RSKSOP-175 Headspace gas chromatography for dissolved gas measurement

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phases of headspace vial used in gas chromatography


calculations

using known temperature of sample, bottle volume, concentrations of gas in headspace (as determined gc), , henry s law constant, concentration of original water sample calculated. total gas concentration (tc) in original water sample calculated determining concentration of headspace , converting partial pressure , solving aqueous concentration partitioned in gas phase (cah) , concentration remaining in aqueous phase (ca). total concentration of gas in original sample (tc) sum of concentration partitioned in gas phase (cah) , concentration remaining in aqueous phase (ca).


tc = cah + ca


henry s law states mole fraction of dissolved gas (xg) equal partial pressure of gas (pg) @ equilibrium divided henry s law constant (h). gas solubility coefficients used calculate henry s law constant.


xg=pg/h


after manipulating equations , substituting volumes of each phase, molar concentration of water (55.5 mol/l) , molecular weight of gas analyte (mw), final equation solved for.


tc=[(55.5 mol/l)* pg/h*〖10〗^3 mg/g]+ ([vh/((vb-vh) )]*cg*mw(g/mol)/22.4(l/mol) *[273k/((t+273k)]* 〖10〗^3 mg/g)


where vb bottle volume , vh volume of headspace. cg volumetric concentration of gas. full calculation examples reference rsk-175sop.


practical considerations

one of major concerns method reproducibility. due nature of calculations, method reliant on temperatures constant , volumes exact. when gases spiked manually gc, speed , technique in analyst plays role in reproducibility. if 1 analyst faster in removing gas vial , injecting onto instrument, important have same analyst run on calibration prepped, otherwise error more introduced. headspace auto-sampler may remove of error, constant heat , variable temperature on instrument becomes issue.








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