Chromatography Forum

I've recently begun using a Waters Xevo TQ MS. Over the weekend, the diverter valve began leaking. We do not have backup parts for this valve, and I don't want to wait until Tuesday (at a minimum) to get the replacement parts. Does anyone know if it's kosher to bypass the divert valve by using a union to attach the PEEK line from the source directly to the PEEK output line from the column? My only concern is that I'm not 100% certain that I'm not going to expose myself to the voltage applied to the ESI source, because I'm not certain where the grounding occurs on this instrument. It's not really apparent, like on the Sciex or Varian systems I've used (e.g there isn't an obvious metal ZDV union where the column output line meets the capillary heading to the source). My hope is that the diverter valve isn't serving as a grounding union as well as being a divert valve. I hope that makes sense.

If you're (quite reasonably!) worried about it, you could attach an earthing wire between the steel column into which you will have attached your peek tubing from the ESI source, and some convenient part of the instrument's casework. In the event you're using a plastic column, add a steel zero-dead-volume union.

My thought whilst being unable to sleep last night was to tie off a grounding wire between a steel fitting attached to the divert valve and the steel ZDV I plan to use to bypass said divert valve. A ghetto fix, but perhaps necessary. I'll see if my service engineer can shed some light on it this morning. Thanks for the reply, lmh.

I don't have a Xevo TQ but I have a TQD and a QuattroMicro both from Waters. In both cases you can bypass the valve with no issues other than having to do without solvent delay. I strongly advise not to connect steel tubing to the ESI source, I did once and received an unpleasant electric shock.

Good to know. I only used a steel ZDV union to connect the column effluent line to the PEEK capillary going to the source. I figured I would do it out of an abundance of caution.

Why don't you all just turn off the ESI voltages when fiddling with connections to the source?!? In that moment, obviously you can't run any measurement. Earthing is not required as long as the tubing connected to the source is made of PEEK.
LMH:

other than having to do without solvent delay

When you are bypassing the valve temporarily, you don't have to take care of anything. The valve may switch or not, flow is not affected. When using a ZDV instead of the valve, dolvent delay should be virtually unaffected.

Earthing is not required as long as the tubing connected to the source is made of PEEK.

I respectfully disagree. Many sources have a metal union attached to the source or the MS that serves as ground between the source and the column effluent, due to the fact that the voltage from the source can be transmitted back along the PEEK capillary from the source. I have personal knowledge of this, thanks to a Sciex applications guy who thought it would be funny to turn on the needle voltage while I was holding the ungrounded capillary.

I would have thought that an aqueous mobile phase would have enough conductivity to give you a bit of a shock is you touch an ungrounded metal fitting in the flow path.

Peter

But shouldn't you have a lot of drain voltage loss to grounding if the conductivity of aqueous eluent is significant?

(1) Yes there is enough conductivity in an aqueous solvent to give you a shock if one end of the tubing is attached to 3kV and the other end is attached to you.
(2) The resistance of this length of solvent is enough, however, to ensure that electrospray still works. The power-supply providing the 3kV has to be able to do it at X mA to cope with the conductivity back along the solvent, as well as the charge provided to the droplets (some physicist can work out whether the latter is actually trivial or not).
(3) Yes, conductivity of aqueous solvent matters. I'm no good at electrochemistry, but wonder whether this is why well-meaning attempts to modify pH to enhance ionisation often don't work; if it involves post-column addition to neutralise something, effectively it adds a load of salt. Again, I'd love to hear more detail from someone who actually knows what they're talking about (i.e. not me!).

All manufacturers will have earthed their system somewhere. The question is merely where. I don't know if Waters ion-sources are earthed inside, or whether they rely on the steel, earthed divert valve to do the earthing.

Thank you for explaining that more succinctly than I could have, lmh. In any case, my divert valve has been repaired and I am now running without my makeshift grounding apparatus (whether it was necessary in the first place, I don't know - but I didn't get jolted by the MS!).

I'm glad you're still alive! I'd be very upset to think that there was a pair of boots somewhere in front of a mass spec, with a wisp of smoke emerging from each...