Chemistry of Winemaking ADVANCES IN CHEMISTRY SERIES No. 137 A. Dinsmoor Webb, Editor A symposium sponsored by the Division of Agricultural and Food Chemistry of the American Chem ical Society. An excellent source of scientific facts and applications covering the entire spectrum of winemaking from vineyard to wine cellar. Thirteen papers give a compre hensive overview of all phases of production, spotlighting specific procedures and equipment used in commercial and home winemaking. Topics include: • the chemistry of grapes and red wine color • malo-lactic fermentation; phenolic substances • distilling wine into brandy; use of wooden containers in wine mat uration 311 pages (1974) $16.95 cloth bound. ISBN 0-8412-0208-7. Post paid in U.S. and Canada, plus 40 cents elsewhere. Order from: Special Issues Sales American Chemical Society 1155 Sixteenth St., N.W. Washington, D.C. 20036
crease the width of early eluting so lutes over and above the width that results from the usual processes in the column. Snyder (15) defined a term called the linear capacity of the system which is described as the weight of the sample per gram of packing which causes a 10% reduction in the specific retention volume relative to the constant retention volume ob served for smaller samples. If con stant injection volume is used, it is advisable to operate at sample weights which are lower than the lin ear capacity, since this will ensure better efficiencies. The linear capaci ty of a chromatographic system can be obtained by experimentation (14). In general, however, as the surface area of the support increases, larger sample injections can be made. It is also possible that partition coefficients can be solute concentra tion dependent, giving rise to asym metrically shaped elution peaks. When the peaks tail (Figure 4, A), the position of the peak maxima de creases with increased solute concen tration; when the peaks front (Figure 4, B), the peak maxima increase with increased solute concentration. In both bases the asymmetry results in a peak broadening which reduces the resolution between peaks. It is thus desired at times to run the experiment with several different solute concentrations to see whether or not the peak width, peak asymme try, and retention time are constant. Summary The efficiency H, then, is a func tion of the mobile phase velocity, the solute's partition coefficient (or the capacity ratio), the diffusion coeffi cient of the solute in the stationary and mobile phases, the support parti cle size and nature, the amount of stationary phase on the support (film thickness), and the surface area of the support in adsorption chromatog raphy. To lower H and increase the resolu tion, several courses of action are available. The following is only a rough guideline to some of the ap proaches the investigator can take. [A much more comprehensive and sys tematic discussion of improving the resolution in high-speed LC was re cently given by Snyder (16, 17)]. • Decrease the carrier velocity. This, however, means longer analysis time. Also, one can increase the mo bile phase velocity provided that the column length is increased propor tionately. • Decrease amount of stationary phase in liquid-liquid system (but not to the point where adsorption on the naked support begins to adversely af fect H).
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A
Β
Figure 4. Peak asymmetry A Tailing Β Fronting
Φ Decrease the size of the support particle. It must be emphasized here that when using small particles (i.e., less than 20 μτη), packing becomes a problem and special care must be ex ercised when filling the column. This point will be discussed shortly. • Use a mobile phase having low viscosity so that the diffusion of so lutes in it is rapid. Also, for a desired mobile phase velocity, lower viscosity means lower inlet pressures. • Lower the viscosity of the mobile phase by increasing the temperature. However, the effect of temperature on the plate height is less clear cut. Schmit et al. (18) indicated the H improves with increasing tempera ture, whereas Knox and Vasvari (19) found no dependence of H on the temperature. An increase in the tem perature results in a decrease of k' (or the retention time) and most fre quently in α. That decrease in LC is most often more pronounced than any improvement in H, and the resolution can deteriorate with increasing tem perature (Equation 4). • Use a support where the likeli hood of large stagnant pockets of mo bile phase are minimized (pellicular packing or microspheres with small pores). As mentioned previously, the HETP can be decreased by decreas ing the size of the packing. Η varies roughly as the particle diameter to the power of about 1.8. As the parti cle size decreases, the permeability of the column decreases, and higher inlet pressures are needed to drive the mobile phase. The permeability is roughly a function of the square of the particle diameter. However, be cause of the higher efficiency of the
