Discharge in ES-?

[Sofia]

I would like some help and experienced opinion about discharge in negative electrospray (or what it might be). When I switch mode from positive to negative ES the probe tip starts to glow (not always but sometimes). The glowing occurs, above all, when the LC-flow is off and disappears when the flow is on. What am I doing wrong? How can I avoid it? Is there any procedure that I should do when switching between positive and negative mode (either way)?

Thank you for your time!

[Kostas Petritis]

Sofia,

In general you should be operating in lower voltages in negative ion mode in comparison with positive mode because of the discharges. So before you switch polarity decrease your electrospray voltage.

The fact that you do not see the glowing when the LC flow is off is normal as you do not have any electrospray at that time so no discharges.

There are some people that are using compressed air instead of nitrogen for the nebulization which decrease a little bit the discharge problem in negative ion mode (but go with what the manufacturer recommends).

[Sofia]

Thank you for your reply Kostas!

I think you misunderstood one part of my question; the glowing often disappears when the LC-flow is on - not off. That puzzels me a lot.

Sofia

[MG]

Sofia, I have experienced just what you describe. Under certain conditions in ESI(-), I have seen corona discharge with no solvent flow, and the discharge goes away when flow is present. I recommend optimizing your ESI voltage while flowing at the flow rate you intend to use for analysis, for best signal-to-noise ratio. Easiest way is to do flow injections of a standard solution at discrete voltages. Alternately you can use a syringe-pump and mixing-tee between your LC and MS, and infuse analyte into your flow stream at a constant rate; and vary the voltage while doing SIM or MRM on the ion(s) of interest.

If you see a broad range of optimum voltages, use the lowest voltage that gives the best S/N. If corona discharge is present at a given voltage, you probably won't have good S/N at that voltage. After running enough negative ion methods, you will develop a feel for the range of appropriate voltages. As Kostas pointed out, these values will generally be lower than the normal values used in positive mode.

[Kostas Petritis]

The maximum voltage you can use in negative ion mode depends on several parameters such as flow rate, mobile phase solvents and additives etc. In general I try not to exceed 4000 V for normal flows (50 uL/min or higher) or 2000 V in nano flow rates (400 nL/min or lower) (but as I said it depends on other parameters so take these values with a grain of salt)

[Sofia]

Dear MG and Kostas,
I thank you both very much for helpful answers!

Just one more question; have any of you any experience in how much this glowing damages the capillary tip? And if it gets damaged; what should a routine procedure be? Swich to negative mode only after the LC-flow is on?

Thank you!!
Sofia

[Kostas Petritis]

Sofia,

Extended period of discharging wouldn't be good for your capillary tip but that is not the point.

From every respect you shouldn't operate under conditions where you have discharges. Be aware that these discharges are not always visible (i.e. what you describe as glowing). Electrospray instability can be a good indication of discharging. Furthermore, depending on your conditions, discharge might happen during your gradient elution (not very common).

As I mentioned before, in order to avoid such discharges during polarity changes the best way is to decrease your ESI voltage before polarity changes. This can be by having a negative ion mode method with adequate ESI voltage that you load, or if you do it manually first lower your voltage then change polarity.

[Sofia]

Thank you Kostas! I understand - and should of course have understood from the earlier answers, but I was too engaged in what had already happened to the capillary tip...

/Sofia

[MG]

If your instrument reads back electrospray current, that is very helpful for determining if you have a discharge condition. During discharge, I have noticed that these readings will be much higher, 10x or more, above normal operating currents.

[Sofia]

Still another question: is there a lower limit (below 1 or 0,5 kV for example), below which it´s "inappropriate" to be - even if the signal is best in that region?

[MG]

The lowest voltage I've used was approx -2kV in ESI negative. That was for a standard LC flow and a standard "pneumatically assisted" ESI source. But I think lower voltages than that are common in nanospray ESI. If your method gives good performance and meets your needs at the low voltage, I see no reason to call it inappropriate.

If you are running at 0.5 or 1.0 kV, I am a little surprised that higher voltages don't give improved performance. But not having seen all your method conditions and the optimization experiments you have done, there may be some reason why that is the case that I am not aware of.