Supplements electrospray ionization

Figure A.3A.3 LC/MS analysis of a dietary supplement consisting of extract of Trifolium pratense (red clover). Reversed-phase C18 HPLC and negative ion electrospray ionization mass spectrometry were used with a quadrupole mass spectrometer analyzer (Agilent also see Table A.3A.1). The map illustrates the abundance of information provided by this hyphenated technique with HPLC mass chromatograms in one dimension and mass spectra in another dimension.

Prasain JK, Jones K, Kirk M, Wilson L, Smith-Johnson M, Weaver C and Barnes S, Profiling and quantification of isoflavonoids in kudzu dietary supplements by high-performance liquid chromatography and electrospray ionization tandem mass spectrometry. J Agric Food Chem 51 4213-4218 (2003). 

It may be that in light of advances made using both desorption electrospray ionization (DESI) [20,21] and direct analysis in real time (DART) [22,23] that the use of IT-SIMS protocols may be supplemented in some applications. These techniques differ in that they sample surface contaminants at atmospheric pressure and do not necessarily require collection of a sample, or transporting the sample into the vacuum environment of the mass spectrometer. However, preliminary experiments have shown that these techniques are tremendously matrix dependent, even more so than SIMS, and thus the generation of even semiquantitative data for environmental surfaces like soil samples remains challenging. Comparisons of ultimate detection limits between the techniques have not yet been accomplished. 

The next generation of HPLC detectors, namely mass spectrometers, are now available and being deployed for water-soluble vitamin determinations in dietary supplements with electrospray ionization interface (ESI) (Holler et al. 2006). There remain obstacles to overcome for multivitamin analyses in complex food matrices, as co-elution of vitamins or excipients can compromise... 

The dominating method of ion formation in metabolic flux analysis is electron impact. It might be supplemented in the future by novel methods, such as matrix assisted laser desorption and electrospray. Additional techniques such as chemical ionization, fast atom bombardment or inductively coupled plasma ionization are only of minor importance and not further discussed in this context. 

Ionization — Although electron impact (El) and chemical ionization (Cl) both in the negative and positive mode have traditionally been used, these have been supplemented by other procedures that are suitable for liquid samples including fast atom bombardment (FAB), electrospray (ES), and matrix-assisted laser desorption ionization (MALDI), which is particularly valuable for analysis of high-molecular-mass samples. Additional comments are provided below. 

The criteria for optimum performance of atmospheric pressure chemical ionization and electrospray are different. For electrospray, it is important to control the flow rate and composition of the sheath-flow liquid, the mobile phase flow rate, and the dimensions of the electrospray and sheath-flow capillaries, as well as their relative position with respect to each other. Sample ionization with carbon dioxide alone does not occur in the absence of sheath-flow liquid. For atmospheric pressure chemical ionization, apart from those factors that affect the performance of the nebulizer, the flow dynamics throughout the interface and temperature in the ionization region, are the most important parameters. Moistened air is sometimes used as a makeup gas to maintain stable operation of the atmospheric pressure chemical ionization interface with HaO ions supplementing solvent ions in the reaction gas plasma. The ionization efficiency of both methods is similar to liquid chromatography. 

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