Direct injection of large volumes of plasma in a column-switching system for the analysis of local anaesthetics. I. Optimization of semi-permeable surface precolumns in the system and characterization of some interference peaks

New approach to large-volume injection in reversed-phase high performance liquid chromatography: Determination of atrazine and hydroxyatrazine in soil samples

A well established method of direct injection of larger than conventional sample volumes ranging from 0.1 mL to 10 mL in HPLC is the injection valve method in which a loop of tubing is totally or partially filled with sample. Recent HPLC pumps have a flow-rate setting accuracy of +/- 1-2% over a flow-rate range from 0.1 mL/min to 10 mL/min and the flow stability is 0.2% or less. Quarternary low pressure gradient pumps are widely available and used, but all their hydraulic lines are seldom utilised. The idea of using one line of a common commercial HPLC quaternary low-pressure pump for direct on-column injection (pumping) of large sample volumes ranging from 1 mL to 100 mL was tested. This approach was evaluated during practical work on the development of an RP-HPLC method of determination of residual atrazine and hydroxyatrazine. In lysimetric environmental experiments hydroxyatrazine was formed in situ in a soil column by hydrolysis of atrazine. The results proved the applicability of this approach not only in experiments with model mixtures of analytes at microg/L levels in solutions. Analysis of 20 mL of soil leachates and extracts of soil samples containing atrazine and hydroxyatrazine at the 10 microg/kg level (in dry soil) revealed that good figures-of-merit were preserved, even in the presence of a large excess of humic substances.

Simultaneous determination of bupivacaine, mepivacain, prilocaine and ropivacain in human serum by liquid chromatography–tandem mass spectrometry

A liquid chromatography-tandem mass spectrometric (LC-MS-MS) method with a rapid and simple sample preparation was developed and validated for the simultaneous determination of the local anesthetics bupivacaine, mepivacaine, prilocaine and ropivacaine in human serum. An external calibration was used. The mass spectrometer was operated in the multiple reaction monitoring mode. A good quadratic response over the range of 1.0-200.0 ng/ml was demonstrated. The accuracy for bupivacaine ranged from 93.2 to 105.7%, for mepivacaine from 96.2 to 104.3%, for prilocaine from 94.6 to 105.7% and for ropivacaine from 94.3 to 104.0%, respectively. The limit of quantification was 1.0 ng/ml for all substances. This method is suitable for pharmacokinetic studies.

On-line sample cleanup and chiral separation of gemifloxacin in a urinary solution using chiral crown ether as a chiral selector in microchip electrophoresis

In chiral capillary electrophoresis of primary amine enantiomers using (+)-18-crown-6-tetracarboxylic acid (18C6H4) as a chiral selector, the presence of alkaline metal ions in the sample solution as well as in the run buffer is undesirable due to their strong competitive binding with 18C6H4. A channel-coupled microchip electrophoresis device was designed to clean up alkaline metal ions from a sample matrix for the chiral analysis of amine. In the first channel, the metal ions in the sample were monitored by indirect detection using quinine as a chromophore and drained to the waste. In the second separation channel, gemifloxacin enantiomers, free of the alkaline metal ions, were successfully separated using only a small amount of the chiral selector (50 microM 18C6H4).

Integrated on-line sample clean-up using cation exchange restricted access sorbent for the LC determination of atropine in human plasma coupled to UV detection

A new, simple and fully automated liquid chromatographic (LC) method with UV detection has been developed for the direct determination of atropine in plasma. Sample clean-up was based on the use of cation exchange restricted access material (RAM) in a pre-column, coupled to LC by means of a column switching system. After direct injection of a 200 microl-volume of plasma sample, the biological matrix was washed out for 10 min using a washing liquid composed of 2 mM lithium perchlorate adjusted to pH 3.0 and methanol (97:3; v/v). By rotation of the switching valve, atropine was then eluted in the back-flush mode for 2 min and transferred to the analytical column packed with octadecyl silica by the LC mobile phase constituted of a mixture of acetonitrile and potassium phosphate buffer (pH 3.0; 50 mM) containing 2 mM sodium heptanesulfonate (16:84; v/v). The UV detection was performed at 220 nm. The method was validated according to a new approach based on accuracy profile over a concentration range from 25 ng/ml, corresponding to the limit of quantitation, to 1000 ng/ml. The method was then applied for the determination of atropine in plasma after intravenous administration to hospitalised patients.

Restricted access materials and large particle supports for on-line sample preparation: an attractive approach for biological fluids analysis

An analytical process generally involves four main steps: (1) sample preparation; (2) analytical separation; (3) detection; and (4) data handling. In the bioanalytical field, sample preparation is often considered as the time-limiting step. Indeed, the extraction techniques commonly used for biological matrices such as liquid-liquid extraction (LLE) and solid-phase extraction (SPE) are achieved in the off-line mode. In order to perform a high throughput analysis, efforts have been engaged in developing a faster sample purification process. Among different strategies, the introduction of special extraction sorbents, such as the restricted access media (RAM) and large particle supports (LPS), allowing the direct and repetitive injection of complex biological matrices, represents a very attractive approach. Integrated in a liquid chromatography (LC) system, these extraction supports lead to the automation, simplification and speeding up of the sample preparation process. In this paper, RAM and LPS are reviewed and particular attention is given to commercially available supports. Applications of these extraction supports, are presented in single column and column-switching configurations, for the direct analysis of compounds in various biological fluids.

Role of biological matrices during the analysis of chiral drugs by liquid chromatography

The review article covers advances of chiral drugs analysis by high-performance liquid chromatography (HPLC) methods achieved during last 10 years. Emphasis is given to various aspects of influence of biological matrix in pharmacodynamics, pharmacokinetics, HPLC analysis. Discussed is composition of main biological matrices from the point of view of potential interferences to above-mentioned fields of study. Beside typical analytical approaches to chiral recognition in HPLC, sample pretreatment and/or clean-up by conventional extraction procedures, column switching (CSW) techniques using restricted access materials (RAMs), microdialysis (MCD) is discussed. Measurement of unbound drug concentration and discussion of column maintenance and remedy is an additional source of information and field where knowledge on complex properties and interactions of biological matrix is usefully applicable.

Fully automated LC method for the determination of sotalol in human plasma using restricted access material with cation exchange properties for sample clean-up

A simple and rapid fully automated bio-analytical method for the liquid chromatographic (LC) determination of sotalol in human plasma has been described. The method is based on the use of a new kind of porous silica restricted access material (RAM) with cation exchange properties for sample clean-up. 100 microl of plasma samples were directly injected into the precolumn coupled on-line to a reversed-phase column (RP-Select B) by means of column switching system. The plasma matrix was washed out for 10 min using a washing liquid composed of 2 mM lithium perchlorate and methanol (97:3; v/v). By rotation of the switching valve, the analytes were then eluted in back-flush mode for 2 min and transferred to the analytical column by the LC mobile phase constituted of a mixture of methanol and 50 mM potassium phosphate buffer (pH 7.0) containing 1 mM 1-octanesulphonic acid sodium salt (20:80; v/v). The flow-rate was 1.0 ml/min and sotalol was detected using fluorescence detection at 235 and 300 nm as excitation and emission wavelengths, respectively. The method was then validated using a new approach based on accuracy profile over a concentration range from 5 to 500 ng/ml. The limit of quantitation (LOQ) was 5 ng/ml and the total analysis time was 19 min.

Use of online-dual-column extraction in conjunction with chiral liquid chromatography tandem mass spectrometry for determination of terbutaline enantiomers in human plasma

An online sample extraction chiral bioanalytical method was developed and validated for the quantification of terbutaline, a beta2-selective adrenoceptor agonist, spiked into human plasma by using two extraction columns and a chiral stationary phase (CSP) in conjunction with liquid chromatography tandem mass spectrometry (LC-MS/MS). In this method, two Oasis HLB extraction columns were used in parallel for plasma sample purification and a Chirobiotic T CSP was used for enantiomeric separation. Atmospheric pressure chemical ionization MS/MS was employed in multiple reaction monitoring mode for the detection and quantification. Subsequent to the addition of an internal standard solution, the plasma samples were directly injected onto the system for extraction and analysis. This method allowed the use of one of the extraction columns for purification while the other was being equilibrated. Hence, the time required for reconditioning the extraction columns did not contribute to the total analysis time per sample, which resulted in a shorter run time and higher throughput. A lower limit of quantification of 1.0 ng/mL was achieved using only 50 microliter of human plasma. The method was validated with a dynamic range of 1.0-200 ng/mL. The intra- and interday precision was no more than 11% CV and the assay accuracy was between 94-106%.

Simultaneous determination of enantiomers of structurally related anticholinergic analogs in human serum by liquid chromatography-electrospray ionization mass spectrometry with on-line sample cleanup

Trihexyphenidyl, biperiden and procyclidine are anticholinergic drugs produced as racemates for the treatment of Parkinson's disease. This paper describes a simple and sensitive LC-MS method for the simultaneous determination of these compounds in human serum. An on-line sample clean-up procedure was used, where serum samples were directly injected into a "restricted-access media" pre-column. After the exclusion of the serum proteins, the drug molecules were eluted to a beta-cyclodextrin analytical column for chiral separation. The quantitation was done by electrospray ionization MS using diphenidol as an internal standard. The method developed has limits of detection of 1 ng/ml, at least two-orders-of-magnitude linear dynamic ranges (r>0.999), and RSDs of less than 10%. The system can be completely automated for increased sample throughput and unattended analyses.

Quantitation of entacapone glucuronide in rat plasma by on-line coupled restricted access media column and liquid chromatography-tandem mass spectrometry

A column-switching liquid chromatography-electrospray ionization-tandem mass spectrometric (LC-ESI-MS-MS) method was developed for the direct analysis of entacapone glucuronide in plasma. The plasma samples (5 microl) were injected onto a C18-alkyl-diol silica (ADS) column and the matrix compounds were washed to waste with a mixture of 20 mM ammonium acetate solution at pH 4.0-acetonitrile (97:3). The retained analyte fraction containing (E)- and (Z)-isomers of glucuronides of entacapone and tolcapone glucuronide (internal standard) was backflushed to the analytical C18 column, with a mixture of 20 mM ammonium acetate-acetonitrile (85:15) for the final separation at pH 7.0. The eluate was directed to the mass spectrometer after splitting (1:100). The mass spectrometer was operated in the negative ion mode and the deprotonated molecules [M-H]- were chosen as precursor ions for the analytes and internal standard. Collisionally induced dissociation of [M-H] in MS-MS resulted in loss of the neutral glucuronide moiety and in the appearance of intensive negatively charged aglycones [M-H-Glu]-, which were chosen as the product ions for single reaction monitoring. Quantitative studies showed a wide dynamic range (0.0025-100 microg/ml) with correlation coefficients better than 0.995. The method was repeatable within-day (relative standard deviation, RSD<7%) and between-day (RSD<14%) and the recovery (78-103%) was better than with the traditional, laborious pretreatment method. The use of tandem mass spectrometry permitted low limits of detection (1 ng/ml of entacapone glucuronide). The method was applied for the quantitation of (E)- and (Z)-isomers of entacapone glucuronide in plasma of rats used in absorption studies.

Simultaneous measurement of plasma ropivacaine and bupivacaine concentrations by HPLC with UV detection

The authors developed a high-performance liquid chromatography (HPLC) assay for the simultaneous determination of plasma ropivacaine and bupivacaine concentrations using ultraviolet (UV) detection and a simple solid-phase extraction procedure. The absolute retention times of ropivacaine, bupivacaine, and the internal standard pentycaine were 1.9, 3.0, and 5.6 minutes, respectively. The assay had a linearity of 2000 ng/mL, a sensitivity of 5 ng/mL, an average recovery of 98%, and an average day-to-day imprecision of <10% for both drugs. A patient correlation study (n = 23) using this HPLC method and an established gas chromatographic assay revealed a slope of 1.01, an intercept of -10.6 ng/mL, and a correlation coefficient of 0.99 for ropivacaine; and a slope of 0.96, an intercept of 14.7 ng/mL, and a correlation coefficient of 0.99 for bupivacaine. Of the 60 different drugs tested, only quinidine and lidocaine extracted but did not interfere with the measurement of the drugs of interest. The authors conclude that the method described here is ideally suited for the therapeutic monitoring of plasma ropivacaine and bupivacaine concentrations.

Advances in sample preparation in electromigration, chromatographic and mass spectrometric separation methods

The quality of sample preparation is a key factor in determining the success of analysis. While analysis of pharmaceutically important compounds in biological matrixes has driven forward the development of sample clean-up procedures in last 20 years, today's chemists face an additional challenge: sample preparation and analysis of complex biochemical samples for characterization of genotypic or phenotypic information contained in DNA and proteins. This review focuses on various sample pretreatment methods designed to meet the requirements for the analysis of biopolymers and small drugs in complex matrices. We discuss the advances in development of solid-phase extraction (SPE) sorbents, on-line SPE, membrane-based sample preparation, and sample clean-up of biopolymers prior to their analysis by mass spectrometry.

Analysis of neuropeptide Y and its metabolites by high-performance liquid chromatography-electrospray ionization mass spectrometry and integrated sample clean-up with a novel restricted-access sulphonic acid cation exchanger

A novel restricted access cation exchanger with sulphonic acid groups at the internal surface was proven to be highly suitable in the sample clean up of peptides on-line coupled to HPLC-electrospray ionization (ESI)-MS. Neuropeptide Y (NPY) and several of its fragments in plasma were subjected to the sample clean-up procedure. The peptides were eluted by a step gradient from the restricted access column, applying 10 mM phosphate buffer pH 3.5 from 5 to 20% (v/v) of acetonitrile with 1 M NaCl and transferred to a Micra ODS II column (33x4.6 mm). The separation of the peptides and their fragments was performed by a linear gradient from 20 to 60% (v/v) acetonitrile in water with 0.1% formic acid and 0.01% trifluoroacetic acid in 4 min at a flow-rate of 0.75 ml/min. An integrated and completely automated system composed of sample clean up-HPLC-ESI-MS was used to analyze real life samples. The sample volumes ranged between 20 and 100 microl. Peaks due to the fragments NPY 1-36, 3-36 and 13-36 in porcine plasma were identified by ESI-MS. The limit of detection was in the 5 nmol/ml range. The total analysis required 21 min and allowed the direct injection of plasma.

Semi-permeable surface analytical reversed-phase column for the improved trace analysis of acidic pesticides in water with coupled-column reversed-phase liquid chromatography with UV detection. Determination of bromoxynil and bentazone in surface water

The coupled-column (LC-LC) configuration consisting of a 3 microm C18 column (50 x 4.6 mm I.D.) as the first column and a 5 microm C18 semi-permeable-surface (SPS) column (150 x 4.6 mm I.D.) as the second column appeared to be successful for the screening of acidic pesticides in surface water samples. In comparison to LC-LC employing two C18 columns, the combination of C18/SPS-C18 significantly decreased the baseline deviation caused by the hump of the co-extracted humic substances when using UV detection (217 nm). The developed LC-LC procedure allowed the simultaneous determination of the target analytes bentazone and bromoxynil in uncleaned extracts of surface water samples to a level of 0.05 microg/l in less than 15 min. In combination with a simple solid-phase extraction step (200 ml of water on a 500 mg C18-bonded silica) the analytical procedure provides a high sample throughput. During a period of about five months more than 200 ditch-water samples originating from agricultural locations were analyzed with the developed procedure. Validation of the method was performed by randomly analyzing recoveries of water samples spiked at levels of 0.1 microg/l (n=10), 0.5 microg/l (n=7) and 2.5 microg/l (n=4). Weighted regression of the recovery data showed that the method provides overall recoveries of 95 and 100% for bentazone and bromoxynil, respectively, with corresponding intra-laboratory reproducibilities of 10 and 11%, respectively. Confirmation of the analytes in part of the samples extracts was carried out with GC-negative ion chemical ionization MS involving a derivatization step with bis(trifluoromethyl)benzyl bromide. No false negatives or positives were observed.

Solid-phase extraction: method development, sorbents, and coupling with liquid chromatography

The objective of this review is to provide updated information about the most important features of the new solid-phase extraction (SPE) materials, their interaction mode and their potential for modern SPE. First, the recent developments are given in formats, phases, automation, high throughput purpose and set-up of new types of procedures. Emphasis is then placed on the large choice of sorbents for trapping analytes over a wide range of polarities, such as highly cross-linked copolymers, functionalized copolymers, graphitized carbons or some specific n-alkylsilicas. The method development is given which is based on prediction from liquid chromatographic retention data or solvation parameters in order to determine the main parameters of any sequence (type and amount of sorbent, sample volume which can be applied without loss of recovery, composition and volume of the clean-up solution, composition and volume of the desorption solution). Obtaining extracts free from matrix interferences in a few steps--one step when possible--is now included in the development of SPE procedure. New selective phases such as mixed-mode and restricted access matrix sorbents or emerging phases such as immunosorbents or molecularly imprinted polymers are reviewed. Selectivity obtained by combining two sorbents is described with the use of ion-exchange or ion-pair sorbents. Special attention is given to complete automation of the SPE sequence with its on-line coupling with liquid chromatography followed by various detection modes. This represents a fast, modern and reliable approach to trace analysis. Many examples illustrate the various features of modern SPE which are discussed in this review. They have been selected in both biological and environmental areas.

High-performance liquid chromatographic determination of ropivacaine, 3-hydroxy-ropivacaine, 4-hydroxy-ropivacaine and 2',6'-pipecoloxylidide in plasma

A sensitive HPLC method has been developed for the determination of ropivacaine, 3-hydroxy-ropivacaine, 4-hydroxy-ropivacaine and 2',6'-pipecoloxylidide in plasma. The procedure involved extraction from plasma with a mixture of n-heptane-ethyl acetate and a back-extraction into an acidified aqueous solution. The chromatography was achieved using a LiChrospher RPB C8 column with a mobile phase consisting of a mixture of acetonitrile and pH 2.1, 0.01 M potassium dihydrogenphosphate, the latter phase containing 0.005 M 1-heptanesulfonic acid for ropivacaine metabolites analysis. The extraction yields of ropivacaine, 3-hydroxy-ropivacaine, 4-hydroxy-ropivacaine and 2',6'-pipecoloxylidide were 94.7%, 79.4%, 79.4% and 77.7%, respectively. The limits of detection of ropivacaine, 3-hydroxy-ropivacaine, 4-hydroxy-ropivacaine and 2',6'-pipecoloxylidide in plasma samples were 0.9 ng/ml, 3 ng/ml, 5 ng/ml and 1 ng/ml, respectively.

Evaluation of liquid chromatographic behavior of restricted-access media precolumns in the course of direct injection of large volumes of plasma samples in column-switching systems

The chromatographic behavior of an alkyl-diol silica (ADS, 25 x 4 mm I.D.) and a semipermeable surface (SPS, 10 x 10 mm I.D.) supports two types of restricted-access media (RAM), which served as precolumns in column-switching systems for direct injection of large volumes of plasma samples (500 microl), was studied with regard to peak performance, retention and column back pressure. The adsorption of matrix proteins both on sealings (porous frits and sieves) and packings was also examined. Columns of ADS and SPS were unchanged after the injection of 10-20 ml human plasma under normal working conditions. Even when changes occurred on the precolumns (>50 ml of plasma in total), it was still possible to regenerate the column performance by replacing the column sieves, or by washing and removing columns from the system for a period, since the changes were more related to the blockage of sealings and/or the adsorption of proteins on the hydrophilic surfaces. Proteins could eventually be unspecifically adsorbed on the hydrophobic ligand of the support. It was found on one ADS column that the retention decreased by 20% and the pressure increased 30 bar after an intensive loading of 75 ml plasma (injection volume, 500 microl) without reconditioning procedure. Studies showed that the column sealings played the most important role for the lifetime of RAM columns. For ADS columns, using sieves without polytetrafluoroethylene (PTFE) nets were the best. No significant difference in column life span between SPS and ADS was found.

Ion-pair chromatography of methotrexate in a column-switching system using an alkyl-diol silica precolumn for direct injection of plasma

The retention behaviour of methotrexate as an ion-pair with tetrabutylammonium in a column-switching system, based on an alkyl-diol silica C8 precolumn, combined with an analytical column, LiChrospher RP 18, was studied. Methotrexate is mainly present as a divalent anion at pH 7.4, however, the retention data was consistent with the formation of a 1 + 1 ion pair with the counter ion. The concentration of the tetrabutylammonium and the acetonitrile in the mobile phase could be used to regulate the retention in the system. Relevant chromatographic parameters to estimate the enrichment effect in the column-switching system are also identified and discussed. The column-switching system was applied to direct injection of plasma (100 microliters) giving a limit of detection of 10 ng/ml for methotrexate using UV detection at 307 nm.