Sample injection port liners

Programmed-temperature vaporizers are flexible sample-introduction devices offering a variety of modes of operation such as spHt/sphtless, cool-sample introduction, and solvent elimination. Usually the sample is introduced onto a cool injection port liner so that no sample discrimination occurs as in hot injections. After injection, the temperature is increased to vaporize the sample. 

The purge activation time (or the sample transfer time) depends on the sample solvent and carrier gas flow relative to the volume of the injection port liner and the boiling points of the sample components. For most applications, a purge activation time of 50-120 sec is better than 25-50 sec. Early activation results in the loss of sample, while late activation results in peak tailing. A more accurate method of determining purge activation time is to divide the volume of the injector liner by the flow rate (F) of the carrier gas and multiply this value by 1.5 or 2.0. (Do not use a packed liner.)... 

Ranid instrumental techniques were used to elucidate off-flavor problems in raw and processed rice products, raw and roasted peanuts, and corn-soy food blends. Less than a gram of the solid material was secured in a standard or special injection port liner of the gas chromatograph. Then, the volatiles from the sample were steam distilled in situ and identified by combined gas chromatography/mass spectrometry. 

The heat from the injection port liner combined with the GC column flow causes the volatiles contained in the sample to be thermally desorbed directly onto the GC column. This reduces or eliminates interfering components of the sample matrix. An example of this additive specific extraction is shown in Figure 2-2. This technique also can be used to obtain the purity and identity of neat additive standards which are not readily soluble. By altering the injection port temperature, an analyst can extract various types of additives without thermally degrading the sample matrix. There are no limitations on the additive/polymer combinations which can be analyzed. Another advantage of this technique is that it requires only a few milligrams of sample typically 2-5 milligrams per analysis. 

The additive standards for the MS library were dissolved in a suitable solvent and a sample was injected into the GC/MS system, or a sample was transferred into an injection port liner as is done for real world samples. Additive standards were mn at temperature program rates of 10 °C or 15 °C due to the lack of the necessity for multi-component separation. The actual temperature program used is listed on each Total Ion Chromatogram (TIC). 

The injection port liner is used to minimize dead volume when a sample is injected. Several are shown in Figure 20-27. Those shown are 0.8 mm i.d. with an internal volume of 12 pL. The solvent expansion type prevents flashback when injecting large volumes of dilute samples. 

Long-term reproducibility is affected by eveutual deterioration of resistive filaments or sample wires. All components exposed to sample during pyrolysis (GC injection port liners and quartz sample tubes if used) often require acid cleaning, solvent washing, and oven drying. Active pyrolyzer elements (coils and ribbons in filament pyrolyzers) can be heated without sample to ranove contamination (lOOO C for 2 sec is usually adequate). Curie-poiut wires are inexpensive enough to be discarded after use. 

Thermal Desorption (direct) Direct thermal desorption from sample located within the injection port liner (see discussion below). (15)... 

New accessories are continually made available. This unit discusses the basics necessary to conduct SPME analysis for flavor analysis. An automated sampling and injection system is available from Varian. Supelco offers a manual sampling stand setup. Injection liners are available that reduce the injection port volume to presumably produce sharper peaks. Predrilled septa for the GC are available to reduce septum coring. 

Injection ports for OT columns are usually much more complex because OT columns cannot accept very large samples. By using different liners inside the body of the port and different conditions, a variety of modes of sample introduction can be accommodated with one design. A typical one is shown in Figure 8.3. It is set up for split injection the sample is deposited in the region designated S, where it is volatilized. 

The optimum operating conditions for GC analysis must be determined prior to the GC-MS analysis. In general, this is accomplished by injecting the sample solution several times while varying such parameters as carrier gas, splitting ratio of carrier gas at the injection port fitted with a split liner, carrier gas flow rate, and oven temperature profile. In addition, the type of capillary column used and the concentration and volume of the sample to be injected also have to be optimized. 

Specially designed injection port inlet sleeves have been available on the market for direct and hot on-column injection.In the direct injection mode, 2 -mm-lD inlet sleeves are commonly used, which permit a sufficient space for sample evaporation however, the on-column mode is usually performed by inserting a 26-gauge needle inside a 0.53-mm-lD column. Direct injection is more favorable because it is less problematic than the hot on-column mode. Because the liner can trap nonvolatile residues before entering the column, this technique is suitable for dirty samples. Compared to the splitless mode, the direct injection is advantageous, involving less adsorption of the solutes and better sensitivity. However, with this technique, the adsorption of the sample may occur on the inlet sleeve during the evaporation process.In this case, the hot on-coIumn mode offers more benefits. 

Various precolumn concentration procedures used in conjunction with thermal focusing are desirable for biochemical applications. Novotny and Farlow [65] developed a simple technique, where an off-line injection of a relatively large dilute sample onto a small precolumn results in effective concentration. In this procedure, a small volume of deactivated solid support is packed into the glass liner of an injection system. After the volatile solvent is removed, the liner is quickly introduced into the injection port, and the desorbed sample is trapped for several minutes in the cool column. Chromatograms obtained through the following temperature program-... 

Brown et al (20) examined volatiles from roasted peanuts by Introducing glass liners containing ground samples of rocisted nuts into the injection port of a gas chromatograph where volatiles were vaporized in situ. 

On-column injection is another admission mode, which prevents component discrimination due to volatility or molecular weight. The sample is applied directly into the column head. This might be seen as an ideal injection, but with a large number of samples there is the risk of severe column contamination. This is caused by non-volatile sample components which deposit inside the column and eventually clog it. Split/splitless injection ports include a liner just before the column, to act as a filter for these interfering substances. In the case of on-column injection, a pre-column, or retention gap, which is a short capillary with no timer coating, is used for this purpose. 

Split injection is the oldest, simplest, and easiest injection technique to use. The procedure involves injecting 1 /xL of the sample by a standard syringe into a heated injection port that contains a deactivated glass liner. The sample is rapidly vaporized, and only a fraction, usually 1-2%, of the vapor enters the column (see Fig. 6.8). The rest of the vaporized sample and a large flow of carrier gas passes out through a split or purge valve. 

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