Teledyne Leeman Labs Blog

We often get asked if there is an ideal photo multiplier tube (PMT) voltage for the QuickTrace^® M-8000. The truth is the optimal value is unique to each instrument’s detector. It varies from instrument to instrument. During the initialization process, after connecting to the QuickTrace software, the M-8000 will select the voltage that is right for the PMT inside that system. This is the same process as clicking the Auto Select Voltage button in the Instrument Control menu. So, why is there another Set Volts option in that same menu?

The Beer-Lambert Law states that atomic absorbance is linear to concentration. But the detection range of atomic spectrometers is limited by the sensitivity of their detectors. In our case, the photomultiplier is extremely sensitive to the point that its voltage may need to be modified to reduce such limitations. By lowering the PMT voltage (and therefore the sensitivity), the concentration range of a developed method can be extended, and linearity can be maintained when calibrating to higher concentrations. For the QuickTrace M-8000, adjustment of the PMT voltage is often used when the highest calibration standard’s peak height exceeds ~6,000 hF units.

By adjusting the instrument parameters, calibrations and samples can be evaluated for mercury (Hg) over any desired range, from sub-ppt to 400 ppb. There are four parameters that work together to determine a method’s range on the QuickTrace M-8000:

• Carrier gas flow rate
• Peristaltic pump speed
• Sample uptake time
• PMT voltage

Recently, I was working on developing an ISO method 17852 worksheet analyzing wastewater using the M-8000. I had adjusted the flow rate, pump speeds, and sample uptake to get good peak plateaus and low carryover in just 80s. But my highest calibration point (10 ug/L Hg) was within my instrument’s non-linear range. Since I was happy with the parameters, peak shape, and timing, the simplest way to avoid the non-linear range was to lower the PMT voltage. But by how much?

The original 10 µg/L Hg sample had a peak height of 9,200k at a PMT voltage of 773V. I did a study to find the PMT voltage where a 10 µg/L Hg standard would have a peak height of ~2,500 hF units. This target is not arbitrary; I picked ~25% of the current peak height. I wanted the peak height to be well below the threshold, but in practice anything below ~50% would have worked for this method.

I did this by lowering the PMT voltage manually using the Set Volts option in the software, then running a Peak Profile in the Method Editor. I recorded the peak height and PMT values until I found one that was close to my target value. This would scale down the intensity, ensuring the calibration curve linearity will not be limited by the detection range of the system. Peak profiles, PMT settings, and peak heights are shown in Table 1. Note that digestion chemicals (in my case the ISO 17852 matrix) suppress the fluorescence of the samples from 2,500 hF to 1,700 hF, or about 30%, compared to a 10 µg/L Hg in 3% hydrochloric acid only.

For most instruments, the PMT calibrates to somewhere between 600-900 V automatically upon startup. I recommend keeping a daily log of your PMT voltage and a peak profile of your highest calibration point. With this information tracked, you can easily trace any changes. Each QuickTrace M-8000 detector has a unique optimal PMT setting but can be adjusted as needed. So, do you know your QuickTrace M-8000’s PMT voltage?

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