Ionisation processes

Three types of ionisation process can be distinguished as contributing to the ions generated in the SSIMS experiment. These are illustrated very simplistic-ally by eqns (11.6)-(11.8) and describe direct desorption of pre-charged material (eqn (11.6)), cationisation/anionisation (eqn (11.7)) and electron ionisation (eqn (11.8))  

Desorption of precharged material (i.e. salts) is a highly efficient process, since energy is not translated into both an ionisation step and a desorption step existing ions in the solid specimen are simply transferred to the gaseous phase. This effect can be seen in the SIMS spectra of quaternary ammonium salts. Cationisation or anionisation of neutral molecules by attachment [35, 36] 

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Some of the gas atoms or molecules must be stripped of one or more of their electrons. The energy required to accomplish this, called the ionisation potential, is measured in electron volts. In MHD flows of interest, the required energy is suppHed by heating the gas. Thus the ionisation process is referred to as thermal ionisation. 

The smaller cluster ions 83", 84" and 85 + have been examined by Zakrzewski and von Niessen at the HF/6-3H-G level [82]. The lowest cationic states are predicted to be 82, and A" for 83 (Cyv), 84 (I>4h) and (Cs), respectively. The ionisation processes may result in significant structural relaxation leading to the sequence of states different from that of the vertical states. The calculated lowest adiabatic ionisation energies, using the GI method with a very large ANO basis set, are 9.53, 8.05, and 8.20 eV for 83, 84 and 85 , respectively. 

Principles and Characteristics Ionisation processes are the basis for mass-spectrometric detection. Each of the ionisation techniques occupies its own position in mass spectrometry. The optimum performance of any ionisation method (and therefore the result) will depend critically on the characteristics and reliability of the mass spectrometer. Ionisation may occur in the gas, liquid or condensed phase, and may be either hard or soft , i.e. with or without extensive... 

For the El ion source, the generated total ion stream is directly proportional to the gas pressure in the impact field, which provides a basic condition for quantitative analysis. Compounds can only safely be quantified if influences on the sensitivity of detection, such as ion-molecule reactions and competition in the ionisation process, can be excluded by experimental evidence. 

Ionisation processes in IMS occur in the gas phase through chemical reactions between sample molecules and a reservoir of reactive ions, i.e. the reactant ions. Formation of product ions in IMS bears resemblance to the chemistry in both APCI-MS and ECD technologies. Much yet needs to be learned about the kinetics of proton transfers and the structures of protonated gas-phase ions. Parallels have been drawn between IMS and CI-MS [277]. However, there are essential differences in ion identities between IMS, APCI-MS and CI-MS (see ref. [278]). The limited availability of IMS-MS (or IMMS) instruments during the last 35 years has impeded development of a comprehensive model for APCI. At the present time, the underlying basis of APCI and other ion-molecule events that occur in IMS remains vague. Rival techniques are MS and GC-MS. There are vast differences in the principles of ion separation in MS versus IMS. 

Most dyes, including sulfonated azo dyes, are nonvolatile or thermally unstable, and therefore are not amenable to GC or gas-phase ionisation processes. Therefore, GC-MS techniques cannot be used. GC-MS and TGA were applied for the identification of acrylated polyurethanes in coatings on optical fibres [295]. Although GC-MS is not suited for the analysis of polymers, the technique can be used for the study of the products of pyrolysis in air, e.g. related to smoke behaviour of CPVC/ABS and PVC/ABS blends [263],... 

The mobile phase in LC-MS may play several roles active carrier (to be removed prior to MS), transfer medium (for nonvolatile and/or thermally labile analytes from the liquid to the gas state), or essential constituent (analyte ionisation). As LC is often selected for the separation of involatile and thermally labile samples, ionisation methods different from those predominantly used in GC-MS are required. Only a few of the ionisation methods originally developed in MS, notably El and Cl, have found application in LC-MS, whereas other methods have been modified (e.g. FAB, PI) or remained incompatible (e.g. FD). Other ionisation methods (TSP, ESI, APCI, SSI) have even emerged in close relationship to LC-MS interfacing. With these methods, ion formation is achieved within the LC-MS interface, i.e. during the liquid- to gas-phase transition process. LC-MS ionisation processes involve either gas-phase ionisation (El), gas-phase chemical reactions (Cl, APCI) or ion evaporation (TSP, ESP, SSI). Van Baar [519] has reviewed ionisation methods (TSP, APCI, ESI and CF-FAB) in LC-MS. 

Table 7.63 lists LC-MS interface types and the species formed in the various ionisation processes. 

LC-tandem MS was recently used for polymer/additive characterisation. In cases of soft ionisation processes (e.g. ESI, APCI, etc.), MS/MS is often necessary to confirm the ionic species. QITMS has the potential to improve the detection limits for organotin analysis compared to QMS. HPLC-UV and LC-API-MS/MS have been employed for the characterisation of the products of photodegradation of benzotriazole-based UV absorbers (Tinuvin P/328/900) under mild conditions [642]. Among the photoproducts identified... 

HPLC High-performance liquid (ionisation process)... 

With APCI, the LC column eluent is nebulised in a heated vaporiser. Once vaporised, the plasma of eluent and sample components enters the APCI source where it encounters electrons emitted from the tip of a corona discharge needle. The eluent molecules are ionised and act as the reagent gas in a chemical ionisation process, transferring charge to the analyte. 

These different classes of molecules reflect the variety of reaction conditions which are present in ISM. Thus ion-molecule reactions dominate in the dark clouds these reactions are driven by ionisation processes and lead to unsaturated molecules, ions and radicals. 

Ion Relative abundance at 140 eV (%) Appearance potentials ieV) Probable ionisation process ... 

Several of the primary ionisation processes listed in Table 4 yield H and Cl atoms as well as ions, and similar processes are observed with HBr and HI. Atoms may also be formed in secondary ion-molecule reactions which are discussed below. As shown in Table 5, the ionisation potentials of the halogen atoms and their hydrides are such that the charge transfer reaction... 

The comparatively high ionisation potential of sulphur hexafluoride and its inertness toward attack by thermal hydrogen atoms have lead to its use as a specific scavenger for electrons in several irradiated systems. This has already been illustrated in section 1.7.2. The ionisation processes in SF6 have been studied by beam techniques171, but to date there has been no investigation of its radiolysis per se. Such a study would be well worthwhile. 

TSI. The liquid is converted into a vapour jet and small droplets are generated with the help of a heated vapouriser tube. A buffer dissolved in the eluent assists the ionisation process through the formation of adduct ions, which are produced via statistical charging of individual droplets. Due to the softness of the procedure, no structurally characteristic fragments, which could aid identification of unknown compounds, are formed. 

One of the most serious drawbacks that has been observed in the ionisation processes with soft ionisation techniques is the very soft generation of ions. This process, which predominately leads to molecular ions or adduct ions but no fragments for identification, however, was also used to improve and speed-up MS analysis. 

Applying product and parent ion scans in the FIA-MS-MS(+) mode, the unequivocal identification of the AE blend constituents is not possible. The reason for this failure is the simultaneous presence of C12 and Ci4 AE and AES compounds in the mixture that, under positive APCI ionisation conditions, were ionised with the same patterns of [CnH2n+iO(CH2-CH2-0) + NH4]+ ions (A m/z 44) and the same ion masses according to n, the alkyl chain lengths, and x, the polyether chain lengths. The destructive ionisation process of AES in APCI(+) mode therefore imagines a differentiation of the A m/z 44 compounds but results are not reliable. 

For examination of these interferences during the ionisation of surfactants in the FIA-MS mode we tried to verify and quantify potential effects in the ionisation process. Ionisation efficiencies of... 

In the negative FIA-MS spectra, the sulfate, phosphate and carboxylate of these anionic alkylphenol ether derivatives in parallel to the aliphatic ethoxy surfactants show equally spaced signals with Am/z 44. However, according to the ionisation method applied—APCI or ESI in the negative or positive mode—and also that observed in the ionisation process of AES, equally spaced signals came either from the anionic compounds themselves or from the alkylphenol ethers... 

The product ion spectra generated from the aromatic di-NPEC homologues ionised in the positive mode, however, suffered from the destructive ionisation of anionics, as also observed in the ionisation process of the aliphatic alkylether sulfates [22]. 

While under negative CID conditions, no structural information could be obtained by CID from NPE0-S04 compounds on a MS-MS instrument despite the variation of CID parameters, although the behaviour of the homologue ion mlz 590 (equivalent to the negative ion 667) under positive CID conditions was comparable with NPEO product ion spectra because of the destructive ionisation process of the precursor NPE0-S04 [15]. 

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