GC-FID/MS

Hi huyenchalmers,

Can you let us know what instrument and software you are using? The second signal is usually just a matter of setting it up in the signals page. I will be able to give you more info once I know what you have.

Regards

James

Hi James!

We have GC (Agilent 7890A) and MS (Agilent 5975C), spectral interpretation by using the NIST MS Search programme version 2.0.

It is very good if you can help us with this.

Best regards,

Huyen

Good morning,

I work for Agilent and support GCMSD and CFT splitters. Depending on how you are splitting, CFT plate, or simple union etc, back to basics checking connections is the place to start. Keep in mind that the MSD has a vacuum on the end of the column so with the same size columns more flow will go to the MSD. Are you splitting using a CFT splitter? Let me know more about the splitter input and I maybe able to make better suggestions.

Regards

James

I see that your run is over 2 hours long? This maybe difficult to determine the issues you are having through the Agilent community.  Do you see a solvent peak on the FID after injection?

James

Hi, huyenchalmers~

Because your detectors have two different exit pressures (ambient - FID and vacuum - MSD), you will not have retention time alignment with equal lengths of the same column after the splitter.  You need to calculate different lengths/inner diameters to provide equivalent restriction between the two detectors.  If both of your detectors had ambient exit pressures, this approach would be correct. 

Separations are typically done pre-splitter, so many people choose to use uncoated column (restrictors) with sub-250 micron inner diameters between the splitter and the detectors to minimize the amount of column in their GC oven.   Other factors worth considering are holdup time (how long it will take your sample to travel the additional length of column) and the split ratio between detectors.  If an equal split is desired, your calculations will be different that the case of a 10:1 FID/MS split. 

Un-purged splitters ship with a disk that contains a spreadsheet to help you balance the restrictors along with selections of restrictors lengths with 4-5 different inner diameters.  While we generally recommend a purged splitter with MS detectors, an unpurged one will work well once the restrictors are balanced and all connections are leak-free. 

Best of luck, and keep us posted!

Abbey

Hi!

We have run the same sample with short time (about 14 min long) but the observation is more or less same with long time (about 2 hours long). That is why we can not discover what is wrong.

We set up solvent delay at 3 min. So we can not see the solvent peak in MS. But we see a big peak after 3 min in FID detector which is not observed in MS spectrum. I have no idea what this big peak is actually.

Best,

Huyen

Hi Abbey!

Thanks for information!

Since our two columns have the same length/inner diameter, should we use the flowrate for calculation to provide equivalent restriction between the two detectors?

How to input the split ratio between detectors and a purged splitter?

This is the reason why we need help with the setup of method for GC-FID/MS.

Best,

Huyen

Of course,  Happy to help.

You will not be able to achieve an equal split with an MS and FID using the same length/ID columns.  You will need to use the excel spreadsheet on the software disk that shipped with your splitter.  You more restriction to the MS when compared to the FID.   A pressure is applied from the inlet, and the restriction of the two columns will create the split. 

Abbey

Of course,  Happy to help.

You will not be able to achieve an equal split with an MS and FID using the same length/ID columns.  You will need to use the excel spreadsheet on the software disk that shipped with your splitter.  You more restriction to the MS when compared to the FID.   A pressure is applied from the inlet, and the restriction of the two columns will create the split. 

Abbey