Design and construction of a simple Knudsen Effusion Mass Spectrometer (KEMS) system for vapour pressure measurements of low volatility organics

A design of and initial results from a Knudsen Effusion Mass Spectrometer (KEMS) are presented. The design was adapted from high temperature alloy studies with a view to using it to measure vapour pressures for low volatility organics. The system uses a temperature controlled cell with an effusive orifice. This produces a molecular beam which is sampled by a quadropole mass spectrometer with electron impact ionization calibrated to a known vapour pressure. We have determined P_{(298 K)} and Δ H_sub of the first 5 saturated straight chain dicarboxylic acids: 2.15±1.19×10 ^-2 Pa and 75±19 KJ mol ⁻¹ respectively for oxalic acid, 5.73±1.14×10 ^-4 Pa and 91±4 KJ mol ⁻¹ for Malonic acid, 1.13±0.47×10 ^-4 Pa and 93±6 KJ mol ⁻¹ for Succinic acid, 4.21±1.66×10 ^-4 Pa and 123±22 KJ mol ⁻¹ for Glutaric acid and 6.09±3.85×10 ^-6 Pa and 125±40 KJ mol ⁻¹ for Adipic acid.