Performance of instruments and aspects of methodology and validation in quantitative capillary electrophoresis an update

The novel Outer Plug Standardization (OPS) calibration strategy applied in a new method for the high-throughput determination of Cl-, NO3- and SO4-2 in rainwater samples

This paper presents the Outer Plug Standardization - OPS, a novel technique as an alternative for the classic internal standardization using multiple injection in capillary zone electrophoresis is proposed herein. This technique was applied in a new method for the determination of chloride, nitrate and sulfate in rainwater samples. After the injection accuracy was tested and proved to be a minor error source (average 1.26% RSD), the OPS was applied and it improved the intra-day and inter-day precision of the analytical method by 32.5% and 24.7%, respectively. Using a capillary with effective length of 23.5 cm the electrophoretic separation of the three inorganic anions could be achieved in <1 min, with detection limits of 0.05, 0.09 and 0.11 mg L^-1 for chloride, nitrate and sulfate, respectively. Also, the statistical t-test was applied to the results obtained for 82 rainwater samples that were collected and analyzed applying both the method developed in this study and the official APHA 4140 method where no statistical difference was noted within a 95% confidence level.

Rapid separation of seed gliadins by reversed-phase ultra performance liquid chromatography (RP-UPLC) and its application in wheat cultivar and germplasm identification

To separate gliadin from wheat flour, a novel and stability-indicating reversed-phase ultra performance liquid chromatography (RP-UPLC) method is established and optimized. A comparative analysis of routine capillary electrophoresis (CE), reversed-phase high-performance liquid chromatography (RP-HPLC), and RP-UPLC was performed and the results showed that the resolution and efficiency of RP-UPLC were significantly higher than those of CE and RP-HPLC. Characteristic RP-UPLC patterns of different bread wheat variety and related species were readily identified. These results demonstrated that our RP-UPLC procedure resulted in significant improvements in sensitivity, speed, and resolution, and thus is highly useful in wheat cultivar and germplasm identification.

Analysis of substances to be used as internal standards in MEKC

The use of internal standards (ISs) improves the quantitative performance of CE. However, ISs chosen for use in CZE often cannot be used for micellar EKC (MEKC). Therefore 22 substances were investigated as potential ISs in MEKC. These substances were selected based upon a literature search. The substances were investigated using a method similar to the standard operating conditions for MEKC as recommended by S. Terabe. Furthermore, the migration time and the corrected migration time (k(S)) were determined for each substance to establish the migration position relative to other peaks in the electropherograms. A combination of eight substances, selected according to the obtained results (t(m) = 4 up to 12 min), was tested for practical benefit and applicability. The peak area precision was in the range of 0.8 and 1.2% (n = 60), and the peaks were well shaped for all the investigated substances. The selected substances covered a wide migration time window and therefore they can be regarded as suitable for future analysis at any required migration position.

Decreasing analysis time in capillary electrophoresis: Validation and comparison of quantitative performances in several approaches

Capillary electrophoresis (CE) can be used for the rapid determination of pharmaceuticals, particularly in routine quality control analysis. This paper focuses on several approaches aimed at decreasing the analysis time with commercially available instrumentation by (i) application of a high electric field through a reduced capillary, (ii) use of a dynamically coated capillary to increase the electroosmotic flow, (iii) short-end injection (SEI) technique, and (iv) application of multiple sample injections. Moreover, SEIs were combined with the three other approaches. A pharmaceutical formulation containing lidocaine as an active component was selected, and the methods were validated according to the ICH guidelines. The seven approaches investigated fulfilled different statistical requirements and demonstrated their linearity and trueness, with good recoveries and confidence limits always inferior to 1.5%. Furthermore, relative standard deviation (RSD) values for repeatability and intermediate precision were inferior to 1.1 and 1.8%, respectively. These results confirmed that each approach is of utmost interest to increase the analyte throughput in CE.

Capillary electrophoresis-a high performance analytical separation technique

Capillary electrophoresis (CE) is often one of the preferred techniques in pharmaceutical quality control and in clinical chemistry, particularly considering the high selectivity and lower costs compared to HPLC. The precision of CE is as good as in liquid chromatography (LC). The sample-throughput is high due to short analysis times. Efforts for sample pre-treatment are usually minor in CE. Urine and even blood plasma can be directly injected without further pre-treatment. After summarising the basic principles of CE, general strategies for method development are described to achieve selective, efficient, precise, fast, sensitive, and validated methods. Sample pre-treatment requirements are discussed. Standard buffer recipes, surfactants used in micellar electrokinetic chromatography (MEKC), chiral selectors, useful buffer additives, actions to deal with complex matrices, and aspects of validation have been collected. Other techniques that can be performed with CE instruments, such as capillary isoelectric focusing (CIEF), capillary isotachophoresis (CITP), and capillary electrochromatography (CEC), are briefly discussed.

Determination of low-molecular-mass carboxylic acids in atmospheric aerosol and vehicle emission samples by capillary electrophoresis

A method is developed for the determination of a large number of airborne and vehicle-emitted low-molecular-mass mono- and dicarboxylic acids using capillary electrophoresis with indirect UV detection. A background electrolyte (BGE) consisting of 2,6-naphthalenedicarboxylic acid and tetradecylmethylammonium bromide, adjusted to pH 6.2 with 2,2-bis(hydroxymethyl)-2,2',2"-nitrilotriethanol, is employed. Separations are robust using the buffered BGE, proper rinse steps, and constant current mode with migration time variations less than 3% RSD on a day-to-day basis, using different capillaries and performed by different analysts. Detection limits are at the tens of microg/l level using pressure injection. A comparison of the CE method with ion chromatography is also made.

Quantitative and qualitative precision improvements by effective mobility-scale data transformation in capillary electrophoresis analysis

By transforming the time-based x-axis of electropherograms in capillary zone electrophoresis (CZE) into the corresponding effective mobility-scale, we propose a simple and robust data representation for a better qualitative and quantitative capillary electrophoresis (CE) analysis. The time scale of the raw electrophoretic data (detection signal versus time) is transformed into an effective electrophoretic mobility scale (mu eff-scale) with account of the electroosmotic flow (EOF) peak or of an internal standard of known effective mobility. With the new scaling (detection signals versus effective mobility), the obtained electropherograms are more representative of the velocity-based electrophoretic separation and the comparison of complete electropherograms is directly possible. This is of importance when tracking peaks in real samples where alteration in EOF stability can occur or when comparing electrophoretic runs from different experimental setups (independence in column length and voltage). Beside the qualitative possibilities, a quantitative improvement is achieved in the mu eff-scale with significant better peak area reproducibility and equal to more precision in quantitative analysis than with the primary time-scale integration.

Laser-induced dispersed fluorescence detection of polycyclic aromatic compounds in soil extracts separated by capillary electrochromatography

Polycyclic aromatic hydrocarbons (PAHs) and nitrogen containing aromatic compounds (NCACs) are characterized in soil extracts and laboratory standards by capillary electrochromatography (CEC) with laser-induced dispersed fluorescence (LIDF) detection using a liquid-nitrogen cooled charge-coupled device detector. The LIDF detection technique provides information on compound identity and, when coupled with the high separation efficiencies of the CEC technique, proves useful in the analysis of complex mixtures. Differences in fluorescence spectra also provide a means of identifying co-eluting compounds by using deconvolution algorithms. Detection limits range from 0.5 to 96x10(-10) M for selected PAHs and 0.9-3.7x10(-10) M for selected NCACs. Soil extracts are also injected onto the CEC column to evaluate chromatographic method performance with respect to complex samples and the ability to withstand exposure to environmental samples.

Determination of enantiomeric excess by capillary electrophoresis

Capillary electrophoresis (CE) is becoming an established method for the determination of chiral trace impurities. This paper provides an overview of the state of the art of CE for such determinations. Detection limits of 0.1% impurity is widely accepted as a minimum requirement for chiral trace impurity determinations. This can be relatively easily achieved with CE. However, determination of lower concentrations requires careful optimization of the separation system. Four factors that are of particular significance for trace enantiomeric determinations: resolution, limit of detection, linear range and type of detection, are discussed. Further, the advantages and disadvantages of derivatization in this context are treated as well as the separation approach, ie., direct chiral separation or separation after the formation of diastereomers. It is concluded that the limit of impurity detection can be about 0.05% when UV detection is employed. Using laser-induced fluorescence detection, a quantitative determination at the 0.005% level is often possible.

Fast chiral separations using sulfated beta-cyclodextrin and short-end injection in capillary electrophoresis

The general applicability of sulfated beta-cyclodextrin as chiral selector and short-end injection in capillary electrophoresis (CE) as a powerful screening tool for fast and efficient chiral separation of Ormeloxifene enantiomers and racemic Ormeloxifene analogues is demonstrated. Using the short-end injection procedure, all of the 16 racemic compounds studied were successfully separated with high efficiencies and with analysis times of less than 1.2 min. Furthermore, long-end injections of eight analogues named C1-C8 afforded separations with extremely high efficiencies. A statistical evaluation of the resolution values obtained in short-end and long-end injections of compounds C1-C8 showed that the sensitivity of the CE method towards structural changes in the studied molecules is intact when the chiral analysis is performed with short-end injection compared to conventional long-end injection.

Parameters affecting reproducibility in capillary electrophoresis

It is well known that poor quantitative reproducibility substantially limits the practical implementation of capillary electrophoresis (CE) separations in chemical analysis. The principal sources of variance in observed peak areas are irreproducible flow rate, which influences on-column detector response, and inconsistent injection volume or amount. An overview of studies by researchers to address the reproducibility issue will be presented. In addition, current efforts in our laboratory to assess sources of quantitative variance for separations of dansylated amino acids using an automated CE system are presented and related when appropriate to the body of existing knowledge on this important topic. A comparison of different injection methods (hydrostatic vs. electrokinetic) and approaches (e.g., high vs. low pressure), the effect of random changes in electroosmotic flow (EOF) due to air bubbles in the CE capillary, and choice of certain peak integration parameters in terms of peak area reproducibility are presented. Under optimum conditions relative standard deviation (RSD) values in raw peak area are typically 2.0%. With nonoptimum conditions (e.g., with air bubbles in capillary), RSD values can substantially degrade. However, normalizing with retention times, internal standards, or observed electrophoretic current produces RSD values in a range of 1.4-2.3%.

Determination of oligonucleotide ISIS 2922 in nanoparticulate delivery systems by capillary zone electrophoresis

ISIS 2922 is an antisense oligonucleotide with antiviral activity against cytomegalovirus. However, its rapid degradation in biological fluids and its low capacity for diffusion across cell membranes limit its therapeutical use. One possibility to overcome these drawbacks consists of using nanoparticles as drug carriers. The aim of this study was to develop an analytical method for determining the amount of ISIS 2922 loaded into albumin nanoparticles. For this purpose, capillary zone electrophoresis (CZE) was performed on a fused-silica capillary filled with borate buffer (12.5 mM, pH 9.5). Paracetamol was used as an internal standard and a diode-array detection system was set at 270 nm. Under these conditions, the limit of quantitation of ISIS 2922 was 1.27 microg and the precision and accuracy of the method did not exceed 7%. Moreover, the use of paracetamol as internal standard and the quantification by means of a 'corrected area' procedure enabled us to reduce the peak variability and accurately determine the amount of oligonucleotide loaded in the albumin nanoparticles. In summary, this assay is a selective and sensitive CZE method for the accurate quantitation of ISIS 2922 oligonucleotide in albumin nanoparticles.

Determination of ephedrine, theophylline and phenobarbital in a tablet dosage form by capillary electrophoresis

A capillary electrophoresis method was developed to separate and quantitate ephedrine (ED), theophylline (TP) and phenobarbital (PB) in a tablet dosage form. Tablets were ground and extracted with methanol using ultrasonication. Aliquots of standard stock solution were hydrodynamically injected for 5 s at the anodic end. Separation was performed on a fused silica capillary (72 cm x 50 microm i.d.; 50 cm to detector) at an applied voltage of 20 kV with a phosphate run buffer (pH 8.0, 50 mM). Analysis was performed at ambient temperature (23+/-1 degrees C) and the total run time was 9 min with detection at 220 nm. Calibration curves were prepared for ED, TP and PB with methyl p-hydroxy benzoate as internal standard. For each analyte, the correlation coefficients were >0.999 (n = 4). The RSD% of ten replicate injections for each analyte were <1%. The method was applied to the quantitation of ED, TP and PB in a commercial tablet dosage form.

Development of a capillary electrophoresis assay based on free sulfate determination for the direct monitoring of sulfoesterase activity

A capillary electrophoresis assay of sulfoesterase activity was developed that overcomes the main drawbacks encountered with the usual methods for sulfate determination in complex biological medium. Conditions are described allowing direct measurement of inorganic sulfate that is enzymatically produced in the reaction mixture. The main features of this method are electrokinetic sample introduction, which allows selective extraction of sulfate from the matrix into the separation capillary, counter-electroosmotic flow migration mode, indirect absorbance detection and use of an internal standard for quantitative performances. Likewise, perfect linearity was obtained for concentrations of sulfate up to 40 ppm. The limits of detection and quantification were 0.2 and 0.6 ppm, respectively. The run-to-run and day-to-day precision are 1 and 4.5%, respectively, for sulfate concentrations varying from 35 ppm down to 1 ppm. The accuracy was established for the synthetic p-nitrocatechol sulfate substrate by comparison with the classical spectrophotometric assay. The method was applied to the kinetic monitoring of the activity of a sulfoesterase extracted from the marine mollusc Pecten maximus on fucoidan, a bioactive sulfated fucose-based polysaccharide derived from brown algae. For the first time, a sulfoesterase activity was shown to be effective on such sulfated polysaccharides.

Capillary electrophoresis and capillary electrochromatography of organic pollutants

Capillary electrophoresis (CE) has a unique capability for separation of analytes of environmental concern, particularly those that are more polar and ionic, based on the complementary separation principle of electrophoresis. In the past few years, CE has been selectively used to analyze various classes of compounds having current or potential environmental relevance. This review outlines the current status of CE for the determination of environmental pollutants, based predominantly on research results published from the beginning of 1997 to early 1999. Covered are environmental pollutants of all types except pesticides and inorganics. Certain naturally produced toxins are also covered because of their significant impacts upon human health and the environment. CE methods, as with all methods, must be judged on their ability to provide approaches that are reliable, sensitive, selective, and rapid, while meeting "green chemistry" initiatives for pollution prevention. We also compare CE methods to benchmark environmental techniques involving gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and high performance liquid chromatography (HPLC).

Micellar electrokinetic capillary chromatography as a powerful tool for pharmacological investigations without sample pretreatment: a precise technique providing cost advantages and limits of detection to the low nanomolar range

A number of pharmaceuticals (e.g., acetaminophen, salicylic acid, sulfamethoxazole, theophylline, tolbutamide and trimethoprim) have been determined in human plasma by micellar electrokinetic chromatography (MEKC), without sample pretreatment, using underivatized fused-silica capillaries. The total analysis time was only 10 min. A sodium dodecyl sulfate (SDS)-containing borate buffer (60 mM with 200 mM SDS) at pH 10 was used. Between runs, proteins adsorbed to the capillary wall are removed by rinsing with SDS buffer and either acetonitrile (e.g., 50% v/v) or isopropanol (e.g., 10% v/v). Other rinsing procedures are discussed (salts, enzyme-containing solutions, organic solvents, sodium hydroxide, hydrofluoric acid). The separation system is tested in a concentration range between 10 ng/mL and 100 microg/mL; a detection limit of about 20 ng/mL can readily be obtained. The sensitivity was substantially improved using isopropanol as buffer additive. A day-to-day precision for relative peak areas of 1-2% relative standard deviation (RSD, n > 40) was reached in the upper concentration range. Under repeatability conditions, these values could also be obtained for low microg/mL concentrations. Thus, not only drug monitoring but also pharmacokinetic investigations from blood plasma become possible without further sample pretreatment.

How to achieve higher repeatability and reproducibility in capillary electrophoresis

The influence of experimental parameters on precision of migration times and accuracy of integration has been studied. The repeatability of migration times strongly depends on the proper selection of the rinse steps between each run of a CE system. The rinse steps have to be optimized additionally with each separation system. This is especially important for systems providing an anodic electroosmotic flow. The errors introduced by the integration software were studied by transferring the same data sets to different commercial available integration softwares. A strategy for the transfer of raw data files between several softwares is described. By using systems with identical peak areas or with identical peak heights it could be shown that the newly introduced softwares can cope better with the leading or tailing peaks. The RSD of quantitation strongly depends on the signal-to-noise ratio. At S/N ratios larger than 35 no differences between the various softwares studied could be observed. At lower S/N ratios the newly released softwares are to be preferred.

Micellar electrokinetic capillary chromatography method for direct determination of glucuronides of entacapone and its (Z)-isomer in human urine

This paper describes the validation of a micellar electrokinetic capillary chromatography method for the direct determination of the 3-O-glucuronides of entacapone and its (Z)-isomer, the main urinary metabolites of entacapone in humans. Entacapone is a novel drug which, as a potent inhibitor of catechol-O-methyltransferase (COMT), is used as an adjunct in the standard therapy of Parkinson's disease. The 3-O-glucuronide of another COMT inhibitor, nitecapone, was used as internal standard (I.S.). The validation experiments were performed by using spiked urine samples that were extracted with Sep-Pak C18 cartridges before analysis. Determinations were carried out in a buffer of pH 7.0 containing 25 mM of phosphate, 50 mM of borate and 20 mM of sodium dodecyl sulfate, and by applying 15 kV over a 67 cm (60 cm to the detector) x 75 microns fused-silica capillary. UV detection was at 335 nm. The validity of the method was assessed by investigating the identity of the analytes, selectivity, limit of quantitation, linearity, within-day precision, extraction recovery, between-day precision and accuracy, electroosmotic flow stability and analyte stability. The method proved to be reproducible, sufficiently selective and accurate. Extraction recoveries of the analytes were > 94%. The limit of quantitation (LOQ) was 2 micrograms/ml and the assay was linear in the range 2-150 micrograms/ml with correlation coefficients better than 0.999 for both glucuronides. The repeatability of the method, expressed as the ratio of corrected peak area of the analytes to that of I.S., gave RSD values of < 5% even at the LOQ. Between-day precision (RSD) was < 7.5% for both glucuronides at 7.5 micrograms/ml. Determination of the glucuronide concentrations in urine samples of 34 patients treated with entacapone either orally (200 mg) or intravenously (25 mg) showed the method to be suitable for monitoring the concentrations of the glucuronide of entacapone after both oral and intravenous administration and those of the glucuronide of its (Z)-isomer after oral administration. The limited long term stability of the system requires, however, frequent recalibration in applications involving long sample series.

Separation of the glucuronides of entacapone and its (Z)-isomer in urine by micellar electrokinetic capillary chromatography

A micellar electrokinetic capillary chromatography (MECC) method was developed for the separation of the 3-O-glucuronides of entacapone and its (Z)-isomer, the two main urinary metabolites of entacapone in humans. Entacapone is a novel, potent inhibitor of catechol-O-methyltransferase (COMT) intended for use as an adjunct in the treatment of Parkinson's disease. Urine samples spiked with synthetic 3-O-glucuronides were used to study the effects of running buffer pH, composition and applied voltage on separation of the closely migrating glucuronides. The 3-O-glucuronide of nitecapone, was used as internal standard. The greatest improvement in separation was achieved by increasing the running buffer ionic concentration. Changes in pH had little effect on the separation, whereas increase in sodium dodecyl sulfate (SDS) concentration slightly improved resolution. Baseline separation and good selectivity relative to urine components were achieved by using a phosphate (25 mM)-borate (50 mM)-SDS (20 mM) running buffer, pH 7.0, in a 75 microm x 60/67 cm fused-silica capillary at 15 kV and a 335 nm cut-off filter in the UV detector. The limits of detection (LOD) at a signal-to-noise ratio of 3 were about 0.25 microg/ml (5.2 x 10(-7) M) (injection 0.5 p.s.i./8 s). The linear detection range was 2-100 microg/ml (r2>0.999). Good repeatability of injection and relative migration times were obtained.

Stereoselective determination of S-naproxen in tablets by capillary electrophoresis

A capillary electrophoresis (CE) method was developed for the stereoselective determination of the non-steroidal anti-inflammatory drug (NSAID), S-naproxen, in tablets. Several beta-cyclodextrin derivatives (CDs) were tested as chiral selectors, including sulfobutyl-beta-CD (SBCD), carboxymethyl-beta-CD (CMCD), dimethyl-beta-CD (DMCD) and trimethyl-beta-CD (TMCD), in a phosphoric acid/triethanolamine pH 3 buffer. Under these conditions, the analyte was mainly present in an uncharged form and therefore, the use a neutral CD (DMCD or TMCD) alone could not lead to enantiomeric separation. On the contrary, by addition of a charged CD (SBCD or CMCD) to the running buffer, giving the analyte enantiomers an adequate mobility, chiral resolution could be achieved, although the resolution values obtained in this case were not quite satisfactory (Rs < 1.5). Dual systems, based on the use of mixtures of charged and neutral CDs, were then investigated. The SBCD/TMCD system was found to be particularly well suited to the enantioseparation of naproxen and after optimisation of the concentrations of both CDs, a resolution value of 5.4 could be obtained. The method was validated for the determination of R-naproxen (enantiomeric impurity) in the 0.1-2% range, using the racemic mixture of the analyte. A second validation was performed in the 50-150% range for the quantitation of S-naproxen. In both cases, good results with respect to linearity, precision and accuracy were obtained.

Separation of ibuprofen, codeine phosphate, their degradation products and impurities by capillary electrophoresis. II. Validation

A micellar electrokinetic chromatography method for the determination of ibuprofen and codeine phosphate hemihydrate and their degradation products and impurities in a commercial tablet formulation has been validated. The validation has been performed according to the International Conference of Harmonisation's guidance on the validation of analytical methods, and selectivity, linearity, accuracy, precision, detection limit, quantitation limit, robustness and range test were performed to determine the suitability of the method. It was possible to use the fractional factorial design model from the optimisation of the method to draw conclusions about its robustness. The results confirm that the method is highly suitable for its intended purpose.

Analyte identification in capillary electrophoretic separation techniques

A review on applications of on-line hyphenation in capillary electrophoresis and capillary electrochromatography for the identification of migrating analytes is presented. There is an urgent need for unambiguous analyte identification by combining spectral information and observed migration times, because the parameters influencing the migration times and separation efficiencies in these separation techniques are not easily controlled, especially when real samples containing unknown interferences have to be analyzed. The spectrometric techniques covered here are ultraviolet and visible radiation (UV/Vis) absorption, fluorescence including fluorescence line-narrowing spectroscopy, Raman spectroscopy, nuclear magnetic resonance and mass spectrometry. Attention is essentially confined to literature reports in which the extra information provided by the detector is really used for identification purposes, especially in real-life samples, while the interfacing as such and analyte detectabilities in standard solutions are only briefly discussed. This article covers an extensive fraction of the literature published on this topic until the beginning of 1998.

Strategies for capillary electrophoresis: method development and validation for pharmaceutical and biological applications

This review is in support of the development of selective, reproducible and validated capillary electrophoretis (CE) methods. Focusing on pharmaceutical and biological applications, the successful use of CE is demonstrated by more than 800 references, mainly from 1994 until 1998. Approximately 80 recent reviews have been catalogued. These articles sum up the existing strategies for method development in CE, especially in the search for generally accepted concepts, but also looking for new, promising reagents and ideas. General strategies for method development were derived not only with regard to selectivity and efficiency, but also with regard to precision, short analysis time, limit of detection, sample pretreatment requirements and validation. Standard buffer recipes, surfactants used in micellar electrokinetic capillary chromatography (MEKC), chiral selectors, useful buffer additives, polymeric separation media, electroosmotic flow (EOF) modifiers, dynamic and permanent coatings, actions to deal with complex matrices and aspects of validation are collected in 20 tables. Detailed schemes for the development of MEKC methods and chiral separations, for optimizing separation efficiency, means of troubleshooting, and other important information for key decisions during method development are given in 19 diagrams. Method development for peptide and protein separations, possibilities to influence the EOF and how to stabilize it, as well as indirect detection are considered in special sections.

Determination of pharmaceuticals in plasma by capillary electrophoresis without sample pretreatment reproducibility, limit of quantitation and limit of detection

Pharmaceuticals in human plasma are determined on underivatized fused-silica capillaries by micellar electrokinetic capillary chromatography (MEKC) without sample pretreatment. Our best method to date uses as running buffer a sodium dodecyl sulfate (SDS) containing borate buffer (60 mM with 200 mM SDS) at pH 10. Between runs, proteins adsorbed to the capillary wall are removed by an acetonitrile and SDS-buffer rinsing regimen (50% v/v each). A day-to-day precision for relative peak areas of about 2% relative standard deviation (RSD; n > 40) has been reached. Different rinsing approaches are discussed (salts, enzyme-containing solutions, organic solvents, hydrofluoric acid). The separation system is tested in a concentration range between approximately 100 mg/L-10 mg/L. Correlations between the limit of quantitation, the limit of detection and the signal/noise are discussed. The applicability of the system is demonstrated for the pharmaceuticals acetaminophen, salicylic acid, sulfamethoxazole, tolbutamide, and trimethoprim.