Peaks suddenly disappeared or became irregular at lower cal

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

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Hi , I am facing some problem with the GC-MS. I am wondering if anyone could give me a reply.

I ran calibration standards on the GCMS one month before. The practice was to run it every month as a monthly calibration for EPA8270. This month, I loaded the calibration standards (freshly prepared) yet when running on the GC, I found out the response was really low for this month for the lower calibration levels. There was even no peak at the lowest level for one compound phthalic anhydride. Either that or the peaks were jagged and not sharp.

I will appreciate it if anyone can advice me.

Thank you for reading.
How did the internal standard peak areas compare to previous standards? If they are within 15% then it is probably breakdown caused by something in the inlet or column. If they are much lower, then you may need to retune and adjust the electron multiplier voltage up a little to compensate for loss of sensitivity.

Do you start with a fresh inlet liner and remove a few centimeters of column before each run?
The past is there to guide us into the future, not to dwell in.
Thank you James Ball for the reply.

I had used a fresh liner. The internal standards peaks were within 20% though. I think you are right. There may be breakdown in the column or inlet. However, if column bleeds then there should be peaks of the packing in the spectrum which I never see . Hence, there may be breakdown at the inlet. But I am not sure why is there breakdown too.
Different things cause different compounds to be lost. Heavy hydrocarbon contamination from a sample that contains grease can cause loss of sensitivity of compounds when it gets stuck on the first few cm of the column. Inorganic residues can also cause some compounds to degrade. pH shifts can cause loss of either acid or base compounds. I have see a time when a highly basic sample caused benzoic acid to tail so badly the peak was barely visible and over two minutes wide, then I have had samples with high levels of humic acids to cause loss of analytes like benzidine.

We change the liner ever day, the gold seal about once a week and replace the 1m section of guard column about once a week, if it lasts that long. When replacing the guard we remove about 3-4cm of the analytical column to remove anything caught in the stationary phase that won't bake off. Ever few weeks we also swab out or even scrub out with a brush, the inlet. Every few months we have to replace the split vent lines too, it is amazing what contamination in that will do to cause loss of analytes.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
...
We change the liner ever day, the gold seal about once a week and replace the 1m section of guard column about once a week, if it lasts that long. When replacing the guard we remove about 3-4cm of the analytical column to remove anything caught in the stationary phase that won't bake off. Ever few weeks we also swab out or even scrub out with a brush, the inlet. Every few months we have to replace the split vent lines too, it is amazing what contamination in that will do to cause loss of analytes.
I must say, I am impressed by this description of maintenance. It really shows the difference between extraction to a solvent versus Purge and Trap. 8260 inlet maintenance is far easier. I calibrate every 6 months to a year; when I clean the source and change the filament. I go for at least a year on a gold seal. However, I'm only putting -1500 samples through it in a year.
I wish I only had annual maintenance on my purge and trap instruments. But the samples I get don't seem to allow it.
Today's batch includes one with 1/2" of floating creosote on it. Last week one was 20% toluene...
At least no one asked for SIM lately.
James_Ball wrote:
We change the liner ever day, the gold seal about once a week and replace the 1m section of guard column about once a week, if it lasts that long. When replacing the guard we remove about 3-4cm of the analytical column to remove anything caught in the stationary phase that won't bake off. Ever few weeks we also swab out or even scrub out with a brush, the inlet. Every few months we have to replace the split vent lines too, it is amazing what contamination in that will do to cause loss of analytes.


I agree with James. I think in your case it may possibly be related to the split-vent lines. Especially if you are using a "pulsed splitless" injection method.

I saw once that I had lost almost all response on ONE internal standard compound for Method Blanks and LCS only. As soon as I injected a sample with "matrix" in it, the signal for that chemical would come back. I used a new liner about every 10 samples as we were running diluted soil extracts. I removed the split line from the inlet (Agilent 6890) and noticed what looked like black tar gooping out of the line and into the injection port. I cleaned this well with MeCl2 and Hexane and replaced it. The split-vent trap was clean as it was down the line farther and most of the "tar" condensed out in the first few inches of cold copper split-vent line. I was getting a selective adsorption of specific chemicals during the pulse injection. It was cured after cleaning the split-vent line.

I observed that standard splitless and Split are far less affected by this. If you can imagine, as the inlet pressure is regulated via back-pressure from the mass flow controller at the back of the instrument, a bit of your sample is actually pushed into the split-vent and pulled back into the injection port as the sample is expanding and vaporizing in the inlet. This is not the same as "back-flashing" when you inject more than the liner volume and your sample vapor goes up into the cold carrier gas line. In my case, this was not the problem.

I suggest cleaning/replacing the split-vent as James does. You may not need to do it as often but it's definitely considered a maintenance item. I would have a spare copper line (Agilent systems again) ready to go so you can swap them in a few minutes. I would leave the top covers off of my Agilent GC as they are a pain to take off and get back on.
~Ty~
LALman wrote:
James_Ball wrote:
...
We change the liner ever day, the gold seal about once a week and replace the 1m section of guard column about once a week, if it lasts that long. When replacing the guard we remove about 3-4cm of the analytical column to remove anything caught in the stationary phase that won't bake off. Ever few weeks we also swab out or even scrub out with a brush, the inlet. Every few months we have to replace the split vent lines too, it is amazing what contamination in that will do to cause loss of analytes.
I must say, I am impressed by this description of maintenance. It really shows the difference between extraction to a solvent versus Purge and Trap. 8260 inlet maintenance is far easier. I calibrate every 6 months to a year; when I clean the source and change the filament. I go for at least a year on a gold seal. However, I'm only putting -1500 samples through it in a year.


Yup. I have 8260/624/524 instruments that get no maintenance at all for up to 18 months at a time. Purge and Trap is much less dirty, and most of the crud get captured at the trap or sparge tube instead of the inlet. If a sample foams then you can get a mess to clean out all the way to the inlet, but I switched to 25ml sparge tubes without frits to help with that problem.
The past is there to guide us into the future, not to dwell in.
Steve Reimer wrote:
I wish I only had annual maintenance on my purge and trap instruments. But the samples I get don't seem to allow it.
Today's batch includes one with 1/2" of floating creosote on it. Last week one was 20% toluene...
At least no one asked for SIM lately.

The majority of my samples are water and soil from storage tank leak work. I do the sonication in methanol extraction for grease trap and other nasty samples that have trap/inlet killing potential. Had an innocouous sample full of durane that purged onto my trap and ruined it. So, I dilute first and then run at lower dilutions if it is not too dirty.
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