ISTD Problems w/ TD-GC-MS running EPA Method TO-15

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

7 posts Page 1 of 1
Hi Chromforum,

I've been running EPA Method TO-15 for chlorinated VOCs associated with soil vapor intrusion into indoor spaces on a TD-GC-MS without any serious persistent problems for the better part of two years now. However, as of late I've started experiencing a very major problem with ISTD response that I haven't seen before. Most of the tricks I use to fix GC-MS problems and troubleshoot haven't helped so far.

I realize this isn't a typical matrix and method that is seen here (at least judging by the topics from the past two years), but I am open to input/help/suggestions from anybody who has ever had a similar issue in their analytical careers. Below I'll present my problem, followed by my hardware and general settings as well as some context.

I'm seeing the ISTD response vary somewhat wildly, with an overall decreasing trend, over the course of 10 diagnostic CCVs around the midpoint of my usual calibration range. I've tabulated this data and I've seen RSDs around 20% for my ISTD responses, which has proven to be quite troublesome when calibrating the instrument for low RSDs as required by the method (the method requires RSDs to be < 30%). What's even stranger is that analyte response for these CCVs is very stable--the RSD's are around 3%. This leads me to believe that there isn't a problem with the source (which I have cleaned and whose filaments I replaced just a week ago), but rather with the ISTD or maybe somewhere along the ISTD flowpath. Curiously enough, the first sample within a batch of say, 10 diagnostic CCVs, always has a much higher ISTD response than the subsequent samples. I'm talking about starting at 5 million area counts and dropping to 2.5 million area counts (and the ISTD response will then never go above 4 million area counts later in the batch).

I've checked for leaks on all three subsystems using both a conventional leak detector and the manual tune/profile mode with a can of dust-off and cannot find anything suspect. The air/water levels are acceptable (<5% for N2, O2, CO2, and H2O), and as far as I can tell the column is healthy. The TIC is clean and there is no sign of contamination and no peak tailing.

I'm running the method on a TD-GC-MS. Specifically, I'm using a Markes International Unity II/CIA Advantage-HL as my thermal desorption/autosampler system, with a 7890B/5977B as my GC-MS. I inject onto the column from the TD using an inert transfer line that's about 18" (not sure if this is considered a direct injection). The column is a Restek Rxi 624Sil MS column, 30 m long with a 0.25 mm internal diameter and a 1.4um film thickness. The trap used in the TD is a quartz trap, and desorbs from 30 degrees C up to 300 degrees C with a 40 degrees C/min temperature ramp. My GC oven ramps from 35 C to 220 C using successively larger temperature increase rates.

To whomever is reading this, thanks for taking the time. Hopefully somebody out there is running a similar method or instrumentation. Please let me know if I've left out any key details, I really want to paint the full picture here.
Does the 5977 have the turbo molecular pump or the oil diffusion pump?

How often do you change the oil in the rough pump?

I haven't done TD but thinking of a few things on the detector side at the moment.
The past is there to guide us into the future, not to dwell in.
Hi James,

I'm using an oil diffusion pump (Pfeiffer Duo). I replaced the oil two weeks ago with vacuum pump oil and was very careful to avoid under- or overfilling.
dank-voc-chem wrote:
Hi James,

I'm using an oil diffusion pump (Pfeiffer Duo). I replaced the oil two weeks ago with vacuum pump oil and was very careful to avoid under- or overfilling.


The Duo is the roughing pump, what about the one under the vacuum chamber for the high vacuum? That will either be a turbo molecular pump or an oil diffusion. If turbo then very little maintenance needed, if oil diffusion then the oil may need changing, though I can normally go a couple years between changes without problem, but if moisture gets in there it can cause stability problems.

Just want to rule out any vacuum problems causing sensitivity problems. If the Internal standard is going up and down but the targets are not, then the %RSD on the calibration should be high, if both Internal Standard and target analytes are going up and down then the targets will have good %RSD because the internal response is changing the same amount and "correcting" for any changes in sensitivity, as they should.

Another thing that can cause similar problems could be the split vent trap causing the split flow to be inconsistent. That also needs to be changed out on a regular basis, along with cleaning or replacing the copper line from the inlet to the split vent trap.
The past is there to guide us into the future, not to dwell in.
Hi James,

My bad--it's a turbo pump. It's been operating at a fairly low pressure, usually around 9.25E-06 torr, so as far as I can tell there's no issue there. As for the split vent and potential issues there, I'll definitely have to give that closer look. It could be that the solenoid controlling the flow through the split vent is being troublesome. Thank you so much for your input!
dank-voc-chem wrote:
Hi James,

My bad--it's a turbo pump. It's been operating at a fairly low pressure, usually around 9.25E-06 torr, so as far as I can tell there's no issue there. As for the split vent and potential issues there, I'll definitely have to give that closer look. It could be that the solenoid controlling the flow through the split vent is being troublesome. Thank you so much for your input!


The solenoid can go bad, we have had a 7890 that we replaced that board on at least twice, the rest are running great. The carbon trap before the solenoid though can clog and that is a much easier and less expensive fix to try first.
The past is there to guide us into the future, not to dwell in.
How are you spiking your ISTD and your CCV’s?
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