Effect of Pr-Ca substitution on the transport and magnetic behavior of LaMnO3 perovskite

The effect of simultaneous substitution of a fluctuating cation and a divalent cation in LaMnO₃ perovskite modifies the properties of the material to exhibit large valence colossal magnetoresistance (CMR) effect. A good example of these properties is (La_1−2x Pr _x Ca _x )MnO₃ (LPCMO) type CMR material. In this communication it is reported that, with the increase in x (for x=0.1, 0.15, 0.2), the T _c varies between 100 and 120 K with improvisation in metal-insulator transition. Interestingly, resistance increases with x from few hundred ohms to few kilo ohms with corresponding decrease in the unit cell volume. The results of the studies using X-ray diffraction (XRD), electrical resistivity, magnetoresistance and ac susceptibility measurements on LPCMO samples for understanding the structural, transport and magnetic properties are discussed in detail.     

Structure and absolute configuration of an unsaturated anteiso fatty acid from Bacillus megaterium

Gas chromatography in combination with electron ionization mass spectrometry (GC/EI-MS) was used to determine the fatty acids of a membrane lipid from Bacillus megaterium. Special attention was put on the structure and absolute configuration of a monoenoic fatty acid previously described in this sample. GC/EI-MS operated in the selected ion monitoring mode was used to determine twelve fatty acids in the bacterium. Methyl esters were prepared to verify the presence of a 14-methylhexadecenoic acid (a17:1) isomer. The position of the double bond of the a17:1 isomer and four further monoenoic fatty acids was elucidated by means of their picolinyl esters produced by the transesterification of the phospholipid. For the a17:1 isomer, the double bond was located between C-5 and C-6. Silver ion liquid chromatography was used to verify that the double bond was in cis-configuration. The bacterial 14-methylhexadec-5-enoic acid (a17:1Δ5) is chiral due to the stereogenic C-14 carbon. Initial enantioselective measurements were carried out with isomers of a17:1Δ5 which were available in form of racemic and (S)-enantiopure cis- and trans-isomers of a17:1Δ12 previously synthesized. The cis-a17:1Δ12 enantiomers were partly resolved on a chiral stationary phase coated with 50% heptakis(6-O-tert.-butyldimethylsilyl-2,3-di-O-methyl)-β-cyclodextrin in OV-1701 (β-TBDM). However, resolution of the enantiomers of the trans-isomer of a17:1Δ12 failed. Only one peak was also observed for the a17:1Δ5 isomer from B. megaterium. Thus, it remained unclear whether the compound a17:1Δ5 was racemic or enantiopure in the sample. To clarify this point, we separated the cis-monoenoic fraction from the saturated fatty acids. Then, the monoenoic fraction was hydrogenated in order to transform a17:1Δ5 into 14-methylhexadecanoic acid (a17:0). This chiral fatty acid was known to be sufficiently enantioseparated on the β-TBDM column and was found to be (S)-enantiopure in the sample. Hence, these measurements verified that the B. megaterium sample contained enantiopure (S)-a17:1Δ5.     

Enantiomeric resolution of persistent compounds of technical toxaphene (CTTs) on t-butyldimethylsilylated β-cyclodextrin phases

Summary Eight of the most important single compounds of technical toxaphene were separated on t-butyldimethylsilylated β-cyclodextrin (β-BSCD) diluted in a medium polar phase using gas chromatography with electron capture detectors (GC-ECD). The enantiomeric resolution of all compounds was obtained in one GC run. The β-BSCD phase also separated the enantiomers of oxychlordane, cis- and trans-chlordane as well as α-HCH. Problems in the enantioselective determination of CTTs in biological samples are discussed. Finally, the enantioselective determination of the two most recalcitrant CTTs in biological samples was achieved using electron capture negative ionization mass spectrometry (GC-ECNI-MS) in the single ion monitoring (SIM) mode. Presented at the 21^st ISC held in Stutgart, Germany, 15^th–20^th September, 1996.     

关于K个复相关矩阵相等的检验

§ 1.Ｉｎｔｒｏｄｕｃｔｉｏｎ　ＷｅａｒｅｇｉｖｅｎｋｉｎｄｅｐｅｎｄｅｎｔＷｉｓｈａｒｔｄｅｎｓｉｔｉｅｓｏｆｔｈｅ (ｐ +ｑ)× (ｐ +ｑ)ｒａｎｄｏｍｓｙｍｍｅｔｒｉｃｐｏｓｉｔｉｖｅｄｅｆｉｎｉｔｅｍａｔｒｉｃｅｓＧ1,… ,Ｇｋｔｏｂｅｇ(Ｇｉ) =Ｋｅｘｐ -12 ｔｒＲ- 1ｉ Ｇｉ Ｇｉ12 (ｎｉ- ｑ-ｐ- 1) ,(1 )ｗｈｅｒｅｉ=1 ,… ,ｋ,ａｎｄＲｉｄｅｎｏｔｅｓｔｈｅｐｏｐｕｌａｔｉｏｎｃｏｒｒｅｌａｔｉｏｎｍａｔｒｉｘｏｆｔｈｅｉ ｔｈｐｏｐｕｌａｔｉｏｎａｎｄＫａｓａｇｅｎｅｒｉｃｌｅｔｔｅｒｄｅｎｏｔｅ…     

Enantioseparation of chiral organochlorines on permethylated beta- and gamma-cyclodextrin, as well as 1:1 mixtures of them

Gas chromatography columns coated with 10% permethylated beta- and gamma-cyclodextrin in 85% dimethyl-15% diphenyl polysiloxane (beta- and gamma-PMCD, respectively) and 1:1 mixures are prepared and tested with regard to the enantioseparation of chiral chloropesticides. On the columns with the individual O-methylated cyclodextrins (O-tCDs), the enantiomers of aaeeee-hexachlorocyclohexane (alpha-HCH), e-aeee-1,3,4,5,6-pentachlorocyclohex-1-ene (beta-PCCH), and e-aaee-1,3,4,5,6-pentachlorocyclohex-1-ene (gamma-PCCH), cis- and trans-chlordane, and cis-heptachlor epoxide are separated on both columns, with the exception of the latter being separated only on beta-PMCD. On the column coated with 5% beta- and 5% gamma-PMCD, the resulting separation factor (a) is virtually 1/2 of the arithmetric mean of the elution-dependent separation factors on the individual O-tCDs. In case of reversed elution order on beta- and gamma-PMCD, the enantiomers are not resolved on the mixed columns as is the case with cis-chlordane. Likewise, the lower resolution of the gamma-PCCH enantiomers on the mixed columns prove the reversed elution order on beta- and gamma-PMCD without having enantioenriched standards available. On the column coated with 5% beta- and 5% gamma-PMCD, similar retention times to those observed on both 10% beta-PMCD and 10% gamma-PMCD are obtained. On the column coated with 10% beta- and 10% gamma-PMCD, significantly longer retention times are obtained compared with the columns that contain a total of 10% chiral stationary phase (CSP). This indicates that a relevant part of the interaction of the analytes with the chiral selector is non-enantioselective and, thus, only delays the elution of both enantiomers. Moreover, these non-enantioselective interactions prevent a direct comparison of CSPs with different amounts of the chiral selector. However, this is possible by using mixed phases of two CSPs with similar properties. Using this system, it is demonstrated that for the organochlorine compounds studied, no higher separation factor is observed on the mixed CSPs than on the individual O-tCD with the higher separation factor. Estimations allow a prediction that enantioseparations of organohalogen compounds can be achieved on columns coated with as little as 1% of the CSP.     

Offene W?gung hygroskopischer K?rper durch Extrapolation

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