Often following a first installation or a detector gas refill, the pulse height of incoming 14C ions can change.
It is important to set an appropriate discriminator level to account for this change.
\uD83D\uDCD8 How to set the discriminator level without an oscilloscope
Put in a tuning standard and, for best results, let the 12C current stabilize.
Set a single run sufficiently long to perform the study (e.g. Cycle time 10s, 250 Cycles)
Select a starting discriminator high enough that most to all of the 14C counts are filtered out
Reduce the discriminator by 0.1 V, dwelling on each setting for 5 cycles before moving on to the next
Compute the average 14C/12C ratio over each set of 5 cycles and plot them against the discriminator setting.
The average pulse height will be roughly where the 14C/12C ratio of the standard is cut in half.
Set the discriminator to 2/3 this average pulse height
Example case:
An OxII standard was inserted.
A Single run with 250 ten second long cycles was run and the currents were allowed to settle.
Starting at Cycle 100, the discriminator was set to 2.2 V
Every 5 cycles, the discriminator was lowered by 0.1 V until electronic noise began to saturate the count-rate (0.7 - 0.5 V). See the following plot for the view in ACS of this test.
The above data was copied into Microsoft Excel. The average 14C/12C ratio was computed for each segment of 5 cycles and plotted against the corresponding discriminator value as seen below:
Examining the graph, the 14C/12C ratio is cut roughly in half between 2.0 to 2.1 V on the discriminator. This is where the average pulse height is.
Two thirds of 2.1 V yields a setting of 1.4 V as a final discriminator setting.
Is my average pulse height too low?
Note, the average pulse height of a clean detector should ideally lie between 3 to 4 V.
If it drops below 2 V, replacement of the detector gas is strongly recommended.
What causes the pulse height to change?
Particularly in newer systems or following a new detector window installation, impurities in the detector gas due to outgassing can degrade the average pulse height.