Hanghui Liu

Analytical Chemistry in a Drop. Solvent Extraction in a Microdrop

An organic microdrop (∼1.3 μL) is suspended inside a flowing aqueous drop from which the analyte is extracted. The drop-in-drop system is achieved by a multitube assembly. The aqueous phase is continuously delivered to the outer drop and is aspirated away from the bottom meniscus of the drop. After the sampling/extraction period, a wash solution replaces the sample/reagent in the aqueous layer, resulting in a clear outer aqueous drop housing a colored organic drop containing the extracted material. This also results in an automatic backwash. The color intensity of the organic drop, related to the analyte concentration, is monitored by a light-emitting diode based absorbance detector. After the analytical cycle, the organic drop is removed and replaced by a new one. The performance of the system is illustrated with the determination of sodium dodecyl sulfate (a methylene blue active substance) extracted as an ion pair into chloroform. This unique microextraction system is simple and flexible, permits automated backwashing, consumes only microquantities of organic solvents, and is capable of being coupled with other analytical systems. This concept should prove valuable for preconcentration and matrix isolation in a microscale.

Light emitting diode based flow-through optical absorption detectors.

Simple inexpensive high performance optical absorption detectors are possible using light emitting diodes (LEDs) as light sources. The designs presented in the literature are reviewed. Designs used by the investigators are described in detail with respect to construction, electronic design, performance and cost; these have not previously been described in the literature. Characteristics of commercially available LEDs are tabulated. At the simple end, a single beam, dc driven, transmittance output detector can be constructed within the body of a LED. At the high performance end, fully referenced, computer interfaced detectors are described that are pulsed at high speeds to attain measurement standard deviations in the range of 2-3 x 10(-6) absorbance.

Analytical chemistry in a drop

Abstract A liquid drop has some unique features, namely, reproducibility, renewability and the lack of containment walls. The value of these features are highlighted through a series of novel liquid drop-based systems: truly renewable gas sampling interfaces, windowless optical cells and reaction vessels for flow-injection turbidimetry and solvent extraction systems with nested drop arrangements. The use of a drop minimizes sample/ reagent consumption and, like the dropping mercury electrode of Heyrovsky, allows a fresh reaction surface for every sample. Drops are of particular value in solving problems in biphasic systems.

Fluorometric fiber optic drop sensor for atmospheric hydrogen sulfide

A fluorometric technique based on a liquid drop excited from its interior by an optical fiber is described for the measurement of low concentrations of atmospheric hydrogen sulfide (H(2)S). A drop of alkaline fluorescein mercuric acetate (FMA) solution is suspended in a flowing air sample stream and serves as a renewable sensor. An optical fiber contained within the conduit that forms the drop, brings in the excitation beam; the fluorescence emission is measured by an inexpensive photodiode positioned close to the drop. As H(2)S in the sample is collected by the alkaline drop, it reacts rapidly with FMA resulting in a significant decrease in fluorescence intensity, proportional to the concentration of H(2)S sampled. The chemistry of this uniquely selective reaction has been well established for many years; the present technique permits a simple fast inexpensive near real-time measurement with very little reagent consumption. Even without prolonged sampling/preconcentration steps, limits of detection (LODs) in the double digit ppbv range is readily attainable.

A liquid drop: A windowless optical cell and a reactor without walls for flow injection analysis

Abstract The design and characteristics of a dynamically growing and falling liquid drop based optical detection system are described for use with flow injection analysis (FIA). Several aspects of the analytical potential of a liquid drop as an optical cell are demonstrated by the detection of sulfate through the precipitation reaction between barium ion and sulfate ion in an FIA system. Deposition of precipitate on the windows of a conventional flow cell usually constitutes a vexing problem; this is avoided due to the windowless nature of the drop based cell. Gradient dilution techniques are conveniently implemented without precise external timing: with small drops, a single FIA peak is spread over a multitude of drops. Each individual drop represents a different dilution ratio and the dynamic range of measurement can be greatly increased based on the choice of the specific drop used for quantitation. Both absorbance/turbidimetric detection and nephelometric detection can be made on a single drop using a single optical fiber for light collection. Different configurations for mixing and reaction of the sample and reagents in a drop based system has been studied.

Dual-wavelength photometry with light emitting diodes. Compensation of refractive index and turbidity effects in flow-injection analysis

Abstract An approach to compensate for both refractive index and turbidity effects is described. A light emitting diode-based dual-wavelength, double-beam (fully referenced), dual-flow-cell photometric detection system is used. The only requirement is that the analyte yields a product (that can be detected by absorbance measurement) as a result of a reaction which does not affect the optical properties of the suspended matter. The principle is mathematically established and experimentally demonstrated by determining micromolar concentrations of bromthymol blue in samples containing up to 1.5% whole milk and 60% ethanol with an absolute error of

Flow-injection extraction without phase separation based on dual-wavelength spectrophotometry

Abstract A new detection technique for flow-injection extraction is introduced. The absorbance is read radially, on the same PTFE tube that constitutes the reaction coil, near its distal terminus. With an effective illuminated detector volume of ∼ 60 nl, the optical aperture is much smaller than the segment length, enabling the detector to reliably measure signals for each phase. The detector used is a light emitting diode (LED) based dual-wavelength photometric system utilizing personal computer (PC) based data acquisition and processing. One non-specific wavelength is used to recognize the phases and the other wavelength monitors the analyte concentration. Accurate and reliable phase recognition is achievable with conventional segmentors and peristaltic pumping. Applied to the determination of anionic surfactants by ion-pairing with methylene blue (MB) and extraction into chloroform, a linear response is observed with a limit of detection (LOD) of 0.03 ppm C-12 linear alkylbenzene sulfonate (LAS) for a 65 μl injected sample, compared to an LOD of 0.025 ppm quoted for the standard manual method attainable by subjecting several hundred ml of the sample to extraction.

High performance optical absorbance detectors based on low noise switched integrators.

Optical absorption detection is the most common analytical measurement principle in liquid phase analysis. The current state-of-the-art of commercially available detectors exhibit peak-to-peak (p-p) noise levels in the range of 1 x 10(-5)-2 x 10(-5) absorbance units (10-20 microAU). Using circuitry based on newly available switched integrator integrated circuit (IC) packages, it is possible to construct inexpensive absorbance detectors with p-p noise levels as low as 3 microAU under actual use conditions. The necessary electronics are described and performance data are reported with light emitting diodes (LEDs) as light sources. Even in the capillary format with a rectangular capillary (50 x 1000 microm cross section) with a slitwidth <50 microm and with the 1000 microm dimension as the nominal pathlength, p-p noise levels of 10 microAU are observed, from which a concentration limit of detection (LOD) of 10 nM for bromothymol blue (BTB) can be estimated with a 660 nm light source.

Expanding the linear dynamic range for multiple reaction monitoring in quantitative liquid chromatography-tandem mass spectrometry utilizing natural isotopologue transitions.

We describe a method for expanding the linear dynamic range for multiple reaction monitoring (MRM) in quantitative liquid chromatography/tandem mass spectrometry (LC-MS/MS) using additional transitions for isotopologues. In addition to the regular transition for the highest possible sensitivity, a transition corresponding to the less abundant isotopologue ions was utilized. This decreases saturation at the ion detector; the sensitivity reduction increases the upper dynamic limit. We demonstrated this for a rat plasma assay for a candidate flavor compound; the linear dynamic range increased by an order of magnitude from 3 to 6,000 ng/mL with the regular MRM alone to 3-60,000 ng/mL using additionally the isotopologue transition.

Black box linearization for greater linear dynamic range: the effect of power transforms on the representation of data.

Power transformations are commonly used in image processing techniques to manipulate image contrast. Many analytical results, including chromatograms, are essentially presented as images, often to convey qualitative information. Power transformations have remarkable effects on the appearance of the image, in chromatography, for example, increasing apparent resolution between peaks by the factor √n and apparent column efficiency (plate counts) by a factor of n for an nth-power transform. The profile of a Gaussian peak is not qualitatively changed, but the peak becomes narrower, whereas for an exponentially tailing peak, asymmetry at the 10% peak height level changes markedly. Using several examples we show that power transforms also increase signal-to-noise ratio and make it easier to discern an event of detection. However, they may not improve the limit of detection. Power responses are intrinsic to some detection schemes, and in others they are imbedded in instrument firmware to increase apparent linear range that the casual user may not be aware of. The consequences are demonstrated and discussed.