lectivity for L-Phe-L-Val and L-Val-L-Phe; at least its effect was greater for aliphatic than for aromatic amino acids. A greater effect was also found for the ionization of valine relative to alanine.The retentions of the dipeptides and the selectivities of the separations were also examined as a function of buffer composition. Pietrzyk and co-workers (14-16) discussed as a function of pH the different mechanisms for retention by more conventional (nonchiral) packings. In the present study, there appears to be an additional factor influencing the separations as evidenced by the following. The McIlvaine buffer, which contains both phosphate and citrate, gave greater retention and selectivity than a buffer of either of its componenb. This difference may be the result of a difference in ionic strength. The greater ionic strength may have resulted in greater dehydration of ions which may, in turn, have increased the selectivity of the separation, as cited by Niederwieser (17). However, Krummen and Frei (18) have also shown the opposite trend to be possible, depending upon the range of the ionic strength involved.The pyrophosphate buffer gave the smallest retention. However, it gave almost as good a selectivity as the McIlvaine buffer, even though its retention values were smaller. Hence, one wonders if ion pairing or a conformational change was the source of the differences. As a result, the use of pyrophosphate as an eluent is being explored. In the future, it will be of interest to immobilize different tripeptides so as to examine in more detail the effects of chirality on the k and a values for selected isomeric dipeptides. It will also be interesting to examine the effects of having Zn (19,20) or cupric ion (21) in acidic buffers, such as those of ammonium acetate, so as to take advantage of ligand exchange behavior. ACKNOWLEDGMENTWe thank Mercedes Galobardes for her preliminary work. We also thank Spectrophysics for the loan of a SP8770 pump.The results from preliminary lnvestlgatlons Into the use of segmented, flowing extraction of aqueous effluents from a reversed-phase HPLC system for flow cell FTIR detection are presented. The use of a hydrophoblc membrane phase separator facllltated the operatlon of the RP-HPLC/FTIR instrument. By lncorporatlng this technlque wlth chloroform as the extraction solvent, one observes slgnlflcantly improved Infrared transparency over the aqueous, elution solvent system. Further Improvements were observed with carbon tetrachloride, not only in the Infrared transparency of the solvent system but also In the wider range of compounds that were extracted from the methanol/water elutlon solvent system. Infrared spectra obtalned match standard reference spectra peak-for-peak.'Current address: The Procter and Gamble Co., 6210 Center Hill Rd., Cincinnati, OH 45224.Recently we have reported several developments involving the Fourier transform infrared (FTIR) spectrometer as a detector for normal-phase high-performance liquid chromatography (NP-HPLC) employing a flow cell interface (1-...

No abstract

Results which have evolved from an evaluation of HPLC/FTIR employing semipreparative, analytical, and microbore scale separations are presented. Normal-phase chromatography on silica gel has been carried out with Freon-113 elution. Model mixtures of aliphatic and aromatic hydrocarbons and nonpolar constituents In coal liquefaction process solvents have been separated. Best results have been obtained with the semipreparative and microbore modes because of the higher sample concentration developed at the detector. This study has shown the feasibility of microbore HPLC/FTIR which will enable chromatographers to economically use exotic solvents with large infrared windows.Identification of nonvolatile components in a thorough complex mixture analysis has always been one of the most demanding tasks for the analytical chemist to perform. Approaches to these analyses have been facilitated by the increasing technological level of chromatography. High-performance liquid chromatography (HPLC) has been particularly enhanced by the development of more efficient columns and by the employment of a variety of highly specific detectors. Continuing sophistication in small-bore column technology may lend itself nicely to those complex separation problems that require very high column efficiency in order to achieve the necessary high resolution (1). Multiple, simultaneous, selective detection allows detailed information to be obtained for chromatographically unresolved components (2). Unconventional information-rich spectroscopic detectors can potentially provide highly specific data for less than adequate complex mixture separations (3). The application of infrared (4), nuclear magnetic resonance (5), inductively coupled plasma atomic emission (6), and mass spectrometry (7) to chromatography, however, is not without experimental problems. Compatible spectroscopic-chromatographic solvents, minimal dead volume between column and detector, and reasonable detection limits are but a few of these prob-

No abstract

No abstract