Pls. check my method for preparation of std. gas and advice me

Chromatography Forum: GC Archives: Pls. check my method for preparation of std. gas and advice me     By Mustafa El Said on Monday, February 18, 2002 - 12:30 pm:

I urgentlly need Standered He gas & no time to order for std. gas Mix. from manfact. so, I have doen some things, but, I do'nt know exactlly if this method is correct or not, so it w'll be so kindlly from any scientest to give me his advice, : my method based on the follwing equations:

PV = nRT (gas equation)

since i'll use one cylender to mex the gases so the volume w'll be constant.
R =const. & T =const.

so, P = n RT / V by taking RT/V=X=const.
so, P = nX since; n (no. of moles) = Wt./MWt.
so,P = (Wt./MWt) *X

if I have 2 gases as H2 & He with diff. pressures so, P(H2) = [Wt.(H2) / MWt.(H2)]*X .............eq. 2

and, P(He) = [Wt.(He) / MWt.(He)]*X .............eq. 2

dividing eq. 1 by eq. 2
P(H2) / P(He) =[Wt.(H2) / MWt.(H2)] / [Wt.(He) / MWt.(He)]

so, Wt.(H2) / Wt.(He) =P(H2) * MWt.(H2) / P(He) * MWt.(He) .................eq.I

I applyed this (eq.I) by the following method ,in an evacuted cylender I have applyed a 5 psi He and inc. this prss. to 1015 by H2 ,i.e. 5 psi He + 1010 psi H2 and since MWt. of H2 ~ 2 & MWt. of He ~ 4
and by applying (eq.I)
Wt.(H2) / Wt.(He) = (1010 * 2) / (5 * 4)
= 101
i.e. Wt.(H2) : Wt.(He) = 101 : 1
since the sum of all partes of the ratio is, 101+1 = 102
so, the percentage of Wt.(He) to the sample is, (1/102) *100 = 0.98 Wt. %

and by taking diff. pressures to each He & H2 , I can obtaine diff. standerdes to make the caliberation to the GC to detect the He percent. in any gas sample by using N2 as a carrier gas.

also, I want to ask about the separation between He & H2 by Rt. Ms. 13X plot Or suplco SPB-1 if it is posible or not , scince I do'nt try yet.

Many thanks

Mustafa El Said     By Anonymous on Wednesday, February 20, 2002 - 06:29 am:

You are correct except for saying weight%. Your calibration is done by mol% or partial pressures or volume/volume % if you prefer (They are the same). Mass or molecular weights are not needed. Absolute pressure of He in the cylinder is 15 + 5 psi. Absolute pressure of H2/He mixture is 15 + 1010 psi. Therefore He comprises 20/1025 as a mol fraction, or 2 (mol %). That's all it is. It is usual to report results as mol%.

Don't even think about H2/He separation on SBP-1. It will work on a Mol Sieve 5A PLOT 25m x .32mm capillary at 50C and below (at 16 psi nitrogen carrier, 50C, the r.t was about 1.18 min for He and 1.27 min for H2). I have never tried a Mol Sieve 13X PLOT. I suspect H2/He separation is poor.     By Anonymous on Friday, February 22, 2002 - 10:35 am:

If the pressure is measured absolute (psia), as is probably the case if you are blending into an evacuated cylinder, the concentration should be 5 psia He/ 1015 psia mixture = 0.5 mol% He.

Be advised that the calibration curve via TCD wil be non-linear, especially at higher concentrations.