Droplets forming on corona needle (ESCI+ source) [LCMS]

[pwilson416]

Hello,

The primary user of our Waters UPLC Xevo TQD was let go a while back and we have been sort of teaching ourselves how to use it. We have an ESCI+ source (ES+/- and apci +/-). We don't typically use APCI and thus don't use the corona needle. However I don't think forming droplets on it are intended and it probably indicates too much fluid is being discharged from the ESI needle. We run at 0.4 ml per minute with 10% acetonitrile to 90% acetonitrile with 0.1% formic acid. MS parameters are as follows:

Desolvation temp: 350 C
Source temp: 150 C
Desolvation gas flow: 650 L/hr
Cone gas flow: 20 L/hr

I'd prefer not to decrease flow rate as that would increase our run times. Would increasing cone or desolvation gas flow help nebulize the droplets better?

Thanks for any help.

[Rndirk]

You can and probably should just take the needle out when only doing ESI. It might just be normal to have droplets on there if not in ESCI/APCI mode, I haven't tried.

0.4mL/min is a normal flow, this is OK. Typical settings I use (note that this is Xevo TQ-XS not TQD) for 0.4-0.5mL/min methods:

Desolvation temp: 500-600°C
Source temp: 150 C
Desolvation gas flow: 650-1000 L/hr
(Cone gas flow: 150-200 L/hr)

I'm not saying these settings will be optimal on your instrument. I suggest to optimize them while infusing your compound(s) in the mobile phase, and see what each parameter does to the signal. The desolv temperature is rather easy: go as high as you can as long as your analyte signal does not go down. Source temperature: Waters told me to just leave it at 150°C for everything. The desolv gas flow will depend on the angle the ESI source is positioned, and on the flow of your method. For higher LC flows you want to use a higher gas flow. The cone gas flow has a minimum value of 150 L/h on the TQ-XS for some reason, so you probably can't compare it with the TQD.

Would increasing cone or desolvation gas flow help nebulize the droplets better?

I'd say the biggest influence is the temperature and the desolv gas flow, not the cone gas flow.

[pwilson416]

Thank you.

I took the needle out last night and it's working fine. I will try increasing desolvation temp when the instrument is free later today and see if that helps since it looks like we are on the lower end.