Providing Gases for LC-MS Systems in a Safe, Convenient, and Cost-Effective Manner

A liquid chromatographic system with a mass spectrometric detector (LC-MS) requires a supply of nitrogen as the curtain gas, pure zero grade air as the source gas, and dry (–40 °F dewpoint) air as the source exhaust gas. In many facilities, these gases are provided by a series of compressed gas tanks; while the use of gas tanks can meet the overall requirements of the system, this approach imposes a number of serious operational, safety, and economic disadvantages. From an operational standpoint, the tanks must be replaced periodically, requiring operator involvement and reducing the overall uptime of the system. In addition, the handling of compressed gas tanks introduces a significant safety risk and is an expensive way to supply the necessary gases.

A considerably safer, more convenient, and cost-effective method of providing the necessary gases is via in-house gas generation. An in-house gas generator is a compact system that can be located in the facility directly alongside the LC-MS system and operate on a continuous basis with a minimal amount of operator interaction. This paper describes how gas generation systems can provide the various gases from laboratory air, discusses the benefits that arise from their use, and portrays a number of cases in which significant benefits have been achieved from self-generation of the gases.

In-house generation of gases for LC-MS

In-house generation of gases such as nitrogen, zero air, and source exhaust air from laboratory air involves a number of discrete processes to separate the various components of air into a series of gas streams of the desired purity that can be directly ported into the LC-MS. A variety of gas generation systems are available that can provide a single component (e.g., nitrogen), and many facilities acquire a generator for a single gas since the other gases (e.g., dry air) may already be available in the facility.

Figure 1 - Schematic representation of Parker Balston model LCMS-5000 tri-gas generator. (Reprinted with permission from Parker Balston Source LC/MS TriGas Generator Series Model LC-MS 5000 Technical Bulletin, 2006.)

In recent years, the various components of gas generation systems have been integrated into a single system to generate the assorted gases that provide all of the gases required for LC-MS. This offers a significantly greater degree of convenience, system control, and cost savings for the chromatographer. The general design of an integrated system (Parker Balston model LCMS-5000 tri-gas generator [Parker-Hannifin Corp., Filtration and Separation Div., Haverhill, MA]) is presented in Figure 1 (gas generation systems that are designed to provide only one of the gases [e.g., nitrogen] contain only those components relevant for generation of the desired gas).

The input for the generator is laboratory air, which is divided into three streams to generate the desired gases. The generator includes the following major components to produce the various gases:

  • Compressor—pressurizes ambient laboratory air to 110–140 psi (7.5–9.6 bar). An oil-less rotary scroll compressor is employed, which consists of two identical spirals offset 180° with respect to the other so that the scroll mesh is used to compress the air. An after-cooler cools the temperature of the discharged air. The cooling serves to extend the system life and condenses much of the water in the compressed air, which is sent to an electric drain trap and is automatically discarded.
  • Coalescing filters—remove additional water and particulate matter (as small as 0.01 μm) from the compressed laboratory air. Drains collect the liquid that is accumulated, and automated valves allow the liquid to be sent to waste. This filter protects the hollow fiber membrane that generates nitrogen (see below) and associated components from contamination that may foul the operation.
  • Activated carbon module—removes hydrocarbon contaminants that may be present in the air. One module is located before the hollow fiber membrane, and a second carbon module is located after the hollow fiber membrane to ensure that research-grade purity gas is supplied to the instrument. A filter is placed after the carbon module to trap any carbon particles.
  • Figure 2 - The hollow fiber membrane bundle separates nitrogen from air. Oxygen and water vapor permeate the membrane, allowing nitrogen to flow through the tubes. (Reprinted with permission from Parker Balston Analytical Gas Systems Bulletin AGSB, 2006, p. 32.)

  • Hollow fiber membrane bundle (for nitrogen)—consists of a series of hollow fiber membranes that permit oxygen and water vapor to permeate it and escape through the sweep port while the nitrogen flows through the tube, as shown in Figure 2. While each individual fiber membrane has a small internal diameter, a number of fibers are bundled together to provide an extremely large surface area for permeation of oxygen and water. After the membrane bundle, the nitrogen passes through another carbon filter and is allowed to flow directly to the outlet port at a flow rate of up to 10 L/min at a pressure of 80 psi.
  • Dryer membrane (for dried air)—permits water vapor to permeate the hollow fibers of the membrane, resulting in dry air. A small portion of the dry air is redirected along the fibers to sweep out additional water vapor. Dried air from the dryer membrane is allowed to flow directly to the outlet port. Air with a dewpoint of –40 °F is delivered at a flow rate of 28 slpm at a pressure of 100 psig.
  • Catalyst module (for zero air)—oxidizes hydrocarbons into CO2 and H2O. A coiled copper after-cooler and fan are employed to cool the hot outlet air. The cooled zero air passes to the zero air outlet port at a flow rate of up to 23 L/min at a pressure of 110 psig.

The detailed specifications of the various gases provided are summarized in Table 1. The in-house gas generator can supply a continuous stream of the requisite gas(es) for the LC-MS system at the flow rate and pressure necessary to maintain operation of an LC-MS system, and systems are available to provide the necessary gases for a single or two LC-MS systems.

Operation of an in-house gas generator is straightforward. Once the system is installed, no day-to-day maintenance or user interaction is required. The system contains no moving parts, except for the compressor. On a periodic basis, the filters should be replaced; typically they are replaced after approx. 5000 hr of operation or on a yearly basis to ensure optimum performance.

Comparing in-house generation of gases with the use of tank gases for LC-MS

An in-house gas generator offers a number of significant benefits to the operator of an LC-MS system relative to the use of tank gas. These include a dramatic improvement in safety, a reduction in the inconvenience of changing and handling tanks, and a significant lowering of the cost of supplying the gases.