Leaks are anathema to chromatographers. Wasteful of ever more scarce and expensive gas or liquid mobile phase, leaks have been blamed for detector noise, baseline instability, inaccurate flow calibration, column degradation, and potential explosion or toxicity hazards. Conventional laboratory wisdom states that any leak is to be avoided, even trace leaks that don't materially affect flow measurements or gas consumption.

In gas chromatography (GC), the gas supply lines are bathed in a mixture of 78% nitrogen, 21% oxygen, and 0.9% argon, plus some water and a smattering of trace-level gases, which is to say, room air. Room air can flow or diffuse past a leaky fitting, diaphragm, or seal into regulators, supply lines, and inlets, so the potential exists for oxygen and water to enter carrier gas lines through poorly made-up fittings, permeable regulator diaphragms, or leaking septa. The result: expensive, high-purity 99.9999% carrier gas gets downgraded to something less pure. Good laboratory practice calls for careful attention to fittings, checking for leaks with high-sensitivity electronic leak detectors, the use of appropriately rated high-purity gas regulators and valves, and regular septum and inlet seal replacement. Installation of gas filters that trap oxygen, hydrocarbons, and moisture is highly recommended as a stop-gap measure against the traces of contamination that might make their way into the carrier gas despite all the other precautions.

The effects of gas impurities are relative to the sensitivity of the instrumentation and columns to the contaminants. Some capillary GC columns — the "wax" types for example — can degrade rapidly in the presence of sub-part-per-million (<10^-6 volumetric concentration) levels of oxygen when operated at elevated temperatures. Certain detection methods, such as flame ionization detection (FID), are not sensitive to air contamination but they are quite sensitive to the hydrocarbon content of their fuel gases. Other detection methods, such as discharge ionization detection (DID), are exquisitely sensitive to contaminants. Gas sampling valves for very high sensitivity work can require various purge arrangements that flood the valves with carrier gas externally as well as internally to prevent even the slightest influx of air into the active valve passageways. The fact that it is necessary to resort to such measures clearly demonstrates the significance of extremely small gas leaks.

Contamination or Leakage?

Carrier gas leaks and poor quality carrier gas can yield similar problems: They both cause contaminants to enter the column and detector. In the course of investigating issues with carrier gas quality I wondered, how much air does enter a carrier gas line against a detectable leak? The concept of air flowing into a fitting against the outward flow of exiting carrier gas seems counterintuitive; could this really happen?

Recently. I investigated what apparently were several locally sourced cylinders of contaminated ultrahigh purity (UHP) helium that had been installed on a group of process analyzers in a remote area halfway around the world from my location in the western U.S. A little experimentation in the laboratory with a test analyzer quickly revealed that the helium carrier gas in the remote analyzers seemed to contain something like 1000 ppm of nitrogen, orders of magnitude more than the 1 ppm or less that should have been present. But this finding left open the question of the origin of the contaminating nitrogen: Was it in the cylinders themselves or had it entered the carrier gas downstream from the cylinders due to a leak or a defective seal? All the evidence pointed to poor quality cylinders. The rates of gas consumption were normal, no leaks were evident when examined during a service visit, high-purity gas regulators had been installed, and the side effects of the contamination were very similar for each of the analyzers. If leaks were involved, then it seemed unlikely that the leakage would be nearly the same for each of three independent analyzers with their own individual carrier gas tanks.

A quick experiment further exonerated any reasonable leakage as a source of nitrogen contamination. With no carrier gas filters in place, I simply loosened the carrier gas bulkhead fitting on the laboratory test system until a large leak could be detected with a handheld electronic leak detector set on its low sensitivity range. This had no effect on the chromatography and did not produce the characteristic baseline-upset symptoms of high levels of nitrogen contamination. I went even further and loosened the carrier gas fitting enough to cause an audible hiss as the gas escaped. At this leak rate, the cylinder pressure dropped from around 2300 psig (15.8 mPa) to 2150 psig (14.8 mPa) in 2 h, which would have emptied the tank in short order. Yet, no noticeable effects were seen in the chromatography. The resulting nitrogen contamination in the carrier, if any, was nowhere near 1000 ppm.

This left me wondering: Just how much air does move into the carrier gas stream against an outgoing leak, and how much does the air influx depend upon the size of the leak? As I thought about it, I recalled an inadvertent but similar result from an earlier "GC Connections" installment (1). At that time, I had been looking into carrier leaks inside a GC system and had found an incorrectly assembled and leaky bulkhead fitting at the back of the instrument. Repairing the fitting had no effect on the chromatography back then, either. In that case, the leak was found in the column inlet.