Enzyme inhibition-based determination of pesticide residues in vegetable and soil in centrifugal microfluidic devices.

Pesticide residue is of concern as an environmental pollutant when present at medium to high concentrations. Such residue was quantified in both vegetable and soil samples by an enzyme inhibition technique. The multistep reactions were integrated into centrifugal microfluidic devices allowing automated simultaneous analysis of several samples or of replicates. The small sample size inherent to microfluidic devices allowed for less reagent to be used including less of the expensive enzyme which is key to this method. Liquid-solid magnetically actuated extraction, filtration, sedimentation, and detection were all integrated on the same device. Several parameters were optimized including the concentration of enzyme, substrate, chromatic agent, and reaction time. In this environmental application of centrifugal microfluidics, the percent inhibition of enzyme activity is logarithmically proportional to the demonstration pesticide concentration (in this case carbofuran). This meant that as the pesticide concentration increased in the samples, the reaction was more inhibited and the final product absorbed less light at 525 nm. Two versions of the centrifugal microfluidic devices were made. One version was designed for the analysis of vegetable samples (cabbage) and the other for the analysis of soil samples. Each version provided results that were statistically similar to the conventional benchtop method with a carbofuran limit of detection of 0.1 ppm or 0.1 µg g(-1) (5 ng absolute limit of detection).

Spectrophotometric determination of aqueous sulfide on a pneumatically enhanced centrifugal microfluidic platform.

A pneumatically enhanced centrifugal microfluidic platform was developed for rapid spectrophotometric determination of aqueous sulfide. This platform performs an automated analysis based on the reaction between hydrogen sulfide and N,N-dimethyl-p-phenylenediamine in the presence of iron(III) chloride to form Methylene Blue. The platform design minimizes the number of integrated valves required, compared to other centrifugal systems, significantly improving the ease of fabrication. The sequential analytical procedure and spectrophotometric analyses were performed directly on-disk, demonstrating significant advantages in portability and cost over conventional analytical methods. This method allows for rapid and precise determination of aqueous sulfide in the concentration range of 0.4-2.0 mg L(-1), which can be extended by a pneumatically induced, on-disk serial dilution to 6.0 mg L(-1). A detection limit of 0.4 mg L(-1) was calculated for this pneumatically enhanced method.

Automated liquid-liquid extraction by pneumatic recirculation on a centrifugal microfluidic platform.

In this technical note, a liquid-liquid extraction technique was performed using pneumatic liquid recirculation on a centrifugal microfluidic device. Non-contact pneumatic pumping enabled a multi-cycle liquid-liquid extraction process using aqueous iodine in a potassium iodide solution and hexadecane while requiring a minimal amount of space on the device. The extraction process was completely automated on the device following sample introduction and required only 50 s for each extraction cycle. The pumping rate achieved during liquid recirculation was 120 ± 10 µL/min. A recycling process such as the one demonstrated would be difficult to implement in a conventional centrifugal microfluidic system.

A valveless pneumatic fluid transfer technique applied to standard additions on a centrifugal microfluidic platform.

This paper demonstrates a valveless pneumatic fluid transfer technique applicable to centrifugal microfluidic platforms. The technique involves using compressed gas to generate a pneumatic force, which works together with the centrifugal force to control and direct fluid flow. Fluid can be pneumatically transferred from chamber to chamber, greatly decreasing the number of conventional valves required in a multistep process. By varying the rotational frequency of the centrifugal microfluidic platform while pneumatic force is applied, sequential fluid transfer steps can be achieved. The effectiveness of this fluid transfer method is demonstrated by performing a standard additions calibration. This technique is shown to be robust, easy to implement, and greatly reduces the design limitations traditionally associated with centrifugal microfluidic platforms.

Pneumatic flow switching on centrifugal microfluidic platforms in motion.

This paper describes a flow switching technique applicable to centrifugal microfluidic platforms, using a regulated stream of compressed gas. This pneumatic flow switching technique allows for flow control at a T-shaped junction between one inlet channel and two outlet channels. This technique provides a noncontact, robust, and efficient method for switching the direction of fluid flow while a disk is rotating at relatively low frequencies. The switching operation can be implemented reproducibly with applied gas flow rates between 17 and 58 L min(-1) and rotational frequencies between 400 rpm (6.6 Hz) and 1200 rpm (20 Hz).

Pneumatically pumping fluids radially inward on centrifugal microfluidic platforms in motion.

This paper describes a pumping technique applicable to centrifugal microfluidic platforms, involving the use of a regulated stream of compressed gas to pump liquid radially inward and toward the center of the platform while spinning. This technique provides a noncontact method for pumping fluids and is highly efficient, requiring only approximately 60 s to reach completion. This pumping operation can be attained with an applied gas flow rate of 58.8 L min(-1), while the platform is rotated at frequencies less than 180 rpm (3.0 Hz).

Non-contact addition, metering, and distribution of liquids into centrifugal microfluidic devices in motion.

A three-dimensional structure was constructed to demonstrate contact-free liquid addition and distribution to a conventional centrifugal microfluidic platform. The structure allowed a continuous stream of liquid to be added to the rotating platform where it was dispensed and metered to a series of reservoirs with 9% relative standard deviation of the volume. An off-platform generated gradient was used to demonstrate this design's potential to make gradient centrifugal microfluidic chromatography possible without the use of platform space for liquid storage. The system demonstrates a convenient and versatile "World-to-Chip" or more appropriately, World-to-Disk, macro-to-micro interface.

A rapid prototyping technique for valves and filters in centrifugal microfluidic devices.

Using short lengths of fused silica capillary tubing embedded in the disk, a system for valving and filtering samples on centrifugal microfluidic devices has been designed and implemented. Sedimentation of turbid samples and transfer of the clear supernatant was also accomplished. Also demonstrated is the transfer of the liquid through a second capillary valve to the final reservoir. By controlling rotational speed, sedimentation and multiple step valving operations are readily implemented and easily prototyped.

Magnetically driven solid sample preparation for centrifugal microfluidic devices.

A prototype for solid sample preparation on centrifugal microfluidic devices has been designed and characterized. The system uses NdFeB magnets in both the centrifugal device and a fixed base. As the centrifugal device rotates, the magnets move and spin in their chambers creating a pulverizing mechanical motion. This technique was successfully applied to the dissolution of potassium ferricyanide (K(3)[Fe(CN)(6)]), a hard colored crystal. A 0.10 g sample was completely dissolved in 3 s in 1.0 mL of water while rotating at 1000 rpm. This is a 300-fold improvement over static dissolution.

Speciation of chromium by high-performance thin-layer chromatography with direct determination by laser ablation inductively coupled plasma mass spectrometry.

It is of considerable importance to be able to distinguish metallic species because their toxicity depends on their chemical form. Therefore, the analysis of environmental samples can be enhanced by the combination of high-performance thin-layer chromatography (HPTLC) with laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). In this study, Cr (3+) and Cr (6+) were separated on silica gel HPTLC plates using aqueous mobile phases. Separation was achieved in seconds with retardation factors ( R f ) of 0 and 1 for Cr (3+) and Cr (6+), respectively. LA was used to volatilize the chromium species directly from the chromatographic material prior to ICPMS detection. A linear calibration was obtained, and detection limits (3sigma) of 6 ng for Cr (6+) and 0.4 ng for Cr (3+) were achieved with precision ranging from 3 to 40% at the 95% confidence level. The silicon present in the stationary phase was used as an internal standard. This procedure allows for a rapid separation and quantification, requires only 0.5 muL of sample, and lower detection limits can be achieved through preconcentration.

Rapid prototyping of pneumatically actuated hydrocarbon gel valves for centrifugal microfluidic devices.

A novel, easy to prototype hydrocarbon gel-based active valve was developed for use in centrifugal microfluidic devices. The valve has been demonstrated to restrict flow by an additional 1000 revolutions per minute (RPM) when compared to a passive capillary valve of the same size located at the same radius. Opening of the valve is accomplished in a contactless manner using a stream of focused compressed air. The ease of fabrication, low cost and small dimensions of the gel valve offer the potential for integration of multiple valves of this type into multi-process centrifugal microfluidic systems.

Volumetric measurements by image segmentation on centrifugal microfluidic platforms in motion.

An image segmentation based method was developed to perform volumetric measurements of liquid aliquots in centrifugal microfluidic platforms in motion. The method was designed to be as automated as possible to allow its applicability to the large variety of available design features that tend to be included on such platforms. Experiments have indicated a relative standard deviation (RSD) of 0.3% for replicate measurements and 1% for same volume aliquots injected into different sized chambers. The versatility of the method in regards to chamber shape and size, liquid colour and platform rotational frequency was demonstrated. This flexibility should allow it to be used for a variety of applications including real time metering of volumes in platforms, quantitative monitoring of a design's performance in real time and could result in the elimination of metering chambers for some applications.

Automated liquid-solid extraction of pyrene from soil on centrifugal microfluidic devices.

Organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) are present in the environment in increasing concentrations and so are of growing concern. Monitoring these species, particularly on-site, can be both difficult and expensive. This paper presents a novel miniaturised magnetically actuated liquid-solid extraction unit integrated in tandem with a filtration unit and a detection unit on a single centrifugal microfluidic device. A demonstration analyte, pyrene, was rapidly extracted and quantified by UV-absorbance from multiple soil samples. The system showed excellent performance for a system designed for field use. Characterization of two types of passive valves was performed along with an extraction time efficiency study. The system provides a factor of 150 reduction in sample weights and extraction solvent volumes and provides statistically similar recoveries to the conventional method with a pyrene detection limit of 1 ppm (0.03 microg absolute detection limit). The reduction in time and solvent and the potential for field use suggest that this device type may be valuable for environmental monitoring.

Rapid simultaneous determination of nitrate and nitrite on a centrifugal microfluidic device.

A centrifugal microfluidic device was developed for the rapid sequential determination of two critical environmental species, nitrate and nitrite, in water samples. The nitrate is reduced to nitrite and the nitrite is derivatized. The analytes are determined spectrophotometrically through the disc with a 1.4mm pathlength. The detection limits are 0.05 and 0.16 mg L(-1) for nitrite and nitrate respectively. The use of powdered reagents, the 100 microL sample required and the design of the device suggest that it would be suitable for field use.

Miniaturised centrifugal solid phase extraction platforms for in-field sampling, pre-concentration and spectrometric detection of organic pollutants in aqueous samples.

Great variations in pollutant concentrations are observed in the environment and pre-concentration is often required to detect trace contaminants in water samples. This paper presents a novel solid phase-extraction device integrated onto a centrifugal microfluidic platform for rapid on-site pre-concentration and screening of organic contaminants in aqueous samples. In-column fluorescence and absorbance measurements are obtained directly from an analyte trapped on the top of a solid phase extraction microcolumn. Results are presented for the representative fluorophore fluorescein and the polycyclic aromatic hydrocarbon anthracene. An absolute detection limit of 20 ng was obtained for anthracene using a simple light emitting diode for fluorescence excitation. One of the main advantages of this device is that only a simple motor is needed to induce liquid flow, making simultaneous on-site extraction and measurement of multiple samples easy while minimizing sample losses and contamination.

A centrifugal microanalysis system for the determination of nitrite and hexavalent chromium.

A centrifugal microanalysis system using spectrophotometric detection has been developed for the determination of nitrite and chromium VI. The system was designed to illustrate the potential of centrifugal microfluidic systems for on-site (field) analysis of waters. The performance was significantly better than manual/visual field test systems and approaches that of bench top systems with detection limits of 0.008 mg L(-1) NO(2)(-)-N and 0.03 mg L(-1) Cr(6+). The system used pre-loaded solid reagents on the disc, providing ease of use, and could process up to 24 samples simultaneously. Precision was enhanced by the use of a multiwavelength ratiometric blank estimation technique.

Analysis of mercury in sequential micrometer segments of single hair strands of fish-eaters.

Although it has been established that mercury (Hg) can be detected in single hair strands using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), calibration remains a challenge due to the lack of well-characterized matrix-matched standards. We concurrently evaluated two strategies for quantifying Hg signals in single hair strands using LA-ICP-MS. The main objective was to obtain time-resolved Hg concentrations in single hair strands of fish-eaters that would correspond to the changes of their body burden over time. Experiments were conducted using hair samples collected from 10 individuals. The first experiment involved the construction of a calibration curve with four powdered hair standard reference materials (SRMs) with a range of Hg concentrations (0.573-23.2 mg/kg). An internal standard, sulfur, as 34S, was applied to correct for ablation efficiency for both the hair strands and the SRMs. Results showed a linear relationship (R2 = 0.899) between the ratio of 202Hg to 34S obtained by LA-ICP-MS and the certified total Hg concentration in the SRMs. Using this calibration curve, average Hg concentrations of 10 shots within a 1-cm segment of a hair strand were calculated and then compared to the total Hg concentrations in the matched 1-cm segment as measured by cold vapor atomic absorption spectrometry (CV-AAS). A significant difference (p < 0.05) was observed. The difference could be attributed to the highly variable ablation/sampling process caused by the use of the laser on the hair powder SRM pellets and the difference in the physical properties of the SRMs. An alternative approach was adopted to quantify consecutive 202Hg to 34S ratios by calibrating the signals against the average Hg concentration of the matched hair segment as measured by CV-AAS. Consecutive daily Hg deposition in single hairs of fish eaters was determined. Results showed that apparent daily changes in Hg concentrations within a hair segment that corresponds to 1 month of hair growth. In addition, a significant decreasing or increasing time-trend was observed. The difference between the minimum and maximum Hg concentration within each individual corresponded to a change of 26-40%. Our results showed that LA-ICP-MS can be used to reconstruct time-resolved Hg exposure in micrometer segments of a single hair strand.