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Direct injection

  • Page ID
    61165
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    Direct injection is the oldest injection mode in gas chromatography. It is a so-called evaporation injection. The sample (liquid) is introduced into a hot evaporation chamber where it vaporizes explosively. After evaporation, the sample is transferred to the column by the carrier gas.

    Direct injection is a relatively simple and qualitatively and quantitatively reliable injection mode, which is exclusively used in combination with packed columns or wide-bore capillary columns.

    Direct Injector for Packed Columns

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    Direct injection is not suitable for other capillary columns due to their small sample capacity, at least not as long as the sample is a liquid introduced by means of a syringe. In combination with a gas valve (analysis of gases) it is possible to apply direct injection to capillary columns.

    Advantages Direct Injection

    • Simplicity
    • Detection of trace components is maximized because the entire sample is admitted to the column.

    If direct injection is used with programmed temperature operation, then the volume capacity can be increased by one or two orders of magnitude due to the focusing effect of programming, thereby allowing larger injection volumes. This results in improved detection limits.

    Because the standard split/splitless injector is much more flexible, this type of injector is presently usually preferred over the direct injector in combination with capillary columns.

    Direct vs Split Injection

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    Direct injectors of the type normally used with packed columns can usually be adapted for use with wide bore open tubular columns simply by an appropriate choice of glass liner and column fitting. In this case it is important that the liner should have a compatible volume to that of the volume capacity of the column and that stagnant flow regions should be avoided.

    Positioning of 'wide bore' column in direct injector

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    The length of the column installed inside a direct injector should be as small as possible (a few millimeters). A larger insertion length results in extra dead volume and loss of efficiency.

    For wide bore columns there are special liners available. In these fused silica liners, in which there is a neck with a passage opening somewhat smaller than the external diameter of the column, the dead volume is significantly smaller if the column is inserted at the bottom of the narrow passage.

    Trace analysis on wide-bore with direct injection

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    Direct injection is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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