Gas purifier manufacturers - CRS included - like to illustrate how you can save money with GC carrier or FID gases by using our purifiers. We like to show how you can turn cheaper, lower-grade gas into ultra-high-purity gas yourself and avoid buying the more expensive grade cylinders. Typically this is illustrated by taking the difference in price between a cylinder of high- and low-grade gas and multiplying by the number of cylinders you can purify, based on the filter’s capacity.
One of the push-back arguments is that if you’re getting the results you need without obvious interference from a contaminant peak in the baseline, what do you really get by moving from, say, 99.995% to 99.999+% purity? (Based on contaminants like oxygen and water; trace inert gases like nitrogen or argon aren’t usually removed.)
Well we say, besides the price of gas, there are two hidden costs that are hard to put a dollar value on, but in our opinion are worth the price of a purifier: what we’ll call Aggravation Cost, and Column Replacement Cost.
First, the Aggravation Cost: If you’re like most chromatographers, you like to present good clean data, and it’s aggravating if day-in and day-out you need multiple clean-up runs before your first good baseline, or a sample is rushed through that gets the report out, but with lots of noise around the peak of interest, etc., etc… Figure 1 shows the difference in baseline from an FID signal with fuel gases run through dedicated lab lines, before and after installation of point-of-use purifiers just upstream of the instrument. Relief!
Figure 1. A) FID baseline signal with UHP air and hydrogen through lab stainless steel lines. B) FID signal with ZPure™ filters for air and hydrogen immediately upstream of the instrument.
Then there is Column Replacement Cost: This one’s tough to quantify. Basic facts are known; with capillary columns just 1 ppm oxygen in the carrier gas leads to a large elevation in the baseline level and noise1. Stationary phase degradation means your columns don’t last as long, so replacement costs are higher. It’s easy in theory to design an experiment: run identical columns, one with ultra-pure carrier gas through a purifier, the other with 1 ppm oxygen, through multiple program runs and see how long each lasts. (Repeat with 0.5 and 2 ppm O2 for a multi-point curve.) The practical difficulties are such, however, that I’m not aware of any published articles showing column lifetime versus carrier gas purity. But I agree with the common wisdom that columns last significantly longer when high-purity gas is used, and gas purifiers are low-cost insurance.
To sum it up, helium is a significant factor in GC lab costs today and prices continue to rise. Be sure you get the best gas for your chromatography at the lowest price with CRS gas purifiers in-line. That’s how we supply gas in our own lab!
- E.J. Guthrie and J.J. Harland, LCGC 13(6), 446–455 (1995).