Choosing the Optimum Makeup Gas for GC with Flame Ionization Detection
While nitrogen can be supplied via an in-house nitrogen generator or from tanks from an external supplier, the use of an inhouse generator can provide the necessary gas on a 24/7 basis with a significant increase in safety and convenience. In addition, the use of an in-house generator can provide the gas at a lower cost than tank gas and reduce the impact on the environment, as heavy tanks need not be transported from a bottling site.
The flame ionization detector (FID) is commonly used in gas chromatography (GC) and can detect almost all organic compounds; it is especially useful for monitoring compounds with a high carbon concentration. Compounds that are eluted from the column are ionized using a hydrogen/oxygen flame, and the resultant ion current is monitored. The optimum flow rate for the detector is 50 to 500 mL/min, significantly greater than the typical flow rate for the optimization of the chromatographic separation (1 to 2 mL/min). A makeup gas is added to the eluant stream as it departs from the column to increase the flow rate in the detector and to ensure a sufficient stream of electrons to ionize the eluant and maximize the sensitivity of the detector.
Nature of the makeup gas
The makeup gas should contain low levels of compounds that can be ionized by the flame so that the background signal is minimized, and it should not react with the eluant or the ions that are formed in the flame. Historically, helium has been used as the makeup gas. Helium is generated by the radioactive decay of thorium and uranium. Trace levels of helium are present in the atmosphere, and significant levels are found in natural gas in some gas fields.
Helium is isolated from natural gas by fractional distillation. In the US, helium is extracted from natural gas in Texas, Kansas, Oklahoma, and Wyoming, and the U.S. Department of the Interior’s Bureau of Land Management maintains a strategic stockpile (the National Helium Reserve) to provide a reliable supply. A finite amount of helium is available and is dependent on the extraction of natural gas. In recent years, the drawdown has been significantly greater than its replenishment, resulting in limited availability and higher cost.
Many chromatographers use nitrogen as a makeup gas, as it is readily available, inexpensive, and improves the shape of the flame in the FID, enhancing the sensitivity. High-purity nitrogen can be generated via the fractional distillation of air and is supplied in a high-pressure tank.
As an alternative, nitrogen can be generated in-house from compressed air.
How does an in-house N2 generator isolate N2 from compressed air?
The design of a typical in-house nitrogen generator (Parker Balston Model MGG-2500) is presented in Figure 1.
In-house generation of nitrogen consists of seven stages:
- Prefiltration—A high-efficiency coalescing filter is used to remove water, oil, and particulate matter (to 0.01 μ) from compressed laboratory air to avoid damaging downstream components. The water and oil are removed from the filter by an automatic drain.
- Adsorbtion 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. If halogenated hydrocarbons are present in the compressed air at a level exceeding 5 ppm, an auxiliary activated carbon scrubber can be used to avoid poisoning the catalyst.
- Particulate filtration—A BQ-grade filter (99.99% efficient at 0.01 μ) 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, resulting in a concentration of less than 0.05 ppm as methane.
- 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 ultrahigh-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. Air flows through the tube as shown in Figure 2. Gases that permeate at a rapid rate (CO2, O2, H2, H2O, He) are removed from the compressed air at a higher rate than nitrogen is. A single-membrane fiber has a very small internal diameter and a nitrogen module with a large number of fibers bundled together to provide a large surface area and to generate high nitrogen output.
The nitrogen obtained from an in-house generator is 99.9999+% pure with respect to hydrocarbons (measured as methane) and 99+% pure with respect to oxygen. In addition to providing N2, zero air that contains
The purified makeup gas is ported directly to the detector using a permanent connection to provide gas on a 24/7 basis with no user interaction. The generator provides warnings if the air supply falls, excessive airflow is observed, or the temperature of the catalyst heater is incorrect.
Advantages of an in-house N2 generator
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There are many advantages to using an in-house makeup gas generator instead of tank gas for FID detection in gas chromatography. (Table 1).
Safety—When an in-house generator is employed, it is hard-plumbed directly into the gas chromatograph and delivers nitrogen at a flow and a pressure that meet the needs of the detector. The maximum pressure at the outlet is dependent on the inlet air pressure, and the maximum flow rate is 400 mL/min. In contrast, when a tank is used, the maximum pressure is upward of 2,000 psi. There is a significant hazard in transporting a tank from the storage area to the laboratory and connecting it to the chromatography system. Serious personal injury and/or damage to the facility could occur if control of the tank is lost during transportation or installation. If the tank valve is damaged, a large amount of nitrogen gas could escape into the laboratory, potentially causing an asphyxiation hazard.
Convenience—Because the in-house generator is hard-plumbed into the GC, makeup gas is available on a 24/7 basis. In contrast, when a tank is used, the operator must monitor the level of gas to ensure that there is a sufficient amount to perform the desired analyses (e.g., if an overnight series of analyses is to be performed). An in-house generator can produce sufficient N2 for several detectors.
Cost—An in-house generator can provide makeup gas at a considerably lower cost than that of tank gas. The generator described above requires 100 V/4 amps. If it is used on a 24/7 basis at a power cost of 10c/kwh, the approximate cost per day is $1.15. In contrast, when tank gas is used, the facility must bear the direct cost of the tank and demurrage charges. In addition, use of a tank involves incidental costs such as the cost of the time required to obtain and install a tank, order new tanks, maintain inventory, and related activities. While the actual cost savings from the use of an in-house generator depend on a broad range of factors, many users report that the payback period is a year or less and the overall operating costs can be reduced by 50 percent or more.
Environmental—The use of an in-house generator has a significant benefit to the environment because it uses a minimal amount of energy. In contrast, when a gas tank is employed to supply makeup gas, air must be cooled for fractional distillation; once the purified nitrogen is obtained, it must be compressed and tanks must be transported from the supplier’s site to the end user. Similarly, once a tank is emptied, it must be returned for refilling; transportation of tanks requires a significant amount of energy.
Elimination of contamination—An in-house generator eliminates the possibility of introducing foreign materials when a new tank is installed. When a tank is used to provide nitrogen, the user must periodically break the connection between the supply and the detector to install a new tank, potentially leading to the introduction of materials into the atmosphere, which could have a deleterious effect on the FID measurement.
The limited availability and increased cost of helium have prompted many chromatographers who use FID detectors to switch to N2 for makeup gas. While high-purity nitrogen tanks are available, the use of an in-house generator provides a number of significant advantages. An in-house generator eliminates the safety issues related to handling gas tanks and is much less expensive; many users report that an in-house system pays for itself in less than a year. Once an in-house generator is installed and plumbed into the GC, it can provide the makeup gas with essentially no user interaction. Because the generator is plumbed directly into the GC, the possibility of contamination is dramatically reduced, while it is possible that foreign materials could enter the detector when tanks are replaced.