Ionization techniques APCI

Matrix effects complicate not only GC but also LC-MS analysis. Matrix components may impact selective determination of coeluting analytes (a rare effect in tandem MS) and mainly interfere in the spraying, ionization, and evaporation processes leading to analyte signal suppression or less often enhancement. To compensate for the latter effects, matrix-matched standardization is mostly used after failure of more convenient approaches, such as application of a different ionization technique (APCI is usually better than ESI in this respect), altered LC separation, or improved sample preparation selectivity. In the case of a limited number of analytes, the use of deuterated internal standards or echo-peak... 

Thus, either the emitted light or the ions formed can be used to examine samples. For example, the mass spectrometric ionization technique of atmospheric-pressure chemical ionization (APCI) utilizes a corona discharge to enhance the number of ions formed. Carbon arc discharges have been used to generate ions of otherwise analytically intractable inorganic substances, with the ions being examined by mass spectrometry. 

TSP, ESI and APCI effect ionization from solution and in these cases it is not possible to separate a description of the processes involved in the ionization of an analyte from a description of the interface. These ionization techniques will therefore be described in detail in Chapter 4. 

For many years, electron ionization, then more usually known as electron impact, was the only ionization method used in analytical mass spectrometry and the spectra encountered showed exclusively the positively charged species produced during this process. Electron ionization also produces negatively charged ions although these are not usually of interest as they have almost no structural significance. Other ionization techniques, such as Cl, FAB, thermospray, electrospray and APCI, however, can be made to yield negative ions which are of analytical utility. 

Particular emphasis has been placed upon electrospray and atmospheric-pressure chemical ionization (APCI) which, in addition to being the currently most widely used interfaces, are ionization techniques in their own right. 

Factors may be classified as quantitative when they take particular values, e.g. concentration or temperature, or qualitative when their presence or absence is of interest. As mentioned previously, for an LC-MS experiment the factors could include the composition of the mobile phase employed, its pH and flow rate [3], the nature and concentration of any mobile-phase additive, e.g. buffer or ion-pair reagent, the make-up of the solution in which the sample is injected [4], the ionization technique, spray voltage for electrospray, nebulizer temperature for APCI, nebulizing gas pressure, mass spectrometer source temperature, cone voltage in the mass spectrometer source, and the nature and pressure of gas in the collision cell if MS-MS is employed. For quantification, the assessment of results is likely to be on the basis of the selectivity and sensitivity of the analysis, i.e. the chromatographic separation and the maximum production of molecular species or product ions if MS-MS is employed. 

The vast majority of LC-MS analyses cnrrently in use employ either electrospray ionization or APCI. In the previons example, electrospray ionization was employed because of the highly polar nature of the analytes bnt, as discussed above in Sections 4.7 and 4.8, this and APCI are, to a large extent, complementary, with APCI being used for low- to medium-polarity analytes and electrospray for medinm- to high-polarity analytes. There are many compounds, therefore, for which the best ionization technique is not immediately obvious and their relative merits must be investigated. 

Atmospheric-pressure chemical ionization (APCI) and electrospray ionization are both soft ionization techniques which give rise, almost exclusively, to the production of molecular species. Structural information. 

The specificity of the ions/decompositions must be considered. Both electrospray ionization and APCI are soft ionization techniques and the resulting mass... 

The introduction and development of more recent ionization techniques, such as ESI, APCI and MALDI, have allowed the study of nonvolatile and thermally unstable compounds. 

Electrospray ionization [21] is one of the most widely utilized ionization techniques employed today for the analysis of thermally fragile molecules. As such, it has assumed an important role in the analysis of biologically important molecules. ESI is a desorption ionization technique. This means that ions are formed before or during the transition from the liquid phase and need not be volatilized in advance of the ionization event (as is the case for El, Cl, etc.). Like APCI and APPI, ESI occurs at atmospheric pressure outside the vacuum chamber of the mass spectrometer (Fig. 11.5). A solution of the analyte passes through... 

The ionization techniques most widely used for LC-MS, however, are termed soft ionization in that they produce primarily molecular species with little fragmentation. It is unlikely that the molecular weight alone will allow a structural assignment to be made and it is therefore desirable to be able to generate structural information from such techniques. There are two ways in which this may be done, one of which, the so-called cone-voltage or in-source fragmentation, is associated specifically with the ionization techniques of electiospray and APCI and is discussed later in Section 4.7.4. The other, termed mass spectrometry-mass spectrometry (MS-MS) or tandem mass spectrometry, is applicable to all forms of ionization, provided that appropriate hardware is available, and is described here. 

The similarity of the hardware required for APCI to that required for electrospray may be seen from comparing Figures 4.9 and 4.22 and makes changing between the techniques convenient from a practical point of view. The complementary nature of the two ionization techniques may, therefore, be readily utilized. 

APCI is a soft ionization technique which usually enables the molecular weight of the analyte under study to be determined. 

For which types of compound is APCI likely to be the most appropriate ionization technique and for which is electrospray likely to be more effective ... 

The polarity and thermal instability of biopolymers, together with the almost exclusive formation of singly charged ions renders APCI an inappropriate ionization technique for their study. Much of the early work involving electrospray ionization, on the other hand, was connected with the analysis of this type of molecule, in particular determining the molecular weight of proteins for which it is particularly effective. 

The fact that APCI and electrospray are soft ionization techniques is often advantageous because the molecular ion alone, in conjunction with HPLC separation, often provides adequate selectivity and sensitivity to allow an analytical method to be developed. Again, method development is important, particularly when more than one analyte is to be determined, when the effect of experimental parameters, such as pH, flow rate, etc., is not likely to be the same for each. Electrospray, in particular, is susceptible to matrix effects and the method of standard additions is often required to provide adequate accuracy and precision. 

With the development of sophisticated ionization techniques including electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), HPLC-MS techniques have been successfully applied to the online analysis of ginsenosides in extracts and biological fluids (Fuzzati, 2004). In terms of sensitivity and specificity, an MS detector is better than UV or ELSD. Among the various MS methods, the HPLC-MS-MS (or just LC-MS-MS) technique is to date the most sensitive method for detection and quantification of ginsenosides. 

LC-APCI-MS has been shown to very useful for the characterization of both neutral ginsenosides as well as thermolabile malonyl-ginsenosides in ginseng extracts (Ma et al., 2005). However, LC-MS with ESI interface is a highly sensitive and soft ionization technique for the LC-MS analysis of thermolabile compounds and is considered to be the best method for the analysis of ginsenosides as it can overcome most problems associated with the thermolabile malonyl-ginsenosides and low molecular ion abim-dance levels. LC-ESI-MS is characterized by abundant adduct formation... 

Atmospheric pressure photoionization (APPI) was recently introduced in the world of atmospheric pressure ionization techniques to analyze nonpolar molecules that are not efficiently ionized either by ESI or by APCI. Photoionization (PI) was already exploited some 30 years ago as a detection method for GC and LC, but only in recent times it has been used as an ionization method for mass spectrometry [52],... 

A considerable amount of information has been accumulated during the review period with respect to fragmentation studies of flavonoid aglycones and their glycosides using ionization techniques such as El and CID (Figure 2.17). Tandem mass spectrometry with soft ionization methods such as FAB, ESI, and APCI have been used for the structural characterization of a variety of flavonoids, and both deprotonation ° ° and... 

---