Cold on-column injector

Figure 3.8 Alternative designs for cold on-column injectors, h, Injector with a duck bill valve (Hewlett-Packard), B, an injector with provision for secondary cooling of the column inlet (Carlo-Erba), and C, a temperature- programmable on-column injector with its own oven isolated from the column oven (Varian Associates).

GC of aqueous simulant or water extract of olive oil using cold on-column injector. 1,4 butane diol internal standard Solvent extraction with dichloromethane and concurrent derivatisation with 9-(methylaminomethyl)anthracene. Fluorescent derivatives analysed by HPLC with fluorescence detection... 

Figure 2.5—Injectors, a) A split/splitless injector (the split is regulated by valve 2). The exit labelled 1 is called the septum purge, b) A cold on-column injector. A typical feature of a chromatogram obtained in the splitless mode is the interference of the solvent with the analytes. This can be avoided using a selective detector.

Cold (on-column) injectors can be used in capillary GC for the analysis of relatively high boiling (low volatile) solutes, so that only the upper temperature limit of the column itself remains relevant. 

Procedure Use a gas chromatograph equipped with a flame-ionization detector, a cold-on-column injector, a suitable deactivated precolumn, and a 10-m x 0.32-mm (id) capillary column coated with an apolar stationary phase 0.12-p.L film thickness. Program the column to heat to 60°, hold for 1 min, heat to 300° at 20°/min, and hold for 3 min. Set the flame-ionization detector to 320°. Chromatograph five injections of the Standard Preparation. Measure the responses. The relative standard deviation for each peak should be below 2%. The peaks for brassicasterol and campesterol should be baseline resolved (Rs > 1.0) and show no tailing. Measure the response of the Internal Standard Solution and all the individual sterols eluting in the relative retention window of 0.98 to 1.13. 

Cold on-column injector all the sample is injected onto the column. It is used for thermally unstable compounds and high-boiling solvents. 

As the name indicates, the cold on-column injector allows the injection of the sample directly as a liquid onto the column, of which the inlet and/or outlet section is... 

Fig. 1 Schematic diagram of a typical cold on-column injector with provision for secondary coohng of the column inlet. 1 Rotating valve, 2 carrier gas, 3 capillary column, 4 secondary cooling.

Figure 3.8. Cold-on column injector with a duckbill valve sealing mechanism ( Hewlett-Packard Co.).

As the name indicates, the cold on-column injector allows the injection of the sample directly as a liquid onto the column, of which the inlet and/or outlet section is maintained at a lower temperature than the oven. After the solutes are focused in the inlet section of column, the sample is then vaporized as the oven temperature is increased. In this way, all of the sample components are transferred into the column, so that the sample discrimination attributed to the syringe needle heating during... 

Fig. 2 Schematic diagram of a cross section of a cold on-column injector with a duck-bill valve. 1 Cool tower needle guide, 2A disk septum for automated injections, 2B isolation valve for manual injections, 3 frit, 4 carrier gas, 5 oven wall, 6 ferrule, 7 capillary column, 8 column nut.

For a splitless injection, the temperature will be at least 250°C. However, the cold on-column injector seems to be best suited for the following reasons ... 

Adsorption or catalytic decomposition of labile substances by the syringe needle can be a problem for some compounds using hot vaporizing injectors [25]. For open tubular columns deactivated fused silica syringe needles and cold on-column Injection techniques are used to minimize this problem. Alternatively, syringes fitted with a needle shroud for cold-needle injection can be used [26]. 

Like cold on-column injection the PTV has a low thermal mass, so can be heated and cooled quickly. However, unlike cold on-column the sample is injected into an injector liner and not direct onto the column. Unlike conventional split/spUtless injection, the aim is to make use of volatility selectivity of the system rather than to inhibit it. By selecting an appropriate starting temperature, the anal34es condense in the liner if desired the solvent may be flushed away through... 

In GC the injection may take place at a temperature that is lower than that of the column oven. The solute bands will be concentrated in a small volume and may be brought into the column by a subsequent heating of the cold zone. If this zone is part of the column itself we talk about cold (on-column) injection , if it is part of a separate injector unit we talk about cold trap injection. A similar band compression effect may be achieved in a different way by leaving the first part of the capillary column uncoated (i.e. no stationary phase present). The solute band will then be compressed at the point where the stationary phase starts to be present in the column. This band compression technique is usually referred to by the unfortunate term retention gap [706]. 

Figure 4.14 Sample injection ports, (a) Rash-vaporizer, (b) Split injector with septum purge for capillary columns, (c) Direct -cold on-column injection onto a capillary column showing rotating valve and insertion of needle into the base of the column. (Reproduced by permission of Dr Alfred I liithig Verlag from J. High Res. i hromuiogr., Chromalogr. Contniun., 2. 35.X (1979).)...

It has been shovm that direct injectors, such as the cold on-column where the liquid sample enters the inlet of a capillary column, are able to produce quantitative results with improved reproducibility and accuracy [17, 18 ]. Moreover, non-derivatized compounds with low physico-chemical stability can efficiently be analyzed using this type of injector. 

For sample introduction in HTGC, split, splitless, on-column injection, or combinations of the latter, the programmable temperature vaporizing (PTV) injector are suitable. Howeveq with the exception of cold on-column injection, none of these introduction systems fulfill the requirements of an adequate HTGC injection device and guarantees reliable results. 

All heated injector systems are well-known sources of error, due to the possibility of artifact formation, and in many cases, split, splitless, or PTV injection provides discrimination of high-boiling compounds by thermal degradation. Only cold on-column injection ensures quantitative sample transfer from the injection port into the capillary column without thermal stress of the samples or discrimination of high-boiling compounds. 

Rgure 5 Programmed temperature vaporizer injector. (From Hinshaw JV and Seferovic W (1986) Programmed-temperature split-splitless injection of triglycerides comparison to cold on-column injection. Journal of High Resolution Chromatography 9 69-77.)... 

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