Other Ionisation Techniques

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

Recently, Lattimer et al. [22,95] advocated the use of mass spectrometry for direct analysis of nonvolatile compounding agents in polymer matrices as an alternative to extraction procedures. FAB-MS was thus applied as a means for surface desorption/ionisation of vulcanisates. FAB is often not as effective as other ionisation methods (El, Cl, FI, FD), and FAB-MS is not considered particularly useful for extracted rubber additives analysis compared to other methods that are available [36], The effectiveness of the FAB technique has been demonstrated for the analysis of a live-component additive mixture [96]. 

From the characteristics of the methods, it would appear that FD-MS can profitably be applied to poly-mer/additive dissolutions (without precipitation of the polymer or separation of the additive components). The FD approach was considered to be too difficult and fraught with inherent complications to be of routine use in the characterisation of anionic surfactants. The technique does, however, have a niche application in the area of nonpolar compound classes such as hydrocarbons and lubricants, compounds which are difficult to study using other mass-spectrometry ionisation techniques. 

The process of ion formation is extremely soft usually no fragmentation occurs. Mass spectra in ESI+ mode are dominated by pseudo-molecular ions (M+, [M + H] + or [M + Na]+) and cluster ions formed by the addition of one or more solvent molecules. For samples with molecular masses up to ca. 1000 Da (as in case of polymer additives), the ions produced from ESI are similar to those formed by other soft ionisation techniques, namely a protonated molecular ion (MH+) in the positive ionisation mode for basic compounds such... 

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

The ionspray (ISP, or pneumatically assisted electrospray) LC-MS interface offers all the benefits of electrospray ionisation with the additional advantages of accommodating a wide liquid flow range (up to 1 rnl.rnin ) and improved ion current stability [536]. In most LC-MS applications, one aims at introducing the highest possible flow-rate to the interface. While early ESI interfaces show best performance at 5-l() iLrnin, ion-spray interfaces are optimised for flow-rates between 50 and 200 xLmin 1. A gradient capillary HPLC system (320 xm i.d., 3-5 xLmin 1) is ideally suited for direct coupling to an electrospray mass spectrometer [537]. In sample-limited cases, nano-ISP interfaces are applied which can efficiently be operated at sub-p,Lmin 1 flow-rates [538,539]. These flow-rates are directly compatible with micro- and capillary HPLC systems, and with other separation techniques (CE, CEC). 

Mass spectrometry (MS) is an analytical method based on the determination of atomic or molecular masses of individual species in a sample. Information acquired allows determination of the nature, composition, and even structure of the analyte. Mass spectrometers can be classified into categories based on the mass separation technique used. Some of the instruments date back to the beginning of the twentieth century and were used for the study of charged particles or ionised atoms using magnetic fields, while others of modest performance, such as bench-top models often used in conjunction with chromatography, rely on different principles for mass analysis. Continuous improvements to the instruments, miniaturisation and advances in new ionisation techniques have made MS one of the methods with the widest application range because of its flexibility and extreme sensitivity. 

One of the main advantages of these soft ionisation techniques is that they lead to the formation of multiply charged, pseudomolecular ions (z can be greater than 30). Hence the mass range of the spectrometer can be extended to over 105 Da (to include proteins, polysaccharides and other polymers) (Fig. 16.20). These ionisation devices are often coupled to the mass spectrometer through a heated capillary transmitting the ions. 

The most important piece of information which may be obtained from a mass spectrum is the molecular weight. However, certain classes of compounds do not show molecular ions and in other cases it is not always possible to identify unequivocally the molecular ion. Therefore a family of so called soft ionisation techniques has been developed. These generate a molecular ion or quasi-molecular ion and fragmentation is kept to a minimum. The most commonly used technique is chemical ionisation (Cl)... 

Thus most polymers can be both taken into the gas phase and ionized allowing detection by mass spectrometry. There are some polymers that are nol routinely observed and although many generalities regarding ionisation, by either MALDI or ESI, can be made it is unadvisable at this point to extrapolate conditions between polymer types. The polymer chemist is now quite likely to obtain a mass spectrum of their sample to complement NMR, GPC, IR, DSC data, etc. It is necessary to consider what this information is, how it can be used and does it render other analytical techniques redundant ... 

Larhrib, H. Wells, M.H. Rubinstein, M.H., et al. Characterization of PEGs using matrix-assisted laser desorption/ ionisation mass spectrometry and other related techniques. Int. J. Pharm. 1997, 14, 187-198. 

Amoxicillin trihydrate did not give a useful mass spectrum by the conventional electron impact ionisation technique, even with a specialised in beam procedure which gave spectra from several other penicillins [26], Laser desorption [27] and desorption chemical ionisation [28] both gave pseudomolecular ions and the latter technique also gave significant fragmentation. 

In the early years, the petroleum industry acted as a catalyst for the development of this physical method. However, over the past twenty-five years, organic mass spectrometry has been the subject of a series of major developments. Some of them, such as the advent of commercially available interfaces for gas-liquid, high performance liquid, and supercritical fluid chromatographs, as well as novel ionisation techniques particularly well suited for high molecular weight, non-volatile macromolecules were welcome and well-accepted by the food analysts. Others, such as the introduction of relatively inexpensive quadrupole mass filters that proved to be reliable and... 

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