Large-Volume Injectors

FIGURE 13.10 Generic setup for (a) on-line SPE-HPLC implemented with a simple valve-switching system (G. Maio, R. Morello, F. Arnold, and K.-S. Boos, Analysis of Antimycotic Drugs in Biofluids by On-Line SPE-LC Application note LPN 1859-01 06/06 Diones Corporation, Sunnyvale, CA 94088-3603 Figure 2, p. 1, 2006. With permission.) and for (b) on-line SPE-GC implemented with a large volume injector with a solvent venting option. 

FIGURE 2.8 Basic arrangement of an HPLC-GC device with a sample loop interface. RG, retention gap Cl, retaining column C2, analytical column LVl, large volume injector and SVE, solvent vapor exit. 

Programmed-temperature vaporizer electronic pressure-controlled sample inlet systems -ms grade columns, solid-phase microextraction sampling techniques, large-volume injectors, integrated guard columns, multidimensional GC, sol-gel columns, improvements in silphenylene phases, advances in GC-MS, more affordable benchtop GC-MS systems... 

Together with the techniques described above, other techniques using hot injectors for the transfer of large-volumes in capillary gas chromatography have been developed. Transfer of large-volume solvents in a programmed temperature vaporizing... 

Several sample preparation techniques are performed inside the inlet system. Large-volume injection can be carried out by a number of methods including programmed temperature vaporisation (PTV). Automated SPE may be interfaced to GC using a PTV injector for large volume injection. SPE-PTV-GC with on-column injection is suited to analysis of thermola-bile compounds. 

Principles and Characteristics Although early published methods using SPE for sample preparation avoided use of GC because of the reported lack of cleanliness of the extraction device, SPE-GC is now a mature technique. Off-line SPE-GC is well documented [62,63] but less attractive, mainly in terms of analyte detectability (only an aliquot of the extract is injected into the chromatograph), precision, miniaturisation and automation, and solvent consumption. The interface of SPE with GC consists of a transfer capillary introduced into a retention gap via an on-column injector. Automated SPE may be interfaced to GC-MS using a PTV injector for large-volume injection [64]. LVI actually is the basic and critical step in any SPE-to-GC transfer of analytes. Suitable solvents for LVI-GC include pentane, hexane, methyl- and ethylacetate, and diethyl or methyl-f-butyl ether. Large-volume PTV permits injection of some 100 iL of sample extract, a 100-fold increase compared to conventional GC injection. Consequently, detection limits can be improved by a factor of 100, without... 

Because the expected concentration level is so low, the standard procedure for bromate using IC calls for a 250-/./L sample loop on the injector, an unusually large volume for a sample loop. The procedure for the common anions listed above utilizes a 50-/./L loop. It is a therefore a common task in this laboratory to change the sample loop regularly as these different anions are determined. 

There are three injection techniques for introducing a sample into a GC equipped with a capillary column split injection, splitless injection, and on-column injection. Split injection is the most often used injection technique. When a certain amount of FAME sample (1 to 3 ll) is introduced into the GC injector that is normally set at a temperature much higher than the boiling point of the solvent, the solvent vaporizes instantly in the carrier gas and creates a large volume of gas that contains all of the injected FAME in it. The carrier gas that contains the FAME is then divided into two streams from the injector one is directed onto the column, and the second is vented to the atmosphere, clearing the sample out of the injection chamber momentarily. This way, only a limited amount of sample is introduced into the column, to avoid column overloading, and injection time is short, to avoid peak broadening. 

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