Sample inlets split injection

One method that prevents overloading of narrow-bore capillary columns is split injection. The inlets are usually bimodal split/splitless inlets, and either mode can be selected for a given analytical method. In the split mode, the sample is rapidly vaporized in the inlet and a portion is introduced into the column in a narrow band with carrier gas, while the rest of the sample is vented (Dybowski and Kaiser, 2002). The amount introduced can vary for each method and is chosen as a ratio, e.g., 1 100. Easily vaporized compounds may preferentially vent, leading to the introduction of a nonrepresentative sample to the column (Watson, 1999). When the splitless mode is chosen, the entire sample is introduced into the column and the vent is opened after a predetermined period of time, to flush the excess solvent from the injector (Dybowski and Kaiser, 2002). 

In particular, it has been shown that both the nature of the solvent and the column inlet temperature during the injection is of primary importance in split injection. By influencing the true splitting ratio, they give rise to erroneous results when working with external standards which are not dissolved in the same solvent as the sample or when the column temperature is not precisely reproduced [11]. 

The attractive features of splitless injection techniques are that they allow the analysis of dilute samples without preconcentration (trace analysis) and the analysis of dirty samples, since the injector is easily dismantled for cleaning. Success with individual samples, however, depends on the selection of experimental variables of which the most important sample size, sample solvent, syringe position, sampling time, initial column temperature, injection temperature and carrier gas flow rate, often must be optimized by trial and error. These conditions, once established, are not necessarily transferable to another splitless injector of a different design. Also, the absolute accuracy of retention times in splitless injection is generally less than that found for split injection. For splitless injection the reproducibility of retention times depends not only on chromatographic interactions but also on the reproducibility of the sampling period and the evaporation time of the solvent in the column inlet, if solvent effects (section 3.5.6.2) are employed. The choice of solvent, volume injected and the constancy of thermal zones will all influence retention time precision beyond those for split injection. For quantitative analysis the precision of repeated sample injections is normally acceptable but the method is subject to numerous systematic errors that may... 

For large volume splitless injection the sample is introduced at a temperature below or close to the pressure corrected solvent boiling point with the split vent closed. Solvent vapors are discharged through the separation column. Compared with the split injection configuration, volatile compounds are trapped in the solvent swollen stationary phase at the column inlet rather than lost through the split vent. Since the flow rate of gas through the vaporization chamber is the same as the carrier gas flow rate, solvent elimination is slow and this method is not widely used. The maximum volume of sample that can be introduced is about 20 - 30 p.1. 

The method of sample introduction into the column is important for preserving the chromatographic efficiency [10]. In packed-column SFC, an HPLC injection valve capable of withstanding high pressures is employed with a 1-, 5- or 10-/xl sample loop. In the case of capillary SFC, a submicrolitre ( O.l-jul) injection valve is used either for time-split injections or together with an inlet splitter for reduction in the sample size. This is necessary because of the low sample capacity of 50 fim I.D. capillary columns, commonly used for... 

Figure 6. Selective detection of tetrachloromethane (TCM) contaminations by using SIM mode (measuring conditions Bruker Dallonik GC/MS EM 640S with direct inlet split I 10 direct injection of 0.2 mL gas samples, 30 m HP 5 ms. d, = i)25 mm.. /ih = 0 75 inin). The upper part of the figure incorporates the 3D visualisation of TCM distribution of soil gas (sampling depth 1.80 m combination of IMS and GC-MS data) of part of the contaminated area in the neighborhood of the housing estate...

Different sample introduction methods can be used in GC. If the sample is a liquid or a solid dissolved in an appropriate solvent, it may be introduced by a syringe into the injector. The choice of injection system depends on the column type and the sample composition. In packed columns, the sample is injected directly into the column inlet (Figure 2.3). In smaller inner diameter columns, split injection, splitless injection, and on-column injection techniques are used for liquid samples. 

The purpose of this injection technique is to introduce the entire injected sample into the column and use it for trace determination. Different techniques can be used, but the most common is the solvent effect technique, which uses the same instrumentation as used for spht injection (Figure 2.4). In splitless injection, the sample is introduced into the heated liner as in split injection and brought into the gas phase. Contrary to the spht injection, the splitter outlet valve is now dosed. Hence, the total sample volume (1-2 ml of gas) is transferred to the column. When splitiess injection is carried out, the column inlet temperature is kept at a temperature that is 20-50 °C lower than the solvent Bp. Hence, when the sample arrives at the column inlet, the solvent condenses as a thick film on the column wall. This film will act as a plug of stationary phase into which the sample components will be dissolved. Following the sample transfer to the column, which will take 2 min when 2 pi is injected and the carrier gas flow rate is 1 ml min , the column oven temperature is increased. The solvent evaporates first from the column entrance and thereafter the analytes, which will subsequently be separated in the column. The sphtter valve is opened when the whole sample has been transferred to the column in order to wipe out remains of the sample before the next injection. This injection technique is used for trace determinations and can only be carried out in combination with temperature programming. 

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