Higer temperature, higher ionization efficiency?

[Anthony_Ng]

Hi all,

We have come across different vendors' LC-MS system. Some of them have 450oC in the spray chamber and others can reach up to 600oC.

For a general situation, is the ionization efficiency gets better if the temperature reach higher? (for both case of ESI and APCI)

And, what other factors (e.g. voltage, flow rate), can I alter to increase the ionization efficiency?

Secondly, in order to increase sensitivity (and S/N ratio), should I consider increasing EM voltage as the last resort? (Please correct me if I am wrong: I think increasing EM voltage will increase the signal but also increase the background noise, just like increasing ISO value in photography).

Thanks in advance!

[lmh]

(1) the temperature is there to ensure all solvent is dried away. If you still have solvent present, it will reduce signal, but in extreme cases can also (in some instruments, e.g. single quads) create noise, because tiny droplets getting through the analyzer will be measured as a signal, but arrive at random times, and therefore be assigned random masses.

(2) once the solvent is gone, there is no benefit in further heating.

(3) in some cases, some compounds can be heat-sensitive, and will be lost if you use excessive spray-chamber temperatures. However, heat sensitivity in the lab isn't the same as heat sensitivity in a spray chamber; compounds survive temperatures in a spray chamber that would fry them in seconds in the lab (obvious example is proteins)

(4) ESI gives a signal proportional to concentration, not amount, so reducing flow (for the same amount of sample injected) will increase signal.

(5) ESI creates a plume of ions that is sampled by the capillary that acts as interface between spray chamber and high-vacuum. If your instrument allows it, you can experiment with gas flows and needle positioning in order to get the best bit of the plume of ions close to the end of the capillary.

(6) In critical cases, you should also optimise ion transfer. Make sure lens voltages are appropriate to draw ions of the mass that interests you into the instrument. Your manufacturer should be able to help with procedures for this.

(7) I personally wouldn't fiddle with the EM voltage. Manufacturers tend to optimise this pretty well as part of their calibration routine. Increasing it is unlikely to improve things vastly (if it did, the manufacturer would already have done it!), but it will decrease the lifespan of your EM horn, which is expensive.