Part I of this post talked about ways to adjust gas chromatography (link to blog post) approach and GC system setup for different sample types, with a focus on sample introduction. In part II, we'll zoom in on five methods of sample introduction that address common sample challenges, with example applications for further reading.
How Do PTV and On-Column Methods Differ From Split/Splitless?
The flash vaporization temperature of a split/splitless inlet can take some analytes well past their boiling point, risking damage, while leaving others too cool to vaporize, excluding them from the analysis. Both on-column and PTV address these issues, with some differences in flexibility and reproducibility.
A cold on-column injector introduces the sample directly onto the column without heat, or below the analyte boiling point(s), condensing it into a narrow volume. Then the column and inlet are heated to release the sample into the gas phase at the lowest possible temperature. This approach avoids the risk of decomposing labile components, such as triglycerides, above their boiling points, while also preventing discrimination of heavier sample fractions. Cold on-column injection ensures sample integrity and excellent analyte recovery for both volatile and heavy compounds.
PTV inlets combine the advantages of split/splitless and on-column approaches. PTV ramps up temperature gradually, starting gently and covering a wider range than split/splitless flash vaporization. This makes it possible to both limit heat degradation and analyte discrimination with the right program. Probably the most flexible GC injection technique, PTV may not achieve the outstanding reproducibility of on-column injection in some cases.
How is On-Column Insertion Used?
As examples of GC with on-column insertion, see this webinar and application note outlining GC analysis of biodiesel using a cold on-column inlet and flame ionization detection.
Headspace GC analysis is an excellent method for determining very light volatiles and semi-volatiles in liquid, solid, or gas samples. This approach has grown in popularity for its ease-of-use and potential for automation, handling even very complex sample matrices with little or no need for sample preparation. Setting up headspace technique with autosampling enables unattended weekend-long operations, with highly reproducible sampling even at microvolumes.
"Headspace" is room in a sample vial above the sample itself. Light volatiles diffuse into whatever gas occupies the headspace, providing a hands-off equivalent of matrix extraction. Headspace analysis can be easily automated using an autosampler, making this an ideal approach for QC labs and sample screening. Note that analytes with higher boiling temperatures cannot be detected by this method.
How is Headspace Analysis Used?
The following application notes provide examples of GC headspace analysis in several fields:
When using dynamic headspace a packing is deployed to trap the volatiles in the sample before injecting into the GC. This is mainly used if static headspace is not sufficient to determine the volatile compounds at the lowest levels.
There are several forms of dynamic headspace GC:
Purge and trap for mainly liquid and soil matrices
Thermal desorption for mainly air
ITEX containing a small packing in the syringe itself
How is Purge-and-Trap GC Used?
The purge-and-trap technique was first developed as an efficient means to extract trace levels of volatile organic compound (VOC) pollutants from water and soil samples. Purge-and-trap sample extraction captures target compounds on a concentrator column, providing a convenient means not only for their removal, but also concentration. In purge-and-trap GC, volatiles captured on the concentrator column are eluted and separated by GC for analysis.
Thermal desorption techniques have a very wide variety of applications, such as indoor air monitoring or flavor analysis. This technique is also used for analyzing material emissions.
Again the volatile compounds are trapped, but in this technique they are trapped two times. The first time, they are trapped on a sorbent tube. This trapping can be performed passively, for example by wearing a sorbent tube in the workplace, and the indoor air contaminants will get trapped on the tube. Also, active sampling is performed by pumping air or sample through the tube at a fixed flow rate.
The second time the compounds are trapped in the introduction system itself. These are subsequently heated and injected onto the GC system.
See the following application note for an example of this technique: