This article will cover the general function of GC inlets and describe some common attributes, differences and provide a context for future articles covering inlets in more detail.
A GC inlet is the interface through which sample is introduced into the GC column. Sometimes I hear people call inlets “injectors”. I think the word “injector” is better suited to something that actively injects sample into the inlet, like an automatic liquid sampler (ALS), or the dude in the lab coat holding the 10 µL syringe.
With the many operational requirements placed on an inlet, it is to me amazing that they work as well as they do. They must provide a pressure seal for carrier gas, be inert, allow entry of syringe to deliver sample and be easy to maintain. The ideal inlet would have the following attributes:
- it would seal to gas pressures above 100 psig (690 kPa)
- it would be completely inert toward fragile (labile) sample components
- it would never get dirty or would not change performance when dirty
- it would work from cryogenic temperatures (e.g., -80 °C) to very high temperatures (e.g., 600 °C)
- it would heat and cool quickly, yet have uniform temperature profile
- it would always transfer a representative sample to the column (show no discrimination)
- it would accommodate injection of a wide range of sample volumes
- it would accommodate auxiliary sampling devices (e.g., headspace samplers) with no degradation of peak shape (no dead volumes or sample dilution)
- it would provide a means of reducing sample load to the column (split) when necessary
- it would be inexpensive
There is no such inlet. Some come close, but only if the right choices in use are made by the operator. Occasionally, one can just get lucky, but it is better to become aware of the various attributes of inlets, the related consumables (liners, liner packing, seals, etc.), proper use and the interplay between setpoints and injection parameters.
Table 1: Comparison of inlet attributes. Assumes injections are made with fast automatic liquid sampler. 1 = Temperature programmed. 2 = Only with solvent or stationary phase focusing. 3 = Low discrimination.
Most of the time, one chooses an inlet based on one or two dominant attributes that greatly depend on specific sample and analytical needs. Table 1 lists inlet types in terms of some of these important attributes. The comparisons are presented in the form of Low, Medium and High relative to each other. A “High” does not mean that the inlet is perfect in that attribute, just that it is better than the rest.
For temperature programmed (TP) split and splitless modes, injections are presumed to be done at lower temperatures, after which the inlet temperature is programmed up to transfer sample components to the column.
In Table 1, representative sampling refers to the ability of an inlet to transfer each sample component to the column in the same proportion that it exists in the injected sample. If there were ten components in a sample at equal concentrations, for example, an inlet that provides a representative sampling of the sample components would transfer equal amounts of each component into the column.
Figure 1: Discrimination curves showing low end and high end discrimination. The x axis is carbon number for n-alkanes, but could just as well be solute elution temperature during the temperature program, analyte boiling point or some other measure of volatility. Low end discrimination is probably the result of loss out the septum purge line (splitless injection). High end discrimination is worse with the lower temperature inlet because high-boiling analytes were not transferred quantitatively to the column before the splitless purge time was up and split vent turned on.
Discrimination is a more commonly-used term when speaking of inlet performance than is “representative sampling”. It is a measure of the deviation from ideal representative sampling one actually gets. Figure 1 illustrates a typical plot that is used to visualize inlet discrimination. A straight line at 100% recovery line (100% of the expected amount of each sample component is observed) would indicate perfect representative sampling. As can be seen in the figure, the low inlet temperature curve shows both low end and high end discrimination, whereas the high temperature inlet line shows primarily low end discrimination. Low end discrimination with splitless injection mode is often related to loss of volatiles out the inlet purge line (inlet overload). Losses at the high end in this case were the result of incomplete transfer of high-boilers to the column at lower inlet temperatures (transfer of analytes from the inlet is not instantaneous and is slower for higher-boiling analytes).
Figure 2 shows chromatograms relating to the discrimination data of Figure 1. The lower peak heights at high elution temperatures for the low inlet temperature chromatogram (inverted) are obvious. I will deal in more detail in the future with inlet discrimination — there are too many factors that influence inlet discrimination to cover here.
Figure 2: Chromatograms corresponding to the discrimination plots of Figure 1. The chromatogram for the lower temperature inlet is inverted.
Inertness in Table 1 refers to the compatibility of the inlet with fragile sample components. Fragile (labile) analytes can be encouraged to degrade by high temperatures, chemically active surfaces, or both. Many pharmaceuticals, pesticides and natural products are considered labile.
“Ruggedness for Dirty Samples” refers to the ability of an inlet to withstand high sample matrix without impacting subsequent analyses. No inlet is great in this regard, but some are better than others. Included in this attribute is a related aspect of “column protection” wherein non-volatile sample components are retained within the inlet instead of being transferred to the column where they might have greater influence on subsequent analyses.
Inlets that can accommodate a wide range of sample volumes are listed in Table 1 as having High “Sample Volume Flexibility” . This is important for general methods used for many different types of samples and/or with large differences in analyte concentrations. For these methods, the ability to change injection volumes and/or split ratios without causing problems in chromatography or precision is important.
Compatibility with Fast GC is on the table only because many people I talk to express interest in maximizing the speed of GC analysis. The faster the analysis, the narrower the initial peak width needs to be to resolve early eluting components. Certain inlets and modes (e.g., split) are better for this than others.
Some inlets and modes are well suited for analysis of major components, and for impurities down to about 0.01-0.1% of the major component. These are designated as High under the category “Bulk Analysis” .
Those inlets rated High in “Trace Analysis” are those suited to the analysis of analyte concentrations at or below one part in a million (ppm) in the sample. These inlets and injection modes are designed to transfer the complete injected sample volume into the column.
If one were doing trace (e.g., parts per billion, ppb) analysis of labile pesticides in food extracts (high sample matrix), then it is clear from Table 1 that a temperature programmed splitless inlet would be the preferred choice. On the other hand, fast GC purity profiling of bulk chemicals in a process would best be done with hot split injection.
This series of articles is produced in collaboration with Dr Matthew S. Klee, internationally recognized for contributions to the theory and practice of gas chromatography. His experience in chemical, pharmaceutical and instrument companies spans over 30 years. During this time, Dr Klee’s work has focused on elucidation and practical demonstration of the many processes involved with GC analysis, with the ultimate goal of improving the ease of use of GC systems, ruggedness of methods and overall quality of results.