Retention gap, injection

An alternative approach is to use a splitless injection system. If the valve in Fig. 1 is closed, then all the sample passes into the column and there is no split ipso facto, the device is a splitless injector. When used in the splitless mode, however, it is usual to employ a somewhat wider capillary column, which will allow the penetration of a small-diameter injection syringe and thus permit on-column injection. Under these circumstances, there can be no differential sampling of the form described. This procedure, however, introduces other injection problems that can affect both resolution and quantitative accuracy that need to be addressed (See the entries Retention Gap Injection Method and Solute Focusing Injector Method). 

Figure 5 Helium flow rate and solvent peak profile for injections of ethyl acetate into a 0.53-mm ID retention gap. Injection time, 20 sec injection speed, 160 pl/min evaporation rate, 130 pl/min 10 pi were left as solvent film in the retention gap at end of injection. Helium flow is measured by means of a flow meter in the carrier gas tubing, and the solvent flow by means of an FID at the end of the retention gap. (From Ref. 68.)...

Traditionally, LC and GC are used as separate steps in the sample analysis sequence, with collection in between, and then followed by transfer. A major limitation of off-line LC-GC is that only a small aliquot of the LC fraction is injected into the GC p. (e.g. 1 - 2 p.1 from 1 ml). Therefore, increasing attention is now given to the on-line combination of LC and GC. This involves the transfer of large volumes of eluent into capillary GC. In order to achieve this, the so-called on-column interface (retention gap) or a programmed temperature vaporizor (PTV) in front of the GC column are used. Nearly all on-line LC-GC applications involve normal-phase (NP) LC, because the introduction of relatively large volumes of apolar, relatively volatile mobile phases into the GC unit is easier than for aqueous solvents. On-line LC-GC does not only increase the sensitivity but also saves time and improves precision. 

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... 

A retention gap is used to improve peak shapes under certain conditions. If you introduce a large volume of sample (>2 pL) by splitless or on-column injection (described in the next section), microdroplets of liquid solvent can persist inside the column for the first few meters. Solutes dissolved in the droplets are carried along with them and give rise to a series of ragged bands. The retention gap allows solvent to evaporate prior to entering the chromatography column. Use at least 1 m of retention gap per microliter of solvent. Even small volumes of solvent that have a very different polarity from the stationary phase can cause irregular solute peak shapes. The retention gap helps separate solvent from solute to improve peak shapes. 

However, the most frequently used system is trace enrichment in a short LC column or SPE precolumn. The column is usually filled with C18 or PLRP-S and dried with nitrogen prior to elution. The analytes are eluted with an organic solvent, usually ethyl acetate, which is injected into the gas chromatograph through an on-column interface by a retention gap. In the chromatograph there is also a retaining precolumn, to minimize losses of the most volatile compounds, an analytical column and, between the precolumn and the analytical column, a solvent vapour exit (SVE) to eliminate the vapour (Figure 13.18). 

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