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Michael Lämmerhofer

Publications and source records attributed to Michael Lämmerhofer.

At least 19 recordsLinked to original sources
PubMed2006

Validated method for the determination of the ethanol consumption markers ethyl glucuronide, ethyl phosphate, and ethyl sulfate in human urine by reversed-phase/weak anion exchange liquid chromatography-tandem mass spectrometry.

A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the determination of the ethanol consumption markers ethyl glucuronide (EtG), ethyl phosphate (EtP), and ethyl sulfate (EtS) in human urine was developed. A reversed-phase/weak anion exchange type stationary phase demonstrated particular suitability for the analysis of these highly polar acids. Sample preparation was minimized to centrifugation and dilution of urine prior to injection. The method was validated in the range of 5-750 microg.L(-1) with 1:20 and 1:1000 diluted urine, which corresponds to actual concentration ranges of 0.1-15 and 5-750 mg.L(-1) in undiluted samples. Method validation was carried out using six different lots of human urine. Over the entire calibration range intraday and interday precision (each n = 5, three concentration levels per dilution factor) adopted values between 0.6 and 4.7% and 0.8 and 12.1%, relative standard deviation, respectively. Corresponding accuracy values ranged between 94.2 and 113.5% and 86.6 and 110.9%, respectively. Matrix effects (absolute/relative) were found to be present in minor extent (approximately -30% to +15% MS/MS signal alterations) and were well corrected by the employed isotopically labeled internal standards. The validated assay was applied to urine samples of a drinking study as well as postmortem specimens. It was possible to assess the principal potential of EtP as ethanol consumption marker. Elevated concentration levels were found in real samples; however, EtP seems to be less sensitive compared to the previously known ethanol phase II conjugates EtG and EtS.

Adult↗
PubMed2006

Monolithic silica-based capillary column with strong chiral cation-exchange type surface modification for enantioselective non-aqueous capillary electrochromatography.

A silica-based monolithic stationary phase prepared by the sol-gel process in a 100 microm I.D. fused-silica (FS) capillary has been modified chemically with 3-mercaptopropyl trimethoxysilane followed by immobilization of a strong cation-exchange (SCX) type chiral selector, (S)-N-(4-allyloxy-3,5-dichlorobenzoyl)-2-amino-3,3-dimethylbutane phosphonic acid, by radical addition reaction onto the reactive sulfhydryl surface. After a fine-tuning of the mobile phase composition, the enantioselective capillary column was evaluated for the separation of various chiral basic drugs by enantioselective non-aqueous capillary electrochromatography (CEC), in comparison to capillary column analogs packed with 3.5 microm silica particles having attached the same selector. The performance of the monolithic silica column was further compared to corresponding polymethacrylate-based organic polymer monoliths. The study indicated that strong counter-ions such as 2-aminobutanol or N,N,N',N'-tetramethylethylenediamine are needed, although they reduce the electroosmotic flow velocity and separation factors in comparison to less efficient counter-ions, in order to allow the elution of the oppositely charged solutes in the ion-exchange retention mode within reasonable run time and as sharp zones. In contrast, weak counter-ions such as N,N-diisopropylethylamine (Huenig base) provided stronger electroosmotic flow and much better separation factors, but relatively poor peak efficiencies. Overall, with the chemically functionalized monolithic silica column the high quality separations of packed column analogs could be approximated, with regards to both separation factors and peak performances. On the other hand, the monolithic capillary column certainly outperformed the packed column in terms of system robustness under capillary electrochromatography conditions and showed excellent column longevity. The enantioselective strong cation-exchange-type monolithic silica column performed also well in comparison to the organic polymer monolith.

Cation Exchange Resins↗
PubMed2006

Determination of ethyl sulfate--a marker for recent ethanol consumption--in human urine by CE with indirect UV detection.

A CE method for the determination of the ethanol consumption marker ethyl sulfate (EtS) in human urine was developed. Analysis was performed in negative polarity mode with a background electrolyte composed of 15 mM maleic acid, 1 mM phthalic acid, and 0.05 mM cetyltrimethylammonium bromide (CTAB) at pH 2.5 and indirect UV detection at 220 nm (300 nm reference wavelength). This buffer system provided selective separation conditions for EtS and vinylsulfonic acid, employed as internal standard, from urine matrix components. Sample pretreatment of urine was minimized to a 1:5 dilution with water. The optimized CE method was validated in the range of 5-700 mg/L using seven lots of urine. Intra- and inter-day precision and accuracy values, determined at 5, 60, and 700 mg/L with each lot of urine, fulfilled the requirements according to common guidelines for bioanalytical method validation. The application to forensic urine samples collected at autopsies as well as a successful cross-validation with a LC-MS/MS-based method confirmed the overall validity and real-world suitability of the developed expeditious CE assay (sample throughput 130 per day).

Adult↗
PubMed2006

Stereoselective separations of chiral phosphinic acid pseudodipeptides by CEC using silica monoliths modified with an anion-exchange-type chiral selector.

A nonaqueous CEC method for the simultaneous separation of the four stereoisomers of the N-benzyloxycarbonyl phosphinic pseudodipeptide methyl ester benzyloxycarbonyl-homophenylalanine Z-hPhepsi(PO2HCH2)Phe-OCH3 as well as of the corresponding N-2,4-dinitrophenyl (DNP)-derivative with free C-terminal carboxylic group DNP-hPhepsi(PO2HCH2)Phe-OH was developed. For this purpose, a monolithic silica capillary column modified with a cinchona alkaloid-derived anion-exchange-type chiral selector, namely O-9-(tert-butylcarbamoyl)quinidine (tBuCQD) was prepared. The mobile phase composition (ACN/methanol ratio, counterion type) was thoroughly optimized to end up with baseline resolution of all four stereoisomers with critical resolution of as high as about 2. The CEC method proved to be superior over the corresponding HPLC separations primarily due to significantly enhanced plate numbers (between 200,000 and 600,000 m(-1) in CEC). Diastereoselectivity contributions arising from electrophoretic mobility differences of the diastereomers facilitated the separation of the later eluted diastereomeric peak pair (peaks III and IV), but had a negative influence on the selectivity of the earlier eluted diastereomeric peak pair (peaks I and II). The stereoselective CEC assay allowed the assessment of the stereoisomeric purity of the individual isomers which were obtained by preparative HPLC on a CHIRALPAK QD-AX column that is based on the same tBuCQD selector. The present study demonstrates that there exist problems which are hard to solve by HPLC, yet can be conveniently solved by CEC. Moreover, it was intended to prove by this practical application that CEC with monolithic columns is robust enough to be used for solving real-life problems.

Anion Exchange Resins↗
PubMed2006

Silica-based monolithic columns with mixed-mode reversed-phase/weak anion-exchange selectivity principle for high-performance liquid chromatography.

This article describes the synthesis, chromatographic characterization, and performance evaluation of analytical (100 x 4.6 mm id) and semipreparative (100 x 10 mm id) monolithic silica columns with mixed-mode RP/weak anion-exchange (RP/WAX) surface modification. The monolithic RP/WAX columns were obtained by immobilization of N-(10-undecenoyl)-3-aminoquinuclidine onto thiol-modified monolithic silica columns (Chromolith) by a radical addition reaction. Their chromatographic characterization by Engelhardt and Tanaka tests revealed slightly lower hydrophobic selectivities than C-8 phases, as well as higher polarity and also improved shape selectivity than RP-18e silica rods. The surface modification enabled separation by both RP and anion-exchange chromatography principles, and thus showed complementary selectivities to the RP-18e monoliths. The mixed-mode monoliths have been tested for the separation of peptides and turned out to be particularly useful for hydrophilic acidic peptides, which are usually insufficiently retained on RP-18e monolithic columns. Compared to a corresponding particulate RP/WAX column (5 microm, 10 nm pore diameter), the analytical RP/WAX monolith caused lower system pressure drops and showed, as expected, higher efficiency (e.g. by a factor of about 2.5 lower C-term for a tetrapeptide). The upscaling from the analytical to semipreparative column dimension was also successful.

Anions↗
PubMed2006

Quinine carbamate chiral stationary phases: systematic optimization of steric selector-selectand binding increments and enantioselectivity by quantitative structure-enantioselectivity relationship studies.

A series of quinine carbamate-based chiral stationary phases (CSPs) differing solely in the carbamate residue have been devised and a congeneric set of N-3,5-dinitrobenzoyl (DNB) amino acids (AAs) was separated into enantiomers on these CSPs by HPLC using a buffered hydro-organic mobile phase. Some details on retention and chiral recognition mechanisms have been investigated by application of quantitative structure-property relationship (QSPR) studies using the linear free energy relationship methodology, i.e., the extrathermodynamic approach. Retention factors of the high affinity enantiomer (log k2) and enantioselectivities (log alpha) were correlated with Taft's steric parameter as structural descriptor for the variability in the carbamate and AA residues, and statistically significant QSPR models could be obtained. They confirmed that the variance in the dependent variable (log k2, log alpha) is mainly associated with the steric bulkiness of the selectors' carbamate and of the AA residues. The retention factor of the second eluted enantiomers and the enantioselectivity first increased with steric bulkiness of the carbamate and AA residues, which may be explained by stronger dispersive interactions. After reaching an optimum, the dependent variable, however, declined with further increase of the steric bulkiness of the substituents, probably because of steric hindrance. The variability of the retention factors of the first eluted enantiomer could not be explained by steric descriptors. Instead, it has become obvious that the retention arises mainly from interactions of the DNB-AA and the quinine carbamate backbone, as it turned out to be more or less constant and solely to a minute amount modulated by the carbamate residue and the AA side chain. The QSPR models were fully in agreement with an earlier postulated chromatographically and spectroscopically derived hypothetical selector-selectand binding model.

Amino Acids↗
PubMed2005

Alternative high-performance liquid chromatographic peptide separation and purification concept using a new mixed-mode reversed-phase/weak anion-exchange type stationary phase.

This article describes a new complementary peptide separation and purification concept that makes use of a novel mixed-mode reversed-phase/weak anion-exchange (RP/WAX) type stationary phase. The RP/WAX is based on N-(10-undecenoyl)-3-aminoquinuclidine selector, which is covalently immobilized on thiol-modified silica particles (5 microm, 100 A pore diameter) by radical addition reaction. Remaining thiol groups are capped by radical addition with 1-hexene. This newly developed separation material contains two distinct binding domains in a single chromatographic interactive ligand: a lipophilic alkyl chain for hydrophobic interactions with lipophilic moieties of the solute, such as in the reversed-phase chromatography, and a cationic site for anion-exchange chromatography with oppositely charged solutes, which also enables repulsive ionic interactions with positively charged functional groups, leading to ion-exclusion phenomena. The beneficial effect that may result from the combination of the two chromatographic modes is exemplified by the application of this new separation material for the chromatographic separation of the N- and C-terminally protected tetrapeptide N-acetyl-Ile-Glu-Gly-Arg-p-nitroanilide from its side products. Mobile phase variables have been thoroughly investigated to optimize the separation and to get a deeper insight into the retention and separation mechanism, which turned out to be more complex than any of the individual chromatography modes alone. A significant anion-exchange retention contribution at optimal pH of 4.5 was found only for acetate but not for formate as counter-ion. In loadability studies using acetate, peptide masses up to 200 mg could be injected onto an analytical 250 mm x 4 mm i.d. RP/WAX column (5 microm) still without touching bands of major impurity and target peptide peaks. The corresponding loadability tests with formate allowed the injection of only 25% of this amount. The analysis of the purified peptide by capillary high-performance liquid chromatography (HPLC)-UV and HPLC-ESI-MS employing RP-18 columns revealed that the known major impurities have all been removed by a single chromatographic step employing the RP/WAX stationary phase. The better selectivity and enhanced sample loading capacity in comparison to RP-HPLC resulted in an improved productivity of the new purification protocol. For example, the yield of pure peptide per chromatographic run on RP/WAX phase was by a factor of about 15 higher compared to the standard gradient elution RP-purification protocol.

Anion Exchange Resins↗
PubMed2005

Determination of chlorpyrifos metabolites in human urine by reversed-phase/weak anion exchange liquid chromatography-electrospray ionisation-tandem mass spectrometry.

A liquid chromatography-electrospray ionisation-tandem mass spectrometry (LC-ESI-MS/MS) method for the quantification of major chlorpyrifos (CP) metabolites, i.e. diethyl thiophosphate (DETP), diethyl phosphate (DEP), and 3,5,6-trichloro-2-pyridinol (TCP), in human urine was developed. Simultaneous separation of the parent compound and its primary biotransformation products was achieved within 20 min in gradient elution mode employing a mixed-mode reversed-phase/weak anion exchange (RP/WAX) separation principle. The analytical method was developed for a toxicokinetic study of an acute poisoning incidence with a CP containing pesticide formulation. An initial mass spectrometric screening performed with unprocessed urine samples revealed that CP is not excreted unchanged by the kidney. Hence, the quantitative assay was validated for DETP (quantifier transition: m/z 169-->95, qualifier transition: m/z 169-->141), DEP (m/z 153-->79, 153-->125), and TCP (m/z 196-->35, 198-->35) taking dibutyl phosphate (DBP) (m/z 209-->79, 209-->153) as internal standard. Clean-up of urine samples prior to LC-ESI-MS/MS analysis was carried out by a liquid-liquid extraction step with a mixture of ethylacetate and acetonitrile (70:30; v/v). Linearity was observed between 0.25 and 75 mgL(-1), and the signal-to-noise ratio at 0.25 mgL(-1) was better than six for the individual analytes. Recoveries, precision, and accuracies were all adequate across the validated range of 1-75 mgL(-1) for the present toxicological case study.

Chlorpyrifos↗
PubMed2005

A case study of acute human chlorpyrifos poisoning: novel aspects on metabolism and toxicokinetics derived from liquid chromatography-tandem mass spectrometry analysis of urine samples.

The metabolic fate of the organophosphorothioate-type insecticide chlorpyrifos (CP) in an acutely intoxicated 59 years old female was investigated by liquid chromatography-electrospray ionisation-tandem mass spectrometry (LC-ESI-MS/MS) analysis of urine samples. Fifteen metabolites of CP and its bioactivated intermediate chlorpyrifos-oxon (CPO), respectively, of which only three have been described in man so far, were identified on the basis of characteristic MS/MS transitions, precursor/product ion and/or neutral loss scans, chlorine isotopomer patterns, and partly by synthesis of reference compounds and subsequent structure confirmation. Three distinct biotransformation routes of CP are proposed: (1) cleavage reactions at the aromatic phosphoester bond, (2) cleavage reactions at the alkyl phosphoester bonds, and (3) glutathione (GSH) dependent nucleophilic substitution of the 6-chlorine at the aromatic moiety. Route (2) has not been reported in humans before and (3) is a hitherto completely unknown scheme of CP metabolism. Urinary markers of the latter were chiefly cysteine S-conjugates of mono-dechlorinated CP, CPO, mono-O-deethyl CP, and mono-O-deethyl CPO as well as the 6-mercapturic acid conjugate of 3,5-dichloro-2-pyridinol. The presence of 3,5-dichloro-6-methylthio-2-pyridinol as well as its O-glucuronide suggests further a cysteine S-conjugate beta-lyase mediated degradation. In addition to the qualitative LC-MS/MS screening the renal elimination profiles of the primary products of scheme (1), i.e. diethyl thiophosphate (DETP), diethyl phosphate (DEP), and 3,5,6-trichloro-2-pyridinol (TCP), were monitored over 14 days (n=21). A biphasic first-order excretion mechanism with half-lives of 21.5h (initial fast excretion phase) and 119.5h (terminal phase) for the sum of free DETP and DEP was found. TCP was hardly eliminated in its free form (O-glucuronide identified as phase II conjugate) and half-lives calculated for the total amount of TCP (acidic hydrolysis of urine samples) were 40.8 and 150.7h. The present study gives a more detailed view on the biotransformation of CP and together with the obtained kinetic data adds novel aspects to the limited knowledge of human metabolism of this xenobiotic, in particular at high dosage.

Biomarkers↗
PubMed2005

Chiral separations by capillary electromigration techniques in nonaqueous media. I. Enantioselective nonaqueous capillary electrophoresis.

Enantiomer separations by CE employing nonaqueous conditions are reviewed. The general focus of this article is directed towards solvent effects on chiral recognition and the separation mechanism. After a general discussion of solvent effects on the individual processes involved in CE enantiomer separation, specifics for various selector classes are discussed together with a few applications of enantioselective nonaqueous CE.

Electrophoresis, Capillary↗
PubMed2005

Chiral separations by capillary electromigration techniques in nonaqueous media. II. Enantioselective nonaqueous capillary electrochromatography.

A review on the advantages, peculiarities, and the potential of enantioselective capillary electrochromatography (CEC) in nonaqueous media is presented. Some fundamentals on CEC with particular focus on enantioselective CEC are discussed. The strategies, concepts, preferentially utilized chiral selectors and column technologies that have been utilized to succeed in highly efficient enantiomer separations by nonaqueous CEC are described thoroughly.

Chromatography, Micellar Electrokinetic Capillary↗
PubMed2005

Enantiomer separation of a powerful chiral auxiliary, 2-methoxy-2-(1-naphthyl)propionic acid by liquid chromatography using chiral anion exchanger-type stationary phases in polar-organic mode; investigation of molecular recognition aspects.

The enantiodiscriminating potential of the weak anion exchange-type quinine-based chiral stationary phases (CSPs) for direct enantiomer separation of racemic 2-methoxy-2-(1-naphthyl)propionic acid (selectand, SA) was studied. The influence of structure variations of the selector (SO) in the carbamate functional group and/or in the C6' position of quinoline moiety on retention and enantioselectivity was investigated. Systematic chromatographic studies were made to gain more insight into the overall chiral recognition mechanism for a given mobile phase. In this context, the tert-butylcarbamoyl quinine and the corresponding diisopropylphenyl-derived selector provided the highest resolution and enantioselectivity under polar-organic conditions with the elution order of (R) before the (S) enantiomer. When the bulkiness of the substituents in the C6' position of the SO was increased, the selectivity was decreased in all cases. Alkylation of the nitrogen atom in the carbamate functionality of the SO resulted in the complete loss of enantiomer separation, confirming the crucial importance of the hydrogen-bond formation involved in the stereodiscriminating events. In addition, ten different mono-, bi-, or trivalent acids, necessary as competitor molecules (counter-ions) of the mobile phase, were screened to judge their influence on retention and overall enantioselectivity. Among them, acetic acid, formic acid, N-acetylglycine, and glycolic acid proved to be the most promising counter-ions with R(S) values of 6.35, 6.81, 8.19, and 7.34, respectively. On the basis of chromatographic data, a tentative molecular recognition model was proposed. Simultaneous ion-pairing and hydrogen bonding, in concert with pi-pi stacking and steric interactions, were expected to be responsible for chiral recognition mechanism. This was partially corroborated by structural and/or conformational analysis of the tert-butylcarbamoyl quinine-2-methoxy-2-(1-naphthyl)propionic acid (SO-SA) complex.

Journal Article↗
PubMed2005

Investigations of molecular recognition aspects related to the enantiomer separation of 2-methoxy-2-(1-naphthyl)propionic acid using quinine carbamate as chiral selector: An NMR and FT-IR spectroscopic as well as X-ray crystallographic study.

The chiral recognition mechanism of a cinchona alkaloid-based chiral stationary phase (CSP) showing high enantiomer discrimination potential for 2-methoxy-2-(1-naphthyl)propionic acid (MalphaNP acid) was investigated. Conformational and structural analyses of the 1:1 complexes of 9-O-(tert-butylcarbamoyl) quinine selector (SO) and MalphaNP acid (selectand, SA) were carried out employing NMR spectroscopy in solution, Fourier-transform infrared (FT-IR) spectroscopy, and solid-state X-ray diffraction analysis. Intramolecular NOEs of a soluble analogue of the CSP afforded the conformational states of the free and complexed form of the selector. The (1)H-NMR spectra revealed that the free form of the SO constitutes anti-open as well as anti-closed and/or syn-closed conformers. Upon complexation with the (S)-MalphaNP acid enantiomer to form the more stable diastereomeric associate, a conformational transition of the selector takes place, resulting in the synthesis of the anti-open conformer nearly exclusively. FT-IR spectra reveal that, besides the primary ion-pairing interaction, stereoselective hydrogen bonding stabilizes the more stable complex via the amide hydrogen of the SO. X-ray diffraction analysis of 9-O-(tert-butylcarbamoyl)quinine and (S)-MalphaNP acid complex further revealed the occurrence of a bidentate H-bond-mediated ionic interaction between SO and SA as well as the lack of pi-pi interaction in the 1:1 complex, and corroborated the conclusions derived from spectroscopic and chromatographic studies.

Journal Article↗
PubMed2005

Polymethacrylate-type monoliths functionalized with chiral amino phosphonic acid-derived strong cation exchange moieties for enantioselective nonaqueous capillary electrochromatography and investigation of the chemical composition of the monolithic polymer.

In situ prepared monolithic poly(glycidyl methacrylate-co-ethylene dimethacrylate) (poly(GMA-co-EDMA)) capillary columns were activated to reactive thiol-monoliths and subsequently functionalized with (S)-N-(4-allyloxy-3,5-dichlorobenzoyl)-2-amino-3,3-dimethylbutanephosphonic acid as chiral selector by radical addition to afford enantioselective strong cation exchanger (SCX) capillary columns (100 microm inner diameter (ID)). These monolithic capillaries were devised for the enantioseparation of chiral bases by nonaqueous and aqueous capillary electrochromatography (CEC) and the results obtained for mefloquine and its tert-butylcarbamate as test compounds were compared to those obtained with particulate silica-based analogs (packed columns). Despite abolishment of nonspecific ionic interactions between the cationic solutes and residual silanols that may diminish separation factors of the silica-based chiral SCX particles, the poly(GMA-co-EDMA)-supported SCX monolith did not, as expected, show better enantioselectivities, which was assumed to be due to detrimental nonspecific interactions between the analytes and the lipophilic polymer backbone. In order to minimize these unfavorable contributions, less lipophilic monoliths were developed by copolymerization of different amounts of the hydrophilic monomer 2-hydroxyethyl methacrylate (HEMA) with GMA and EDMA, leading to GMA-co-HEMA-co-EDMA-terpolymeric monoliths. By this increase of the hydrophilicity of the monolithic support the enantioselectivity of the resultant SCX stationary phase could be enhanced and reached values comparable to the packed silica-based enantioselective SCX capillaries. Additionally, the mobile phase composition and other variables were examined and it could be shown that the separation factors are considerably affected by diverse parameters such as acetonitrile-methanol ratio and type and concentration of the counterion. Mefloquine enantiomers could be separated with alpha-values up to 1.56 and a maximum plate count of ca. 60,000 m(-1) could be achieved.

Cation Exchange Resins↗
PubMed2004

Development of reactive thiol-modified monolithic capillaries and in-column surface functionalization by radical addition of a chromatographic ligand for capillary electrochromatography.

Reactive thiol-modified capillary columns for capillary electrochromatography (CEC) were developed by transforming the pendent 2,3-epoxypropyl groups of poly(glycidyl methacrylate-co-ethylene dimethacrylate) (poly(GMA-co-EDMA)) monoliths into 3-mercapto-2-hydroxy-propyl residues by a nucleophilic substitution reaction, employing sodium-hydrogen sulfide as nucleophilic reagent. Conditions for this modification reaction were systematically optimized with respect to different parameters, such as reaction temperature, pH-value, reaction time, type and concentration of organic modifier, and concentration of the sodium-hydrogen sulfide solution. The amount of thiol groups that was generated on the monolith surface was determined directly in the capillaries by a disulfide-exchange reaction employing 2,2'-dipyridyl disulfide (DPDS). This reaction in the capillary liberates pyridine-2-thione in equimolar amount to the surface sulfhydryls, which was collected into a vial and determined photometrically at 343 nm by RP-HPLC. About 17% of the total lateral epoxide moieties of the monolithic substrate could be transformed to reactive sulfhydryl groups, which corresponds to about 0.7 mmol g(-1) monolithic polymer, with a column-to-column repeatability of 3.2% R.S.D. The reactive thiol groups can be utilized to attach any chromatographic ligand with appropriate anchor in a second step, e.g. by radical addition, graft polymerization, nucleophilic substitution, disulfide formation or Michael addition reaction. To demonstrate the feasibility of the concept, we chose an anion exchange type chromatographic ligand based on a quinine derivative, O-9-tert-butylcarbamoylquinine (t-BuCQN) which was attached to the monolith in a radical addition reaction, for a further in-column surface functionalisation. About 78% of the sulfhydryl groups were derivatized with t-BuCQN as determined from differential DPDS assays before and after the selector immobilization reaction. The applicability of these surface-functionalised monolithic capillary columns could be shown by an electrochromatographic separation of the enantiomers of N-3,5-dinitrobenzoyl-leucine, which performed fairly well compared to an analogous capillary that was fabricated by an in situ copolymerization approach.

Chromatography, High Pressure Liquid↗
PubMed2004

Direct high-performance liquid chromatographic method for enantioselective and diastereoselective determination of selected pyrethroic acids.

This study reports on the direct HPLC stereoisomer separation of selected pyrethroic acids employing commercial cinchona alkaloid derived chiral stationary phases (CSPs). cis/trans-Chrysanthemic acid (cis/trans-CA), cis/trans-chrysanthemum dicarboxylic acid (cis/trans-CDCA), cis/trans-permethrinic acid (cis/trans-PA), and fenvaleric acid (FA) were resolved into the individual stereoisomers, i.e. enantiomers and diastereomers as well. To achieve satisfactory baseline separation an optimisation of the variables of the chromatographic method including chemical structure of the cinchona carbamate CSP, mobile phase composition, and flow rate was required. All four stereoisomers of PA were successfully separated in a single run (alphacis = 1.20, alphatrans = 1.26, critical Rs = 1.65) with an acetonitrile (ACN)-based polar-organic eluent. The complete baseline resolution of all CA stereoisomers succeeded in polar-organic (alphacis = 1.20, alphatrans = 1.35, critical Rs = 3.03) as well as in acetonitrile-based reversed-phase media (alphacis = 1.24, alphatrans = 1.22, critical Rs = 2.73). The latter elution mode was also found to be suitable for the enantio- as well as diastereoselective resolution of CDCA (alphacis = 1.09, alphatrans = 1.50, critical Rs = 1.43), which is to the best of our knowledge the first reported enantiomer separation of this analyte. The enantiomers of FA could be baseline separated employing also reversed-phase mode (alpha = 1.16, Rs = 2.91). These separation methods may be applied for quality control processes in the production of stereoisomerically pure insecticides as well as stereoselective toxicokinetic studies, as CDCA, PA, and FA are suitable biomarkers for monitoring human pyrethroid burden.

Chromatography, High Pressure Liquid↗