Laser ionisation, analytical method

An alternative to chemical ionisation is resonant (and non-resonant) laser ionisation methods [179], i.e. selective and soft laser photoionisation, such as REMPI. A particularly interesting setup is the combination of REMPI with TOFMS for monitoring coffee brew headspace. This chapter deals with technical features and applications of time-resolved analytical methods with particular focus on PTR-MS and resonant and laser ionisation methods (REMPI-TOFMS). 

The results are reported of a study of plate-out in PVC extrusion carried out using several analytical techniques, including DSC, SEM-EDX, FTIR spectroscopy and laser ionisation mass spectrometry. A special die and calibrator unit developed to investigate plate-out are illustrated and the reproducibility of the method evaluated. The effects of water content and anti-plate-out additives, such as alumina and silica, on plate-out are discussed and mechanisms explaining the formation of plate-out are proposed. 11 refs. 

A variety of laser techniques have been applied for direct analysis of additives in polymers. Table 3.1 shows the relation between various induced phenomena and proposed analytical methods, broadly classitied as laser spectroscopy and laser mass spectrometry. Some specific features for lasers are multiphoton absorption spectroscopy (based on the frequency dependency), multiphoton ionisation spectroscopy (with resonance enhancement) and laser mass spectrometries (laser wavelength and intensity dependency desorption/ionisation). 

In laser vaporisation experiments, generating a plume , the laser s frequency may be synchronised with the resonance line of the element (analyte) to be analysed. The basic principles are (i) absorption of the radiation by the analyte (LAAS laser atomic absorption spectrometry) (ii) fluorescence (LIE, laser-induced fluorescence LEAFS) or (Hi) production of ionisation products (ions and electrons). LIF is an analytical method of high precision that is suitable for the measurement of diatomic species in the plume. Excitation spectroscopy or laser-excited fluorescence is not concerned with the spectral composition of the fluorescence but with how the overall intensity of emission varies with the wavelength of excitation. 

Desorption/ionisation (DI) methods may be classified as one or two-step processes with or without the use of lasers. Direct formation of ions by photon-solid interaction involves complicated energy deposition phenomena, which are largely unknown and hence completely uncontrollable. Most of the emitted particles are neutrals, not ions. Laser ionisation of the analyte isolated in the gas phase from the surrounding matrix constituents can be addressed more selectively than the solid target-matrix combination. One-step desorption/ionisation does not fuUy exploit the advantages offered by laser techniques. 

REMPI) is due to a successive absorption of two or more laser photons. The sum of the photon energies must exceed the ionisation energy. REMPI is the most sophisticated form of photoionisation, where the wavelength of an excitation laser is varied such that a second photon (from the same or another laser) ionises only when the first photon is resonant with a specific molecular level. This provides a combination of optical spectroscopy and mass spectrometry (i.e. a 2D analytical method). The intensity... 

Two slightly different laser desorption/ionisation methods were developed simultaneously by Karas and Hillenkamp [1] and Tanaka et al. [2]. Whereas Karas and Hillenkamp used small organic matrix molecules to assist and facilitate the desorption and ionisation of analytes (MALDI), Tanaka et al. used ultra-fine metal powders and glycerol. Zumbiihl et al. first analysed natural triterpenoid resins, dammar and mastic, both... 

While fast atom bombardment (FAB) [66] and TSI [25] built up the basis for a substance-specific analysis of the low-volatile surfactants within the late 1980s and early 1990s, these techniques nowadays have been replaced successfully by the API methods [22], ESI and APCI, and matrix assisted laser desorption ionisation (MALDI). In the analyses of anionic surfactants, the negative ionisation mode can be applied in FIA-MS and LC-MS providing a more selective determination for these types of compounds than other analytical approaches. Application of positive ionisation to anionics of ethoxylate type compounds led to the abstraction of the anionic moiety in the molecule while the alkyl or alkylaryl ethoxylate moiety is ionised in the form of AE or APEO ions. Identification of most anionic surfactants by MS-MS was observed to be more complicated than the identification of non-ionic surfactants. Product ion spectra often suffer from a reduced number of negative product ions and, in addition, product ions that are observed are less characteristic than positively generated product ions of non-ionics. The most important obstacle in the identification and quantification of surfactants and their metabolites, however, is the lack of commercially available standards. The problems with identification will be aggravated by an absence of universally applicable product ion libraries. 

A recently introduced technique for the separation of larger molecules is matrix-assisted faser Resorption-ionisation mass spectrometry (MALDI-MS). Developed by Karas et al. [4, 5] in 1988, it has been successfully used to determine the mass of biomolecules up to 500.000 Da. This method is based on the principle that the dissolved specimen is mixed with a matrix, and then crystallizes. After drying, a laser pulse is directed onto the solid matrix to photo-excite the matrix material,resulting in desorption and soft ionisation of the analyte.The molar mass is then determined by the lime ef ilight (TOF). 

A reliable analysis of multicomponent mixtures is particularly difficult and the situation becomes more complicated and less meaningful with increasing mixture size. Nevertheless, the analysis of peptide libraries is possible by mass spectroscopic means.366-572 mixture size increases, the main aim of any analysis is to ensure that the vast majority of expected library members are represented in the mixture. The use of MS in conjunction with other analytical techniques (HPLC-MS, GC-MS, MS-MS) have been used successfully in library characterisation. 3.425.573 Electronspray-MS is a particularly mild method for ionisation. s Matrix-assisted Laser Desorption Ionisation - time-of-flight (MALDI-TOF) mass spectrometry is a tool of increasing importance in multicomponent analysis. Its reduced tendency to preferential ionisation results in an increased likelihood of observing ions from all components.5 575... 

The choice of an ionisation method depends on the analyte characteristics and the required type of analytical information. Classically, hard ionisation methods such as electron ionisation (El) or chemical ionisation (Cl) make use of their fragmentation capabilities to gain stractural information, typically of small organic molecules. In contrast, soft ionisation techniques such as electrospray ionisation or laser desorption are used to obtain mass spectra of intact molecules with little or no fragmentation, being capable of analysing complex multi-component mixtures. 

Matrix-assisted laser desertion ionization (MALDI) was first described by Karas and Hillenkamp in 1988 [469]. At that time it was a revolutionary method for the ionisation and analysis of large biomolecules. Now many more MS methods have been devised, but the analysis of complete protein masses is still only possible by MALDI [470]. With MALDI, a matrix (crystals of small organic molecules) with a small amount of analyte is ionised by a short laser pulse at a wavelength close to the adsorption band of the matrix molecules. This produces predominantly singly charged molecular ions, which are detected by the TOE spectrometer. The ana-... 

Laser-enhanced ionisation spectroscopy (LEIS) is essentially a very sensitive mono-element analysis method (as AFS or AAS) with limits of detection (LCDs) often in the 1-100 pgmL range [96,97]. LEIS is based on the measurement of the increase in ionisation of the analyte in a flame, furnace, or glow discharge by laser irradiation as a result of selective population of a level of the term diagram. In LEIS, one or two dye lasers are tuned to a wavelength characteristic of an electronic transition of the species of interest. The laser beam(s) are directed... 

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