Gas Generators – Part 2A – Nitrogen Generators15 May 2023
Chromatography companies and analytical laboratories often look at nitrogen generators as belonging to one of two different categories, those for LC and those for GC. The main difference between the two is the flow rate and purity needed for each analysis.
Nitrogen in LC-MS applications may be used for a variety of purposes including nebulizer gas, curtain gas, collision gas and for sample introduction/solvent removal. LC-MS typically needs much higher nitrogen flow rates than GC. At these higher flow rates, output purity is often lower. However, it is not unusual for 95% pure nitrogen to be acceptable for LC-MS instruments. Having said this, please review your instrument manual for required flow rate, line pressure and purity.
When used as a gas for GC, flow rates are often much lower, but the purity required is often much higher (than for LC-MS). When used as a carrier gas, 99.9999% purity is not uncommon. When used as a make-up gas, you may see 99.999% recommended by instrument manufacturers. Even though these generators are capable of ultra-high purity, you still may want to include an oxygen and moisture indicating gas filter between the generator and instrument(s) just in case something, such as a leak, compromises the purity of the nitrogen.
Before we discuss nitrogen generators and their maintenance consumables, I will provide a brief description on how these generators work. Air, which is approximately 78% nitrogen, needs to be supplied to the generator. This air should be clean, dry and between 60psi and 125psi.
There are several different techniques which are common for producing high purity nitrogen from air. One technique is called membrane separation. As the name implies, semi-permeable membranes are used to separate the nitrogen molecules from the other (impurity) gas molecules. A second technique is called pressure swing adsorption (PSA). Once again, as the name implies, adsorption is used to purify the nitrogen. Compressed air is forced through packing material where the nitrogen molecules pass through the packing while other gas molecules are trapped/adsorbed.
I will begin with GC because Restek sells one nitrogen generator capable of 99.9999% purity. The Restek part # is 20697 (Parker # UHPN2-1100). This generator uses a combination of filtration and PSA (described above) to achieve ultra-high purity output.
A source of clean, compressed air is required. Specifications for air are listed on page 7 of the manual (an excerpt is pasted below).
Compressed Air - The Parker Balston Nitrogen Generator requires a source of clean, dry compressed air for optimal operation. The compressed air should be as close to instrument quality as possible and supplied at a dewpoint less than or equal to 60°F (15°C), and at a pressure between 60 psig and 125 psig (4.1 barg and 8.6 barg). If the incoming air pressure is less than 60 psig (4.1 barg), an alarm will beep, and the system may go into a modified start-up mode (see "Pressure Interruptions" for more details). The supply air should be at room temperature and relatively free of water, compressor oil, hydrocarbons, and particulate matter. If the compressed air supply has excessive oil and water carryover from the compressor, install a prefilter upstream from the generator.
- Hydrocarbon removal.
- Removal of oxygen, carbon dioxide and water vapor.
- Final filtration.
You may be asking yourself “What maintenance is required for this generator?” On an annual basis, several filters will need replaced. These filters can be found in Restek 21655 (Parker # MK7694).
- 1st stage prefilter (Parker # 100-12-BXE)
- 2nd stage prefilter (Parker # 050-05-BX)
- 3rd stage prefilter (Parker # 050-05-BX)
- Final filter (Parker # GS050-05-95)
In addition, every three years the hydrocarbon filter will also need replaced. The Parker # is 76133.
As you can see, obtaining high purity nitrogen is as simple as a source of compressed air and replacement of four filters every year. To me, that sounds easier than replacing gas cylinders.
In my next post, Gas Generators – Part 2B – Nitrogen Generators, I will discuss the generators (and maintenance kits) which Restek offers for LC-MS. Stay tuned.