Chromatography Forum

Hello all,

Recently we need to do performance check for the GC-MS (6890-5973N, EI mode). But no one in our lab have experience about using Chemstation to perform s/n check.

I tried to inject 1ul of 1ppb OFN solution, scan from 50-300amu, click Instrument Control --> Qualify --> Sensitivity Check.

After around 6 min, it come out a report but no peak of m/z 272 at all. And the s/n is 15.

May I know:
1. The normal range of s/n of 5973N? (some data sheet said 60:1, some said 20:1, how about yours GC-MS?)
2. Is my step correct? (Instrument Control --> Qualify --> Sensitivity Check)
3. I found that in the Data Analysis --> Tools --> Sensitivity. What is the difference between this and pt.2?
4. If no m/z 272 found. What action should I do? (e.g. clean ion source?)

Thanks for your help

Hi,

I don't know why you have to do a sensitivity check on your device. I wouldn't care about certain masses.
The 5973N has a built-in vial with PFTBA-solution which can be used for checking the MS.
All I do from time to time is a AUTOTUNE and EVALUATION. The system opens a valve for a short time which lets evaporate a small amount of PFTBA. If the three characteristic masses are found, the system should be ok and there's no need to assume that other masses will not be found.
You can do an autotune every week, let the system print a report and make a statistic of the counts at the three main masses. The value should be (relatively) constant.

Greetinx,

- Cmdr. Keen

Hi Anthony,
When using the OFN to perform the sensitivity check , that tells you not only how well the MS is performing, but also the condition of the column, and injector port and if there are any septum leaks.

You need to use the OFN_SN.m method that came with the install, as this method ups the electron multipler, so that electronic noise is included, the threshold value is set to 0 too.


, but it also uses a specific macro, that performs a signal to noise by finding the 272 ion, and then plots a EIC that is .05 amu to either side of the 272 ion, not the typical -.3 to +.7 amu EIC, that you typically see if you used tools->signal to noise check.

my suggestion is copy C:\msdchem\1\5973N\ofn_sn.m
to C:\msdchem\1\METHODS

load up the C:\msdchem\1\METHODS\ofn_sn.m method, check that the tune file is correct, and that the EM is set to +400.

Inject some OFN.

The method automatically printouts out a Sign to noise sheet, in which it calculates the RMS sig2Noise ratio.

, also OFN check is used typically for a brand new system, new column etc.. , so daily use will degrade response.

I suggest buying some OFN, and making a dilution to a level that you need your instrument to work to. eg you don't need an instrument to do trace analysis, if the samples being analysed aren't trace analysis quantities.


Alex

It can be very difficult to pass OFN signal to noise, trust me.

Have you recently cleaned your source and done inlet maintenance? If not, do. Or don't run OFN, because you won't get good results if you don't have a nice clean system.

I would recommend reducing the inlet temperature and final oven temperature a bit from what the OFN_SN.m defaults are. Drop your inlet T and final oven T 20 degrees.

Use pulsed splitless if you aren't aleady.

Inject several times, you'll probably find that your signal to noise jumps a pretty decent amount over a few injections. On tricky systems I sometimes make a couple different methods with different gain values in each method, I then run a sequence with the different methods to determine what gain setting is optimal for the particular system. This may be a bit overkill for your purposes ;P.