Using G3180B 2-way splitter with makeup excel calculator

The MS is more sensitive than the FID and too much sample can result in ion source or detector overload.  Too much sample into the MS will also result in more ion source cleaning and other system maintenance.  The smallest peak in the lowest concentration standard only needs to be about 3-5x the average baseline noise.   ( see: Which Electron Ionization Ion Source? Part 2 of 4 )    Are you sure you want approximately a 1:1 split?    If you aren't sure, start with 1:4  MS:FID or something like that. 

Since the restrictor pieces into the MS and FID are at constant pressure, the flows in those segments will change with the oven profile so the split ratio will also change somewhat during the run.  They can't each be run in constant flow as they are connected to the same supply but one exits to vacuum and one to atmospheric pressure so need different programmed pressures.  This can be minimized a bit by having shorter restrictors so less column pieces exposed to the oven temperature. However they need to be long enough, too. 

3.8 psi at the splitter is from the beginnings of this type of splitting. It's quite arbitrary.  The important thing is for the two restrictor lengths to work out so that the flow into the MS is from 0.8 to 1.4 ml/min and the flow to the FID is less than or equal to 30 ml/min.  The inlet pressure at the oven maximum temperature with the outlet at the splitter pressure needs to not exceed 100 psi.

It is difficult to accurately control very low pressures like 1.0 psi, so 3.8 psi, 4 or 5 or so is better, remembering that the higher the spitter pressure the longer or narrower the restrictors need to be.

Also remember that with a splitter you cannot turn off the column flow and splitter pressure or else the MS vacuum may suck air backwards from the FID and splitter and oxidize the hot ion source and quadrupole.

It may take some experimentation to get it perfect.

Thanks Paul. Yes, 1:1 was chosen as what I presumed a general default, so appreciate the recommendation on better general purpose split ratio along with a clearer explanation of the other design goals compared to what i could find in manuals or online.  So under those constraints I'm guessing 0.779m of .10 mm capillary to the MS and 0.907m of 0.18 mm ID capillary to the FID would be a good starting point (or possibly extrapolating to a shorter length of 0.15 mm capillary even though the calculator doesn't include it)? 

As for turning off flows, I assume this risk is when turning of both flows, correct?  Is it fine for the column flow to stop provided there's still 5psi supplied to the splitter? 

Turning off the inlet flow only when doing inlet maintenance is good, but turning it off just to reduce helium usage is not.  In this configuration if the inlet is turned off and the splitter is still at 5psi, the flow will go backwards through the column and try to exit the inlet. 

You can also use the GC Calculators in MassHunter Acquisition to figure this out, it's just a bit more complicated.

5 psi on 0.779 m of 100µm at 250°C to vacuum yields 0.371 ml/min

5 psi on 0.907 m of 180µm at 250°C to atmosphere yields 1.48 ml/min - a 1:4 split ratio.

5 psi on 0.44m of 150µm at 250°C to atmosphere yields just about 1.48 ml/min -- the same 1:4 split ratio

Make sure to consider the analytical column flow. Are you using a 0.25 mmid column at about 1 ml/min?  Or a 0.32 mmid column at about 3 ml/min?  The analytical column flow must be less than the total flow out of the two restrictors so that some flow is added at the splitter plate.  If not, the pressure regulation at the splitter plate won't be as accurate and the excess flow may even go up the splitter flow line and contaminate it.

Thanks! Yes, 0.25 mmid column at 1 ml/min.  And very much appreciate you showing an example of how to do this in the GC calculators as well (the manuals briefly mention using the calculators, but your example really helps tie it all together).

This explanation may help explain why an MS/FID method may need more adjustment - The detectors are vacuum and atm, but the method's Column "Control Mode" in the software has the restrictors set to constant flow. When you say to have the pressure be 4 or 5 psi at the splitter, does that mean this setting should be set at constant pressure for the restrictors?

The restrictor from CFT splitter to MS is set as column 3 above – When this is set to constant pressure control, then the makeup gas supply will be adjusted to maintain the pressure at the splitter as the temperature changes?

Electronic Pneumatic Control devices control pressure.  "Constant Flow" is pressure programming done using calculations based on the restriction of the column/restrictor and the temperature. With two restrictor segments coming out of the splitter, one to vacuum and one to atmospheric pressure, if you want the split ratio between the two detectors to be constant through the run, the splitter control should be set to Constant Pressure. 

If you set one restrictor to Constant Flow and the other is reliant on it, as in your example, the split ratio between the detectors will change with oven temperature.

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So at 100°C the split ratio is 3.466/0.4 = 8.665:1  and at 250°C the split ratio is 4.793/0.4 = 11.198:1 

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If the splitter is set to 6.0 PSI constant pressure:

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At 100°C the split ratio is 3.015/0.372 = 8.105:1  and at 250°C the split ratio is 1.709/0.211 = 8.099:1 -- essentially the same.

The MSD prefers constant flow into it as changes in the ion source pressure affect ion creation and ion transmission.   FID+MSD splitter applications require some dancing around with the parameters knowing what each change causes.