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Atmospheric Pressure Sampling for Laser Ablation Based Nanoscale Imaging Mass Spectrometry: Ions or Neutrals?† Liang Zhu, Johannes Stadler, Thomas A. Schmitz, Frank Krumeich, and Renato Zenobi* Department of Chemistry and Applied Biosciences, ETH Zu¨rich, 8093 Zu¨rich, Switzerland ReceiVed: June 6, 2010; ReVised Manuscript ReceiVed: August 25, 2010
Although the ratio of neutrals-to-ions in a typical laser ablation event was reported to be on the order of 1000 or greater, most imaging mass spectrometry (IMS) studies collect the minor ionic component instead of the abundant neutrals for subsequent mass analysis. In this report, we present a fundamentally different strategy, sampling neutrals from atmospheric pressure laser ablation into the vacuum of the mass spectrometer, followed by postionization, and mass spectrometric analysis. We compare its overall sampling efficiency (transfer efficiency + ionization efficiency) with that of ion sampling. The products from many single ablation events (creating a crater of ∼1 µm in diameter each) were deposited on a collection plate placed on the vacuum side of the sampling capillary. The sample surface and the collection plate were carefully examined both prior to and after laser ablation with scanning electron microscopy (SEM) and scanning probe microscopy (SPM). Volumetric measurements gave an estimate of the sampling efficiency of (1 ( 0.7) × 10-4 overall. It was found that with a proper collection geometry, ablated neutral molecules can be efficiently directed to the inlet of the sampling capillary: several % are available for further MS analysis. We observed that the fraction of the ablated mass that leaves the sample in the form of particles was not sampled into the vacuum but was instead deposited between the ablation site and the capillary inlet. By comparing our results with those of other IMS techniques using ion sampling, we conclude that the overall sampling efficiencies are similar. Advantages provided by the neutral sampling approach include a large amount of analyte available for collection, the potential for improving the ionization efficiency, and the elimination of sample pretreatment steps. 1. Introduction Analytical methods capable of acquiring micro- or nanoscale chemical information on solid samples in the form of mass spectra, such as laser ablation based imaging mass spectrometry (IMS),1–7 are of great importance for materials and life science research. In a laser ablation event, the laser pulse typically not only serves to desorb/ablate material from the sample surface but also can simultaneously ionize the ablation products, depending on the physical/chemical properties of the compounds, on the sample pretreatment (with or without the addition of a MALDI matrix), and on the laser irradiation parameters. Information about the spatial distribution is obtained in IMS by acquiring numerous mass spectra across the sample surface, either simultaneously in “microscope mode”8 or sequentially by scanning the ablation laser. In the latter “microprobe mode”, it is mainly the focal spot size of the laser beam that determines the spatial resolution of the IMS measurements. Conventional laser desorption techniques, such as laser desorption/ionization (LDI)1,9 and matrix-assisted laser desorption/ionization (MALDI),10,11 can operate at atmospheric pressure (AP), which provides easy sample access. These methods routinely achieve a spatial resolution of around 50-100 µm12,13 or even down to 25-40 µm with oversampling techniques.14 In a typical laser ablation event, many more neutral species than ions are produced, even under optimized conditions.15 Thus, the introduction of a postionization step following ablation offers a higher overall ionization efficiency and thus a better detection †
Part of the “Alfons Baiker Festschrift”. * Corresponding author. Fax: (+) 41-44-632-1292. E-mail: zenobi@ org.chem.ethz.ch.
sensitivity. Representative implementations of this concept for desorption/ablation at atmospheric pressure include laser ablation inductively coupled plasma MS (LA-ICP-MS) for imaging of elemental species,16,17 laser ablation electrospray ionization,18,19 laser desorption atmospheric pressure chemical ionization,20,21 and others.22 Their spatial resolution is defined by the diameter of the focused laser beam and is in the range of tens of micrometers.19–21 This spatial resolution is still not sufficient to address certain problems in the analysis of microelectronic elements and of subcellular structures. Recently, a custom-built scanning AP MALDI ion source has achieved a spatial resolution of ∼3 × 1.2 µm on a film of red dye under single shot conditions using tight coaxial laser focusing when coupled to a linear ion trap MS.23 Further improvement in spatial resolution of IMS techniques relies not only on the ability to locally sample analytes from a surface, which is defined by the size of laser focus, but also on the inherent sensitivity of the mass spectrometer, because the smaller the ablation spot the lower the amount of material that is available for analysis. The sensitivity of the mass spectrometer may thus in some cases be the limiting factor. Additionally, a reproducible and homogeneous matrix deposition is required for high spatial resolution MALDI imaging to avoid location-specific ion yields (so-called “hot spots”).24 In some cases, the application of matrix can lead to blurring of the localization of some sample components. Assuming that the sensitivity of the mass analysis is high enough, it is the classical optical diffraction limit (∼λ/2) that hinders further development in the spatial resolution of laser ablation based IMS techniques. To circumvent this limitation, scanning near-field optical microscopy (SNOM)25 has been
10.1021/jp105178q 2011 American Chemical Society Published on Web 09/23/2010
Nanoscale Imaging Mass Spectrometry successfully used for laser ablation on the micro- and nanoscale for IMS.26–28 Relying on either the apertureless (enhancement of radiation intensity in a confined area based on the “lightning rod” effect)29–31 or the aperture working mode (by coupling a pulsed laser into the blunt end of a tapered optical fiber probe),26–28,32 it is possible to ablate material from an area with a diameter on the order of 100 nm and obtain high-resolution topographic information about the sample in the same experiment. For example, either by using metallized AFM probes25 or through nanometer-sized tip apertures,32 craters as small as 70 nm fwhm have been produced reproducibly on a dye film. Goeringer et al.25 suggested that the ionic species produced in such near-field laser ablation could be sampled and acquired for further MS analysis. In practice, this group has acquired corresponding mass spectra from craters of ∼2 µm diamter on a layer of Rhodamine 6G,33 by directly sampling the species ionized by the same laser photons. This approach of ion sampling is also utilized in ref 23, with an auxiliary use of matrix. However, it is well established that even in the best case, many more neutrals than ions are produced from a single laser ablation event, with an ion-to-neutral ratio between 10-3 and 10-5.15,34,35 The implementation of a postionization source at the laser ablation site is not easy, due to the presence of the near-field probes or sophisticated optical components in the vicinity. To not waste the overwhelming numbers of neutral species created in the laser ablation, a sampling device that uses the vacuum drag effect has been proposed for transporting these neutral species into the mass spectrometer. By sampling the ablated neutral species created with the aid of a sharp silver needle in a sealed chamber for inductively coupled plasma mass spectrometry (LA-ICP-MS) at ambient pressure, elemental analysis of metallic alloy films on the hundred nanometer scale was claimed.36,37 However, although the authors speak of a “near-field” laser ablation experiment, the distance between the tip to the sample surface was so large (∼200 nm)37 that true near-field effects are very unlikely. The neutral sampling strategy is experimentally pursued in our laboratory for the analysis of molecular solids: with the help of a heated sampling capillary, the neutral products from nearfield ablation events are collected into the system, ionized and mass analyzed with an ion trap/time-of flight mass spectrometer. A lateral resolution of 5 µm on solid anthracene has so far been demonstrated.28 The features of this approach include the potential to sample the majority of the ablation products, a good ionization efficiency of certain compounds via electron impact (EI), e.g., compounds with high ionization potential, and no need for sample preparation, such as the addition of MALDI matrix. To further increase the sensitivity of our SNOM-MS setup and thus improve the lateral resolution, not only the characteristics of near-field laser ablation38 but also the sampling efficiency of neutral ablation products need to be investigated. The aim of this study is to get an estimate of the sampling efficiency when neutral ablation products are collected, and to compare it with results of other available nanoscale LDI-MS techniques. Herein, the sampling efficiency is defined as the fraction of solid sample material that ends up as ions inside the mass spectrometer. It is the product of the overall transfer efficiency (which includes a crucial factor, the collection efficiency) and the ionization efficiency. Optical techniques, such as light scattering methods39 and shadowgraphy,40 have been applied to study the real-time behavior of far-field laser plume propagation on various substrates, even with a sampling tube in the vicinity.41 However, in near-field experiments, the
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1007 obstruction by the near-field probes renders the observation of the laser plume virtually impossible. It is thus more informative to perform off-line measurements to obtain information on nanoscale laser ablation events with a sampling capillary in place. The importance of the local positioning of the sampling capillary with respect to the laser ablation site was evaluated. Furthermore, relying on experimental results and on theoretical calculations, a comparison of the sampling efficiency afforded by our SNOM-MS setup and that of other laser ablation based IMS techniques that sample ionic products was made. 2. Experimental Section The general concept and an introduction to the SNOM-MS instrument used here have been given in our previous work.28 Briefly, with a heated sampling capillary located close to the ablation site (∼100-200 µm), the neutral products from nearfield ablation via an aperture SNOM tip are collected, transported into the system, ionized, and mass analyzed with an ion trap/time-of flight mass spectrometer. The SNOM probes (cone angle of ∼30°) for the near-field laser ablations were fabricated using the so-called “tube etching” method with conditions reported previously.38 A frequency-tripled, Q-switched Nd:YLF laser (Triton, Spectra Physics, Mountain View, CA; 349 nm, ∼10 ns nominal pulse width) was coupled into the far blunt end of the optical fiber, whose tip aperture was kept in feedback by a commercial SNOM instrument (Topometrix Lumina, Veeco Metrology Inc., Santa Barbara, CA) equipped with a shear-force scan head (maximum piezo range 100 × 100 µm). Laser ablation by single shots was controlled by a pulse/delay generator (model 555, Berkeley Nucleonics Corp., San Rafael, CA). In the experiments, hundreds of laser ablations were done on the sample surface, by continuous laser ablation either at a given repetition rate during tip scanning (scan rate 10 µm/s, 100 × 100 pixels, pixel size 0.1 µm) or in a point-to-point mode. The “point-to-point mode” refers to performing laser ablation at individual sample locations by moving the SNOM tip from one spot to another. The solid sample was fixed on a simple sample holder tilted by 15°, to direct the ablation products toward the sampling capillary. An intermediate-polarity deactivated fused-silica GC capillary with an inner diameter of 250 µm (BGB Analytik AG, Switzerland) was used for sampling the ablation products. The temperature of this capillary was kept at ∼120 °C to prevent adsorption of material onto its inner surface. The capillary inlet was placed as close as possible to the ablation spot with a micromanipulator (Nikon Narishige MO-338, Tokyo, Japan). The relative positioning between the sample surface, the SNOM tip, and the sampling capillary inlet can be observed by two CCD cameras from the side and along the axis of the sampling capillary, respectively. Thin glass slides (22 mm × 22 mm × 0.4 mm, VWR International AG, Switzerland) were first cleaned with piranha solution (3:1 (v/v) H2SO4/H2O2) for ∼30 min, and then rinsed with water and methanol, dried, and stored for further use. For the purpose of collecting material sampled into the MS system, a piece of glass slide was put between the exit of the sampling capillary and the skimmer in the intermediatepressure stage (∼0.6 mbar), as illustrated in Figure 1. The morphology of the sample surface prior to and after hundreds of shots, and that of the glass slide with collected deposits was characterized with scanning electron microscopy (Zeiss 1530 Gemini; V ) 1 kV). AFM and confocal Raman measurements were also conducted on a dedicated AFM/Raman system (Ntegra Spectra upright, NTMDT, Moscow, Russia; 1 s acquisition time for the Raman measurements shown below)
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Figure 1. Schematic of our setup (not drawn to scale) for transporting neutral products from atmospheric pressure near-field laser ablation into the vacuum environment of a mass spectrometer.
Figure 2. AFM scan (A) and SEM image (B) of an array of 6 × 7 single shots on a R6G sample surface in the absence of the sampling capillary. The inset in B is a zoomed view of a crater produced by a single ablation event (scale bar in the inset is 400 nm).
equipped with a 100 × 0.7 NA objective (Mitutoyo, Kanagawa, Japan), a 632.8 nm HeNe laser for exciting Raman scattering, a 100 × 100 × 10 µm piezo scanner, and noncontact cantilever based AFM probes (ATEC-NC, NanoSensors, Neuchaˆtel, Switzerland) for atomic force microscopy. The system is equipped with an additional white light illumination and a CCD camera to acquire optical microphotographs with a 105 × 80 µm field of view. Unless otherwise noted, all chemicals and materials were purchased at analytical grade from Fluka (Buchs, Switzerland) and used without any further purification. Samples of Rhodamine 6G (R6G, MW ) 479.0) and brilliant cresyl blue (BCB, MW ) 386.0) were made from fine powders with a hydraulic press by applying a force of about 1 × 105 N for ∼3 min. 3. Results and Discussion This study started with an investigation of the R6G sample surface after single ablation shots in the absence of the sampling capillary, with both AFM and SEM. On the basis of previous experimental results, laser energies on the target of around ∼38 nJ/pulse (corresponding to ∼3 × 105 W/cm2, estimated for a laser pulse width of 10 ns and an illuminated area with 1 µm diameter) were found to be suitable for generating craters of
e1 µm fwhm diameter after single shots. Figure 2 shows an array of 6 × 7 craters created on the R6G surface. As shown in the AFM scan recorded afterward (Figure 2A), regular craters with a reproducible depth (∼0.6 µm) were observed, demonstrating the possibility to perform microprobe sampling from solid surfaces using this setup. Quite interesting results were obtained when the same R6G surface was examined using SEM. The conical shape of a single crater (inset in Figure 2B) suggests a Gaussian-like laser beam profile rather than a flat-top exiting the SNOM tip aperture. As shown in Figure 2B, the ablation site is surrounded by debris consisting of tiny particles (size ∼20-50 nm) and their agglomerates. Owing to the wide cone angle (∼30°) of the tip itself, which obstructs the plume propagation, the transient high vapor pressure created between the SNOM tip and the sample surface forces ablated material out to the sides. It seems that, even without the drag force induced by the vacuum inlet in place, products from single shot laser ablation events travel as far as 30 µm in horizontal direction at atmospheric conditions. The much smaller value compared to the far field ablation case (normally in the range of hundreds of micrometers42) can be explained on the basis of shock wave theory, in which a deviation from the free plume expansion occurs when the mass of the ambient background air at the
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Figure 3. (A) and (B) Optical microphotographs of a BCB surface after firing 1000 single shots on it without (A) and with (B) the sampling capillary in place, respectively. (C) and (D) SEM images of the pits from (A) and (B). The two insets in (D) show zoomed views of the top-right and the bottom-right corners of the pit. (E) Zoomed view of the rim area underneath the pit in (C). (F) High resolution AFM scan of pit from (B). Note that the dashed lines and arrow for “sampling” in (D) are only for guiding the eye.
plume periphery becomes comparable with the plume mass.43 Since the mass of ablated material in a near-field event is certainly much lower, this leads to a smaller plume expansion due to the resistance of ambient background gas. It has recently been found, by indirect observation of the deposited material on SNOM tips, that the ablation products in near-field ablation events travel up to a maximum of ∼35-40 µm in height.38 This suggests a fairly wide angular spread of the ablation plume in our case. The more pronounced redeposition of ablated material in the top right corner in Figure 2B (corresponding to the lower “elevation” on the sample) compared to that in the bottom part of Figure 2B (the higher “elevation”) is attributed to the intended direction of the ablation plume, due to the sample tilting, as shown in Figure 1. This experimental observation confirms that the laser ablation plume can indeed be coaxed to move toward the sampling capillary, facilitating more efficient sampling of the ablation products for mass spectrometric analysis. As noted before, the viscous flow induced by the pressure gradient close to the inlet of the sampling capillary continuously transports neutral ablation products from the atmospheric pressure laser plume into the vacuum of the mass spectrometer. To estimate the collection and thus the transfer efficiency, a piece of clean glass slide was mounted as a collection plate at the exit of the sampling capillary on the vacuum side, while laser ablation on the sample surface took place at atmospheric pressure. Attempts to detect anthracene on such collection plates
either by X-ray photoelectron spectroscopy or by energydispersive X-ray spectroscopy in SEM were not successful, due to the contamination by dust which is also partially carbon based. For better identification of the collected material, a sample of BCB instead of anthracene was chosen for its clear spectroscopic fingerprint. To accumulate enough material for estimating the amount of sampled substance, a large number of laser ablation events was accumulated. For the BCB sample, a total of 1000 shots were fired, with a laser repetition rate of 5 Hz during tip scanning (∼46 nJ/pulse, scan rate 10 µm/s, 100 × 100 pixels, pixel size 0.1 µm). The firing of the laser ablation in these experiments was synchronized to the tip scan. We first present a detailed analysis of the ablation site, using microphotography, AFM, and SEM imaging. As shown in Figure 3A,B, a larger hollow structure (referred to as “pit” to differentiate it from “crater” generated by a single laser ablation) on the BCB surface was formed without and with the sampling capillary in place, respectively. A clear difference between the patterns of debris redeposited around the edge of the pit is observed. Note that the blurred, purple triangular object in both microphotographs is the AFM probe, not the sampling capillary. Looking closely at the pit by SEM shows the pit itself is evenly/ unevenly surrounded by debris of ablated material without/with the sampling capillary in place, respectively, as shown in Figure 3C,D. The slightly trapezoidal (instead of square) shape of the pit might be caused by redeposition of ablated material from previous ablation events while the pit was formed. This
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Figure 4. Optical microphotograph (A), intensity map of the Raman band at 582 cm-1 (B), and high resolution AFM scan (C) of the material on the collection plate. This particular plate collected the material from the laser ablation events, producing the pit shown in Figure 3B. The insets in (A) are zoomed views of the center and the edge of the collection area by SEM (the SEM image of the edge is rotated clockwise by 90°).
explanation is supported by the zoomed views of both the topright and the bottom-right corners, as shown in the two insets in Figure 3D. It is evident that the top-right corner of the pit is heavily coated by layers of material. As observed in Figure 3B,D, there is a gap in the otherwise circular redeposition pattern toward the direction of the sampling capillary, indicating the successful collection of ablated material there. The redeposition of ablated material in the rim area underneath the pit (as shown in Figure 3E) is hardly perceptible in Figure 3D. The spiderweblike structure in Figure 3E consists of agglomerates of particles, which are embedded within a smooth layer of material presumably originating from condensation of ablated molecules/clusters. Similar observations were made on R6G surfaces after hundreds of laser ablation shots. Furthermore, an AFM overview scan (Figure 3F) gives us an estimate of the volume of material ablated from the sample surface. By taking the average depth of several line profiles across the pit, we estimated the volume ablated from the pit on the BCB sample in Figure 3B/D/F to be 460 µm3 ( 15%. The error was derived from the uncertainty in the dimensions of the pits observed in the SEM images. The value of 460 µm3 refers to the material removed from the pit and ignores the material redeposited around it. We now turn to the estimation of the volume of material deposited on the collection plate on the vacuum side. One thing to consider is that our setup is continuously sampling dust from air, leading to a considerable amount of dust deposition on the collection plate during the measurements. To minimize the influence from dust, laser ablation on the BCB surface took place immediately after the collection glass slide had been put in place and the pressure had stabilized. Moreover, the glass slide was taken out of the system instantly after the ablation events on the BCB surface. For consistency, the glass slides with the collected products from the pit in Figure 3D were examined using different analytical techniques. Figure 4A shows a microphotograph of collected material. The scratch was created accidently; the asymmetric shape of the collection area might be caused by an asymmetry of the exit orifice of the sampling capillary. The diameter of the core collection area was found to be ∼30 µm, much smaller than the inner diameter (250 µm) of the sampling capillary. This is mainly attributed to the confinement of material on the centerline of the sampling capillary due to the gas flow and an apparently efficient
desorption of material from the hot inner capillary wall. The right inset in Figure 4A shows a SEM examination of one edge of the collection area, demonstrating a gradually fading material redeposition. The smooth structure of redeposited material both in the center and at the edge of the collection zone may indicate that the collection originated from condensation of ablated molecules/clusters (aggregates of molecules) instead of ejected particles. In other words, most ejected particles from the same ablation event probably were not sampled into the system. This behavior will be discussed later. Moreover, a Raman mapping of the collection area was performed. The reconstructed intensity map of the Raman band at 582 cm-1 is shown in Figure 4B, identifying the collected material as intact BCB. Afterward, a high resolution AFM scan of the same area was performed, as shown in Figure 4C. As described above, the collected material is a mixture of ablated BCB molecules and dust; thus the overall volume cannot be directly used for calculation of the collection and transfer efficiency. Efforts to subtract the volume of dust collected for the same time were not satisfactory: in control experiments, the volume of dust deposited was found to be irreproducible. Since the Raman intensities of BCB across the collection site seem similar, we assume that there is a layer of BCB of the same thickness on top of the collected dust. Therefore, the following strategy for estimating the volume of collected BCB was adopted: the average thickness of the edge of the collection zone (20 ( 10 nm, supposed to be pure BCB since dust is heavier and thus more confined to the center of the collection area) was multiplied by the total area of the collection zone (3300 ( 165 µm2), resulting in a volume of collected BCB of ∼66 ( 33 µm3. The errors are standard deviations of the dimensions determined by AFM measurements. The collection efficiency is in this case estimated to be 14 ( 7% under the assumption that all the sample material exiting the suction capillary was deposited on the collection slide. This assumption is justified on the basis of the following facts: the surface of the glass slide was put right behind the exit of the suction capillary; the BCB molecules exiting the capillary undergo expansion and fly supersonically, implying enough energy to bombard the glass slide and deposit onto the surface; the surface roughness resulting from the deposition of (irregularly shaped) aerosols is also beneficial for collecting BCB molecules. In a series of
Nanoscale Imaging Mass Spectrometry similar experiments, collection efficiencies between a few percent and 14% were determined. The value depended strongly on geometrical factors such as the relative distance between the orifice of the sampling capillary and the ablation site. We will use 10 ( 5% for further quantitative estimates below. The volume instead of the mass was calculated here, to avoid difficulties in estimating the density difference between the ablated and collected material. As mentioned before, the glass slide was put right behind the exit of the sampling capillary on the vacuum side; thus the collection efficiency estimated here is only the fraction of ablated material that gets into the vacuum before the skimmer that leads to the MS system. In this intermediate pressure region, the beam of neutral molecules undergoes expansion and passes through the skimmer before entering the ion trap for subsequent ionization via the EI source. A skimmer with a small aperture (0.5 mm) is used to extract the centerline of the beam. As suggested by Campargue,44 by setting the distance between nozzle orifice and skimmer aperture to an optimum value, the centerline intensity (s-1 · sr-1) at the skimmer aperture can be maximized. By inserting our experimental parameters used in this study into these empirical equations,45,46 we calculate the attenuation factor of background gas (air) from the capillary orifice to the skimmer aperture at optimum position downstream (∼3.5 mm upstream of the Mach disk) to be ∼10.4%. Furthermore, for the case of a seeded beam (neutral BCB molecules with air as the carrier gas), an enrichment of the heavier species on the centerline can be expected. The enrichment factor depends on the ratio of molecular weights of the heavy and the light species.47 An enrichment factor of ∼10× was experimentally estimated for our nozzle-skimmer interface for a mixture of two gases (SF6 MW ) 146.0 in CH4 MW ) 16.0).48 If we use this enrichment factor for the current case of neutral BCB molecules seeded in air, which appears justified because the MW ratio of BCB and air is very similar to that of SF6 and CH4, the above considerations suggest that the neutral BCB molecules exiting the sampling capillary will almost quantitatively enter the skimmer aperture and will be transported into the ion trap for subsequent ionization. This seems reasonable, also because of the narrow distribution of BCB molecules in the centerline of the beam that exits the sampling capillary (as shown in Figure 4A), and due to the slower radical diffusion of the heavy BCB molecules during the expansion in the nozzle-skimmer region. With a transfer efficiency of approximately (10 ( 5)% and an ionization efficiency via EI of approximately 10-3 ((50%) the overall sampling efficiency (transfer efficiency × ionization efficiency) for BCB from atmospheric pressure to the ionic form in the trap is estimated to be 10-4 ((70%) using our current setup. This can be compared with the efficiency of sampling ionic ablation products in other high spatial resolution LDI techniques for which such data are available. As shown in Table 1, the sampling efficiencies of both SNOM-MS and scanning AP MALDI-MS are similar (and actually fairly high), although their sampling strategy is fundamentally different. For anthracene,28 almost no redeposition around the ablation pit was observed, suggesting a higher fraction of molecular desorption (as opposed to particle ablation). The sampling efficiency for anthracene might thus be even higher, although no quantitative estimate was obtained. The sampling efficiency in AP LDI-MS is lower, supposedly because of the poor ionization yield of R6G without any MALDI matrix. In the case of “tip enhanced (TE)”-LA ICP-MS, its relatively high efficiency is attributed to the high transfer out of a sealed chamber whose only exit is
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1011 TABLE 1: Comparisons of Sampling Efficiencies of Different AP Laser Ablation Based Analytical Techniques with a Potential for High Spatial Resolution Imaging Mass Spectrometrya transfer efficiency SNOM-MS [this work]
ionization efficiency
overall sampling efficiency
∼0.1, neutrals from ∼10-3, via EI (1 ( 0.7) × 10-4 AP to ion trap source in vacuum
AP scanning ∼10-2, ions from MALDI-MS23 AP to ion trap
∼10-2, ion yield at AP
∼10-4
AP LDI-MS33
n/a
n/a
∼10-6
“TE”-LA ICP-MS37
50%-100%, in a sealed chamber
n/a
∼2.7 × 10-5
a
The overall sampling efficiency is taken to be the product of the transfer efficiency and the ionization efficiency. The value for “TE”-LA ICP-MS is based on the assumption of a 100% ion-tocount conversion.
to the MS system, and to an extremely efficient ionization of the elemental analytes in the ICP plasma torch. As mentioned earlier, sampling ions produced at atmospheric pressure for laser ablation based nanoscale imaging mass spectrometry is a popular choice, mainly due to the ease of ion collection. However, focusing ions at atmospheric pressure toward a vacuum aperture using only dc fields is counteracted by Gauss’s law and the subsequent divergence of ion trajectories on the vacuum side of a simple aperture.49 It has been reported that the vacuum drag force due to the pressure gradient dominates over the effects of electric fields in the vicinity of a sampling capillary inlet.50,51 In other words, optimizing the geometry in the region between the ablation site and the sampling inlet is crucial for sampling ions efficiently. In both scanning AP MALDI-MS and AP LDI-MS, the transport of ions through the metal capillary of a linear trap quadrupole (LTQ) mass spectrometer is limited by space-charge effects,52 which is of course not an issue for neutral molecules. Certainly, the capability of directing ions by electric fields and the ability of focusing ions in a vacuum with specific RF ion optics are important advantages when ions are sampled. Although the effective sampling volume within the ablation plume is enhanced by the electric field in the case of ion collection, several advantages provided by the neutral sampling approach, such as the larger fraction of analyte available for collection, possibilities for improving the ionization efficiency, and the elimination of sample pretreatment can compensate for the absence of an increased effective sampling volume. The distance between the ablation site and the sampling capillary inlet appears to be the most important parameter for efficient sample uptake for both ion and neutral sampling at atmospheric pressure. To the best of our knowledge, studies on the behavior of sampling nanoscale laser ablation plumes by vacuum drag have not been carried out, due to the difficulty of applying optical techniques capable of imaging the plume propagation at such a small scale. Therefore, we used SEM to systematically study the redeposition patterns around the ablation pit on the BCB surface with the sampling capillary positioned at various distances. Figure 5A shows an SEM overview image of a typical ablation pit after 1000 shots on a BCB surface, in which a quite long “tail” of redeposited material can be observed. The sampling capillary was in place when the pit was being formed by laser ablation. As visible in the zoomed views at different locations along the “tail” (Figure 5B-F), the redeposited material is composed of particles and their ag-
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Figure 5. (A) An SEM overview of a typical ablation pit, showing the “tailing” of redeposited material on a BCB sample surface. (B)-(F) Zoomed views at different locations of the “tail” inward to the ablation site. The scale bars in (B)-(E) are 1 µm while the one in (F) is 2 µm. Note that the symbol of the sampling capillary in (A) is only for guiding the eye.
glomerates. Furthermore, the amount and the density of this material clearly diminish with increasing distance from the ablation pit. The region where the redeposited material disappears is thought to be the position of the sampling capillary inlet during laser ablation. In Figure 5A, the measured distance d between the center of ablation pit and the position of the capillary inlet was approximately 128 µm, which is fairly close, considering the geometrical constraints in our setup: this distance corresponds to the length of a perpendicular line between the axis of the SNOM tip and the circular face of the capillary inlet. Due to the tapered shape of the SNOM tip (cone angle of ∼30°), the uppermost edge of the capillary is closer to the SNOM tip than the bottom edge. In other experiments, we put the sampling capillary inlet even closer (measured to be ∼85 µm) to the ablation site. However, this distance was so short that during the measurement, the top edge of the sampling capillary touched the SNOM tip cone. According to simple calculations using the geometry of our setup, the closest distance achievable without touching the SNOM tip in a vibrationless environment should be ∼86 µm. Another finding from our measurements is that locating the sampling capillary more than 250 µm away from the ablation site will lead to little or no ablation products on the collection plate. This emphasizes the great importance of positioning the sampling capillary properly. In practice, this is not easy to do with our current setup, which explains the poor reproducibility of the in-vacuum deposition experiments; also, most of the material on the collection plate is thought to be composed of molecules or small clusters, since these are easier to be sampled into the capillary inlet by vacuum drag effect. In contrast, ablated particles are not easily sampled into the suction capillary: these are observed between the ablation site and the capillary inlet, as shown in Figure 5. 4. Summary and Conclusions We estimated the sampling efficiency (especially the collection efficiency) of neutral products from atmospheric pressure
near-field laser ablation via vacuum drag, for ionization and mass spectrometric analysis. The collection of neutral ablated material on a trapping plate located in the intermediate pressure stage was examined and compared with the dimension of the ablation pit. This provides an estimate of the overall sampling efficiency (transfer efficiency × ionization efficiency), which was determined to be ∼10-4. It was found that by using an optimized collection geometry, although not easy to implement, ablated neutrals in molecular form can be efficiently directed to the inlet of the sampling capillary. The results presented here indicate that keeping a small distance (∼100-200 µm) between the ablation site and the sampling capillary is critical for efficient sample uptake. It was also found that ablated particles were not sampled into the vacuum but were deposited between the ablation site and the inlet. According to our estimations, the sampling efficiencies for both neutral and ion sampling in different embodiments of laser ablation MS are similar. The experimental results presented here are useful for improving the understanding of nanoscale laser ablation and guide future developments in high spatial resolution imaging MS techniques. Increasing the ionization efficiency in the mass spectrometer is another option for improving the sensitivity of our SNOM-MS setup. Secondary ionization, such as pulsed irradiation or a lowenergy radioactive source, could be employed to boost the ion yield in instruments using ion or neutral sampling. Acknowledgment. The SEM measurements were carried out at the electron microscopy center of ETH Zurich. We also thank Mr. D. Stapfer for support with the technical drawings. References and Notes (1) Hillenkamp, F.; Unsold, E.; Kaufmann, R.; Nitsche, R. Nature 1975, 256, 119–120. (2) McDonnell, L. A.; Heeren, R. M. A. Mass Spectrom. ReV. 2007, 26, 606–643. (3) Esquenazi, E.; Yang, Y. L.; Watrous, J.; Gerwick, W. H.; Dorrestein, P. C. Nat. Prod. Rep. 2009, 26, 1521–1534.
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