Electrospray ionization mobile phases compatible with

Because plasma and urine are both aqueous matrixes, reverse-phase or polar organic mode enantiomeric separations are usually preferred as these approaches usually requires less elaborate sample preparation. Protein-, cyclodextrin-, and macrocyclic glycopeptide-based chiral stationary phases are the most commonly employed CSPs in the reverse phase mode. Also reverse phase and polar organic mode are more compatible mobile phases for mass spectrometers using electrospray ionization. Normal phase enantiomeric separations require more sample preparation (usually with at least one evaporation-to-dryness step). Therefore, normal phase CSPs are only used when a satisfactory enantiomeric separation cannot be obtained in reverse phase or polar organic mode. 

Because all library compounds must be monitored simultaneously, the compounds must be selected so that they have unique molecular weights. Also, one compound in the mixture should not suppress the ionization of another. Therefore, this approach is probably restricted to the screening of small combinatorial libraries that are similar in chemical structure and ionization efficiencies. Finally, the binding buffer used for affinity chromatography must be compatible with on-line APCI or electrospray mass spectrometry. This means that the mobile phase must be volatile and usually of low ionic strength (i.e., typically <40 mM for electrospray ionization). 

It has become painfully obvious that most of the excellent approaches and techniques that have been developed for use in liquid chromatography are not applicable to liquid chromatography/mass spectrometry (LC/MS) with atmospheric pressure ionization. Chapter 5 described the reagents and the range of mobile-phase compositions that are compatible with electrospray and atmospheric pressure chemical ionization (APCI), and these are limited to volatile components that do not cause significant ion suppression. Certain problems that are not significant with standard LC separations become difficult to deal with because of the limitations placed on the mobile phase by atmospheric pressure ionization (API) LC/MS. 

For method transfer, the following rule is of great practical relevance with any change in column diameters at constant stationary and mobile phase and temperature, the linear velocity u must always be kept constant through a respective adaptation of the flow rate f according to Eq. 2.7. A reduction of the column diameter from 4.6 to 2.1 mm implies a reduction in the flow rate to 0.2 ml/min (exactly 0.208 ml/min) if the original flow rate was 1 ml/min. This reduces the solvent consumption and the environmental footprint, but also improves the compatibility of the method with electrospray ionization mass spectrometry (ESI-MS). This is discussed in more detail in Sections 1.1 and 1.2 on LC/MS coupling. 

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