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

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

The FID is a two dimensional signal and NIST will not be able to read FID data. Perhaps we need more information here, can you see the FID signal in data analyses ?

Hi!

Yes, we see some peaks in FID signal which are not corresponding to MS spectrum.

I think we need to change the analytic method.

Do you have the same system like us? The sample is sent from inlet to splitter which splits it in two columns, one connects with FID and another with MS. It seems no sample goes to the column connected with FID.

Best regards

Hi,

Yes, we have CFT splitter (G3181 60500) in our system, and two same size columns (HP-5MS) (5m x 0.25mm x 0.25µm).

We actually do not know how to control splitter input. Could you suggest us the way to do it. Here is a part information of our method:

Column #1
Agilent 19091S-433: USF729013H
HP-5MS  5% Phenyl Methyl Silox
325 °C: 30 m x 250 µm x 0.25 µm
In: Front SS Inlet He
Out: Vacuum

(Initial)                                    45 °C
Pressure                                     7.3614 psi
Flow                                         1 mL/min
Average Velocity                             36.354 cm/sec
Holdup Time                                  1.3754 min
Flow Program                                 On
    1 mL/min for 0 min
Run Time                                     139.75 min

Column #2
Agilent 19091S-433: USF731114H
HP-5MS  5% Phenyl Methyl Silox
325 °C: 30 m x 250 µm x 0.25 µm
In: Front SS Inlet He
Out: Front Detector FID

(Initial)                                    45 °C
Pressure                                     7.3614 psi
Flow                                         0.5561 mL/min
Average Velocity                             15.963 cm/sec
Holdup Time                                  3.1321 min
Pressure Program                             On
    7.3614 psi for 0 min
Run Time                                     139.75 min

Front Detector FID
Heater                                       On    250 °C
H2 Flow                                      On    25 mL/min
Air Flow                                     On    400 mL/min
Makeup Flow                                  On    25 mL/min
Const Col + Makeup                           Off
Flame                                        On
Electrometer                                 On

Best regards,

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