Mass Spectrometric Detection of Short-Lived Drug Metabolites Generated in an Electrochemical Microfluidic Chip

Electrocatalytic CO2 Reduction: Monitoring of Catalytically Active, Downgraded, and Upgraded Cobalt Complexes

The premise of most studies on the homogeneous electrocatalytic CO2 reduction reaction (CO2RR) is a good understanding of the reaction mechanisms. Yet, analyzing the reaction intermediates formed at the working electrode is challenging and not always attainable. Here, we present a new, general approach to studying the reaction intermediates applied for CO2RR catalyzed by a series of cobalt complexes. The cobalt complexes were based on the TPA-ligands (TPA = tris(2-pyridylmethyl)amine) modified by amino groups in the secondary coordination sphere. By combining the electrochemical experiments, electrochemistry-coupled electrospray ionization mass spectrometry, with density functional theory (DFT) calculations, we identify and spectroscopically characterize the key reaction intermediates in the CO2RR and the competing hydrogen-evolution reaction (HER). Additionally, the experiments revealed the rarely reported in situ changes in the secondary coordination sphere of the cobalt complexes by the CO2-initiated transformation of the amino substituents to carbamates. This launched an even faster alternative HER pathway. The interplay of three catalytic cycles, as derived from the experiments and supported by the DFT calculations, explains the trends that cobalt complexes exhibit during the CO2RR and HER. Additionally, this study demonstrates the need for a molecular perspective in the electrocatalytic activation of small molecules efficiently obtained by the EC-ESI-MS technique.

Absolute Quantitation of Peptides and Proteins by Coulometric Mass Spectrometry After Derivatization

Peptide/protein quantitation using mass spectrometry (MS) is advantageous due to its high sensitivity. Traditional absolute peptide quantitation methods rely on making calibration curves using peptide standards or isotope-labelled peptide standards, which are expensive and take time to synthesize. A method which can eliminate the need for using standards would be beneficial. Recently, we developed coulometric mass spectrometry (CMS) which can be used to quantify peptides that are oxidizable (e.g., those containing tyrosine or tryptophan), without using peptide standard. The method is based on electrochemical oxidation of peptides followed by MS to measure the oxidation yield. However, it cannot be directly used to quantify peptides without oxidizable residues. To extend this method for quantifying peptides/proteins in general, in this study, we adopted a derivatization strategy, in which a target peptide is first tagged with an electroactive reagent such as monocarboxymethylene blue NHS ester (MCMB-NHS ester), followed with quantitation by CMS. To illustrate the power of this method, we have analyzed peptides MG and RPPGFSPFR. The quantification error was less than 5%. Using RPPGFSPFR as an example, the quantitation sensitivity of the technique was found to be 0.25 pmol. Furthermore, we also used the strategy to quantify proteins cytochrome C and β-casein with an error of 2-26%.

The Combination of Electrochemistry and Microfluidic Technology in Drug Metabolism Studies

Drugs are metabolized within the liver (pH 7.4) by phase I and phase II metabolism. During the process, reactive metabolites can be formed that react covalently with biomolecules and induce toxicity. Identifying and detecting reactive metabolites is an important part of drug development. Preclinical and clinical investigations are conducted to assess the toxicity and safety of a new drug candidate. Electrochemistry coupled to mass spectrometry is an ideal complementary technique to the current preclinical studies, a pure instrumental approach without any purification steps and tedious protocols. The combination of microfluidics with electrochemistry towards the mimicry of drug metabolism offers portability, low volume of reagents and faster reaction times. This review explores the development of microfluidic electrochemical cells for mimicking drug metabolism.

Microfluidic Electrochemistry Meets Trapped Ion Mobility Spectrometry and High-Resolution Mass Spectrometry-In Situ Generation, Separation, and Detection of Isomeric Conjugates of Paracetamol and Ethoxyquin

Over the last 3 decades, electrochemistry (EC) has been successfully applied in phase I and phase II metabolism simulation studies. The electrochemically generated phase I metabolite-like oxidation products can react with selected reagents to form phase II conjugates. During conjugate formation, the generation of isomeric compounds is possible. Such isomeric conjugates are often separated by high-performance liquid chromatography (HPLC). Here, we demonstrate a powerful approach that combines EC with ion mobility spectrometry to separate possible isomeric conjugates. In detail, we present the hyphenation of a microfluidic electrochemical chip with an integrated mixer coupled online to trapped ion mobility spectrometry (TIMS) and time-of-flight high-resolution mass spectrometry (ToF-HRMS), briefly chipEC-TIMS-ToF-HRMS. This novel method achieves results in several minutes, which is much faster than traditional separation approaches like HPLC, and was applied to the drug paracetamol and the controversial feed preservative ethoxyquin. The analytes were oxidized in situ in the electrochemical microfluidic chip under formation of reactive intermediates and mixed with different thiol-containing reagents to form conjugates. These were analyzed by TIMS-ToF-HRMS to identify possible isomers. It was observed that the oxidation products of both paracetamol and ethoxyquin form two isomeric conjugates, which are characterized by different ion mobilities, with each reagent. Therefore, using this hyphenated technique, it is possible to not only form reactive oxidation products and their conjugates in situ but also separate and detect these isomeric conjugates within only a few minutes.

Capture of Electrochemically Generated Fleeting Carbazole Radical Cations and Elucidation of Carbazole Dimerization Mechanism by Mass Spectrometry

The capture of reactive intermediates is important for the elucidation of reaction mechanisms. We report the first observation of electrochemically generated, short-lived radical cations of carbazole (t_1/2 ≈ 97 μs) and two N-substituted carbazole derivatives by mass spectrometry. In addition, online investigation of the reactivity of electrochemically generated carbazole radical cations supports that the carbazole dimerization mechanism involves the reaction of one radical cation with one neutral molecule rather than the previously proposed coupling of two radical cations.

Electrocatalytic redox neutral [3 + 2] annulation of N-cyclopropylanilines and alkenes

Although synthetic organic electrochemistry (EC) has advanced significantly, net redox neutral electrosynthesis is quite rare. Two approaches have been employed to achieve this type of electrosynthesis. One relies on turnover of the product by the reactant in a chain mechanism. The other involves both oxidation on the anode and reduction on the cathode in which the radical cation or the radical anion of the product has to migrate between two electrodes. Herein, a home-built electrochemistry/mass spectrometry (EC/MS) platform was used to generate an N-cyclopropylaniline radical cation electrochemically and to monitor its reactivity toward alkenes by mass spectrometry (MS), which led to the discovery of a new redox neutral reaction of intermolecular [3 + 2] annulation of N-cyclopropylanilines and alkenes to provide an aniline-substituted 5-membered carbocycle via direct electrolysis (yield up to 81%). A chain mechanism, involving the regeneration of the substrate radical cation and the formation of the neutral product, is shown to be responsible for promoting such a redox neutral annulation reaction, as supported by experimental evidence of EC/MS.

Reactive Metabolites: Generation and Estimation with Electrochemistry Based Analytical Strategy as an Emerging Screening Tool

Background:

Analytical scientists have constantly been in search for more efficient and economical methods for drug simulation studies. Owing to great progress in this field, there are various techniques available nowadays that mimic drug metabolism in the hepatic microenvironment. The conventional in vitro and in vivo studies pose inherent methodological drawbacks due to which alternative analytical approaches are devised for different drug metabolism experiments.

Methods:

Electrochemistry has gained attention due to its benefits over conventional metabolism studies. Because of the protein binding nature of reactive metabolites, it is difficult to identify them directly after formation, although the use of trapping agents aids in their successful identification. Furthermore, various scientific reports confirmed the successful simulation of drug metabolism studies by electrochemical cells. Electrochemical cells coupled with chromatography and mass spectrometry made it easy for direct detection of reactive metabolites. In this review, an insight into the application of electrochemical techniques for metabolism simulation studies has been provided. The sole use of electrochemical cells, as well as their setups on coupling to liquid chromatography and mass spectrometry has been discussed. The importance of metabolism prediction in early drug discovery and development stages along with a brief overview of other conventional methods has also been highlighted.

Conclusion:

To the best of our knowledge, this is the first article to review the electrochemistry based strategy for the analysis of reactive metabolites. The outcome of this ‘first of its kind’ review will significantly help the researchers in the application of electrochemistry based bioanalysis for metabolite detection.

Structure-Based Reactivity Profiles of Reactive Metabolites with Glutathione

Therapeutic agents can be transformed into reactive metabolites under the action of various metabolic enzymes in vivo and then covalently combine with biological macromolecules (such as protein or DNA), resulting in increasing toxicity. The screening of reactive metabolites in drug discovery and development stages and monitoring of biotransformation in post-market drugs has become an important research field. Generally, reactive metabolites are electrophilic and can be captured by small nucleophiles. Glutathione (GSH) is a small peptide composed of three amino acids (i.e., glutamic acid, cysteine, and glycine). It has a thiol group which can react with electrophilic groups of reactive metabolic intermediates (such as benzoquinone, N-acetyl-p-benzoquinoneimine, and Michael acceptor) to form a stable binding conjugate. This paper aims to provide a review on structure-based reactivity profiles of reactive metabolites with GSH. Furthermore, this review also reveals the relationship between drugs' molecular structures and reactive metabolic toxicity from the perspective of metabolism, giving a reference for drug design and development.

Spectroelectrochemistry, the future of visualizing electrode processes by hyphenating electrochemistry with spectroscopic techniques

Reviewing the future of electrochemistry combined with infrared, Raman, and nuclear magnetic resonance spectroscopy as well as mass spectrometry.

Absolute Quantitation of Oxidizable Peptides by Coulometric Mass Spectrometry

Quantitation methods for peptides using mass spectrometry have advanced rapidly. These methods rely on using standard and/or isotope-labeled peptides, which might be difficult or expensive to synthesize. To tackle this challenge, we present a new approach for absolute quantitation without the use of standards or calibration curves based on coulometry combined with mass spectrometry (MS). In this approach, which we call coulometric mass spectrometry (CMS), the mass spectrum of a target peptide containing one or more tyrosine residues is recorded before and after undergoing electrochemical oxidation. We record the total integrated oxidation current from the electrochemical measurement, which according to the Faraday's Law of coulometry, provides the number of moles of oxidized peptide. The ion intensity ratio of the target peptide before and after oxidation provides an excellent estimate of the fraction of the peptide that has been oxidized, from which the total amount of peptide is calculated. The striking strength of CMS is that it needs no standard peptide, but CMS does require the peptide to contain a known number of oxidizable groups. To illustrate the power of this method, we analyzed various tyrosine-containing peptides such as GGYR, DRVY, oxytocin, [Arg⁸]-vasotocin and angiotensinogen 1-14 with a quantification error ranging from - 7.5 to + 2.4%. This approach is also applicable to quantifying phosphopeptides and could be useful in proteomics research.

Application of microfluidic chip technology in pharmaceutical analysis: A review

The development of pharmaceutical analytical methods represents one of the most significant aspects of drug development. Recent advances in microfabrication and microfluidics could provide new approaches for drug analysis, including drug screening, active testing and the study of metabolism. Microfluidic chip technologies, such as lab-on-a-chip technology, three-dimensional (3D) cell culture, organs-on-chip and droplet techniques, have all been developed rapidly. Microfluidic chips coupled with various kinds of detection techniques are suitable for the high-throughput screening, detection and mechanistic study of drugs. This review highlights the latest (2010–2018) microfluidic technology for drug analysis and discusses the potential future development in this field.

Lab-on-a-Chip: Frontier Science in the Classroom

Lab-on-a-chip technology is brought into the classroom through development of a lesson series with hands-on practicals. Students can discover the principles of microfluidics with different practicals covering laminar flow, micromixing, and droplet generation, as well as trapping and counting beads. A quite affordable novel production technique using scissor-cut and laser-cut lamination sheets is presented, which provides good insight into how scientific lab-on-a-chip devices are produced. In this way high school students can now produce lab-on-a-chip devices using lamination sheets and their own lab-on-a-chip design. We begin with a review of previous reports on the use of lab-on-a-chip technology in classrooms, followed by an overview of the practicals and projects we have developed with student safety in mind. We conclude with an educational scenario and some initial promising results for student learning outcomes.

Instrumentation and applications of electrochemistry coupled to mass spectrometry for studying xenobiotic metabolism: A review

The knowledge of metabolic pathways and biotransformation of xenobiotics, artificial substances foreign to the entire biological system, is crucial for elucidation of degradation routes of potentially toxic substances. Nowadays, there are many methods to simulate xenobiotic metabolism in the human body in vitro. In this review, the metabolism of various substances in the human body is described, followed by a summary of methods used for prediction of metabolic pathways and biotransformation. Above all, focus is placed on the coupling of electrochemistry to mass spectrometry, which is still a relatively new technique. This promising tool can mimic both oxidative phase I and conjugative phase II metabolism. Different experimental arrangements, with or without a separation step, and various applications of this technique are illustrated and critically reviewed.

Recent advances in real-time and in situ analysis of an electrode–electrolyte interface by mass spectrometry

Novelty: Recent advances in real-time and in situ monitoring of an electrode–electrolyte interface by mass spectrometry are reviewed.

On-chip integration of organic synthesis and HPLC/MS analysis for monitoring stereoselective transformations at the micro-scale

We present a microfluidic system, seamlessly integrating microflow and microbatch synthesis with a HPLC/nano-ESI-MS functionality on a single glass chip. The microfluidic approach allows to efficiently steer and dispense sample streams down to the nanoliter-range for studying reactions in quasi real-time. In a proof-of-concept study, the system was applied to explore amino-catalyzed reactions, including asymmetric iminium-catalyzed Friedel-Crafts alkylations in microflow and micro confined reaction vessels.

Oxidation and adduct formation of xenobiotics in a microfluidic electrochemical cell with boron doped diamond electrodes and an integrated passive gradient rotation mixer

Reactive xenobiotic metabolites and their adduct formation with biomolecules such as proteins are important to study as they can be detrimental to human health. Here, we present a microfluidic electrochemical cell with integrated micromixer to study phase I and phase II metabolism as well as protein adduct formation of xenobiotics in a purely instrumental approach. The newly developed microfluidic device enables both the generation of reactive metabolites through electrochemical oxidation and subsequent adduct formation with biomolecules in a chemical microreactor. This allows us to study the detoxification of reactive species with glutathione and to predict potential toxicity of xenobiotics as a result of protein modification. Efficient mixing in microfluidic systems is a slow process due to the typically laminar flow conditions in shallow channels. Therefore, a passive gradient rotation micromixer has been designed that is capable of mixing liquids efficiently in a 790 pL volume within tens of milliseconds. The mixing principle relies on turning the concentration gradient that is initially established by bringing together two streams of liquid, to take advantage of the short diffusion distances in the shallow microchannels of thin-layer flow cells. The mixer is located immediately downstream of the working electrode of an electrochemical cell with integrated boron doped diamond electrodes. In conjunction with mass spectrometry, the two microreactors integrated in a single device provide a powerful tool to study the metabolism and toxicity of xenobiotics, which was demonstrated by the investigation of the model compound 1-hydroxypyrene.

Comparison of in Silico, Electrochemical, in Vitro and in Vivo Metabolism of a Homologous Series of (Radio)fluorinated σ1 Receptor Ligands Designed for Positron Emission Tomography

The imaging of σ₁ receptors in the brain by fluorinated radiotracers will be used for the validation of σ₁ receptors as drug targets as well as for differential diagnosis of diseases in the central nervous system. The biotransformation of four homologous fluorinated PET tracers 1'-benzyl-3-(ω-fluoromethyl to ω-fluorobutyl)-3H-spiro[2]benzofuran-1,4'-piperidine] ([¹⁸ F]1-4) was investigated. In silico studies using fast metabolizer (FAME) software, electrochemical oxidations, in vitro studies with rat liver microsomes, and in vivo metabolism studies after application of the PET tracers [¹⁸ F]1-4 to mice were performed. Combined liquid chromatography and mass spectrometry (HPLC-MS) analysis allowed structural identification of non-radioactive metabolites. Radio-HPLC and radio-TLC provided information about the presence of unchanged parent radiotracers and their radiometabolites. Radiometabolites were not found in the brain after application of [¹⁸ F]2-4, but liver, plasma, and urine samples contained several radiometabolites. Less than 2 % of the injected dose of [¹⁸ F]4 reached the brain, rendering [¹⁸ F]4 less appropriate as a PET tracer than [¹⁸ F]2 and [¹⁸ F]3. Compounds [¹⁸ F]2 and [¹⁸ F]3 possess the most promising properties for imaging of σ₁ receptors in the brain. High σ₁ affinity (K_i =0.59 nm), low lipophilicity (logD_7.4 =2.57), high brain penetration (4.6 % of injected dose after 30 min), and the absence of radiometabolites in the brain favor the fluoroethyl derivative [¹⁸ F]2 slightly over the fluoropropyl derivative [¹⁸ F]3 for human use.

A Disposable Microfluidic Device with a Screen Printed Electrode for Mimicking Phase II Metabolism

Human metabolism is investigated using several in vitro methods. However, the current methodologies are often expensive, tedious and complicated. Over the last decade, the combination of electrochemistry (EC) with mass spectrometry (MS) has a simpler and a cheaper alternative to mimic the human metabolism. This paper describes the development of a disposable microfluidic device with a screen-printed electrode (SPE) for monitoring phase II GSH reactions. The proposed chip has the potential to be used as a primary screening tool, thus complementing the current in vitro methods.

Electrochemical Protein Cleavage in a Microfluidic Cell with Integrated Boron Doped Diamond Electrodes

Specific electrochemical cleavage of peptide bonds at the C-terminal side of tyrosine and tryptophan generates peptides amenable to liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis for protein identification. To this end we developed a microfluidic electrochemical cell of 160 nL volume that combines a cell geometry optimized for a high electrochemical conversion efficiency (>95%) with an integrated boron doped diamond (BDD) working electrode offering a wide potential window in aqueous solution and reduced adsorption of peptides and proteins. Efficient cleavage of the proteins bovine insulin and chicken egg white lysozyme was observed at 4 out of 4 and 7 out of 9 of the predicted cleavage sites, respectively. Chicken egg white lysozyme was identified based on 5 electrochemically generated peptides using a proteomics database searching algorithm. These results show that electrochemical peptide bond cleavage in a microfluidic cell is a novel, fully instrumental approach toward protein analysis and eventually proteomics studies in conjunction with mass spectrometry.

Paraquat-Melanin Redox-Cycling: Evidence from Electrochemical Reverse Engineering

Parkinson's disease is a neurodegenerative disorder associated with oxidative stress and the death of melanin-containing neurons of the substantia nigra. Epidemiological evidence links exposure to the pesticide paraquat (PQ) to Parkinson's disease, and this link has been explained by a redox cycling mechanism that induces oxidative stress. Here, we used a novel electrochemistry-based reverse engineering methodology to test the hypothesis that PQ can undergo reductive redox cycling with melanin. In this method, (i) an insoluble natural melanin (from Sepia melanin) and a synthetic model melanin (having a cysteinyldopamine-melanin core and dopamine-melanin shell) were entrapped in a nonconducting hydrogel film adjacent to an electrode, (ii) the film-coated electrode was immersed in solutions containing PQ (putative redox cycling reductant) and a redox cycling oxidant (ferrocene dimethanol), (iii) sequences of input potentials (i.e., voltages) were imposed to the underlying electrode to systematically engage reductive and oxidative redox cycling, and (iv) output response currents were analyzed for signatures of redox cycling. The response characteristics of the PQ-melanin systems to various input potential sequences support the hypothesis that PQ can directly donate electrons to melanin. This observation of PQ-melanin redox interactions demonstrates an association between two components that have been individually linked to oxidative stress and Parkinson's disease. Potentially, melanin's redox activity could be an important component in understanding the etiology of neurological disorders such as Parkinson's disease.

Multi-channel cell co-culture for drug development based on glass microfluidic chip-mass spectrometry coupled platform

RATIONALE

Cell-based drug assay plays an essential role in drug development. By coupling a microfluidic chip with mass spectrometry (MS), we developed a multifunctional platform. Cell co-culture, cell apoptosis assay, fluorescence and MS detection of intracellular drug absorption could be simultaneously conducted on this platform.

METHODS

Three micro-channels were fabricated through photolithography technology to conduct the cell co-culture. Cell apoptosis after drug treatment was assayed by fluorescent probes (Hoechst 33342). Intracellular Dox absorption was analyzed by confocal fluorescent microscopy. With a high voltage (~ 4.5 kV) applied onto the microfluidic chip, the ionization spray was successfully generated by dropping isopropanol onto it. By coupling with a Shimadzu LCMS-2010 A mass spectrometer, intracellular CPA absorption was detected on the microfluidic chip.

RESULTS

The microfluidic chip-MS coupled platform showed high biocompatibility. Distinction of cell apoptosis between co-cultured and mono-cultured cells was detected. The results of intracellular drug absorption well explained the different cell apoptosis rate.

CONCLUSIONS

Cell-based drug assay was facilely and successfully conducted on the microfluidic chip-MS coupled platform. This technology we have devised could promote MS application in the field of drug development. Copyright © 2016 John Wiley & Sons, Ltd.

Exploiting plug-and-play electrochemistry for drug discovery

Electrochemistry has emerged as a powerful analytical technique for chemical analysis of living cells, biologically active molecules and metabolites. Electrochemical biosensor, microfluidics and mass spectrometry are the most frequently used methods for electrochemical detection and monitory, which comprise a collection of extremely useful measurement tools for various fields of biology and medicine. Most recently, electrochemistry has been shown to be coupled with nanotechnology and genetic engineering to generate new enabling technologies, providing rapid, selective, and sensitive detection and diagnosis platforms. The primary focus of this review is to highlight the utility of electrochemical strategies and their conjunction with other approaches for drug metabolism and discovery. Current challenges and possible future developments and applications of electrochemistry in drug studies are also discussed.

Observation of electrochemically generated nitrenium ions by desorption electrospray ionization mass spectrometry

We report the observation of the electrochemically generated nitrenium ions of 4,4'-dimethyoxydiphenylamine and di-p-tolylamine in solution by mass spectrometry. This setup takes inspiration from desorption electrospray ionization mass spectrometry to sample directly from the surface of a rotating waterwheel working electrode for mass spectrometric analysis. Detection of the 4,4'-dimethyoxydiphenylamine nitrenium ion was expected based upon para-methoxy resonance stabilization, whereas observation of the di-p-tolylamine nitrenium ion might be unexpected because resonance stabilization from the para-substituted position is unavailable. However, the short timescale analysis of the setup allows for the isolation of the di-p-tolylamine nitrenium ion, which is electrogenerated in solution and detected mass spectrometrically.

Metabolic activation and drug-induced liver injury: in vitro approaches for the safety risk assessment of new drugs

Drug-induced liver injury (DILI) is a significant leading cause of hepatic dysfunction, drug failure during clinical trials and post-market withdrawal of approved drugs. Many cases of DILI are unexpected reactions of an idiosyncratic nature that occur in a small group of susceptible individuals. Intensive research efforts have been made to understand better the idiosyncratic DILI and to identify potential risk factors. Metabolic bioactivation of drugs to form reactive metabolites is considered an initiation mechanism for idiosyncratic DILI. Reactive species may interact irreversibly with cell macromolecules (covalent binding, oxidative damage), and alter their structure and activity. This review focuses on proposed in vitro screening strategies to predict and reduce idiosyncratic hepatotoxicity associated with drug bioactivation. Compound incubation with metabolically competent biological systems (liver-derived cells, subcellular fractions), in combination with methods to reveal the formation of reactive intermediates (e.g., formation of adducts with liver proteins, metabolite trapping or enzyme inhibition assays), are approaches commonly used to screen the reactivity of new molecules in early drug development. Several cell-based assays have also been proposed for the safety risk assessment of bioactivable compounds. Copyright © 2015 John Wiley & Sons, Ltd.

Comparison of TiO2 photocatalysis, electrochemically assisted Fenton reaction and direct electrochemistry for simulation of phase I metabolism reactions of drugs

The feasibility of titanium dioxide (TiO2) photocatalysis, electrochemically assisted Fenton reaction (EC-Fenton) and direct electrochemical oxidation (EC) for simulation of phase I metabolism of drugs was studied by comparing the reaction products of buspirone, promazine, testosterone and 7-ethoxycoumarin with phase I metabolites of the same compounds produced in vitro by human liver microsomes (HLM). Reaction products were analysed by UHPLC-MS. TiO2 photocatalysis simulated the in vitro phase I metabolism in HLM more comprehensively than did EC-Fenton or EC. Even though TiO2 photocatalysis, EC-Fenton and EC do not allow comprehensive prediction of phase I metabolism, all three methods produce several important metabolites without the need for demanding purification steps to remove the biological matrix. Importantly, TiO2 photocatalysis produces aliphatic and aromatic hydroxylation products where direct EC fails. Furthermore, TiO2 photocatalysis is an extremely rapid, simple and inexpensive way to generate oxidation products in a clean matrix and the reaction can be simply initiated and quenched by switching the UV lamp on/off.

Capacitive deionization on-chip as a method for microfluidic sample preparation

Desalination as a sample preparation step is essential for noise reduction and reproducibility of mass spectrometry measurements. A specific example is the analysis of proteins for medical research and clinical applications. Salts and buffers that are present in samples need to be removed before analysis to improve the signal-to-noise ratio. Capacitive deionization is an electrostatic desalination (CDI) technique which uses two porous electrodes facing each other to remove ions from a solution. Upon the application of a potential of 0.5 V ions migrate to the electrodes and are stored in the electrical double layer. In this article we demonstrate CDI on a chip, and desalinate a solution by the removal of 23% of Na(+) and Cl(-) ions, while the concentration of a larger molecule (FITC-dextran) remains unchanged. For the first time impedance spectroscopy is introduced to monitor the salt concentration in situ in real-time in between the two desalination electrodes.