The goal when running a gas chromatograph (GC) with a flame ionization detector (FID) is to achieve as stable a baseline as possible. The more the baseline can be differentiated from the emerging peak, the better.
Hydrocarbons and other contaminants present in the air used for an FID will adversely affect the baseline. The sensitivity of an FID versus air purity under isothermal conditions was studied and reported on by Professor Harold M. McNair at the Department of Chemistry, Virginia Polytechnic Institute, and State University at Blacksburg, Virginia. Air from four different sources was studied:
- Zero air generated by a Parker Balston Zero Air Gas Generator
- Breathing air from a cylinder, above 2000psi
- Breathing air from a cylinder, below 100 psi
- Filtered house air
The results are presented in Figure 1.
Figure 1: Baseline Signals. Baselines are raw data (mA) on equivalent scales.
As shown in Figure 1, zero air provides the best baseline. (The term zero air is defined as air containing < 0.1ppm total hydrocarbons, including methane.)
Breathing air or filtered house air
If breathing air from cylinders or filtered air from an in-house compressor is used, the trapped hydrocarbons must be removed. Most labs use purifiers that contain adsorbents intended to remove these hydrocarbons. The typical purifier uses activated carbon to stop hydrocarbons and may also use molecular sieves and or silica gel adsorbents. These purifiers remove most hydrocarbons (C3+) but do not remove methane. Methane and very light single or double carbon compounds can move through an adsorbent purifier in hours to days. At best, these purifiers only slow light contaminants.
For the best baseline, zero grade air should be used because it contains less than 0.1ppm of total hydrocarbons. There are two ways to obtain zero grade air, cylinders or a zero air generator.
Challenges of cylinders
- Zero grade air cylinders are expensive.
- High-pressure cylinders must be properly handled and stored, otherwise leaks or explosions could occur causing personal injury and/or property damage.
- Downtime, from changing tanks or not having enough gas on hand, interrupts testing.
- Inflexible delivery schedules or delayed deliveries.
- Price increases, rental fees, long-term contracts, and extra administrative work add to the expense.
Advantages of an on-site Parker Balston Zero Air Generator
- Provides the highest quality zero air for an FID by catalytically reducing the total hydrocarbon content to less than 0.05ppm.
- Zero air generators are safe - no need to handle heavy gas cylinders which pose risks of injury or laboratory damage.
- In-house gas generation eliminates reliance on an external delivery service.
- Gas delivery is continuous, automatic, reliable, and relatively inexpensive.
- Cost of operation is very low, with a typical payback period of less than one year.
The mode of operation of an on-site generator is straightforward and involves very little operator interaction. A pre-filter is included to remove moisture, particulates and other liquids. A heated hydrocarbon catalyst module that breaks hydrocarbons down into H2O and CO2 follows the pre-filter. Neither of these compounds will respond on an FID. As the air exits the catalyst module, it passes through an after cooler, and then a final filter to remove particulate contamination larger than 0.5 micron. Figure 2 shows the flow schematic for the Parker zero air generator.
Figure 2: Flow schematic for Parker Zero Air Generator.
A Parker Balston Zero Air Generator is recommended to provide the highest quality zero air for an FID while eliminating the challenges of zero air cylinders. A Parker Balston Zero Air Generator produces a consistent supply of ultra-high purity zero air containing less than 0.05ppm total hydrocarbons from a standard compressed air supply on a continual basis. Using zero air produced from a zero air generator increases the signal to noise ratio for the baseline allowing for the best stability. In addition, the cleaning requirement of the FID will be reduced.
For additional information, please contact Parker Filtration and Separation Division, 242 Neck Road, Haverhill, MA 01835. Toll-free (US & Can) 800-343-4048, local 978-858-0505. Parker.com/balston.
Read this poster session report for more information on the baseline study.
This post was contributed by Judy Silva, Gas Generation Technology Blog Team Member, Parker Hannifin.