Programmed-temperature sample introduction

Gas chromatographic analysis starts with introduction of the sample on the column, with or without sample preparation steps. The choice of inlet system will be dictated primarily by the characteristics of the sample after any preparation steps outside the inlet. Clearly, sample preparation has a profound influence on the choice of injection technique. For example, analysts may skip the solvent evaporation step after extraction by eliminating solvent in the inlet with splitless transfer into the column. Sample introduction techniques are essentially of two types conventional and programmed temperature sample introduction. Vogt et al. [89] first described the latter in 1979. Injection of samples, which... 

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. 

Traditionally, LC and GC are used as separate steps in the sample analysis sequence, with collection in between, and then followed by transfer. A major limitation of off-line LC-GC is that only a small aliquot of the LC fraction is injected into the GC p. (e.g. 1 - 2 p.1 from 1 ml). Therefore, increasing attention is now given to the on-line combination of LC and GC. This involves the transfer of large volumes of eluent into capillary GC. In order to achieve this, the so-called on-column interface (retention gap) or a programmed temperature vaporizor (PTV) in front of the GC column are used. Nearly all on-line LC-GC applications involve normal-phase (NP) LC, because the introduction of relatively large volumes of apolar, relatively volatile mobile phases into the GC unit is easier than for aqueous solvents. On-line LC-GC does not only increase the sensitivity but also saves time and improves precision. 

There are several types of sample introduction systems available for GC analysis. These include gas sampling valves, split and splitless injectors, on-column injection systems, programmed-temperature injectors, and concentrating devices. The sample introduction device used depends on the application. 

Introduction of the sample into a cool inlet characterizes programmed temperature vaporization. A temperature program is then utilized to vaporize the sample and introduce it into the column (Dybowski and Kaiser, 2002). 

Burtiset al, 1987). Current gas chromatographs have the capability of very complex temperature programs that can minimize the time per run and maximize the resolution of the compounds of interest. The minimum and maximum temperatures are usually the only nonvariable parameters. The solvent usually determines the minimum temperature because it must be volatized for sample introduction. The maximum temperature is determined by the stationary phase, because it may break down at high temperatures, which can result in an elevated baseline of the chromatograph. Thus, the manufacturer recommends a maximum temperature to maximize column life and minimize breakdown. 

J. Staniewski, H.G. Janssen, C. A. Cramers and J. A. Rijks, Programmed-temperature injector for large-volume sample introduction in capillary gas chromatography and for liquid chromatography-gas chromatography interfacing , J. Microcolumn Sep. 4 331-338(1993). 

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. 

When using electrothermal evaporation for sample introduction, the development of a suitable temperature program for the elements to be determined in a well-defined type of sample is of prime importance. In the case of liquid samples a small sample aliquot (10-50 pL) is brought into the electrothermal device with a syringe or with the aid of an automated sampler and several steps are performed. 

The low concentration levels of the compounds in environmental samples impose specific requirements in terms of sample injection for GC analysis. In addition to the common injection techniques of capillary GC (split, splitless, on-column, and programmed temperature vaporized (PTV) injection), some other sample introduction methods coupled to GC such as solid-phase microextraction (SPME), headspace, etc., have favored the versatility of GC and reduced the time required for sample preparation. These techniques have an advantage over the conventional injection methods, which is that a preconcentration step prior to GC... 

Figure 3.2 Automated headspace sampling systems. Descriptions of each system are provided in text. Reproduced from Schomburg, G., Temperature programmed sample transfer, in Sample Introduction in Capillary Gas Chromatography (ed. P. Sandra), pp. 55-76 published by Huethig...

Column temperature - A. With cryofocusing at the sample introduction and temperature program... 

The equipment for packed-column SFC is very similar to that for HPLC, whereas the instrumentation for capillary SFC is more reminiscent of gas chromatography. In Figure 43 a schematic diagram of packed-column SFC instrumentation is shown. It consists of a high-pressure pump, sample introduction system, oven, detector, restrictor, and recorder. In addition, a microcomputer is installed that controls temperature, flow, pressure, and density programming. 

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