Chromatography Forum

Peter apps:

I just ran a SPME sample with MEOH alone. I got slight peaks close to where my compounds are supposed to be. I dont have much verification where my compounds really are because I am very inconsistant with my chromatograms.

Peter apps:

I just ran a SPME sample with MEOH alone. I got slight peaks close to where my compounds are supposed to be. I dont have much verification where my compounds really are because I am very inconsistant with my chromatograms.

With a non-selective detector like an FID the retention time is the only criterion that you have to identify which peak is which. It is absolutely essential that retention times are consistent from run to run.

Vague terms like "slight" and "close to" are not really helpful in troubleshooting. I need to know; what were the retention times of the MIB and Geosmin peaks when you ran the liquid injection of methanol solution ? What were the retention times of the MIB and geosmin peaks when you ran your highest standard using SPME, what were the retention times of the MIB and geosmin peaks when you ran your lowest standard using SPME ?. How big were the "slight" peaks form the methanol blank and how big are the peaks form your standards (in whatever units of peak area your integration software produces).

At this stage it looks as if the problem rests on which peaks are assigned to the analytes.

Peter

You folks may have already covered this, but keep in mind the retention times obtained from the GC/MS will likely vary a bit from the FID due to the vacuum on the MS verse the atmospheric outlet of the FID.

Thanks everyone for your support and suggestions. This is wonderful!

Peter apps,
I will come back with times in a bit but my frustration lies within the fact that several months back when my GCMS was down, I used the FID as my back up and got fantastic peaks, real clear retention times and excellent calibration curves. Then my MS was back and running, so the FID sat for a month and a half, mostly on, but I did turn it off to use the colun once I had problems with my GCMS again.
Now, the GCMS is down again, I replaced the flame tip, ferrule, re-installed the column and I cannot get data anywhere close to what I have 2 months ago.
This is where I am trying to grasp what went wrong?
I reverted to using the old flame tip but I still am getting very vague results, loss of sensitivity compared to 2 months back peaks.
I just wanted to put the whole situation into context.
THANKS!

Peter apps:

I just ran a SPME sample with MEOH alone. I got slight peaks close to where my compounds are supposed to be. I dont have much verification where my compounds really are because I am very inconsistant with my chromatograms.

With a non-selective detector like an FID the retention time is the only criterion that you have to identify which peak is which. It is absolutely essential that retention times are consistent from run to run.

Vague terms like "slight" and "close to" are not really helpful in troubleshooting. I need to know; what were the retention times of the MIB and Geosmin peaks when you ran the liquid injection of methanol solution ? What were the retention times of the MIB and geosmin peaks when you ran your highest standard using SPME, what were the retention times of the MIB and geosmin peaks when you ran your lowest standard using SPME ?. How big were the "slight" peaks form the methanol blank and how big are the peaks form your standards (in whatever units of peak area your integration software produces).

At this stage it looks as if the problem rests on which peaks are assigned to the analytes.

Peter

Retention time data:

MIB and Geosmin direct liquid injection with stock 100 ppm
MIB mintue 9.1 (29,000uV peak size)
Geosmin 11.8 (65,000 uV) peak size


MIB and Gesomins SPME 100ppt standard
MIB 9.1 (280uV peak height)
Geosmin 12.25 (260 uV )

Slight peaks with MEOH run
MIB 9.3 RT (250uV)
Geosmin 12.2 (100uV)

The discrepancy between the geosmin retentions for the liquid and SPME injections suggests that the peak that you are calling geosmin in the SPME run is not geosmin.

You have not provided the retention times for the lowest standard that I asked for.

You still need to run a water plus methanol blank (see my post of 6 June).

Your previous results would require the FID to be detecting at least an order of magnitude less than its known performance parameters suggest is possible.

Peter

But wont there be a slight variation in retention times from direct injection to SPME injection, or should I just use the direct injection RT's for SPME data?

But wont there be a slight variation in retention times from direct injection to SPME injection, or should I just use the direct injection RT's for SPME data?

I would expect differences in retention between SPME and liquid injections. How big the differences are will depend on the operating conditions. If there are differences in retention then the peak that you are calling MIB in the SPME run, which elutes at the same time as with the liquid injection, is not MIB.

You have still not posted the retention times of the MIB and geosmin peaks for your lowest standard; which produces larger peaks than the high standard so finding the peaks should not be a problem.

What are the retention times for a standard in water made up to 100 times as concentrated as your current highest standard ?

You also need to calculate how much of each analyte is in your lowest and highest standards.

Peter