Ionisation overview

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]. 

To summarise, a fractionation step allows the isolation of the compounds of interest from the other molecular constituents, particularly from the fatty acids that are well-ionised. To compensate for the low ionisation yield of some compounds, such as TAGs, the solutions may be doped with a cation. Samples are then directly infused into the ion electrospray source of the mass spectrometer. A first spectrum provides an overview of the main molecular compounds present in the solution based on the peaks related to molecular cations. The MS/MS experiment is then performed to elucidate the structure of each high molecular compound. Table 4.2 shows the different methods of sample preparation and analysis of nonvolatile compounds as esters and TAGs from reference beeswax, animal fats and archaeological samples. 

The soft API techniques such as APCI and ESI, which are predominantly applied nowadays fulfil the most desirable criteria for an ionisation method in MS-MS, especially for mixture analysis, where each compound contained in the mixture produce as few—ideally only one—ions of different mass-to-charge ratio (m/z) as possible upon ionisation. The result would be a quite simple FIA-MS overview spectrum with few interferences in the parent ion to be selected... 

In environmental analytical applications where analyte concentrations, e.g. surfactants or their metabolites, are quite low, extraction and concentration steps become essential. Solid phase extraction (SPE) with cartridges, disks or SPME fibres (solid phase micro extraction) because of its good variety of SP materials available has become the method of choice for the analysis of surfactants in water samples in combination with FIA as well as LC—MS analysis. SPE followed by sequential selective elution provides far-reaching pre-separations if eluents with different polarities and their mixtures are applied. The compounds under these conditions are separated in the MS spectrometer by their m/z ratios providing an overview of the ionisable compounds contained in a sample. Identification in the sense it has been mentioned before, however, requires the generation of fragments. 

The FIA-MS screening approach using soft ionisation interfaces prior to any CID procedure provides an overview of the MS separation procedure, which is based on the different m/z ratios of the molecular or cluster ions generated. With the help of this very fast screening method—positive or negative FIA-MS by-passing the analytical column—the surfactant chemist is able to characterise complex blends and formulations without difficulty (Fig. 2.5.1) while the experienced analyst is able to make initial statements about the presence of frequently used and therefore most important surfactants in environmental samples (Fig. 2.5.2) despite the presence of complex matrices. The information provided by ESI or APCI—FIA—MS overview spectra for a first characterisation [8,17-19], which were also available with non-API soft ionising interfaces such as FAB [20] or TSI [9] in industrial blends as well as environmental samples, were obtained from ... 

Fig. 2.5.4. (a) APCI-FIA-MS(-) and (b) APCI-FIA-MS(+) overview spectrum combined with general structural formula demonstrating differences in the ionisation behaviour of NPEO-SO4 (C9H -0fiH4-0-(CH2-CH2-())r-S()3). 

As an example of an anionic surfactant mixture frequently contained in detergent formulations, an AES blend with the general formula C H2 i i—O—(CH2—CH2—O) —SO3 was examined in the negative FLAMS mode. Because of the considerable differences observed between both API ionisation mode overview spectra, the ESI—FIA—MS(—) and the APCI—FIA—MS(—) spectra are reproduced in Fig. 2.5.3(a) and (b), respectively. Ionisation of this blend in the positive APCI—FIA—MS mode, not presented here, leads to the destruction of the AES molecules by scission of the O—SO3 bond. Instead of the ions of the anionic surfactant mixture of AES, ions of AE can then be observed imaging the presence of non-ionic surfactants of AE type. 

Fig. 2.5.12. APCI-FIA-MS(+) overview spectra of industrial surfactant blends used as pure blends or mixtures in the examination of ionisation interferences, (a) C13-AE, (b) cationic (alkyl benzyl dimethyl ammonium quat) surfactant, (c) amphoteric C12-alkylamido betaine, and (d) non-ionic FADA all recorded from methanolic solutions. 

In the qualitative analyses of surfactants, the FIA-MS screening method applying both soft ionising API interface types, APCI and ESI, provides the overview spectra that contain the molecular ions or adduct... 

Fig. 2.9.6. (Inset) ESI-FIA-MS(+) overview spectrum of (a) PEG homologues and (b) PPG homologues contained in wastewater effluent SPE extract (7) ESI-LC-MS(+) RIC and selected mass traces of (l)-(3) PEG and (4)-(6) PPG homologues from mixture of (a) and (b) CiS-SPE with selective elution, compounds ionised as [M + NH4]+...

Fig. 2.9.7. (a) ESI-FIA-MS(+) and (b) APCI-FIA-MS(+) overview spectra of synthetically produced mixture of di-carboxylated PEG homologues (f) APCI-LC-MS(+) and (j) APCI-LC-MS(-) RICs of mixture as in (a,b) (c-e) selected mass traces of di-carboxylated PEG homologues under positive and (g-i) negative ionisation. Gradient elution separated by RP-Ci8 column [24]. 

Fig. 2.9.10. Interdependences of temperature and ions examined in the APCI-FIA-MS(+) process. Overview spectra of AP blend ionised at source temperatures ((a) 400°C and (b) 200°C) resulting in [M + NH4]+ and [M + H]+ ions and [M + NH4P ions of PPG...

Fig. 2.9.17. APCI-FIA-MSC+) overview spectrum of fatty acid EO/PO polyglycolether blend ionised as [M + NH4]1 ions [16]. 

Fig. 2.9.22. APCI—FIA—MS(+) overview spectrum of methylated EO/PO polyglycolether blend ionised in the form of [M + NH4P ions [37].

The homologues of the methylated non-ionic EO/PO surfactant blend were ionised as [M + NH4]+ ions. A mixture of these isomeric compounds, which could not be defined by their structure because separation was impossible, was ionised with its [M + NH4]+ ion at m/z 568. The mixture of different ions hidden behind this defined m/z ratio was submitted to fragmentation by the application of APCI—FIA—MS— MS(+). The product ion spectrum of the selected isomer as shown with its structure in Fig. 2.9.23 is presented together with the interpretation of the fragmentation behaviour of the isomer. One of the main difficulties that complicated the determination of the structure was that one EO unit in the ethoxylate chain in combination with an additional methylene group in the alkyl chain is equivalent to one PO unit in the ethoxylate chain (cf. table of structural combinations). The overview spectrum of the blend was complex because of this variation in homologues and isomers. The product ion spectrum was also complex, because product ions obtained by FIA from isomers with different EO/PO sequences could be observed complicating the spectrum. The statistical variations of the EO and PO units in the ethoxylate chain of the parent ions of isomers with m/z 568 under CID... 

The qualitative determination of anionic surfactants in environmental samples such as water extracts by flow injection analysis coupled with MS (FIA-MS) applying a screening approach in the negative ionisation mode sometimes may be very effective. Using atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI), coupled with FIA or LC in combination with MS, anionic surfactants are either predominantly or sometimes exclusively ionised in the negative mode. Therefore, overview spectra obtained by FIA—MS(—) often are very clear and free from disturbing matrix components that are ionisable only in the positive mode. However, the advantage of clear... 

FIA-MS overview spectra in the APCI(-) and ESI(—) mode proved that the AES blend examined, contained two mixtures of homologues with Ci2 and C14 alkyl chains (Fig. 2.11.7(a)). FIA-MS(—) spectra confirmed that, in addition to AES compounds, the ASs precursor compounds with n = 12 and 14 and x = 0 (mJz 265 and 293) were also present. In the AES molecules, the Ci2 and C14 alkyl chains were coupled with ethoxy chains that could be ionised to a quite different extent. While APCI(—) ionised AESs up to 14 or 9 polyglycol ether units for the Ci2 and C14 homologues, respectively, ESI(—) ionised AESs up to 6 or 4 PEG units, all equally spaced by Am/z 44 (cf. Table 2.11.1). 

For comparison of the different ionisation methods and detection modes, the results obtained as FIA overview spectra are presented in Figs. 2.11.7 and 2.11.8. Reconstructed ion chromatograms (RIC) of APCI and ESI combined with selected mass traces of all LC separations and, in parallel, the selected standardised mass traces of the C42 and C14 homologues containing three ethoxy chain links recorded in the negative mode are presented in Fig. 2.11.9. These results again demonstrate the quite large variation in the ionisation efficiency of... 

Fig. 2.11.8. APCI-FIA-MS(+) overview spectrum of an AES blend as in Fig. 2.11.7 (C H2 -i-(0-CH2-CH2) -OS03H) confirming the destructive ionisation results obtained under positive APCI ionisation [61]. 

The behaviour of AEC under API-MS conditions in both positive and negative ionisation modes was the same as that observed with AES. Under APCI and ESI-FIA-MS(-), ionisation spectra of this commercial blend of AEC contained the deprotonated molecular [M - H] ions as presented in the overview spectrum in Fig. 2.11.12. The signals observed represent the equally spaced ions of the C8Hi7-0-(CH2-CH2-0)n-CH2-C00 homologues (re = 5-17 mlz 407-891, equally spaced by Am/z 44) [61]. The same mixture ionised in the positive APCI mode, however, contained a more complex pattern of signals consisting... 

The FIA-MS(—) overview spectrum is presented in Fig. 2.11.16. It shows the di-NPEC homologue ions all equally spaced by Am/z 44. The ion starting at m/z 535 contains three ethoxylate units while the homologue at m/z 1019 represents a molecule with 14 EO units. In the APCI-LC-MS(+/—) mode, RICs and the selected mass traces at m/z 760 or 799 represent the di-NPEC homologue with nine EO units in the form of [M — CH2 — C02 + NH4]+ or [M — H] ions as shown in Fig. 2.11.17. Under positive ionisation conditions, the detection of the NPEO homologues ionised as [M + NH4]+ ions are favoured in the reconstructed ion mass trace (b) [22,61]. 

Knotek and Feibelman [94] examined the modification to a surface when exposed to ionising radiation and assesed the damage that can be produced. They addressed the stability of ionically bonded surfaces, where the KF mechanism applies, and concluded that Auger induced decomposition only occurs when the cation in the solid is ionised to relatively deep core levels. In the case of non-maximal oxides as with NiO, Freund s group [95] showed that whilst desorption of neutral NO and CO from NiO(lOO) and (111) surfaces has thresholds at the C Is, N Is and O Is core levels, it proceeds mainly on the basis of the MGR model, involving an excited state of the adsorbate. An overview of electronic desorption presented by Feibelman in 1983 [96] examined particularly the stability of the multiple-hole final state configuration leading to desorption. The presence of multiple holes, and associated hole-hole correlation... 

Fig. 8.22. The vanishing radiative width in the Ba spectrum. Experimental data are shown which correspond to oscillator strength measurements below the ionisation threshold by the magneto-optical method described in chapter 4, which also gives an overview of the Ba spectrum including the broad 5d8p perturber responsible for the vanishing radiative width. The inset shows a comparison between measured values (squares) and an MQDT extrapolation (triangles) on a logarithmic scale (see also fig. 4.3 - after J.-P. Connerade et al. [136]).

The two chapters that were selected for this topic one on GC-ion trap mass spectrometry, by SabUer and Fujii and the other by Schroder on LC-MS in environmental analysis give an excellent contribution to the application of GC-MS and LC-MS to environmental analysis. Both chapters include many practical aspects and examples in the environmental field and also cover the historical perspective of the techniques and show the perspective on ionisation and scanning modes. Advances achieved in GC-ion trap by the use of external ion sources and GC/MS/MS possi-bihties are discussed. The LC-MS chapter provides an overview of the first applications of LC/MS interfacing systems, such as moving belt, direct Uquid introduction (DLI) and particle beam (PB), and then on the more recent soft ionisation techniques, like thermospray and atmospheric pressure ionisation interfacing systems. 

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