ASTM E260 Standard Practice for Packed Column Gas Chromatography
10. Sample Injection Procedures
10.1 Injection Technique - Useful analyses are obtained only by injecting representative samples into the gas chromatograph. Since chromatographic samples are small, the best practices and procedures must be followed.
10.1.1 If elution is to be otherwise, the sample injection must be almost instantaneous in order to introduce the sample as a plug. Avoid unnecessary sample dilution and inadvertent trapping.
10.2 Sample Size - Sample sizes used for analysis with 2- to 5-mm ID packed columns are in the range of 0.1 to 10 µL multi-component liquids. Gas samples usually range from 10 µL to 2 mL.
10.2.1 There is frequently a reduction in retention times with sample size when the column is overloaded. Therefore, when identifying by comparison with a known sample, the same amount of each component must be used in an amount that does not overload the column. The sample size that overloads the column is that size which decreases the efficiency (Section 11) by 10 % compared to a smaller sample size. Sample overload is sometimes shown by peaks with sloping fronts and backs that are almost perpendicular to the baseline. (Another reason for this peak shape is insufficient vaporization of the sample. Either the injection port or the column temperatures, or both, are too low.)
10.3 Sample Injection Devices - Samples may be introduced by syringes, automatic sample injectors using syringes, or sample valves. (There are also devices that introduce capsules containing sample into the injection port.) For rigorous quantitative work, any sample introduction device should be flushed and filled at least three times with the sample immediately before use.
10.3.1 The most common method for liquids is the use of a syringe injection technique through a self-sealing septum. In the usual 10-µL syringe, there is approximately 0.8 µL of dead volume in the syringe needle. This volume is additional to the volume in the syringe barrel and can be measured by withdrawing the entire sample volume into the syringe barrel.
10.3.1.1 First, pump the sample back and forth vigorously in the syringe to wet the plunger and to remove air bubbles. Then withdraw the sample back into the syringe so that the entire volume can be read on the volume marks of the syringe barrel. When preparing the syringe for injection, never leave the sample solution in the needle. This technique will minimize sample boiling out of the needle when it is inserted into the hot injection port.
10.3.1.2 In an alternative procedure, called the solvent-flush technique, load the syringe in the following order: solvent, air, sample solution, and air, with only air remaining inside the needle. When the sample is injected, the solvent is the last to leave the syringe, and it rinses out sample residue in the needle.
10.3.1.3 Wipe the syringe needle off before injection. Insert the needle carefully through the GC septum, inject the sample at once, and withdraw the needle in one continuous motion.
10.3.1.4 Often the tip of the needle becomes bent, forming a fish hook that will tear the septum on subsequent injections. This can be detected by brushing the end of the needle gently over the end of a finger. A few strokes on a sharpening stone will remove the fish hook.
10.3.1.5 Syringes should be cleaned with a solvent that will remove all traces of contamination. Consult the manufacturer's literature for further information. Many chromatographic supply vendors sell suitable cleaning solutions and kits. Liquid sample valves, in both automated and manual versions, are also available.
10.3.1.6 Gas samples are most conveniently injected using gas-tight syringes. These devices are quite satisfactory for survey work because the sample size can easily be verified. However, the syringe needle is more likely to clog with pieces of septum material when gas samples are injected than when liquids are injected. If no chromatographic peaks are observed, test the syringe by injecting air into a liquid. If no bubbles are seen, unclog the needle with a wire or by filling the syringe without the needle and forcing solvent through the needle. However, repeatable results in gas analysis will only be obtained using gas sample valves that have a fixed sample loop. The latter valves can easily be automated.
10.3.1.7 A sealed, friable, or puncturable ampule containing a weighed amount of sample may be placed in the injection chamber. The ampule is physically broken to release the sample, which is then swept into the gas chromatographic column by the carrier gas.
10.4 Sample Container - Care must be taken in the design and construction of sample containers so that none of the components of interest are in any way changed or removed from the sample by reaction, diffusion, or adsorption. Stopcock grease and material desorbed from plastic cap liners are frequent sample contaminants.
