1999; Johnson et al. 2000; Konermann et al. 2008). In the first step, the gas is adsorbed onto the surface
of the membrane; in the second step the analyte molecules enter the membrane (permeation); and the third step is desorption of the molecules into the vacuum on the other side of the membrane. The gas transmission rate (k trans) across the membrane is given by Fick’s law of diffusion (Hoch and Kok 1963) $$ k_\texttrans = (P \, A \, \Updelta p)/l $$ (3)and is Thiazovivin ic50 defined by the gas permeability (P) constant,2 the area of the membrane inlet (A), the partial pressure difference across the membrane (∆p), and the membrane thickness (l). As the partial pressure of gases on the low pressure (vacuum) side of the membrane is very small, the transmission rate is proportional to the gas concentration in the liquid phase. The overall sensitivity (gain factor) of detection is greater for thinner membranes and membrane types with high permeability. There are also effects due to relative diffusion of different molecular weight gases and “stickiness” of gas such as CO2. Therefore, for
quantitative measurements, calibrations need to be performed for each different analyte using a volume of a liquid or calibration gas. The choice of membrane depends on the experiment. If high sensitivity is required then a highly permeable membrane and a large inlet area are advantageous to facilitate a higher rate of gas sampling.
It may also be possible in some circumstances to operate with a higher vacuum to influence greater gas Belinostat solubility dmso transmission. In contrast, if long term sampling is required with near constant background gas concentrations, then a low consumption (i.e., thicker) membrane is required and/or use of a small sampling area. Most membranes have a good chemical resistance and if measurements are undertaken at elevated pressure (e.g., 20 bar) a supported membrane with an embedded metal grid can be used. A range of membranes suitable for MIMS applications are the following: silicone Methane monooxygenase membranes (MEM-213, Mem Pro); Teflon films such as FET or AF (DuPont); silicone rubber; oxygen electrode membranes3; HDPE plastic films (various sources); silicon membranes with embedded metal grid (Franatech GmbH, Germany). Thus, the choice of MIMS sensitivity versus gas concentration stability is an important factor in the experimental design. Isotopic enrichment Isotopes are defined as atoms with the same number of protons, but a different number of neutrons and thus differ in atomic weight. There are 80 elements with stable isotopes (26 with only one isotope) and 94 elements that occur naturally on earth. The MIMS approach makes use of the stable isotopes which can be found at natural abundance or purchased from many suppliers in varying enrichments. Table 1 lists many elements that are useful to study with photosynthesis and selleck inhibitor respiration in plants.
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