This material was prepared with the support of U.S. Department of Energy Grant No. DE-FG05-84ER13292; however, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE.

problems. First, certain bonded phases change in nature as well as in the amount of stationary phase; both of these effects result in irreproducible separations. Second, the degradation products from the bonded phase can contaminate a fraction isolated during a separation; such adulteration is of special consideration in preparative chromatography.We have developed two new classes of bonded phases that result in significantly increased stability of HPLC column packings during use. The first class uses bifunctional (or bidentate-type) silanes, which contain one reactive site on each of two silicon atoms of the silane. These two silicon atoms are connected by a bridging group, such as -Oor -(CH2)"-The type of bridging group can be varied, to change the spacing between the silicon atoms of the silane for the most favorable reaction with the SiOH groups on the surface of silica supports. Bonded-phase packings with methyl, vinyl, phenyl, isopropyl, and tert-butyl functional groups show similar properties-but greater stability at low pH-than their corresponding dimethylsilyl derivatives.A second class of more stable bonded-phase materials uses monofunctional silanes, which contain one or two bulky groups (e.g., isopropyl or tert-butyl) on the silicon atom of the silane. These bulky groups provide steric protection to the Si-O-Si bond on the surface of the silica supports against hydrolysis at low pH, while still providing equivalent retention and column efficiency, compared to those of conventional monofunctional silane bonded-phase packings.ACKNOWLEDGMENT Special thanks are given to R. F. Carver for performing the solid-state NMR experiments. We also thank G. R. Wooler and J. B. Marshall for their work in performing many of the syntheses and chromatographic experiments.

The determination of the basic drug Amperozide, one of its main metabolites, and a third related compound in human blood plasma is described. All three compounds include an amine functionality. The process is governed by a moditled ASTED instrument (Gilson). The dialysis unit is exchanged for a supported liquid membrane (SLM), which gives a simultaneous cleanup and enrichment of the analytes. The ASI'ED is coupled to column liquid chromatographic equipment with W detection. An efficient cleanup is achieved, and there is no visible difference between the chromatograms after enrichment of a plasma blank and an aqueous blank. To increase the recovery, the same sample can be passed by the membrane several times, while the receiving phase is kept stagnant. An enrichment with three passages takes 15-20 min. The limit of detection for Amperozide in blood plasma is then -30 ng/mL. The use of the SLM technique for estimation of the degree of protein binding is discussed. Preliminary results agree reasonably with measurements with other techniques. The optimization of the enrichment with supported liquid membranes is described and different membrane supports are compared.When biological samples are analyzed, a cleanup procedure is usually necessary before any chromatographic determination. The most frequently used methods are liquid-liquid solvent extraction, solid phase extraction, SPE, or protein precipitation.' The former often gives a good cleanup from the matrix, but is either laborious or difficult to automate, and occasionally problems with emulsions can occur. The use of large amounts of organic solvents should also be avoided for environmental and health reasons. SPE is a convenient method?a but suitable packing materials are not always available, and the cleanup is sometimes not sufficiently efficient. With protein precipitation the sample is diluted, which increases the detection limit, and many endogenous compounds are still present in the sample after the removal of the proteins. A new pretreatment method is the use of restricted 5

A method for continuous sampling of acidic herbicides in natural waters based on supported liquid membrane extractions has been developed. A porous PTFE membrane was impregnated with an organic solvent, ensuring only uncharged molecules passed through the membrane. Selecting the pH in the donor and acceptor phases selectively enriched acidic substances on the acceptor side. The herbicides (bentazon, 2,4-D, dicamba, dichlorprop, MCPA, and mecoprop) were continuously sampled during 24 h. After collection, the samples were taken to the laboratory for quantification using liquid chromatography with UV detection. The extraction efficiences for the herbicides were ca. 0.70, resulting in detection limits of ca. 40 ng/L, sample volumes of 1 L (24-h sampling), in clean aqueous samples. The detection limits in natural waters are somewhat higher and are estimated to ca. 0.1 pg/L. Changes in both air and water temperatures had no significant effect on the extraction efficiencies.

In spite of the great technical importance of organometallic based epitaxial growth there is still a lack of more detailed understanding of the conditions prevailing on the growing surfaces. This lack of knowledge is in contrast to the case of molecular beam epitaxy (MBE) for which reflection high-energy electron diffraction (RHEED) patterns tell in considerable detail the surface reconstructions which occur in different conditions. With the invention of the reflectance-difference (RD) technique Aspnes opened the possibility for similar control of MOVPE (metalorganic vapour phase epitaxy) growth because the optical techniques do not require high-vacuum conditions as do electron beam experiments. However, there has been, to date, very limited progress in connecting MBE results with MOVPE growth. In this paper we are able to connect (i) the RD results obtained in a growth system (VCE, vacuum chemical epitaxy) where pressure can be varied in the range from low-pressure MOVPE ( mbar) to 1O mbar with (ii) RD and RHEED data obtained from a CBE system (chemical beam epitaxy) where pressure can be varied in the range from iO mbar to 106 mbar. Using this comparison we identify RD features normally seen for VCE and MOVPE conditions with specific surface reconstructions as recognised by RHEED in our CBE system. We then identify the less arsenic-rich surface conditions into which a GaAs surface spontaneously converts when the arsenic source is removed. An identification is proposed for the surface reconstruction conditions which correspond to the critical surface stoichiometry, in terms of the RD response and the material properties, recently demonstrated for VCE growth. Finally, we compare the quality and the character of real-time RD-detected growth oscillations as obtained for CBE, VCE and MOVPE growth.