Microcolumns

Microcolumns use less solvent and, because the sample is diluted to a lesser extent, produce larger signals at the detector. These columns are made from fused silica capillaries with internal diameters of 44—200 pm and lengths of up to several meters. Microcolumns packed with 3-5-pm particles have been prepared with column efficiencies of up to 250,000 theoretical plates. 

Open tubular microcolumns also have been developed, with internal diameters of 1-50 pm and lengths of approximately 1 m. These columns, which contain no packing material, may be capable of obtaining column efficiencies of up to 1 million theoretical plates.The development of open tubular columns, however, has been limited by the difficulty of preparing columns with internal diameters less than 10 pm. 

Procedures for trapping accelerant vapors in the headspace of a closed container on charcoal that is either encased in a porous pouch or impregnated into a flexible membrane have been described (124). Trace amounts of explosive compounds can be trapped from hplc effluents onto a porous polymer microcolumn for confirmatory gc examination (125). 

The increasing use of microcolumns has moved chromatography towards uniflca-tion. Giddings was the first to point out (18) that there was no distinction between... 

Better solubility and faster diffusion available in high-temperature p.LC R. Trones, A. Iveland and T. Greibrokk, J. Microcolumn Sep. 7, 505 (1995)... 

M. Novotny, Recent advances in microcolumn liquid cliromatography , Awa/. Chem. 60 500A(1988). 

J. A. Lippeit, B. Xin, N. Wu and M. L. Lee, East ultraliigh-pressure liquid chromatography on-column UV and time-of-flight mass spectrometric detection , 7. Microcolumn. Sep. 11 631 (1999). 

J. Staniewski, H.G. Janssen, C. A. Cramers and J. A. Rijks, Progi ammed-temperature injectoi for lai ge-volume sample inti oduction in capillaiy gas clnomatography and for liquid clnomatography-gas cln omatogi aphy interfacing , 7. Microcolumn Sep. 4 331-338(1993). 

H. G. J. Mol, H.-G. Janssen, C. A. Cramers and U. A. Th Brinkman, On-line sample enrichment-capillary gas clir omatography of aqueous samples using geometr ically deformed open-tubular extraction columns , 7. Microcolumn Sep. 7 247-257 (1995). 

L. Mondello, G. Dugo and K. D. Baitle, On-line microbore high perfoimance liquid cliiomatography-capillaiy gas cliiomatography foi food and water analyses a review , J. Microcolumn Sep. 8 275-310 (1996). 

L. Mondello, M. Catalfamo, P. Dugo and G. Dugo, Multidimensional capillary GC-GC for the analysis of real complex samples. Part IE Enantiomeric distribution of monoteipene hydrocarbons and monoteipene alcohols of cold-pressed and distilled lime oils , J. Microcolumn Sep. 10 203-212 (1998). 

J. Beens and R. Tijssen, An on-line coupled HPLC-HRGC system for the quantitative chai acterization of oil fractions in the middle distillate range , J. Microcolumn Sep. 7 345-354(1995). 

L. A. Holland and J. W. Jorgenson, Separ ation of nanoliter samples of biological amines by a comprehensive two-dimensional microcolumn liquid chr omatography system . Awn/. Chem. 67 3275-3283 (1995). 

E. M. Ean as and S. R. Rissato, Influence of temperature, pressure, modifier and collection mode on superaitical CO2 extraction efficiencies of diuron from sugar cane and orange samples , J. Microcolumn Sep. 10 473-478 (1998). 

H. Daimon and Y. Hirata, Trapping efficiency and solute focusing in on-line supercritical fluid exti action/capillai y supercritical fluid chi omatography , 7. Microcolumn Sep. 5 531-535 (1993). 

E. S. Erancis, M. Wu, R B. Eamswoith and M. L. Lee, Supercritical fluid extraaion/gas cliromatography with thermal desorption modulator interface and niti O-specific detection for the analysis of explosives , 7. Microcolumn Sep. 7 23-28 (1995). 

U. Ullsten and K. E. Mai kides, Automated on-line solid phase adsoiption/supera itical fluid exti action/superciitical fluid cltromatogi aphy of analytes from polar solvents , 7. Microcolumn Sep. 6 385-393 (1994). 

F. M. Ean as and M. A. Ruggiero, On-line coupling of supercritical fluid exti action to capillaiy column electi odriven separation techniques , J. Microcolumn Sep. 12 61-67 (2000). 

R. E. Robinson, D. Tong, R. Moulder, K. D. Battle and A. A. Clifford, Unified open tubular column cliromatography sequential gas chromatography, at normal pressures and supercritical fluid cliromatography on the same column , J. Microcolumn. Sep. 3 403-409(1991). 

D. Tong and K. D. Battle, Band broadening during mobile phase change in unified cliromtography (GC-SEC) , 7. Microcolumn Sep. 5 237-243 (1993). 

V. L. McGuffin, C. E. Evans and S. H. Chen, Ditect examination of separation processes in liquid-cliromatography effect of temperature and pressure on solute retention , 7. Microcolumn Sep. 5 3-10 (1993). 

MICROCOLUMN REVERSE PHASE HIGH PEREORMANCE LIQUID CHROMATOGRAPHY-CAPILLARY ZONE ELECTROPHORESIS... 

A six-port valve was first used to interface the SEC microcolumn to the CZE capillary in a valve-loop design. UV-VIS detection was employed in this experiment. The overall run time was 2 h, with the CZE runs requiring 9 min. As in the reverse phase HPLC-CZE technique, runs were overlapped in the second dimension to reduce the apparent run time. The main disadvantage of this yu-SEC-CZE method was the valve that was used for interfacing. The six-port valve contributed a substantial extracolumn volume, and required a fixed volume of 900 nL of effluent from the chromatographic column for each CZE run. The large fixed volume imposed restrictions on the operating conditions of both of the separation methods. Specifically, to fill the 900 nL volume, the SEC flow rate had to be far above the optimum level and therefore the SEC efficiency was decreased (22). 

The second interface design that was developed for use with yu-SEC-CZE used the internal rotor of a valve for the collection of effluent from the SEC microcolumn. The volume collected was reduced to 500 nL, which increased the resolution when compared to the valve-loop interface (20). However, a fixed volume again presented the same restrictions on the SEC and CZE operating parameters. An entirely different approach to the interface design was necessary to optimize the conditions in both of the microcolumns. 

Figure 9.7 Schematic illustration of the flow-gating interface. A channeled Teflon gasket was sandwiched between two stainless steel plates to allow for flow into the electrophoresis capillary, either from the flush buffer reservoir or from the LC microcolumn during an electiokinetic injection.

Figure 9.11 Schematic illustration of the transparent interface used to link the HPLC capillary to the CZE capillary. Reprinted from Analytical Chemistry, 69, T. E. Hooker and J. W. Jorgenson, A transparent flow gating interface for the coupling of microcolumn EC with CZE in a comprehensive two-dimensional system , pp 4134-4142, copyright 1997, with permission from the American Chemical Society.

Figure 10.14 Schematic representation of the SFSPE/SFC set-up developed by Murugaverl and Vooi hees (67). Reprinted from Journal of Microcolumn Separation, 3, B. Mumgaverl and K. J. Vooi hees, On-line supercritical fluid exti aaion/chromatography system for ti ace analysis of pesticides in soybean oil and rendered fats , pp. 11-16, 1991, with permission from John Wiley and Sons, Inc.

E. Ibanez, J. Palacios and G. Reglero, Analysis of tocopherols by on-line coupling supercritical fluid extraction-superaitical fluid chromatography , ]. Microcolumn Sep. 11 605-611 (1999). 

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