An overloaded chromatogram 5 10 15 20 time min

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An Overloaded Chromatogram 0 5 10 15 20 Time (min) Detector Response Overloaded Peaks
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The following figure shows a little better chromatography with fewer overloaded peaks. The second eluting peak (about 6 minutes) is clearly not overloaded while the group between 10 and 14 minutes still shows overloading characteristics: long drawn-out tailing and much less than baseline separation with peaks that elute nearby (the 11 and 12 minute peaks, for instance). A "not so" Overloaded Chromatogram 0 5 10 15 20 Time (min) Detector Response Normal Packed Column Injector The normal sequence of events in a GC injection is as follows. We will assume in this explanation that some analytes are dissolved in a (liquid) solvent although much of this process also holds for gas GC injections too: A small amount of liquid (microliters) is injected through a silicon rubber septum (using a special microliter syringe) into the hot (usually 200 o C) GC injector that is lined with an inert glass tube. The injector is kept hot by a relatively large, metal heater block that is thermostatically controlled. The sample is immediately vaporized and a pressurized, inert, carrier gas-which is continually flowing from a gas regulator through the injector and into the GC column-sweeps the gaseous sample, solvent, analyte and all, onto the column. In the packed column injector, ALL the vaporized sample enters onto the column. This is how all packed column injectors work; everything that is injected goes onto the column. One modification of this is a small ancillary flow of carrier gas that bathes the underside of the injector's septum so that hot vaporized sample gases can't interact and possibly stick to the septum. This improves peak shape and reproducibility. This last feature is called the septum
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purge. The following figure is a schematic of a packed column injector, sometimes called a direct or flash injector. The septum purge is not shown here although the carrier gas regulator and
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  • Fall '06
  • CHASTEEN
  • Chromatography, pH, Column, packed column, Overloaded Peaks, packed column injector

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