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

Abstract

Abstract.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 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.

Piezoelectric crystal microbalance measurements of enthalpy of sublimation of C<sub>2</sub>–C<sub>9</sub> dicarboxylic acids

“…A good temperature range to monitor enthalpy variation is 30-55 • C, far away from the point where succinic acid changes its structure (∼ 137 • C) (Vanderzee and Westrum, 1970). In this range, the average enthalpy of sublimation measured is 113.3 ± 1.3 kJ mol −1 , in agreement within 5 % with the previous works (Chattopadhyay and Ziemann, 2007;Davies and Thomas, 1960; Table 4). Considering the vacuum environment and an upper temperature larger than 55 • C, the retrieved enthalpy may not be reliable for the transformation (initial phase) of succinic acid crystalline form (monocline/triclinic prisms) into cyclic anhydride, a ring structure (pyramidal crystal), losing one water molecule (Orchin et al, 2005;Vanderzee and Westrum, 1970).…”

“…In this dehydration reaction, its molecular structure changes from monocline to rhombic crystals and becomes anhydrous (Bahl and Bahl, 2010). In our experiment, we considered a monocline dehydrate oxalic acid and the obtained enthalpy results (Table 4) differ to previous works, relative to the anhydrous form (Booth et al, 2009). On the contrary, our results, i.e.…”

“…Our results were compared with previous works (Table 4) that use a similar effusion method and theoretical approach (i.e. Albyn 2001;Booth et al, 2009;Ribeiro da Silva et al, 2001) and with works that analyse the same dicarboxylic acids used in our study (Chattopadhyay et al, 2007;Bilde et al, 2003 …”

“…On the right side and left side are the contact points for the proximity electronics. (Chattopadhyay, 2007). In this work our accuracy has been improved as described in Sect.…”

“…the thermal desorption particle beam mass spectrometry (TDPBMS) method (Chattopadhyay and Ziemann, 2007), the Knudsen effusion mass-loss method (Ribeiro da Silva et al, 2001), Knudsen effusion mass spectrometry (KEMS) (Booth et al, 2009), the tandem differential mobility analyser (TDMA) technique (Bilde et al, 2003), and the effusion method (Davies and Thomas, 1960;Granovskaya, 1948). Discrepancies between results obtained by the different methodologies were found to be up to 2 orders of magnitude, and this makes the acids' characterization even more difficult.…”