Soft ionization technique

The analysis of penicillins by mass spectrometry (qv) has developed with the advent of novel techniques such as fast atom bombardment. The use of soft ionization techniques has enabled the analysis of thermally labile nonvolatile compounds. These techniques have proven extremely valuable in providing abundant molecular weight information from underivatized penicillins, both as free acids and as metal salts (15). 

Alternative ( soft ) ionization techniques are not usually required for aromatic isothiazoles because of the stability of the molecular ions under electron impact. This is not the case for the fully saturated ring systems, which fragment readily. The sultam (25) has no significant molecular ion under electron impact conditions, but using field desorption techniques the M + lY ion. is the base peak (73X3861) and enables the molecular weight to be confirmed. 

Chemical ionization (Cl) is a technique that has been developed specifically to enhance the production of molecular species, i.e. to reduce the fragmentation associated with ionization. A number of such techniques exist and these are known collectively as soft ionization techniques . 

In this chapter, the main aspects of mass spectrometry that are necessary for the application of LC-MS have been described. In particular, the use of selected-ion monitoring (SIM) for the development of sensitive and specific assays, and the use of MS-MS for generating structural information from species generated by soft ionization techniques, have been highlighted. Some important aspects of both qualitative and quantitative data analysis have been described and the power of using mass profiles to enhance selectivity and sensitivity has been demonstrated. 

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

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. 

This is a task for which electrospray ionization is well suited although, as discussed earlier in Section 4.7, this is a soft ionization technique that yields almost exclusively molecular ions with little fragmentation and consequently, in the case of biopolymers, little sequence information directly. 

MS-MS is a term that covers a number of techniques in which two stages of mass spectrometry are used to investigate the relationship between ions found in a mass spectrum. In particular, the product-ion scan is used to derive structural information from a molecular ion generated by a soft ionization technique such as electrospray and, as such, is an alternative to CVF. The advantage of the product-ion scan over CVF is that it allows a specific ion to be selected and its fragmentation to be studied in isolation, while CVF bring about the fragmentation of all species in the ion source and this may hinder interpretation of the data obtained. 

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

The fact that APCl 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. 

Soft ionization technique An ionization technique that produces molecular species with few, if any, fragment ions. 

The success of the soft ionization techniques (DCI, FAB, and LSIMS) presents several possibilities for detection of brevetoxins in complex matrices. Positive-ion DCI was used for the analysis of PbTx-3 metabolites generated in vitro by isolated rat hepatocytes (see below). Unmetabolized parent was conclusively identified and metabolites were tentatively identified, pending confirmation by alternate methods (see below). 

In line with the policy of Advances to provide periodic coverage of major developments in physical methodology for the study of carbohydrates, A. Dell (London) here surveys the use of fast-atom-bombardment mass spectrometry in application to carbohydrates. This technique has achieved rapid prominence as the soft ionization technique of choice for structural investigation of complex carbohydrate sequences in biological samples. The author s extensive personal involvement in this field makes her chapter a critical, state-of-the-art overview for the specialist, as well as a valuable primer for the reader unfamiliar with this technique. 

ESI-MS has emerged as a powerful technique for the characterization of biomolecules, and is the most versatile ionization technique in existence today. This highly sensitive and soft ionization technique allows mass spectrometric analysis of thermolabile, non-volatile, and polar compounds and produces intact ions from large and complex species in solution. In addition, it has the ability to introduce liquid samples to a mass detector with minimum manipulation. Volatile acids (such as formic acid and acetic acid) are often added to the mobile phase as well to protonate anthocyanins. A chromatogram with only the base peak for every mass spectrum provides more readily interpretable data because of fewer interference peaks. Cleaner mass spectra are achieved if anthocyanins are isolated from other phenolics by the use of C18 solid phase purification. - ... 

The recommended technique for the determination of oxime carbamates and their metabolites by HPLC/MS and HPLC/MS/MS is positive ESI. Electrospray is a soft ionization technique and is suitable for thermally labile compounds. Ions are produced in the liquid phase at quasi-ambient temperature and atmospheric pressure, thus leaving the fragile pesticides intact. For oxime carbamates, the molecular adducts that can be monitored during HPLC/MS analysis with electrospray in positive mode are [M- -H]+, [M- -Na]+, or [M- -NH4]+, depending on the nature of mobile phase used. ... 

The introduction of soft ionization techniques, such as plasma desorption (PD),[1] field desorption (FD)[2] and fast atom bombardment (FAB),[3] marked the beginning of a new era for MS. In fact, they allowed MS to extend its applications to wide classes of nonvolatile, polar, thermally unstable and high molecular weight analytes. This opened up new horizons for MS in many unexpected fields, such as biology, biomedicine and biotechnology, in which this methodology had not previously found any possible application. 

Mass Spectra Obtained by Soft Ionization Techniques... 

Blakley C.R., Carmody J.J., and Vestal M.L., 1980. A new soft ionization technique for mass spectrometry of complex molecules. J Am Chem Soc 102 5931. 

What features of a mass spectrum indicate that a soft ionization technique has been employed (few if any fragment ions). 

Fortunately, this background is often less of a problem than might be anticipated from the above. The majority of ionization techniques employed in LC-MS are soft ionization techniques which provide primarily molecular ions that occur at relatively high values of mass-to-charge ratio (m/z), rather than fragment ions which occur at relatively low m/z values. In the majority of cases, the molecular weight of the analyte is higher than those of the solvent impurities and the effect of these may therefore be minimized. 

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