Separation of sucrose polyesters by capillary supercritical-fluid chromatography/flame ionization detection with robot-pulled capillary restrictors

Abstract

Fatty acid esters of sucrose (sucrose polyesters, or SPE's) were successfully separated on an open-tubular column uslng supercrltlcal carbon dloxlde as moblle phase. Room-temperature spllttlng Injection was used wlth an uncoated, fused silica Inlet tube to focus solute on the head of the column, slmllar to the retention gap method In GC. Artlfact-free detectlon was posslble by the use of a robotmade, thin-walled, tapered-caplllary restrlctor as the Interface between the column and a flame Ionization detector. The column temperature was found to have a pronounced effect on the selectlvlty and resolution of the chromatogram. Standards were analyzed to find the effect of carbon number dlstrlbutlon and of unsaturallon on retention tlmes of sucrose octaesters. Expected relative amounts of sucrose octaesters In real samples were calculated based on the random dlstrlbutlon of fatty acids from a starting materlal of known composltlon. A model chromatogram was calculated and plotted and was used to help Interpret the laboratory chromatograms. For sucrose octaesters, carbon number dlstrlbutlon leads to discernible peaks, wlth the peak wldth determined by the dlstrlbutlon of double bonds.Sucrose polyester (SPE) (1) is produced by the catalytic reaction of sucrose and long-chain fatty acid methyl esters, resulting in the esterification of up to all eight of the sucrose hydroxyls. SPE is of interest in both the food and medical science areas because, when eaten, it is neither hydrolyzed nor absorbed from the gastrointestinal tract. Thus, it represents a noncaloric source of edible fat (2). In addition, clinical studies have shown that SPE, taken orally, can lower the level of plasma cholesterol in humans (3,4).Very little work has been published on the analysis of SPE. This is closely tied to two fundamental problems: First, commerical sources of fatty acids used to prepare SPE often have four or more different components at major levels. Second, there is a range in the degree of esterification of the molecules in any SPE sample, with hexa-, hepta-, and octaesters all usually present at major levels. Thus, dozens of (empirical) compounds and thousands of isomers exist in SPE finished product. Only thin-layer chromatography has been capable of determining ester distributions (5) but is incapable of telling additional information such as carbon number distribution within a particular ester family or the extent of unsaturation. Size exclusion chromatography (SEC) has been used to determine SPE in diets and feces, but SEC elutes all SPE molecules in a single peak (5). It is possible that SEC could give, at least, limited data on the range and average degree of esterification of SPE material, but no work on this or any other liquid chromatography approach has been reported. Molecular weights are too high for gas chromatography. Thus, a battery of tests, including SEC, TLC, GC (of residuals), iodine value, and hydroxyl value, are usually run to characterize SPE.We have investigated the use of capillary supercritical-fluid chromatog...

Applications of Supercritical Fluids in Industrial Analysis

“…Non-volatile compounds may precipitate in this restrictor, due to the long pressure-reduction zone. The robot-drawn fused-silica tapered restrictor (Chester et al, 1985) represented a significant improvement, but the hairthin tip with a 1-2 ~m orifice breaks easily. A more robust construction was the integral restrictor , made by polishing the end of a fused-silica tube carefully closed by heat.…”

New detection strategies through supercritical fluid chromatography

“…In subsequent communications [19,20], his group dramatically increased the molecular-weight range through even more delicate restrictors prepared by a robot device to ensure their reproducibility. Thin-walled, fused silica capillary restrictors, with 3-4 pm orifice, were used to introduce solutes as large as 7,000-8,000 daltons into the flame for successful detection.…”

Use of SFC/MS for the confirmation of peak identities in the SFC/FID chromatogram of olestra

“…An alternative method which is now in routine use for monitoring olestra samples is open tubular supercritical fluid chromatography with flame ionization detection (SFC-FID) [9,10]. The full range of olestra components, from fatty acid methyl esters to sucrose octaesters, can be eluted in a single pressure programmed run.…”

“…Based upon injection of standards and other data, the three large humps were postulated to be TMSderivatized sucrose hexaester, TMSderivatized sucrose heptaester, and sucrose octa-J , Conditions were similar to those described in the Experimental section, with these exceptions: flowsplit injection (-lO:l), no retention gap, no wasteport restrictor, integral flow restrictor, pressure rampedfrom 125 atm to 375 atm at 10 atm m i d , 90°C oven temperature. ester [9,10]. The fatty acids esterified to each sucrose have a variety of chain lengths (primarily 16-and 18-carbon acids) and a variety of degrees of unsaturation.…”