On-column injection
The on-column injection is a special injection technique that is used to introduce an analysis sample into a gas chromatographic system. The injector is not heated during the injection; the liquid sample is injected directly into the beginning of the column ( on-column ). This injection method is used when analyzing samples that are heavily diluted with low-boiling solvents . When using capillary columns, very sharp peaks are obtained.
Designs of the injector
The injector can either be constructed without a septum or provided with a septum made of elastic material. In the case of the septeless variant, the injection channel is usually opened by a slide for the injection and then closed again. The disadvantage of this design is the difficulty in achieving a complete seal and preventing the entry of air during the injection. Alternatively, a septum made of elastic material is possible, which closes automatically when the needle is pulled out. With this method, frequent piercing of the syringe needle can cause particles from the septum to enter the column and interfere with the analysis.
Dimensions of the cannulas and capillaries
In the application of on-column injection technique has the cannula of the syringe extending into the column, so capillary columns can not be used with very small inner diameters. The thinnest on-column needles have an outside diameter of 0.17 mm. For them you need a column with an inner diameter of at least 220 μm. However, needles of this diameter are not stable enough to pierce a septum in the injector head. In practice, septum-free injectors or syringes with an outer diameter of 0.47 mm and capillaries with an inner diameter of 0.53 mm are used. Occasionally, empty columns ( retention gap ) are also used for injection, which are connected to the actual separation column and have a larger internal diameter than this.
Working method
Ideally, the solvent in the sample should have a boiling point that is up to 100 ° C lower than that of the analyte. The sample solution must be very dilute, otherwise the column will be clearly overloaded ( overfloating ). Concentrations of 0.001 to 200 ppm per component are preferred. The sample solution is injected directly in liquid form onto the beginning of the column. Usually the temperature of the injector and the column is below the boiling point of the solvent in the sample. Depending on the type of solvent used, it may be necessary to cool the injector for this. The analytes can be concentrated at the beginning of the column by choosing a temperature during the injection that is only slightly below the boiling point of the solvent. This then already evaporates during the slow injection, the analytes remain at the beginning of the column. This solvent focusing ( cold trapping ) makes the peaks narrower and higher, so that the separation efficiency increases. The injector is then heated up using a temperature program in the column oven. The maximum oven temperature should remain below the boiling point of the sample components.
advantages
Since with the on-column injection there is no evaporation of the sample in the injector space with subsequent recondensation at the beginning of the column as with the split / splitless injection , all problems that arise from such recondensation are avoided. Above all, the discrimination against high-boiling components and the decomposition effects of unstable compounds at high temperatures (such as thermally induced rearrangements) should be mentioned. It is therefore possible to quantify analytes from a sample which differ greatly in their boiling range (e.g. short and long-chain hydrocarbons ).
See also
Individual evidence
- ↑ a b Bruno Kolb, Gaschromatographie in Bilder , 2nd edition, Viley VCH-Verlag, Weinheim, 2003, p. 159
- ↑ Wolfgang Gottwald: GC for users , VCH Verlagsgesellschaft , 1995, ISBN 3527286810
literature
- Dean Rood: Troubleshooting in capillary gas chromatography , Hüthig Buch Verlag, Heidelberg, 1991, ISBN 3-7785-2104-7