Sensitivity in GC-MS

[allene]

Hi,

In all GC-MS instrumens, tuning is done with cal gas and S/N and other parameters are checked. Why HCB (hexachlorobenzene) is used for sensitivity check of GC-MS?

In my GC-MS (Leco Pegasus III), cal gas test is passed (S/N is high) but sensitivity using HCB is low (S/N of 10 is obtained for 1 microliter injection of 50 ppb of HCB instead of 2 ppb). What parameters should be checked? I verified all sections of GC such as injector, liner, septum, transfer line,… and increase detector voltage up to 1800 V. All things are apparently correct but the problem still remains.

I would greatly appreciate any help.

Thanks

[dblux_]

... Why HCB (hexachlorobenzene) is used for sensitivity check of GC-MS? ...

Is it really ? Not octafluoronaphthalene ?

[allene]

Both octafluoronaphthalene and hexachlorobenze (HCB) are usually used, but HCB is more common.

any idea about the problem?

[Peter Apps]

Aren't the two "sensitivities" measuring different things ? The tune has a high concentration of analyte, the minimum detectable quantity test has a very low one.

Can you confirm that you are running the minimum quantity test under exactly the conditions specified by Leco ? Is the retention time for the PCB what it should be ?

Peter

[allene]

Thanks Peter,

What does this mean if I have good S/N in tuning with cal gas but low S/N when using HCB test? Is it related to detector?

Yes, I set all Leco recommended conditions. The retention time of HCB is normal. I could see the HCB peak at high concentration.

I think the noise of my chromatogram is high in all m/z (even without sample injection). This high level of noise maybe related to detector, but S/N is passed in tuning. Column is new and no contamination is present and the nois is seen in all mass range.

I also injected Naphthalene and antecedent. Unfortunately I could not get good signal (S/N>5 or 10) at concentration lower than 10 ppm which is very high for GC-MS.
I don,t know, what is the cause of low s/n in sample analysis while everything is apparently ok and tuning is passed.

[Peter Apps]

A low signal:noise value can come from high noise just as well as from low signal. Possible sources of noise are column bleed, contaminated carrier gas, septum bleed, etc etc. There may also be a problem with the detector or elsewhere in the electronics.

Are you doing the minimum quantity test in the TOF eqivalent of full scan, or SIM ? If the noise is from chemical background (e.g. bleed etc) it should be much lower in SIM ( as long as the ion you choose is not in the contaminants). If the SIM noise is similar to the full scan noise it suggests a detector or electronics problem.

Have you approached Leco for help with this ?

Peter

[dblux_]

Have you executed this sensitivity test earlier with good results ?

Try to combine or exclude any of your maintenance procedures on S/N ratio.

[allene]

Actually I purchased the GC-MS as a second-hand instrument which has been out of service for 6 years due to detector crashing. I replaced the detector with a second-hand one (an electronic-man of our lab replaced it).

I changed all possible contamination source with new one such as column, septum, carrier gas and liner. I don't think the noise comes from column bleed or other contamination because no specific ion mass is seen in the mass spectrum: the intensity of all ion masses (m/z 50-650) are approximately equal. I think it comes from detector, but I don't know is it possible that the noise to come from detector while s/n is passed in tuning with cal gas or not?

I have low s/n in both TIM an SIM mode, but the HCB test was done in SIM mode. Unfortunately, there is no previous record about S/N (I've purchase the instrument as a second-hand and the vendor doesn't know about it). But it is clear that S/N of 10 for 10 ppm of naphthalene or anthracene (in TIM) or not getting S/N of 10 for 20 ppb of HCB (which should be get for 2 ppb) in SIM mode is not satisfactory for GC-MS.

I was deprived of the Leco service because I replaced the detector myself !

[dblux_]

I hope your instrument will be repaired someday.

A while ago there was a discussion about faulty Pegasus III:

viewtopic.php?f=3&t=17300&start=0

Is the vacuum in spectrometer chamber within specification ?

[Peter Apps]

Do you have gas scrubbers on your carrier gas immediately upstream of the GC , and are they fresh ? Changing gas cylinders just gives a fresh set of the same contaminants. What about your gas lines, what are they made of and how were they cleaned ? Do you have any other instruments using the same gas.

There is a good chance that you are seeing detector or electronic noise. I cannot see why Leco would pass up the chance of selling a new detector and charging to install it, but it asking a lot to expect them to troubleshoot over the phone for example - where are you located ?

Peter