slight scare this a.m.

[Jumpshooter]

When I turnt on my pump and lamp to warm up, I noticed that the solvent level in my mobile phase had gone below the length of the suction discs. Presumably the instrument had been pumping air for at least 10 mins. The system is programmed to auto-shut off after the last injection has gone through--probably after a short wait delay time. I noticed that when I starte up the pump this a.m., the pressure was only at 2 bars on a flow rate of 1 mL per min. after about 5 mins of pumping. So I let it keep pumping while I finished my sample extractions---came back 30 mins later and pressure = 70 bar. Were there air bubbles in my sys or was the problem somthing else--will the chromatography turn out OK now that my samples are ready to inject?

[Bryan Evans]

It happens - the good news is mobile phase is being pumped through the system.

You'll know right away if you do some test injections and your retention is off. If you notice something is wrong - replace the column with a zero dead volume connector - and pump H2O/organic mixture throughout the system (pump through all lines for both a binary or quat pump).

The pumps are happiest when they are not dry.

[zokitano]

During observing 2 bars pressure with 1 mL/min flow, is your mobile phase passed through the column, or the column was switched off?

If the pump pumped mobile phase through your column, at particular time during pumping there might be some air bubbles in your column. Because you haven't observed rapid change of the pressure in the system, after stabilization, probably you didn't have problems with bubbles.

Regards

[Bruce Hamilton]

This must explain my behaviour - I've been scared silly... It happens to my HPLC pumps a couple of times a month or more.

Note that having air in the pump is not the same as having air in your column. Most HPLC pumps are piston based, so the air just compresses and expands, and the column stays wet. So don't just add water to the reservoir and restart, open the purge valve first and pump to waste. Disconnect the column if you don't have a purge value.

I pump at the maximum flow rate to clear all the air from the degasser and lines for about 5-10 minutes, but you can also use a syringe to fill the degasser first if you want - I don't, mainly because applying vacuum to a system that operates at slight hydraulic head pressue can create new leaks.

5-10 minutes of pumping will flush the air out of the pump/degasser system and you can reset to normal flowrate and close the purge value.

Modern HPLCs ( at least the Agilent 1100s I now use ) will not be affected by such air incursions, but the check valves and piston cylinders in old pumps ( Waters 6000A ) were really difficult to remove air from once they had run dry for several minutes. The inlet valves would stick, cylinder surfaces would become hydrophobic, and IPA had to be flushed through to get them operating again.

My pumps also have passive or active seal washes, which ensure the back of the piston seals are slightly lubricated, greatly reducing damage from such premature "senior moments".

Bruce Hamilton