Compressed gases such as nitrogen and zero air are critical components in laboratories that run liquid chromatography with mass spectrometry (LC-MS). Nitrogen is used to nebulize the mobile phase and also serves as an inert gas when fragment ions are generated in the mass spectrometer. Zero air is often used in the analysis for aromatic hydrocarbons.
The source and rate of consumption of nitrogen and zero air are key factors in optimizing performance.
There are two ways to obtain nitrogen and zero air for LC-MS analysis:
- Cylinders containing zero air and nitrogen
- In-house gas generation
The challenge of cylinders
Many laboratories use high-pressure gas cylinders to supply gases to analytical instruments. These cylinders are typically filled to 2000 psi. Although cylinders are commonly used, they have several disadvantages:
- Safety risks: High-pressure cylinders must be carefully handled (leaks or explosions could cause injury).
- 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.
The advantages of in-house generation
The safety hazards and inconveniences of cylinders can be eliminated by switching to an in-house gas generator. Both purified nitrogen and zero air can be readily obtained from compressed air using an in-house generator on a 24/7 basis, whenever needed. Advantages include:
- In-house generators are safe. They operate at low pressures and store small volumes of pressurized gas.
- 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, especially compared to high-pressure gas cylinders.
In-house generation of gases
In-house generation of nitrogen from compressed ambient air involves the separation of the gas from oxygen, water vapor, and particulate matter and porting it directly to the LC-MS system. A Parker Nitrogen Generator produces a continuous supply of up to 99.5% pure nitrogen at a maximum pressure of 145 psig. Dust and particulate matter are removed by a pre-filtration system. An oxygen monitor with an audible alarm is also provided to signal a high oxygen concentration. Zero air can be generated by passing compressed air through pre-filters which serve to remove oil, water, dust and particulate matter. Hydrocarbons that may be present are removed by passing the filtered air through a heated catalytic convertor to form CO2 and H2O. The compressed air is then passed through another particulate filter to remove dust and other solids and delivered to the analyzer. A Parker Zero Air Generator produces air with less than 0.01 ppm total hydrocarbons at flow rates of up to 280 slpm.
The cost of operating an in-house gas generator is extremely low, since the only raw materials required are air and electricity. Running and maintaining a gas generator for an LC-MS system typically costs only a few hundred dollars a year. Return on investment takes only about 12 months, depending on the specific usage and required purity. This is a huge ongoing savings compared to the costs of tanks. When considering factors such as electricity used, maintenance, cylinder cost, demurrage, labor, order processing, shipping, invoice processing, and inventory control, operating a tank delivery system is significantly more expensive than using in-house gas generation.
This post was contributed by Judy Silva, Gas Generation Technology Blog Team Member, Parker Hannifin.