Sample application splitless

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. 

Numerous types of GC injectors have been manufactured over the past four decades. The most commonly used injection techniques have been reviewed and described by Grob, who correctly states that analysts must fully understand the techniques before they can make the most appropriate choice for their particular application(s). For most GC capillary column applications, the split/splitless, programmed-temperature vaporization (PTV) and on-column injectors remain the most popular. However, over the last few years, technology has progressed rapidly to provide injectors that allow more of the sample extract on to the GC column without overloading it. 

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. 

Researchers should therefore study the performance of the derivatization reactions used with their samples to avoid the possibility that biases are introduced due to the derivatization reactions (QC samples of the type employed in LC-MS can provide much useful data in this respect) (44). Another issue is the need for extensive column cleanup. In most cases, injection occurs in splitless mode. Introduction of excess derivatization reagents may damage the injection liner and the analytical column. Frequent replacement of the liner is thus necessary. Cleanup of the column can be done by back-flush (when this feature is available) or by application of high temperatures (bake-out). 

Thermal desorption from the SPE cartridge is a further possibility [77,122]. In this approach, the sample is introduced at a controlled speed into the packed liner of a PTV injector set to a low temperature with the water eliminated via the split vent. Salts and involatile polar material are rinsed from the sorbent with water and the sorbent dried by purging with a high carrier gas flow rate. The trapped analytes are subsequently desorbed in the splitless mode by rapidly heating the PTV to the injection temperature. The most commonly used sorbents are Tenax and Carbofrit. The method is restricted to a narrow range of applications by the low breakthrough volume of polar analytes on the... 

Today, micro-LC is a valuable analytical tool for sample-constrained applications such as proteomics and bioanalysis. The microliter flow rates are ideally suited for direct, splitless coupling with electrospray ionization mass spectrometry (ESI-MS). Many benefits can be demonstrated from well-established HPLC theory, which allows for direct method transfer to micro-LC. 

FIGURE 9.7 First application of splitless injection for the analysis of steroids (a) synthetic sample (b) mixture of natural and synthetic samples (c) Natural sample (reprinted with permission from Reference 9, which provides experimental details). 

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