Q&A: Gases

Nov 01, 2012
Volume 25, Issue 11

In this month's instalment, John Hinshaw addresses a number of frequently asked questions about gases and their delivery to a gas chromatography instrument.

Gases for gas chromatography (GC) have become a hot topic in recent months, primarily because of concerns over short supplies of helium. Already one of the top discussion items, hydrogen as a carrier gas has garnered much of the attention as chromatographers' awareness of these issues continue to expand. Just last month (October 2012), on-line web seminars from Agilent and CHROMacademy were dedicated to conversion from helium to hydrogen carrier gas. A web search on 'helium hydrogen carrier' yields guidelines and instructions from every major GC manufacturer and supplier, as well as myriad topical threads on all of the GC blogs, boards and discussion groups. Go to Pittcon, Analytica, the Eastern Analytical Symposium (EAS) or any other conference where GC is on the agenda and the helium issue will be featured prominently.

Among all the discourse I have noticed that the essential related topic of good practices for deployment of carrier gases, and for that matter all gases used in GC, is largely missing. Although much of this good advice is easy enough to find in instrument installation guides and supplier catalogues, the connection between obtaining the information and putting it into practice is often missed in many laboratories. Most laboratories will install gas filters in-line, but many will fail to obtain the right type of regulator, make the gas connections correctly, or maintain the filters and check the regulators on a regular schedule.

Questions That Should Be Asked Frequently

Here are some guidelines and recommendations about GC gases, in a question-and-answer format. This is not an exhaustive list, but rather it covers some of the more frequently asked questions as well as some that are not asked as often as they should be. The list starts at the gas source and moves onward to the instrument. Questions about the instrument internals are not addressed because of space limitations.

What Are the Recommended Gas Purities for Carrier and Detector Gases?: The exact requirements for gas purity should follow the instrument manufacturer's guidelines as found in their site preparation and installation manuals. If that information is not available, then Tables 1 and 2 will serve as a general guideline. The gas purities are stated as percent levels rather than referring to supplier–specific names, which can be ambiguous or inconsistent.

How Pure Are My Gases, Really?: The purity of a gas when it reaches the back of the instrument depends on the supply quality, regulators, filters, fittings and connecting tubing. Filters will clean up minor contamination, but they are not intended to take gas to a higher purity level. Most of the time the purity of cylinder gas is as labelled on the bottle, but occasionally a contaminated cylinder may make it to delivery. Although it is bad practice on the part of cylinder users, a cylinder might be left open to the atmosphere for hours when empty and removed from service. If not cleaned up by evacuation and baking before filling with gas to 2450 psig (166 mPa), such a cylinder will contain approximately 6000 ppm of air, which degrades the gas purity to 99.4%. Although it is extremely unlikely to arrive in a cylinder at the receiving dock, this level of contamination represents a conceivable upper limit. When placed in service the resulting onslaught of oxygen, water and possibly hydrocarbons will completely exhaust a high-capacity gas filter before the contaminated tank is empty.

This potential for contamination is an excellent reason to use indicating filters on all gas supply lines. The indicator will change colour as the filter reaches capacity. As long as the colour change is noticed, a new filter is installed and a pure gas supply is restored, the GC instrument will be spared the indignity of gas contamination and resultant high detector background, irregular baselines and accompanying loss of signal-to-noise and repeatability.

It is possible, but expensive, to order purity analyses of individual cylinders. This step is only significant when it is difficult to observe the filters or replace the cylinder, such as at remote unattended locations.

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