Chromatography Forum

Hi everyone,

I am noticing that my MS peaks (injected using a sampling valve) are too large to be in the linear range, even when I use the lowest gain allowed by ChemStation (0.25, which corresponds to a voltage of around 1300).

I'd like to just operate in relative voltage mode and decrease the EM voltage to about 1000 or lower in order to reduce the peak size, but I'm wondering if there's a reason why I shouldn't go too low. I understand that a voltage that is too high may results in a shorter lifespan of the instrument, but is there a problem with going too low (other than simply a lack of sensitivity, which I think should be fine given the large peaks at my current voltage)?

Thanks!

I have ran both ways without any problems. I think there is a way to link gain to something that will automatically adjust the multiplier as it gets weaker over time but I have never used that. Try the relative voltage, and see how it works. If you go too low, you just won't get any peaks.

In my opinion, there are better ways to get your peaks in the right range. For example dilution.

Is this in SIM mode or full scan?

In my opinion, there are better ways to get your peaks in the right range. For example dilution.

Is this in SIM mode or full scan?

Thanks for the reply! This is in full scan.

Why do you think dilution is better than lowering the voltage?

For instance, you risk getting into trouble with overloading the column. You're exposing the system to more than what's needed (analytes + matrix). You'll have to retune (?) the lower EM voltage, and again if you want to run a different analysis.

For instance, you risk getting into trouble with overloading the column. You're exposing the system to more than what's needed (analytes + matrix). You'll have to retune (?) the lower EM voltage, and again if you want to run a different analysis.

Actually lowering the voltage doesn't always require retuning. If you run an autotune then use a negative relative EM voltage say -100V it will just lower the intensities.

Column overload will present itself as distorted peaks, and as long as you do not see signs of that it will be ok. Detector overloading will present itself as flat top peaks or even negative dips within a peak where the detector shuts down for a scan to prevent damage. Turning up the gain can cause detector overload even when the column is well within its limits. The autotunes also sometimes set the detector voltage higher than needed for certain analyses. Best to run a low calibration standard and adjust the voltage to make for a suitable peak size, then run a high calibration standard to make sure you are not overloading either the column or detector. If the detector overloads with the high standard, then you need to lower the voltage but increase the amount on column of the low standard by lowering the split ratio or increasing the injected volume.

It is a balancing act to get everything right to cover the largest operating range for the instrument, if you need a large range. If you are looking for an analyte that will always be within 10% of a known value, then it is easy to just lower the voltage until you are getting good results as that will help extend the life of the detector.