Splitless inlets

Hewlett-Packard Model 6890 gas chromatograph with capillary split/splitless inlet with HP5973 mass-selective detector equipped with an autosampler Shimadzu GC17A gas chromatograph with capillary split/splitless inlet with flame thermionic detector equipped with an AOC-17 autoinjector... 

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. 

Fig. 14.5. Schematic of splitless inlet with purge valve closed showing the major components. Note the position of the purge valve in comparison to Fig. 14.4. Reprinted with permission from the 6890 Gas Chromatograph Operating Manual. Copyright (2004), Agilent Technologies.

One method that prevents overloading of narrow-bore capillary columns is split injection. The inlets are usually bimodal split/splitless inlets, and either mode can be selected for a given analytical method. In the split mode, the sample is rapidly vaporized in the inlet and a portion is introduced into the column in a narrow band with carrier gas, while the rest of the sample is vented (Dybowski and Kaiser, 2002). The amount introduced can vary for each method and is chosen as a ratio, e.g., 1 100. Easily vaporized compounds may preferentially vent, leading to the introduction of a nonrepresentative sample to the column (Watson, 1999). When the splitless mode is chosen, the entire sample is introduced into the column and the vent is opened after a predetermined period of time, to flush the excess solvent from the injector (Dybowski and Kaiser, 2002). 

Septum bleed is worse than in split mode because all the flow goes through the column. Most of the splitless inlet systems that are presently available use a vented septum technique to avoid this problem. 

For semi volatile compounds, inlet optimization is very simple. Classical splitless inlet conditions, followed by an initial column temperature cool enough to refocus the analyte peaks following the desorption, work well. Thus, a typical condition would be a temperature of about 250° C, a head pressure sufficient to maintain optimum GC column flow and an initial column temperature at least 100°C below the normal boiling point of the analyte. For semivolatile analytes, a classical splitless inlet liner can be used, as the cool column will refocus these peaks. The desorption time in the inlet must be determined by experimentation, but typically, runs between 1 and 5 minutes. 

Phthalates can be present in the chromatographic system. The most important contamination is located in the inlet and gas supply system. Split or splitless inlets may contain septa, liners and o-rings that are contaminated with phthalates. Another critical factor is the quality of caps for autosampler vials. These caps can also contain phthalates. 

As the QuEChERS-extracts are solved in acetonitrile, they are directly amenable to GC- and LC-applications. However, since acetonitrile is rather difficult to handle by GC using split/splitless inlets, the use of a PTV with solvent vent possibility is highly recommended. Should a PTV not be available and the desired pesticide detection limits cannot be achieved using the split/splitless technique, extract concentration followed by a solvent exchange, if necessary, may be considered. If GC-MSD is employed, a simple evaporative concentration of the extracts by a factor of four should be sufficient. To achieve this, e.g., a 4-mL extract (acidified to pH 5) is transferred into a test tube and reduced to ca. 1 mL at 40 °C using... 

Injector Split/splitless inlet with an electronic pressure control, split ratio range 1 20-1 50... 

Glass Liners Ferrules and Fittings Setting the Inlet Temperature Setting the Flows and Split Ratio Discrimination and Linearity of Splitting SPLITLESS INLET... 

The splitless inlet, which employs the same instrumentation as the split inlet, provides a means for improving sensitivity by transferring nearly the entire injected... 

FIGURE 9.8 Diagrams of splitless inlet with (a) purge off and (b) purge on. (Reprinted with permission from 6890 Gas Chromatograph User s Manual, Agilent Technologies,... 

The splitless inlet is most commonly used for trace analysis however, it has several deficiencies as it is a hot, vaporizing device. In the late 1970s, the... 

If the inlet temperature is maintained hot and constant throughout the analysis, the PTV inlet operates in exactly the same manner as a classical split or splitless inlet. These operations are described elsewhere in this chapter. 

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