Scott D. Noblitt

High-sensitivity microchip electrophoresis determination of inorganic anions and oxalate in atmospheric aerosols with adjustable selectivity and conductivity detection

A sensitive and selective separation of common anionic constituents of atmospheric aerosols, sulfate, nitrate, chloride, and oxalate, is presented using microchip electrophoresis. The optimized separation is achieved in under 1 min and at low background electrolyte ionic strength (2.9 mM) by combining a metal-binding electrolyte anion (17 mM picolinic acid), a sulfate-binding electrolyte cation (19 mM HEPBS), a zwitterionic surfactant with affinity towards weakly solvated anions (19 mM N-tetradecyl,N,N-dimethyl-3-ammonio-1-propansulfonate), and operation in counter-electroosmotic flow (EOF) mode. The separation is performed at pH 4.7, permitting pH manipulation of oxalates mobility. The majority of low-concentration organic acids are not observed at these conditions, allowing for rapid subsequent injections without the presence of interfering peaks. Because the mobilities of sulfate, nitrate, and oxalate are independently controlled, other minor constituents of aerosols can be analyzed, including nitrite, fluoride, and formate if desired using similar separation conditions. Contact conductivity detection is utilized, and the limit of detection for oxalate (S/N=3) is 180 nM without stacking. Sensitivity can be increased with field-amplified sample stacking by injecting from dilute electrolyte with a detection limit of 19 nM achieved. The high-sensitivity, counter-EOF operation, and short analysis time make this separation well-suited to continuous online monitoring of aerosol composition.

Interfacing Microchip Electrophoresis to a Growth Tube Particle Collector for Semicontinuous Monitoring of Aerosol Composition

Semicontinuous monitoring of aerosol chemical composition has continually increased in demand because of the high spatial and temporal variability of atmospheric particles and the effects these aerosols have on human health and the environment. To address this demand, we describe the preliminary development of a semicontinuous aerosol composition analyzer consisting of a growth tube particle collector coupled to a microfluidic device for chemical analysis. The growth tube enlarges particles through water condensation in a laminar flow, permitting inertial collection into the microchip sample reservoir. Analysis is done by electrophoresis with conductivity detection. To avoid hydrodynamic interference from the sampling pressure, the microchip was operated isobarically by sealing the buffer reservoirs from the atmosphere and interconnecting all the reservoirs with air ducts. The collector samples at 1 L min(-1) and deposits particles into 30 microL of solution. Sample accumulates with time, and sequential injections are performed as aerosol concentration increases. For extended analyses, a sample rinsing system flushes the sample collection reservoir periodically. For inorganic anions, temporal resolution of 1 min and estimated detection limits of 70-140 ng m(-3) min were obtained. The system was used to measure sulfate and nitrate, and results were compared to a particle-into-liquid-sampler running in parallel. Results indicate that the prototype growth tube-microchip system (termed aerosol chip electrophoresis, ACE) could provide a useful complement to existing aerosol monitoring technologies, especially when less expensive and/or rapid analyses are desired.

Rapid Analysis of Perchlorate in Drinking Water at Parts per Billion Levels Using Microchip Electrophoresis

A microchip capillary electrophoresis (MCE) system has been developed for the determination of perchlorate in drinking water. The United States Environmental Protection Agency (USEPA) recently proposed a health advisory limit for perchlorate in drinking water of 15 parts per billion (ppb), a level requiring large, sophisticated instrumentation, such as ion chromatography coupled with mass spectrometry (IC-MS), for detection. An inexpensive, portable system is desired for routine online monitoring applications of perchlorate in drinking water. Here, we present an MCE method using contact conductivity detection for perchlorate determination. The method has several advantages, including reduced analysis times relative to IC, inherent portability, high selectivity, and minimal sample pretreatment. Resolution of perchlorate from more abundant ions was achieved using zwitterionic, sulfobetaine surfactants, N-hexadecyl-N,N-dimethyl-3-ammonio-1-propane sulfonate (HDAPS) and N-tetradecyl-N,N-dimethyl-3-ammonio-1-propane sulfonate (TDAPS). The system performance and the optimization of the separation chemistry, including the use of these surfactants to resolve perchlorate from other anions, are discussed in this work. The system is capable of detection limits of 3.4 +/- 1.8 ppb (n = 6) in standards and 5.6 +/- 1.7 ppb (n = 6) in drinking water.

Pseudomonas moraviensis subsp. stanleyae, a bacterial endophyte of hyperaccumulator Stanleya pinnata, is capable of efficient selenite reduction to elemental selenium under aerobic conditions

To identify bacteria with high selenium tolerance and reduction capacity for bioremediation of wastewater and nanoselenium particle production.

Improving the Compatibility of Contact Conductivity Detection with Microchip Electrophoresis Using a Bubble Cell

A new approach for improving the compatibility between contact conductivity detection and microchip electrophoresis was developed. Contact conductivity has traditionally been limited by the interaction of the separation voltage with the detection electrodes because the applied field creates a voltage difference between the electrodes, leading to unwanted electrochemical reactions. To minimize the voltage drop between the conductivity electrodes and therefore improve compatibility, a novel bubble cell detection zone was designed. The bubble cell permitted higher separation field strengths (600 V/cm) and reduced background noise by minimizing unwanted electrochemical reactions. The impact of the bubble cell on separation efficiency was measured by imaging fluorescein during electrophoresis. A bubble cell four times as wide as the separation channel led to a decrease of only 3% in separation efficiency at the point of detection. Increasing the bubble cell width caused larger decreases in separation efficiency, and a 4-fold expansion provided the best compromise between loss of separation efficiency and maintaining higher field strengths. A commercial chromatography conductivity detector (Dionex CD20) was used to evaluate the performance of contact conductivity detection with the bubble cell. Mass detection limits (S/N = 3) were as low as 89 +/- 9 amol, providing concentration detection limits as low as 71 +/- 7 nM with gated injection. The linear range was measured to be greater than 2 orders of magnitude, from 1.3 to 600 microM for sulfamate. The bubble cell improves the compatibility and applicability of contact conductivity detection in microchip electrophoresis, and similar designs may have broader application in electrochemical detection as the expanded detection zone provides increased electrode surface area and reduced analyte velocity in addition to the reduction of separation field effects.

Electrophoretic separations in poly(dimethylsiloxane) microchips using a mixture of ionic and zwitterionic surfactants

The use of mixtures of ionic and zwitterionic surfactants in poly(dimethylsiloxane) (PDMS) microchips is reported. The effect of surfactant concentration on electroosmotic flow (EOF) was studied for a single anionic surfactant (sodium dodecyl sulfate, SDS), a single zwitterionic surfactant (N‐tetradecylammonium‐N,N‐dimethyl‐3‐ammonio‐1‐propanesulfonate, TDAPS), and a mixed SDS/TDAPS surfactant system. SDS increased the EOF as reported previously while TDAPS showed an initial increase in EOF followed by a reduction at higher concentrations. When TDAPS was added to a solution containing SDS, the EOF decreased in a concentration‐dependent manner. The EOF for all three surfactant systems followed expected pH trends, with increasing EOF at higher pH. The mixed surfactant system allowed tuning of the EOF across a range of pH and concentration conditions. After establishing the EOF behavior, the adsorption/desorption kinetics were measured and showed a slower adsorption/desorption rate for TDAPS than SDS. Finally, the separation and electrochemical detection of model catecholamines in buffer and reduced glutathione in red blood cell lysate using the mixed surfactant system were explored. The mixed surfactant system provided shorter analysis times and/or improved resolution when compared to the single surfactant systems.

Sensitive, selective analysis of selenium oxoanions using microchip electrophoresis with contact conductivity detection.

The common selenium oxoanions selenite (SeO32–) and selenate (SeO42–) are toxic at intake levels slightly below 1 mg day–1. These anions are currently monitored by a variety of traditional analytical techniques that are time-consuming, expensive, require large sample volumes, and/or lack portability. To address the need for a fast and inexpensive analysis of selenium oxoanions, we present the first microchip capillary zone electrophoresis (MCE) separation targeting these species in the presence of chloride, sulfate, nitrate, nitrite, chlorate, sulfamate, methanesulfonate, and fluoride, which can be simultaneously monitored. The chemistry was designed to give high selectivity in nonideal matrices. Interference from common weak acids is avoided by operating near pH 4. Separation resolution from chloride was enhanced to improve tolerance of high-salinity matrices. As a result, selenate can be quantified in the presence of up to 1.5 mM NaCl, and selenite analysis is even more robust against chloride. Using contact conductivity detection, detection limits for samples with conductivity equal to the background electrolyte are 53 nM (4.2 ppb Se) and 380 nM (30 ppb) for selenate and selenite, respectively. Analysis time, including injection, is ∼2 min. The MCE method was validated against ion chromatography (IC) using spiked samples of dilute BBL broth and slightly outperformed the IC in accuracy while requiring <10% of the analysis time. The applicability of the technique to real samples was shown by monitoring the consumption of selenite by bacteria incubated in LB broth.

Electrophoretic separations in poly(dimethylsiloxane) microchips using mixtures of ionic, nonionic and zwitterionic surfactants: Microfluidics and Miniaturization

The use of surfactant mixtures to affect both EOF and separation selectivity in electrophoresis with PDMS substrates is reported, and capacitively coupled contactless conductivity detection is introduced for EOF measurement on PDMS microchips. First, the EOF was measured for two nonionic surfactants (Tween 20 and Triton X‐100), mixed ionic/nonionic surfactant systems (SDS/Tween 20 and SDS/Triton X‐100), and finally for the first time, mixed zwitterionic/nonionic surfactant systems (TDAPS/Tween 20 and TDAPS/Triton X‐100). EOF for the nonionic surfactants decreased with increasing surfactant concentration. The addition of SDS or TDAPS to a nonionic surfactant increased EOF. After establishing the EOF behavior, the separation of model catecholamines was explored to show the impact on separations. Similar analyte resolution with greater peak heights was achieved with mixed surfactant systems containing Tween 20 and TDAPS relative to the single surfactant system. Finally, the detection of catecholamine release from PC12 cells by stimulation with 80 mM K+ was performed to demonstrate the usefulness of mixed surfactant systems to provide resolution of biological compounds in complex samples.

Protonated diamines as anion-binding agents and their utility in capillary electrophoresis separations

Capillary zone electrophoresis is a proven method for separating small ions because of the inherent charge and differences in mobility of these analytes. Despite its resolving power, CZE can be insufficient for separating ions with similar mobilities. One remedy is to modify mobilities via the addition of background electrolyte complexation agents. However, this approach is not straightforward for inorganic anions, which lack complexation options. To address this shortfall, the diprotonated diamine moiety was investigated for complexation of dianions. Dicationic diamines significantly complexed dianions, and this interaction was not purely electrostatic in nature because affinities varied with dianion identity. Aqueous association constants were measured with affinity capillary electrophoresis (ACE) and found to be similar in magnitude but different in selectivity to those of dianions with magnesium ion. Binding was also investigated for zwitterionic buffers containing the protonated diamine moiety. Zwitterions exhibited binding constants as high as 18 M−1 (30‐mM ionic strength). This work discusses the observed binding constants and their potential usefulness in CZE separations of inorganic anions. Also covered are improvements to ACE methodology and an evaluation of some of the assumptions employed.

IR-Compatible PDMS microfluidic devices for monitoring of enzyme kinetics

Coupling infrared (IR) spectroscopy to microfluidic devices provides a powerful tool for characterizing complex chemical and biochemical reactions. Examples of microfluidic devices coupled with infrared spectroscopy have been limited, however, largely due to the difficulties associated with fabricating systems in common infrared transparent materials like CaF2. Recent reports have shown that polydimethylsiloxane (PDMS) can be used as an IR transparent substrate when fabricated with thin layers. The use of soft lithography with PDMS expands the library of possible designs that can be achieved for IR measurements in microfluidics. In initial reports with thin PDMS, the target analytes were small molecules; however, IR spectroscopy offers a powerful tool to study protein structure and reactions. Here, a PDMS microfluidic device compatible with IR spectroscopy was fabricated by means of spin-coating of PDMS pre-polymer to obtain thin PDMS microfluidic features. The device was comprised of only PDMS and IR absorption of PDMS was significantly minimized due to the thickness (∼40 μm) of the PDMS layer. The use of thin PDMS allowed for measuring the amide I and II vibrational bands of proteins that have been difficult to measure in other microfluidic devices. To demonstrate the power of the system, the microfluidic device was successfully used to measure the enzyme kinetics as one class of important biochemical reactions with broad use in a variety of fields from medicine to biotechnology. As a model, the reaction of glucose oxidase with glucose was tracked by following the formation of gluconic acid. Michaelis-Menten kinetics from the device were compared with bulk solution measurements and found to be in good agreement.