Microscopic Chemical Imaging with Laser Desorption Mass

Interface Anal. 1992, 18, 824-826. ... raw data are transferred via a GPIB interface to a desktop ..... virtual inverse of the C4H5+ image, confirming...
1 downloads 0 Views 169KB Size
Download PDF
Anal. Chem. 1997, 69, 3741-3746

Microscopic Chemical Imaging with Laser Desorption Mass Spectrometry Michael R. Savina* and Keith R. Lykke†

Materials Science and Chemistry Divisions, Argonne National Laboratory, Argonne, Illinois 60439

Mass spectrometric chemical images of surfaces were obtained using a laser desorption/laser postionization time-of-flight mass spectrometer. Using a single laser to both desorb and ionize surface species results in a large variability in the ion signal as the laser is rastered from spot to spot on the surface. The variation is greatly reduced when the detected species are photoionized desorbed neutrals rather than ions produced directly by the desorption laser. A Schwarzschild microscope mounted outside of the vacuum chamber is used to focus the desorption laser on the sample. Ion dispersion and detection are accomplished by a simple time-of-flight mass analyzer. The spatial resolution of the system is on the order of 1 µm. The design and use of the imaging laser microprobe mass spectrometer are presented, along with images of organic and inorganic surfaces. Mass spectrometric chemical imaging is an increasingly important surface analysis technique. The most common technique is secondary ion mass spectrometry (SIMS),1-4 in which a primary ion beam rasters a surface, resulting in the ejection of atoms, molecules, and molecular fragments. A small fraction of the desorbed material is in the form of ions, which are dispersed and detected by a mass spectrometer, usually a time-of-flight mass spectrometer (TOF-MS). In contrast, lasers, which are common desorption/ionization sources for microprobe mass spectrometry,5-7 have found relatively little use in imaging applications. One reason for this is the large spot-to-spot variability in ion yield of laser desorption/ionization compared to that of SIMS.8 The lasersurface interaction produces an ion yield that depends in a highly nonlinear way on the amount of energy absorbed by the sample. On all but the smoothest surfaces, small changes in morphology from spot to spot lead to variation in optical reflectivity, which in turn produces variations in laser energy deposition. Likewise, spatial chemical variationssthe very parameter under investigationsleads to optical absorptivity variations across the surface, again producing uneven laser energy deposition. The practical † Present address: Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899. (1) Benninghoven, A. Surf. Sci. 1994, 299/300, 246-260. (2) Benninghoven, A.; Hagenhoff, B.; Niehuis, E. Anal. Chem. 1993, 65, 630A640A. (3) Winograd, N. Anal. Chem. 1993, 65, 622A-629A. (4) Chabala, J. M.; Soni, K. K.; Li, J.; Gavrilov, K. L.; Levi-Setti, R. Int. J. Mass Spectrom. Ion Processes 1995, 143, 191-212. (5) Denoyer, E.; Van Grieken, R.; Adams, F.; Natusch, D. F. S. Anal. Chem. 1982, 54, 26A-41A. (6) Hercules, D. M.; Day, R. J.; Balasanmugam, K.; Dang, T. A.; Li, C. P. Anal. Chem. 1982, 54, 280A-305A. (7) Zenobi, R. Int. J. Mass Spectrom. Ion Processes 1995, 145, 51-77. (8) Van Vaeck, L.; Struyf, H.; Van Roy, W.; Adams, F. Mass Spectrom. Rev. 1994, 13, 189-208.

S0003-2700(97)00115-7 CCC: $14.00

© 1997 American Chemical Society

consequence of these effects is to produce a speckled or streaked image. Nonetheless, there are several examples of imaging laser desorption mass spectrometry (LDMS) in the literature. Wilk and Hercules9 obtained LDMS images of metals, organic dyes, and various iron and sulfur compounds in coal. The desorption laser position was fixed while the sample was translated from spot to spot with stepper motors. The images contained 31 × 31 pixels and had a spatial resolution of 2.5 µm. The technique has also been used to image thin-layer chromatography plates.10,11 The increased penetration depth compared to that of SIMS, coupled with the ability to incorporate the MALDI technique directly into the analytical scheme, makes LDMS particularly well suited for this type of imaging. One method of reducing the spot-to-spot ion yield fluctuations is to use a postionization technique in which ions are formed in the gas phase from atoms and molecules desorbed as neutrals. Laser3,12-14 and electron15,16 postionization have been used with ion-beam desorption for imaging applications, primarily to lower detection limits by raising useful yields (ions detected/incident primary ion). We have found that a substantial benefit of postionization for imaging with laser desorption is that the ion yield becomes far less sensitive to chemical and morphological variations in the sample. Direct ion yields are extremely sensitive to these parameters because ionization, itself a highly nonlinear process, is coupled directly to desorption. Decoupling these processes greatly reduces the spot-to-spot variations in ion yield inherent in direct LDMS and makes imaging much more feasible. In this article, we demonstrate the use of laser postionization of laser-desorbed neutrals in an imaging time-of-flight mass spectrometer. The mass spectrometer itself is quite simple, with nearly all of the optical components mounted outside the vacuum chamber. The ionization laser used in this work was a Ti:sapphire femtosecond system, the main advantage of which is that it produces high-intensity pulses at high repetition rates (up to 1 kHz), thereby decreasing image acquisition times. The spatial resolution of the system was on the order of 1-2 µm. Photopatterned films of Rhodamine 6G were used as model systems, and (9) Wilk, Z. A.; Hercules, D. M. Anal. Chem. 1987, 59, 1819-1825. (10) Gusev, A. I.; Proctor, A.; Rabinovich, Y. I.; Hercules, D. M. Anal. Chem. 1995, 67, 1805-1814. (11) Gusev, A. I.; Vasseur, O. J.; Proctor, A.; Sharkey, A. G.; Hercules, D. M. Anal. Chem. 1995, 67, 4565-4570. (12) Wood, M.; Zhou, Y.; Brummel, C. L.; Winograd, N. Anal. Chem. 1994, 66, 2425-2432. (13) Terhorst, M.; Mollers, R.; Niehuis, E.; Benninghoven, A. Surf. Interface Anal. 1992, 18, 824-826. (14) Brummel, C. L.; Willey, K. F.; Wickerman, J. C.; Winograd, N. Int. J. Mass Spectrom. Ion Processes 1994, 143, 257-270. (15) Oechsner, H. Appl. Surf. Sci. 1993, 70/71, 250-260. (16) Oechsner, H. Int. J. Mass Spectrom. Ion Processes 1995, 143, 271-282.

Analytical Chemistry, Vol. 69, No. 18, September 15, 1997 3741

Figure 1. Schematic of the imaging laser desorption mass spectrometer. The Schwarzschild microscope focuses the desorption laser and provides an optical image for the video monitor. The turning mirror is the only optical component mounted inside the vacuum chamber. A HeNe guide laser is used to locate the spot when a UV desorption laser is used.

a practical application of the system is demonstrated in the chemical imaging of a brass/rubber fracture surface. EXPERIMENTAL SECTION Apparatus. All spectra and images were obtained using a simple home-built time-of-flight mass spectrometer (shown in Figure 1). The sample holder is an electrode and is typically biased at 6-8 kV. The ion extraction region consists of two fine mesh grids. The first grid is 4 mm above the sample and is typically biased at ∼95% of the sample potential, while the second grid is held at ground. The drift tube is 1.2 m long and contains an einzel lens (not shown in Figure 1) for focusing the ion beam on an 18 mm diameter microchannel plate detector (Galileo Electro-Optics Corp., Sturbridge, MA). A single turbo pump maintains the chamber pressure at ∼3 × 10-9 Torr. The detector output is amplified and sent to a 300 MHz digital storage oscilloscope (LeCroy 9450A, Chestnut Ridge, NJ), which is triggered by a photodiode monitoring the ionization laser. The raw data are transferred via a GPIB interface to a desktop computer. Two different desorption lasers were used. The first was a frequency-doubled diode-pumped Nd:YLF (Spectra-Physics, Mountain View, CA) that produced ∼20 µJ pulses at 524 nm, with a pulse width of 6 ns, operating at a repetition rate of 300 Hz. The second was a frequency-doubled diode-pumped Nd:YVO4 (SpectraPhysics) that produced 50 µJ pulses at 532 nm, with a pulse width of 6 ns, operating at a repetition rate of 300 Hz. The 532 nm output of the Nd:YVO4 laser was frequency-doubled to give 266 nm light for some experiments. Both lasers are capable of repetition rates in excess of 1 kHz, but our slow scanning and data collection hardware made higher speed operation superfluous. The desorption laser optical path consists of six main components: a Schwarzschild microscope objective for focusing the 3742

Analytical Chemistry, Vol. 69, No. 18, September 15, 1997

beam, a diagonal mirror mounted inside the vacuum chamber to turn the beam onto the sample, a 13 cm focal length quartz lens for filling the Schwarzschild aperture, a computer-controlled scan mirror for rastering the beam (Oriel Encoder Mikes with Model 18011 controller, Darmstadt, Germany), an electronic shutter (Uniblitz, Vincent Associates, Rochester, NY) for stopping the beam while the mirror is moving between raster spots (not shown in Figure 1), and a neutral density filter for attenuating the beam (not shown in Figure 1). The shutter and scan mirror are controlled by the desktop computer via the RS-232 interface. The Schwarzschild objective has several advantages for this type of system. The large diameter of the concave mirror gives it a combination of long working distance and low f number (12.4 cm at f/2.7), allowing it to be mounted outside the vacuum chamber and still achieve a tight focus on the sample. It is achromatic, has greater depth of focus, and gives a slightly smaller focal spot diameter than a similar refractive element. Significantly, it is free of aberrations to the fifth order, which is an important consideration at low f numbers. In addition, the Schwarzschild objective provides an optical image of the sample surface. The ionization laser was a chirped-pulse-amplified Ti:sapphire system, portions of which have been described elsewhere.17 The Ti:sapphire oscillator18 is pumped by an Ar ion laser and produces 50 fs pulses with 6 nJ/pulse at a wavelength of ∼800 nm and a repetition rate of 86 MHz. The pulses are chirped in a singlegrating stretcher to give a pulse width of ∼100 ps and amplified in a Ti:sapphire regenerative amplifier19 pumped by a frequencydoubled Nd:YAG. The beam is then further amplified in a five(17) Nicolussi, G. K.; Pellin, M. J.; Lykke, K. R.; Trevor, J. L.; Mencer, D. E.; Davis, A. M. Surf. Interface Anal. 1996, 24, 363-370. (18) Asaki, M. T.; Huang, C.-P.; Garvey, D.; Zhou, J.; Kaptyn, H. C.; Murnane, M. M. Opt. Lett. 1993, 18, 977-979. (19) Squier, J.; Salin, F.; Mourou, G.; Harter, D. Opt. Lett. 1991, 16, 1986.

pass Ti:sapphire amplifier pumped by a frequency-doubled Nd: YLF prior to recompression in a two-grating compressor to give ∼100 fs pulses with 2 mJ/pulse at 300 Hz. (The maximum repetition rate of this laser system is 2 kHz.) The output beam was expanded to a diameter of 2 cm and focused 0.5-1 mm above the sample surface with a 30 cm focal length plano-convex quartz lens (f/15), leading to a nominal 1/e2 beam waist of 22 µm at focus and a focal plane intensity of 5 × 1015 W/cm2. (These are calculated values and do not include the aberrations of the focusing lens.) In practice, the focal point was rarely positioned inside the plume of laser-desorbed material, since such high intensity produced severe fragmentation of the molecules of interest (see below). The time delay between desorption and ionization lasers was typically ∼1 µs. A Questek 2500vβ KrF excimer laser was also used for some postionization experiments. The laser produced 20 ns pulses at 248 nm with ∼60 mJ/pulse at a repetition rate of 45 Hz. The collimated beam was truncated through a 2 mm iris to give a beam with an irradiance of ∼4 × 106 W/cm2. Sample Preparation. Patterned Rhodamine 6G films were prepared by contact photolithography. A saturated solution (∼20 mg/mL) of Rhodamine 6G (Exciton Rhodamine 590 chloride, purity unknown) in 2,4-pentanedione was passed through a 0.2 µm filter and spun-cast on a polished stainless steel substrate at 2000 rpm for 30 s. A 400 mesh copper hexagonal TEM grid (SPI Supplies, West Chester, PA) was laid flat on the film, and the sample was pulsed 100 times in air with the 532 nm output of a Q-switched Nd:YAG laser (8 ns pulse width) at an irradiance of 5 × 105 W/cm2. The copper grid was removed to reveal a sharp hexagonal grid (50 µm hexagons with 7 µm bars) with essentially no Rhodamine 6G in the hexagon interiors. RESULTS AND DISCUSSION Photopatterned Rhodamine 6G (R6G) films were used to investigate the spatial resolution and imaging capabilities of the system. R6G absorbs strongly in the green light emitted by the desorption lasers used in this study20 and can be photoionized with little fragmentation under low-irradiance conditions. The 524 nm direct ion (i.e., single laser) and 248 nm photoion mass spectra are shown in Figure 2. The static ion extraction grids in our system do not improve mass resolution for direct ion spectra, so, for the mass spectrum of Figure 2A, both grids in Figure 1 were held at ground potential, while the sample block was charged to 7 kV. Despite the fact that the R6G film was cast as the hydrochloride, the desorption/ionization process produces the free base parent ion at m/z 442.2, with a major fragment at m/z 428.2. Given the natural isotope peaks and poor resolution, it is impossible to determine if any of the protonated molecule (from the hydrochloride) is present at m/z 443.2. Figure 2B is the mass spectrum of R6G desorbed at 524 nm and ionized at 248 nm. This experiment was run with the sample block charged to 8 kV, the first grid charged to 7.66 kV, and the second grid grounded. The 8 kV sample bias is the maximum achievable with our power supply, and the optimal charge for the first grid was determined empirically by examining the effect of grid bias on the mass resolution of the spectrum. The ionization irradiance was kept low to avoid excessive fragmentation. The resolution is improved by the spatial (20) Green, F. J. The Sigma-Aldrich Handbook of Stains, Dyes and Indicators; Aldirch Chemical Co.: Milwaukee, WI, 1990.

Figure 2. (A) Direct ion mass spectrum of Rhodamine 6G at a desorption laser wavelength of 524 nm. (B) A 248 nm (KrF) photoion mass spectrum of the same sample at the same desorption wavelength. The KrF ionization laser intensity was ∼4 × 106 W/cm2. In the photoion case, two-stage ion extraction was used to improve mass resolution. The parent ion regions are shown as insets.

focusing effect of the grids, and the m/z 428.2 ion is still the major fragment. Figure 3 shows mass spectra of R6G desorbed at 524 nm and ionized with 100 fs pulses at 800 nm, at two different ionization irradiances. The free base molecular ion peak has the expected [M + 1]+ and [M + 2]+ isotope peaks, and the same structure is seen more plainly in the m/z 428.2 manifold. The resolution is greatly improved over the 248 nm ionization case because of the difference in temporal pulse widths. For 248 nm ionization, the full width at half-maximum of the m/z 428.2 peak in time space is 20 ns, which is the temporal pulse width of the KrF laser. In this instrument, with static ion extraction grids, the long laser pulse width sets the limiting mass resolution of the system. In the case of 100 fs laser ionization, the mass resolution is governed primarily by the spatial focusing ability of the extraction grids. In the case of the low-power ionization (1012 W/cm2), the focal point of the beam was translated along the optical axis to a point 1 cm outside the desorption plume, giving an ionization beam diameter inside the plume of 1500 µm and a mass resolution of 1500 at m/z 428.2 (Figure 3A inset). For the high-power ionization (5 × 1015 W/cm2), the focal point of the beam was positioned inside the desorption plume to give an ionization beam waist of 22 µm and a mass resolution of 2300 (Figure 3B inset). The difference in mass resolution between parts A and B of Figure 3 is due to the difference in ionization volumes. A smaller initial spread in the ion packet along the ion axis of the instrument requires less focusing by the extraction grids and, therefore, results in a higher mass resolution. However, while the mass resolution is higher for the tight-focus, high-irradiance ionization, the decreased fragmentation and greatly increased overlap between the ionization laser and desorption plume make the loosely focused, lower irradiance ionization more suitable for imaging. The ion intensity Analytical Chemistry, Vol. 69, No. 18, September 15, 1997

3743

Figure 4. Direct (A) and photoion (B) images of the m/z 428.2 signal from a photopatterened Rhodamine 6G film. The hexagons are 50 µm across, with 7 µm bars. Fifty laser shots were summed for each pixel. (A) contains 90 × 55 pixels at 2.6 µm/per pixel. (B) contains 100 × 100 pixels at 1.8 µm/pixel horizontal, 1.2 µm/pixel vertical. The desorption laser wavelength was 532 nm in both cases. The ionization laser in the photoion image was a 100 fs Ti:sapphire laser operating at 800 nm and focused outside the desorption plume as in Figure 3A.

Figure 3. Photoion mass spectra of Rhodamine 6G desorbed at 524 nm and ionized with the femtosecond laser at 800 nm at two different ionization irradiances. (A) Photoionization irradiance ) 1012 W/cm2, obtained by moving the focal plane of the ionization laser beam (f/15) 1 cm outside the desorption plume to give a beam diameter in the plume of 1500 µm. (B) Photoionization irradiance ) 5 × 1015 W/cm2, obtained by focusing the femtosecond laser (f/15) inside the desorption plume. The irradiances are calculated assuming a Gaussian beam.

is concentrated in fewer mass channels, yielding a higher signalto-noise ratio for the imaged ion. Furthermore, the larger ionization volume leads to a wider field of view, since the desorption laser can be moved farther on the surface before the overlap between the ionization laser and desorption plume is attenuated to the point at which the ion yield is noticeably diminished. In practice, we have found that, in our system, in which the desorption laser is rastered across the sample but the ionization laser is fixed in space, the desorption laser can be moved 150-200 µm off center without noticeably affecting the ion yield when the ionization laser is intentionally defocused to produce a large ionization volume. The two photoionization irradiances in Figure 3 span the range from multiphoton to tunneling ionization mechanisms. Ionization of organic molecules by highly intense subpicosecond laser pulses has been studied by several groups.14,21-24 In the multiphoton case, the molecule is assumed to absorb photons until the ionization potential is reached and the electron is ejected. In the tunneling ionization case, the electric field of the laser is so intense as to distort the molecular Coulombic field until the electron can tunnel through the suppressed barrier on the time scale of the optical period of the photon. The crossover between these two regimes is demarked by the Keldysh parameter, γ, which is the (21) Aicher, K. P.; Wilhelm, U.; Grotemeyer, J. J. Am. Soc. Mass Spectrom. 1995, 6, 1059-1068. (22) DeWitt, M. J.; Levis, R. J. J. Chem. Phys. 1995, 102, 8670-8673. (23) Ledingham, K. W. D.; Kilic, H. S.; Kosmidis, C.; Deas, R. M.; Marshall, A.; McCanny, T.; Singhal, R. P.; Langley, A. J.; Shaikh, W. Rapid Commun. Mass Spectrom. 1995, 9, 1522-1527. (24) Mollers, R.; Terhorst, M.; Niehuis, E.; Benninghoven, A. Org. Mass Spectrom. 1992, 27, 1393-1395.

3744 Analytical Chemistry, Vol. 69, No. 18, September 15, 1997

ratio of the tunneling time of the electron to the period of the photon.25 For γ < 1, the electron can tunnel through the barrier with high probability, while for γ > 1, the tunneling time is too long and the ionization mechanism is multiphoton absorption. The transition between mechanisms is far from sharp, and in the intermediate regime of γ = 1, both mechanisms must contribute to the ion formation. Figure 3 shows the ionization of R6G at 1012 and 5 × 1015 W/cm2. Assuming an ionization potential of ∼7 eV for R6G,26 this corresponds to γ ) 7.6 and 0.11, respectively. Figure 3B shows that the molecule is severely fragmented at the higher intensity. This means either that tunneling ionization is not a soft ionization process for this molecule or that there is still appreciable multiphoton absorption at this intensity (i.e., γ is too high), leading to excess energy deposition and, ultimately, to fragmentation. Indeed, as long as the laser pulse rise time is finite and the absorption cross section of the molecule at the laser wavelength is non-zero, there will always be multiphoton absorption during the rise and fall of the pulse, so that fragmentation may be unavoidable in any case. Similarly, it has been pointed out that multiphoton ionization must always occur in the lowintensity wings of the Gaussian laser beam.27 Figure 4 shows both direct and photoion images of a photopatterned R6G film. Resolution was bracketed by indirectly measuring the desorbed spot size. The desorption laser was scanned horizontally across an unpatterned R6G film, returned to the starting point, stepped a known distance vertically, and then scanned again. When the step size was smaller than the desorbed spot size, i.e., when the rows overlapped, scanning the first row removed material from the second row, and the total ion intensity of the first row was higher than that of the second row. When the step size was greater than or equal to desorbed spot size, there was no depletion between rows. In the direct ion case (Figure 4A), the relatively high irradiance necessary to both desorb and ionize the material results in a spatial resolution of between 2.5 and 3 µm. This is roughly the calculated 1/e2 beam diameter of 2.6 µm (assuming a Gaussian input beam28). The image is speckled with dim pixels due to spot-to-spot variations in direct ion yield, presumably arising from morphological variations as discussed above. In addition, the image shows intensity (25) Keldysh, L. V. Sov. Phys. JETP 1965, 20, 1307-1314. (26) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, 861. (27) Wise, M. L.; Emerson, A. B.; Downey, S. W. Anal. Chem. 1995, 67, 40334039.

gradients (bright and dim regions), which we attribute, in part, to the inability of the preobjective scanning system to maintain focus over the relatively large imaged area (∼250 µm × 150 µm). Since the Schwarzschild is not a flat-field objective, the focal plane will drift somewhat as the input beam is scanned off-axis.28 In contrast, the photoion image (Figure 4B) is free of both speckles and gradients. The spot-to-spot variation is greatly reduced, and the system is much more tolerant of the small changes in focus across the imaged region due to preobjective scanning. The spatial resolution is improved over the direct ion image of Figure 4A because the irradiance required to desorb neutrals is much lower than that required to both desorb and ionize surface species. The lower irradiance leads to a lower peak temperature and, hence, a reduced thermal gradient in the material, resulting in less diffusion of the pulse energy to regions outside the laser focus on the time scale of the desorption event. The desorbed spot diameter in Figure 4B was between 1.2 and 2.1 µm, smaller than the calculated 1/e2 beam diameter of 2.6 µm. In this case, the desorption laser irradiance was low enough that only the central portion of the laser spot was sufficiently intense to desorb the R6G. Thus, the diffraction limit need not be a lower limit to resolution for laser desorption imaging. A second consequence of the lower desorption irradiance is that the sample sustains far less damage. Fifty laser shots per pixel at the relatively high irradiance necessary for direct ion imaging was sufficient to remove nearly the entire film. In the photoion image, 50 shots/pixel removes a noticeable amount of material, but enough remained so that the scanned region could be imaged several more times. It should be noted that the use of an ion beam for imaging would damage the surface far less than the laser, and the sample could be imaged an indefinite number of times. The acquisition time of the images was long, about 2.5 h/100 × 100 pixel image, due to a slow scan mirror and slow data transfer from the oscilloscope to the computer. The time could be reduced to a few minutes by replacing the mirror’s motorized scan mount with a pair of galvanometers and the oscilloscope with gated integrators. A practical application of the system is the imaging of a brass/ vulcanized rubber fracture surface in Figure 5. The sample was prepared by placing a layer of unvulcanized natural rubber between two brass sheets and then vulcanizing the rubber with heat and pressure to form an adhesive joint. The brass strips were gripped and pulled parallel to the joint, to produce the fracture surface of Figure 5. The optical image (Figure 5A) shows a dark diagonal band of thick rubber, which was previously adhered to the other brass sheet, and which is flanked on both sides by a brass/rubber interface. The sample thus provides an opportunity to study both the brass and rubber failure surfaces. The mass spectra of the rubber surface and brass surfaces show several differences. The rubber spectrum is shown in Figure 6A and is dominated by organic fragments such as CH3+, C2H3+, C3H3+, and C4H5+. Zinc is also present on the surface of the thick rubber strip, presumably from the brass sheet that was originally adhered to it. The mass spectrum of the brass region (Figure 6B) shows comparatively little organic content and is, instead, dominated by Cu+, Zn+, Zn2+, and SO+. The H+, C+, C2+, and (28) Hopkins, R. E.; Stephenson, D. In Optical Scanning; Marshall, G. E., Ed.; Marcel Dekker, Inc.: New York, 1991; Vol. 31, pp 27-81.

Figure 5. Optical and photoion images of a brass/vulcanized rubber fracture surface. Desorption and ionization conditions were the same as in Figure 4B. The field of view is 200 µm × 250 µm, digitized into 100 × 100 pixels with 50 laser shots/pixel. (A) Optical image of the scanned region. (B) Sum of ion intensities from m/z 27 to 70, which was the total mass range monitored. The scale is abitrary, representing the digitized signal from the oscilloscope. (C) m/z 39 (C3H3+) ion fraction image. (D) m/z 48 (SO+) ion fraction image. (E) m/z 53 (C4H5+) ion fraction image. (F) m/z 63 (Cu+) ion fraction image. Images C-F were normalized by image B.

Figure 6. Photoion mass spectra of the rubber (A) and brass (B) regions of a brass/vulcanized rubber fracture surface. The spots on the sample at which the spectra were taken are marked in Figure 5A. A total of 350 laser pulses were summed for each spectrum. Desorption laser: 532 nm, 8 ns pulse. Ionization laser: 800 nm, 100 fs pulse, irradiance ) 1012-1013 W/cm2.

H2O+ peaks in both spectra are due mainly to the background gas in the vacuum chamber at a pressure of 3 × 10-9 Torr. Figure 5B-F shows photoion images from the sample. Figure 5B is the total ion image, i.e., the sum of ion intensities from m/z 27 to 70 (the range over which ions were collected). The images in Figure 5C-F were normalized by the total ion signal (Figure Analytical Chemistry, Vol. 69, No. 18, September 15, 1997

3745

5B), making them ion fraction images rather than simple ion intensity images as in Figure 4. Thus, Figure 5C-F shows the fraction of ions produced at a particular spot that are due to a particular species. This was done to reduce the matrix effect and put the contrast on a rational basis. Any complex multicomponent surface will show variations in optical absorptivity and hence lasersurface coupling from spot to spot. The effect is, as stated above, to change the desorption yield for all species. Normalization reduces this matrix effect, though by far the biggest reduction comes from postionization. In fact, it was not possible to obtain a direct ion image of this surface since the dynamic range of ion yields over even small areas ranged from zero to off-scale on our detector. The C3H3+ image (m/z 39, Figure 5C) shows rubber over the entire sample but concentrated on the thick rubber strip, as expected from the spectra of Figure 6. In contrast, Figure 5E shows that the C4H5+ fragment (m/z 53) is found almost exclusively on the rubber surface, with very little in the brass interfacial region. The Cu+ image (m/z 63, Figure 5F) is the virtual inverse of the C4H5+ image, confirming the observation that C4H5+ is found only where the rubber layer is thick. Finally, the SO+ (m/z 48) image of Figure 5D correlates strongly with the Cu+ image and inversely with the C4H5+ image, indicating that it resides only on the brass side of the failure interface. Taken together, the chemical images indicate that SO-containing moieties may contribute to the adhesive failure of the joint. They may result from the oxidation of carbon-sulfur bonds, but the fact that they reside only on the brass side of the failure interface suggests that the brass surface plays a role in its formation. Further, the bright region at the upper left in the C4H5+ image (and the corresponding dark region in the SO+ and Cu+ images) indicates that there is a relatively thick rubber layer in this area, though this is not evident in the optical image. Thus, in addition to the implication of SO-containing species in the adhesive failure of the joint, the chemical image reveals a region of cohesive failure in the rubber, presumably due to simple mechanical failure since no chemical markers are found in this

3746

Analytical Chemistry, Vol. 69, No. 18, September 15, 1997

area. Finally, the fact that Cu+, SO+, and C3H3+srepresenting the brass substrate, brass/rubber interface, and rubberscan be seen simultaneously in some areas points to an important difference between ion beams and lasers as desorption sources for imaging. Whereas ion beams run in the static mode are sensitive only to the uppermost monolayers of the surface, the laser can remove material from deeper in the sample and allows simultaneous analysis of both the surface and near-surface regions. CONCLUSIONS Postionization of desorbed neutrals greatly facilitates laser desorption mass spectral imaging by increasing the instrumental sensitivity and reducing the inherent spot-to-spot variation in ion yield associated with direct ion imaging. Intense femtosecond laser postionization increases mass resolution in the simple timeof-flight mass analyzer by reducing the time spread in the ion production. In addition, the 800 nm femtosecond laser ionizes nonselectively and runs at high repetition rates to reduce imaging time. The spatial resolution of our instrument with a 532 nm desorption laser is between 1 and 2 µm, and our experiments show that the ultimate resolution need not be limited by the focusing optics since the desorbed spot diameter can be smaller than the beam waist when the desorption irradiance is kept low. ACKNOWLEDGMENT The authors are grateful to Dong Kim of the Goodyear Tire and Rubber Co. of Akron, OH, and R. Giles Dillingham of the University of Cincinnati for providing the brass/rubber sample. This work was supported by the U.S. Department of Energy, BESMaterials Sciences, under Contract W-31-109-ENG-38. Received for review January 28, 1997. Accepted July 2, 1997.X AC970115U X

Abstract published in Advance ACS Abstracts, August 15, 1997.