Drinking Through a Dirty Straw: Five Myths About Gas Management Systems

Aug. 18, 2005
When gas chromatography problems arise, most people think the gas in the cylinder is the problem. It’s more likely to be the gas management system, as Frank Kandl of Airgas discusses in this myth-busting primer.
Since many problems in gas chromatography immediately follow a change in cylinders, the gas in the cylinder is often suspected as the source of the problem. Yet, elevated baseline noise, ghost peaks and excessive column bleed are often caused by contaminants introduced by an errant gas management system, not the cylinder. Think of it as drinking clean water through a dirty straw. Here are five myths about gas management systems and the truth behind them.Myth No. 1: A better grade of gas will improve performance.Fact: This is only true if the gas entering the instrument is the same quality as the gas in the cylinder. If the gas – no matter how pure – travels through a “dirty” system, it becomes contaminated. If the system is clean, chromatographers may be able to improve the baseline to detect at lower levels or switch to a lower purity level to save money. The impurities that can most adversely affect a GC system are oxygen, moisture and hydrocarbons. To prevent these from causing harm, build a better system that will 1) contain and remove contaminants before they can do damage, 2) provide a visual indication when contaminants enter, and 3) identify the contaminant.Myth No. 2: All regulators with stainless steel diaphragms are suitable.Fact: Not so. High purity regulators use stainless steel diaphragms since they do not absorb contaminants. However, body design – bar stock or forged body – is actually more important. A bar stock regulator has a small internal volume and a straight gas path. If contaminants do enter, it is easy to predict when they will emit. By contrast, a forged body has a large internal cavity with no direct path, allowing contaminants to become trapped and emit unpredictably. Only use a regulator made from bar stock that has been cleaned for chromatographic service. Never use lubricants or Teflon tape on the CGA nut, since these cause leaks and contamination as well.Myth No. 3: It takes four or more hours to recover a baseline after cylinder change-out.Fact: This recovery time is only necessary if contaminants enter the system. Yet, even with proper safeguards, a cylinder change-out can take a half-hour or more. Because of this downtime in testing, lab managers often replace the cylinder when convenient rather than necessary. They will change a cylinder with more than 500 psig of pressure remaining on a Friday afternoon — to avoid the expected downtime.Consider adding an automatic changeover system. When one cylinder is depleted, the system switches to the other side, allowing the “empty” to be returned with a minimum of residual (150 psig). This allows for uninterrupted service, eliminating waste and downtime. Cylinder regulators can also minimize this recovery time if they have components to contain and remove the contaminants that enter during cylinder change out.Myth No. 4: Leaks only go outward.Fact: Many lab managers believe that leaks only go outward. However, when the gas is flowing, the leak generates a vacuum that can suck in oxygen, moisture and hydrocarbons from the atmosphere. Therefore, make sure that joints and fittings are made correctly by using compatible materials. For example, avoid combining stainless steel tubing and brass regulators or ferrules. For fittings, only use two-piece ferrules tightened to manufacturer specifications. Also, if the system has brazed joints, they must be made using the fluxless brazing method. Orbital welding is a viable alternative as it creates a metal-to-metal fusion joint that has no filler metal. Pipe thread joints must only use Teflon tape without a lubricant or sealer.Another tip to prevent leaks and contamination is to use diaphragm packless valves, since they use multiple metal diaphragms, require no lubrication and are capable of passing helium leak tests. Also, when using flexible pigtails, avoid those with a Teflon core and use those that are leak tight with internal stainless steel corrugated bellows.Myth No. 5: Purifiers solve everything.Fact: Purifiers safeguard against contamination. They are not a practical method to clean gas. In fact, using some types can actually introduce impurities at unpredictable moments. There are three types: surface absorbent, chemically absorbent and self-indicating chemically absorbent purifiers. Surface absorbent purifiers are the worst choice. When they become 40 to 60 percent saturated, contaminants introduced during change outs can dislodge trapped contaminants. Since typically the purifier is located directly in front of the GC, the column is the next stop for these contaminants. Chemically absorbent purifiers lock up contaminates, so they cannot become dislodged later. However, if the purifier is non-indicating, it is difficult to measure its saturation level. At some point, the medium will stop working, allowing contaminants to pass through unabsorbed. By contrast, a chemically indicating purifier alerts the user when it is time to change the purifier, and even what contaminant is in the gas stream.Remember, it is easy to contaminate a system and time consuming to regain a baseline once contaminated. Building the system correctly can save time and money, and provide the consistency that chemists require in their results.
About the Author

Frank Kandl | national technical manager of specialty gas equipment