This month we complete a list of questions about gases that began in an earlier installment.
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 installments 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
questions 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 I is an expanded version of a table included in the previous
installment (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 I: 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 due
to 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.
When Should I Change or Replace My Gas Filters?
Gas filters for GC either include an end-of-life indicator or they are specified to purify a set gas volume. The indicating
types are more efficient, in the sense that they don't need to be changed until they indicate the approach of exhaustion.
A nonindicating filter that expires after some number of tanks of gas must be changed at that point, whether truly exhausted
or not. A nonindicating filter also creates the extra burden of noting for each filter how many tanks of gas have gone through
it — assuming of course that it will purify more than one tank. Nonindicating filters usually will include a specification
for just how contaminated the incoming gas can be to meet the filter lifetime specification. However, feeding less-contaminated
gas — as printed on the gas tank analysis — cannot be assumed to yield a longer filter lifetime. The filter is supposed to
remove extra contamination that may be entrained into the gas and not improve on the purity of the gas that is in the tank.