13 C NMR as an analytical tool for the detection of carbonic acid and pKa determination.

NMR spectroscopy has become a standard technique in studies both on carbon capture and storage. 13 C NMR allows the detection of two peaks for carbonated aqueous samples: one for CO2(aq) and another one for the species H2 CO3 , HCO3 - , and CO3 2- -herein collectively named Hx CO3 x-2 . The chemical shift of this second peak depends on the molar fraction of the three species in equilibrium and has been used to assess the equilibrium between HCO3 - and CO3 2- . The detection of H2 CO3 at low pH solutions is hindered, because of the concurrent liberation of CO2 when the medium is acidified. Herein, a valved NMR tube facilitates the detection of the Hx CO3 x-2 peak across a wide pH range, even at pH 1.8 where the dominant species is H2 CO3 . The method employed the formation of frozen layers of NaH13 CO3 and acid solutions within the tube, which are mixed as the tube reaches room temperature. At this point, the tube is already securely sealed, preventing any loss of CO2 to the atmosphere. A spectrophotometry approach allowed the measurement of the actual pH inside the pressurized NMR tube. The chemical shift for H2 CO3 was determined as 160.33 ± 0.03 ppm, which is in good agreement with value obtained by DFT calculations combined with Car-Parrinello molecular dynamics. The H2 CO3 pKa value determined by the present method was 3.41 ± 0.03, for 15% D2 O aqueous medium and 0.8 mol/L ionic strength. The proposed method can be extended to studies about analogs such as alkyl carbonic and carbamic acids.

Improving hydrodynamic injection in capillary electrophoresis by using the integral of pressure.

A careful analysis of the typical devices and conditions used during hydrodynamic injection in capillary electrophoresis shows that the Hagen-Poiseuille model for the laminar flow is valid, even during the transitions of pressure. Therefore, the monitoring of pressure becomes a reliable approach to evaluate the effective injected volume, because the volume is proportional to the integral of pressure (IoP) over time. A piezoresistive sensor was used to monitor the air pressure at headspace of the sample vial. A set of 18 injections at 50 mbar and different times were used to evaluate the use of the normalization of the peak areas of the analytes by the IoP to compensate for imperfection during the injection. There was a significant decrease in relative standard deviation (RSD), and the proposed approach presented results similar to the use of internal standard. In addition, a microcontroller was used not only to monitor the pressure but also to command a peristaltic pump and a solenoid valve creating a system that dynamically controls the applied pressure and stops the injection when the desired value of IoP is reached. The system was used in a proof of concept in which different combinations of pressure and time were used: 10 mbar × 50 s, 25 mbar × 20 s, 50 mbar × 10 s, 125 mbar × 4 s, and 250 mbar × 2 s. Despite the constraints posed by the flowrates of the peristaltic pump and the solenoid valve, the microcontroller effectively conducted the injections across this extensive range of conditions, resulting in an IoP RSD of 2.7%.

Qualitative and quantitative aspects of time-, charge-, and mobility-based electropherograms.

The migration process in capillary electrophoresis is obtained by using a high-voltage power supply, and the basic idea is to keep the control on the migration velocity of the analytes by controlling either the applied voltage or current. The effectiveness of this control has impact on the resulting electropherogram and, thus, in the identification and quantification of the analytes. Although the usual electropherogram is the record of the detector signal as a function of time, other two domains should be considered: charge and mobility. Both mathematical modeling and experimental results were used to evaluate the two different approaches for controlling the electrophoretic migration and the resulting time-, charge-, and mobility-based electropherograms. The main conclusions are (1) the current-controlled mode is superior to the voltage-controlled mode; (2) when the first mode cannot be implemented, the electrophoretic current should be monitored to improve the identification and quantification procedures; and (3) the consistent monitoring of the electrophoretic current allows the implementation of the charge-based electropherogram and the mobility spectrum. The first one is advantageous because the peak position is more reproducible, and the peak area is more resistant to change than the ones from the time-based electropherogram. The mobility spectrum has the additional advantage of being more informative about the mobility of the analytes. Although peak area is less robust, the spectrum may also be used for quantitation when the number of plates is greater than 103 .

Determination of topiramate by capillary electrophoresis with capacitively-coupled contactless conductivity detection: A powerful tool for therapeutic monitoring in epileptic patients.

Topiramate (TPM) is the main antiepileptic drug used for the control of partial and generalized seizures in both adults and children. In association with clinical observations, the analysis of plasmatic concentration of TPM is of utmost importance for the individual adjustment of the administered dose to the patient. In the present work, a bioanalytical method was developed and validated for TPM analysis in plasma samples by capillary electrophoresis with capacitively-coupled contactless conductivity detection (CE-C4 D). A simple background electrolyte composed of 15 mmol/L triethylamine, hydrodynamic injections (0.8 psi for 5 s) and a moderate separation voltage (20 kV) were used, rendering relatively short analysis times (<3 min). The sample pre-treatment was carried out by liquid-liquid extraction using methyl terc-butyl ether as solvent and 200 µL of plasma. The method was validated according to the official guidelines from the European Medicine Agency and showed linearity in plasmatic concentration range from 1 to 30 µg/mL, which covers the clinically-relevant interval. The lower limit of quantification of 1 µg/mL obtained also allows following patients with low dosage of the drug. The method was successfully applied to analysis of plasma samples and allowed the identification of 80% under-medicated patients in the analyzed patient pool.

Analysis of Methanol in the Presence of Ethanol, Using a Hybrid Capillary Electrophoresis Device with Electrochemical Derivatization and Conductivity Detection.

Concurrently with ethanol, many other compounds can be formed during the fermentation of grains and fruits. Among those, methanol is particularly important (because of its toxicity) and is typically formed at concentrations much lower than ethanol, presenting a particular challenge that demands the implementation of separation techniques. Aiming to provide an alternative to traditional chromatographic approaches, a hybrid electrophoresis device with electrochemical preprocessing and contactless conductivity detection (hybrid EC-CE-C4D) is herein described. The device was applied to perform the electro-oxidation of primary alcohols, followed by the separation and detection of the respective carboxylates. According to the presented results, the optimum conditions were obtained when the sample was diluted with 2 mmol L-1 HNO3 and then electro-oxidized by applying a potential of 1.4 V for 60 s. The oxidation products were then electrokinetically injected by applying a potential of 3 kV for 4 s and separated using a potential of 3 kV and a background running electrolyte (BGE) consisting of 10 mmol L-1 N-cyclohexyl-2-aminoethanesulfonic acid (CHES) and 5 mmol L-1 sodium hydroxide (NaOH). n-Propanol was used as an internal standard and the three carboxylate peaks were resolved with baseline separation within <3 min, defining linear calibration curves in the range of 0.10-5.0 mmol L-1. Limits of detection (LODs) of 20, 40, and 50 µmol L-1 were obtained for ethanol, n-propanol, and methanol, respectively. To demonstrate the applicability of the proposed strategy, a laboratory-made sample (moonshine) was used. Aliquots collected along the beginning of the fractional distillation presented a decreasing methanol ratio (from 4% to <0.5%) and a growing ethanol ratio (from 80% to 100%) in the collected volume.

Getting started with open-hardware: development and control of microfluidic devices.

Understanding basic concepts of electronics and computer programming allows researchers to get the most out of the equipment found in their laboratories. Although a number of platforms have been specifically designed for the general public and are supported by a vast array of on-line tutorials, this subject is not normally included in university chemistry curricula. Aiming to provide the basic concepts of hardware and software, this article is focused on the design and use of a simple module to control a series of PDMS-based valves. The module is based on a low-cost microprocessor (Teensy) and open-source software (Arduino). The microvalves were fabricated using thin sheets of PDMS and patterned using CO2 laser engraving, providing a simple and efficient way to fabricate devices without the traditional photolithographic process or facilities. Synchronization of valve control enabled the development of two simple devices to perform injection (1.6 ± 0.4 µL/stroke) and mixing of different solutions. Furthermore, a practical demonstration of the utility of this system for microscale chemical sample handling and analysis was achieved performing an on-chip acid-base titration, followed by conductivity detection with an open-source low-cost detection system. Overall, the system provided a very reproducible (98%) platform to perform fluid delivery at the microfluidic scale.

Miniaturized flow system based on enzyme modified PMMA microreactor for amperometric determination of glucose.

This paper describes the development of a microfluidic system having as main component an enzymatic reactor constituted by a microchannel assembled in poly(methyl methacrylate) (PMMA) substrate connected to an amperometric detector. A CO2 laser engraving machine was used to make the channels, which in sequence were thermally sealed. The internal surfaces of the microchannels were chemically modified with polyethyleneimine (PEI), which showed good effectiveness for the immobilization of the glucose oxidase enzyme using glutaraldehyde as crosslinking agent, producing a very effective microreactor for the detection of glucose. The hydrogen peroxide generated by the enzymatic reaction was detected in an electrochemical flow cell localized outside of the reactor using a platinum disk as the working electrode. The proposed system was applied to the differential amperometric determination of glucose content in soft drinks showing good repeatability (DPR=1.72%, n=50), low detection limit (1.40×10(-6)molL(-1)), high sampling frequency (calculated as 345 samples h(-1)), and relatively good stability for long-term use. The results were in close agreement with those obtained by the classical spectrophotometric method utilized to quantify glucose in biological fluids.