Chromatography Forum

Hi,

I'm working on an Agilent 7890B, which is coupled to a 5977A and I'm experiencing pressure issues.

At the start of a sequence, I have a foreline pressure of ~45mTorr and notice it rises above 100mTorr, approximately 90 seconds into the run. The pressure then returns to normal before it reaches the solvent delay (3mins). I suspect a leak, so I've changed the septum, liner and column ferrules and I've also vented the MS and checked the seals, but the issue persists. Any suggestions on where to look next?

I don't recall ever seeing THAT big of a spike, but it's normal for the pressure to spike when the solvent comes off the column. This can be especially true if you're using a larger diameter column(.25mm or larger) and/or splitless injection. I'll often see the hivac on my 5975 jump to 10^-3 and the foreline jump considerably(mine reacts a bit slower since I have a diffusion pump) when the solvent comes out.

In fact, this is one of many reasons why we set a solvent delay. If you were using a 5971(which I've been using more than a 5975 these past few weeks) and had the MS turned on, it would likely throw an "excess source pressure" error at the same time.

One other thing to consider-I've seen pressures both higher than normal and "bouncing" when I had oil in the foreline. Since it only happens on injection in your case, I suspect this isn't the problem, but it's something worth considering.

Make a blank run (without injection), monitor the vacuum. Hope this will clarify the reason of pressure change.

Make a blank run (without injection), monitor the vacuum. Hope this will clarify the reason of pressure change.

I agree, if the pressure doesn't change when run without injection then it is most likely the solvent causing the pressure spike.

What solvent are you injecting and how much?

Make a blank run (without injection), monitor the vacuum. Hope this will clarify the reason of pressure change.

I agree, if the pressure doesn't change when run without injection then it is most likely the solvent causing the pressure spike.

What solvent are you injecting and how much?

I ran a blank (no-injection) and monitored the pressure and it didn't change! I'm running a splitless injection of 1uL of acetonitrile into a 0.25mm diameter column.

Make a blank run (without injection), monitor the vacuum. Hope this will clarify the reason of pressure change.

I agree, if the pressure doesn't change when run without injection then it is most likely the solvent causing the pressure spike.

What solvent are you injecting and how much?

I ran a blank (no-injection) and monitored the pressure and it didn't change! I'm running a splitless injection of 1uL of acetonitrile into a 0.25mm diameter column.

The 1ul of Acetonitrile will turn into 500ul or more vapor when it hits the source so that will give a nice spike in pressure for a moment. Methylene Chloride is even worse.

As long as it is cleared out by the time the filament turns on after the solvent delay you should have no problems.

As said, this is perfectly normal for a 1µL splitless injection of pretty much any volatile solvent.

If you were watching the hi-vac, I would be surprised if you saw 10^-3. I was doing some work today on the 5971(I've been using it for most of my personal samples since everyone else uses the 5975) and did a 1µL splitless with methanol. It's not easy to see the convectron gauge(foreline gauge) with the instrument running, but the somewhat laggy hi-vac gauge made it up to the mid 10^-4 range(this instrument does high 10^-6 with no sample running). In my experience, the micro-gauge on the 5975/77 responds VERY quickly.