Gases Q&A, Part II

Jan 01, 2013
Volume 31, Issue 1, pg 36–44

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?

Table I: Filter and gas selection matrix
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.

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.

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