Column deactivation techniques

The column deactivation techniques are highly important in that the trace analysis of certain compounds should not be limited by the column to a greater degree than by the sensitivity of our detection devices. The lack of deactivation may become particularly visible while separating polar solutes (primary amines, acids. 

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]. 

The United States Pharmacopeia (USP) test (467) describes three different approaches to measuring organic volatile impurities in pharmaceuticals. Method I uses a wide-bore coated open tubular column (G-27, 5% phenyl-95 % methylpolysiloxane) with a silica guard column deactivated with phe-nylmethyl siloxane and a flame-ionization detector. The samples are dissolved in water and about 1 p is injected. Limits are set for benzene, chloroform, 1,4-dioxane, methylene chloride, and trichloroethylene. Methods V and VI are nearly identical to method I except for varying the chromatographic conditions. For the measurement of methylene chloride in coated tablets, the headspace techniques described above are recommended. 

For the assay of piperine in pepper, a 25 m by 0.5 mm I.D. glass capillary deactivated by high temperature silanization and coated with 0V-1 was used. The samples were injected by the on-column injection technique at 100°C, as described by Grob and Grob Jr.. The standard deviation of the whole procedure, sample preparation and chromatographic analysis, was 2.5 %. 

The on-column injection techniques, in which the solvent is generally vaporized in a few meters of uncoated deactivated capillary (retention gap) and vented via an early vapor exit valve, are of good accuracy and reproducibility.A schematic diagram for on-column injection is shown in Fig. 1. The on-column large-volume injection... 

The derivatization is commonly done for medium and low volatility compounds. The volatile compounds are more frequently analyzed by on-line techniques without derivatization, although on-line derivatization is occasionally utilized (such procedures were discussed in Section 2.7). When the off-line derivatization is done for the pyrolysate generated in a furnace, no particular precautions are recommended. For Curie-point or filament type pyrolysers, the pyrolysate can be collected in a deactivated piece of capillary column connected to the pyrolyser and cooled either in an ice bath or at lower temperatures needed for capturing more volatile compounds. 

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