Eric Denoyer' Rene Van Grieken Fred Adams David F. S. Natusch' Department of Chemistry University of Antwerpen (VIA) 2610 Wilrijk, Belglum
LaserMimpmbeMass Spectrometry ~r Basicprinciplesand Mrmancecharaks Due primarily to the recent advent of commercially available instrumentation, considerable interest is currently being focused on laser microprobe mass spectrometry (LAMMA) as an analytical tool. The technique makes use of a high-intensity laser pulse to vaporize and ionize a small amount of a solid sample, and the elemental and molecular ions produced are then mass analyzed in a time-offlight (TOF) mass spectrometer. The most important claims made for the technique are its high sensitivity (down to -1O-*O g), its speed of operation, its applicability to both inorganic and organic (including biological) samples, and its microbeam capahility (spatial resolution -1 pm). It is the intention of this article to describe the operational characteristics of present state-of-the-art instrumentation and to assess the extent to which its performance claims can be realized in practice. The analytical utility of the technique is evaluated critically, and examples of several areas of application are presented. Next month the application of the technique to structural studies of organic and inorganic compounds will be discussed.
Hlstorical Development The advantages of the laser as an ion source were recognized in the early 19608,and since that time some 460 publications have appeared on the subject (I). In tracing the historical development of LAMMA it is appar-
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On leave from Colorado State University, Fort Collins. Coio. 80523
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ent that advances have been deter mined primarily by the success of research workers in achieving an effective compromise between ion production (sensitivity), mass spectral resolution (specificity), and spatial resolution. In fact, high ion production can be achieved by using a long-duration laser pulse of large spatial diameter, but such a pulse gives rise to a broad kinetic energy distribution of the ions produced so that successful mass analysis requires double-focusing instruments with very low mass spectral transmission. Historically, the first laser mass spectrometry studies were reported by Honig and Woolston (2).They used a pulsed ruby laser with a 150-pm-diameter beam and a pulse length of 50 ps in conjunction with a douhle-focusing mass spectrometer. The samples were irradiated a t an angle of 4 5 O to their surface and the ions extracted at 135" in a reflectance configuration. Sample craters 125 pm deep and 150 pm in diameter resulted from the vaporization of -2 X 1 0 ' atoms and produced high ion currents (-10-5 A). Moreover, effective mass analysis was limited due to considerable space charge broadening and hence limited mass resolution. This and several similar studies indicated that the broad kinetic energy spread previously observed for laserproduced ions was due to the use of long laser pulse lengths and large spot sizes and, in part, to the use of reflection geometry. Consequently, later research concentrated on reducing laser pulse lengths (by Q-switching), spot diameters (by focusing the laser
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
beam), and on the use of a transmission configuration for ion production and extraction (I). Following this philosophy Fenner and Daly (3)reported the first true laser microprobe mass spectrometer that utilized a Q-switched ruby laser having pulse times in the nanosecond range and a beam diameter of 20 pm. These workers employed a transmission configuration for ion production and extraction into a TOF maw spectrometer (pathlength 1m) fitted with an electrostatic sector energy selector and a scintillation detector. Space charging effects were found to be much less than thme reported by Honig and Woolston (2)but the initial ion kinetic energy distribution was still extremely broad. Furthermore, incorporation of the energy selector to maximize mass resolution W A m = 20 a t rnle = 30) resulted in reduction of ion transmission by a factor of 103 to give detection limits of only to 10-10 g. The next advance came with the development by Hillenkamp et al. ( 4 ) of a modified laser focusing system that used microscope optics to focus the laser beam to a diameter of 0.5 pm (close to the diffraction limit). This configuration also allowed microscopic visualization of the sample. Using this system, in conjunction with a transmission configuration for laser excitation and ion extraction into a TOF mass spectrometer, it was found that earlier predictions concerning the broad kinetic energy distribution of laser-induced microplasmas do not hold true for thin dielectric materials, and detection limits as low as g 0003-2700/61/0351-026A$O1.0010
t 3 1981 American Chemical Society
Renort
Analog
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’ Transillurninatlor
Figure 1. Schematic diagram of LAMMA-500 laser microprobe ma5. spectrometer
(0.2 ppm) were obtained for Li doped into thin (0.1-1.0 pm) epoxy resin films. Further improvement in spectrometer design resulted from the introduction by Wechsung et al. (5) of a “time focusing” ion reflector into the TOF mass spectrometer, thereby increasing the mass resolution to rnlAm = 800900 a t rnle = 200. It is essentially this instrumentation in conjunction with the optical design of Hillenkamp et al. ( 4 ) that constitutes the LAMMA-500 instrument (Leybold Heraeus, Koln, West Germany). This instrument represents the state of the art and is the only instrument presently available on a commercial basis. Instrumentation. A schematic diagram of the LAMMA-500 laser microprobe mass spectrometer system is presented in Figure 1, and a detailed description is provided in the literature ( 5 , 6 ) . The system is equipped with two lasers. The first of these is a low-poyered (2 mW) visible (A = 613 nm) He-Ne search laser that can be focused to a spot for illuminating the analytical region. The second is a Qswitched (pulse duration = 15 ns) neodymium-YAG laser whose output is quadrupled in frequency to provide a high power density (1O’O - 10” W cm-* for a 0.5-pm diameter spot) a t 265 nm for vaporization and ionization. The power of this laser beam can be attenuated to 1% by means of a 25step optical filter sequence. The optical paths of the two lasers are collinear (Figure 1)so that they may be focused onto the analytical region by an optical microscope fitted with UV-transparent glycerol immersion lenses. The sample may be viewed with a binocular microscope using top or trans-illumination, in either phase contrast or interference contrast modes, via the same optical path as followed by the lasers. All optics prior to the sample are external to the vacuum system, and magnifications in the range lOOX to 1250X are available. The sample is mounted in vacuum (1X 10-6 torr) on a movable x-y stage and viewed through a thin (0.2-0.5 mm) quartz vacuum seal (Figure 1).
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
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Figure 2. Laser microprobe mass spectra of ccfonene at high and low energy
Once the analytical region is chosen, the transmitted light condenser lens (which is located in the vacuum system) is pneumatically exchanged for ion optics. Since ions are extracted for mass analysis a t 180' from the path of the incident laser beam it is necessary for the ionization laser to pass through the analyticalregion, which can he no thicker than a few pm. Activation of the excitation laser vaporizes analyte material to produce a microplasma that consists of neutral fragments together with elementary, molecular, and fragment ions, having predominantly unit charge. For the highest laser power densities the kinetic energy spread of the ions is typically 0-60 eV with a plateau between 10 and 20 eV. Ions in this energy range are selectively accelerated by an einsel-type ion lens (3-5 kV) into the drift tube of a 1.8-m TOF mass spectrometer with high extraction and transmission efficiency (up to 50%of the ions produced in the microplasma). The remaining energy spread is largely compensated for by an ion reflector in the drift tube. The mass spectrum obtained from a laser pulse is detected by an open Cu-Be secondary electron multiplier (gain 104-106)after additional acceleration to increase ion-to-electron conversion efficiency and to decrease the mass discrimination of the detector (5). The analog signal is digitized for storage in a 2048-channel 100-MHz transient recorder having 8-bit precision and a time resolution of 10 ns. Spectra can be displayed on an oscilloscope, recorded as an analog trace, or transferred to a computer for storage or data treatment. Mass spectra of either positive or negative ions may be 28A
recorded simply by switching the polarity of the electric fields. The power of each laser pulse is monitored and displayed directly for reference purposes.
Performance Characteristics The primary advantage claimed for the laser microprobe mass analyzer is its ability to analyze, very rapidly and with high sensitivity, both organic and inorganic species present in a micro region of the sample with a variable ionization source in either positive or negative ion detection modes. All of these claims are correct; however, the analytical information available depends on a number of instrumental, theoretical and practical factors, which are described and critically examined herein. Sample Requirements. Undoubtedly a major practical limitation of the LAMMA-500 instrument for many analyses is its requirement for thin samples through which the laser can pass. In the case of biological samples, for which the instrument was originally developed, thin sections can he prepared in the same way as for transmission electron microscopy (TEM). Since the absence of low-temperature facilities may allow ion migration to occur in the sample, the inbedding procedures common in electron microscopy must be applied, and essentially the same target preparation problems are encountered. Once prepared, however, such samples are readily amenable to examination by LAMMA. Similarly, thin sections of other materials can be mounted directly provided that they have sufficient mechanical stability. These latter are generally quite difficult to pre-
ANALYTICAL CHEMISTRY, VOL. 54. NO. 1. JANUARY 1982
pare in the case of friahle inorganic materials. Particles must be supported on a surface. The simplest approach is to mount particulate samples on a TEM grid that has been coated with a thin transparent organic film (-0.1 pm) of slightly adhesive Formvar, collodion or Mylar. Ionization of this film material, however, gives rise to a fairly complex and often substantial background spectrum that in some cases may interfere with the analyte spectrum. Sometimes it is possible to dispense with the supporting film in cases where particles will adhere sufficiently to the TEM grid, but such samples are usually mechanically unstable. Another approach is to mount particles on a very thin metal foil (e.g., AI, Au, Sn), but such procedures require that the laser vaporize the sample and the foil in the same laser pulse, which usually results in reduced mass spectral resolution and sensitivity if the foil is not sufficiently thin. Metal foils less than 1wm in thickness are commercially available, hut tend to be extremely delicate and difficult to work with. Macroscopic samples up to a few millimeters in size may be mounted in the LAMMA-500 instrument via a specially designed stage, and their analysis can be achieved in several ways, none of which are really satisfactory or universally applicable. The first of these is to focus the laser at grazing incidence to the sample: however, it is almost impossible to reproduce the analytical volume and power density absorbed. Also, ionization of too much material results in a large microplasma whose time spread and ion energy distribution on entering the
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mass spectrometer can severely limit mass spectral resolution and produce 3eak deformation. A second approach s to utilize multiple laser shots until a iole is produced in the sample and a Spectrum obtained from the final shot; however, preferential vaporization and migration of some species are likely to oroduce nonrepresentative analytical results. A third approach is to vaporme regions of the sample in the ahience of an electric field on the ion ens. The vaporized material deposits 3artially on the quartz window (Figure 1) and can then he ionized in the nornal way. Again, such a procedure is inlikely to provide a representative analysis. Finally, if the sample is .ransparent to both the pilot and ionzation laser wavelengths, it is someimes possible to focus on the reverse ride of the sample and produce ionizaion from that region. This procedure ias been used to analyze inclusions in iome mineral samples (7). The actual analytical volume inrolved in an analysis is a function of 30th the morphology and composition >fthe sample. For a very thin (
