Atmospheric pressure User ionization

Currently no comprehensive libraries have been developed for methods, such as ESI (see Section 2.1.15) and atmospheric pressure chemical ionization (APCI see Section 2.1.8), used to ionize compounds after LC separation. Generally these types of library are user generated for a specific purpose. 

MS/MS identification of co-evolving compounds, combined with the weight-loss data obtained from TGA, furnishes a means of delineating complex thermolytic pathways. The compatibility of TGA (an atmospheric pressure technique) with Atmospheric Pressure Chemical Ionization (APCI), as offered on the TAGA 6000 MS/MS system, permits an interface which is easily coupled and decoupled making the system user-friendly and well suited to routine quality control and trouble-shooting applications. 

Liquid chromatography-mass spectrometry The initial attempts to couple LC with MS lacked important attributes for trace analysis sensitivity, robustness, and reliable quantitation. Moreover, the cost of the early LC-MS instruments was prohibitive for most laboratories. The revolutionary introduction of atmospheric pressure ionization (API) techniques, mainly electrospray (ESI) and atmospheric pressure chemical ionization (APCI), resulted in greater applicability of LC-MS and manufacture of more reliable, affordable, and user-friendly instruments. Thus, LC-MS is now becoming an indispensable part of the analytical strategy in many routine laboratories, enabling direct, selective, and sensitive multiclass, multiresidue analysis of more polar, low volatile, and/or thermolabile pesticides, such as carbamates, phenylureas, sulfonylureas, imidazoles, triazoles, imidazolinones, chlorophenoxy acids, and many others. 

After the development of larger and more efficient vacuum pumps, more user-friendly LC/MS interfaces of thermospray,4 5 and atmosphere pressure ionization,6 7 LC/MS earned its place in bio-analytical laboratories. The resulting device was a powerful instrument that required significantly more capital investment than HPLC/UV, GC, or GC/MS. 

Supercritical fluid chromatography-mass spectrometry is closely related to HPLC-MS except that SFC is normal phase and HPLC tends to be used in reversed phase mode. In SFC-MS, the mass spectrometer is used in atmospheric pressure ionization (APCI) mode, where there is little fragmentation and the user generally sees a. M + lorM—1 peak (M = molecular weight) is generally prevalent. 

Since the pioneering work by Gray (1975), mass spectrometry of elements ionized in an inductively coupled gas plasma at atmospheric pressure (ICP-MS) has gained a steadily increasing application for trace-element analysis. The determination of mercury by this technique seems to be quite free from interfering polyatomic mass fragments formed by constituents of the plasma and sample matrices, which may disturb the determination of elements with lower mass units (Houk, 1986 Delves, 1988 Lyon et al., 1988 Templeton et al., 1989 Olesik, 1991). However, especially in the case of mercury, several users of ICP-MS have experienced severe memory effects from samples with high mercury levels. 

For a number of years (1987-1992), thermospray LC-MS was the most frequently applied interface for LC-MS. It has demonstrated its applicability in both qualitative and quantitative analysis in various application areas. With the advent of the more robust LC-MS interfaces, based on atmospheric-pressure ionization, the use of thermospray interfacing and ionization rapidly decreased. The newer technology pointed out the limitations of the thermospray system, e.g. in the analysis of thermolabile compounds, ionic compounds, high molecular-mass compounds, as well as in robustness and user-friendliness. Therefore, thermospray as an ionization and interface technique for LC-MS is now history. Thermospray nebulization will continue to be used, e.g. in nebulization for ICP-MS. 

Similarly, mass spectrometry (MS) lias had a dramatic impact, and is, arguably, the tool that continues to see the greatest pace of technological development. From its earliest days (23) the mass-spec experiment has evolved from a substantial technological challenge to one that now can be almost trivial. One particularly important development that spurred this growth was the implementation of new ionization techniques (24), most notably that of electrospray ionization, which permits sample introduction at atmospheric pressure (25). The large number of instruments today that provide walk-up access to mass data, literally, at moments notice, even for the novice user attests to these advances as well as to the immense value of this tool. 

The most common ionization sources interfaced to UHPLC are electrospray ionization (ESI) and atmospheric chemical ionization (APCI) in both positive and negative ion modes, because they can both be operated at atmospheric pressure and offer a user-friendly way to couple UHPLC with MS (26). Most commercial MS systems are equipped with these two interfaces, allowing the easy switch between them. 

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