by Joseph Rotter
Chemistry
Advances in GC Valves Extend Service Life and Improve Repeatability
apid advances have been made in the quality, reliability, and sensitivity of detectors and other components and techniques used in gas chromatography. However, little attention has been paid to the instrumentation side. As a result, gas-sampling valve technology, little changed for the past half-century, has become the limiting factor constraining equipment improvements in long-term repeatability, sensitivity, and maintenance frequency.
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One of the challenges with traditional GC valves is wear. Since a valve is one of the few moving parts on an instrument, premature failure leads to unscheduled maintenance and downtime. Another challenge is data integrity. Worn or leaking valves that allow sample loss, contamination, or carryover deliver inaccurate results, requiring more experiments and longer run times.
Now, advances in GC valve technology are changing this picture, not only improving repeatability and extending maintenance cycles, but also enabling the much lower detection limits required for sensitive chromatography analyses.
Repeatability Repeatability is defined as how consistently the same actions will pro-
duce the same results. In the case of gas-sampling valves, this means delivering the same sample size at the same flow rate and pressure time after time. For proper gas analysis, it is important that the GC valve de- liver good repeatability from the start and maintain good repeatability over time.
Uneven valve wear and leakage are the two main culprits causing both poor repeatability and shorter valve life. Wear due to abrasion and fric- tion impacts flow path uniformity, degrading results over time. Chemical activity can also impair the flow path integrity. Carrier or sample gases contaminated with particles can cause wear on the valve surfaces. All of these lead to faster valve degradation, premature failure, and additional material and labor expenses.
Note that the common practice of replacing just the rotor in a worn valve does next to nothing to remedy wear problems. The new rotor may be pristine, but putting it in a worn valve body perpetuates the previous problem. The difference in surface finishes means the parts will not get a tight fit. Even if the valve head is remachined for a better match, there are still labor costs and instrument downtime to consider. A new rotor may seem like a quick fix, but it won’t last as long as the original
valve, leading to more frequent maintenance cycles. A better solution to reducing maintenance is to use a longer-lasting valve in the first place.
Perhaps the most insidious villain working against good GC perfor- mance is leaking. Leak paths in valves and fittings can cause all kinds of problems. Inbound leaks and diffusion of air/atmosphere into the valve contaminate the sample, changing results from one sample to the next. While it may seem that a pressurized line would preclude atmospheric contamination, in fact affinity for the contaminant can override it. Think of a jar filled with sand. While no more sand can be added, adding water to the jar will change the mass of its contents.
Outbound leaks of sample to atmosphere also impact repeatability through sample loss, which changes flow pressure and sample concentra- tion. This makes trace detection difficult because sample sizes are so small and detection limits so low that even the smallest variation can skew the results. Worst of all, if toxic or hazardous gases are used in the process, outboard leaks can cause serious safety and environmental issues.
Wear and leaks are related problems. Wear can lead to leaking over time, and leaks can admit particles or chemicals that accelerate wear. Thus the most important advances in GC valve technology are those that protect against wear, reduce leaks, and extend service life.
Rotary valves The oldest and most widely used GC valve technology is the conical
rotary valve, essentially a rotor matched to a V-shaped stator. Until re- cently, this design was little changed since the 1960s, but the demand for lower detection limits and longer life has led to some significant improvements in the last few years.
New, proprietary surface treatments for the valve body decrease wear caused by friction and abrasion so the valve operates as required for longer. The right treatment depends on the intended application. For example, metal surfaces in valves used to find parts-per-billion of organo-sulfur compounds and mercury need the ultimate in passivat- ing coatings. These reduce moisture contamination, improve system performance, and eliminate surface adsorption of active compounds on stainless steel, reducing wear. A coating that improves hardness and corrosion resistance of stainless steel might be the choice for sulfur, mercaptan, ammonia, and mercury sampling.
Using a valve with the right rotor material for the application will also extend life. A valve manufacturer can help the OEM designer or end
AMERICAN LABORATORY • 11 • SEPTEMBER 2014
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