The cost and supply of helium have caused chromatographers to use nitrogen gas generation as a cost-effective alternative.
Helium has been used as makeup gas in gas chromatography for decades, but the global shortage of helium has caused costs to skyrocket and availability to decline. Helium is an inert gas generated by the radioactive decay of thorium and uranium. Trace levels are found in the atmosphere and higher levels in natural gas fields. The U.S. Bureau of Land Management (BLM) maintains a strategic stockpile to provide a reliable supply of helium. Because of increased demand, the price of helium has steadily increased over the last few years. The BLM set the price of helium at $95/1Mcf at the start of 2014, up from $84 in the fall, an increase of 13%. During the same period, laboratory users of helium cylinders have experienced shortages, gas rationing, and price increases as high as 20 to 25%.
Because of the cost and supply issues associated with helium, many chromatographers are turning to nitrogen as a makeup gas. Nitrogen is readily available, inexpensive, and improves the shape of the flame in the Flame Ionization Detector (FID), which enhances the sensitivity. Nitrogen can be purchased or generated in-house. When purchasing high purity nitrogen it can be generated via the fractional distillation of air and is commonly supplied in high-pressure tanks. Nitrogen generated in-house will be generated from compressed air on a continuous basis, and at a much lower cost. Generating nitrogen in-house also eliminates the need for dependence on outside suppliers, and the inconvenience and safety hazards of storing and handling high-pressure cylinders.
How in-house nitrogen generation works
The design of a typical in-house nitrogen generator consists of seven stages:
- Pre-filtration. A high-efficiency coalescing filter removes water, oil, and particulate matter from compressed laboratory air.
- Adsorption of volatile organic compounds. Activated carbon is placed upstream of the catalyst to remove trace amounts of halogenated hydrocarbons and other volatile organic compounds that could poison the catalyst.
- Particulate filtration. A BQ-grade filter is placed after the activated carbon filter to ensure that activated carbon particles are removed from the stream.
- Hydrocarbon removal. A cartridge heater in a stainless steel vessel with a proprietary catalyst blend oxidizes the hydrocarbons in the air into CO2 and H2O.
- Cooling. A copper coil allows the makeup gas to cool after passing through the heated catalyst module.
- Filtration. The cooled gas is passed through an ultra-high efficiency membrane to remove particulate contamination in the gas from the catalyst module.
- Nitrogen purification via a semi-permeable membrane. A hollow-fiber membrane separates the nitrogen from the other gases in the compressed air.
Nitrogen obtained from an in-house generator is 99.9999+% pure with respect to hydrocarbons (measured as methane) and 99+% with respect to oxygen. In addition to nitrogen, an in-house generator can provide zero air, which contains <0.05 ppm of hydrocarbon (measured as methane), and can be used for other components of the chromatographic system.
The purified nitrogen makeup gas is directly ported to the detector to provide gas on a 24/7 basis, requiring no user interaction. The generator provides warnings if the air supply falls, excessive air flow is observed, or the temperature of the catalyst heater is incorrect.
Other advantages of in-house nitrogen gas generation include:
Safety. The gas generator is hard-plumbed directly into the gas chromatograph and delivers nitrogen at a flow and pressure that meets the needs of the detector; no high-pressure tanks are required.
Environmental benefits. An in-house generator uses a minimum amount of energy, which benefits the environment. Additionally, helium is a non-renewable natural resource. Using replacement gases will help to conserve it.
No risk of contamination. An in-house generator eliminates the possibility of introducing foreign materials (a risk when a new tank is installed).
Cost. An in-house generator can provide makeup gas at a considerably lower cost than tank gas. The payback period on purchase and installation of an in-house nitrogen generator is about one year.
The limited availability and increased cost of helium have prompted many chromatographers who use FID detectors to switch to nitrogen for makeup gas. Nitrogen generated directly in the lab provides a number of significant advantages over high-purity nitrogen available in tanks. An in-house generator increases productivity and also eliminates the hassle of dealing with deliveries and the safety risks related to handling gas tanks.
Therefore, in-house nitrogen gas generation is a highly efficient and cost-effective way to increase the quality of your analytical operations, at a significantly lower cost. It also gives you greater control over your operational efficiency, with faster results—an added value that clients appreciate and that makes you more competitive in the marketplace.
This post was contributed by the Gas Generation Technology Blog Team, Parker Hannifin.
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