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mation factor analysis (which can relate one data space to another by vector rotation) to retention data from three different reversed-phase columns. They found that the data from the three columns shared a common factor space and that three factors are sufficient to describe the retention data. The eigenvector matrix associated with analytes from singular value decomposition was character-
istic of the retention behavior of the compounds and independent of the mobile phase and column, and the same is true for the mobile-phase eigenvector. On the basis of these observations, they developed a reference retention index system for chromatographic solutes and solvents across different columns with a mean retention error of prediction of 4%. (J. Chromatogr. .ci. 1196,34,101-10)
Diagram of the interface: 1, ion source block; 2, ceramic block; 3, heating wires; 4, thermocouple; 5, Vespel ferrule; 6, SFC/MS probe; 7, capillary column; 8, end of transfer line. (Adapted with permission of Huthig GmbH.)
tance between the restrictor and ion source was > 10 mm were higher interface temperatures required for efficient transfer of the more polar compounds. A distance of —11 mm from the beginning of Most coupled SFC/MS systems are based the heated zone was found to result in maximum sensitivity. When noise caused on quadrupole mass analyzers, which are relatively simple and inexpensive but have by C0 2 was removed from the total ion current chromatogram, S/N was further limited resolving power and mass range. improved by a factor of 4. Under optiMagnetic sector instruments can provide higher resolving power, greatly expanded mized conditions, the estimated detection limit for M-pentadecane is —30 ppm, mass range, and better sensitivity. However, these instruments are expensive, and and spectra obtained in electron impact ionization mode were very similar to liinterfacing them with a chromatographic inlet is complicated. Manuel A. A. Mertens brary spectra. and colleagues at Eindhoven University They also used the system to separate of Technology (The Netherlands) and and identify a set of low molecular mass, Koninklijke/Shell-Laboratorium (The thermally unstable peroxides that can be Netherlands) have described an interfound in many large-scale industrial proface for coupling a magnetic sector mass cesses; they were able to distinguish more spectrometer to a supercritical fluid chro- than 50 different compounds. However, bematograph bcised on direct introduction cause of the restricted scan range, strucof the CO; mobile phase into the ion ture elucidation was sometimes troublesome. With additional information from GC/MS, the majority of peaks could be They optimized the interface using a identified. The SFC/MS system also demtest mixture of low-to-medium mass compounds dissolved in hexane. For relatively onstrated good performance for high molecular mass compounds such as polyacrylow-boiling compounds in the mixture, the restrictor temperature had virtually no lates and polymer additives. (J. High Resolut. Chromatogr. 1996,19,17-22) influence on S/N. Only when the dis-
Interfacing a magnetic sector MS to SFC
296 A
Analytical Chemistry News & Features, May 1, 1996
Enantiomeric separations using CEC In capillary electrochromatography, a relatively new technique, an electricfieldis applied between both ends of a 5- to 500-um i.d. capillary column with a stationary phase that is either bonded to the capillary walls or packed as small particles. The mobile phase is driven through the capillary by electroosmosis, and solutes are separated according to how they partition between the phases and the ratio of charge to friction coefficient (electrophoretic mobility). Richard N Zare and colleagues at Stanford University and Ecole Nationale Superieure de Chimie de Parrs (France) studied some fundamental aspects of CEC bv separating neutral comrjounds in caDillaries oacked with octadecylsilica particles They found no loss of efficiency up to a linear electroosmotic velocity of 1.3 mm/s for retained analytes with capacity factors of 0.72.5. Neutral enantiomers of the drug chlorthalidone were successfully separated by adding the chiral agent hydroxypropyl-P-cyclodextrin (HP3CD) to the acetonitrile/ phosphate buffer mobile phase and using an ODS column and by using a chiral HPBCD stationary phase with an archiral acetonitrile/ phosphate buffer mobile phase. The separation using the HPBCD column achieved higher selectivity and resolution in less time (/ Chromatogr A 1996 727 145-56)
Influence of % acetonitrile on separation of chlorthalidone on HPfiCD-bonded silica. (Adapted with permission of Elsevier Science.)
