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2012 III / AMINO ACIDS / Thin-Layer (Planar) Chromatography acetyl-cysteinyl and with OPA/N-L(D)-isobutyrylcysteinyl AA derivatives gave excellent resolution of enantiomers. Consequently, the CDR technique is the primary importance in a number of practical applications of the separation of enantiomeric AAs. The interaction of AAs with the enantiomerically pure reagents takes place at ambient temperature, without racemization, resulting in the formation of stable diastereomer derivatives. Online LC-MS In the case of AAs, thermospray ionization has been displaced by the milder techniques of electrospray (ES) and atmospheric pressure chemical ionization (APCI), converting analyte molecules without fragmentation into ions. The analyte should contain the AAs in a stable form: either in the free condition or in the form of stable derivatives, such as phenylthiohydantoins (PTH) or PTCs. SigniRcantly reduced Sow rates are essential (100}300 nL min\1) for stable ES and APCI operation. In automated Edman microsequencing, the ES-MS of PTH derivatives. The protonated molecules were measured with a linear response in the 50}1000 fmol level. Future Trends Efforts are needed to extend the life time, plate number and reproducibility of columns, and to standardize testing methods. The extended use of thermostated columns is desirable in order to obtain reproducibility in absolute and relative retention times. LC-MS will be more widely used in laboratories as the cost of these instruments falls to the level of GC-MS, and/or an all-purpose interface becomes available. See also: II/Chromatography: Liquid: Derivatization; Mechanisms: Reversed Phase. Further Reading Blau K and Halket J (eds) (1993) Handboook of Derivatives for Chromatography. Chichester: John Wiley. BruK ckner H, Langer M, LuK pke M, Westhauser T and Godel H (1995) Liquid chromatographic determinati