Laser Spectroscopy Theory

LASER MICROPROBE MASS SPECTROMETERS 1141

Laser Microprobe Mass Spectrometers Luc Van Vaeck and Freddy Adams, University of Antwerpen (UIA), Wilrijk, Belgium

MASS SPECTROMETRY Methods & Instrumentation

Copyright © 1999 Academic Press

Introduction The defining attribute of laser microprobe mass spectrometry (LMMS) is the use of a focused laser to irradiate a 5–10 µm spot of a solid sample at a power density above 106 W cm–2. The photon–solid interaction yields ions which are mass analysed by time-of-flight (TOF) or Fourier transform (FT) MS. The technique is sometimes referred to as laser probe microanalysis (LPA or LPMA), laser ionization mass analysis (LIMA) and laser microprobe mass analysis (LAMMA). The use of lasers to ionize solids in mass spectrometry dates back to the early 1960s, when the first high-power pulsed lasers became available. These highly directional and intense monochromatic beams can be easily introduced in the confined space of ion sources without disturbing the electrical fields. Moreover, upon the photon interaction with nonconducting materials, no charging of the sample occurs. First, lasers became an interesting alternative for sparksource MS to quantify elements in dielectrical specimens. Later, the ultrafast heating of the solid by laser irradiation was exploited for the desorption and ionization (DI) of labile organic compounds without thermal decomposition. The growing interest in microanalysis triggered the application of a focused laser in the 1970s. The initial aim was again elemental analysis in nonconducting samples but the main strength of LMMS was found to be the moleculespecific information on the local constituents. Organic and inorganic compounds are characterized by a combination of fragments and adduct ions. The former arise from the structure-specific breakdown of the analyte. The latter simply consist of the intact analyte attached to one or more of these stable fragments. The understanding of material properties on a local level implies knowledge of the chemistry and, hence, of the molecules present. Therefore the molecular specificity of LMMS is a major advantage in comparison with most micro- and surface analysis techniques, which characterize relative elemental abundances, bonds present or functional groups. Quantitation in LMMS is difficult because adequate

reference materials are not always available. However, qualitative identification of local constituents can often be achieved by deductive reasoning only. As a result, LMMS became appreciated as a versatile tool in diverse problem-solving applications in science and technology. The main sample requirement is its stability in the vacuum.

Instrumentation The simple construction and operation, excellent transmission, and the registration of full mass spectra made a TOF analyser the obvious choice for the early LMMS instruments. Later it was found that the time definition of the ion production from the laser pulse was less suited to TOF MS than initially assumed. Also, the complex mass spectra required a significantly higher mass resolution and mass accuracy than that available in TOF LMMS. Hence, FT LMMS was developed. Although the laser microbeam ionization was retained, the species detected and their relative intensities differ significantly from TOF LMMS. The reason is that each MS may capture a different fraction of the initial ion population, depending on its acceptance with respect to the initial kinetic energy (Ekin), their time and place of formation in the ion source. Therefore, a clear distinction between TOF and FT LMMS is mandatory. TOF LMMS

Figure 1 depicts a commercial instrument. The sample is mounted inside the vacuum chamber of the MS. The DI is commonly performed by 266 nm UV pulses from a frequency-quadrupled Nd:YAG (neodymium–yttrium–aluminium–garnet) laser. This instrument allows repositioning of the sample and the optics for analysis in transmission or in reflection. In the former case, the laser hits the lower surface of the sample while the upper surface faces the MS. This suits thin films or particles of about 1 µm on a polymer film. Reflection means that the beam impinges on the sample side facing the MS. The surface of bulk samples can thus be characterized. Micropositioners allow one to move the spot of interest on

1142 LASER MICROPROBE MASS SPECTROMETERS

the sample into the waist of the ionizing beam, which is visualized by a collinear He–Ne laser. The UV-irradiated spot is typically 1–3 µm in TOF LMMS so that power densities between 106 and 1011 W cm–2 can be attained with mJ pulses of 10–15 ns. Refractive objectives allow spot sizes down to the diffraction limit of 0.5 µm. Reflective optics (as in Figure 1) allow larger working distances and are free from chromatic aberrations so that use of other wavelengths is facilitated. Ionization with a tunable dye laser allows resonant one-step DI or postionization of the laser ablated neutrals. Specificity and sensitivity can thus be improved substantially but wavelength selection becomes cumbersome for unknown compounds. The principles of a TOF mass analyser are covered in another article. Commercial TOF LMMS permits a mass resolution of about 500, a much lower value than theory predicts, and this figure strongly depends on the analyte. The reason is quite fundamental. The application of TOF implies that ions with different m/z and therefore different velocities must arrive at the same time at the entrance to the drift tube. The pulsed laser ionization does not meet this requirement as ion formation may continue long after the laser pulse, especially for organic compounds but also for inorganic analytes. As a result, mass resolution and mass accuracy may become problematic. The asset of an unlimited m/z range is cancelled by the low mass resolution. A major advantage is the inherent panoramic registration.

FT LMMS

Figure 2 illustrates an instrument with an external ion source and shows the different microprobe related devices such as the optical interface, the micropositioners, the sample observation and the exchange system. This set-up features laser irradiation of the sample in reflection with a spot of 5 µm. Electrostatic fields transport ions through a differentially pumped transfer line from the source at 10–6 torr to the FT MS cell at 10–10 torr inside a 4.7 tesla magnetic field (B). Inherent advantages of FT MS are the routinely obtainable high mass resolution of over 100 000 at m/z 1000, and up to a few million below m/z 100, and the mass accuracy of better than 1 ppm up m/z 1000. Internal calibration of the m/z scale by adding a reference compound is not necessary as is the case in magnetic high–resolution mass spectrometers. A remarkable feature of FT MS is that better mass resolution means better sensitivity. This contrasts with other types of MS where resolution is increased by selecting a central fraction of the ion beam, thereby sacrificing sensitivity. The direct link of sensitivity and mass resolution in FT MS results from the influence of space charge effects, field imperfections and pressure in the cell. All these cause a faster decay of the coherence in the ion packets after excitation, which decreases mass resolution, but they also limit the quantitative trapping and/or the radius increase of the ion orbit during excitation, which diminish detection sensitivity. FT MS is also limited in the

Figure 1 Schematic diagram of the LIMA 2A TOF LMMS. Reprinted from the LIMA technical documentation with permission of Kratos Analytical.

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detection of low and high m/z by its detection bandwidth and B, respectively. A range between m/z 15 and 10 000 is common. The use of FT MS in microanalysis implies that the technique is operated close to its detection limits. Therefore, the vacuum in the cell becomes a prime reason for using an external ion source be used for LMMS as opposed to a single- or dual-cell instrument, where the sample is placed in the FT MS cell. Additionally, the Ekin of laser ions can exceed the energy range imposed by the optimal trapping potentials of typically less than 2 V. Biasing the sample holder or placing additional electrodes around the sample inside the cell disturbs the trapping field. Cooling of the ions by ion–molecule interactions is not always efficient. The use of micropositioners, laser and observation optics inside the narrow bore of the strong magnetic field, is not obvious. However, injection of external ions requires temporary deactivation of the potential gradient in the hole of the first trapping plate. Upon reflection against the second trapping plate, ions come back and may escape unless the entrance is blocked. The use of an electrostatic transfer line causes a time dispersion of ions of different m/z as in TOF MS, i.e. the so-called TOF effect. To trap a high m/z ion together with a low m/z ion, the high m/z ion, which arrives later, must enter the cell before the low m/z ion (which arrived earlier and has been reflected by the second trapping plate) leaves the cell. In practice, the m/z ratio of high-to-low m/z ions that can be trapped simultaneously in the cell is about a factor of 3–4, depending on the time of ion formation and life time of

the specific ions. Adapting the time between the laser pulse and the closure of the injection lenses, allows one to shift the m/z window along the m/z range in successive experiments.

Analytical characteristics Table 1 surveys the common techniques for microand surface analysis of solids. Both static secondary ion mass spectrometry (SSIMS) and LMMS provide molecular information on local organic and inorganic compounds. However, the primary interaction of keV ions with the sample in SSIMS as opposed to eV-range photons in LMMS makes the direct structural linkage of the detected signals to the sample composition less obvious in SSIMS. Hence, comparison with reference spectra is usually required in SSIMS, while LMMS allows a deductive interpretation. In fact, both methods are complementary with respect to their typical applications. SSIMS can detect high m/z ions from polymers, while LMMS yields more detailed structural information on analytes of up to a few kDa. SSIMS generates primarily ions from the upper monolayer, making surface contamination a problem in real–life applications. In contrast, ions in LMMS originate from the upper 10–50 nm surface layer, although the crater depth goes up to 0.1–1 µm. SSIMS allows imaging, while LMMS performs spot analysis. The reproducibility of LMMS strongly depends on the positioning of the sample surface in the waist of the UV beam. Hence, mapping by motorizing the

Figure 2 Schematic diagram of the FT LMMS with external ion source. Adapted from Struyf H, Van Roy W, Van Vaeck L, Van Grieken R, Gijbels R and Caravatti P (1993) Analytica Chimica Acta, 283: 139–151 with permission of Elsevier Science.

Overview of some important microanalytical techniques

Dynamic SIMS

Static SIMS

LMMS

EPXMA

Auger

ESCA

Raman

FT-IR

Stigmatic

Scanning

TOF

TOF

Probe input beam

20 keV electrons

Electrons ≤ 3 keV

Photons X-ray, UV

Photons visible

Photons IR

≤ 20 keV ions

40–60 keV ions

20 keV

UV photons (few eV)

Detected beam

X-rays

Electrons

Electrons

Photons

Photons

Parameter

WDS, EDS

Energy

Energy

Wavelength

Wavelength

Resolution of detector Typically analysed area Depth of information

WDS 20 eV EDS 150 eV ±1 µm

1–15 eV

0.3–1 eV

≤1 µm

1–3 nm

≥150 µm ≥5 µm (µESCA) 1–10 nm

0.7 cm–1 (IR) 2 cm–1 ( µIR) 5–10 µm

400–104

0.2 µm

0.7 cm–1 (sp) 8 cm–1 (im) 1 µm

Image resolution

SEM/X ±1 µm STEM/X< µm WDS 100 ppm EDS 1000 ppm WDS Z ≥ 4 EDS Z ≥ 11a No

50–100 nm

10 µm 5–50 nm (PAS) µm

Detection limit

Detection range Direct isotope information Compound speciation Organic characterization Destructive with sputtering In-depth profiling With sputtering Quantification Easy analysis of insulators Sample in vaccum required a

FT

+/– ions

+/– ions

m/z 250–500

m/z 103–104

≤850

104–106

2–250 µm

≥ 20 nm

0.1 µm

1–3 µm

5 µm

10 µm

0.5 nm

1 nm

Monolayer

(0.1–1 µm)

n.a.

0.5 mm

20 nm

0.5 µm

1 µm

>1 µm

Major

n.a. 1 mm (µIR) ppm

≤ ppm

10–100 ppm

Monolayer

10–3 –10–15

10–11–10–12g

All but H, He All but H, He

n.a.

n.a.

H–U

H-unlimited

H-unlimited

15–15 000

No

No

No

No

Yes

Yes

Yes

No

No

Yes

Yes

Yes

No

Yes

Yes

No

No

No

Yes

Yes

Yes

Yes

Yes

(No) n.a. No n.a. Yes No

No Yes No Yes Yes No

No Yes No Yes Yes Yes

No n.a. No n.a. (Yes) Yes

No n.a. No n.a. Yes Yes

Yes n.a. Yes n.a. Difficult No

Yes

Yes

Yes

No

No

Yes

≥1%

No Yes (µESCA) ≥1%

Yes n.a. No n.a. Difficult No Yes

+/– ions

m/z

Yes n.a. Yes n.a. Very difficult Yes Yes

EDS ranges to lower elements with windowless detector. PAS=photoacoustic single detection; WDS=wavelength dispersive spectrometry; EDS=energy dispersive spectrometry; SEM=scanning electron microscopy; STEM=scanning transmission microscopy; sp=spectrum; im=imaging mode; µIR=microscope FT-IR; µESCA=microESCA; n.a.=not applicable.

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Table 1

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sample positioners is only feasible for extremely flat samples. As to the kind of materials analysed, LMMS requires stability in a vacuum of 10–6 torr but SSIMS needs a pressure of under 10–8 torr, which prevents the analysis of ‘volatile’ organic compounds. Charge build-up in dielectrical samples occurs in SSIMS, not in LMMS. Quantification in MS requires adequate reference samples. For LMMS this means that not only the chemical composition but also the UV absorption, reflective and refractive properties of each microvolume must be comparable to ensure that the energy deposition and ion yield are similar. Hence, preparation of suitable reference materials often becomes the bottleneck. Biological sections or ambient aerosols are chemically and optically heterogeneous, so that quantification is not feasible. On the other hand, element diffusion in the wafers from semiconductor applications can be quantitatively studied.

Diagnostic use of the mass spectra Model of ion formation

Deduction of the analyte composition from the signals requires that one must be able to extrapolate the mass spectra from a limited database and ideally predict the ions from a given analyte. Therefore, practical concepts about ion formation are mandatory. Figures 3–5 survey a tentative model for DI in LMMS, applicable to both organic and inorganic compounds. Basically, the ionization event is described in terms of the three prime parameters in MS, namely local energy, pressure and time. First of all, laser impact is assumed to create different local energy regimes in physically distinct regions within and around the irradiated spot. Figure 3 depicts schematically the occurrence of atomization and destructive pyrolysis in hot spots, direct ejection of fragments and molecular aggregates from surrounding regions. Ultrafast thermal processes allow preservation of the molecular structure of even thermolabile analytes. The initial species will undergo fragmentation or subsequent ion–molecule interactions. Thermionic emission of e.g. alkali ions, yields the necessary flux of ions to form adducts with the initially released neutrals and ion-pairs. This occurs primarily after the laser pulse in the selvedge, i.e. the gas phase just above the sample. Secondly, the local pressure must be considered. As shown in Figure 4, the released neutrals and ions give rise to a gradually expanding microcloud. Its initial density depends on the ‘volatility’ of the analyte, i.e. the number of species released at a given power density. The importance of ion–molecule interactions

Figure 3 Relationship between the energy regimes at the surface and the ion formation process according to the tentative model for DI in LMMS. Reprinted from Van Vaeck L, Struyf H, Van Roy W and Adams F (1994) Mass Spectrometry Reviews, 13: 189–208 with permission of John Wiley and Sons.

depends on the local pressure in this cloud, which determines the collision probability. In low-density regions, ions follow unimolecular behaviour and fragmentation only depends on the internal energy. Finally, the timescale of the ionization is an essential feature of the approach since it must match the time window of detectable ions by a given mass analyser. The time evolution of the DI process is assumed to include two distinct phases. The schematic picture in Figure 5 shows the that the ion–generation process comprises two consecutive contributions. An initial intense ion current is produced during the first 10–25 ns, compatible with detection in TOF LMMS. However, selvedge ionization may already start in that initial period but will reach its maximum later and finally continues for microseconds after the end

Figure 4 Effect of the local pressure on the type of ions formed according to the tentative model for DI in LMMS. Reprinted from Van Vaeck L, Struyf H, Van Roy W and Adams F (1994) Mass Spectrometry Reviews 13: 189–208 with permission of John Wiley and Sons.

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TOF and FT LMMS spectra as to the species detected and their relative intensities. Moreover, it allows linkage of LMMS to the older literature on the soft ionization of organic compounds by defocused lasers and magnetic analysers, where fragmentation was virtually absent. Inorganic speciation

Figure 5 Influence of the time domain of ion formation and of the analyser on the mass spectra recorded according to the tentative model for DI in LMMS. Reprinted from Van Vaeck L, Struyf H, Van Roy W and Adams F (1994) Mass Spectrometry, Reviews, 13: 189–208 with permission of John Wiley and Sons.

of the laser pulse. This process may be small, but integrated over time, this contribution often prevails over the prompt DI. This rather slow DI component is not compatible with detection in TOF LMMS but is inherently included in the signal when magnetic mass spectrometers or FT MS ion traps are used. This tentative DI model does not give an accurate description of all the physical processes involved, but up to now, it allows a consistent explanation of the formation of the ions detected. Also, it unifies the processes of organic and inorganic ion formation and allows rationalization of the differences between

Figure 6 illustrates how FT LMMS signals can be used for analyte identification. The low m/z fragments and their adducts to the original molecule readily allow tentative identification of an unknown compound by deductive reasoning alone. This represents a major advantage in e.g. industrial materialsscience applications, where many not well characterized reagents are possible causes of anomalous behaviour. Recording reference spectra of all possible candidates for the given anomaly could be time consuming if one cannot select deductively the most likely one. The most demanding speciation task is the distinction between analytes with the same elements but in different ratios, e.g. sodium sulfate, sulfite and thiosulfate. The positive fragments Na+, Na2O, Na2O⋅H+ and Na2O⋅Na+ as well as the signals from Na2SO3⋅Na+ and Na2SO4⋅Na+ are observed by FT LMMS for all three analogues. However, only thiosulfate produces additional peaks for cationized Na2S, which characterize the presence of the

Figure 6 Scheme for the deductive identification of the analyte molecular composition from mass spectra taken by FT LMMS. Reprinted from Struyf H, Van Vaeck L, Poels K and Van Grieken R (1998) Journal of the American Society for Mass Spectrometry 9: 482–497 with permission of Elsevier Science.

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sodium–sulfur bond in this analogue. Adduct ions of Na2S2O3 are also seen at low intensity. Distinction can be based on the ratio of the ions Na2SO3⋅Na+ to Na2SO4⋅Na+, which is 0.82 ± 0.13 for sulfite and 0.30 ± 0.06 for sulfate. The production of cationized sulfate from sulfite and vice versa is rationalized by the relatively high-energy regime in the hot spots, where the stability of the formed ion prevails. Nevertheless, the softer regime of the periphery is assumed to produce the cationized analyte and thereby restores the logical connection with the original composition. The major signals in the negative mode are found at the nominal m/z values of 64 and 80 and are common for all three analogues. However, these signals are uniquely due to SO and SO for sulfite and sulfate, while thiosulfate produces additional contributions from S and S2O– also at m/z 64 and 80 respectively. The FT LMMS high mass resolution is needed to exploit such distinctive features. Identification of organic compounds

Deductive identification is even more important in organic microanalysis because of the numerous structures for each molecular weight. Figure 7 shows the positive and negative TOF LMMS data of a microscopic residue, obtained from the eluate of a single peak from analytical HPLC. The nanogram amount of available material was not consumed significantly after microprobe analysis although it was insufficient for conventional MS. The latter required the combination of fractions from numerous elutions to yield the mass spectrum of miconazole, the parent drug. In contrast, TOF LMMS detected the protonated molecules by the cluster of isotope peaks around m/z 503. Subsequent loss of HCOOH yields the fragments around m/z 457, while the accompanying lower m/z signals are readily rationalized by the known ring formation and rearrangement mechanisms in organic MS. Complementary information comes with the negative ions. Specifically the fragment at m/z 153 serves to specify further the presence of the imidazole methacrylate function in the molecule. This example highlights two features of the LMMS technique, namely the minute material consumption and the deductive identification of labile compounds without thermal destruction as a result of the ultrafast heating rate of the solid. Comparison of mass spectra recorded by TOF LMMS and FT LMMS

Several characteristic differences are observed between the mass spectra, recorded by FT LMMS and TOF LMMS from the same analyte under the same experimental conditions with respect to laser

wavelength, pulse duration and power density. While FT LMMS offers superior mass resolution and mass accuracy in comparison to TOF LMMS, the TOF effect in FT LMMS with an external source (see earlier sections on TOF LMMS and FT LMMS) means that only partial mass spectra in given m/z windows can be recorded (TOF LMMS always yields mass spectra that cover the entire m/z range). As well as these directly instrument-related data between FT LMMS and TOF LMMS, the mass spectral patterns still reflect additional distinctive features. Specifically, more intense signals from the adduct ions in comparison to the summed intensities of the fragments are seen in FT LMMS as compared to TOF LMMS. This is related to the increased sampling of the selvedge contribution to the initial ion population, created by the laser impact. Also the existence of the two contributions, i.e. direct ion emission from the solid and the selvedge recombination in the gas phase, could be observed by selectively tuning the ion source in FT LMMS so that only one of the contributions had initial energies within the 1 eV trapping energy of the cell. Note that this increased adduct ion detection in FT LMMS applies to both organic and inorganic compounds. For instance, cation and anion adducts of molecules and even dimers are generally detected in FT LMMS from virtually all salts such as Na2SO4, Na3PO4, oxides and binary salts, as opposed to TOF LMMS. Additionally, FT LMMS shows increased signal intensities specifically for fragment ions associated with the fragmentation of adduct ions with low internal energy. Such ions are observed to undergo the loss of small neutral molecules, while ions with high internal energy are subject to more drastic cleavages and rearrangement, which may reduce structural specificity. Finally, TOF LMMS is often handicapped in the analysis of high molecular mass and polar substances. These compounds are hard to desorb from the solid without the aid of matrix-assisted techniques, that require dissolution of the sample in a UV-absorbing matrix and, therefore, are incompatible with the direct local analysis of as-received solid samples. Especially for such strongly adsorbed compounds, the slow release of the analyte over a long time interval (up to several microseconds) occurs (continuing postlaser desorption). The adducts formed upon recombination with for instance Na+ (also thermionically emitted for long periods after the laser pulse) no longer fulfil the TOF LMMS requirement of ion formation within 25 ns. As a result, these ions give rise to broad unresolved peaks or even to a continuous background. However, in FT LMMS, ions formed within a period of 50 µs to several hundreds of µs (depending on the m/z) after the laser pulse are still trapped in the FT LMMS cell

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Figure 7 Positive and negative mass spectrum recorded by TOF LMMS from the residue of the eluate of a single peak in analytical HPLC. Reprinted from Van Vaeck L and Lauwers W (1989), Advances in Mass Spectrometry, 11a: 348–349 with permission of Heyden and Son.

together with the ions formed during the laser pulse. As a result, the rang e of compounds to which FT LMMS can be applied, significantly extends towards higher molecular mass and polarity as compared to TOF LMMS.

Selected applications Review of the literature reveals that LMMS is applied to a wide variety of problems in the field of

bioscience, environmental chemistry and materials research. Complete coverage is not feasible in this contribution, which only aims to demonstrate the strengths of the method. Specifically, LMMS excels at yielding, within a relatively short period, qualitative information on the organic or inorganic local surface components from the most diverse samples, often with negligible sample preparation. The transmission type TOF LMMS instruments especially suits biomedical section samples as commonly used for optical and electron microscopy.

LASER MICROPROBE MASS SPECTROMETERS 1149

The lateral resolution of 0.5 µm allows experiments at the subcellular level. Initial work has concentrated on the localization of elements in specific sites of the tissue. Typical examples involve the assessment of aluminium levels in the bones of chronic haemodialysis patients, the study of lead accumulation in kidneys and calcifications in intraperitoneal soft tissue as a result of chronic lead intoxication, and the detection of several heavy metals in the amalgam tattoos of the oral mucose membrane and human gingiva in direct contact with dental alloys. The relatively low lateral resolution, the lack of automated mapping and the almost impossible quantization strongly hinder the use of LMMS, especially in view of analytical electron microscopy (AEM) with X-ray analysis and the emerging possibilities of nuclear microscopy. As a result, experiments have become gradually directed towards applications where the direct extraction of molecular information from the biological sector could be exploited. Interpretation of the mass spectra in TOF LMMS with low mass resolution may become problematic when the local microvolume constitutes of a complex multicomponent system. As a result, applications were especially successful when LMMS was used to characterize the micrometre-size microliths and foreign bodies, often found in histological sections. In this way, the spheroliths in the Bowman’s membrane of patients suffering from primary atypical bandkeratopathy and the intrarenal microliths, formed after the administration of high doses of cyclosporin were proven to consist of hydroxyapatite. Implants often lead to long–term problems by dispersion of wear particles in tissues, leaching of specific compounds and sometimes subsequent chemical transformation. The pathogenesis of the aseptic loosening of joint prostheses was related to the presence of zirconium oxide in the granular foreign bodies of surrounding tissue. This compound was added to the bone cement to enhance the contrast in later X-ray radiographs. Examples of characterization of organic molecules include the detection of deposits from an anti-leprosy drug in the spleen of treated mice. The study of biomedical sections is not the exclusive field of TOF LMMS, although the lower lateral resolution of FT LMMS limits the number of applications. However, the much better specificity from the superior mass resolution and mass accuracy pays off with increasing complexity of the local composition. FT LMMS is particularly appreciated because of the separation of isobaric ions in the tissue, such as CaO+ and Fe+ ions, the facile assignment of a signal as an ‘organic’ ion or an ‘inorganic’ cluster on the basis of accurate m/z values, and the increased contribution of ions

from the selvedge, which decreases the influence of the local laser power density on the mass spectra. Practical applications widen the range, initiated by TOF LMMS. For instance, the foreign bodies in the inflammed tissue around implants were identified as wear particles from a titanium knee-implant. Other experiments involved the verification of the local molecular composition deduced from relative element abundances in AEM. The apoptotic cell death in the tissue around vein grafts was associated with the presence of hydroxyapatite because of the intense Ca and P signals in AEM. FT LMMS clearly showed the erroneous nature of this conclusion since both elements did not belong to the same molecule. This clearly illustrates the importance of molecular information in microanalysis. Figure 8 illustrates the application of FT LMMS for the in situ identification of pigments in a lichen, Haemmatomma ventosum. The interest in such natural products relates to the understanding of their role in the interception of sunlight and their possible interaction in energy transfer. The optical micrograph shows the dark-red dish-like fruiting bodies or apothecia on the light coloured thallus. The combination of positive and negative ion mass spectra taken from the apothecia gives quite a detailed picture of the molecule. Specifically, the molecular mass is available from the potassium adduct and the numerous fragments serve to deduce structural features. The material is analysed as it is found in nature, without any prior sample preparation. As to materials research, diverse problem-solving examples of LMMS are described. Typical examples involve the identification of local heterogeneities in poorly dispersed rubbers as one of the ingredients of the formulation, the tracing of the origin of occasional organic and inorganic contaminants at the surface of microelectronic devices and the study of segregation and formation of specific compounds in the joints of welded or heat–treated oxide-dispersion-strengthened alloys. Particularly interesting is the study of the dispersion of the magnetic elements inside the polyethylene terephthalate matrix at the surface of faulty floppy discs. Here the capability to detect both inorganic ions and organic structural fragments in the same spectrum is essential for trouble shooting. A final example illustrates how LMMS can contribute to the fundamental understanding of processing of materials. Specific ally, aluminium strips and plates are produced by hot rolling the primary alloy ingots under high pressure and temperature. The lubricating emulsions contain numerous additives, whose composition is empirically optimized. Sometimes, given constituents adhere or interact with the

1150 LASER MICROPROBE MASS SPECTROMETERS

Figure 8 In situ FT LMMS analysis of a pigment in the apothecia of a microlichen. Reprinted from Van Roy W, Matthey A and Van Vaeck L (1996) Rapid Communications in Mass Spectrometry 10: 562–572 with permission of John Wiley and Sons.

LASER MICROPROBE MASS SPECTROMETERS 1151

Figure 9 FT LMMS analysis of an additive coating on aluminium. Reprinted from Poels K, Van Vaeck L, Van Espen P, Terryn H and Adams F (1996) Rapid Communications in Mass Spectrometry 10: 1351–1360 with permission of John Wiley and Sons.

1152 LASER MICROPROBE MASS SPECTROMETERS

surface and cause optical defects upon subsequent coating or surface treatment. FT LMMS was used to study the possible additive–metal interactions. The data in Figure 9 for triethanolamine (TEA) oleate on aluminium show the normally expected simple adduct and fragments, as well as particularly interesting signals due to ions, which include multiply charged anions with Al3+. Since only singly charged aluminium is formed in laser microbeam DI, these Al3+-containing ions must exist as such in the solid and undergo direct ejection. This implies that TEA oleate is likely to bind effectively to the metal, while this additive is often considered as a chemically nonaggressive surfactant in the lubricating emulsions. To conclude, it should be mentioned that one of the major breakthroughs in the field of organic MS emerged from the research on the initial TOF LMMS instruments. Indeed, the now booming field of matrix assisted laser desorption ionization (MALDI) for the characterization of high molecular mass compounds up to 230 kDa found its roots in the laboratory of Prof. Hillenkamp, which was one of the driving forces behind the development of the initial TOF LMMS instruments. It shows the ingenuity of chemists in exploiting the powerful combination of laser ionization and MS. As to local analysis, the second generation of LMMS, FT LMMS seems to represent a major step in the search for a versatile microprobe, enabling us to characterize the molecular composition of organic and inorganic compounds at the surface of almost any type of solid, electrically conducting or not, with minimal sample preparation.

List of symbols Ekin = initial kinetic energy of ions. See also: FT-Raman Spectroscopy, Applications; Time of Flight Mass Spectrometers.

Further reading Eeckhaoudt S, Van Vaeck L, Gijbels R and Van Grieken R (1994) Laser microprobe mass spectrometry in biology and biomedicine. Scanning Electron Microscopy supplement 8: 335–358.

Poels K, Van Vaeck L, Van Espen P, Terryn H and Adams F (1996) Feasibility of Fourier transform laser microprobe mass spectrometry for the analysis of lubricating emulsions on rolled aluminium. Rapid Communications in Mass Spectrometry 10: 1351–1360. Struyf H, Van Roy W, Van Vaeck L, Van Grieken R, Gijbels R and Caravatti P (1993) A new laser microprobe Fourier transform mass spectrometer with external ion source for organic and inorganic microanalysis. Analytica Chimica Acta 283: 139–151. Struyf H, Van Vaeck L, Poels K and Van Grieken R (1998) Fourier transform laser microprobe mass spectromery for the molecular identification of inorganic compounds. Journal of the American Society for Mass Spectrometry 9: 482–497. Van Roy W, Matthey A and Van Vaeck L (1996) In situ analysis of lichen pigments by Fourier transform laser microprobe mass spectrometry with external ion source. Rapid Communications in Mass Spectrometry 10: 562–572. Van Vaeck L, Gijbels R and Lauwers W (1989) Laser microprobe mass spectrometry: an alternative for structural characterisation of polar and thermolabile organic compounds. In: Longiévalle P (ed) Advances in Mass Spectrometry, Vol. 11A, pp. 348–349. London: Heyden. Van Vaeck L, Struyf H, Van Roy W and Adams F (1994) Organic and inorganic analysis with laser microprobe mass spectrometry. Part 1: Instrumentation and methodology. Mass Spectrometry Reviews 13: 189–209. Van Vaeck L, Struyf H, Van Roy W and Adams F (1994) Organic and inorganic analysis with laser microprobe mass spectrometry. Part 2: Applications. Mass Spectrometry Reviews 13: 209–232. Verbueken A, Bruynseels F, Van Grieken R and Adams F (1988) Laser Microprobe Mass Spectrometry. In: Adams F, Gijbels R and Van Grieken R (eds) Inorganic Mass Spectrometry, pp. 173–256. New York: Wiley. Vertes A, Gijbels R and Adams F (eds) (1993) Laser ionisation mass analysis. Chemical Analysis Series, Vol. 124. New York: Wiley.