Laser microprobe mass spectrometry: Description and selected applications

Applied Surface Science 31 (1988) 103-117 North-Holland, Amsterdam

103

REVIEW LASER MICROPROBE MASS SPECTROMETRY: DESCRIPTION AND SELECTED APPLICATIONS

David Center

*

S. SIMONS for Analytical Chemistry, National Bureau of Standardq

Received 19 August 1986; accepted for publication

Gaithersburg

MD 20899, USA

16 October 1986

Laser microprobe mass spectrometry (LAMMS) uses a high power density pulsed laser beam to ablate a microvolume of material. The fraction of this material that is ionized can be detected using a time-of-flight mass spectrometer. Two different instrumental configurations, “ transmission” and “reflection”, that satisfy different analytical requirements are characterized by the geometry of ion collection from the specimen. The features of LAMMS include a spatial resolution of about 1 pm, high mass range, isotopic selectivity, ppm detection limits for many elements from picograms of material, and molecular structure information from organic materials. The major application areas for this technique are in the analysis of biological tissues and cells, organic materials, particles and aerosols, and surfaces of metals, semiconductors, and dielectric materials.

1. Introduction Lasers have been used as an ionization source for solids analysis by mass spectrometry for more than two decades [l]. However, it was not until 1975 that an instrument was developed exploiting the capability of a laser beam to be focused to micrometer dimensions [2]. Several years later, a commercial instrument, the LAMMA-500, was introduced by Leybold-Heraeus [3,4]. Recently, applications have proliferated as laser microprobe mass spectrometry (LAMMS) has demonstrated its versatility as a “soft” ionization source for organic mass spectrometry as well as an elemental analyzer for inorganic and biological matrices [5]. At least two additional companies, Cambridge Mass Spectrometry and VG Scientific, have also entered the commercial LAMMS market. * This article was one of a series of invited review papers presented at the ASTM symposium on “ Recent Developments in Surface Spectroscopy/Analysis”, held as part of the 1986 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, which took place in Atlantic City, New Jersey on 12 March 1986.

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2. Description of instrument For the purpose of this article a laser microprobe mass spectrometer will be defined as an instrument that can produce a complete mass spectrum from a single laser pulse that ablates an area smaller than 10 pm in diameter. Two instrumental configurations for laser microprobes have been developed to satisfy different analytical requirements. They are characterized by the geometry of ion collection from the specimen. In the “transmission” geometry ions are extracted along the same axis and on the opposite side of the specimen from the incident laser beam [2,4]. This geometry has the advantage that the optical objective lens can be placed very close to the specimen resulting in a laser spot size that is near the diffraction limit [6]. The specimen shape is restricted because the sample must usually be perforated by the laser beam in order that ions may be extracted into the mass spectrometer. Typical specimen types for a transmission geometry instrument are free-standing thin films, thin biological tissue sections, and small particles mounted on a thin-film substrate. These samples are often supported by transmission electron microscope (TEM) grids which makes it convenient to perform sequential analyses in an electron microscope followed by the laser microprobe [7]. In the more recently developed “reflection” geometry instruments, ions are extracted from the same side of the specimen on which the laser beam is incident [8,9]. In this case, certain compromises are generally required to minimize the perturbation of the ion extraction field by the laser focusing lens. One clever design combines the optical and ion lenses into a single unit that allows both laser incidence and ion extraction to be perpendicular to the sample surface [lo]. The additional complexity of reflection geometry is offset by the greater flexibility of sample types that can be accepted, since specimen perforation is no longer required. Fig. 1 shows a schematic of the LAMMA-500, an instrument of the transmission type. Two separate laser systems are included. A low power He-Ne laser is used as a spotting or pilot laser to align a specific feature of the sample for analysis. An adjustable microscope crosshair could serve the same purpose. The pulsed Nd : YAG laser, which is typically frequencyquadrupled into the UV (X = 265 nm), is the ablating and ionizing laser. It delivers an adjustable power density of between lo8 and 10” W/cm* that is controlled by a combination of laser amplification and filter attenuation. Both lasers are focused through a microscope head and objective lens that also serve for operator viewing and incident light illumination. The sample, mounted on a TEM grid, is placed in the vacuum system of the spectrometer immediately behind a thin quartz cover glass that forms an air-to-vacuum seal. Ions generated from a single pulsed laser event are accelerated to a fixed energy, typically 3 keV, extracted through a simple immersion lens, and focused with an einzel lens to maximize transmission to the detector. The ions

D.S. Simons / LAMMS:

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105

TRANSIENT RECORDER TRIGGER

Pk;$f&--DIODE

q jT

11

FREGUENCY CONVERTER

k%?E*,“, LASER

DISPLAY

STRIPCHART RECORDER

Fig. 1. Schematic

drawing

of LAMMA-500.

pass through a time-of-flight (TOF) mass spectrometer that produces a temporal separation of mass/charge ratio at the detector. The spectrometer incorporates an ion reflecting element that compensates for differences in initial kinetic energy of ions of the same m/z, thereby improving the mass resolution [ll]. Ions are detected by a Venetian blind secondary electron multiplier operating as a current amplifier. The TOF spectrometer produces a complete mass spectrum from a single laser pulse. Recording of the spectrum is accomplished with a transient waveform recorder. This device incorporates an A/D converter capable of sampling the input waveform in 10 ns increments (100 MHz digitization rate) with 8-bit amplitude resolution. It then stores the digitized spectrum in an internal memory. Such high speed sampling is necessary because the time width of individual mass peaks can be less than 50 ns. The duration of the signal that is useful for elemental analysis (m/z l-300) is about 80 ps. A memory capacity of at least 8 kbytes is required if this mass range is to be sampled at 100 MHz. Although transient recorders that were available several years ago contained only 2 kbytes of memory, newer models have memory capacities of 32 kbytes or more, as well as superior dynamic response characteristics [12,13]. The spectrum stored in the transient recorder is instantaneously displayed on a CRT, and it can also be transferred to a computer for further manipulation such as mass scale calibration and peak area integration. A magnified view of the sample region for transmission geometry is depicted in fig. 2. The illustrated target is a small particle supported by a thin film on a copper TEM grid. The focused laser pulse enters from the left, passes

106

D.S. Simons / LAMMS:

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-TOF

Fig. 2. Schematic view of sample region of LAMMA-500, showing TEM grid with particle on carbon film.

through the quartz cover glass into the vacuum of the sample chamber, and ablates the particle at the focal point. The depth-of-field is so shallow with a 32 x objective lens that the power density of the laser at the vacuum side of the cover glass is usually not sufficient to cause damage to the glass. A variety of species are produced when the specimen is ablated by the laser, including neutral and charged atoms and molecules, some in excited states, and sometimes macroscopic fragments as well. At sufficiently high laser power density an analytically useful percentage of the ablated material is in the form of elemental or molecular ions, usually singly-charged, that can be extracted and then detected in a TOF mass spectrometer. Detection of ions of either polarity is possible, though unfortunately not simultaneously. Under optimum conditions we estimate that one ion is detectable for every 103-lo4 atoms that are vaporized, with variations depending upon the particular element and sample matrix. From fig. 2 one can readily see that a certain amount of the supporting film will be vaporized along with the particle, contributing an unwanted background in the mass spectrum. It is therefore desirable that the film produce as few ion peaks as possible. Films in common use for this application are

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107

collodion (nitrocellulose), Formvar (polyvinyl formal), and evaporated carbon, with thicknesses of lo-20 nm. We have found that Formvar has the lowest background and is most suitable for single particle analysis.

3. Features of laser microprobe mass spectrometry A number of special features combined in a single instrument make laser microprobe mass spectrometry an attractive analytical technique: (a) The spatial resolution can be below 1 pm in transmission geometry instruments. Perforation holes of 0.4 pm diameter have been demonstrated in thin biological tissue samples [6]. In reflection geometry instruments the minimum crater diameter is several micrometers [9,14]. (b) In principle, an unlimited range of m/z can be collected from a single pulsed laser ablation event. The protonated molecular ion of the polypeptide mellitin has been detected at m/z 2845, and inorganic clusters of (CsI),Cs+ have been observed up to m/z 15 000 [15]. (c) Isotopic selectivity is possible for all elements owing to a typical mass resolution of 500 (FWHM). (d) Elemental detection limits are in the ppm range, and can be achieved with the consumption of only picograms of material. Furthermore, many different elements can be detected following a single laser pulse. Detection limits below 1 ppm have been quoted for some elements in thin sections of doped epoxy that mimic biological tissue [16]. In glass microparticles detection limits are typically 10 ppm by weight [17]. In bulk metal targets analyzed with reflection geometry instruments, detection limits are in the range of l-10 ppm [14]. (e) The mass of material consumed during an analysis can be as small as 0.1 pg. This would be’ the case when a 1 pm diameter hole is made in a 0.1 pm thick section of biological tissue. (f) Information about molecular structure can be derived from laser-induced mass spectra of organic materials. This is usually achieved through a laser desorption process brought about by reducing the power density of the laser beam to a level just above the threshold for ion emission [18,19]. In one case high mass molecular ions were produced from thick sections in transmission geometry at high laser power density [20]. The largest single deficiency of laser microprobe mass spectrometry has been the lack of understanding of the ion formation process which has limited the capability to provide reliable quantitative compositional information. Further difficulties arise from the variability of laser energy per pulse when the laser is operated in a single-shot mode, and from the variability of energy absorption by the sample from pulse to pulse. The nonlinear response of the ion detection system can lead to systematic errors, though progress has been made in modeling this effect [12]. In spite of these difficulties, accuracies as

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D.S. Simons / LAMMS: description and selected applications

good as 10% relative have been reported 2%-3% for isotopic ratios [22].

for elemental

determinations

[21], and

4. Applications The above-mentioned features of laser microprobe mass spectrometry suggest some of the major application areas to which this technique has been applied: (a) Biomedical microprobe analysis of inorganic species in tissues and cells. (b) Organic mass spectrometry and organic microprobe analysis, primarily of nonvolatile and thermally labile compounds. (c) Analysis of particles and aerosols. (d) Microanalysis of surfaces of metals, semiconductors, and dielectric materials. A bibliography of applications published through 1982 is available [5], as are reports of three international workshops [23-251. The following are several examples of applications taken primarily from work done at the National Bureau of Standards. 4. I. Particle analysis - elemental NBS has developed a technique for producing glass microspheres that are valuable for studying performance characteristics of chemical microanalysis equipment [26]. Fig. 3 shows a portion of a mass spectrum collected from a single 1.5 pm diameter microsphere known to contain five major cations: aluminum (7.5 at%), silicon (16.9 at%), calcium (6.8 at%), iron (4.8 at%), and barium (2.5 at%). It illustrates a number of the properties of laser microprobe spectra of inorganic materials. Note that the sodium and potassium signals in the spectrum are larger than that of silicon, even though the latter is the most abundant element, aside from oxygen, in the particle. Potassium is present at less than 1% in the bulk of the particle according to energy dispersive X-ray spectrometry. The very high sensitivity of alkali elements results from their low first ionization potentials. In fact, potassium is one of the few elements that can be ionized by a single photon process with 265 nm light. It is also possible that these elements are surface-associated and more easily ionized for that reason. Aluminum is less than half as abundant as silicon in this material but the Al+ signal is larger than Si+. The large variation in relative sensitivity among the elements is one of the factors that complicates quantitative analysis by LAMMS. An additional complication is the variation of relative sensitivity with laser power [27]. Several of the largest peaks in fig. 3 have the same apparent amplitude. In fact, their true amplitudes have exceeded the full-scale range of the transient

D.S. Simons / LAMMS:

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109

x

c :

Q, -

!z

BoOH

Si

L 1

AlCoO

w

/

,/,.,,,,,,I

I,

I’

40

65

I’I’I’I’I’I

100

80

120

140

160

180

200

Mass

Fig. 3. Portion

of positive

ion mass spectrum

of single microsphere

of K309 glass.

recorder. If this range is increased, weak peaks may no longer be detectable since the dynamic range in a spectrum is set at 255 : 1 by the S-bit resolution of the transient recorder. Off-scale peaks are not a problem if minor isotopes of the same element are on-scale. However, this approach will not work for monoisotopic elements such as aluminum. A more general solution to the dynamic range problem is the parallel recording of the spectrum by two transient recorders operating at different full-scale sensitivities [12]. 4.2. Particle analysis - molecular Molecular ions are often found in the spectrum, as illustrated by the presence of BaO+, BaOH+, AlCaO+, and CaO+ in fig. 3. In this respect laser microprobe spectra of oxygen-rich material resemble secondary ion mass spectra taken with an oxygen primary ion beam. The abundance of molecular ions can be affected by sample type, laser conditions, and tuning of the focusing lens of the mass spectrometer [28]. One can exploit the molecular ions that are seen in laser microprobe spectra by using them as a “fingerprint” for identification of compound stoichiometry, even in single particles [28-301. In one study positive and negative ion mass spectra of micrometer-sized particles of five nickel compounds were examined to determine the feasibility of nickel speciation [31]. The compounds

D.S. Smons / LAMMS:

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description and selected applications

investigated were nickel metal, nickel oxide (NiO), nickel sulfate heptahydrate (NiSO,. 7H,O), nickel sulfide (NiS), and nickel subsulfide (Ni,S,). The objective was to identify nickel compounds emanating from pollution sources.

NICKEL

NICKEL

SULFATE

;!

HEPTAHYDAATE

METAL

0

z :

;

NICKEL

(III

SULFIDE

I

b., NICKEL

OXIDE

NICKEL

Fig. 4. Positive

ion mass spectra

of single micrometer-sized particle5 nickel compounds, from ref. [31].

SUESULFIDE

of nickel

metal

and

four

D.S. Simons / LA MMS: descriptionand selectedapplicahons

111

Fig. 4 shows representative positive ion spectra of the five compounds. Three of the five can be distinguished uniquely, based on the presence or absence of specific molecular ions. The spectra of nickel sulfide and subsulfide were indistinguishable on qualitative grounds. Even when intensity ratios of molecular ions were compared for the two compounds, the within-compound variability from one particle to another was as great as the variability between compounds. The difficulty in making this distinction was primarily attributed to variability in laser energy deposition from particle to particle. Heterogeneity in composition among particles of the same compounds was another possibility that could not be excluded. 4.3. Isotopic ratio measurements An application area of LAMMS that has been little exploited is the measurement of isotopic ratios from nanograms of material. Precisions and accuracies in the range of several percent have been demonstrated, which are not as good as those attainable by thermal ionization mass spectrometry (TIMS) or secondary ion mass spectrometry (SIMS). However, in certain cases LAMMS may be the analytical method of choice for an element that is difficult to ionize efficiently by other means. For this reason the LAMMAwas used to measure osmium isotopic ratios as a necessary step in the determination of the radioactive half-life of 187Re. The nuclide lg7Re decays by beta emission to 1870s with a half-life of about 45 billion years, a value based on measurements that were normalized to accepted ages of meteorites and rocks. An experiment was conducted to make a direct, accurate determination of this half-life in a controlled laboratory experiment by observing the ingrowth with time of the decay product ‘870s relative to 19’Os and 1920s spikes that were added to a purified rhenium solution [22]. For this experiment a technique was developed to obtain isotopic analyses of nanogram quantities of osmium with relative precision and accuracy of 2%-3% (lo) on isotope ratios between 0.3 and 3.0. Fig. 5 shows a portion of the mass spectrum in the osmium region taken from a purified and dried aliquot of the rhenium solution that was extracted

187

Osmium

Fig. 5. Positive ion mass spectrum of osmium isotopes from single laser pulse, showing lR70s and control spikes of 1900s and 19’Os.

radiogenic

112

D.S. Simons / LAMMS:

description

and selected applications

I

I

1

I

3

Time

(yrs]*

Fig. 6. Growth curve showing linear increase with time of ‘870s relative to ‘900s spike following purification of rhenium.

approximately two years after the growth of “‘0s had commenced. Only picograms of osmium were consumed to acquire this spectrum. About 100 such spectra were added together to improve the statistics of the ratio measurement. Fig. 6 shows the growth curve for “‘0s that was determined from ratios measured on three aliquots of the rhenium solution taken at different times. From the slope of this curve, a half-life of (4.35 + 0.13) x 10” years was determined for r*‘Re. This value was corroborated by measurements made on similar aliquots of the rhenium solution by inductively coupled plasma mass spectrometry (ICPMS) [22]. 4.4. Organic mass spectrometiy Many applications of LAMMS have exploited the capability to ionize organic molecules with little fragmentation [5]. In one study a monolayer of paranitrobenzoic acid (pNBA) was deposited on the surface of a silver island film, as depicted in fig. 7 [32]. These films are known to enhance the Raman scattering spectra of adsorbed organic molecules via a resonant increase in the local electromagnetic field strength, and it was of interest to see whether the surface microstructure of the film affected the ion desorption of the adsorbate. The experiment was carried out in a transmission mode instrument using laser wavelengths of 265 and 532 nm. Some of the spectra taken at 265 nm are shown in fig. 8. The upper mass spectrum of negative ions is from an uncoated silver island film. Characteristic ions of Ag- (107, 109) Cll(35, 37) and SiO; and SiO; (60, 76) from the quartz substrate are observed. Fig. 8b shows the negative ion spectrum of the coated film taken well above the power density threshold for ion detection. Additional peaks at m/z 166 (M-H)), 122

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D.S. Simons / LA MMS: description and selected applications

Quartz Substrate

Ag Islands Coated with Monolayer Para-nitrobenzoic Acid (PNBA)

of

Photon Pulse A = 265 or 532 nm lo6 to lo* W/cm2 I\-

Ag-

,-“‘I\ 1 ‘\

Fig. 7. Schematic

view of sample geometry

NO?-

for pNBA desorption

from silver island film

60

(Cl

(M-HI,166

I

23

40

60

80 M/Z

100

120

/

1

140

r III

I,,

I60

180

200

(amu)

Fig. 8. Negative ion mass spectra of (a) uncoated silver island film, (b) film coated with monolayer of paranitrobenzoic acid, laser energy density 400 ml/cm’, and (c) coated film as in (b), laser energy density 100 ml/cm’.

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(M-H-CO,)), and 46 (NO,)) are now seen. In fig. 8c the laser power density has been reduced to a level where only ions characteristic of the adsorbate are detectable. In the latter condition spectra could be obtained from five successive laser pulses aimed at the same area, suggesting that about lo9 molecules are desorbed by each shot. Spectra similar to fig. 8c could not be obtained from a monolayer of pNBA on a continuous silver film, indicating that the island nature of the film enhanced the detectability of the adsorbate. However, no wavelength dependence of the spectra was observed between the two wavelengths, 265 and 532 nm, thereby proving that the ion desorption enhancement by island films comes from a mechanism different from that responsible for the increase in the Raman signal. The authors conclude that the desorption mechanism is most likely purely thermal, and that the silver islands provide an efficient way to absorb the incident laser energy and to localize the thermal energy in the vicinity of the adsorbate. 4.5. Surface microanalysis The microanalysis of surfaces of metals and semiconductors requires an instrument of the reflection type for reasons discussed earlier. Especially in these cases SIMS can often provide similar information to LAMMS. However, the ability to collect an entire mass spectrum from one laser pulse and to

1

1”“I” 50pm

Ca

Fig. 9. Positive

ion mass

spectrum from cell wall of aluminum-lithium impurities present, from ref. [33].

Powder

alloy

showing

major

D.S. Simons / LAMMS:

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115

obtain interpretable organic mass spectra make LAMMS especially valuable for failure analysis where the species causing a problem are unknown. In one case aluminum-lithium alloy particles were produced by high pressure gas atomization resulting in spheres of 50 pm diameter with a cellular structure [33]. A high impurity concentration was suspected at intercellular boundaries. The laser mass spectrum from the region within a cell showed major lithium and aluminum, with only trace impurities. Fig. 9 shows a spectrum from the intercellular boundary. Major impurities of Mg, Ca, Fe, and Cu, and a minor level of Ga, are observed. In addition, the laser microprobe was able to show that lithium had been preferentially partitioned to the cell walls.

5. New directions Two recently developed modifications of laser microprobe instrumentation offer the promise of increased sensitivity and improved quantitative capability. Schueler, Odom, and Evans have added a second Nd: YAG pulsed laser, focused above and parallel to the sample surface, that ionizes the ablated neutrals via a nonresonant multiphoton ionization process [34]. Their goal is to decouple the ablation and ionization steps and to achieve a high, uniform ionization efficiency by operating the ionizing laser at short wavelength (265 The ions produced by the nm) and high power density (up to 10 l2 W/cm’). ablating laser can be rejected by adjusting the potential on the ion reflector so that only ions produced by the second laser are detected. J.F. Muller and coworkers have added a tunable dye laser to a laser microprobe in order to investigate the effects of a continuously variable wavelength on both organic and inorganic materials [35,36]. For some organic materials they have noted a decreasing energy threshold for production of the molecular ion as the UV wavelength is decreased. For metallic samples resonance ionization phenomena have been observed, resulting in a factor of 50 improvement in the sensitivity for copper in thin epoxy resin films [37]. The latter effect is somewhat surprising since the tunable laser is used for both ablation and ionization. The resonant process may be occurring in the solid state in this case, rather than in the gas phase as has usually been observed 1381.

Acknowledgements The author gratefully acknowledges the assistance of Inga Musselman and Robert Fletcher in the preparation of this manuscript. Permission of Cambridge Mass Spectrometry, Ltd, to use fig. 9 is also appreciated.

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References [l] R.E. Honig and J.R. Woolston, Appl. Phys. Letters 2 (1963) 138. [2] F. Hillenkamp, E. Unsold, R. Kaufmann and R. Nitsche, Appl. Phys. 8 (1975) 341. [3] Certain commercial instruments are identified in this paper to specify adequately the experimental procedure. Such identification does not imply endorsement by the National Bureau of Standards, nor does it imply that the equipment identified is necessarily the best available for the purpose. [4] R. Kaufmann, F. Hillenkamp and R. Wcchsung, Eur. Spectrosc. News 20 (1978) 41. [5] R.J. Conzemius, D.S. Simons, 2. Shankai and G.D. Byrd, in: Microbeam Analysis - 1983, Ed. R. Gooley (San Francisco Press, San Francisco, 1983) p. 301. [6] R. Kaufmann, in: Microbeam Analysis - 1982, Ed. K.F.J. Heinrich (San Francisco Press, San Francisco, 1982) p. 341. [7] E.B. Steel, D.S. Simons, J.A. Small and D.E. Newbury, in: Microbeam Analysis - 1984, Eds. A.D. Romig, Jr. and J.I. Goldstein (San Francisco Press, San Francisco, 1984) p. 27. [8] T. Dingle, B.W. Griffiths and J.C. Ruckman, Vacuum 31 (1981) 571. [9] H.J. Heinen, S. Meier, H. Vogt and R. Wechsung, Intern. J. Mass Spectrom. Ion Phys. 47 (1983) 19. (lo] T. Dingle, B.W. Griffiths, J.C. Ruckman and C.A. Evans Jr., in: Microbeam Analysis - 1982, Ed. K.F.J. Heimich (San Francisco Press, San Francisco, 1982) p. 365. [ll] B.A. Mamyrin, V.I. Karataev, D.V. Shmikk and V.A. Zagulin, Soviet Phys-JETP 7 (1973) 45. [12] D.S. Simons, Intern. J. Mass Spectrom. Ion Processes 55 (1983) 15. [13] R.A. Fletcher and D.S. Simons, in: Microbeam Analysis - 1985. Ed. J.T. Armstrong (San Francisco Press, San Francisco, 1985) p. 319. [14] T. Dingle and B.W. Griffiths, in: Microbeam Analysis - 1984, Eds. A.D. Romig, Jr. and J.I. Goldstein (San Francisco Press, San Francisco, 1984) p. 23. [15] F. Hillenkamp, in: Secondary Ion Mass Spectrometry SIMS V, Eds. A. Benninghoven, R.J. Colton, D.S. Simons and H.W. Werner (Springer, Berlin, 1986) p. 471. [16] R. Kaufmann, F. Hillenkamp, R. Wechsung, H.J. Heinen and M. Schlirmann, in: Scanning Electron Microscopy/l979/11, Ed. 0. Johari (SEM, AMF O’Hare, IL, 1979) p. 279. [17] P. Surkyn and F. Adams, J. Trace Microprobe Tech. 1 (1982) 79. [18] H.J. Heinen, Intern. J. Mass Spectrom. Ion Phys. 38 (1981) 309. [19] D.M. Hercules, R.J. Day, K. Balasanmugam, T.A. Dang and C.P. Li, Anal. Chem. 54 (1982) 280A. [20] B. Lindner and U. Seydel, Anal. Chem. 57 (1985) 895. [21] T. Dingle and B.W. Griffiths, in: Microbeam Analysis - 1985, Ed. J.T. Armstrong (San Francisco Press, San Francisco, 1985) p. 315. [22] M. Lindner, D.A. Leich, R.J. Borg, G.P. Russ, J.M. Bazan, D.S. Simons and A.R. Date, Nature 320 (1986) 246. [23] Proc. LAMMA Symp., Dusseldorf, Fed. Rep. of Germany, 1980 [Z. Anal Chem. 308 (1981) 193-3201. [24] Proc. LAMMA Workshop, Borstel, Fed. Rep. of Germany, September l-2, 1983, available from Leybold-Heraeus GmbH. [25] Proc. 3rd Intern. Laser Microprobe Mass Spectrometry Workshop, Antwerp, Belgium, August 26-27, 1986. [26] J.A. Small, K.F.J. Heinrich, C.E. Fiori, R.L. Myklebust, D.E. Newbury and M.F. Dilmore, in: Scanning Electron Microscopy/l978/1, Ed. 0. Johari (SEM, AMF O’Hare, IL, 1978) p. 445. (271 R. Kaufmann, P. Wieser and R. Wurster, in: Scanning Electron Microscopy/l980/11, Ed. 0. Johari (SEM, AMF O’Hare, IL, 1980) p. 607.

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E. Michiels and R. Giebels, Spectrochim. Acta 38B (1983) 1347. F.J. Bruynseels and R.E. Van Grieken, Spectrochim. Acta 38B (1983) 853. F.J. Bruynseels and R.E. Van Grieken, Anal. Chem. 56 (1984) 871. I.H. Musselman, R.W. Linton and D.S. Simons, in: Microbeam Analysis - 1985, Ed. J.T. Armstrong (San Francisco Press, San Francisco, 1985) p. 337. R.A. Fletcher, I. Chabay, D.A. Weitz and J.C. Chung, Chem. Phys. Letters 104 (1984) 615. Cambridge Mass Spectrometry, Ltd., Application Note. B.W. Schueler, R.W. Odom and C.A. Evans, Jr., in: Proc. 3rd Intern. Laser Microprobe Mass Spectrometry Workshop, Antwerp, Belgium, August 26-27, 1986. G. Rrier, F. Verdun and J.F. Muller, Z. Anal. Chem. 322 (1985) 179. J.F. Muller, G. Rrier, F. Verdun and D. Muller, Intern. J. Mass Spectrom. Ion Processes 64 (1985) 127. F.R. Verdun, J.F. Muller and G. Krier, in: Proc. 3rd Intern. Laser Microprobe Mass Spectrometry Workshop, Antwerp, Belgium, August 26-27, 1986. J.D. Fassett, L.J. Moore, J.C. Travis and J.R. Devoe, Science 230 (1985) 262.