Frequently asked Questions

Q) What is the shelf life of an Electron Multiplier?

A) Contained in its original packaging, the shelf-life of an ETP multiplier is guaranteed for up to two years from the date of shipment.

Q) How do I get rid of background noise?

A) Excessive noise due to absorbed moisture may be rectified by leaving the multiplier under high vacuum for 24 hours. Do not increase the applied voltage to a multiplier to overcome background noise as this can cause permanent loss of gain.

Q) How do I store a multiplier outside original packaging?

A) It should be kept in a clean, dust-free, low-humidity environment, such as a glass desiccator.
Care should be taken to minimize exposure of the multiplier to dust or other airborne contamination. Dust particles within the detector may result in increased background noise during operation.

Q) How do I precondition a Multiplier?

A) ETP Ion Detect generally require no preconditioning if stored correctly.

Q) What operating pressures can be used?

A) Operating pressure should be as low as possible for best lifetime and minimum noise. To avoid risk of damage due to discharge or arcing, the multiplier should not be operated at a pressure in excess of 1 x 10-4 Torr.

Q) What voltage should I apply to my Multiplier?

A) Use the minimum voltage possible to achieve operational parameters. Applying the maximum rated supply voltage to a new multiplier can cause damage and permanent loss of gain. Do not increase the applied voltage to a multiplier to overcome background noise as this can cause permanent loss of gain.

Q) How do I gain optimum life from an ETP multiplier?

A) Operational life can depend on the following factors:
Operating Environment: Lower operating pressure and lower partial pressure of hydrocarbons in the vacuum chamber will result in increased operational life, all other conditions remaining the same.
Operating Gain: Operation should always be at the lowest applied voltage consistent with good signal acquisition. In general, the lower the gain, the longer the life of the multiplier. The gain of an electron multiplier will fall gradually over time, requiring the applied voltage to be periodically increased. This is a normal part of the multiplier aging process. 
Output Current: Multiplier lifetime is inversely proportional to average output current. Excessive input signals should be avoided, especially when operating at high gain.

Q) What happens to the multiplier if the vacuum fails?

A) Rapid degradation of multiplier performance may result from failure of the vacuum pumping system. This may cause severe contamination of the detector or damage due to arcing caused by the sudden increase in chamber pressure. Degradation caused by vacuum failure may be permanent and irreversible, especially if the multiplier supply voltage was ON during the vacuum failure.

Q) Can I clean my multiplier?

A) ETP does not recommend washing or cleaning of multipliers.

Q) At what rate is a multiplier “used up”?

A) There are many reasons that a multiplier may exhibit an apparently shorter life. Firstly, it needs to be understood that the life of a multiplier is properly described in terms of usage (it is not measured in terms of years). The measure of 'usage' is the total amount of charge (accumulated number of electrons in the output signal) that has been drawn from the multiplier. So, if a multiplier is in a system that operates only 8 hours per day, it will have an 'apparent life' that is three times longer than one in a system operating 24 hours per day, even though it will have had the same usage. 

The electrical current, I, seen at the output of the multiplier is determined by:
I= qNG

where q is the charge on an electron, N is the number of ions per second being detected, and G is the gain of the multiplier. So, the amount of electrical current supplied by the multiplier is directly related to the number of ions detected, and the operating gain of the device.

Consequently, if a multiplier is operated at higher gain than is required for good signal-to-noise, then its life will be used up more quickly than it would be if it were operated at a lower gain. For example, if the ion count rate remains constant and the multiplier gain is increased by a factor of 2, its output current will be twice as large, and it will be used up at twice the previous rate.

This effect can be particularly dramatic in pulse-counting systems. In pulse-counting applications, good detection efficiency occurs when the large majority of output pulses from the multiplier are above the set threshold of the detection electronics. For longer multiplier life it is desirable to operate at as low a gain as possible while maintaining sufficient pulse amplitude to exceed the threshold of the detection electronics. In this case, the optimal operation is obtained by setting the threshold as low as possible without increasing the background noise level. This will allow good detection efficiency to be obtained

Q) What is the principal aging mechanism for an Electron Multiplier?

A) The principal aging mechanism for an electron multiplier is the "stitching" of hydrocarbon contaminants in the vacuum onto the high gain surfaces of the multiplier during use. By removing or reducing the partial pressure of hydrocarbon contaminants in the vacuum, this mechanism of gain loss can be slowed significantly.
The main source of hydrocarbon contamination is usually the pumping system. Back streaming rotary pump oil is a recurring culprit for introducing contaminants into the vacuum chamber (even in systems incorporating turbo molecular pumps). A fore line trap between the rotary backing pump and the main (turbo or diffusion) pump is recommended. An oil free pumping system gives the best results.

In diffusion-pumped vacuum systems, chilled baffles above the diffusion pump will prevent pump oils from entering the vacuum chamber, and extend multiplier operating life.
How effective these steps are in extending multiplier life for a particular system depends on how "dirty" the system was initially, and on how effective the trapping measures are in removing the contaminants. This, of course, will vary from system to system, so it is difficult to make a quantitative prediction.