Cold trapping, injection

Yamashita S, Ozawa R, Yamaguchi K, et al. 1992. Analysis of volatile organic compounds in air by gas chromatography with thermal desorption cold- trap injection and atomic emission and mass selective detection. Journal of High Resolution Chromatography 15 549-551. 

Sampling and Analysis. A frozen slice of bread was cut in pieces and stacked in an enlarged sample flask of an aroma isolation apparatus according to MacLeod and Ames (74). Volatile compounds were trapped on Tenax TA and afterwards thermally desorbed and cold trap injected in a Carlo Erba GC 6000 vega equipped with a Supelcowax 10 capillary column (60 m x 0.25 mm i.d.) and a flame ionisation detector. Similar GC conditions were used for GC-MS identification of volatile compounds by dr. M.A. Posthumus (Dept. Organic Chemistry, VG MM7070F mass spectrometer at 70 eV El, 75). 

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]. 

Spruit, H.E.T., Trap, H.C., Langenberg, J.P., Benschop, H.P. (2001). Bioanalysis of the enantiomers of ( )-sarin using automated thermal cold-trap injection combined with two-dimensional gas chromatography. J. Anal. Toxicol. 25 57-61. 

When properly installed, the analytical system involving thermodesorption cold trap injection and two-dimensional chromatography can be used routinely for analysis of the stereoisomers of soman and sarin in blood and tissue samples at minimum detectable concentrations of 1 -5 pg of stereoisomer per ml blood or gram tissue. In recent toxicokinetic experiments in pigs, the analytical system comprised chiral gas chromatography on a Chirasil-L-Val column with splitless injection and detection... 

When columns of the same polarity are used, the elution order of components in GC are not changed and there is no need for trapping. However, when columns of different polarities are used trapping or heart-cutting must be employed. Trapping can be used in trace analysis for enrichment of samples by repetitive preseparation before the main separation is initiated and the total amount or part of a mixture can then be effectively and quantitatively transferred to a second column. The main considerations for a trap are that it should attain either very high or very low temperatures over a short period of time and be chemically inactive. The enrichment is usually carried out with a cold trap, plus an open vent after this, where the trace components are held within the trap and the excess carrier gas is vented. Then, in the re-injection mode the vent behind the trap is closed, the trap is heated and the trapped compounds can be rapidly flushed from the trap and introduced into the second column. Peak broadening and peak distortion, which could occur in the preseparation, are suppressed or eliminated by this re-injection procedure (18). 

It is possible to collect the volatiles in a cold trap [22]. A more favoured technique is the collection of the gases by adsorption on some support such as one of the Chromosorbs, or Tenax GC [13,22,23]. The volatiles are then desorbed by heating and injected into a gas chromatograph. 

In addition to the analytical columns (columns used mainly for analytical work), so-called preparative columns may also be encountered. Preparative columns are used when the purpose of the experiment is to prepare a pure sample of a particular substance (from a mixture containing the substance) by GC for use in other laboratory work. The procedure for this involves the individual condensation of the mixture components of interest in a cold trap as they pass from the detector and as their peak is being traced on the recorder. While analytical columns can be suitable for this, the amount of pure substance generated is typically very small, since what is being collected is only a fraction of the extremely small volume injected. Thus, columns with very large diameters (on the order of inches) and capable of very large injection volumes (on the order of milliliters) are manufactured for the preparative work. Also, the detector used must not destroy the sample, like the flame ionization detector (Section 12.6) does, for example. Thus, the thermal conductivity detector (Section 12.6) is used most often with preparative gas chromatography. 

CS055 Smith, R. N. High-pressure liquid chromatography after injection of solid plant material and cold trapping. J Chromatogr 1975 115 101. 

Collection of air samples in stainless steel canisters whose surfaces have been passivated is another common collection technique for VOCs. (Aluminum has also been used but the stability of polar organics in them is poor Gholson et al., 1990.) Indeed, this method is used not only for sampling air but in medical applications as well, where they have been used to sample organics in a single breath (Pleil and Lindstrom, 1995). Passivation of the canisters is often carried out using a process called SUMMA and hence referred to as SUMMA canisters. The canisters also have to be thoroughly cleaned before use an example of one such procedure is described by Blake et al. (1994). The sample is then typically preconcentrated by transfer to a cold trap prior to injection onto the GC column (e.g., see Blake et al., 1994). 

Splitless injection is required for very dilute solutions. It offers high resolution but is poor for quantitative analysis because less volatile compounds can be lost during injection. It is better than split injection for compounds of moderate thermal stability because the injection temperature is lower. Splitless injection introduces sample onto the column slowly, so solvent trapping or cold trapping is required. Therefore, splitless injection cannot be used for isothermal chromatography. Samples containing less than 100 ppm of each analyte can be analyzed with a column fdm thickness < 1 p.m with splitless injection. Samples containing 100-1 000 ppm of each analyte require a column film thickness 1 p.m. 

Explain how solid-phase microextraction works. Why is cold trapping necessary during injection with this technique Is all the analyte in an unknown extracted into the fiber in solid-phase microextraction ... 

Figure 9-2 A gas-liquid chromatogram of a mixture of the isomeric butanols at constant column temperature. A tiny peak on the far left is a trace of air injected with the sample. The retention times of the various isomers are in the same order as the boiling points, which are, from left to right, 82°, 99.5°, 108°, and 117°. The areas under each peak correspond to the relative amounts of material present. Raising the column temperature at a preprogrammed rate while developing the chromatogram speeds up the removal of the slower-moving components and sharpens their peaks. Also, by diversion of the gas stream to appropriate cold traps it is possible to collect pure fractions of each component.

Cold trap heating should be done as quickly as possible for injection-like resolution. 

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