Chemical ionization principles

Moini, M. in Gross, M. L. and Caprioli, R. M. (eds) (2007) The Encyclopedia of Mass Spectrometry, Atmospheric Pressure Chemical Ionization Principles, Instrumentation, and Applications,... 

In the following chapters, the basic principles of HPLC and MS, in as far as they relate to the LC-MS combination, will be discussed and seven of the most important types of interface which have been made available commercially will be considered. Particular attention will be paid to the electrospray and atmospheric-pressure chemical ionization interfaces as these are the ones most widely used today. The use of LC-MS for identification and quantitation will be described and appropriate applications will be discussed. 

MS involves the separation of ions based on their mass-to-charge ratio (m/z). The concept was invented a century ago1 with a dramatic impact on analytical chemistry.2-3 The fundamental principle of MS requires vaporization of the molecules in the gas phase and in ionization. Early ionization methods such as electron impact (El) and chemical ionization (Cl)4-5 were limited to small organic molecules that were volatile and stable to heat and amenable to transfer into high vacuum. Introduction of the fast-atom-bombardment (FAB) method of ionization6... 

Figure 14.1 Schematic view of a mass spectrometer. Its basic parts are ion source, mass analyzer, and detector. Selected principles realized in modern mass spectrometers are assigned El—electron impact. Cl—chemical ionization, FAB—fast atom bombardment, ESI—electrospray ionization, MALDI—matrix-assisted laser desorption/ionization. Different combinations of ion formation with mass separation can be realized.

Figure 14.3 Principle of atmospheric pressure chemical ionization. The dissolved analyte is sprayed through a capillary. Evaporation of the solvent is supported by a heated gas stream. Within the source, a plasma is formed by a Corona discharge needle, which creates the charged reagent gas (here HgO+j. The ionization of the analyte (M) is performed by the transfer of the charge (proton) via ion-molecule reactions.

Chemical ionization is, as might be expected from its name, more chemically interesting and is closely allied to ion cyclotron resonance, which will be discussed in the next section. The principle of chemical ionization is simple. The molecule to be studied is injected into the ionizing region of the mass spectrometer in the presence of 0.5-1.5 mm Hg pressure of a gas, usually methane. Electron impact causes ionization of the methane, which is present in relatively large concentration. The ionization products of methane then react with the compound to be analyzed and convert it to ions. The gas mixture then exits into a low-pressure zone (10 4 mm) and the ions are analyzed according to mje in the usual way. 

The method is based on the principle of chromatographic separation of components of a mixture on a GC column, followed by their identification from their mass spectra. The compounds are separated on a suitable GC column, following which, the components eluted from the column are subjected to electron-impact or chemical ionization. The fragmented and molecular ions are identified from their characteristic mass spectra. Thus, the substances present in the sample are determined from their characteristic primary and secondary ions and also from their retention times. 

Other early attempts at quantification from first principles included use of the Dobretsov equation for surface ionization through nonequilibrium thermodynamics [87], use of quantum mechanical models [88,89], and others, including surface bond breaking and dissociative [90] or chemical ionization [91]. None of these led to successful quantification schemes. An evaluation of several of these methods was made by Rudat and Morrison [92]. 

MS/MS is an empirical method of analysis. As is evident from the exanples presented, the interpretation of a daughter ion MS/MS spectrum is often based on the same correlation principles derived frcm electron and chemical ionization mass spectrometry. More often, the comparison of the spectrum obtained to that of the authentic capound is used for identification. This is a fundamentally unsatisfying procedure. While electron and chemical ionization spectra can be compared to a spectral library which has been compiled over the past thirty years, no comparable library of MS/MS spectra exists. Data systems may be used within individual... 

Selected ion flow tube mass spectrometry (SIFT-MS) is an analytical technique used for direct and quantitative determination of VOCs in mixtures of gases. SIFT-MS was introduced in 1976 by N. G. Adams and D. Smith. The technique can be applied for parallel real-time monitoring of a few substances [126]. A scheme of the SIFT-MS system is presented in Fig. 14.9 [127]. The principle of gas mixture analysis is based on the reaction of reagent ions with molecules of analyte within a specific time (a few milliseconds). In this method, chemical ionization is applied reagent ions are generated in the ion source by a suitable ionization gas (nitrogen, oxygen, or water vapor). Of aU the obtained ions, only cations of the desired m/z... 

Three popular ionization techniques are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and matrix-assisted laser desorption (MALDI). Electrospray is the most widely used ionization technique when performing LC-MS, and has proved to be a most versatile tool for soft ionization [72] of a large variety of analytes such as them described in paper I. Figure 6 shows the principle of the ESI. 

E DeHoffmann, J Charette, V Straoobant. Chemical ionization. In Mass Spectrometry Principles and Applications. Paris Wiley, 1996. 

The general principle of mass spectrometry (MS) is to produce, separate and detect gas phase ions. Traditionally, thermal vaporization methods are used to transfer molecules into the gas phase. The classical methods for ionization are electron impact (El) and chemical ionization (Cl). Most biomolecules, however, undergo severe decomposition and fragmentation under the conditions of both methods. Consequently, the capabilities of mass spectrometry have been limited to molecules the size of dinucleotides [1]. Analysis of oligonucleotides with a mass range of up to 3000 Da became feasible with the development of plasma desorption (PD) methods [2]. However, until the invention of soft ionization techniques such as ESI- and MALDI MS, mass spectrometric tools were not widely considered for routine applications in biological sciences. 

Atmospheric pressure chemical ionization (Bruins, 1991) was developed starting from the assumption that the yield of a gas-phase reaction depends not only on the partial pressure of the two reactants, but also on the total pressure of the reaction environment. For this reason, the passage from the operative pressure of 0.1-1 Torr, present inside a classical Cl source, to atmospheric pressure would, in principle, lead to a relevant increase in ion production, which consequently leads to a relevant sensitivity increase. Furthermore, the presence of air at atmospheric pressure can play a positive role in promoting ionization processes. 

Munson and Field reported in 1966 on a technique of ionizing molecules by gas phase ion-molecule reactions, which they called chemical ionization (Cl). In this way, break-up of the molecules can be greatly reduced or even avoided. Thus, measured ion currents can be correlated with the densities of the respective parent neutral compounds, allowing for on-line monitoring of rather complex gas mixtures. The fundamental principles of gas phase ion chemistry on which Cl is based, as well as the instrumentation for Cl, have been reviewed in great detail by Harrison." The wide variety of Cl methods that has been developed includes Medium Pressure Mass Spectrometry, Fourier Transform Mass Spectrometry, Quadrupole Ion Trap Mass Spectrometry, Pulsed Positive Ion-Negative Ion Chemical Ionization, and Atmospheric Pressure Ionization Mass Spectrometry (API-MS). Of these, API-MS has developed into a very reliable and widely used technique for analysis of VOCs in flavor release studies and human breath. A variety of API-MS applications in these fields of research has been described in a recent volume by Roberts and Taylor. ... 

Finally, the Appendix shows schematic representations of the principle of operation of some of the ionization processes presented. " Among these are the theory of ESI, atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI). Also shown are schematics of the ionizers and typical experimental conditions for the APCI and APPI sources as well as that of an ESI-APCl mixed source. It should be noted that these schematics are for sources that are interfaced with a mass spectrometer but are similar to IMS interfaces. 

The technique of atmospheric-pressure chemical ionization (APCl) also serves to analyze LC effluents by mass spectrometry. It is applicable to relatively less polar and thermally stable compounds with an upper mass range of 1500 Da. The principle of ionization in APCI is identical to that described for conventional Cl, with the difference that APCI is performed at attnospheric pressure, at which many more ion-molecule collisions can occur between the sample molecules and reagent ions. Therefore, the ionization efficiency and detection sensitive are improved significantly. 

Figure 2.15. Block diagram of atmospheric pressure chemical ionization source. (Reproduced from C. Dass, Principles and Practice of Biological Mass Spectrometry, Wiley-Inter-science, 2001.)...

The emergence of thermospray ionization heralded a first ideal interface for a wide range of molecules [30,31]. With the introduction of this interface, LC/MS was accepted as a routine analytical technique. A major beneficiary of this interface was the pharmaceutical industry, which used this system to characterize drugs and metabolites. The construction and basic principle of thermospray ion source was discussed in Section 2.14 briefly, it consists of a heated probe, a desolvation chamber, and an ion extraction skimmer. When passed through a resistively heated capillary, the HPLC effluent, emerges as a mist of fine droplets into a heated desolvation chamber. Ionization of the solute molecules occurs by direct evaporation of the preformed ions or solvent-mediated chemical ionization. Thus, unlike the interfaces discussed above, the thermospray system acts as an ion source as well as an interface. Thermospray is ideally suited to coupling with conventional wide-bore columns. It is, however, confined primarily to reversed-phase HPLC separations, and it is less compatible with nonvolatile... 

Similar to any mass spectrometric experiment, ions that are intended to be converted to neutrals in NR MS should be first generated by appropriate ionization methods. In principle, all ionization methods described in Chapter 2.28 may be used for the generation of ions for NR MS studies. However, only a limited number of ionization techniques have found practical use for this purpose. They are electron impact, chemical ionization, fast atom bombardment, and secondary ion mass spectrometry. One of the reasons for not using other methods is that NR MS experiments are mostly carried out on sector instruments. The mass spectrometers of this type are usually equipped with relatively old methods of ionization. The second reason for using these methods is that they provide high ion fluxes of ions of interest. This condition is crucial for many NR MS experiments because of the overall low total efficiency (<0.1%) of the neutralization-reionization process. 

In principle however, this method should be portable to other mass spectrometer systems capable of chemical ionization and tandem mass spectrometry. An [M+54] ion was reported using atmospheric pressure chemical ionization (APCI) with a predominantly acetonitrile solvent while analyzing extremely long-chain polyunsaturated fatty acids (16). Such an observation is promising for the use of this method for the analysis of low- or nonvolatile lipids, such as triglycerides and phospholipids. 

Suppression of ionization efficiency is important when the total ionizing capability of the ionization technique is limited, so that there is a competition for ionization among compounds that are present in the ion source simultaneously. In principle such a saturation effect must be operative for all ionization techniques, but in practice it is most important for electrospray ionization (Section 5.3.6), slightly less important for atmospheric pressure chemical ionization (Section 5.3.4), atmospheric pressure photoionization (Section 5.3.5) and matrix assisted laser desorption ionization (Section 5.2.2) it does not appear to be problematic under commonly used conditions for electron ionization and chemical ionization (Section 5.2.1) or thermospray (Section 5.3.2). Enhancement of ionization efficiency for an analyte by a co-eluting compound is less commonly observed and is, in general, not well understood. 

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