The purpose of this Application Note is to answer some of the questions that are commonly asked regarding the Larimax Range of Residual Gas Analyzers (RGA's). It is of course no substitute for discussing your individual requirements with your local Larimax Distributor who has been trained to help you select the most appropriate system for your needs and then get the best performance from your Larimax unit in service.

Most vacuum systems are fitted with a total pressure measurement device such as an ion gauge, Penning gauge or Pirani gauge. These gauges will indicate how much gas remains in the vacuum system but will not give any information on which gases are present. Often this is not a limitation to the user until the required level of vacuum cannot be reached. All a vacuum gauge can do is to confirm this but a Larimax quadrupole mass spectrometer can identify the problem in seconds. The increase in pressure may be due to an air leak, hydrocarbon contamination, excess water vapor or maybe the presence of some other species such as a cleaning solvent. The quadrupole will permit unambiguous identification and assist in the elimination of the problem thus preventing unnecessary delays. 

If the mass spectrum produced by your Larimax RGA is plotted on a printer or displayed on a screen, a series of peaks will be seen corresponding to the relative amounts of charged particles  formed in the mass spectrometer analyzer from the gas molecules present within your vacuum system. A characteristic pattern of peaks is produced under certain circumstances. For example if there is an air leak on the system there will be large peaks present at masses 28 and 32 indicating the presence of nitrogen and oxygen. These characteristic spectra are called fingerprint spectra and more examples are given in the Application Note "Residual Gas Analysis - All You Need To Know".

Yes, all Larimax RGA's have built in facilities for helium leak detection and there is automatic tuning to mass 4. Units are provided with both visual displays of increasing partial pressure and also an audible alarm so that leaks can be identified when spraying helium at suspect joints even when the operator is out of sight of the instrument front panel. There are of course dedicated instruments manufactured for this task but a major advantage of the Larimax approach is that any gas can be used by simply re-tuning from mass 4 for helium to the appropriate mass for your preferred search gas.

An Larimax RGA can be used on all vacuum systems as a combined residual gas analyzer/ leak detector/total pressure gauge. In many cases our RGA can provide essential information about the vacuum process. Typical applications include:

 The maximum operating pressure is dictated by the need to protect filaments and electron multipliers from damage but also due to divergence from linearity of response due to a physical effect relating to the mean free path of the molecules. At higher pressures, the charged particles (ions) formed in the ion source of the RGA are much more likely to collide with neutral molecules than they are to pass through the mass filter and be detected. The resulting mass spectrum is grossly distorted to the point of being unusable.

The maximum pressure with the Faraday detector is 10-4mbar and 10-5mbar with the electron multiplier (SEM).

Various high pressure sampling methods are available, see your Larimax Distributor for details.

The lowest detectable partial pressure is currently 10-13mbar and this is achieved using the electron multiplier detector that is an option on all Larimax RGA's 

This depends on several factors including the choice of Larimax instrument and the pumping speed of the vacuum system on which our analyzer is mounted. The following table, giving leak rates in mbar ls-1, is therefore only a guide:

This will depend on your requirements, if in doubt choose the dual detector version.

Faraday detectors are robust, may be operated at higher pressures and are more stable. However, due to the small ion currents involved, they have slower response times at higher amplifier gains. 

Secondary Electron Multipliers (SEMs) have faster response times and are more sensitive than Faraday detectors. SEM's are more expensive, they cannot be used at higher pressures and they need to be calibrated against the Faraday detector to adjust for changes in their gain factor. 

Yes, Larimax can offer multiplexer units for their range of RGA's that permit up to 8 RF Head/Analyzer combinations to be connected to one control unit. 

This will depend on the operating conditions, usually the higher the operating pressure the shorter the filament lifetime. The presence of oxygen and corrosive gases will accelerate the aging process as will frequent use of the "Degas" facility. If the normal working pressure is around 10-7 mbar then a filament will typically last for many months. If the filament is inadvertently switched on at atmospheric pressure it will burn out immediately! 

Once the analyzer has been de-mounted from the vacuum system, the filaments can be exchanged in a few minutes. The filaments are factory pre-aligned so that there are no special skills required to correctly position them. The tools required are provided in the spares kit which can be purchased from Larimax.

When a filament burns out there is a visual indication consisting of either a flashing red light or an on screen "O/C" display depending on the instrument.  

Providing that our instruments are operated in accordance with the suggestions in our Instruction Manuals, there is no routine servicing required. The analyzer will become contaminated during use however and after a period of time, which depends on the operating conditions and the performance required, it will need cleaning. This cleaning procedure is explained in the manual but we also offer a factory reconditioning and exchange analyzer service. Ask your Larimax Distributor for details.

Cleanliness is the key to all successful vacuum work and our analyzer should only be handled by operators wearing suitable gloves such as those included in the spares kit. 

A spares kit is included with every new instrument but it is recommended that an additional pack of filaments is purchased so as to avoid unnecessary instrument downtime.

If you are going to bake-out your vacuum system, you should remove the RF Head from the analyzer to prevent thermal damage to the electronics. We recommend that the maximum bake-out temperature for the analyzer should be 250 C. A maximum of 350 C can be used but the life of the feedthrough assembly will be reduced. The potential problem is due to the small different thermal expansion properties of the metal and glass parts of the electrical feedthrough. If the baked system is cooled too quickly the metal pins will contract faster than the ceramic disc resulting in the parting of the two surfaces to introduce a leak into the system.

Most Larimax RGA's have an ion source degas facility which automatically increases the rate of electron bombardment so as to eject gas molecules adhered to source components. This facility, which makes our units compatible with true UHV operation, is automatically timed on some units so that operators can leave the instrument unattended.

Larimax instruments show the mass spectrum directly on a built-in VDU or on the P.C. monitor display with calibrated axes.

It is very easy to check that the mass scale is correctly aligned by looking for expected groups of peaks, e.g water peaks at 17 and 18. The Analogue Display of Larimax RGA's shows these peaks as simulated chart recorder traces and a quick examination can show if the peak centers are located above the correct mass markers. If not then the easy adjustment routine is explained in the Instruction Manual. 

 After initial warm up, the peak location stability is better than +/- 0.1 amu over 8 hours. 

All Larimax RGA's are calibrated against standard ion gauges so that their pressure measurement ability is within specification. Normally total pressure measurement on quadrupoles can differ somewhat from that measured by an ion gauge. This is due to the inability of the mass filter to collect all of the ions of different masses simultaneously.

The ability to detect small partial pressures i.e. sensitivity, and the requirement to separate adjacent masses, which is called resolving power or resolution of the mass spectrometer, are mutually interactive. In order to increase the resolution one must decrease the sensitivity. Larimax RGA's are factory set such that the resolution is 10% valley across the whole mass range. This means that if two adjacent peaks e.g. 44 and 45 are of equal height, then the valley between them will be no greater than 10% of either peak height. Trim-pots on the RF Head allow the operator to re-adjust the resolution such as may be desirable if more sensitivity is required in instances where adjacent peak resolution is not involved. Or conversely where more resolution is necessary but sample volume is not a problem. 

If two gases are present with the same nominal molecular weight e.g. nitrogen and carbon monoxide both m/z 28, it will not be possible to separate the two molecular ions. A resolution of 2500 is needed to do this so that small mass differences can be detected. In some instances, under suitable conditions, it is possible to examine the fragmentation patterns of the two gases and then select minor mass fragments to aid identification. For instance in a 50:50 mixture of nitrogen and carbon monoxide, the mass 12 peak can only be produced from the carbon compound therefore allowing the amount of its contribution to the 28 peak to be estimated and deducted leaving the remainder as being due to the nitrogen.