Electrospray ionization convenience

The use of TFA as a mobile-phase additive in LC-MS can be problematical when using electrospray ionization. In negative ion detection, the high concentration of TFA anion can suppress analyte ionization. In positive ion detection, TFA forms such strong ion pairs with peptides that ejection of peptide pseudo-molecular ions into the gas phase is suppressed. This problem can be alleviated by postcolumn addition of a weaker, less volatile acid such as propionic acid.14 This TFA fix allows TFA to be used with electrospray sources interfaced with quadrupole MS systems. A more convenient solution to the TFA problem in LC-MS is to simply replace TFA with acetic or formic acid. Several reversed-phase columns are commercially available that have sufficient phase coverage and reduced levels of active silanols such that they provide satisfactory peptide peak shapes using the weaker organic acid additives.15... 

The development of a robust and reliable online LC/MS combination via atmospheric pressure ionization (API) has greatly advanced the science of quantitative analysis of polar and thermally labile compounds. The speed of analysis, simplicity, and enhanced selectivity are the highlights of this combination. HPLC is more conveniently coupled to mass spectrometry via attnospheric-pressure chemical ionization (APCI) and electrospray ionization (ESI) interfaces. 

Off-Line Reaction Monitoring For slow enzyme reactions and long-lived intermediates, off-line reaction monitoring is more convenient. In these methods, the known amounts of an enzyme and a substrate are mixed together and incubated at physiological conditions. Aliquots are withdrawn at predetermined intervals, and the reaction is quenched immediately. The time course of the reaction can be monitored with mass spectrometric analysis immediately or later, at a more convenient time, by using either fast atom bombardment (FAB) [7], continuous-flow (CF)-FAB [7], matrix-assisted laser desorption/ionization (MALDI) [8], or electrospray ionization (ESI) [9-11]. Established quantitation procedures can be employed to monitor the concentration of the reactant or product (usually, the latter) (see Chapter 14). As an example, an appropriate internal standard that has no affinity for the enzyme can be added to the reaction mixture to improve... 

Chapter 7, titled Interfacing TLC with Laser-Based Ambient Mass Spectrometry, provides an overview of mass spectrometric techniques that can be coupled with TLC under the most convenient working conditions, that is, at room temperature and atmospheric pressure. The authors introduce readers to electrospray laser desorption ionization (ELDI), plasma-assisted multiwavelength laser desorption/ionization (PAMLDI), laser desorption atmospheric pressure chemical ionization (LD-APCI), laser desorption-dual electrospray and atmospheric pressure chemical ionization I (LD - ESI-I-APCI), laser-induced acoustic desorption electrospray ionization (LIAD-ESI), and laser-induced acoustic desorption-dielectric barrier discharge ionization (LIAD-DBDI). Chapters 6 and 7 are largely complementary because in the former one, main attention is paid to practical applications of a wide number of... 

Lubben, A.T., Mclndoe, J.S., and Weller, A.S. (2008) Coupling an electrospray ionization mass spectrometer with a glovebox a straightforward, powerful, and convenient combination for analysis of air-sensitive organometallics. Organometallics, 27, 3303-3306. 

Electrospray ionization (ESI) ESI requires a polymer solution to be pumped into the mass spectrometer, either as such or, conveniently, after a liquid-chromatographic separation. In the latter case, a narrow, pre-separated fraction of the polymer is introduced into the mass specrometer, which greatly enhances the chances of obtaining useful mass spectra. The liquid is sprayed into the ionization chamber under the simultaneous action of an electric field of several kilovolts. The solvent should have a significant polarity and some ionic additives (salt or buffer) are typically present. It is not strictly necessary for the polymer itself to be dissolved in this polar solvent. Good results have been obtained by adding a separate (immiscible) stream of solvent to the polymer solution through a T-piece just before the ESI interface [12). 

Note Over the last decade, NICI applications have diminished because these analyses have mostly been carried out by negative-ion atmospheric pressure chemical ionization (APCI). APCI is compatible with liquid chromatography (Chap. 14) and can readily be implemented on instruments with electrospray ionization interface (ESI, Chap. 12). Such instruments have an enormous market share, and thus, it is often more convenient and economic to switch between ESI and APCI as required. Although being a type of Cl, APCI will be dealt in conjunction with ESI. 

The similarity of the hardware required for APCl 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. 

Another convenient way to classify ionization sources, rather than from the perspective of odd- or even-electron ion generation, is in relation to where the ions are created relative to the vacuum system that is, either generated at atmospheric pressure or in a vacuum. The two most common atmospheric pressure ionization sources, electrospray and atmospheric pressure chemical ionization, are arguably the most common ionization techniques applied in quantitative mass spectrometry today. However, discussion of earlier ionization sources is useful, as many of these techniques are still commonplace and their understanding provides a framework for appreciation of atmospheric pressure ionization technology and what it has to offer the pharmaceutical industry. 

The need for an ionization source that provided both softer ionization, i.e., less fragmentation of the molecular ion, and a convenient interface with liquid chromatography (at ambient pressure) to mass spectrometry (at high vacuum) helped spur the creation of atmospheric pressure ionization. Two techniques fall under the heading of API, electrospray and atmospheric pressure chemical ionization (APCI), and the technical aspects of each are discussed individually. However, many of the fundamental principals that describe these ionization mechanisms can be applied to both electrospray and APCI sources. 

Although both electrospray and APCI can be described generically as atmospheric pressure ionization techniques, each has its own mode of operation and a wide realm of applications. To a large extent, either technique can be applied to a given analytical method, especially for low-molecular-weight analytes of roughly less than 1000 Da. In fact, it is commonly observed that an industrial analytical laboratory will tend to favor one technique over the other out of convenience rather than analytical rigor. 

Different from the El source, the APCI source contains a heated vaporizer which facilitates rapid desolvation/vaporization of the droplets by a very brief heating period up to 500 °C. Reagent ions obtained from the solvent vapour by a corona discharge ionize the vaporized sample molecules through an ion—molecule reaction. Chemical ionization of sample molecules is very efficient due to the numerous ion—molecule collisions (Figure 16.23). This technique leads to multicharged ions of type (M-F nH)"+ by proton transfer (in the positive mode). Unfortunately this technique is difficult to miniaturize as the required flow rate is higher than that of the electrospray and as such is convenient only for volatile and thermally stable molecules of mass less than 1000 Da. 

The high potentials required for electrospray show that air at atmospheric pressure is not only a convenient, but also a very suitable, ambient gas for ES, particularly when solvents with high surface tension, such as water, are to be electrosprayed. The oxygen molecules in air have a positive electron affinity and readily capture free electrons. Initiation of gas discharges occurs when free electrons present in the gas (due to cosmic ray or background radiation) are accelerated by the high electric field near the capillary to velocities where they can ionize the gas molecules. At near-atmospheric pressures, the collision frequency of the electrons with the gas molecules is very high and interferes with the electron acceleration process. 

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