Sample inlets packed columns

Packed-column instruments offer the simplest sampling devices, as the fundamental problem of the syringe needle diameter is not an issue all common gas chromatographic syringes will fit inside a packed column. This inlet only requires that the samples be introduced without generating leaks, that it be heated to vaporize the sample and that the carrier gas be able to fiow into the column. 

The main advantage of the packed column inlet is that the entire sample that exits the syringe enters the column, making packed column injection highly reproducible. The pneumatics are also very simple and inexpensive. Method development is also very straightforward with only the inlet temperature as an easily adjustable variable. Further, packed columns and inlets typically operate at lower temperatures than capillary inlets, allowing the use of less expensive septa. 

Inlets for syringe sampling are divided kito two main categories one for packed-column and the other for capiHary-column devices. Eor packed columns, all material kijected is carried by the mobile phase onto the column. The inlet is usually an open tube, but sometimes, albeit rarely, the inlet itself may be packed, eg, to assure that the first centimeters of the column do not become contaminated with degradation products or nonvolatile materials that may affect the efficacy of the column. 

Direct Sample passes directly from syringe to hot inlet where it vaporises. Packed column inlets Dilute samples Thermally stable Fair, some focusing required 0.1-2 100... 

For capillary GC, the split/splitless inlet is by far the most common and provides an excellent injection device for most routine applications. For specialized applications, there are several additional inlets available. These include programmed temperature vaporization (PTV) cool on-column and, for packed columns, direct injection. PTV is essentially a split/splitless inlet that has low thermal mass and a heater allowing rapid heating and cooling. Cool injection, which can be performed in both split and splitless mode with the PTV inlet, reduces the possibility of sample degradation in the inlet. Capabilities of the commonly available inlets are summarized in Table 14.3. 

In direct injection, all injected material is carried onto the column by the mobile phase. This eliminates the possibility of sample discrimination in the inlet, but can overload capillary columns and so is more commonly used with packed columns and wide-bore capillary columns. 

In GC we have a real choice between packed columns (dp = 100-200 pm 150-65 mesh) and open columns (dc= 50-500 pm). Capillary columns have the advantage of enhanced speed of analysis (eqn.7.6). In order to exploit this advantage, narrow-bore capillaries (dp< 100 pm) should ideally be used. However, such columns require relatively high inlet pressures (especially for high plate counts) and considerable experimental modifications and have a very low sample capacity [702],... 

The first of the separation techniques to be used in process measurement was gas chromatography (GC) in 1954. The GC has always been a robust instrument and this aided its transfer to the process environment. The differences between laboratory GC and process GC instruments are important. With process GC, the sample is transferred directly from the process stream to the instrument. Instead of an inlet septum, process GC has a valve, which is critical for repetitively and reproducibly transferring a precise volume of sample into the volatiliser and thence into the carrier gas. This valve is also used to intermittently introduce a reference sample for calibration purposes. Instead of one column and a temperature ramp, the set up involves many columns under isothermal conditions. The more usual column types are open tubular, as these are efficient and analysis is more rapid than with packed columns. A pre-column is often used to trap unwanted contaminants, e.g. water, and it is backflushed while the rest of the sample is sent on to the analysis column. The universal detector - thermal conductivity detector (TCD)-is most often used in process GC but also popular are the FID, PID, ECD, FPD and of course MS. Process GC is used extensively in the petroleum industry, in environmental analysis of air and water samples" and in the chemical industry with the incorporation of sample extraction or preparation on-line. It is also applied for on-line monitoring of volatile products during fermentation processes" ... 

Obviously, designs of capillary gas chromatographs must be more carefully executed than those of packed column instmments. The chief reasons for this are the very low flow-rates used and the overall small volumes of capillary columns. Under such circumstances, the units connecting the column to either the inlet or detector parts must virtually be absent of any dead volumes. Inlet systems with clean geometry are also required to introduce the sample as the narrowest possible band into the first column section. A constant dilemma of the manufacturers of modern instruments has been whether to design universal instruments or those usable just for certain column types. It seems now that the production of dedicated capillary instruments is becoming common. Alternatively, instruments can be provided with multiple inlet and detector capabilities. Numerous laboratories also successfully modified the earlier versions of instruments into capillary gas chromatographs. 

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