Programmed solvent split injection

The methods for transferring LC fractions to GC were developed hand in hand with large-volume GC injection. They mostly involved on-column techniques, since these show best performance and follow rather simple rules. The main drawback, the sensitivity to nonevaporating by-products, is not important, since efficient preseparation by LC is also efficient in removing the contaminants . The principal alternative, programmed temperature vaporizing (PTV) injection in solvent split mode, has rarely been applied. 

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. 

The transfer can only be partial for concentrated extracts (split mode), or a total transfer of the sample into the column for trace analysis is performed (splitless mode). Both injection methods require a different parameter setting, choice of inlet liners and oven program start temperature to achieve the optimum performance. Also the possible injection volumes need to be considered. The operating procedures of split injection and total sample transfer (splitless) differ according to whether there is partial or complete transfer of the solvent/sample on to the column. 

Split injection is used for volatile to semi-volatile compounds, and is one of the easiest injection techniques. With this technique, the flow of carrier gas is split between the capillary column and the atmosphere. This split does not occur in case of splitless injection. Splitless injection is used in case residual solvents remain in the sample at low eon-centrations, due to increased sensitivity compared to split injection. Other possibilities of direct injection are on-column injection and Programmed Temperature Vaporisation (PTV) injection. Both techniques allow detection limits to achieve put levels but are large volxune injection techniques. In on-column injection systems, the sample is injected on a pre-colmnn and then the solvent is vented, leaving only the analytes to be injected on the coltunn. In PTV, after the sample injection, the solvent is evaporated at a low temperature in a packed chamber and then removed. This leaves the solutes on the packing. When the injection port is heated the analytes are transferred to the analytical column. ... 

Solvent venting, without splitting Liquid sample from syringe into cold inlet. Solvent is vented at low temperature, condensing nonvolatiles. Heat programming subsequently vaporises the residues, which enter column as in splitless injection Dilute samples thermally labile Broad, some focusing required 1-1000 80-95... 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

Because of the variety of columns and samples that can be analyzed by GC, several injection techniques have been developed. The packed inlet system is designed mainly for packed and wide-bore columns. However, an adapter can be used to enable capillary columns to be used. When injection is carried out in the on-column mode, glass wool can be used for packing the injector. For capillary GC, spht technique is most common, which is used for high concentration samples. This technique allows injection of samples virtually independent of the selection of solvent, at any column temperature, with httle risk of band broadening or disturbing solvent effects. The sphtless technique, on the other hand, is used for trace level analysis. The so-called cold injection techniques (on-column, temperature programed vaporization, cooled needle split) have also been recently developed. " " ... 

---