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Next: 4 Operational and Maintenance Up: GC/MS Buyer's Guide Previous: 2.4 Data Systems

3 Physical Plant Requirements

GC/MS systems range from large scale instruments requiring industrial power sources, occupy large rooms and require specific environmental controls such as air and water cooling to instruments that fit on ordinary laboratory bench tops. Even these systems have certain requirements, however, and the physical layout may affect ergonomic usability.

The GC/MS system can best be thought of as two instruments complete with a computer system and at least one mechanical vacuum pump. For the smaller bench top systems, the GC is typically the larger component, with a typical footprint of about 30 inches square. The MS portion must be installed in a very specific location in relation to the GC, but for benchtop instruments is generally smaller. The computer system will usually consist of a CPU unit, keyboard, mouse, monitor and printer. If additional components, such as an autosampler, headspace sampler, purge and trap unit or cryotrap unit is used, additional bench space will be required for the components and associated controlling electronics.

Power requirements vary for different manufacturers and even different models. Some instruments operate on normal 15 Amp 120 V AC, but some require 20A 120 V AC, and still others require 13 or 15 A 220 V AC. There are numerous components that need power: computer CPU, monitor, printer, Gas Chromatograph, Mass Spectrometer and the mechanical vacuum pump, plus power for additional components. As much of the system is electronic, the system components last longer and are more reliable with so-called clean power; that is, the use of adequate surge protectors is recommended. Some users also install Uninterrupted Power Supplies to further protect the system.

In addition, there are protections that can be installed in the vacuum system, such as pump exhaust mist eliminators/filters and foreline traps that reduce pump oil backstreaming. If toxic materials are used in high throughput, some form of filtration or proper venting of the pump exhaust is highly recommended.

System layout can be linear with the computer on either end (the MS end or GC end), though an L shape is more user friendly than a linear layout. The L shape layout allows the operator to work at the computer station with easy access to the other hardware components, without the excessive movement that is required with a linear layout.

The system will require approximately 50 psi carrier gas for the chromatography system. Unless there is some compelling reason to use another gas, most mass spectrometer users use high grade (5.0) helium. Even with the use of 5.0 grade helium, inline moisture and oxygen traps are the minimum recommended additional treatments. Triple Filters that scrub oxygen, moisture and general hydrocarbons can be obtained in a single unit. These units have replaceable filters that can be changed without opening the carrier gas line to atmosphere.

If chemical ionization is used, reagent gases will also be needed. Common reagent gases include methane and isobutane, though others are used for specialty analysis. If chemical ionization is used routinely, full sized bottles of reagent gas may be required; however, smaller bottles (such as lecture bottles) may be suitable if chemical ionization is only needed occasionally.

Maintenance of the vacuum system, specifically leak detection, will require a leak detection gas. Commercial leak detectors are generally calibrated to detect helium, but argon is also a common choice if the mass spectrometer itself is used as a leak detector.

next up previous
Next: 4 Operational and Maintenance Up: GC/MS Buyer's Guide Previous: 2.4 Data Systems
John S. Riley, DSB Scientific Consulting