Generating Make-Up Gas for GC with an In-House Nitrogen Generator

Since the gas flow required for the separation step in gas chromatography is frequently lower that that required to optimize the detection, nitrogen is used as a make-up gas to increase the gas flow for detection. In many facilities, zero-grade nitrogen make-up gas is provided from a cylinder or tank. While this approach works, an in-house "make-up" gas generator can provide the desired nitrogen with a higher level of purity than bottled nitrogen. In addition, the use of an in-house make-up gas generator can provide a considerably safer, more convenient, and less expensive approach to supply the required gas.

Design of an In-House Nitrogen Generator

Zero-grade nitrogen for make-up gas can be readily obtained from laboratory compressed air using an in-house generator (Parker Hannifin FID MakeUpGas Generator) that includes a heated catalytic converter in which a proprietary catalyst blend is combined with Platinum to remove all hydrocarbons by converting them to CO₂ and water vapor. The convertor is followed by a hollow fiber membrane separator which preferentially allows oxygen and water vapor to quickly permeate the membrane wall while nitrogen travels through the hollow fiber out the end (Figure 1). The hollow fiber has a small internal diameter and thousands of fibers are bundled together to provide a large surface area to provide the desired flow of nitrogen. The makeup gas generator can provide nitrogen with purity of better than 99.9999% with respect to hydrocarbons (<1 ppm) and greater than 99% with respect to oxygen.

Figure 1: Membrane separation technology.

Performance

A chromatographic comparison of the nitrogen that was produced by the MakeUpGas generator and gas that was obtained from bottled fuel air from a commercial supplier is shown in Figure 2. The gas generated by the MakeUpGas generator is much purer than that from bottled fuel air; and provides an extremely flat baseline with essentially no signal due to hydrocarbons, while the zero-grade bottled air provided an irregular baseline with a significant level of hydrocarbons, which could impact the analysis.

Figure 2: Chromatograms comparing baselines produced by a Parker MakeUpGas Generator and bottled fuel air.

Conclusions

In addition to the extremely high level of purity provided by the generator, the use of an in-house generator provides benefits in safety, cost, and convenience. When a MakeUpGas generator is employed, only a small amount of nitrogen is generated at a given instant and a leak would lead to a negligible change in the composition of the laboratory air. In contrast, a leak from a full tank could cause problems. When an in-house generator is employed, gas is available on a 24/7 basis and the possibility of injury or damage during the transportation and installation of a heavy gas tank which can become a guided missile if the valve on a full tank is compromised during transport is eliminated. In addition to the significant safety and convenience benefits, there is an economic benefit from using a MakeUpGas generator. The running cost of operation maintenance of the MakeupGas generator is extremely low; as the raw materials to prepare the required gas are air and electricity.

Parker Hannifin Corporation

Filtration and Separation Division

260 Neck Rd., Haverhill, MA 01835

tel. (800) 343-4048, (978) 858-0505

Website: www.labgasgenerators.com