates, which reduces the amount of solvent present on the surface, effectively as if a lower solvent flow rate was used. The optimization of the sprayer to surface distance displays an interesting shape due to the combined effects of surface coverage and desorption efficiency. When the sprayer to surface distance is increased, the area covered by the spray increases, which increases the amount of material desorbed and collected (assuming that analyte is present and evenly distributed across the entire spray area). However, droplet velocity decreases with distance due to aerodynamic drag forces13 which decreases the ability of droplets to efficiently desorb analyte by momentum transfer. Thus, analyte recovery initially increases as the surface area covered by the spray increases but then falls off again after droplets lose the momentum required for analyte desorption.

Figure 5. A comparison of the direct analysis of a dried orange juice sample by DESI-MS (top spectrum) and the same sample collected by SDC followed by DESI-MS analysis (bottom).

Figure 4. a. The same sample position was held constant, while the collector was moved to a new position after 1-min intervals. After 5 min signal intensity drops to about 10% of the original signal. b. A uniform sample of rhodamine-impregnated porous PTFE was scanned over increasing areas, and the spray was collected. The collected rhodamine was extracted, and the UV absorbance was measured. The collected amount was calculated from the mass extinction coefficient for rhodamine 6G.

d) Collection Efficiency. To obtain the results displayed in Figure 4a the sprayer was trained on a particular sample-surface location, while the collection-surface was replaced at one-minute intervals. This experiment provides an indication of the amount of material removed per unit time. The area obtained by DESIMS decreased to below 10% of the initial value after 5 min. This observation is due to the fact that when surface washing occurs during the typical DESI analysis not all material leaves the surface but is diluted in a surface solvent layer which is only partly removed by subsequent droplet impact events, while the remainder runs away from the impact site as small rivulets as has been observed and reported on by many researchers.15,20,21 In a second experiment, (Figure 4b) the sprayer was scanned at 50 µm/s across a sample surface that was uniformly coated in rhodamine 6G. This experiment investigates the accumulative effect of scanning across fresh surface over time, collected onto the same position on the collection surface. It is an indication of the preconcentration effect that can be achieved by SDC before subsequent analysis. To accurately quantify the total amount of collected material each collected line, as shown in Figure 2, was scraped off the TLC plate and extracted into 2 mL of methanol. The measured absorbance was then converted to grams by applying Beer’s law and using the previously published mass (20) Kertesz, V.; Berkel, G. J. V. Rapid Commun. Mass Spectrom. 2008, 22, 2639–2644. (21) Ifa, D. R.; Wiseman, J. M.; Song, Q. Y.; Cooks, R. G. Int. J. Mass Spectrom. 2007, 259, 8–15.

extinction coefficient for rhodamine 6G.22 This was plotted against the area from which the sample was collected. The area was calculated by multiplying the diameter of the DESI spot size (≈1 mm2) with the length over which the sample stage was moved during collection. As can be seen from Figure 4b the amount of rhodamine collected increases linearly with the area that was sprayed. e) Analysis Options for SDC - Collected Material and Illustration of Applications. After spray-desorption-collection, the collected material can easily be analyzed by the closely related technique of DESI that operates on similar principles of microsolvation and desorption. Figure 5 shows a comparison of the DESI-MS analysis of freshly squeezed orange juice after collection by SDC to a direct analysis of the same sample by DESI-MS. The primary sample surface was prepared by sonicating a strip of porous PTFE in 2 mL of freshly squeezed orange juice which was air-dried before direct DESI-MS and SDC-DESI-MS analyses. For the SDC-DESI-MS analysis approximately 40 mm2 of the surface was subjected to the spray, and the desorbed material was collected onto a second PTFE surface. Both this second surface and the original surface were then analyzed by DESI-MS, and the results are displayed for comparison in Figure 5. The overall increase in signal intensity after collection can clearly be seen across the entire mass range of the spectrum. The intensities of some peaks such as those at m/z 198 and m/z 381 which were close to the level of background ions in the direct DESI-MS analysis increased to intensities well above the noise in the analysis of the collected sample. Some differences in the relative abundances of components in the directly analyzed sample compared to the collected sample can be observed. For example the ion at m/z 144, which is the base peak in the direct analysis increases in intensity by a factor of 20, while the ions at m/z 116, 219, and 381 increase 80, 150, and 130 times, respectively. Solubility in the spray solvent plays a large role in DESI desorption efficiency. However, analyte solubility and desorption should be similar in (22) Du, H.; Fuh, R.-C. A.; Li, J.; Corkan, L. A.; Lindsey, J. S. Photochem. Photobiol. 1998, 68, 141–142.

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Figure 7. Analysis of nanoparticles by SDC-UV. The black curve indicates the absorbance of the directly analyzed sample, while the gray curve indicates the absorption spectrum of the spray-desorption collected sample.

Figure 6. a. The analysis of a men’s fragrance ‘Realm’ by directly spraying some 50 µL of the perfume into the auto sampler vial followed by dilution with ethanol (top chromatogram), compared to the analysis of the same sample sprayed onto a glass plate followed by SDC into a vial before GC-MS analysis. b. The ratio of the normalized intensities of the SDC-GC-MS to direct GC-MS analyses is plotted against the GC run time in minutes and the elution temperature in Celcius.

both SDC and DESI as long as the same solvent is used in both cases. Volatility generally increases with molecular mass. Some discrimination based on volatility is evident in the GC-MS analysis of a perfume sample (Figure 6). However, for polar and ionic compounds ionizable by ESI and therefore DESI, lower mass does not necessarily correlate with increased volatility. Further investigation into the relative collection efficiency of analytes is underway. Since desorption and collection is decoupled from ionization and analysis when doing SDC, the desorbed material need not be analyzed exclusively by DESI. In addition, we demonstrate that analyses by GC-MS and UV spectrometry are possible after SDC and many other methods of analysis should also be possible. GC analysis after SDC is demonstrated here by the analysis of a perfume sample, which was sprayed onto a glass slide. The slide was interrogated by the desorption spray, and the surface material was collected into a GC autosampler vial. This was achieved simply by placing the autosampler vial into the direct path of the spray leaving the surface. Ethanol (250 µL) was added to rinse the collected analytes to the bottom of the vial. Ten µL of this solution was subsequently injected into the GC-MS. A comparison of results obtained by directly diluting the sample in 1678

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ethanol before GC-MS analysis is shown in Figure 6a. Figure 6b shows the ratio of the intensities for the major peaks in the two chromatograms versus elution temperature. This ratio was calculated by dividing the intensities for the peaks obtained by SDCGC-MS analysis to that obtained by the direct GC-MS analysis. Intensities were normalized to the base peak in each chromatogram. The base peak in both chromatograms was the same. In Figure 6b, a value for a particular ion of less than one indicates a higher relative abundance in the GC-MS analysis compared to the SDC-GC-MS, while a value greater than one indicates a higher relative abundance in the SDC collected sample. From Figure 6b we can deduce that there is a volatility dependence on the collection efficiency of components and discrimination against the more volatile compounds can be observed. This is likely due to the high flow rate of nebulizing gas that is used during the collection process which blows off some of the more volatile components. Headspace analysis will be more suited for the analysis of these highly volatile compounds, while SDC-GC-MS could be better suited to the analysis of less volatile components. In Figure 7 we demonstrate the UV analysis of TiO2 nanoparticles after collection by SDC. A sample containing TiO2 nanoparticles with an average size of 20 nm was deposited on an acrylic surface and collected by spraying water at 10 µL/ min. The desorbed material was collected into a second acrylic UV cuvette. The cuvette was filled with a methanol-water solution, and the absorbance of the collected material was measured. This was compared to that obtained by measuring the UV absorbance of a sample prepared by weighing out a similar amount of nanomaterial directly into the cuvette followed by dilution. As can be seen from Figure 7, similar absorbances were obtained irrespective of whether the particles were collected by SDC or directly analyzed by UV. CONCLUSION A general method for collection of chemical species from surfaces is demonstrated. Surface material can be collected noninvasively from field samples by directing a pneumatically assisted spray at the sample surface. Large areas can be scanned and collected onto a small collected area, which allows for preconcentration of low abundance material before subsequent analysis.

Spray-desorption-collection is especially suited for subsequent analysis by desorption ionization methods in mass spectrometry such as DESI. However, as shown, the method is not limited to analysis by MS techniques but can also be used for the collection of samples for subsequent analysis by other separation and spectroscopic methods. The optimized conditions for SDC were different from those typical for a standard DESI analysis. The constraints on solvent and gas flow rates placed on the analysis by the abilities of the vacuum system and the influence of space charge effects on sampling through the atmospheric pressure interface of the mass spectrometer need not be considered for collection. Further, collection efficiency does not depend on ionization efficiency, usually dependent on efficient solvent evaporation and ion desolvation, as can be deduced from the lack of influence observed

for the spray voltage and the negative effect increased desolvation gas has on the collection efficiency. These observations also increase our understanding of the desorption and ionization mechanisms in DESI, where it is now beginning to appear that optimized DESI conditions usually require a trade-off between those settings which effect efficient desorption and those for efficient ionization. ACKNOWLEDGMENT The authors thank Western Michigan University for funding.

Received for review September 7, 2009. Accepted January 27, 2010. AC902013X

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