inconsistent area counts gc-ms pesticides method

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

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Hi All,

I have been struggling with trying to finish an AR method on GCMS, I was hoping for some advice or suggestions.
I am having several problems:
1. When I run a check standard the area counts have an RSD>5%.
2. The area counts decrease each consecutive run (i.e.: if I prep the std once and run it 3 times from the same vial the area counts for a given compound look like this: 14,707 13,680 12,833).
3. I just changed the gold seal & liner prior to this set of data and had a loss in area counts. For the same compound listed above prior to gold seal & liner change: 18,545 18,115 15,900.

Any help at all would be greatly appreciated, I am pretty new to this type of work. I have included the specs and run conditions below. Thank you in advance!


I am running an Agilent 7890A gc coupled to a 5975C MSD, it has a 2 way CFT with active pressure control splitting between an MSD and an FPD. I just cleaned the source at the beginning of the month and changed the column in September. The column is an agilent DB5 30x0.25x0.25. I am using a splitless liner, running the method pulsed splitless. I am letting the std come to room temp and there is no significant temp fluctuations in the lab.

Oven Program:
45 °C for 1 min
then 20 °C/min to 150 °C for 6 min
then 10 °C/min to 200 °C for 1 min
then 10 °C/min to 260 °C for 2 min
then 10 °C/min to 300 °C for 10 min

inlet:
Pulsed Splitless
Heater On 200 °C
Pressure On 28.55 psi
Total Flow On 63.899 mL/min
Septum Purge Flow On 3 mL/min
Gas Saver On 20 mL/min After 3 min
Injection Pulse Pressure 30 psi Until 1 min
Purge Flow to Split Vent 60 mL/min at 0.8 min
When you run the same standard on the next day does the area continue to decrease or does it jump back up and then decrease?
In other words, does the response return when not running?
No, There is just no consistency. For example referring to the same compound as before on nov25 it read 12,290, nov26: 10,107, dec02 (prior to changing gold seal and liner): the avg=17,520, dec5:10,012.
Is this for any and all compounds in the standard or just one?
They are all inconsistent. Some are worse affected than others. However, can't point to a solid trend (i.e. elution time in the run).
Do the area counts also fluctuate on the FPD?

That will determine if it is in the GC or the MS.
The past is there to guide us into the future, not to dwell in.
yes, i see it on both detectors.
Probably taken for granted as yes, but have you changed the septum?

What solvent are you injecting?

When doing inlet maintenance, other than changing liner and gold seal, do you clean the injection port with a brush or swabs?

Have you flushed or changed the split vent line? (the 1/8" copper line from the injection port to the split valve/back pressure controller?

There can be buildup in the small port from the inlet to the split vent line that can cause strange problems, I remove the split vent line and use a pipe cleaner and solvent to make sure that port is clean also. Carbonized buildup in that opening and at the entrance of the copper line can cause loss of analyte even in splitless modes.

Is the column inserted into the inlet about 1-2mm above the top of the gold seal? You can check this by placing a gold seal on the end of the column that is above the column nut and looking at how much it is above the seal. I normally keep an old seal handy just to do this check.
The past is there to guide us into the future, not to dwell in.
Yes, I am changing the septum, liner, gold seal every 70-100 injections. I am trying to keep the use of consumables down but It seems like the area counts get more unpredictable after around 70 injections. Agilent came out and has replaced the EPC cap assembly and split valve. None of this fixed the problem.

I do not clean the inlet manually. would you suggest removing the inlet body and swabbing it out? or just passing it over with a solvent and swab while still installed? I don't know if the split vent line was changed out but i can definitely clean it to see if it helps.

I am using a 1:1 of etoh:isooctane.

I will take a look at the gold seal and column when the instrument isn't running. but I am installing the inlet side at a measurement of 4-5mm from the top of ferrule.

Thank you for your suggestions :)
Do you see any leaks if you run a tune or do a manual leak check (blank injection, scanning from say 15-100 m/z. You can open pftba calibrant valve as a manual event in the MS method and compare your PFTBA abundance to backround air/water. Should be very low.

If you have a leak, open manual tune and scan 50-100 m/z and blow airduster (tetrafluoroethane m/z 69 and 83) on the cft fittings and transfer line, and then look for a response. If you spray at a point where it has to go through a column it can take a minute or so before you see a response.

Do you have an internal standard in your sample? It sounds to me like your problem is either at the CFT splitter or the inlet. It would be interesting to know if the internal standard fluctuation matches your analytes.

EPC probably isn't the problem, and as mentioned it has been replaced so definitely not a problem.


If you really want to figure this out:

Vent the system, remove the columns. Install a column directly from the inlet to the MS (I'd use an hp5-ms 30 x 0.25 x 0.25 for this.) Run a more simple sample: perhaps this https://www.agilent.com/store/en_US/Pro ... 5970-60045

https://www.agilent.com/cs/library/cert ... 951400.pdf

Heres a certificate of analysis with a method

Here is an FPD sample https://www.agilent.com/cs/library/cert ... 495765.pdf

also has a method.

You could first check the MS, measure your RSD using replicate injections of Sample A. Once you confirm that is working, vent the MS and configure a GC only instrument. Install the column to the FPD and run that FPD method and repeat the reproducibility experiment.

If both of them work fine, it was probably the CFT.

You could then hook the 30 m column to the CFT plate, and then attach a few meters of fused silica to the MS and repeat the experiment 1 detector at a time.

Here is a pesticide sample if you want to try that instead:

https://www.agilent.com/store/en_US/Pro ... =Standards

https://www.agilent.com/cs/library/cert ... 494069.pdf certificate of analysis

This one is 17 pesticides at 10 ppm. I'd dilute it down to 1 ppm or run it split.

This one would be good because you could use the same sample to check the MS and the FPD, should be able to see a lot of those on the FPD.. I'd still remove the splitter from the equation and work my way up from first principles. Remove complexity until you get it working, then build it back up.
ABMS8783 wrote:
Yes, I am changing the septum, liner, gold seal every 70-100 injections. I am trying to keep the use of consumables down but It seems like the area counts get more unpredictable after around 70 injections. Agilent came out and has replaced the EPC cap assembly and split valve. None of this fixed the problem.

I do not clean the inlet manually. would you suggest removing the inlet body and swabbing it out? or just passing it over with a solvent and swab while still installed? I don't know if the split vent line was changed out but i can definitely clean it to see if it helps.

I am using a 1:1 of etoh:isooctane.

I will take a look at the gold seal and column when the instrument isn't running. but I am installing the inlet side at a measurement of 4-5mm from the top of ferrule.

Thank you for your suggestions :)


I clean the port by removing the gold seal and running swabs soaked in solvent through the entire port length. Sometimes I wedge two swabs in there together to get a tight fit and harder scrubbing if needed.

The number of sample between cleaning is dependent on the sample matrix more than just number of samples. I have some runs where I clean after 15 injections and the port is absolutely filthy, but running drinking water extracts I can go a few days between cleaning, but almost always change the liner daily. Pesticides are very bad about adsorbing on any carbonized particles in the inlet or breaking down when active sites appear in the inlet. Also I have had samples contaminate the column and have to remove several meters of column before I will see an analyte like Pentachlorophenol again.

I ran 10 samples yesterday and today in my calibration check I have completely lost Aniline and Chloroanilines, but everything else is just fine. The loss can be very selective at times.
The past is there to guide us into the future, not to dwell in.
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