Soft-Landing Ion Mobility of Silver Clusters for Small-Molecule Matrix

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Soft-Landing Ion Mobility of Silver Clusters for Small-Molecule Matrix-Assisted Laser Desorption Ionization Mass Spectrometry and Imaging of Latent Fingerprints Barbara L. Walton and Guido F. Verbeck* Department of Chemistry, University of North Texas, Denton, Texas 76201, United States ABSTRACT: Matrix-assisted laser desorption ionization (MALDI) imaging is gaining popularity, but matrix effects such as mass spectral interference and damage to the sample limit its applications. Replacing traditional matrices with silver particles capable of equivalent or increased photon energy absorption from the incoming laser has proven to be beneficial for low mass analysis. Not only can silver clusters be advantageous for low mass compound detection, but they can be used for imaging as well. Conventional matrix application methods can obstruct samples, such as fingerprints, rendering them useless after mass analysis. The ability to image latent fingerprints without causing damage to the ridge pattern is important as it allows for further characterization of the print. The application of silver clusters by soft-landing ion mobility allows for enhanced MALDI and preservation of fingerprint integrity.

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Most commonly, nanoparticle synthesis a solution-based process using methods that involve numerous chemicals (salts, reducing agents, solvents) and a lengthy procedure, sometimes requiring up to 4 days for complete reaction and drying.31,35−38 Therefore, to minimize sample preparation for MALDI, the ability to produce particles and deposit material directly onto analytes of interest is essential. Silver is known to be very sensitive to oxidation, which can alter its effectiveness for different applications.39−46 Surfaces analyzed up to months after deposition did not show evidence of oxidation. Particle production by laser ablation in liquids and gases has been studied, but this method still requires liquids or some form of stabilizer when the material is collected. 47 Magnetron sputtering of silver targets has been used to implant Ag particles into tissues, but with 500 eV kinetic energy.34 Highenergy ion beams have also been used to implant Au clusters into peptide samples;48 however, both of these instances employ much higher energies than deposition with the instrumentation described in this paper. Pulsed laser deposition has gained popularity to grow films with a variety of properties.49 Besides thin films, laser ablation has also proven to produce clusters of ions when aided by the presence of a background gas of at least 1 Torr.49−55 Ion mobility adds the potential capability to select clusters with the use of split rings that can control ion direction toward or away from the landing surface. Selecting clusters for deposition where specific sizes are necessary can be accomplished by use of split ring optics. Combination of cluster formation via laser ablation with the

ince its inception, matrix-assisted laser desorption ionization (MALDI) has characterized a great number of molecules, from organics to proteins.1−6 While MALDI is one of the most widely used ionization techniques in mass spectrometry, it is not without challenges. Crystal homogeneity is important to the reproducibility of spectra from sample to sample, as well as consistent signal.7 Another big concern is the interference of matrix in portions of the mass spectra that could overshadow the analyte of interest.8 Traditional organic matrices include a variety of aromatic compounds such as derivatives of cinnamic and benzoic acids.2−4,6,9 These different derivatives prove more beneficial for different analytes. The development of alternative matrices has become increasingly popular due to the aforementioned problems along with limited applicability of solvent-based matrices.7,9−15 Some improvements in MALDI have been achieved by employing porous silicon surfaces or silicon films,16−18 metallization of surfaces (with or without organic matrix) by sputtering or evaporation,13,19,20 carbon nanotubes,21,22 graphite,23 graphene,24 titanium oxide nanoclusters,25 and various metallic nanoparticles26−34 as matrices. It has been demonstrated that energy absorption is independent of wavelength for nanoparticles, which makes them an interesting potential matrix. Increased particle size could lead to increased absorption.26 Nanoparticles also have the tendency to be more flexible in terms of sample preparation (pH, solvents, salts, etc.).26 Metallic particles also are advantageous because they limit the amount of extra analyte introduced into the spectra compared to organic matrices. Some organic matrices add multiple peaks to the resultant spectra from being ionized themselves and can interfere with the analyte of interest. © 2014 American Chemical Society

Received: March 25, 2014 Accepted: July 10, 2014 Published: July 10, 2014 8114

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fingerprint, making it useless after analysis. Although there has been some work to show that MALDI is nondestructive,63 it requires washing of the print after analysis. While this method would remove the leftover matrix and leave some of the endogenous lipid content on the surface, it might still remove other soluble material that may be of interest. Some work has been done to image a lifted print, usually with the addition of a magnetic dust on the surface.73−75 The method proposed here would allow for deposition and analysis without damaging the print, allowing for further analysis such as specific site extraction with a manipulator76 and powder developing without needing to wash away extra matrix.

technique of soft-landing ion mobility (SLIM) allows for the creation and deposition of intact ion clusters onto a substrate. Soft landing (SL) is a technique most commonly used for the purification, isolation, and characterization of ionized compounds.56−60 Ion−surface interactions are commonly studied via SL, followed by some type of analysis to observe any selfassembly or aggregation that occurs at kinetic energies between 10 and 100 eV. Typical instruments have a mass spectrometer component that allows for the separation of ions; however, the instrument built and used here, is composed of a drift tube that works to thermalize ions and ion clusters as they travel toward the landing substrate.61 This instrumentation allows for the landing of species at kinetic energies below 1.0 eV, eliminating fragmentation of soft-landed species. Ion deposition occurring over this energy regime also allows for the deposition of ions onto unmodified surfaces with limited (if any) translational motion across the surface.61 Figure 1 depicts soft-landing deposition onto a surface for further characterization.

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EXPERIMENTAL SECTION Materials and Methods. Melatonin (Sigma−Aldrich, St. Louis, MO) was chosen for this work to establish SLIMMALDI as a viable technique for the analysis of low molecular weight compounds due to interest in illicit drug and pharmacological applications. Solvents and additives used included acetonitrile (ACN; Sigma−Aldrich, St. Louis, MO), trifluoroacetic acid (TFA; Sigma−Aldrich, St. Louis, MO), and 18.2 MΩ Millipore water (Millipore, Billerica, MA). Solutions (1 mg/mL) were spotted (1 μL) on stainless steel MALDI slides (Thermo Fisher, San Jose, CA) and allowed to dry. Slides were transferred to the SLIM chamber for particle deposition. Silver nanoparticles were deposited via laser ablation of a silver rod (99.999% purity, SPI Supplies, West Chester, PA) with a 532 nm Nd:YAG laser (Minilite, Continuum, Santa Clara, CA) at 1 and 2 Torr He buffer gas for varying deposition times of 30, 60, and 120 min. Deposition carried out in this manner is most similar to the two-layer (double layer) method,77,78 which is a combination of the dried droplet,79 crushed crystal,80 and fast evaporation methods;81 however, the analyte samples spotted here do not include any organic matrix, as the silver nanoparticle matrix is deposited on top of the sample. After landing, slides were analyzed on a Thermo MALDI LTQ Orbitrap XL (Thermo Fisher Scientific, San Jose, CA). A nitrogen laser (337 nm, 60 Hz) was used for the MALDI process. Slides were also prepared with a 2.5−5 mg/mL solution of α-cyano-4-hydroxycinnamic acid (CHCA) as the matrix [1:1 (v/v) CHCA/ACN with 0.1−1% TFA in H2O), which was spotted in a 1:1 ratio with the sample of interest. The MALDI laser was operated between 5 and 40 μJ, with each collection containing 50 scans, 1 microscan/step, and 100 μm step size. All spectra were collected in positive mode in either linear trap mode or orbitrap mode. Resolution was set to at least 30 000 in the method. Fingerprints were volunteered by members of the laboratory and deposited on stainless steel slides after they touched their faces and necks to ensure sebacous oil transfer. Doped fingerprints were collected after the finger was pressed into a small amount of known compound (approximately 0.5−1.0 mg). Tryptamine (Sigma−Aldrich, St. Louis, MO) and caffeine (Alfa Aesar, Ward Hill, MA), along with melatonin, were used as dopants for fingerprint analysis. Silver deposition occurred over the course of 1 h at 1 Torr He buffer gas. MALDI-MS images were collected at 5−12 μJ nitrogen laser energy and 0 sweep laser shots, with 100 μm steps. Automatic gain control was used for some images, while others were collected with automatic gain control off and with 1−5 laser shots. Spectra were analyzed by Xcalibur (Thermo Fisher Scientific, San Jose, CA), and images were used from ImageQuest Software (Thermo Fisher Scientific, San Jose, CA).

Figure 1. Schematic of soft-landing ion mobility (SLIM) deposition, using laser ablation and a resistively coupled ion mobility cell for creation and deposition of metal clusters.

While matrix is traditionally applied to samples in the liquid phase by a droplet overlayer method, the method proposed here eliminates the need for a solution-based matrix, limiting sample preparation time. This is critical for samples that may be prone to degradation in ambient conditions, as both sample and matrix would require time to dry. Another advantage to the SLIM matrix deposition is its lack of low mass interference in the mass spectra commonly seen with organic matrices. Our method could be comparable to surface-assisted laser desorption ionization (SALDI) where a liquid matrix is not employed, and rather a modified surface assists in ionization.29,62 These advantages make SLIM deposition a possible technique for forensic applications, in particular fingerprint imaging. Fingerprints have been analyzed by several different methods including secondary-ion mass spectrometry (SIMS), infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), gas chromatography/mass spectrometry (GC/MS), high-performance liquid chromatography (HPLC), and MALDI-MS.63−66 All of these methods can provide information, but each has drawbacks of either time constraints or sample preparation methods. MALDI matrix application typically was done via spray-coating of both traditional and nontradiational matrices,65,67−69 or a “wet−dry” method where a solid is dusted over the print and then solution is applied.70−72 Most of these methods are destructive to the 8115

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Figure 2. SEM images of silver deposition after 60 min of landing at 2 Torr: surface after ablating silver at (A) 6.25, (B) 12.5, and (C) 18.75 mJ. This shows that increasing the laser energy for ablation of the silver rod increases the amount of silver that is ablated and therefore deposited onto the surface. It can also be seen that the particle size exhibits more variance with higher energy.

Figure 3. MALDI-MS of melatonin, 1 mg/mL with 1% TFA, with (A) Ag deposition and (D) CHCA matrix, both collected at 15 μJ laser energy, with automatic gain control on and 5 sweep laser shots (LTQ). Also shown are 1 mg/mL tryptamine with 0.1% TFA with (B) Ag deposition and (E) CHCA matrix, both collected at 10 μJ laser energy, and 100-fold dilution of the tryptamine sample with (C) Ag deposition and (F) CHCA matrix, both collected at 10 μJ laser energy, with automatic gain control on and 5 sweep laser shots (LTQ-Orbi).

Soft-Landing Ion Mobility Deposition. The soft-landing ion mobility (SLIM) instrument used for deposition is composed of five concentric stainless steel rings, which are resistively coupled. Each ring has an outer diameter measuring 2.0 in. and an inner diameter measuring 1.25 in., connected by 5 MΩ vacuum-compatible resistors (Caddock Electronics, Riverside, CA). The mobility cell is held together between two guard rings, with 0.05 in. spacing between rings provided by sapphire spheres. The whole assembly is housed in a vacuum chamber equipped with the laser ablation ionization source. A 3 N silver rod is placed at the end of the mobility cell in front of an ion deflector plate held at a constant voltage to repel ions forward into the drift region. The chamber is equipped with

two 6 in. ConFlat quick doors to allow access to the landing region to easily remove samples. Also with this configuration, multiple types and sizes of surfaces can be used. A voltage of +190 V was applied to the backing plate to encourage ion direction toward the landing surface. A field of about 22.8 V/ cm is applied to the drift cell. A 532 nm Nd:YAG laser is moved across the surface of the metal rod in a raster pattern with spot size ∼50 μm. A variety of deposition parameters, such as pressure, deposition time, and laser energy, were initially studied to determine the optimal conditions for particle deposition.51,53,55 A particle characterization study was first done to determine changes in particle size or distribution under different parameters. Pressure was varied between 1 and 10 8116

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Figure 4. Images of an undoped fingerprint example. (A) Photograph of fingerprint after it was impressed onto a stainless steel slide. (B) Image scanned by the MALDI camera, showing the outline of the area to be imaged. (C) MALDI image created by use of m/z 368 for the selected area. (D) Micrograph of the developed print showing the intact ridge pattern.

system is also equipped with two split rings whose voltage can be manipulated to direct ions toward or away from a surface. Half of this steering optic is coupled to the mobility cell, while the other half is connected to a pulsing switch.61 This technique can be used to select particular species from the mobility spectrum, and a pulse generator can direct those particular ions to the surface.

Torr, deposition time between 1 and 60 min, and laser energy between 6.25 and 18.75 mJ. Slides were stored in plastic slide cases after deposition, with analysis usually occurring within 24−48 h after deposition. Analysis of fingerprints as long as 1 month after soft landing still provided quality images. Scanning electron microscopy (SEM) was utilized for the analysis of pre- and post-landed substrates to determine optimal deposition parameters for specific sized particles. Energydispersive X-ray spectroscopy (EDX) was used to determine that the spherical particles on the surfaces did consist of Ag. There was no evidence of oxygen in the particles even after months of storage. Initial landing studies on piranha-cleaned silicon surfaces were conducted and compared to deposited particles on the stainless steel MALDI slides to determine whether substrate material affects particle deposition. Constantpressure laser ablation has been shown to allow control over the size of particles produced, where the size of particles increased with an increase in pressure.53 Carbon clusters have been formed by laser ablation in the presence of argon, where C1+, C2+, and C3+ ions were observed.50 It has also been demonstrated that the ablation of a binary material, such as InP, produces clusters of InmPn in He carrier gas.54 While time and He pressure variations had an effect on the landed particles, the difference in laser fluence showed a more drastic change in particles. Laser ablation of the silver rod was done with the Nd:YAG laser operating at 6.25, 12.5, and 18.75 mJ, and as expected there was a significant increase in the amount of silver on the surface with increasing laser energy. Figure 2 shows SEM images at each laser energy, landing for 60 min and 2 Torr. It appears as though there are no particles on the surface landing with 6.25 mJ laser energy; however, it was confirmed by inductively coupled plasma mass spectrometry (ICPMS) that silver was indeed on the surface but the particles were below the magnification limits of the microscope. When the pressure was kept constant, there was some increase in particle size for varying deposition times, but overall the main factor for particle size was the laser energy. Particles deposited at 12.5 mJ laser energy were between 700 and 1000 nm in diameter, while particles produced at 18.75 mJ were as large as 1700 nm. Surface coverage of silver on the substrate naturally increased over the course of increasing deposition time, no matter which pressure or laser energy was being used. Surface coverage did decrease with increasing pressure overall, however. Therefore, to best cover the samples it was determined that lower pressures would be the most advantageous, as higher pressures increased the likelihood of diffusional broadening of the ions as they travel down the drift tube toward the surface. Our SLIM

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RESULTS AND DISCUSSION Soft-landing silver particles on top of spots of melatonin has shown improved signal, especially over the course of increasing energy of the nitrogen laser. With traditional CHCA matrix, there is significant fragmentation and nearly complete loss of the molecular ion peak even at low laser energies. Another advantage to the silver soft-landed matrix is the lack of organic background noise, as can be seen for CHCA. Figure 3 compares the soft-landed matrix samples with CHCA spotted samples for melatonin and tryptamine. Varying concentrations of analyte and TFA were studied; shown here are 1 mg/mL with 1% TFA solution of melatonin (Figure 3A,D), 1 mg/mL with 0.1% TFA solution of tryptamine (Figure 3B,E), and the 100-fold dilution of the tryptamine solution (Figure 3C,F). The comparison shows that while intensity of the fragment peaks may be higher, the molecular ion of melatonin, m/z 232, is nonexistent with CHCA matrix even at a low laser energy of 15 μJ. The signal-to-noise ratio (SNR) is higher for the Aglanded spectra (Figure 3A) than the CHCA (Figure 3D) (111.56 vs 0.2357) for this peak. At this laser energy, fragments of melatonin observed in both the Ag and CHCA spectra include m/z 189 (-C2H3O), 173 (-C2H4NO; α-cleavage), and 160 (-C3H6NO; β-cleavage), with an additional fragment occurring at m/z 146 (-C4H8NO). These additional fragments are also observed in the spectra for the samples coated with silver, but only when the nitrogen laser is increased to at least 50 μJ. As the laser energy is increased up to 50 μJ, not only does the melatonin peak remain but also an increase in intensity is observed. Similar behavior was observed for samples with 0.1% TFA, as well as dilute melatonin samples. In Figure 3A there are ions present that correspond to the alkali adducts, [M + Na]+ and [M + K] +, and also an [M + 16]+ peak that may correspond to oxidation of the molecular ion. Figure 3 also shows the increased SNR for a tryptamine sample of similar concentration (panels B and E) as well as for the 100-fold diluted tryptamine sample (panels C and F). Here m/z 160 is the molecular ion peak for tryptamine, and subsequent fragments m/z 144 (-NH2; α-cleavage) and 130 (-CH4N; βcleavage) are observed. For the dilute solution (3F), the peaks 8117

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present are from the matrix and overshadow any possible analyte peaks. This work led to a consideration of Ag deposition for MALDI imaging applications. The ability to observe low mass compounds without any significant interference from matrix is critical for an array of samples, but especially for those in low concentrations. The more dilute a sample is, the harder it will be to detect. Fingerprints are such samples, where the chemical makeup is limited to anything that was transferred in the brief moment of contact with a surface. Latent fingerprints are potentially full of information, but collecting chemical evidence without destroying the print in the process is challenging. By use of SLIM deposition of silver, the ridge pattern can be maintained and a variety of analytes can be detected. Since silver deposition does not visibly alter the print, as shown in Figure 4, it allows for post-MALDI examination. It can be seen that at all stages of print analysis the ridge pattern is detectable and generally undamaged. Silver deposition, unlike solutionbased methods, does not mask the print, making it available for a development by traditional powder dusting techniques. The ridge detail within the imaged area is not completely destroyed, as seen in the micrograph in Figure 4D. Fingerprints for this study were volunteered by three individuals. Prints from each person will be referred to as individuals 1, 2, and 3 when necessary. Analysis of the mass spectra collected over the image shows that the ridge pattern is composed of a variety of compounds, some endogenous and some exogenous. Some endogenous species include fatty acids (FAs), diacylglycerols (DAGs), and triacylglycerols (TAGs). Some unsaturated FAs, such as oleic acid (m/z 283.2628) and palmitoleic acid (m/z 255.2099), were observed in the ridge patterns. Also present were some polyunsaturated FAs including the ω-6 compounds eicosadienoic acid (20:2; m/z 309.2787) and dihomo-γ-linoleic acid (DGLA, 20:3; m/z 307.2638) and the ω-9 compound 11-eicosenoic acid (20:1; m/ z 311.2941). These results are similar to previous reports where protonated species were detected.71 The ridge patterns for individuals 1 and 2 were composed of m/z 368.4243, which is attributed to cholesterol with a loss of water.63,82 Also detected in the ridge pattern of these two individuals were m/z 550.6277 and 304.2993, corresponding to dimethyldioactadecylammonium and dimethylbenzylammonium, respectively. These two compounds are commonly found in hair products, antibacterials, and toiletries.63,67 The amount of these compounds varied between persons and days as individuals use different products, which may alter the presence of these species. Fingerprints of two individuals were also impressed on the same slide with portions overlapping, and were able to be differentiated on the basis of the chemistry of each print. Figure 5 shows an image containing two prints, with some compounds being detected only in one print or in differing intensities between the two prints. It can be seen that m/z 368.4243 is present in both individuals 1 and 2; however, the intensity of this compound does vary (Figure 5A). Also observed were DAG and TAG species in different intensities between all three individuals. A DAG species was primarily in individual 2 (Figure 5B), while TAGs were better seen in individual 1 (Figure 5C). The ability to determine different chemistries between sets of prints and conserve the integrity of each print can enable law enforcement to eliminate suspects and potential threats. “Doped” fingerprints were also imaged to determine if additional chemistry added to individuals’ fingers prior to impression onto the slide could be detected. Fingerprints were

Figure 5. Two fingerprints from different individuals. (A) Selected m/ z 368.4243 is clearly present for individual 1, while for individual 2 this peak appears with less intensity (top left of image). (B) Selected m/z 602.5977 is apparent only for individual 2. (C) TAG distribution observed within the print of individual 1, where negative signal corresponds to the negative ridge pattern of the fingerprint.

doped with small amounts drugs, such as tryptamine, melatonin, and caffeine (approximately 1 mg). After application of the compound to the finger, the latent fingerprint impressed onto the slide had voids in the ridge pattern where the drug was located. The amount of illicit materials transferred to the slide is significantly less than the amount applied to the finger itself, in most instances just leaving a residue after impression. These voids were much larger with tryptamine due to its crystalline structure. Images of these prints show the ridge pattern with visible empty cavities. Upon investigation of the corresponding mass spectra for those cavities, it was found that within these voided areas there is evidence of the particular drug. Figure 6 shows the image of a portion of a doped fingerprint. The ridge pattern shown here is imaged by use of m/z 368.4243, a dominant compound prevalent in the print of individual 1. Overlaid on the ridge pattern is another image for m/z 144.0794. This corresponds to a fragment of tryptamine and appears in and around the void areas of the ridge pattern. The fragment is known as the α-cleavage; the β-cleavage is also present at m/z 130.0638. These fragment patterns are common for tryptamines.83 Areas of melatonin and caffeine were also identified on print impressions doped with those compounds.

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CONCLUSION Utilization of SLIM to deposit silver clusters as a MALDI matrix has shown significant benefit over the traditional CHCA matrix. Silver provided an increased signal-to-noise ratio compared to traditional CHCA for melatonin and tryptamine, and it has shown promise for imaging applications. Forensic evidence relies heavily on imaging latent fingerprints, primarily 8118

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Figure 6. MALDI-MS image showing ridge pattern and chemical detail. The ridge pattern (green) can be seen clearly and is composed to m/z 368.4243. The white areas overlaid correspond to m/z 144.0794, which is a fragment of tryptamine. Also shown is the mass spectrum of one of the white areas, showing the dominant fragment, as well as m/z 160.0741, which is the mass of tryptamine.

developing and performing tape lifts in order to collect and preserve them. The ability to analyze the chemistry of a print and retain the ridge pattern is a significant improvement in the preservation of prints used for chemical imaging. Silver deposition by soft-landing ion mobility allows for this, as the ridge pattern is not interrupted by the deposition or subsequent analysis. Prints from different individuals have been able to be imaged, showing some differences between patterns and chemical makeup. Varying the parameters of the soft landing allows for different particle size and amount of silver that is deposited. This type of deposition is nondestructive to the sample and ensures that trace amounts of chemicals that may be on the sample (illicit chemicals on a print) are not disturbed or removed by solution-based techniques or dusting of matrix or particles into the surface. The SLIM instrument can be used to ablate and deposit various metals that may also be effective MALDI matrices. Future work will involve investigating more metals for deposition and different classes of compounds to analyze and image.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the members of the laboratory who volunteered fingerprints for this study, and Stephen Davila and William Hoffmann for their previous work on soft landing. We also thank the Toulouse Graduate School at the University of North Texas for providing B.L.W. with a fellowship to support her work.

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REFERENCES

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Analytical Chemistry

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dx.doi.org/10.1021/ac5010822 | Anal. Chem. 2014, 86, 8114−8120