Automated liquid membrane extraction for high-performance liquid chromatography of Ropivacaine metabolites in urine

Principles, developments and applications of on-line coupling of extraction with chromatography

On-line coupling of extraction and chromatographic separation allows the whole analysis to be performed in a closed system. On-line systems are particularly useful when the analytes are labile, the amount of sample is limited, or very high sensitivity is required. Many on-line systems have been developed both for liquid and for solid samples. This review discusses the different instruments that have been constructed and the factors that need to be considered in the coupling. Selected illustrative applications are described to illustrate the potential of the on-line systems.

Automated, on-line membrane extraction

Over the last few years, membranes have been used to develop new approaches in analytical extraction, concentration and cleanup. An important advantage of membrane processes is that the sample and the extraction phase can be continuously brought into contact without physical mixing, and may be directly interfaced to an analytical instrument. This provides the basis for automated, real-time monitoring. Membrane extraction has been applied to a wide range of organic and inorganic analytes, and has been directly interfaced with chromatography, spectroscopy and mass spectrometry. Implementations of membrane extraction are diverse, encompassing different types of membranes, module designs and configurations. This review highlights some of these, and particularly the unique capabilities in automated, and on-line measurements.

Liquid chromatography–electrospray mass spectrometry determination of free and total concentrations of ropivacaine in human plasma

A specific and sensitive liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) method was developed for the determination of free and total ropivacaine in human plasma. The work-up procedure involved a simple precipitation of plasma proteins with methanol. Etidocaine served as the internal standard. After microscale equilibrium-dialysis, measurement of free ropivacaine levels was performed after direct injection of the dialysate into the chromatograph. The system used a Zorbax eclipse XD8 C8 analytical column packed with 5 microm diameter particles as the stationary phase. The mobile phase consisted of a 15-min gradient (mobile phase A: 0.05% (v/v) trimethylamine in acetonitrile, mobile phase B: 2mM ammonium formate buffer (pH 3)). Mass spectrometric data were acquired in single ion monitoring mode at m/z 275 for ropivacaine and m/z 277 for etidocaine. The drug/internal standard peak area ratios (plasma) or peak areas (dialysate) were linked via a quadratic relationship to concentrations. Precision ranged from 1 to 7.6% accuracy was between 92.6 and 109%. The lower limits of quantitation were 1 microg/l in plasma and 2 microg/l in the dialysate. This method was found suitable for the analysis of plasma samples collected during a clinical trial performed in 30 infants undergoing epidural anaesthesia or continuous psoas compartment block.

Continuous, on-line monitoring of haloacetic acids via membrane extraction

Haloacetic acids are an important class of disinfection byproducts that are being regulated. In this paper we report novel instrumentation for continuous monitoring of the nine haloacetic acids. Hollow fiber liquid-liquid membrane extraction (LLME) and supported liquid membrane extraction (SLME) followed by on-line HPLC-UV detection were studied. With continuous LLME, seven halo-acetic acids could be analyzed and enrichment factor (EF) was around 50. All the nine acids could be extracted and quantified by continuous SLME. Experiments with laboratory standards demonstrated that EF and extraction efficiency could be as high as 500 and 54%, respectively. Relative standard deviations based on seven replicates were between 3.3 and 10.3%, and the MDLs were at sub-ppb levels.

Development of a total analytical system by interfacing membrane extraction, pervaporation and high-performance liquid chromatography

This paper discusses the interfacing of continuous membrane extraction, pervaporation and on-line HPLC-UV detection into a total analytical system (TAS). Organics from a water sample were extracted into an organic solvent, and then concentrated via pervaporation prior to HPLC-UV detection. Factors affecting the system performance were studied. With optimized experimental parameters enrichment factors as high as 192 were obtained, the method detection limits were at low ng/mL levels, and the precisions were better than 5%.

Urine sample preparation of tricyclic antidepressants by means of a supported liquid membrane technique for high-performance liquid chromatographic analysis

Supported liquid membrane (SLM) technique for sample work-up and enrichment was used for determination of tricyclic antidepressant drugs in urine by high-performance liquid chromatography (HPLC) with UV detection. The studied antidepressant drugs were amitriptyline, opipramol, noxiptyline and additionally diethazine was used as possible internal standard. Alkaline phosphoric buffer with urine sample, as the donor solution, was passed over the liquid membrane into which investigated substances were extracted. On the other side of the membrane, analyzed compounds were trapped due to creating non-extractable form in acidic acceptor solution. Enriched and cleaned up drugs were then injected into a HPLC system with ultraviolet detection to analyze of their concentration in acceptor solution. Optimum extraction efficiency was determined by changing acceptor and donor solutions pH, application of different flow rates of donor solution and by using different solvents in the membrane. Also, donor solution volume, extraction time and concentration of analytes were varied to check the linearity of extraction process. The highest extraction efficiency: 43% for opipramol, 56% for noxiptyline, 43% for amitriptyline and 42% for diethazine (R.S.D. values were <6% and n=3) was achieved when 0.05 M phosphate buffer pH 4.0 and 9.5 were used as donor and acceptor solutions, respectively, n-undecane with 5% tri-n-octylphosphine oxide (TOPO) was used as liquid membrane. Limit of quantification (LOQ) for tricyclic antidepressants after enrichment of 100ml of urine sample was about 1 ng/ml.

The XT-Tube Extractor: A Hollow Fiber-Based Supported Liquid Membrane Extractor for Bioanalytical Sample Preparation

A new supported liquid membrane extractor for bioanalytical sample preparation is presented. The extractor consists of a polypropylene hollow fiber mounted inside a PTFE tube by means of a cross-connector and a tee-connector. All parts are commercially available, inexpensive, and easily assembled. An organic solvent in the pores of the fiber forms a liquid membrane that separates the sample, which is pumped along the outside of the fiber, from the acceptor phase, which is pumped inside. The length of the hollow fiber may easily be varied to meet different demands on extractive surface and extract volumes. To test the system, the strongly acidic plasticizer/flame retardant metabolite diphenyl phosphate ester (DPhP), with a pKa value of 0.26, was extracted from urine. DPhP was protonated using 4 M hydrochloric acid and extracted into an acceptor phase at pH 9. Thirty extractions were made with the same liquid membrane without any decrease in extraction efficiency and with a relative standard deviation <7%. An analyte concentration enrichment of 5-10 times was achieved in the extraction step, giving a limit of detection (S/N = 3) of 0.014 microg/mL with LC/ESI-MS and 0.18 microg/mL with CE-UV. The effects on extraction efficiency using different sample pH, organic solvents, sample flow rates, and lengths of the fiber were evaluated.

Investigation of propofol renal elimination by HPLC using supported liquid membrane procedure for sample preparation

One of the least explored subjects in the research on the metabolism of a widely used anaesthetic, propofol, is its excretion in an unchanged form. According to literature, the estimated percentage of applied propofol eliminated intact via kidneys is lower than 0.3%. The present study shows the amount of propofol excreted in an unchanged form with urine collected during the first 48 h after anaesthesia in five patients undergoing elective intracranial procedures. The drug was concentrated and selectively isolated from urine samples by supported liquid membrane technique and determined by HPLC with fluorescence detection. The amount of unchanged propofol eliminated with urine was approximately (0.004 +/- 0.002)% of the total applied dose. The obtained results may suggest that propofol in an unchanged form is not excreted by kidneys at all provided that all propofol determined in presented study originated from conjugates hydrolysis.

Simultaneous determination of unbound ropivacaine in rat blood and brain using microdialysis

To investigate the pharmacokinetics of ropivacaine in rat blood and brain, a sensitive HPLC method and microdialysis were developed for the simultaneous determination of unbound ropivacaine in rat blood and brain. Adult, male Sprague-Dawley rats (290-350 g) were anesthetized with sodium pentobarbital (50 mg/kg, i.p.). Two microdialysis probes were inserted, one into the jugular vein toward right atrium, and one into the brain striatum of rats. Ropivacaine (5 mg/kg, i.v.) was then administered via the femoral vein. Blood and brain dialysates were collected and eluted with a mobile phase containing methanol-acetonitrite-20 mM monosodium phosphoric acid (pH 5.5) (10:40:50, v/v/v) in a liquid chromatographic system. Separation of ropivacaine was achieved by a CN column (Phenomenex Luna, 250x4.6 mm, particle size 5 microm; Torrance, CA, USA) within 10 min. The UV detector wavelength was set at 205 nm and the detection limit of ropivacaine was 20 ng/ml. The intra- and inter-day accuracy and precision of the analyses were less than 10% in the ranges of 0.02-5 microg/ml. The pharmacokinetic data were calculated from the individual animal measurements of dialysate concentration versus time. This method exhibits no endogenous interference and its sensitivity is sufficient for the determination of biological samples. The present results confirm that microdialysis sampling followed by LC separation with UV detection represents a viable approach for the measurement of free ropivacaine in rat brain and plasma.

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.

Analytical applications of membrane extraction in chromatography and electrophoresis

An overview of the analytical applications of membrane-based systems for sample enrichment in chromatography and capillary electrophoresis is presented. A brief introduction to the different types of membranes and the main forces related to the transport through them is also given.

Membrane-based techniques for sample enrichment

Sample preparation techniques based on non-porous membrane extraction generally offer a high degree of selectivity and enrichment power, together with convenient possibilities for direct and automated connections to chromatographic and other analytical instruments. In this review principles and applications for techniques as supported liquid membrane extraction, microporous membrane liquid-liquid extraction, polymeric membrane extraction and membrane extraction with a sorbent interface are described and compared.

Liquid-phase microextraction as a sample preparation technique prior to capillary gas chromatographic-determination of benzodiazepines in biological matrices

Liquid-phase microextraction (LPME) and gas chromatography were applied to determine diazepam and the main metabolite N-desmethyldiazepam in human urine and plasma. The analytes were extracted from 3.0-3.5 ml sample volumes directly into 25 microl of extraction solvent. The microextraction device consisted of a porous hollow fiber of polypropylene attached to two guiding needles inserted through a septum and a 4 ml vial. The hollow fiber filled with extraction solvent was immersed in sample solution. The extraction device was continuously vibrated at 600 rpm for 50 min. An aliquot (1 microl) of the extraction solvent with preconcentrated analytes was injected directly into the capillary gas chromatograph. Thirty samples were extracted simultaneously on the vibrator, providing a high sample capacity. The limits of detection were from 0.020 to 0.115 nmol/ml for diazepam and N-desmethyldiazepam in plasma and urine using a nitrogen-phosphorus detector (NPD).