Foreline Pressure Fluctuates

[EL_Capitannn]

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?

[benhutcherson]

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.

[dblux_]

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

[James_Ball]

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?

[EL_Capitannn]

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.

[James_Ball]

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.

[benhutcherson]

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.