This month we complete a list of questions about gases that began in a previous instalment (1).
The range of questions about gases in gas chromatography (GC) is wide and complex. Even barring direct questions about hydrogen
as a carrier gas, my attempts to address at least most of the core questions has already taken two instalments of GC Connections. I hope this month's additional topics will suffice, but I also encourage readers to correspond with any other questions
they may have on these or any other GC-related subjects.
Clarification on Sealing Tape
More than one reader commented that in the previous column (1), no explicit precaution was given regarding the use of polyfluorocarbon
tape or sealant with swaged fittings. The use of any tape or sealant is clearly proscribed for swaged fittings as well as
for cylinder compression fittings. A better statement would be that polyfluorocarbon tape specifically sold for high-purity
gas distribution is the only type of tube or fitting sealant that can be used for gas chromatography, and the only place that
it is appropriate for use is on pipe-thread fittings where it functions as a sealant and prevents gas from flowing around
the pipe threads themselves. This applies not only to ¼- or ⅛-in. (6- or 3-mm) tubing and associated fittings but also to
larger sizes such as sometimes found in manifolded gas distribution systems (see below for more). Thanks to those readers
for helping to clarify this issue.
More Questions That Should Be Asked Frequently
Here are the remaining questions on my list regarding gases and gas delivery for GC. As before, these questions do not address
anything about the gases after they reach the GC system. Therein lie even more questions that also should be asked frequently.
What Types of Filters Should Gas Chromatographers Use?: Recommendations for filter types vary for different instrument manufacturers as well as for different consumables suppliers
and producers. Filter requirements are driven by the types of injection, column and detection technology in use. In mixed
situations, higher purity requirements override less stringent ones, so always use a filtering scheme that is appropriate
for the component that requires the highest purity level. Table 1 is an expanded version of a table included in the previous
instalment (1) that gives a filter and gas selection matrix for commonly used inlets, columns and detectors. Be sure to choose
filters that are rated for the desired gas purity level and devices in use.
Table 1: Filter and gas selection matrix.
For example, if using a split–splitless inlet with a wide-bore porous-layer open-tubular (PLOT) column and thermal conductivity
detection (TCD), the TCD makeup gas — which has a less strict purity requirement — would be subsumed to the carrier gas because
of its higher purity specification. A separate makeup gas supply is not required with TCD; makeup gas can be pulled from the
higher-purity carrier-gas supply. An electron-capture detector with a split–splitless inlet, capillary columns, and hydrogen
carrier gas would require at least 99.9995% carrier gas and multiple high-purity carrier-gas filters, plus a separate supply
of 99.9995% impurity-free nitrogen or argon plus 5% methane ionization gas with its own set of high-purity gas filters.
In general, most chromatographers will simply install the highest level of filtration and use the highest gas purities that
apply to all in-use or anticipated combinations of inlet, column and detector on a particular instrument population.