Ion-pair methods

In contrast to the various CSPs mentioned so far, but still based on covalently or at least very strongly adsorbed chiral selectors (from macromolecules to small molecules) to, usually, a silica surface, the principle of dynamically coating an achiral premodified silica to CSPs via chiral mobile phase additives (CMPA) has successfully been adapted for enantioseparation. The so-called reverse phase LC systems have predominantly been used, however, ion-pairing methods using nonaqueous mobile phases are also possible. 

L. R. Snyder, J. J. Kirkland, and J. L. Glajch, Practical HPLC Method Development (New York Wiley, 1997) J. W. Dolan, Improving an Ion-Pairing Method, LCGC1996,14, 768. 

One of the first reliable methods for determining the concentrations of PFAs in tissues was the ion pairing method of Hansen et al. [79] (Fig. 6). Concentrations of PFOS in tissues, such as liver and blood plasma have been measured using high-performance liquid chromatography (HPLC) with electrospray mass spectrometry [1], One half milliliter of serum, 5 xL of an internal standard (e.g., tetrahydroperflurooctane sulfonic acid), 1 mL of 0.5 M tetrabutyl... 

A number of HPLC methods are available for the analysis of LSD. Two are presented here, namely (i) an ion-suppression technique, and (ii) an ion-paired method. The majority of techniques that are used exploit the same physicochemical characteristics of the systems as those described here. 

A number of so-called double ion-pair methods have been described for the analysis of hydrophobic amines in which the mobile phase contains both a quaternary ammonium ion and an alkyl sulfate or sulfonate. At first glance, this combination of mobile phase additives is counterintuitive because one would expect the effect of the anionic and cationic additives to cancel. However, the combination of cationic masking agents to reduce peak tailing and an anionic ion-pairing agent to enhance retention is sometimes necessary for the reversed-phase separation of hydro-phobic amines. 

Another widely used method for measuring surface acidity is the so-caUed ion-pair method (59,129). This method is based on the detection of the protonated species, BH, after protonation of a basic molecule according to Equation (2.12) ... 

By utilizing molecules with varying basicity, one may rank the acidity of different types of surface hydroxyls. Another approach is to investigate the thermal stability of the complexes formed. The ion-pair method provides direct information on the proton transfer but its use is restricted to the characterization of surface hydroxyls with relatively high acid strength. 

Orotic acid and orotidine by an ion-pair method with a Beckman ion-pair kit. Urine samples were precleaned on Waters Sep-pak Cl 8 cartridges. 

Another reason for the apparent neglect of organic sample ions in conventional ion chromatography is the success of ion-pair methods for determination of organic ions. Ion-pair methods are discussed in Chapter 9. 

The reader may well ask what nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) have to do with ion chromatography. Nucleic acids are very large biological molecules that are more like linear polymers and bear little similarity to common anions such as chloride and sulfate. Nevertheless, DNA and RNA are anionic by virtue of the phosphate groups attached to each nucleotide sugar unit. Nucleic acids can be separated by chromatography, often by ion-pair methods similar to those described in Chapter 9. For these reasons, it seems appropriate to include a chapter on DNA and RNA chromatography in this book. 

Because of its simplicity, the methylene blue ion pair method has been a standard for environmental analysis for a half-century. The absorbance of the ion pair is maximum at about 658 nm, depending on the specific anionic surfactant. Maximal color extraction is reached after a minute of agitation, and the color is stable in the organic phase for a half-hour (13). The methylene blue method is susceptible to positive and negative interference, and this... 

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