GC Solutions #38: Inlet Liners – Part 3
This month we continue our liner discussion with a focus on liner volume. One of the most important liner design variables is the internal volume. Larger internal volumes are preferred for general use because they minimize the chance of vapor overload (“flashback”). Flashback decreases precision, contaminates the cool upper part of the inlet weldment and tubing with sample components and increases activity. Flashback is most probable when doing hot splitless injections where total inlet flow is low during injections, when using polar solvents, and when injecting larger volumes such as when analyzing trace level sample components. Even though there is slow sample transport and wide initial peak widths with splitless injections, they are corrected typically corrected by solvent and stationary phase focusing.
Large liner volumes, however, are detrimental to methods that require narrow initial peak widths and those that can’t benefit from stationary phase, thermal, or solvent focusing, such as split injections of samples containing early eluting compounds of interest. There are several other applications in this category where small liner volumes are best: small internal diameter columns (i.d. < 200 µm), and slow sample introduction devices that introduce samples already in the gas state (e.g., headspace, sampling valves, thermal desorption, and analytical pyrolysis).
Column flow is low with small-diameter capillary columns, so total flow is low and time to clear the liner volume is long even with a reasonable split ratio. In addition, due to the lower capacity of these columns, it is unlikely that splitless injections would be used (providing solvent focusing) because of the increased probability that condensed solvent (“flooded zone”) would break up causing split peaks, and because higher-level sample component peaks would be distorted due to overload. Hence, sample introduction into small i.d. columns is most likely done in split mode with relatively high split ratios.
As an example, with a 100 µm i.d. column at 0.5 mL/min and a split ratio of 100:1 would have a total inlet flow of only 25.4 mL/min, so a liner volume of 1 mL would take at least 1.2 sec to clear and the initial injection band would have a significant tail. This is too wide an initial peak width for a 100 µm column. A narrow bore liner with an internal volume of 100-200 µL would be a better choice. Unfortunately, most of the narrow bore liners that are commercially available were designed back in the day when it was thought small volume liners were best for splitless injections. Therefore, they typically have wider outside diameters and can generate excessive backpressures at higher total inlet flow rates (such as would be used to minimize loading on such small capacity columns). The higher head pressures of small bore columns saves the situation a bit - the given flow of gas through the restriction will generate a specific backpressure that may be less than the column head pressure and therefore not causing overpressure, however it will affect the ability of inlet pneumatics to adjust to transients such as what happens during injection. So it is important to shop carefully for a liner that complies with both the requirements for loose fitting outer diameter and small internal volume.
In addition, for liquid injections into small diameter liners it would be helpful to have glass wool in the liner (when sample polarity and lability permit it), and to have a taper at the bottom, especially when analyzing samples with a wide volatility range. Unfortunately, there are few (if any) commercially narrow bore (small volume) liners with these attributes.
For sample introduction devices that provide gas samples to the GC, there is absolutely no reason to have a large-volume liner. There are no evaporation or backflash issues to face as one has to with liquid injections. In addition, the gaseous samples are exponentially diluted as they pass through the inlet volume, broadening the already wide initial peak widths further. Of course the sample components are volatile, so there is no chance of focusing once they reach the column unless cryogenic trapping is used. The issue a consumer has, again, is to find a low volume commercial liner that is not tight in the inlet.
With gaseous samples, there is no need for glass wool or a taper at the bottom of the liner. The glass wool will add no benefit and will increase activity toward polar compounds. The taper will add no benefit but would cause no problems.
Next month we discuss liner deactivation.
Dr Matthew S. Klee is 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.