The addition of a Larimax quadrupole mass spectrometer to a vacuum system will yield a lot of useful information to the vacuum user provided that they have some basic knowledge of residual gas analysis (RGA). 

The aim of this application note is to define some of the commonly used terms in RGA and provide a few useful hints on the interpretation of the mass spectra.

The term "residual gas" refers to those gases remaining in a vacuum system when the majority of the atmosphere previously occupying the vessel has been pumped out. Although the residual gases may be present in very small amounts their presence could still be very significant to an industrial process or research experiment where a low level stable background of residual gases is a prerequisite to success.  

It is often found that changes in the residual gases can dramatically affect the macro process taking place in the vacuum system. Frequently encountered problematical residual gas species include oxygen, water vapour and hydrocarbons. 

It is therefore extremely advantageous, if not essential, to utilise a residual gas monitor such as the DQC2000 before, during and after vacuum processes so that the optimum cost effective conditions are met.  

PRINCIPLE OF OPERATION 

All mass spectrometers analyse charged particles called ions using either magnetic or electric fields to separate the ions of different masses. On Larimax instruments, gas molecules are drawn into the source region of the analyser where they are ionised by collisions with electrons emitting from a hot filament. The resulting positively charged particles are selected and focused so that they enter the electric field between the four circular metal rods which comprise the quadrupole mass filter. The electric field is produced by a combination of DC and RF voltages and equations dictate that an ion of unique mass (or more strictly mass to charge ratio) is passed down the filter for any particular DC:RF ratio. Unsuitable ions strike the rods or some other surface and are neutralised. The filtered ions travel in a spiralling but stable trajectory down the centre of the rod system and are collected by the detector. The currents carried by each ion are very small (around 10-19A) so an amplifier system is used to achieve measurable signals.

The so called mass spectra, that are produced by scanning the DC:RF ratio and measuring the corresponding ion current, is characteristic of gas composition and often acts as a "fingerprint" to aid qualitative analysis.

INTERPRETING THE MASS SPECTRUM

All Larimax RGAs have an on-screen display of the mass spectra in the form of a Bar Graph. This readout has an x-axis scale in terms of mass (amu - Atomic Mass Units) and a y-axis which is annotated as partial pressure but stems from the ion current mentioned above.

The first question of the uninitiated is probably "What do all of these lines or bars mean ?"

Although at first sight the mass spectra may appear complex, interpretation is made easier by first identifying the major peaks (tallest bars). The cracking pattern table at the end of this leaflet will be a useful aid to identification.  

Locate the major peak number in the "Base Peak" column of the table. This will identify the component or components contributing to partial pressure at this mass. On some Larimax instruments an automatic Library Search function is provided so that this process can done more easily.

Once each major peak is identified, its associated minor peaks can be eliminated from the spectrum. This process should account for most of the peaks and in RGA it is not usually necessary to assign identities to the remaining small peaks. If positive identification is required then the above procedure should be repeated using smaller and smaller unassigned peaks as the base peak.

OVERLAPPING FRAGMENT PEAKS 

There are occasions were the partial pressure detected at a particular mass number can be the result of contributions from several gas components. This can occur if the same fragment is produced e.g. -CH2OH at m/z 31 from methanol, ethanol, propanol etc or if fragments are chemically different but with the same nominal mass e.g. CO and N2 both at m/z 28.

Sometimes minor peaks can be used to help resolve these overlaps, but it is always an advantage to have a knowledge of which gases and vapours have been introduced into the vacuum system. For example, unless acetone or n-butane have been used in the system or for cleaning components, a peak at mass 43 is likely to have originated from rotary pump oil.

Unless ethylene or ethane has been used, the most likely source of a mass 28 peak is nitrogen or carbon monoxide or a mixture of the two. The minor peaks at 14 and 12 can help to estimate the relative amounts of each although in practice CO will always be present to some extent due to the reaction of carbon compounds with oxygen at the heated filament of the mass spectrometer ion source as well as the filaments of ion gauges etc.

SOME COMMON TERMS USED IN RGA 

Atomic Mass Unit (amu)

The unit used to compare the mass of particles (atoms, fragments, molecules)

1 amu = 1.66055 x 10-27 kg

A pump used in conjunction with a high vacuum pump so as to lower the pressure of the exhaust side and allow correct operation. Required because pumps such as turbomolecular pumps cannot operate with UHV on the inlet side and atmospheric pressures on the outlet. The outlet side needs to be lowered in pressure by a rotary backing pump. 

The movement of gases or vapours under molecular flow conditions in the opposite direction to the gas flow or pumping  

Term used to describe the process of heating a vacuum system to reduce the level of adsorbed and absorbed contaminants

The largest peak in the spectrum of a pure compound

A device, consisting of a vessel containing a liquified gas such as nitrogen, which is included in a vacuum system to improve the pumping performance. Vapours, including backstreaming pump oil, condense on the cold surface, causing a lowering of pressure and contamination risk. Less efficient systems, aimed at removing water vapour, use standard refrigeration coils instead of liquid nitrogen. 

The tabular or graphical representation of the peaks in the mass spectrum of a pure compound. The heights are usually made relative to the base peak which is given the nominal value of 100 (or 1000). 

A mass spectrum characteristic of a vacuum system under a particular set of conditions

A vessel filled with molecular sieve or activated alumina to trap backstreaming pump oil vapours. It is normally fitted between the high vacuum pump and rotary backing pump.

An atom, molecule or molecular fragment which has gained or lost one or more electrons and therefore carries an electrical charge. 

Ingress of gas, usually air, into the vacuum system from the surrounding atmosphere. This kind of leak may be detected using a helium leak detector. 

This term is used to describe the phenomenon of outgassing of vapours and gases, most frequently water and hydrogen, which prevents the attainment of the required operating pressure. A mass spectrometer is required to identify this type of leak since no helium will leak in from the outside as with real leaks.

The average distance between the molecules of a gas under a given set of conditions.

MILLIBAR (mbar)

Unit of pressure which approximates to 1/1000 of an atmosphere 

1 mbar = 0.76 torr 

Minimum Detectable Partial Pressure

The smallest partial pressure that the mass spectrometer can detect

The state of gas flow that exists when the mean free path of molecules is greater than the dimensions of the vacuum vessel such that molecules collide with the vessel walls rather than each other. This typically occurs at pressures below 10-4 mbar 

The contribution that a particular gas component ( more strictly ion of a particular m/z) makes to the total pressure. 

The relative height of the base peak of a component compared to nitrogen measured at the same partial pressure. Different gases are ionised with different efficiencies in the ion source of a mass spectrometer. Nitrogen is given the relative sensitivity value 1.00. The indicated partial pressure of a peak on a mass spectrum should be divided by the relative sensitivity factor to obtain the "true" partial pressure.i.e that pressure which correctly indicates its actual abundance in the gas sample. 

Gas Rel.Sensitivity

H2 0.70

He 0.23

CH4 1.08

H2O 1.17

Ne 0.24

CO 1.09

N2 1.00

O2 0.62

Ar 1.16

CO2 0.90

The ability of the mass spectrometer to separate adjacent mass peaks e.g. the resolving power or resolution is 44 by the 10% valley definition if peaks at masses 44 and 45 are of equal height and the valley between them is 10% of either peak. 

Resolution and Sensitivity are mutually interactive and an increase in one automatically demands a decrease in the other.

The smallest detectable partial pressure change that can be "seen" by the mass spectrometer

The abundance sensitivity is the ability of the mass spectrometer to detect small amounts when these peaks are adjacent to major peaks in the mass spectrum e.g. the nitrogen isotope peak at m/z 29 which follows the base peak at m/z 28

Unit of pressure where 760 torr equals one atmosphere.

1 torr = 1.316 mbar 

The sum of all of the partial pressures detected within the vacuum system. 

The gas flow condition where the mean free path is less than the dimensions of the vacuum systems and collisions between molecules occur rather than impact with the walls of the chamber. This typically hapens at pressures above 10-2mbar.

FINGERPRINT 1.

This is the characteristic mass spectrum of an air leak. Masses 28 and 14 are from nitrogen, 32 and 16 from oxygen, 40 from argon and 44 from carbon dioxide. To locate the leak set your mass spectrometer to Leak Detect mode and spray helium around joints and seams until an increase in helium partial pressure is indicated by the audible alarm. 

The presence of water vapour is indicated by peaks at 18 and 17. The problem can be solved, temporarily at least, by baking the system to 250C for several hours. It is preferable to use an oven and to allow natural slow cooling to avoid damage from thermal shocks.

FINGERPRINT 3

If the characteristic fingerprint of rotary pump oil is significant in your spectrum, then it is an indication that a foreline trap is either not fitted or requires reactivation.

In an otherwise clean system

the outgassing of CO and CO2 from the filaments and ion source can produce the significant peaks in the spectrum. This can be reduced by running the "Degas" facility on your mass spectrometer and ion gauge.

FINGERPRINT 5

A "clean" high vacuum will usually have a mass spectrum with a relatively high hydrogen peak due to gas desorbing from metal surfaces. Mass 28 from carbon monoxide will always be present at a level dependant on the number and type of filaments operating within the vacuum system.