Potential of gas chromatography-atmospheric pressure chemical ionization-time-of-flight mass spectrometry for the determination of sterols in human plasma.

The application of Gas Chromatography (GC)-Atmospheric Pressure Chemical Ionization (APCI)-Time-of-Flight Mass Spectrometry (TOF-MS) is presented for sterol analysis in human plasma. A commercial APCI interface was modified to ensure a well-defined humidity which is essential for controlled ionization. In the first step, optimization regarding flow rates of auxiliary gases was performed by using a mixture of model analytes. Secondly, the qualitative and quantitative analysis of sterols including oxysterols, cholesterol precursors, and plant sterols as trimethylsilyl-derivatives was successfully carried out. The characteristics of APCI together with the very good mass accuracy of TOF-MS data enable the reliable identification of relevant sterols in complex matrices. Linear calibration lines and plausible results for healthy volunteers and patients could be obtained whereas all mass signals were extracted with an extraction width of 20 ppm from the full mass data set. One advantage of high mass accuracy can be seen in the fact that from one recorded run any search for m/z can be performed.

Screening for cerebroprotective agents using an in vivo model of cerebral reversible depolarization in awake rats.

The need to screen cerebroprotective compounds without anesthetic interference prompted the development of a model using hypoxic rats. In this model two outcome measures were used: (1) the time to reach isoelectric electroencephalogram (iEEG), caused by nitrogen gas inhalation in the test chamber, and (2) the time for behavioral recovery measuring the latency of restoration of the head-withdrawal reflex upon vibrissae stimulation. We report here data of blood chemistry, cerebral tissue oxygen measurements, a definition of a proposed scoring system, and the pharmacological results of RGH-2202. The findings with RGH-2202 are used here to show the utility of the screening method. Events during hypoxia: Arterial and venous pO(2), pCO(2), and pH, and brain tissue pO(2)significantly declined. Significant correlations were established among the pO(2)of cerebral tissue, blood, and the test chamber. RGH-2202 significantly and dose-dependently shortened the iEEG time; the compound's Effective Dose(30)was 227.8 mg kg(-1). Events during recovery: Immediately after the iEEG, when the atmosphere in the chamber was replaced with room air, the arterial, venous and brain tissue pO(2)increased above the control level and subsequently recovered to baseline levels. Behavioral recovery occurred before blood chemistry was otherwise normalized. RGH-2202 significantly and dose-dependently shortened the recovery time; the Effective Dose(30)was 8.71 mg kg(-1). The available data define and support the physiological basis of this practicable rat-screening model.

Separation, analyses and syntheses of trimethoprim impurities.

The analysis of several impurities of the chemotherapeutic agent trimethoprim with various methods, including thin layer chromatography, gas chromatography, capillary electrophoresis as well as nuclear magnetic resonance is described. These methods were used to identify new impurities in trimethoprim batches. The main impurities were separated by column chromatography. To ensure the identity of the impurities, de novo syntheses were successfully carried out. With the methods described, it was possible to detect, separate and identify new impurities in trimethoprim batches.

Optimization of electrochemical hydride generation in a miniaturized electrolytic flow cell coupled to microwave-induced plasma atomic emission spectrometry for the determination of selenium.

The optimization of a continuous flow system for electrochemical hydride generation coupled to microwave-induced plasma atomic emission spectrometry (MIP-AES) for the determination of Se is presented. A small electrolytic cell with a porous glassy carbon working electrode was used for hydride generation. When using an Ar MIP operated in a TE101 cavity a detection limit of 0.6 ng mL(-1) (3sigma) could be achieved. The calibration curve was linear up to 1 microgram mL(-1). A standard deviation of less than 2% (10 replicate analyses) could be achieved. It was shown that interferences of transition metals are of the same order of magnitude as with a larger electrolysis cell described earlier, and light elements hardly caused any signal depression as tested. It was possible to distinguish between Se(IV) and Se(VI) species and seleno-DL-methionine, because under optimized conditions of an electrolysis current of 10 mA, a microwave power of 210 W, an Ar flow rate of 15 L h(-1) and a sample flow rate of 2.5 mL min(-1) only Se(IV) was transformed to H2Se and transferred into the plasma. Finally, the possibility of an electrochemical pre-enrichment was shown to enable it to further decrease the detection limit.

Analysis of illicit drugs by nonaqueous capillary electrophoresis and electrochemical detection.

Nonaqueous capillary electrophoresis (NACE) was applied to the determination of illicit drugs. The complete separation of amphetamine, methamphetamine, 3,4-methylene dioxy amphetamine (MDA), 3,4-methylene dioxy methamphetamine (MDMA), mescaline, cocaine and benzoylecgonine was obtained using an acetonitrile based buffer solution containing 10 mM sodium acetate and 1 M acetic acid. Electrochemical detection using a Pt microdisk electrode set to a potential of +1.8 V was found to be selective for MDA, MDMA and mescaline. The detection limits for these compounds were in the low ng/mL range which is between 2 and 3 orders of magnitude lower compared to UV-detection.

Application of non-aqueous capillary electrophoresis with electrochemical detection to the determination of nicotine in tobacco.

Non-aqueous capillary electrophoresis with electrochemical detection (NACE-ED) was applied to the determination of nicotine. The measurements were performed using an acetonitrile-based buffer. Nicotine was shown to yield well defined voltammetric signals suitable for oxidative detection. The precision of NACE-ED regarding migration time and peak height for samples containing 8 micrograms/ml nicotine is expressed by relative standard deviations of 0.1% and 1.6%, respectively (n = 8). The limit of detection for nicotine was 13 ng/ml (286 fg). For nicotine determination in tobacco samples various solutions were studied regarding the extraction efficiency in an ultrasonic bath. The highest extraction efficiency was obtained using a solvent mixture consisting of acetonitrile-acetic acid-water (20:5:75, v/v). The results for nicotine determination in tobacco were evaluated using tobacco reference material with certified nicotine content. Analytical aspects such as accuracy, reproducibility and selectivity were addressed in this work. The measurements were based on the use of a newly developed electrochemical detector cell which was found to enable user-friendly operation of NACE-ED measurements.

Determination of ascorbic acid in nanolitre samples by means of capillary batch injection analysis.

Capillary batch injection analysis (CBIA) is applied to the determination of ascorbic acid in nanolitre samples at neutral pH. It is demonstrated that pulsed amperometric detection is superior to constant-potential amperometric detection with respect to signal stability. The calibration plot is linear within the studied concentration range between 0.5 mM and 5 mM and the reproducibility is characterized by a relative standard deviation of 0.80% for 10 successive injections of 188 nL of 3.5 mM ascorbic acid. The attainable sample throughput is up to 500 per hour.

Sonoelectrochemical analysis of trace metals.

Ultrasonically-enhanced mass transport was exploited to increase preconcentration efficiency in anodic stripping voltammetry. We developed a Nafion-coated mercury thin-film working electrode which is stable under ultrasonic irradiation, making it possible to achieve very low limits of detection for relatively short preconcentration times. This allows the investigation of a variety of biological and environmental samples.

Sonoelectrochemical determination of nucleosides and nucleotides.

We summarize our recent work on sonovoltammetric studies of adenine, guanine, thymine, cytosine and their corresponding nucleosides and nucleotides. Ultrasound is found to be an excellent tool to enhance the reliability of determinations of purine and pyrimidine derivatives and can lead to improved analytical performance characteristics and lower detection limits.

Nafion-coated mercury thin film electrodes for batch-injection analysis with anodic stripping voltammetry.

Batch-injection analysis exhibits the advantages of rapid and simple electroanalysis of microlitre samples. Nafion-coated mercury thin film electrodes have been evaluated for use in batch-injection analysis with anodic stripping voltammetry (BIA-ASV). The advantages of Nafion-coated electrodes in reducing electrode contamination by components of complex matrices are combined with the analysis of small microlitre sample volumes. The measurement of traces of lead and cadmium is used to illustrate the approach. An optimised procedure for formation of Nafion-coated mercury thin film electrodes is evolved. The relative sensitivity for BIA-ASV at electrodes with and without Nafion coatings is 0.9 and 0.8 for cadmium and lead respectively; detection limits are 2 x 10(-9) M and 4 x 10(-9) M. Studies were done concerning the influence of surfactants and their effect was found to be much less with the Nafion film coating. Applications to real environmental samples are demonstrated.

Capillary electrophoresis and capillary flow injection analysis with electrochemical detection.

Measurements by capillary flow injection analysis (CFIA) and capillary electrophoresis (CE) in conjunction with electrochemical detection are described. The detection is based on an end-column electrode arrangement. Several novel electrodes, such as a spherical gold electrode and a dual-microdisk electrode, are presented and characterized regarding their analytical utility. In order to improve the selectivity of CFIA, dual-electrode and multiple-pulse detection are studied using couples of cyanometallates or metallocenes. Capillary electrophoretic experiments with amperometric detection are performed using 50 microm i.d. capillaries without any electrical-field decoupler. The practicality and analytical characteristics of this detection strategy are illustrated for the separation of serotonin and some biological precursors and metabolites of neurotransmitter substances.

Determination of cyanide in microsamples by means of capillary flow injection analysis with amperometric detection.

A new approach for determining cyanide in microsamples is described. The method is based on capillary flow injection analysis (CFIA) with amperometric detection. The sensing electrode is a silver-plated microdisk electrode, where cyanide can react under formation of a dicyanoargentate complex. A remarkably low mass detection limit of 231 fmol cyanide is obtained for an injection volume of 60 nl. The sample throughput of the CFIA-arrangement is comparable with a conventional sized FIA-system. A practical application is given by analyzing the cyanide (amygdalin) concentration in apple kernels.

Trace metal determination in tears by anodic stripping voltammetry in a capillary flow injection system.

The adaptation of a fused silica capillary system to flow injection is described. The incorporated electrochemical cell, consisting of a microelectrode arrangement, is demonstrated to be well suited for conductivity and voltammetric measurements. Microanalytical studies with injection volumes within the nanolitre range were performed. Analytical characteristics of the method, such as accuracy, precision, sensitivity and limit of detection, are discussed. By using capillary flow injection, tear samples were investigated with respect to conductivity and trace-metal concentration. The last was performed via anodic stripping voltammetry at mercury-coated platinum microdisc electrodes. Lead, cadmium and copper were found to be present at concentrations in the lower microgram l-1 range.