Isobaric separation

The advantages of pressure programming are demonstrated in Fig. 10 by comparing isobaric separations at 30 and 100 bar with a separation obtained by pressure-programming at column inlet pressures increasing from 30 to 225 bar (73). At low inlet pressure the early peaks are separated and eluted within 5 min, whereas the last one is eluted with tailing after an additional 5 min. The linear flow velocity is 1.1 cm/sec At 100 bar the... 

At sufficiently high energies and for cases where the radioisotope has a higher Z than the stable isobar, separation by fully stripping with subsequent magnetic analysis can be accomplished. This method has been investigated for the systems "Al""-"Mg "", >an+ 36gi6. k W and Ni "- Co ". Isobar... 

Lasers are very powerful instruments to separate elements. Since the separation of isobars from different elements is the most difficult task in AMS, the use of lasers in connection with AMS could provide a very effective clean-up of background. The basic idea in a recent proof-of-principle experiment at the Rehovot AMS facility was to clean a negative ion beam from unwanted isobaric background ions by selective electrons detachment. S ions which have an electron affinity of 2.08 eV were effectively neutralized by interaction with 2.33 eV photons from a pulsed Nd YAg laser. The same photons did not affect Cl ions whose electron affinity is 3.62 eV. This clearly demonstrated that a laser depletion of S background in C1 measurements is feasible, opening up the possibility for sensitive C1 measurement at small AMS facilities where the ion energy is too low to perform isobar separation. However, for actual applications in AMS measurements, a substantial improvement in overall efficiency of the laser depletion process is necessary. 

Figure 7.8. Separation of Triton X-114 by SFC using programmed elution on a 10 cm x 2 mm I.D. Nucleosil Cig column, 3 p,m packing, at 170°C with UV detection at 278 nm. Separation on the left isobaric conditions at 210 bar with a mobile phase of carbon dioxide and methanol (2 + 0.125) ml/min. Center separation composition gradient from 0.025 to 0.4 ml/min methanol over 8 min with other conditions as above. Separation on the right pressure program from 130 to 375 bar over 8 min with the same mobile phase used for the isobaric separation. (From ref. [137] Copyright Preston Publications, Inc.)...

The positive value indicates that work must be done on the mixture to achieve an isobaric separation. In a real isothermal-isobaric separation of ideal gases, more than this minimum amount of work would be needed, because a real process would be irreversible. Moreover, when separating real mixtures (whose components have inter-molecular forces), the total minimum work would not be given by (4.1.47). However, it could still be determined from O using (4.1.46), provided a reliable model were available for the Gibbs energy of the mixture and each pure. Expressions for G of real mixtures would be more complicated than the ideal-gas expression (4.1.47) but such expressions could be obtained from model equations of state. 

This means that it is thermodynamically possible to separate an ideal-gas mixture into its pure components without doing any work, if the process is performed at constant T and V. But in real separations of ideal gases some amount of work would be needed to overcome irreversibilities. The important lesson here is that the minimum work to perform the isometric separation, given by (4.1.50), differs from that for the isobaric separation, given by (4.1.47). 

This gives the minimum work needed to achieve an isothermal-isobaric separation. If the mixture were an ideal solution, then all the y, = 1, and we would have... 

For forensic analysis, target metabolomics is focused on the quantitative and qualitative analysis of specific metabolites such as drugs in blood, urine, tissue, or hair. In a recent study using IM-MS, metabolites in hair samples were analyzed. Four of the nine hair samples were positive for caffeine and nicotine and one sample tested positive for methamphetamine in the mass spectrum. However, when mobility data was considered the caffeine and nicotine were found to be true positives while the methamphetamine was determined to be a false positive. The mobility values for the unknown with the mass of methamphetamine did not match that of the methamphetamine standard. This is an excellent example of utility of isomeric/isobaric separation in mobility space. 

Another application of target metabolomics illustrative of the importance of isomeric/isobaric separation is in the field of pharmacokinetics. Pharmacokinetic studies focus on the absorption and distribution of drugs and their metabolites in biological systems. " A limiting factor in pharmacokinetic research is a requirement of high-throughput screening of isomeric metabolites. IM-MS is a rapid technique where isomeric separations occur within minutes. For example, IM-MS has been used to separate opiate metabolic isomers in less than 1 minute. ... 

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