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

A simplified protein precipitation/mixed-mode cation-exchange solid-phase extraction, followed by high-speed liquid chromatography/mass spectrometry, for the determination of a basic drug in human plasma

A simplified protein precipitation/mixed-mode cation-exchange solid-phase extraction (PPT/SPE) procedure has been investigated. A mixture of acetonitrile and methanol along with formic acid was used to precipitate plasma proteins prior to selectively extracting the basic drug. After vortexing and centrifugation, the supernatants were directly loaded onto an unconditioned Oasis MCX microElution 96-well extraction plate, where the protonated drug was retained on the negatively charged sorbent while interfering neutral lipids, steroids or other endogenous materials were washed away. Normal wash steps were deemed unnecessary and not used before sample elution. The sample extracts were analyzed under both conventional and high-speed liquid chromatography/tandem mass spectrometry (LC/MS/MS) conditions to examine the feasibility of the PPT/SPE procedure for human plasma sample clean-up. For the conventional LC/MS/MS method, chromatographic separation was achieved on a C18, 2.1 x 50 mm column with gradient elution (k' = 5.5). The mobile phase contained 0.1% formic acid in water and 0.1% formic acid in acetonitrile. For the high-speed LC/MS/MS method, chromatographic separation was achieved on a C18, 2.1 x 10 mm guard column with gradient elution (k' = 2.2, Rt = 0.26 min). The mobile phase contained 0.1% formic acid in water and 0.001% trifluoroacetic acid in acetonitrile. Detection for both conventional and high-speed LC/MS/MS methods was by positive ion electrospray tandem mass spectrometry on a ThermoElectron Finnigan TSQ Quantum Ultra, where enhanced resolution (RP 2000; 0.2 amu) was used for high-speed LC/MS/MS. The standard curve, ranging from 0.5 to 100 ng/mL, was fitted to a 1/x weighted quadratic regression model. This combined PPT/SPE procedure effectively eliminated time-consuming sorbent conditioning and wash steps, which are essential for a conventional mixed-mode SPE procedure, but retained the advantages of both PPT (removal of plasma proteins) and mixed-mode SPE (analyte selectivity). The validation results demonstrated that this PPT/SPE procedure was well suited for both conventional and high-speed LC/MS/MS analyses. In comparison with a conventional mixed-mode SPE procedure, the simplified PPT/SPE process provided comparable sample extract purity. This simple sample clean-up procedure can be applied to other basic compounds with minor modifications of PPT solvents.

Determination of selected mycotoxins in mould cheeses with liquid chromatography coupled to tandem with mass spectrometry

A simple and feasible method is described for analysing nine mycotoxins in cheese matrix. The method involves liquid extraction followed by high performance liquid chromatographic separation and mass spectrometric detection of the analytes, and allows the determination of aflatoxins B1, B2, G1, G2 and M1, ochratoxin A, mycophenolic acid, penicillic acid and roquefortine C simultaneously. Average recoveries of the mycotoxins from spiked samples at concentration levels of 5-200 microg kg(-1) ranged from 96-143%. Within-day relative standard deviations at these concentration levels varied from 2.3-12.1%. The limit of quantification for aflatoxin M1 was 0.6 microg kg(-1) and for the other compounds 5 microg kg(-1). The method developed was applied for analysing these mycotoxins in blue and white mould cheeses purchased from Finnish supermarkets. Roquefortine C was detected in all of the blue mould cheese samples in concentrations of 0.8-12 mg kg(-1). One blue cheese contained also 0.3 mg kg(-1) mycophenolic acid. The other investigated mycotoxins were absent in the samples.

Development of an intravenous microdialysis method for pharmacokinetic investigations in humans

INTRODUCTION

Limited blood volume is a major problem in pharmacokinetic investigations in specific populations, e.g. children. Intravenous microdialysis might help to obtain improved data sets as it is already successfully done in small animals. Since quantification of drugs is crucial in microdialysis, we developed an in vitro method to produce a workable intravenous microdialysis for human use.

METHODS

A specifically designed microdialysis cell consisting of glass was heated to 37 degrees C. The cell was filled with Ringer's solution, plasma or whole blood. A microdialysis probe was inserted into the cell and perfused with Ringer's solution with addition of 4% dextran. The beta-receptor blocker sotalol served as a test drug. The stepwise in vitro evaluation process addressed issues of loss of dialysate, calibration by retrodialysis and relative recovery. These conditions were then applied in an in vivo pilot study to one single healthy volunteer after written informed consent.

RESULTS

To address loss of perfusion fluid 4% of dextran was added and high and constant amounts of dialysate were achieved. To account for changes in the relative recovery a continuous use of retrodialysis by the calibrator atenolol was introduced. The recovery of atenolol was comparable to sotalol. The pharmacokinetic analysis revealed that sotalol concentrations from microdialysates were not different from conventional plasma samples (100+/-11%, n=33) resulting in subsequent comparable pharmacokinetic parameters.

DISCUSSION

This stepwise approach using an in vitro device enabled us to demonstrate the determination of pharmacokinetic parameters of sotalol. The most important evaluation step is represented by the continuous use of retrodialysis by the calibrator atenolol because it can account for changes in the relative recovery of the drug. This approach should be a starting point to simplify pharmacokinetic studies in special populations, e.g. in small children, to improve drug treatment.

Online microdialysis-dynamic nanoelectrospray ionization-mass spectrometry for monitoring neuropeptide secretion

Although mass spectrometric approaches offer a sensitive method for identifying cell-cell signaling peptides, the high salt-containing environment of extracellular solutions often complicates characterization of these microscale samples. Accordingly, we have developed a miniature hollow-fiber microdialysis device optimized for desalting small-volume neuronal samples online, with the device directly connected to a modified dynamic nanoelectrospray ionization assembly interfaced with an ion trap mass spectrometer. Improvements over existing designs include placement of a capillary insert within the microdialysis fiber to minimize volume, as well as the use of a microinjector that enables 1 microl sample injections. We present detailed evaluation of peptide recoveries within the microdialysis fiber by liquid chromatography-electrospray ionization-ion trap-mass spectrometry analysis of tissue homogenate in artificial seawater with and without microdialysis. Analyte recoveries after microdialysis ranged from 6 to 78% with higher recoveries of more hydrophilic peptides, while little correlation between mass and percentage recovery was observed in the range studied (2000 to 6000 Da). Recoveries of peptides were the lowest for the analytes with the highest initial mass spectrometry signal intensity. Finally, we illustrate the utility of this microdialysis device for desalting neuropeptides secreted from preparations of the peptidergic bag cell neurons of the marine mollusk, Aplysia californica. Without microdialysis, the high concentration of salts ( approximately 0.5 M) prevented detection of peptides, whereas following online microdialysis-dynamic nanoelectrospray mass spectrometry of stimulated releasate, three peptides (acidic peptide, acidic peptide 1-24 and delta-bag cell peptide) were detected.

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).

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.

Automated sample preparation based on the sequential injection principle

A molecularly imprinted polymer (MIP) prepared using caffeine, as a template, was validated as a selective sorbent for solid-phase extraction (SPE), within an automated on-line sample preparation method. The polymer produced was packed in a polypropylene cartridge, which was incorporated in a flow system prior to the HPLC analytical instrumentation. The principle of sequential injection was utilised for a rapid automated and efficient SPE procedure on the MIP. Samples, buffers, washing and elution solvents were introduced to the extraction cartridge via a peristaltic pump and a multi-position valve, both controlled by appropriate software developed in-house. The method was optimised in terms of flow rates, extraction time and volume. After extraction, the final eluent from the extraction cartridge was directed to the injection loop and was subsequently analysed on HPLC. The overall set-up facilitated unattended operation, operation and improved both mixing fluidics and method development flexibility. This system may be readily built in the laboratory and can be further used as an automated platform for on-line sample preparation.

Enrichment of proteinaceous materials on a strong cation-exchange diol silica restricted access material: protein–protein displacement and interaction effects

A study of size exclusion and enrichment of proteins employing strong cation-exchange diol silica restricted access material (SCX-RAM) under saturation conditions is presented. Experiments were carried out with bacitracin, protamine, ribonuclease, lysozyme and bovine serum albumin as individual proteinaceous analytes as well as comprehensive binary mixtures and with human urine samples. Protein size dependent capacity features of the SCX-RAM column was observed. Bacitracin demonstrated the highest capacity followed by protamine while adsorption capacities of both ribonuclease and lysozyme were found smaller by a factor of 10. Applying binary protein samples occurring displacement effects were apparent: proteins with strong cationic properties displaced those already adsorbed by the bonded cation-exchange ligands. Bacitracin was displaced in all binary mixture experiments in particular by protamine. Furthermore, the binary mixtures displayed increased adsorption for some proteins due to complex formation. Lysozyme and ribonuclease showed double capacity values when paired with bacitracin. Both phenomena, displacement and enhanced adsorption occurred in the saturated state and led to changes in the urine composition during sample preparation. Injecting urine samples the relative proportions of fractions changed from 4 up to more than 20 times, due to the differences of the protein adsorption capacities on the SCX-RAM column. Analysing urine samples the SCX-RAM column provided extensive long-term stability.

Before the injection--modern methods of sample preparation for separation techniques

The importance of sample preparation methods as the first stage in an analytical procedure is emphasised and examined. Examples are given of the extraction and concentration of analytes from solid, liquid and gas phase matrices, including solvent phase extractions, such as supercritical fluids and superheated water extraction, solid-phase extraction and solid-phase microextraction, headspace analysis and vapour trapping. The potential role of selective extraction methods, including molecular imprinted phases and affinity columns, are considered. For problem samples alternative approaches, such as derivatisation are discussed, and potential new approaches minimising sample preparation are noted.

Systematic error in automated in-tube solid-phase microextraction

The widely employed configuration for automated in-tube solid-phase microextraction (SPME) involves modification of a commercial liquid chromatographic autosampler into an automated extraction device. This popular configuration is demonstrated to result in an inherent systematic error in the quantitation of analyte in a given matrix. The source of error is traced to the accumulation of analyte in the extraction and the pre-extraction segment (i.e., sample loop, metering valve and tubing prior to the metering valve) of the autosampler where the analyte comes in contact with the residual mobile phase. This results in cross-contamination due to sample/mobile phase mixing. The quantity of analyte accumulated in these segments is shown to consistently increase with the increasing number of draw/eject cycles. As a result of the accumulation, the amount of analyte recorded leads to inaccurate quantitative information, leading to overestimation of the limit of detection and limit of quantitation, when automated in-tube SPME is employed as an approach for sample enrichment. Insertion of a 100-microl air plug prior to extraction step was able to significantly minimize sample/mobile phase mixing of analyte with the residual mobile phase in the pre-extraction and extraction step, thus minimizing the systematic error.

The use of the new SPE methods for isolation of some tricyclic antidepressant drugs from human serum

Solid-phase extraction (SPE) methods were applied to isolation of amitriptyline (AMI), imipramine (IMI) and chlorprothixene (CPX) from blood human serum. SPE was carried out using the octadecyl (C(18)) column for isolation of AMI and cyclohexyl (CH) columns in the case of IMI and CPX. The spiked serum samples were used to examine the recoveries of these drugs from C(18) and CH sorbent materials. The volume of serum sample was 500 microl. The recoveries of SPE method using CH cartridge were 100.3+/-1.63% (n=7), 99.7+/-2.3% (n=9) for IMI and CPX, respectively. The recovery of AMI from C(18) cartridge was 99.5+/-1.5% (n=8). Finally, after SPE sample clean-up step the antidepressant drugs were assayed by the own extractive-spectrophotometric methods.

Use of large particles support for fast analysis of methadone and its primary metabolite in human plasma by liquid chromatography-mass spectrometry

A bioanalytical method was developed for the quantitation of methadone (MTD) and its primary metabolite, (EDDP) in plasma. The extraction step was performed within a capillary column packed with large particles (35x0.3 mm I.D.; d(p) 30 micrometer) at high flow-rate conditions (450 microliter/min). The separation was performed on a microbore analytical column (55x2 mm I.D.; d(p) 3 micrometer) coupled to a mass spectrometer (MS). This procedure was based on a column-switching unit. Analytes of interest were retained on the precolumn by hydrophobic interactions and backflushed from the precolumn to the analytical column. The detection was carried out with a MS single quadrupole equipped with an electrospray interface. The total analysis time was 6 min. The limits of quantification were evaluated at 10 and 25 ng/ml for MTD and EDDP, respectively. At this level, good accuracies were obtained for both analytes with repeatability values less than 18%.

Automated LC-MS Method for the Fast Stereoselective Determination of Methadone in Plasma

Methadone (MTD) is a chiral drug widely used for the treatment of opioid dependence for which a rapid analytical method for the determination of each enantiomer would be advantageous. In order to improve method sensitivity and to automate the entire analytical process, a column-switching configuration has been developed. An online extraction system coupled to a cellulose-based chiral stationary phase (CSP), namely Chiralcel OJ-R, was used and detection was performed by mass spectrometry. Fifty microl of plasma were injected into the liquid chromatography-mass spectrometry (LC-MS) system after addition of acetonitrile (ACN) containing methadone deuterated D9 (MTD-D9) (internal standard) and centrifugation. For the rapid extraction step, a large particle size support was selected. A baseline separation of MTD enantiomers was obtained in less than 12 min. Trueness and precision were evaluated with control samples at 500 ng/ml of (R,S)-methadone. Trueness values were 106.6% and 103.0% for (R)-MTD and (S)-MTD, respectively, with a coefficient of variation inferior to 2.5% for both compounds. Finally, a good concordance was found using this method for analysis of plasma samples from patients in maintenance treatment as compared to a previously described HPLC-UV method after liquid-liquid extraction.

A robust approach for the analysis of peptides in the low femtomole range by capillary electrophoresis-tandem mass spectrometry

A capillary electrophoresis-tandem mass spectrometry (CE-MS/MS) approach has been developed for routine application in proteomic studies. Robustness of the coupling is achieved by using a standard coaxial sheath-flow sprayer. Thereby, greater stability than nanoelectrospray ionization-mass spectrometry coupling of sheathless capillary electrophoresis or nanoliquid chromatography (nano-LC) is achieved, resulting in stable operation for several weeks and unattended overnight sequences. The applied sheath flow is reduced to 1-2 microL/min in order to increase sensitivity. Standard peptides and those of digests of standard proteins and gel-separated proteins can be detected in the low femtomole range (full scan and MS/MS). Detection limits are found to be as low as 500 amol. Low femtomole amounts are required for unequivocal identification by MS/MS experiments in the ion trap and subsequent database search. By applying a simple pH-mediated stacking the concentration sensitivity can be lowered to some tens of fmol/microL (nM), depending on capillary size. This sensitivity is close to published values for sheathless CE-MS and nano-LC-MS, respectively (a comparison to reference values is presented). Moreover, with capillaries of about 50 cm in length separations in less than 10 min are possible resulting in a throughput of up to four analyses per hour. This is a factor of 4-12 times faster than nano-LC separation, being the state-of-the-art techniques for proteomic studies.

Mass spectrometric detection in chromatography. Trends and perspectives

The newest results in the use of miscellaneous mass spectrometric detection methods in various chromatographic techniques are compiled and critically evaluated. Examples of application in clinical chemistry, health care, and in the analysis of pharmaceuticals, environmental pollutants, foods and food products are given. The benefits and drawbacks of MS detection are discussed, and future trends are briefly discussed.

On-line microdialysis of proteins with high-salt buffers for direct coupling of electrospray ionization mass spectrometry and liquid chromatography

Mass spectrometry (MS) is one of the most powerful instrumental techniques for protein analysis. The electrospray ionization (ESI) approach is known to be very gentle and at the same time compatible with liquid separation techniques such as HPLC and CE. However, ESI is known to be susceptible to salts and impurities, which often cause a dramatic decrease in sensitivity due to the suppression of the ionization of the product of interest. For this reason, LC-ESI-MS coupling has so far been largely limited to reversed-phase chromatography with its hydro-organic mobile phases. Other chromatographic techniques are typically "linked" to ESI-MS by time consuming, off-line desalting steps. On-line microdialysis has been proposed as a solution to this dilemma. In this paper, we introduce an improved microdialysis system, which enlarges the number of putative applications, thus allowing chromatographic separations of biological compounds to be directly coupled to MS detection with little to no loss in time or chromatographic resolution. Examples include separations by affinity, ion-exchange and size-exclusion chromatography, all of which were connected successfully to the ESI-MS detector via the on-line microdialyzer. We propose that, using this system, any kind of chromatography technique can be coupled to ESI-MS, thus enabling for example application in quality control or process monitoring of many bioproduction and downstream processes.

Analysis of cellular release using capillary electrophoresis and matrix assisted laser desorption/ionization-time of flight-mass spectrometry

In order to increase our understanding of the mechanisms of learning and memory in the central nervous system, it is necessary to know the neurotransmitters and neuromodulators used in the specific neuronal circuits under study. Methods have been developed to identify the peptides released from single neurons and neuronal clusters from the common neuronal model Aplysia californica. Specifically, solid-phase extraction (SPE), capillary electrophoresis (CE) and matrix assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOF-MS) are combined for profiling neuropeptide releasates. A variety of combinations of SPE and CE were coupled off-line with MALDI-TOF-MS to reduce the high physiological salts, to concentrate the analytes, and to reduce the complexity of the mass spectra using separation. With these protocols, peptides and proteins up to 11000 Da were detected in releasates, offering a much wider mass range compared to direct MALDI analysis of the same releasates. A number of expected and unknown neuropeptides, including egg-laying hormone (ELH) and the partially processed delta/gamma-bag cell peptide were observed in the SPE-treated releasates from a single Aplysia-cultured bag cell neuron. However, by adding a CE separation after the SPE step preceding off-line MALDI-TOF-MS detection, the most complete neuropeptide profiles were obtained.