Split-flow injector

The injector was programmed to return to the split mode after 2 min from the beginning of a run. Split flow was set at 50 mL/min. The injector temperature was held constant at 270 °C. Trap temperatures, manifold temperatures, and transfer line temperatures were 250, 50, and 280 °C, respectively. 

Chromatographic Conditions. GC-ECD analyses were performed in an HP 5890 series II GC equipped with an electron capture detector and a split/splitless injector, operated by an HP Chemstation software. PCBs were separated on a 25 m length X 0.32 mm i.d., HP-1 column coated with a 0.17 pm film. The GC oven temperature program was as follows 90 °C hold 2 min, rate 20 °C/min to 170 °C, hold for 7.5 min, rate 3 °C/min, to final temperature 280 °C, and hold for 5 min. Nj was employed as carrier and makeup gas, with a column flow of 1.2 mL/min at 90 °C. Split flow was set at 50 mL/min. 

Procedure (See Chromatography, Appendix IIA.) Use a gas chromatograph capable of split and splitless capillary column injection and equipped with a flame ionization detector and a 25-m x 0.53-mm (id) fused-silica capillary column coated with a 2.0-p.m film of 5% phenyl/95% methylsilicone liquid phase, or equivalent, and a 30-m x 0.32-mm (id) fused-silica capillary column, or equivalent, coated with 1.8-p.m film of (6% cyanopropylphenyl) methylpolysiloxane liquid phase, or equivalent, connected in series, with the first column that was described placed behind the second. Set the injector temperature to 150°, the detector to 250°, and the oven to 40° isothermal. Use helium as the carrier gas at a flow rate of 4.4 mL/min. Set the split flow at a rate of 98 mL/min. 

Fig. 2 An example chromatogram illustrating the determination of headspace oxygen by GC using a PLOT molecular sieve column with thermal conductivity detection. Chromatographic conditions were carrier gas helium (2mLmin ) oven temperature 26 C inlet 160 C, split mode, 10 1 split ratio, split flow of 20mLmin injector 160°C run time lOmin TCD detector 160 C. (From Ref. p. 41. Copyright 2002 Advanstar Communications Inc.)...

The split/splitless detector has been designed for use with open-tubular columns or solid-coated open-tubular (SCOT) columns. Due to the small dimensions of such columns, they have very limited sample load capacity and, thus, for their effective use, require sample sizes that are practically impossible to inject directly. The split injector allows a relatively large sample (a sample size that is practical to inject with modern injection syringes) to be volatilized, and by means of a split-flow arrangement, a proportion of the sample is passed to the column while the remainder is passed to waste. A diagram of a split/splitless injector is shown in Fig. 1. 

Figure 4. Response of the 30 1CD versus flow rate. Detector temperature 170°C, Injector temperature 180 0, column temperature 100°C, filament temperature 250 0, 170 Injector, split ratio 1 65> 12.5 m x 0.20 mm SE-S column,...

A 30 m x 0.25 mm id x 0.25 i.m film thickness HP-5ms capillary column was used, with temperature programming from an initial temperature held at 90°C for 2 min before commencing a 7°Cmin-1 rise to 285°C, with a final time of 20 min. The split/splitless injector was held at 280°C and operated in the splitless mode, with the split valve closed for 1 min following sample injection. The split flow was set at 40 ml min-1, and the mass spectrometer transfer line was maintained at 280°C. Electron impact ionization at 70 eV, with the electron multiplier voltage set at 1500 V, was used, while operating in the single-ion monitoring (SIM) mode. 

Splitless injection uses a similar injector design to split injection but is more suitable for quantitative analysis of trace components in dirty samples, such as biological and environmental extracts [41,49-59]. Conversion of a split to a splitless injector usually requires no more than the installation of a different liner and the interruption of the split flow at the start of the injection using a solenoid valve located in the vent line. The split flow is restarted only at the end of the sampling period. Since the flow of gas through the vaporization chamber is normally the same as the optimum carrier gas flow for the column the transport of sample vapors to the column is relatively slow. During the initial rapid evaporation of the sample there is minimal transfer of vapors into the... 

Figure 3.7. Large volume injection of 100 (il of a mixture of n-alkanes of wide volatility using the PTV injector in the cold split solvent elimination mode. The vaporizing chamber was thermostated at 0°C, split flow 250 ml/min with the split vent closed after 2.5 min. For splitless transfer the vaporizing chamber was heated at 4°C/s to 325°C with the puige flow started after 1.5 min. (From ref. [68] Wiley-VCH).

The increase in the pressure at the inlet of the column causes an increase in the flow rate in the column (Period A). When the pressure returns to its normal value, the flow rate in the column decreases. Sometimes the flow direction reverses momentarily if the pressure at the inlet of the column is higher than that in the injector. The flow rate through the split valve changes in an analogous manner but for a different time and to a different extent. This is important for the splitting of the sample when the cloud of sample vapor reaches the split point. 

The term linearity is used today to indicate that the split ratio is identical for all sample components. This is the basic precondition for ensuring that the small amount of sample material analyzed by the column has the same composition as the sample in the injector. However, it does not mean that the true split flow must be the same as the pre-set split flow. A complementary concept is the idea of discrimination, which is the opposite of linearity. Discrimination is not a very important effect in the headspace technique as the sample is already in vapor form, but it is considerably more significant in liquid injection [11]. 

Small diameter capillary columns require very small samples, often only a fraction of microgram in size and as this sample size is too small for practical injection syringes, a split-flow system must be used. In effect, the sample is vaporized into a gas stream and a fraction of the gas stream (and consequently a fraction of the sample) is allowed to pass through the column. A diagram of a split-flow injector is shown in figure 4.4. 

The split-flow injector is very similar to the packed column injector except that only part of the carrier flow passes to the column, the rest exits to waste. By varying the exit flow-impedance, the split-ratio can be adjusted over a wide range. Without this type of split injection system, the small bore capillary columns would be virtually impossible to use. However, because of the waste of sample and the relatively small mass range obtainable from small bore columns, the large bore capillary column was introduced. 

For normal split injection, the sample is introduced at a temperature below the solvent boiling point with the split exit open. Shortly after withdrawal of the syringe needle the injector is rapidly heated to a temperature high enough to ensure rapid evaporation of the least volatile sample components. Discrimination effects, which are common for hot vaporizing injectors, are virtually eliminated and the sample split ratio and split flow ratio are similar. 

---