Quadrupole Service Self-Help from ESS

Sensitivity	Bad Peak Shape	Gross Charging Effects	Adjusting Resolution
Adjusting Mass Position	Emission / Filament failure	Loss of Signal / Noisy Signal	Leaking Feed-Throughs

Sensitivity

Loss of or reduction in sensitivity is generally the result of contamination in the ion source, on the extraction and focus plates or on the ends of quadrupole rods. Some of these symptoms are also associated with Gross Charging Effects or Bad Peak Shape, depending upon the location of the contamination. Typical Causes: Normal use in sampling systems where heavy elements, high water vapour content, tars or particulates are regularly monitored. Results of vacuum accidents such as evapouration of Tungsten filaments, back-streaming of rotary pump fluids or diffusion pump vapour contamination. Contamination from materials within and RGA monitored process such as Sputtering or vacuum Coating.

Remedial Action:

First of all, the extent of the contamination needs to be assessed by removing the analyser from the vacuum system and separating the ion source from the mass filter. Contamination of source parts such as Source Electrode and lenses is best dealt with by replacing the contaminated parts with new ones. Some results can be obtained by cleaning the electrodes with a fine crocus paper or glass-fibre brush and then cleaning in a solvent such as Ethanol or Acetone in an ultrasonic bath. Care should be taken not to immerse coated filaments such as Thoriated Iridium or Yittria in a solvent bath.

If the quadrupole shows signs of contamination on the rod ends, it will almost certainly require specialist cleaning. Very light contamination may sometimes be removed using an ultrasonic bath and solvent, though this is unlikely to remove visible contamination effectively.

 

Bad Peak Shape

Peak shape is often effected by electrode contamination or quadrupole rod-end contamination although other electronic effects such as poor source regulation or high ripple levels on rod DC supplies and detector supplies is a possibility. Ion Energy settings have considerable effects on peak shape, so check that you are using a compatible Ion Energy setting first.

Remedial Action:

Check that there is no ripple present on the source supplies and source electrodes. Check that the detector supply is stable and test for ripple on the rod DC supplies. If all supplies check out OK, the most likely cause is rod-end contamination of the quadrupole.

If the quadrupole shows signs of contamination on the rod ends, it will almost certainly require specialist cleaning. Very light contamination may sometimes be removed using an ultrasonic bath and solvent, though this is unlikely to remove visible contamination effectively.

 

Gross Charging Effects

Charging effects are often the result of contamination to the extraction and focus electrodes or quadrupole rod end contamination. A charging effect is described by an exponential fall-off in peak intensity when switching mass. On rare occasions a very bad (resistive or intermittent) connection can cause charging since it may allow an electrode to "float". Any un-connected electrodes which are "seen" by the ion beam will charge, so check that everything has a connection or is grounded.

Testing for Gross Charging Effects:

You can test your analyser for a gross charging effect by switching mass. Start in a no signal region (mass 5) for about 5 minutes. Now tune to a stable peak such as Nitrogen. Observe the Ion Current intensity over a period of 10 to 15 minutes, if the signal drops away in an exponential manner, the analyser is charging.

Remedial Action:

First of all, the extent of the contamination needs to be assessed by removing the analyser from the vacuum system and separating the ion source from the mass filter. Contamination of source parts such as Source Electrode and lenses is best dealt with by replacing the contaminated parts with new ones. Some results can be obtained by cleaning the electrodes with a fine crocus paper or glass-fibre brush and then cleaning in a solvent such as Ethanol or Acetone in an ultrasonic bath. Care should be taken not to immerse coated filaments such as Thoriated Iridium or Yittria in a solvent bath.

If the quadrupole shows signs of contamination on the rod ends, it will almost certainly require specialist cleaning. Very light contamination may sometimes be removed using an ultrasonic bath and solvent, though this is unlikely to remove visible contamination effectively.

 

Adjusting Resolution

Resolution is generally set using a low mass and high mass adjustment. These controls vary the RF/DC ratio applied to the quadrupole rods. Consult your instrument manual before attempting to adjust resolution, certain older instruments will rely on internal pre-set potentiometers to set resolution whilst newer devices generally allow resolution setting from software.In either case, you will require a sample which can provide adjacent peak groups at a low mass point and  at a mass close to the full range of the analyser. In analytical systems, PFK is typicaly used to provide a resolution and mass alignment standard. In clean UHV applications a gas standard such as Krypton or Xenon is preferable.

If you do not have your manufacturers instructions, click here for a pdf file which covers most VG Quadrupoles® and the ESS GeneSys and ecoSys instrument ranges.

Under Resolved  Correctly Resolved  Over resolved

  If you are unable to set adequate resolution call Technical Support

 

Adjusting Mass Position

Set resolution correctly first !Mass position is generally set using a low mass and high mass adjustment. These controls vary the level of both RF and DC  applied to the quadrupole rods. Consult your instrument manual before attempting to adjust mass position, certain older instruments will rely on internal pre-set potentiometers to set resolution whilst newer devices generally allow resolution setting from software.In either case, you will require a sample which can provide adjacent peak groups at a low mass point and  at a mass close to the full range of the analyser. In analytical systems, PFK is typicaly used to provide a resolution and mass alignment standard. In clean UHV applications a gas standard such as Krypton or Xenon is preferable.If you do not have your manufacturers instructions, click here for a pdf file which covers most VG Quadrupoles® and the ESS GeneSys and ecoSys instrument ranges.

If you are unable to set your mass alignment call Technical Support

 

Leaking Feed-Throughs

Vacuum feed-throughs can break as a result of accidental damage or begin to leak in processes where analysers are part of a continuous bake-cycle.

In general, it is very difficult to repair feedthroughs by any means other than replacement. A temporary solution in applications where baking is not required, is the use of an epoxy material although care must be taken in critical vacuum applications.

 

Emission / Filament failure

Filament or emission failure can be interrelated though emission failure does not necessarily imply filament failure. The most likely cause of filament failure is a vacuum accident although filaments are a consumable item and will fail through normal use.If the filament is open circuit, emission failure will result, although many other factors influence emission current stabilisation. Source contamination, short circuits, Electron Energy supply failure or source regulation electronics/logic failure are all possible factors

Remedial Actions (Filament Failure):

Disconnect the analyser from the head electronics, consult the manual and locate the filament connections on the feed-through. In most small quadrupoles, there will be a common connection which is linked to filament1 and filament 2, on larger instruments a pair of feed-throughs will be used to connect individual filaments.Test each filament connection with a DVM. To operate correctly, most regulating electronics require filament resistance to be <1.5Ω. If the filament is high resistance or open circuit, replace it.If the filament resistance is OK, use a high voltage tester (Megger) with at least 500V pressure to test for shorts and electrical leakage as follows:Filament to> Source >Focus>RF1>RF2 and ground. If all of these return high resistance values, the filament is probably OK.Remedial Actions (Emission Failure):This section assumes that you have already completed the filament tests described above. use a high voltage tester (Megger) with at least 500V pressure to test for shorts and electrical leakage as follows:Source to>ground>focusResistance measured should be high. If a short or leakage is determined, remove the analyser and rectify the problem.If Ion source resistance tests are OK and the operating vacuum is better than 10-5mBA, the problem is likely to be with the electronics. The most probable cause is failure of the emission current supply or the electron energy supply.

Checking the Filament Current Supply:

Check that the filament is within the operating limits described above and that the source resistance tests have been passed OK.Turn off the electronics and connect a DVM set to DC Amps in one side of the filament circuit. Ensure that the meter is able to read at least 5A DC and that the test electrodes are not touching ground. Turn on the electronics units and select emission. A clear DC current in the range 2 to 4.5A should be shown on the meter. This current will switch off automatically after a short period.If current is registered, check the type of filament you are using and determine the correct current limit. If the supply current is below the current limit for the filament material, it will be necessary to adjust the current limit setting on the supply pcb or from the software.If no current is registered, you have a major supply failure or an open circuit between the analyser and power supply.Checking the Electron Energy Supply:It is not uncommon for Electron Energy supply fuses to blow following a filament failure. If you have completed the source resistance tests and are able to obtain filament current, it is possible that the Electron Energy is not present at the filament.Ensure that the power supply is switched off. Remove the DVM from the filament current test and re-connect the filaments. Re-set the DVM to a 100V DC volts range and re-connect it between one side of the filament and ground. Ensure that the meter electrodes cannot short out to ground. Switch on the supply and select Emission. The DVM should register Electron Energy which is typically -70eV. If you have variable electron energy, the meter should match the intended EE setting.

If Electron voltage is not present, check the EE supply fuse on the power supply pcb and replace if blown. If the EE supply is not present and the fuse is OK, the unit will require electronic repair.

 

Loss of Signal / Noisy Signal

A total loss of signal can result from contamination, RF failure, detector supply failure, or detector failure. Noise on the signal can result from vibration, long collector leads or ripple on the detector supplies.

Remedial Actions:

Since very high voltages are present on the SEM detector supplies of most instruments, we are unable to recommend user testing of the supplies. Here are some simple tests which you can perform without risk of high voltage exposure.

Amplifier Chain Test:

The amplifier chain can be tested by removing the collector lead ***take care not to pick up the detector HT by mistake, if in doubt, do not perform this test*** and then setting the instrument to a high gain range. Touching the centre electrode of the collector lead will induce current in the amplifier chain and should result in a large baseline shift. If the baseline moves, the amplifier chain is probably working normally. If you cannot gain access to the collector lead, select a high Faraday gain range and tap the analyser connecting cable or head electronics, some induced baseline shift should be seen.

HT Supply / SEM test:

A similar test of the HT supply and SEM can be made by selecting a high gain range and rapidly adjusting detector HT, a baseline shift should result.

Noisy Signal:

Since the current from the detector to the amplifier is very small, long collector leads or leads which touch vibrating equipment such as rotary pumps, often induce noise. Keep cables as short as possible and try routing them away from noise sources. On certain older VG Quadrupoles, tying the collector lead back on itself or in a circle can considerably reduce noise.

Since tests on the RF supply should not be undertaken, we recommend that you contact us for Technical Support on any other signal loss problems.