GCMS-FID Split question

Question

Hi I had a question about GC MS Split (gc to FID/MS). I have a thermo splitter that has no auxillary flow to counteract the ms vaccuum, but want to change out the capillaries that come recommended with the splitter. The recommended capillary dimensions for the FID Capillary is 60cm with 170um /0.363 mm and the MS capillary is 240cm with 150um/0.363 mm. I was wondering if I could uses different capillaries and if so if there was an equation I should use to calculate compatibilities(dimensions/lengths).

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Dr. Know · Answer

Calculate Length of Restrictions

The volumetric flow through an empty pipe is determined by the Hagen-Poiseuille law:

, where

F = volumetric flow

ID = internal diameter of the pipe

ΔP = pressure drop between inlet and outlet

L = the pipe length

η = the dynamic viscosity of the gas

Flow and pressure drops are dependent on length and diameter of the pipe.

If one pipe should be replaced by another one with the same "flow resistance", then the pipe flow and pressure drop have to be the same as well as the gas.

That means pipes of different dimensions are equal, if ID⁴/L = const. and the length of the second pipe equals the length of the first one times the ratio of the squares of the diameters: L₂ = L₁· ID₂⁴ / ID₁⁴

GCMS-FID Split question

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Calculate Length of Restrictions

Calculate Length of Restrictions