Solvent effects, injection

Solvent degassing (LC) 553 Solvent demixing (TLC) 660 Solvent effects, splitless injection (GC) 250 Solvent extraction 753 applications 766 hoBogenizer 761 i plitgers 756 micTMethods 764 microwave 761 optimization 753 shake flask 761 solvent reduction 763 Soxhlet 762 Solvent (LC)... 

Hot splitless WCOT 0.5 ppm (FID) without preconcentration Lower injection temperature than split Trace analysis Handles dirty samples Automation Flash vaporisation Optimisation required (splitless time, oven temperature, solvent) Limited number of solvents ( solvent effect ) Thermal degradation possible Discrimination possible Poor direct quantification Unsuitable for very polar substances... 

Principles and Characteristics As mentioned already (Section 3.5.2) solid-phase microextraction involves the use of a micro-fibre which is exposed to the analyte(s) for a prespecified time. GC-MS is an ideal detector after SPME extraction/injection for both qualitative and quantitative analysis. For SPME-GC analysis, the fibre is forced into the chromatography capillary injector, where the entire extraction is desorbed. A high linear flow-rate of the carrier gas along the fibre is essential to ensure complete desorption of the analytes. Because no solvent is injected, and the analytes are rapidly desorbed on to the column, minimum detection limits are improved and resolution is maintained. Online coupling of conventional fibre-based SPME coupled with GC is now becoming routine. Automated SPME takes the sample directly from bottle to gas chromatograph. Split/splitless, on-column and PTV injection are compatible with SPME. SPME can also be used very effectively for sample introduction to fast GC systems, provided that a dedicated injector is used for this purpose [69,70],... 

Grob K (1987) On-column injection in capillary gas chromatography basic techniques, retention times and solvent effects. Springer, Berlin Heidelberg New York, p 67... 

FIGURE 10 Chromatograms showing the effect of sample solvent and injection volume on peak shape.The peak at 5 min is an impurity in a NCE. Column Supelco Discovery RP amide C16 mobile phase, 2.5% acetonitile in water detection, UV 190 nm injection 10, 20,30,50 and 100 pL sample solvent (A) 2.5% (B) 10% (acetonitrile in water). 

Splitless injection involves keeping the injector split vent closed during the time the sample is deposited on the column, after which the vent is reopened and the inlet purged with carrier gas. In splitless injection, the inlet temperature is elevated with respect to the column temperature. The sample is focused at the head of the column with the aid of the solvent effect. The solvent effect is the vaporization of sample and solvent matrix in the injection port, followed by trapping of the analyte in the condensing solvent at the head of the column. This trapping of the analyte serves to refocus the sample bandwidth and is only achieved after proper selection of the solvent, column and injector temperatures. Splitless injection techniques have been reviewed in References 29 and 30. 

Splitless, direct, and cold-on-column techniques all utilize the solvent effect to maximize sample loading and minimize sample band widths. There is a wealth of information on how to best utilize the solvent effect to minimize the starting sample bandwidths in the splitless mode of injection. Several articles review the proper use of the solvent effect [31-36]. Splitless injection is ideal for dilute clean samples it, however, is not suited for heat-sensitive samples. Classical split injection is discussed in a comprehensive review recently published [37]. The solvent effect in split injection has been discussed in two articles [38,39]. 

Produces relatively accurate results for volatile solutes problems with quantitation of high-boiling solutes (matrix effects ) requires re-concentration of the initial bands by cold trapping or solvent effects, which often forces cooling of the column for the injection. This is time-consuming and causes problems with absolute retention times. 

Splitless injection reconcentration of the bands broadened in time requires either lowering of the column temperature at least 60-90°C below the elution temperature of the solutes of interest (cold trapping) or keeping of the column at least 20-25 °C below the solvent boiling point to create solvent effects. 

R. Miller and W. Jennings, Normal and reverse solvent effects in split injection, HRC CC J. High Res. Chromatogr. Chromatogr. Commun., 2 72-73 (1979). 

In LC the solute bands may be concentrated on a pre-column, which is eluted with a weak eluent (low eluotropic strength). A simple way in which the effective injection volume may be reduced considerably is by dissolving or diluting the sample in a solvent that is much weaker than the eluent [707]. This has the effect that the initial capacity factor during... 

Mac-Nod HPLC Column Companion, Section 10 Injection Solvent Effects, http //www.mac-nod.com/cc/cc-10.html (August 1, 2002). 

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