When you click the Adjust Detector Gain button, the software checks that the target abundance for mass #2 is in range. If it is in range, no change will be made to the curve for that polarity. If the target abundance is too low or too high, then the detector gain calibration is performed to achieve the target abundance for mass #2. The EM voltage is set to produce a multiplier of 106. That is, one electron striking the input side of the detector will generate 106 on the output side. If the EM voltage setting exceeds 2600 volts which indicates decreasing electron multiplier performance or instrument performance, the user will be warned that system maintenance may be required.
The HED electron multiplier receives an input current generated by the ions striking it, amplifies that current, and generates a proportional output current.
Gain = Output current / input current
Gain is controlled by the electron multiplier voltage (EMV). The higher the EMV, the higher the gain. The relationship between EMV and gain is log linear. This linear relationship is common for all multipliers. Since the slope of this line is constant, the only adjustment that needs to be made is to the intercept. The gain calibration routine simply adjusts the value of the intercept for the specific instrument. The instrument-specific gain curve coefficients are then stored in the LC/MS or CE/MS.
The use of Gain enables the distribution of methods from instrument to instrument. In principle, using the same gain on two different instruments should result in the same signal response, thereby easing method development and instrument portability. Because the relationship between EMV and gain is log linear, a gain of 2.0 should give twice the abundance of a gain of 1.0.
In general, the detector should be run at the lowest gain that will still produce adequate abundance. High gains increase noise as well as signal and often result in poorer signal-to-noise ratios. Increasing the gain increases the EMV, which shortens the life span of the electron multiplier. The maximum EMV is 3000 V no matter how high the gain is set; a gain of 70 or higher will elicit the 3000 V setting.
As an electron multiplier ages, it slowly becomes less efficient. For a given ion current input, it generates a smaller and smaller output current (abundance). Low abundance caused by an aging electron multiplier is not easily distinguished from low abundance caused by poor ion generation or transmission (low ion current input). It may be tempting to compensate for poor ion generation or transmission by increasing the gain (thereby increasing the EMV) even though the electron multiplier is actually performing correctly at its previous gain. The increased gain will improve abundance but may decrease the signal-to-noise ratio and will shorten the life span of the electron multiplier.
See Also