Split vent

After the purge activation time (which can be optimized), the split mode is turned on and the solvent vapors are purged out of the split vent. If possible, choose a sample solvent that has a boiling point that is at least 20° below the boiling point of the first sample component (if known). 

Split Simple Starting point for method development Use with isothermal and temperature programmed GC Fast very sharp peaks Choice of glass sleeve not trivial Limits detection concentration to ppm Most sample wasted through split vent Loss of low-volatility, labile analytes... 

Split vent. The sample vapors that do not enter the column are ejected through the split vent. A needle valve on this line regulates the total flow of carrier gas into and from the inlet, generating the split ratio, which determines the portion of sample that enters the column. The split ratio is the ratio of the split vent flow to the column flow and provides a measure of the amount of sample that actually enters the column from the injection. A split ratio of 100 1 indicates that a lpl injection from the syringe results in approximately 10 ml of liquid sample reaching the column. 

Pressure regulator. The split inlet is back pressure regulated to ensure a constant head pressure, therefore, a steady flow through the column. For capillary columns, the inlet pressure determines the column flow, as per Eq. (14.11). As the inlet operates, the split vent can be opened or closed or upstream gas flow may change the regulator maintains the desired pressure, therefore the desired column flow. 

Direct injection is the most commonly used technique for sample introduction in GC, typically using combined spht/splitless injectors. In split mode, a portion of the sample passes onto the column and the rest is directed to waste. After sufficient split time to completely flush the injector the split vent may be closed to save gas, although this is optional. The injector is set to a sufficiently high temperature to eliminate discrimination between analytes. The sensitivity of the technique is inversely proportional to the split ratio. 

The simplest method of sampling is to put the sample into a sealed vial and heat it as shown in Figure 11.23. The sample, either in solution or slurried with a relatively involatile solvent with little potential for interference, e.g. water, is put into a sealed vial fitted with a rubber septum and heated and agitated until equilibrium is achieved. Then a fixed volume of head space, e.g. 1 ml is withdrawn. The sample is then injected into a GC in the usual way. If capillary column GC is used a split injection has to be used to facilitate sample injection a flow of 10 1 out of the split vent would ensure that a 1 ml sample could be injected in about 5 s with the flow through the column being 1 ml/min. Several points are important to note ... 

Figure 24-16 shows effects of operating parameters in split and splitless injections. Experiment A is a standard split injection with brisk flow through the split vent in Figure 24-15. The column was kept at 75"C. The injection liner was purged rapidly by carrier gas, and peaks are quite sharp. Experiment B shows the same sample injected in the same way, except the split vent was closed. Then the injection liner was purged slowly, and sample was applied to the column over a long time. Peaks are broad, and they tail badly because fresh carrier gas continuously mixes with vapor in the injector, making it more and more dilute but never completely flushing the sample from the injector. Peak areas in B are much greater than those in A because the entire sample reaches the column in B, whereas only a small fraction of sample reaches the column in A.

Experiment C is the same as B. but the split vent was opened after 30 s to rapidly purge all vapors from the injection liner. The bands in chromatogram C would be similar to those in B, but the bands are truncated after 30 s. Experiment D was the same as C, except that the column was initially cooled to 25 °C to trap solvent and solutes at the beginning of the column. This is the correct condition for splitless injection. Solute peaks are sharp because the solutes were applied to the column in a narrow band of trapped solvent. Detector response in D is different from A-C. Actual peak areas in D are greater than those in A because most of the sample is applied to the column in D. but only a small fraction is applied in A. To make experiment D a proper splitless injection, the sample would need to be much more dilute. 

SIZE coating TYPE HEAD PSI (ml/min) FLOW TYPE TOTAL FLOW SPLIT VENT PURGE VENT ... 

Splitless injection involves keeping the injector split vent closed during the time the sample is deposited on the column, after which the vent is reopened and the inlet purged with carrier gas. In splitless injection, the inlet temperature is elevated with respect to the column temperature. The sample is focused at the head of the column with the aid of the solvent effect. The solvent effect is the vaporization of sample and solvent matrix in the injection port, followed by trapping of the analyte in the condensing solvent at the head of the column. This trapping of the analyte serves to refocus the sample bandwidth and is only achieved after proper selection of the solvent, column and injector temperatures. Splitless injection techniques have been reviewed in References 29 and 30. 

Analysing compounds by GG iri-the split mode - make sure no hazardous materials enter the laboratory atmosphere through the split vent. A charcoal split-vert-trapjna y be required >-to"eliniinate pftential hazards ".--... 

In this separation, a 10-mL sample (large volume) containing a solution in tert-butyl methyl ether (tBME) of the pyrolysate of 1 mg cellulose obtained at 600° C was injected (off-line pyrolysis). The PTV injector was programmed at 20° C initial temperature for 2 min. and ramped with 10° C/min at 250° C and kept at this temperature for 1 min. Then the injector was further heated at 300° C. The split vent purge time was 2.5 min. The oven temperature for the first dimension separation was kept at 35° C for 2.5 min. then heated with 30° C/min. at 55° C and further heated with 3° C/min. to 240° C. The detector used in the first dimension was an MS system, which allowed the identification of a series of compounds from this chromatogram. The peak identification is given in Table 5.2.2. 

Split-splitless, which is performed with a split assembly and a split vent shutoff valve. This method enables larger volumes to be admitted onto the columns and the split activates to reduce tailing by sweeping residual amounts of material out of the system. 

Product analysis of hydrogenation product was done using a Hewlett Packard 5890 gas chromatograph GC), equipped with a DB 1701 (5% crosslinked phenyl-methyl-silicone) megabore column (30 m long, 0.33 ID, 0.25 um film thickness) and a flame ionization detector. The temperature program was 60 °C for 2 min + 8 °/min to 230 °C for 15 min. The column flow rate was 1.5 cc/min helium and split vent flow rate of 60 cc/min helium. The injector and detector temperatures were 250 °C and 265 °C, respectively. Product and by-product identification was done by GC/MS analysis using chemical ionization techniques. 

Split injection is used for concentrated samples where only a small portion of the sample volume (<2 %) is introduced into the column. This is achieved by sending most of the sample to waste at the split vent. Injector temperature is high and the residence time of the sample in the injection port is short. It is used when resolution is most important and when sensitivity is not an issue. 

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