inject - Gas Chromatographic Injectors Introduction The...

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Introduction The great analytical strength of capillary gas chromatography lies in it high resolution. Capillary columns have 1) more theoretical plates (a measure of column resolving power or efficiency) per meter as compared to packed columns and 2) since they have less resistance to flow they can be longer than packed columns. This means that the average capillary column (30 meters long) has approximately 100,000 theoretical plates while the average packed column (3 meters) has only 2500 plates. But with this separation power comes some limitations: 1) Capillary columns, because they have smaller diameters (0.05 to 0.53 mm) than packed columns (2 to 4 mm), require relatively specialized injectors and ancillary flow and pressure controllers and 2) capillary column require a smaller amount of sample than packed columns. While the average sample mass of each component in a mixture that is separable by packed column GC can be in the microgram range (10 -6 grams) per injection, capillary columns routinely only handle 50 nanograms (10 -9 grams) of a particular component or less. Overloaded Chromatography This sample size requirement initially meant that if samples contained components that were too concentrated for a capillary chromatographic analysis, the sample had to be diluted before it was analyzed. Otherwise the column would be overloaded by those high concentrated components. An example of this appears in the first figure below. The clearly overload peaks are indicated. And while some of the other components are in the resolvable (not overloaded) range, having large masses of components can also distort the peak shape of some of the lower mass components. An Overloaded Chromatogram
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inject - Gas Chromatographic Injectors Introduction The...

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