Internal standard and surrogate response

[krshaffe]

I run volatile analysis on a 6890 equipped with a 5975B MS. I have been battling with a problem for over a year now and I'm about ready to give up.
What I am seeing is an increase in internal standard and surrogate response in the presence of other analytes (ie., my calibration standards). It seems to get worse with later eluting compounds. So, from my lowest level standard to my highest the IS and surrogate response increases significantly, but comes back down in my samples. Has anyone ever seen this phenomenon? Any suggestions?

[Peter Apps]

First guess is that you have adsorbtive or absorptive activity in the system somewhere. If this is the case you will see positive deviation from linearity if you plot raw peak area against concentration or mass per peak. By positive deviation I mean that the points will be on a curve that gets steeper as it rises.

Sorptive activity is most likely due to a dirty inlet liner, or a deteriorated column.

Peter

[krshaffe]

Yeah, I made a method where I deleted the internal standards from the compound list and that is exactly what I am seeing...positive deviation from linearity. I just changed the inlet liner and septum because I figured that it might be from active sites. This didn't help, but I do have an extra column that I've been procrastinating about putting in. I'll do that today and see what happens. Thank you.

[Peter Apps]

Hi Katie

Before you put in the new column it will be a good idea to review the status of your carrier gas scrubbers - you do have scrubbers right , because if they are past their useful life the new column will quickly go the same way as the old one. Also leak check all the connections, where gas gets out, air and moisture gets in.

Peter

[krshaffe]

Well, I am seeing a pretty good sized CO2 peak, but it only shows up every now and then. I have checked for leaks before, but I will do it again.

Just for a little background... I worked in a different lab for 5 years and have never had any problems with IS and surrogates before. We had the old 5890 and 5972 MS.

I've only been at my current job for a year and a half, but this was already a problem when I got here. The instrument was set up brand new in Jan of 07 and I got here in Oct of 07. That's really why I haven't changed the column because I thought it was relatively new, however, they could have put the column from the old system that they were replacing. It isn't in the maintenance log...of course.

As far as gas scrubbers...uh, we have an in line hydrocarbon trap and moisture trap. We also have a thermal gas purifier, but the element is out in it, so it is not currently being used. Are these considered "scrubbers"? They could definitely be past their lifetime.

I have already vented the MS and I think I am going to change the column. I checked for leaks and didn't find any. Thank you for your help.

[Peter Apps]

It sounds as if you could easily have problems with carrier gas purity, which will kill the new column.

When you say that you see a CO2 peak, do you mean a chromatographic peak or a mass peak ? If you can see CO2 you must have a serious air leak somewhere. How does an air and water check on the MS look ?

You really need to get the system leak tight and clean with the old column in it before you swap columns.

Peter

[krshaffe]

We have a gas lab here that checks for contaminants and they've been having problems with He purity, so that is definitely a possibility.

By the CO2 peak, I mean a chromatographic peak.

I'll have to do an air and water check on the MS after I pump it down. Also, at my previous job we used UHP grade He as our carrier gas but here, they buy grade 6. Do you think that could be part of the problem?

[Peter Apps]

Hi Katie

The purity of 6 nines helium is fine until you take it out of the cylinder and pipe it to the GC You need a gas scrubber immediatley upstream of the GC to remove the contaminants that the helium picked up on its journey. Also, the plumbing in the GC itself has to be leak tight or the gas just gets contaminated again before it gets into the column. Oxygen and moisture attack stationary phases as soon as you heat the column.

How are you injecting the sample ?, ordinary liquid injection, headspace, 6-port valve ?, and what kind of column are you using ?

Peter

[krshaffe]

Hey there,

It's a purge and trap system, so the samples are desorbed from the trap inside the concentrator.

I usually just check for leaks using methanol to see if it bubbles. Is this sufficient?
Oh, I checked my moisture trap and it's supposed to turn pink when it's time to change it and it hasn't...it's still blue. The hydrocarbon trap could be bad though; I've got one on the way. Are you talking about a different type of scrubber or are these enough.

I'm using a Restek RTX-VMS column... 0.25mmID. I'm used to using a 624 column, but this is what they use here.

[AICMM]

Katie,

All along you are blaming the MS but is it possible that the problem is really with the purge and trap? Have you tried doing a liquid injection of these components directly on the GC/MS? Likewise, the newer Agilent MS's are more sensitive than the older ones so perhaps you should increase the split ratio for this system and see what effect it has on linearity.

With purge and trap it is not at all uncommon to see CO2 since some of the traps will do a bang up job of trapping this component. My argument for or against active sites would be to ask about what compounds are messed up, all late eluting or some late eluting. If all, I doubt reactivity since some of these are very refractory (like PCE for example) while others (oxygenates) will be more reactive. Finally, based on your comments about the heavies, I would ask what you initial and final flow values are in your run.

Best regards

[krshaffe]

AICMM,

It is definitely possible that it is the purge and trap. I did do direct injections of my IS and surrogate mixture, but it was very inconsistent and I really couldn't draw any conclusions from the data. It was all over the place. I couldn't tell if it were me or the instrument.

I have the instrument set at a constant flow of 80mL/min with a split ratio of 60:1 leaving me with about 1.3mL/min through the column. The pressure is around 20. I'm totally open for suggestion if this doesn't sound good. I am used to older equipment...

As far as the later eluting compounds...PCE isn't so bad. The RSD passes at least. Pretty much everything that elutes after PCE is messed up.

I won't be back to work till Monday. Thank you guys!

Peter,

I found a couple of leaks in the gas line and fixed them. I actually found an old leak checker in the lab and used it instead of methanol. I changed the column and I'll test it out on Monday. I'm off Friday...Have a great weekend!

[Peter Apps]

HI Katie

AICMM is right - the P&T is a likely source of the probem (but a P&T is just a fancy GC inlet ), but you still need a leak proof gas system.

A leak seeker is way better than bubble liquids of any sort.

I addition to moisture and hydrocarbons you need to get rid of oxygen. You can get 3-in one scrubbers from most of the chromatogrpahy suppliers. Just make sure that the one you get has glass or metal gas flow paths - you do not want your carrier gas contacting plastic of any kind.

Peter

[Bigbear]

Sounds like an active site to me. How are your aromatics? I had a similar problem and it was my injection port and weldment. Have you ever solvent washed your injection port? It has helped me. As for the weldment the only thing to do is replace it. I'm assuming you have cut the gas supply line to the injector and attachet the P&T transfer line there.
What do you see if you run a gc cycle after direct injecting 1 ul MeOH?

[krshaffe]

After changing the column, inlet sleeve, seal, and septum I am still having the same problem. A while back I was talking with a guy from Agilent and he said that the newer MS's were more sensitive (of course), but that this was possibly causing some type of ion competition in the detector itself. So, what that basically told me was that as instrument technology makes the sensitivity better...the chromatography gets worse... This was not very uplifting news. Does this seem at all possible?

Recap of problem...after new column: I'm using three internal standards and the last one is coming out the worst. 1,4-dichlorobenzene-d4. In my 10ppb standard the response is 153000 and in my 200ppb standard it is 247000. Then when my samples start it goes down to 98000 and remains pretty consistent.

Peter,

In addition to the moisture and hydrocarbon traps, I found a "Superior Helium Purifier" in line also. It was strapped to the back and I overlooked it. I had our gas lab check the He for impurities and it checked out fine, however, they can only see to 5ppm.

Bigbear,

I did not solvent wash the inlet...just changed everything out. This has been a problem since the system was set up...brand new, so I'm trying to think outside of active sites. Yes, the P&T is hooked up to the injector. I'll have to do a direct inject of MeOH sometime next week. I still have to pump out data even with mediocre results. Oh, and most of my aromatics are the problem...

Thank you guys!

[krshaffe]

I decided to do some direct injects today, so I'll let y'all know what I find out.