Ionization techniques chemical

Some mild methods of ionization (e.g., chemical ionization. Cl fast-atom bombardment, FAB electrospray, ES) provide molecular or quasi-molecular ions with so little excess of energy that little or no fragmentation takes place. Thus, there are few, if any, normal fragment ions, and metastable ions are virtually nonexistent. Although these mild ionization techniques are ideal for yielding molecular mass information, they are almost useless for providing details of molecular structure, a decided disadvantage. 

Until 1981, mass spectrometry was limited, generally, to the analysis of volatile, relatively low-molecular-mass samples and was difficult to apply to nonvolatile peptides and proteins without first cutting them chemically into smaller volatile segments. During the past decade, the situation has changed radically with the advent of new ionization techniques and the development of tandem mass spectrometry. Now, the mass spectrometer has a well-deserved place in any laboratory interested in the analysis of peptides and proteins. 

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. 

There are numerous ionization techniques available to the mass spectrome-trist, but for GC/MS almost all analyses are performed using either electron impact ionization or chemical ionization. 

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 . 

Cl is not the only ionization technique where this aspect of interpretation must be considered carefully fast-atom bombardment, thermospray, electrospray and atmospheric-pressure chemical ionization, described below in Sections 3.2.3, 4.6, 4.7 and 4.8, respectively, all produce adducts in the molecular ion region of their spectra. 

The use of the dynamic-FAB probe (see Section 4.4 above) has allowed the successful coupling of HPLC to this ionization technique but there is an upper limit, of around 5000 Da, to the mass of molecules which may be successfully ionized. Problem solving, therefore, often involves the use of chemical methods, such as enzymatic hydrolysis, to produce molecules of a size more appropriate for ionization, before applying techniques such as peptide mapping (see Section 5.3 below). 

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. 

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. 

You already know that MS studies ions. So, once molecules have been introduced inside the mass spectrometer, they must be ionized, i.e. transformed into ions. The ionization process occurs in the ion source by using an ionization technique. There are different ionization techniques depending on the physico-chemical properties of the molecules. 

El and Cl are the most commonly used ionization techniques for the analysis of volatile, nonpolar, low molecular weight (up to 700 900 Da) and thermally stable compounds. These techniques can be also useful for studying non highly volatile molecules, but in these cases, preliminary chemical reactions, such as derivatization, have to be performed in order to obtain derivatives with increased volatility. 

The most important gas phase ionization techniques are electron ionization and chemical ionization. The latter will be described in Section 2.6.2. 

Owing to soft conditions, the mass spectra obtained by this kind of ionization techniques are also characterized by the presence of adduct ions, i.e. ionic species formed by weak interactions between the ions and other chemical species (see below). 

On account of having the same number of electrons and protons, isotopes of a given element have identical chemical properties and reactivity. However, since they differ in the number of neutrons, they have different atomic masses. As MS measures and discriminates mass, isotopes are detected and they are present in every mass spectrum, independent of the ionization technique used, instrumentation, etc. 

During the last decade knowledge of the ion chemistry of nitro compounds in the gas phase has increased significantly, partly due to the more widespread use of specialized techniques. Thus various ionization methods, in particular electron impact ionization and chemical ionization, have been used extensively. In addition, structure investigations as well as studies on fragmentation pathways have involved metastable ion dissociations, collision activation and neutralization/reionization studies, supplementary to studies carried out in order to disclose the associated reaction energetics and reaction dynamics. In general, the application of stable isotopes plays a crucial role in the in-depth elucidation of the reaction mechanisms. 

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. 

---