Coupling techniques, liquid chromatography

MS techniques have met this need in the analysis of involatile, polar surfactants after coupling techniques of liquid chromatographic methods with MS became available. Different types of interfaces for off-line and on-line coupling of liquid chromatography (LC) and MS in the analyses of surfactants had been in use [7,16] while the methods applied at present were performed predominately with soft-ionising atmospheric pressure ionisation (API) interfaces [16-19],... 

In standard FAB, the surface of the matrix solution is depleted of analyte and suffers from radiational damage during elongated measurements. Refreshment of the surface proceeds by diffusion (limited by the viscosity of the matrix) or evaporation. Continuous-flow fast atom bombardment (CF-FAB) continuously refreshes the surface exposed to the atom beam. [107,108] The same effect is obtained in slightly different way by the frit-fast atom bombardment (frit-FAB) technique. [109,110] In addition, both CF-FAB and frit-FAB can be used for online-coupling of liquid chromatography (LC, Chap. 12) [111] or capillary electrophoresis (CE) to a FAB ion source. [112]... 

In atmospheric pressure ionization sources (API) the ions are first formed at atmospheric pressure and then transferred into the vacuum. In addition, some API sources are capable of ionizing neutral molecules in solution or in the gas phase prior to ion transfer to the mass spectrometer. Because no liquid is introduced into the mass spectrometer these sources are particularly attractive for the coupling of liquid chromatography with mass spectrometry. Pneumatically assisted electrospray (ESI), atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI) are the most widely used techniques. 

Much data on the structure of flavonoids in crude or semipurified plant extracts have been obtained by HPLC coupled with MS, in order to obtain information on sugar and acyl moieties not revealed by ultraviolet spectrum, without the need to isolate and hydrolyze the compounds. In the last decade, soft ionization MS techniques have been used in this respect, e.g., thermospray (TSP) and atmospheric pressure ionization (API). However, the most used methods for the determination of phenols in crude plant extracts were the coupling of liquid chromatography (LC) and MS with API techniques such as electrospray ionization (ESI) MS and atmospheric pressure chemical ionization (APCI) MS. ESI and APCI are soft ionization techniques that generate mainly protonated molecules for relatively small metabolites such as flavonoids. 

Coupling of liquid chromatography with mass spectrometry can provide unequivocal on-line spectrometric identification of anthelminthic residues in animal-derived foods. Typical applications of such techniques include the confirmation of moxidectin residues in cattle fat by liquid chromatography-thermospray mass spectrometry (352), and the confirmation of eprinomectin residues in bovine liver tissue by liquid chromatography, electrospray ionization, and multiple reaction monitoring in the MS-MS mode with positive ion detection (370). 

However, due to the fact that polymer structures are becoming more and more complex and analysis time is one of the key efficiency factors, it is not difficult to predict a bright future for on-line coupling of liquid chromatography and spectroscopic techniques. 

Whilst gas chromatography has been used for the analysis of many of the lycoctonine-based alkaloids [52], the larger, less volatile, and more thermally labile MSAL compounds require analytical procedures such as TLC and HPLC for separation and detection. For example, both normal phase liquid chromatography [53] and reversed phase liquid chromatography [54] with UV detection have been used for separation, detection, and quantitation of alkaloids from Delphinium species associated with livestock poisonings in the western US and Canada. The introduction of API techniques has allowed the analysis of all types of diterpene alkaloids by direct MS methods and with MS methods coupled to liquid chromatography. 

Faradaic techniques are those in which oxidation or reduction of analyte species occurs at the electrodes and therefore a measurable current is passed through the electrochemical cell. This discussion will be limited to controlled-potential techniques, primarily volta-metry and amperometry, coupled to liquid chromatography. While other Faradaic electrochemical techniques have been developed and electrochemical techniques in bulk solution are common, the use of liquid chromatography employing these two detection strategies is by far the most common electroanalytical technique in pharmaceutical studies. 

On the other hand, the studies of mixtures necessitating the use of soft ionization techniques, such as SIMS [271], PDMS [272] and LD [273], are possible. Indeed, they can be coupled with liquid chromatography. Thus, quantification should be one of the objectives of these new technologies in the future [274]. However, the search for zero [275] is moving, and in particular the sensitivities obtained with the negative ions are already promising. 

The coupling of liquid chromatography (LC) with mass spectrometry (MS) has undergone much evolution since its initial inception [1,2], Atmospheric pressure ionization techniques such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) opened the door for the ionization and analysis of nonvolatile or thermally labile analytes. This technique revolutionized drug discovery and development allowing for dramatic improvements in sensitivity, selectivity, and speed. This area continues to grow, and significant advances have been and continue to be achieved in all three areas [3-5],... 

A prerequisite for detection, identification, and quantification of any species by MS is that all analytes must be converted into gas-phase ions before they enter the mass analyzer. API techniques are most widely used for metabolite identification, mainly due to their ability to couple to liquid chromatography and generate intact gas-phase molecular ions at very high sensitivity (Rossi, 2002 Voyksner, 1997). 

Automated in-tube solid-phase microextraction (SPME) has recently been coupled with liquid chromatography/electrospray ionisation mass spectrometry (LC/ESI-MS), e. g. for the determination of drugs in urine [60, 62]. In-tube SPME is an extraction technique in which analytes are extracted from the sample directly into an open tubular capillary by repeated draw/eject cycles of sample solution. The analyte is then desorbed with methanol and transferred to an analytical HPLC-column. 

The coupling of liquid chromatography with mass spectrometry (LC/MS) has also been intensively investigated since the early 1980s. but so far without complete success. The new ionization techniques, especially ion spray, made the coupling of HPLC with MS to a routine technique. The decreasing pricesfe.g. of ion trap MS instruments open the use of these instruments in routine HPLC analysis. For more details see-> Mass Spectrometry. In GC/MS it is relatively easy to remove the mobile phase, or to limit and control its effects on the ionization source, but this has proven to be much more difficult in LC/MS. A variety of interfaces has been developed for that purpose ... 

Keywords Crystallization analysis fractionation Field Flow Fractionation High performance liquid chromatography Hyphenated techniques Liquid chromatography Polyolefin analysis SEC-NMR coupling Size exclusion chromatography Temperature rising elution fractionation Two-dimensional liquid chromatography... 

ESI in its current state-of-the-art has not resulted from a straightforward development. It has many predecessors, some of which having been successful at their time, while others were rather short-lived methods replaced as soon as more sensitive or more robust techniques appeared [28]. Nonetheless, the development of all those techniques aimed at both the direct coupling of liquid chromatography to mass spectrometry and the access to highly polar or even ionic analytes. 

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