Alternative methods of ionization

Chemical ionization. Chemical ionization spectra result from ion-molecule reaction between the ionic products of a high pressure reagent gas, commonly methane, with a low pressure sample gas. Because of the low abundance of the sample, almost all of the initial ionization by electron impact is of the reagent gas. When methane is ionized at a source pressure of 1 mm Hg, the normal El products CHl and CHs react with neutral CH4 molecules producing a plasma in which CH5 (48% 2) and C2H5 (41% 2) are the principal species available for further ion-molecule reaction  

The ions CHs and C2H5 are strongly acidic and react with the sample molecule chiefly by proton transfer (as Bronsted acids) or by hydride abstraction (as Lewis acids) stable addition products can also be formed  

A broader appreciation of Cl in organic mass spectrometry and of subtleties in the use of reagent gases may be obtained from the reviews cited. The literature in the biological sciences in considerable. Some recent examples serve to indicate scope in application biogenic amines [121] drugs and metabolites [116,120,122] and [123] in which a listing of 

Atmospheric pressure ionization. A mass spectrometer in which sample ionization is achieved at atmospheric pressure (API) in an external source has been described [127, 128] and applications reported [129]. The source of electrons is a Ni foil and samples, injected in organic solvent, are swept through the reaction chamber (ion source) in a stream of nitrogen carrier gas. Ions and neutral molecules then enter a quadrupole mass analyser via a 25 )u,m diameter aperture. Continuous analysis of the ions, either positive or negative, formed in the reaction chamber, may be achieved by repetitive scanning or by selected ion monitoring. The ion-molecule reactions in the production of sample ions involve the carrier gas and the solvent. Thus the quasi-molecular ion for a compound injected in benzene as solvent may result from the following sequence  

There is currently much interest in field desorption (FD) which describes the process of field ionization from the adsorbed state with desorption as an ion. Field desorption takes place at lower temperatures than are normally required for evaporation of a molecule and since there is also only a low energy transfer in the ionization process itself, molecular ions of high intensity are formed. A number of underivatised polar compounds of low volatility have been investigated without thermal decomposition these include nucleosides and nucleotides [132], pesticides [133] and glycosides [134]. The method has also been extended by combining pyrolysis with FD to distinguish the five bases and some of the nucleotide fragments of deoxyribonucleic acid [135]. 

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There are ill-defined limits on EI/CI usage, based mostly on these issues of volatility and thermal stability. Sometimes these limits can be extended by preparation of a suitable chemical derivative. For example, polar carboxylic acids generally give either no or only a poor yield of molecular ions, but their conversion into methyl esters affords less polar, more volatile materials that can be examined easily by EL In the absence of an alternative method of ionization, EI/CI can still be used with clever manipulation of chemical derivatization techniques. 

Charge exchange was used as an alternative method of ionization in order to form parent ions in an independent manner, and examine the daughter ion spectra of the complementary parent ion M. ... 

An alternate method of ionization is described in the boxed essay "Peptide Mapping and MALDI Mass Spectrometry in Chapter 25. 

All the applications so far described used electron impact ionization for the MS analysis. Alternative methods of ionization. 

Cl is an efficient, and relatively mild, method of ionization which takes place at a relatively high pressure, when compared to other methods of ionization used in mass spectrometry. The kinetics of the ion-molecule reactions involved would suggest that ultimate sensitivity should be obtained when ionization takes place at atmospheric pressure. It is not possible, however, to use the conventional source of electrons, a heated metallic filament, to effect the initial ionization of a reagent gas at such pressures, and an alternative, such as Ni, a emitter, or a corona discharge, must be employed. The corona discharge is used in commercially available APCI systems as it gives greater sensitivity and is less hazardous than the alternative. 

Other methods employ a microplate format followed by fast HPLC. Some researchers approach the determination from a different perspective. For example, an alternative method for ionizable substances is the pSol determination based on an acid-base titration.25 26 Kinetic solubility determinations involve determining the concentration of the compound in the buffer of interest when an induced precipitate first appears. 

Figure 20.4. Alternative method of extrapolation of ionization constants of acetic acid.

The instrumentation and interfaces that had been used up to 1998 in CWC-related LC/MS analysis were summarized previously (4). At that time, sources that operate at atmospheric pressure, using electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI), were displacing their predecessors that used thermospray ionization or continuous flow fast atom bombardment. Atmospheric pressure ionization (API), either ESI or APCI, is now the method of choice in CWC-related analysis and will be the focus of this current review. A small number of recent applications involving alternative types of ionization are also included. For earlier applications of LC/MS to chemical weapons (CW) analysis, using thermospray and other ionization methods, the reader is referred to our previous review (4). The other major trend has been the increasing availability and ease of use of less-expensive bench-top quadrupole and... 

Ll Cl" " (Figure 12) was the most abundant species, and hence the easiest to measure. A reasonably linear segment is apparent over ca. one decade, although this curve is less steep than in the case of Li. This may reflect the more complex Boltzmann population of states for this tetratomic molecule, where a pre-exponential factor is necessary. The departure from linearity is also less abrupt, and occurs at ca. 10.20 eV. We take this to be the adiabatic I.P. of Li2Cl2. Our earlier photoelectron spectroscopic studies (18) yielded 10.22 and 10.17 eV for this quantity, employing two alternative methods of extrapolation. Hence, the inference seems quite plausible that departure from linearity on the semi-logarithmic plot yields a value very close to the adiabatic ionization potential. 

Finally, radiolysis/EPR results forced the reinterpretation of literature spanning more than 25 years on EPR studies of olefin reactions on zeolites in the presence of Lewis acid sites. " Spontaneous oxidation of olefins and products of their Bronsted acid-catalyzed reactions on dehydroxylated (to form Lewis acid sites) ZSM5 and Mordenite vide infra) provides an alternative method of spin labeling, and the technique gives results mostly consistent with those from radiolysis/EPR. However, in the spontaneous oxidation method, the limited degree of control over the ionization process and progress of the catalysis has led to erroneous conclusions (mainly in the earlier work). Interpretation of the radiolysis/EPR experiments is greatly aided by the ability to methodically vary the relative contribution of different products to the EPR spectrum. 

Electron impact (El), the method outlined in Section 9.6.1, is the conventional method of ionizing samples for MS. However, El is not free from disadvantages, such as inability to provide molecular weight information from thermally labile samples, difficulty for study of low-volatility materials, complex spectra arising from interference between molecular and fragment ions, etc. In order to overcome these problems, a large number of alternative ionization methods, collectively known as soft ionization methods have been developed. Reviews of the available techniques have been compiled by Milne and Lacey (1974) and Games (1978). Chemical ionization (Cl), field ionization (FI) and field desorption (FD)... 

The ability of ionizing radiation to kill bacteria has been known for some considerable time, but it was only with the advent of large radiation sources that the possibility of applying this process on an industrial scale arose. Previous to the last war, practically all sterilization was carried out by steam, and it is only in the past few years that important developments have taken place in alternative methods of sterilization. There have been two main developments firstly, sterilization by gases such as ethylene oxide and propylene oxide or mixtures of these two, and secondly, radiation sterilization. Both methods of sterilization are cold processes, and it is possible to sterilize articles made from heat-labile materials, in particular, plastics. The main advantages of radiation sterilization are as follows ... 

Photoionization Photoionization provides an alternative method of ion production for mass spectral analysis. Recent reports by Uchimura and coworkers have demonstrated that shortening the photoionization UV laser pulse from 260 to 60 fs leads to a substantial increase in the TATP molecular ion at m/z 222 [44,45]. At a pulse energy of 11 pJ, the molecular ion was observed to be 15% of the intensity of the m/z 43 base peak in the spectrum. The short pulse ionization method was successfully interfaced with GC [45]. 

Analysis of free fatty acids can be carried out by Gas Chromatography Flame Ionization Detection (GC-FID), this technique being an alternative method of analysis (Hajimahmoodi et... 

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