Ionisation modes

Wide choice of sample introduction, mass analysers and ionisation modes in relation to application... 

Applications Mass spectrometry has often been used more as an auxiliary, rather than a primary, identification method for additives in polymers. Table 6.5 shows the suitability of various ionisation modes for oligomer (and polymer) analysis. 

Ionisation mode Oligomer analysis Polymer MWD Reference... 

The main characteristics of FAB-MS are indicated in Table 6.15. FAB ionisation is relatively simple to perform. However, parameter optimisation and data interpretation of the resulting FAB spectra can be complex. Matrix selection for additive analysis is crucial. Solubility of the additives in the matrix is essential for production of viable spectra. FAB/FIB is well suited to organic compounds which exhibit some polarity, and contain either acidic and/or basic functional groups. Compounds with basic groups run well in positive ionisation mode, and those with acidic centres run well in the negative ionisation... 

Table 6.16 summarises the main characteristics of FI-MS. FT uses high voltages and was once restricted to sensitive double-focusing magnetic sector instruments of relatively high cost. Field ionisation is considered to be the softest ionisation mode. The reproducibility of the non-standard techniques, such as FI-MS and FD-MS, is less well assessed than that of EI-MS. A noticeable drop in FI use occurred after the mid-1980s because of the advent of FAB and other desorption/ionisation methods. FI-MS is only used in a few laboratories worldwide. 

Ionisation mechanism Technique Ions formed Ionisation mode Type of sample... 

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... 

Each of the analyser types has a unique set of figures of merit that makes it optimally suited for particular applications (Table 6.27). The main ionisation modes in relation to various mass spectrometers are summarised in Table 6.28. 

Direct solid-state polymer/additive mass analysis has involved various ionisation modes El (Section 6.2.1), Cl (Section 6.2.2), DCI (Section 6.2.2.1), FAB (Section 6.2.4), FI (Section 6.2.5), FD (Section 6.2.6) and LD. Survey mass spectra obtained with soft ionisation methods (FI-MS, CI-MS) provide diagnostic overviews of chemical composition. The supplemental tandem (MS/MS) and atomic composition (AC-MS) techniques are used to make specific identifications of various organic ingredients. Direct analysis of polymer systems for more than a few thousand daltons has only just begun. Ionisation methods employed are FD, ESI and MALDI. Solid-probe ToF-MS (or DI-HRMS) is a breakthrough [188]. 

LC-MS is now a nature technology and operation of an LC-MS system is no longer the realm of an MS specialist. The proper choice of the LC-MS mode to be used in a specific situation depends on analyte class, sample type and problem (detection, confirmation, identification). On-line LC-MS is used more for specialised applications than for general polymer or rubber compound analysis. This derives from the fact that LC-MS method development (column, solvent system, solvent programme, ionisation mode) is rather time consuming. LC-MS (in particular with API interface) enables analysis of a wide range of polar and nonvolatile compounds which cannot be analysed by GC (icf. Scheme 7.7). 

Variety of ionisation modes required to cover full polarity range... 

Different options are available for LC-MS instruments. The vacuum system of a mass spectrometer typically will accept liquid flows in the range of 10-20 p,L min-1. For higher flow-rates it is necessary to modify the vacuum system (TSP interface), to remove the solvent before entry into the ion source (MB interface) or to split the effluent of the column (DLI interface). In the latter case only a small fraction (10-20 iLrnin ) of the total effluent is introduced into the ion source, where the mobile phase provides for chemical ionisation of the sample. The currently available commercial LC-MS systems (Table 7.48) differ widely in characteristics mass spectrometer (QMS, QQQ, QITMS, ToF-MS, B, B-QITMS, QToF-MS), mass range m/z 25000), resolution (up to 5000), mass accuracy (at best <5ppm), scan speed (up to 13000Das-1), interface (usually ESP/ISP and APCI, nanospray, PB, CF-FAB). There is no single LC-MS interface and ionisation mode that is readily suitable for all compounds... 

Table 7.49 LC-MS interfaces, ionisation modes and mass spectrometers...

Soft ionisation modes, such as API, which leave the (pseudo)molecular ion intact without much fragmentation, offer more sensitivity, and are ideal for quantitative work at low levels (e.g. breakdown products). With the use of soft ionisation techniques in LC-MS, tandem MS... 

Many excellent reviews on the development, instrumentation and applications of LC-MS can be found in the literature [560-563]. Niessen [440] has recently reviewed interface technology and application of mass analysers in LC-MS. Column selection and operating conditions for LC-MS have been reviewed [564]. A guide to LC-MS has recently appeared [565]. Voress [535] has described electrospray instrumentation, Niessen [562] reviewed API, and others [566,567] have reviewed LC-PB-MS. For thermospray ionisation in MS, see refs [568,569]. Nielen and Buytenhuys [570] have discussed the potentials of LC-ESI-ToFMS and LC-MALDI-ToFMS. Miniaturisation (reduction of column i.d.) in LC-MS was recently critically evaluated [571]. LC-MS/MS was also reviewed [572]. Various books on LC-MS have appeared [164,433,434,573-575], some dealing specifically with selected ionisation modes, such as CF-FAB-MS [576] or API-MS [577],... 

The mass spectrometer is mainly used as a mass detector in chromatography (GC, SFC, HPLC, SEC, TLC). With the great variety of interfaces, ionisation modes and mass spectrometers, chromatography-mass spectrometry is highly diversified. High-resolution separations combined with accurate mass measurements and element-sensitive detection (MIP, ICP) have been reported. 

The use of GC-MS in polymer/additive analysis is now well established. Various GC-based polymer/additive protocols have been developed, embracing HTGC-MS, GC-HRMS and fast GC-MS with a wide variety of front-end devices (SHS, DHS, TD, DSI, LD, Py, SPE, SPME, PTV, etc.). Ionisation modes employed are mainly El, Cl (for gases) and ICPI (for liquid and solid samples). Useful instrumental developments are noticed for TD-GC-MS. GC-SMB-MS is a fast analytical tool as opposed to fast chromatography only [104]. GC-ToFMS is now about to take off. GC-REMPI-MS represents a 3D analytical technique based on compound-selective parameters of retention time, resonance ionisation wavelength and molecular mass [105]. 

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