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Low cost 235 nm Ultra-violet Light-emitting diode-based absorbance detector for application in a portable ion chromatography system for nitrite and nitrate monitoring
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... However, the manual bonding of separate layers of the silicon device was both process/resource intensive and challenging regarding optical alignment, thus reducing optical efficiency. Recent examples of integrating optical windows within lower-cost microfluidic systems are seen with the use of fused silica glass windows (12.5 mm × 2 mm) in contact with the low-cost PMMA microfluidic device [123], as seen in Figure 19. The effective device path length was calculated at 96% of the actual optical path (2.15 cm) meaning low-level scattering, potentially due to the non-transmittance of the deep-UV (235 nm) within the PMMA. ...
... Integrated fused silica windows with a PMMA microfluidic device for UV-Visible absorbance detection. Reprinted from [123], copyright © Elsevier 2019. ...
... The current set-up relies on butt-coupled optics through objective lenses that suffers inevitably from background transmission noise, however the team have proposed pig-tailed optical fibers that would provide a simple monolithic, integrated optofluidic solution. In terms of low-cost microfluidic based detection, a recent important development is seen with industrial production of deep-UV light-emitting diodes LEDs (235 nm), as used by Murray et al. [123]. A stable, low-cost UV light source at a fixed wavelength could in many ways act as a catalyst for the development of similar sensitive and stable detection devices. ...
The integration of optical circuits with microfluidic lab-on-chip (LoC) devices has resulted in a new era of potential in terms of both sample manipulation and detection at the micro-scale. On-chip optical components increase both control and analytical capabilities while reducing reliance on expensive laboratory photonic equipment that has limited microfluidic development. Notably, in-situ LoC devices for bio-chemical applications such as diagnostics and environmental monitoring could provide great value as low-cost, portable and highly sensitive systems. Multiple challenges remain however due to the complexity involved with combining photonics with micro-fabricated systems. Here, we aim to highlight the progress that optical on-chip systems have made in recent years regarding the main LoC applications: (1) sample manipulation and (2) detection. At the same time, we aim to address the constraints that limit industrial scaling of this technology. Through evaluating various fabrication methods, material choices and novel approaches of optic and fluidic integration, we aim to illustrate how optic-enabled LoC approaches are providing new possibilities for both sample analysis and manipulation.
... The core of the analytical device is a portable ion chromatography (IC) system based on the method previously reported by Murray [14]. This system employs a novel design of a ultraviolet (UV) light-emitting diode (LED)-based optical detector which enables cost-effective direct in situ detection of nitrite and nitrate in natural waters. ...
... Once full, the syringe pumps empty at a set flow rate enabling chromatographic analysis and analyte detection. The optical detection cell consists of a low-cost, UV absorbance detector which incorporates a 235 nm LED and photodiode [14]. The photodiode is coupled with an ADS1115 analogue to digital converter (ADC). ...
... The samples were stored at 4 • C until further analysis. The sampling was carried out during the month of March 2019, and eight samples were compared in the following days 1, 4, 8, 12, 14, 18, 20, and 22. Analytical determination by portable IC The prototype analytical device is a portable ion chromatography (IC) system based on the method previously reported by [14] as mentioned in the Section 2.1. Therefore, the study parameters have been extracted from this paper. ...
The deteriorating water environment demands new approaches and technologies to achieve sustainable and smart management of urban water systems. Wireless sensor networks represent a promising technology for water quality monitoring and management. The use of wireless sensor networks facilitates the improvement of current centralized systems and traditional manual methods, leading to decentralized smart water quality monitoring systems adaptable to the dynamic and heterogeneous water distribution infrastructure of cities. However, there is a need for a low-cost wireless sensor node solution on the market that enables a cost-effective deployment of this new generation of systems. This paper presents the integration to a wireless sensor network and a preliminary validation in a wastewater treatment plant scenario of a low-cost water quality monitoring device in the close-to-market stage. This device consists of a nitrate and nitrite analyzer based on a novel ion chromatography detection method. The analytical device is integrated using an Internet of Things software platform and tested under real conditions. By doing so, a decentralized smart water quality monitoring system that is conceived and developed for water quality monitoring and management is accomplished. In the presented scenario, such a system allows online near-real-time communication with several devices deployed in multiple water treatment plants and provides preventive and data analytics mechanisms to support decision making. The results obtained comparing laboratory and device measured data demonstrate the reliability of the system and the analytical method implemented in the device.
... Prohibitive costs and technological limitations have hampered these in-situ systems from routine use and adoption on a mass scale [29]. Direct UV systems remain at significant price points and have high power consumption requirements due to the use of UV lamps [30]. As for colorimetric based analysers, these systems are also of a high price point, but they have the added disadvantage of requiring multiple reagents which are often of a hazardous nature. ...
... The automated portable nitrate and nitrite analyser described in this study was based on the IC method coupled with 235 nm LED based absorbance detection recently reported by Murray et al. [30]. A render of the IC system, depicting system components is shown in Fig. 1. ...
... A UVC photodiode (TOCON_C1) with integrated amplifier for photodetection was purchased from Sglux GmbH (Berlin, Germany). The UV optical detection cell was based on the design previously reported [30]. All design work carried out for 3D printed components was performed using Fusion 360 computer aided design software (Autodesk Inc., California, USA). ...
... Integration of Wireless Sensor Network (WSN) and IoT was proposed in [24] to perform a Decentralized Smart Water Quality Monitoring System. The sensor device employed a cost-effective optical sensor based-on an ion chromatography system [25] to detect nitrite and nitrate in the water. The sensor devices are easy to be deployed in multi WWTPs using WSN technology. ...
... Second, compared to the existing real-time monitoring in phytoremediation [13,14], our approach offers a simple hardware configuration and adopts a recent IoT technology. Third, compared to the existing WWTP monitoring systems [15][16][17][18][19][20][21][22][23][24][25][26][27][28], our approach offers an efficient tool to monitor and examine the phytoremediation models by integrating the IoT platform and the mathematical model, which is implemented on the embedded hardware. ...
... R1 and R2 merged into one channel before being combined with the carrier phase. The combined reaction media was then introduced into the 3D printed heated reactor and subsequently into a custom-built optical detection cell, previously reported by Murray et al. [55] for UV-LED based optical detection. In this case, a 660 nm LED (L-53SRC-C) and photodiode (TSL257), obtained from RS components (Dublin, Ireland), were fitted within the 3D printed holders and inserted into the detector housing. ...
... The detector was then placed within a small box to prevent ambient light from having an effect on photodiode readings. Data acquisition and LED operation were achieved using the previously reported electronic prototype board [55]. ...
A multi-material 3D printed microfluidic reactor with integrated heating is presented, which was applied within a manifold for the colorimetric determination of ammonium in natural waters. Graphene doped polymer was used to provide localised heating when connected to a power source, achieving temperatures of up to 120 °C at 12 V, 0.7 A. An electrically insulating layer of acrylonitrile butadiene styrene (ABS) polymer or a new microdiamond-ABS polymer composite was used as a heater coating. The microdiamond polymer composite provided higher thermal conductivity and uniform heating of the serpentine microreactor which resulted in greater temperature control and accuracy in comparison to pure ABS polymer. The developed heater was then applied and demonstrated using a modified Berthelot reaction for ammonium analysis, in which the microreactor was configured at a predetermined optimised temperature. A 5-fold increase in reaction speed was observed compared to previously reported reaction rates. A simple flow injection analysis set up, comprising the microfluidic heater along with an LED-photodiode based optical detector, was assembled for ammonium analysis. Two river water samples and two blind ammonium standards were analysed and estimated concentrations were compared to concentrations determined using benchtop IC. The highest relative error observed following the analysis of the environmental samples was 11% and for the blind standards was 5%.
... To date, 235 nm is the lowest UV wavelength LED available commercially, and it has already been utilized within absorbance detectors in a number of studies [34][35][36][37][38]. Early in 2016, Li et al. reported on the very first 235 nm LEDbased absorbance detector [34]. ...
Liquid chromatography is a prominent analytical technique in separation science and chemical analysis, applied across numerous fields of research and within industrial applications. Over the past few decades, there has been a growing interest in the miniaturization of this technique, which has been particularly enabled through new miniature and portable detection technologies for in-field, at-site, and point-of-need (collectively 'out-of-lab') analyses. Accordingly, significant advances have been made in recent years in the development of miniaturized liquid chromatography with photometric, electrochemical, and mass spectrometric detection, enabling the development of field-deployable and portable instruments for various applications. Herein, recent developments in the miniaturization of detection systems for inclusion within, and/or coupling with, portable liquid chromatographic systems, are reviewed in detail together with critical comments and expected future trends in this area.
... The flow chart of the procedure and method for detecting the toxicity of water according to the different nitrite contents in water is summarized in Fig. 2. To begin, detecting the initial nitrite concentration in the water sample is required to ensure the stability of subsequent detection. This stage takes no more than 20 min and enables you to select from a choice of detecting methods (Abbas and Mostafa 2000, Murray et al., 2019, Wang et al., 2022c. 0.3 mg/L was the lowest concentration of nitrite detected by the biosensor, below which no characteristic peaks would appear. ...
Water toxicity detection, as a valuable supplement to conventional water quality measurement, is an important method for evaluating water environmental quality standards. However, the toxicity of composite pollutants is more complicated due to their mixture effects. This study developed a novel, rapid and interference-resistant detection method for water toxicity based on an electrochemical biosensor using peak current from nitrite oxidation as a signal. Toxicants could weaken the characteristic peak current of nitrite to indicate the magnitude of toxicity. The proof-of-concept study was first conducted using a synthetic water sample containing trichloroacetic acid (TCAA), and then the results were compared with those of the traditional toxicity colorimetric method (CCK-8 kit) and laser confocal microscopy (CLSM). The accuracy of the biosensor was further verified with water samples containing individual pollutants such as Cd2+ (50-150 μg/L), Cr6+ (20-80 μg/L) mixture, triclosan (TCS; 0.1-1.0 μg/L) and TCAA (10-80 μg/L), or a mixture of the above. The viability of the sensor was further validated with the actual water sample from the Tuojiang River. The results demonstrated that although the concentration of a single conventional pollutant in water did not exceed the discharge standard for surface water, the comprehensive toxicity of natural water should not be ignored. This method could be a beneficial supplement to conventional water quality detection to understand the characteristics of the water, and thus contribute to the next stage of water treatment.
... The detector is shown in Figure 1, further design renders and photos of the 3D printed optical detection cell are shown in Figure S1A. The fluidic design of the detector was based upon the design previously reported by Murray et al. [41]. The cell was 56 × 32 × 15 mm in size, the z-shaped channel had a diameter of 1 mm and the optical path of the cell was 2.5 cm. ...
The intestinal epithelium, the fastest renewing tissue in human, is a complex tissue hosting multiple cell types with a dynamic and multiparametric microenvironment, making it particularly challenging to recreate in vitro. Convergence of recent advances in cellular biology and microfabrication technologies have led to the development of various bioengineered systems to model and study the intestinal epithelium. Theses microfabricated in vitro models may constitute an alternative to current approaches for studying the fundamental mechanisms governing intestinal homeostasis and pathologies, as well as for in vitro drug screening and testing. Herein, we review the recent advances in bioengineered in vitro intestinal models.
... Among them, spectrophotometric devices have attracted particular attention since they can be easily and rapidly built by exploiting lowcost light sources and photodetectors, enabling precise and reliable analyte detection in food, environmental and biomedical matrices [27][28][29][30][31]. They can be employed to detect chemical species absorbing in the UV-Vis region, including the absorbance change of a chemical species due to its modification, consumption of reagents or formation of products of specific reactions. ...
A low-cost portable analytical device, suitable for performing reflectance measurements on paper supports is proposed. It consists of a simple module consisting of a matrix of six types of photodiodes (six channels) with maximum sensitivity at six different wavelengths in the visible region. The device was used in combination with paper-based analytical devices (PADs) suitably prepared by 3D printing using polycaprolactone, i.e. a biodegradable and biocompatible polymer. These PADs were used both as substrates for the imbibition of colored solutions and of reagents for colorimetric reactions. The constructive and electronic aspects are described and the instrumental noise, as well as the performance of this device were carefully evaluated in terms of repeatability and reproducibility by using three different food dye and a colorimetric assay for the determination of nitrites with the Griess reagent. The development of this device also included the management of the output signals that, once converted from raw signals into reflectance values, have been elaborated by the Kubelka-Munk theory. Finally, the application to real samples consisting of commercial aqueous solutions of Tartrazine yellow (E102), Ponceau red 4R (E124) and Patent blue V (E131), usually adopted for the coloration and/or decoration of homemade cakes and sugarcoated almonds, gave results in good agreement with values obtained by absorbance measurements performed by a benchtop spectrophotometer. This type of comparison pointed out that also the nitrite detection performed by reflectance measurements on synthetic samples allowed more than reliable results to be obtained.
... The detector is shown in Figure 1, further design renders and photos of the 3D printed optical detection cell are shown in Figure S1A. The fluidic design of the detector was based upon the design previously reported by Murray et al. [41]. The cell was 56 × 32 × 15 mm in size, the z-shaped channel had a diameter of 1 mm and the optical path of the cell was 2.5 cm. ...
An ion chromatography system employing a low-cost 3D printed absorbance detector for indirect UV detection towards portable phosphate analysis of environmental and industrial waters has been developed. The optical detection cell was fabricated using stereolithography 3D printing of nanocomposite material. Chromatographic analysis and detection of phosphate was carried out using a CS5A 4 × 250 mm analytical column with indirect UV detection using a 255 nm light emitting diode. Isocratic elution using a 0.6 mM potassium phthalate eluent combined with 1.44 mM sodium bicarbonate was employed at a flow rate of 0.75 ml min-1 . A linear calibration range of 0.5 to 30 mg L-1 PO4 3- applicable to environmental and wastewater analysis was achieved. For retention time and peak area repeatability, RSD values were 0.68 % and 4.09 %, respectively. Environmental and wastewater samples were analysed with the optimised ion chromatography platform and the results were compared to values obtained by an accredited ion chromatographer. For the analysis of environmental samples, relative errors of < 14 % were achieved. Recovery analysis was also carried out on both freshwater and wastewater samples and recovery results were within the acceptable range for water analysis using standard ion chromatography methods. This article is protected by copyright. All rights reserved.
... The eluent syringe pumps were each coupled to one of two automated 6 port 2-position injection valves provided by VICI AG (Schenkon, Switzerland). LED-based UV absorbance detection was achieved using two low-cost detection cells, previously reported by Murray et al. [27] , employing two 235 nm deep UV-LEDs provided by Crystal IS (Green Island, NY, USA) and two UVC photodiodes (TOCON_C1) with integrated amplifiers purchased from Sglux GmbH (Berlin, Germany). The embedded system consisted of an ARM based microcontroller (Teensy 3.6) which controlled all the timing and actuation of the syringe pumps and 6-way valves as well as the sample intake pump and sample intake syringe pump. ...
A portable and automated IC system with a dual-capability for the analysis of both fresh and saline environmental waters has been developed. Detection of nitrate in complex matrices such as seawater was achieved by the employment of an automated two-dimensional (heart-cut) IC method utilised in tandem with on-column matrix elimination, using a sodium chloride eluent. The system also demonstrated the capability to switch to a second mode of analysis, whereby direct one-dimensional IC analysis was employed to rapidly detect nitrite and nitrate in freshwater, with direct UV LED based absorption detection in under 3 minutes. Calibration curves using a 195 µL sample loop were generated for both freshwater and artificial seawater samples. For marine analysis, an analytical range of 0.1 mg L⁻¹ – 40 mg L⁻¹ NO3⁻ was possible, while an analytical range (0.1 mg L⁻¹ – 15 mg L⁻¹ NO2⁻, 0.2 – 30 mg L⁻¹ NO3⁻) appropriate for freshwater analysis was also achieved. Chromatographic repeatability for both marine and freshwater analysis was verified over 40 sequential runs with RSD values of < 1% demonstrated for both peak area and retention times for each mode of analysis. The selectivity of both methods was demonstrated with interference tests with common anions present in environmental waters. Recovery analysis was carried out on marine samples from Tramore Bay, Co. Waterford, Ireland, and the systems analytical performance was compared with that of an accredited IC following environmental sample analysis.
... The detection and measurement of nitrite could be employed as a screening method for the presence of GSR. Literatures reported various techniques for determination of nitrite, including electrochemical methods [13,14], ion chromatography [15,16], electrophoresis [17,18], and spectrophotometry [19,20]. Among these techniques, the spectrophotometry technique received increasing attention due to its simplicity, convenience, low cost, and good detection limit [21][22][23]. ...
A simple, rapid, and environmentally-friendly spectrophotometric method for nitrite detection was developed. Detection was based on a redox reaction with iodide ions in an acidic condition. The reaction was evaluated by detecting the increase in absorbance of the colored product of iodine at 362 nm wavelength. To obtain a good spectrophotometric performance, the iodide ions concentration, hydrochloric acid concentration, and reaction time were optimized. In the optimal condition, the developed spectrophotometric method provided a linear range of 0.0625 to 4.00 mg L−1 (r = 0.9985), reaction time for 10 min, a limit of detection of 25 µg L−1, and a limit of quantitation of 85 µg L−1. This method showed good repeatability (RSD < 9.21%), high sample throughput (9 samples min−1), and good accuracy (recovery = 88 ± 2 to 99.5 ± 0.4%). The method has the potential to be used in crime scene investigations as a rapid screening test for gunshot residue detection via nitrite detection.
... Among them, spectrophotometric devices have attracted particular attention since they can be easily and rapidly built by exploiting lowcost light sources and photodetectors, enabling precise and reliable analyte detection in food, environmental and biomedical matrices [27][28][29][30][31]. They can be employed to detect chemical species absorbing in the UV-Vis region, including the absorbance change of a chemical species due to its modification, consumption of reagents or formation of products of specific reactions. ...
This investigation was carried out with the aim of stressing that technologically-advanced systems can be prototyped and produced by using 3D printing and inexpensive electronic modules. In particular, open-source technologies are opening new exciting possibilities also in the area of chemical analysis, thus making possible the assembly of customized analytical devices by using widely available and low cost materials. Here, two different field-portable optical analytical instruments, one operating in the transmittance mode and the other in reflectance mode, were built up by combining 3D printed parts with common hardware components, such as Arduino boards, LEDs and photoresistors. They were employed to analyze the antioxidant capacity of several tea infusions by profiting from the Folin-Ciocalteu assay. A performance comparable with that displayed by research-grade spectrophotometers indicates that these easy-to-operate and low-cost devices can provide accurate and precise results, thus demonstrating that highly-sensitive optical instrumentations, useful not only in research and education environments but also in resource-limited settings, can be readily constructed by a DIY (Do-It-Yourself) approach.
Water quality and the management of nitrogenous compounds are of key importance within aquaculture. In this paper, automated and portable analysers for in situ analysis of nitrite and nitrate in water were deployed in freshwater and saline recirculating aquaculture systems (RAS). The analysers were based upon ion chromatography (IC) and employed a NaCl eluent with an anion exchange guard column for low backpressure anion separation, in combination with selective 235 nm ultra-violet light-emitting diode (UV-LED) based absorbance detection. The analysers were monitored and delivered real-time concentration data using a cellular internet of things (IoT) module and cloud-based dashboard. Overall performance and chromatographic repeatability were tested across various temperature profiles and 500 sequential runs within the laboratory. Deployments in freshwater and saline RAS, with concentrations ranging between 0.1–3.6 mg L⁻¹ nitrite and 0.6–392 mg L⁻¹ nitrate, were successful and the analytical performance was comparable to that of accredited lab-based instrumentation.
A gas-phase absorption detector (GPAD) was developed using a surface-mount 235 nm light-emitting diode (LED) as the light source. Modules for housing the LED and photodiode were three-dimensional printed. A stainless-steel tube was used as the detection cell. To perform long path absorption detection, a fused silica full-ball lens (5 mm diameter) was employed to collimate the light produced by the UV-LED with a viewing angle of 120°. The light attenuation coefficient (αb) was measured to be 0.025±0.001 cm⁻¹. The optimum path length with the maximum signal-to-noise ratio was obtained at ca. 80 cm. This result agreed with the theoretical calculation for optimum path length, i.e., 2/αb. The GPAD was coupled with an automated analysis system and applied to detect nitrite in water. The automated system added citric acid and ethanol into a water sample, and nitrite was converted to gaseous nitrous acid (HONO), which has a strong absorption at 235 nm. The produced HONO was purged with air, followed by water removal, and then detected with the GPAD. The method was used to detect nitrate and total dissolved nitrogen (TDN) in water as well. Nitrate was reduced to nitrite through a photo-induced reaction with a conversion efficiency of ca. 60%. TDN was converted to nitrate by using an alkaline persulfate digestion method, followed by photoreduction to nitrite, and detected by using the automated system. The limits of detection were 0.4 μM, 3.9 μM, and 6.7 μM (94 μg N/L) for nitrite, nitrate, and TDN, respectively.
An off-grid, internet-of-things (IoT) connected ion chromatography analyser was developed for the real-time automated measurement of nitrate and nitrite levels in remote surface water applications. The system used KOH eluent with an AG15 guard column to achieve anion separation, in combination with a selective 235 nm UV-LED detector. A self-cleaning 3D printed Sediment Trap was developed to filter the sample before analysis, preventing silt and sediment from causing blockages and facilitating robust performance during long-term deployments. Laboratory tests showed reductions of up to 91 % of total suspended solids and 61 % of turbidity within the sample after a settling time of 5 minutes. In controlled temperature tests (7 to 40°C), the average error in the detected concentration levels remained below 5 %, indicating robust and repeatable performance of the analyser in varying environmental conditions. Two automated in-situ analysers were deployed at remote locations in West Cork, Ireland, for a period of one month. The deployments successfully demonstrated the capability of the system to detect transient pollution events, facilitating improved water-quality management of remote catchments. The analysers were monitored in real-time using a cellular IoT module and cloud-based dashboard.
Nowadays, miniaturization and portability are crucial characteristics that need to be considered for the development of water monitoring systems. In particular, the use of handheld technology, including microfluidics, is exponentially expanding due to its versatility, reduction of reagents and minimization of waste, fast analysis times and portability. Here, a hybrid handheld miniaturized polymer platform with a paper-based microfluidic device was developed for the simultaneous detection of nitrite and nitrate in real samples from both, fresh and seawaters. The platform contains an ionogel-based colorimetric sensor for nitrite detection and a paper-based microfluidic device for the in situ conversion of nitrate to nitrite. The platform was fully characterized in terms of its viability as a portable, cheap and quick pollutant detector at the point of need. The calibration was carried out by multivariate analysis of the color of the sensing areas obtained from a taken picture of the device. The limits of detection and quantification, for nitrite were 0.47 and 0.68 mg L⁻¹, while for nitrate were 2.3 and 3.4 mg L⁻¹, found to be within the limits allowed by the environmental authorities, for these two pollutants. Finally, the platform was validated with real water samples, demonstrating its potential to monitor nitrite and nitrate concentrations on-site as a first surveillance step before performing extensive analysis.
Authentication and quality control of complex samples, such are natural products, presents significant challenges in analytical chemistry. Traditionally, authentication and quality control are performed through targeted approaches (profiling), where for each natural product several quality marker compounds are determined. In contrast, in natural product metabolomic studies, the whole signal recorded with analytical instrumentation can be used. This signal represents the fingerprint for the analysed natural product and contains information of the whole metabolome of the examined natural product. Fingerprints can be recorded either applying solely spectroscopy, mass spectrometry (MS), nuclear magnet resonance (NMR) detection systems or with hyphenated analytical techniques, such are gas chromatography hyphenated with MS or flame ionisation detector (FID), liquid chromatography hyphenated with MS or UV-Vis, etc... Due to complexity and high degree of dimensionality of recorded signals in analytical chemistry, multivariate statistical techniques are utilised to reveal patterns in the recorded signals. While multivariate approaches applied to hyphenated chromatography analytical systems allow data mining and identification of metabolites contributing to the sample discrimination and quality, multivariate approaches applied solely to detection systems offer other advantages, such as fast quality control and authentication of natural products with high sample throughput. Also, application of spectroscopic techniques in metabolomic studies allows development of miniaturised and customised devices for in field and on-site analysis.
This thesis explores different analytical techniques, multivariate approaches and machine learning algorithms for the purpose of simplifying and increasing prediction accuracy in natural product metabolomic studies for purpose of characterisation, authentication and quality control of natural products. Through 4 experimental chapters the following areas were investigated:
1. Development and comparison of different data reduction procedures on gas chromatography hyphenated with electron impact mass spectrometry (GC-EI-MS) data to increase the performance of multivariate statistical approaches in essential oil (EO) authentication, quality control and biological activity prediction.
2. Application of Random Forests machine learning algorithms in the detection of adulterated and natural products with lower quality based on analysis performed with GC-EI-MS and handheld Raman spectroscopy.
3. Characterisation, quality control and authentication, of “extremely complex samples” such are natural product blends and high-value perfumes by determining quality of samples used in their creation.
4. Development of a PLS model based on spectra recorded using an LED-based spectrophotometer for monitoring primary and secondary products in the FuracellTM process.
GC-EI-MS is a most common technique for the analysis of volatile natural products. Single sample, analysed by GC-EI-MS produces a three-way data array, as a function of time, m/z and their intensities. By analysing multiple samples, four-way arrays are created. Most of the multivariate statistical tools cannot handle four-way data arrays and further data reduction is required. The first experimental chapter of this thesis examines and compares three different GC-EI-MS data reduction procedures applied for the purpose of natural product authentication, quality control and prediction of biological activity. The first strategy, and at the same time most commonly applied, is by summing all of the m/z fragments in a single mass scan and plotting them against the time of the mass scan, creating a total ion current chromatogram (TICC), out of which a chemical composition profile is obtained. The second approach is the averaging of the summed responses for each m/z fragment over the total number of scans, the whole time of the analysis, creating a total chromatogram average mass spectrum (TCAMS). In the third approach, GC-EI-MS three-way data array is divided into time dependent sub-windows, where for each sub-window the average mass spectrum (AMS) is calculated. At the end, the AMS of all windows are added into single data set, creating the segmented average mass spectrum (SAMS).
In the first experimental chapter, three strategies for GC-EI-MS data reduction were evaluated for the discrimination of ylang-ylang essential oils based on their distillation time and geographical origin. SAMS showed superior performance compared to the other two data reduction procedures, in principal component analysis (PCA), partial least squares regression (PLS) and discriminatory analysis (PLS-DA) for the prediction and discrimination of ylang-ylang distillation grades and geographical origins, respectively. Also, TCAMS and SAMS were utilised for fast quantification of main compounds in ylang-ylang EOs, without using internal standards. This enabled evaluation of quality of ylang-ylang EOs through comparison with the corresponding ISO standard.
In addition, a high-performance thin layer chromatography approach was utilised for the determination of radical scavenging activity (RSA) and identification of compounds contributing to the RSA. It was shown that increase in distillation time results in ylang-ylang EOs with higher RSA due to higher content of sesquiterpenes, α-(E,E)-farnesene and germacrene D, which are together with eugenol main contributors to the RSA. It was also shown that geographical origin has great influence on the RSA of ylang-ylang EOs. Recorded HPTLC profiles allowed discrimination of YY EO based on their geographical origin in PCA, and prediction of distillation grade utilising PLS.
In the case of prediction of RSA based on three different datasets created from GC-EI-MS data, PLS model created on SAMS showed lowest relative error of prediction (REP) and mean error of prediction (MEP). Data mining on the three datasets created from GC-MS raw data allowed identification of compounds contributing to the RSA. In comparison to the GC-based data sets, ATR-FTIR showed higher accuracy, having lower REP and MEP, as well as root mean square error of prediction (RMSEP). It was also demonstrated that a PLS model created on spectra recorded on smartphone-based handheld Raman spectrometer can be used for the determination of RSA. This is of great importance since analysis and evaluation of biological potency of EOs can be performed directly in-field without any sample pre-treatment.
The second experimental chapter of this thesis illustrates the procedure for the fast quality control of natural products, based on Random Forests machine learning algorithms. In the first part, the application of different GC-EI-MS data reduction procedures, TCAMS and SAMS, followed by Random Forests for the classification of twenty different classes of EOs, at the same time determining the samples with lower quality, was explored. In this work, SAMS showed better performance, where through a calculated proximity matrix it highlighted all EOs with lower quality. Random Forests, PLS-DA and PLS applied on spectra recorded on the smartphone-based portable Raman device, allowed discrimination of pure EOs from the adulterated ones where the adulterant was quantified based on the created PLS model. Also, Random Forests enabled identification of the adulterants.
The third experimental chapter of this thesis examined quality control of blends and mixtures created from several natural products. Application of TCAMS, together with multivariate curve resolution alternating least squares (MCR-ALS), allowed quality control of "extremely complex" samples, such are essential oil blends and perfume mixtures. Singular value decomposition (SVD) applied on TCAMS enabled determining the number of natural products used in creating blends. Resolved TCAMS, through applied Random Forest model, allowed identification of natural products used in creating the blends. Also, PCA on resolved TCAMS allowed determination of distillation grade and geographical origin of ylang-ylang essential oils used in creating high-value perfumes.
Optofluidics, understood as the synergistic combination between microfluidics and photonics, has been at the forefront of the scientific research due to its outmatching properties: on the one hand, microfluidics allows the handling of minute amounts of liquid samples at the microscale. On the other hand, photonics has proved to outmatch other detection methods (e.g. electrochemistry) in terms of sensitivity and selectivity. From the initial single analyte or spiked samples, currently the technology is mature enough for selective detection of a variety of analytes in raw, complex liquid samples. This will pave the way for the applicability of optofluidic devices for applications in the field or at the point of care. Here, we will revisit the current state of the art of optofluidic and photonic lab-on-a-chip systems for the analysis of real and biologically relevant samples: body fluids and water.
A novel biochar electrode Bio-Fe3O4/CF used for electroreduction of nitrate was prepared by the hydrothermal synthesis method. The results showed that the growth of spherical Fe3O4 on the surface of smooth biochar can significantly increase the nitrate reduction rate. Besides, the presence of Cl and Br in the solution could promote the conversion of NH4⁺ to N2, thereby regulating the element nitrogen in the solution. Mechanistic analysis showed that the interconversion of Fe (II) and Fe (III) facilitates the transfer of electrons to nitrate. This study not only provides a biochar electrode material for the efficient removal of nitrate but also simply reveals regulation of halogen in solution, which provides a particular theoretical and data basis for nitrate removal.
Light-emitting diodes (LEDs) could be a potential dosimetry candidate because they are radiation hard, spectrally selective, direct band gap, and low-cost devices. Thus, an LED-based detector prototype was designed and characterized for dosimetry. A 20 × 20 cm2 array of surface mount device LED chips was sandwiched in photovoltaic mode between two intensifying screens to form a dosimetric system. The system was enclosed in a light-tight air cavity using black vinyl tape. The screens converted diagnostic X-ray beams into fluorescent blue light. LEDs, applied in detector mode, converted the fluorescent light into radiation-induced currents. A digital multimeter converted the analog currents into digital voltage signals. Prototype characterization was executed using (a) IEC 61267's RQR 7 (90 kVp) and RQR 8 (100 kVp) beam qualities, and (b) low (25 mAs) and high (80 mAs) beam quantities. A standard dosimeter probe was simultaneously exposed with the prototype to measure the prototype's absorbed dose. In all exposures, the X-ray beams were perpendicularly incident on both the dosimeter and prototype, at a fixed source to detector distance-60 cm. The LED array prototype's minimum detectable dose was 0.139 mGy, and the maximum dose implemented herein was ~ 13 mGy. The prototype was 99.18 % and 98.64 % linearly sensitive to absorbed dose and tube current-time product (mAs), respectively. The system was ± 4.69 % energy, ± 6.8 % dose, and ± 7.7 % dose rate dependent. Two prototype data sets were 89.93 % repeatable. We fabricated an ultrathin (5 mm), lightweight (130 g), and a relatively low-cost LED-based dosimetric prototype. The prototype executed a simple, efficient, and accurate real-time dosimetric mechanism. It could thus be an alternative to the current passive dosimetric systems.
2D nickel phthalocyanine based MOFs (NiPc-MOFs) with excellent conductivity were synthesized through a solvothermal approach. Benefiting from excellent conductivity and a large surface area, 2D NiPc-MOF nanosheets present excellent electrocatalytic activity for nitrite sensing, with an ultra-wide linear concentration from 0.01 mM to 11 500 mM and a low detection limit of 2.3 μM, better than most reported electrochemical nitrite sensors. Significantly, this work reports the synthesis of 2D conductive NiPc-MOFs and develops them as electrochemical biosensors for non-enzymatic nitrite determination for the first time.
3D printing is fast evolving as an additive manufacturing technique that has been adopted in (bio)analytical science because of the ample variety of materials and technologies currently available for highly affordable prototyping. This review focuses on the unique characteristics of 3D printing for manufacturing of optical and electrochemical detection systems, and 3D printed sampling interfaces for analytical purposes using fused deposition modelling, vat polymerization (stereolithography and digital light processing) and photopolymer inkjet printing. The majority of works surveyed within the time span of mid-2018 to mid- 2020 encompassed the fabrication of several components of the detection systems, yet recent reports on totally printed electrochemical detectors are paving the way of 3D printing toward self-dedicated fully printed detectors. From the application viewpoint, the merits and weaknesses of the new sensing platforms as compared with commercially available detectors will be critically analyzed to uncover the actual advantages of using 3D printed materials and devices. Finally, the current state-of-the-art and future perspectives of this emerging technology for fabrication of unique detection systems are highlighted.
In recent years, a trend toward utilizing open access resources for laboratory research has begun. Open-source design strategies for scientific hardware rely upon the use of widely available parts, especially those that can be directly printed using additive manufacturing techniques and electronic components that can be connected to low-cost microcontrollers. Open-source software eliminates the need for expensive commercial licenses and provides the opportunity to design programs for specific needs. In this review, the impact of the “open-source movement” within the field of chemical separations is described, primarily through a comprehensive look at research in this area over the past five years. Topics that are covered include general laboratory equipment, sample preparation techniques, separations-based analysis, detection strategies, electronic system control, and software for data processing. Remaining hurdles and possible opportunities for further adoption of open-source approaches in the context of these separations-related topics are also discussed.
Contamination of water sources world-wide is increasing due to both anthropogenic and naturogenic causes. The ability to correctly monitor and sense levels of key contaminants is a critical step towards the implementation of water quality management strategies designed to reverse this trend and improve water status. Stricter regulatory frameworks and public concern are driving increasing demand for more effective ways to detect pollutants and contaminants within water bodies. This in turn is driving the development of reliable autonomous remote sensing platforms that are capable of providing continuous data on key analytes. The availability of such instruments at an affordable cost will facilitate monitoring at multiple locations, which will enable breaches of regulatory thresholds to be detected much more quickly than conventional approaches based on occasional sampling followed by analysis at centralised laboratory facilities. As a result, the potential damage caused by contamination can be significantly reduced due to much more rapid and effective responses. In this review we will discuss developments in these sensing technologies with a particular focus on nutrient sensing technologies. A comparison of current and emerging technologies will be presented, along with some thoughts on how to move towards much larger-scale, highly distributed monitoring of water quality.
A new miniaturised capillary flow-through deep-UV absorbance detector has been developed using a microscale surface mounted device- type light-emitting diode (LED) (Crystal IS OPTAN 3535-series), emitting at 235 nm and with a half-height band width of 12 nm, and a high-sensitivity Z-shaped flow-cell. Compared with a previously reported TO-39 ball lens LEDs emitting at 235 nm, the new generation LED produced a 20-fold higher optical output and delivered up to 35 times increase in external quantum efficiency (EQE). The Z-cell was based on a reflective rectangular optical path with cross-sectional dimensions of 100 × 100 µm and a physical optical pathlength of 1.2 mm. Inclusion of UV transparent fused-silica ball lenses, between the SMD and the Z-cell, improved light transmission by a factor of 9 and improved the detector signal-to-noise ratio by a factor of 2.2, at the same input current. The detector was housed within an Al-housing fitted with a cooling fan and demonstrated excellent linearity with stray light down to 0.06%, and an effective pathlength of 1.1 mm (92% of nominal pathlength). The resultant detector was fitted successfully into a briefcase-sized portable capillary HPLC system, and practically demonstrated with the detection of a mixture of 13 test compounds at the sub-mg L-1 level in <5 min using gradient elution.
A novel colourimetric method for the detection of nitrite in food samples using digital image colourimetry (DIC) by smartphone is developed. Nitrite directly oxidise 3,3′,5,5′‐tetramethylbenzidine (TMB) to form a yellow TMB diimine (oxTMB). A smartphone was used to capture the image of an inherent colour variation and analyse the image with the RGB (red, green, and blue) model, achieving the quantitative detection of nitrite. Linear response for the detection of nitrite was obtained from 10 μmol L−1 to 440 μmol L−1 and the limit of quantification was 2.34 μmol L−1. The applicability of the method was confirmed by the detection of nitrite in cabbage, pickle, and ham. The recovery was varied in the range from 96.2 % to 108.2 % with a relative standard deviation of less than 5.0 %. The simple, sensitive, and inexpensive method was an alternative for the detection of nitrite in food samples. Redox reaction between NO2− and TMB in acid condition for application in detection of nitrite based on digital image colourimetry by smartphone
The cell volume permissible for a specified degree of loss of efficiency can be computed from response volume considerations. For an open tubular column, the permissible illuminated length can be computed for the unretained peak. Similar estimations can be made for the maximum permissible cell volume with a packed column for a given column efficiency and flow rate. The packed column case does require an assumption on the degree to which the cell behaves as a mixer. An altogether different question is if the data from a long cell, with its considerable advantage in S/N, can be used and the associated dispersion mathematically removed. Experimental data for a variable path length (0-60 mm) HPLC detection cell indicate that an exponential model fits the observed dispersion. Once fit parameters are determined, the same can be applied to a peak, not part of the original training set, obtained in a longer path cell and the effects of dispersion mathematically reversed without major loss of S/N. Results with shorter path cell dispersion characteristics are then obtained with much higher S/N. A comparison is made with Fourier transform-inverse Fourier transform based deconvolution that can be used to achieve the same ends.
Optical detection setup utilizing light emitting diodes (LEDs), 50 nL L-shaped silica capillary detection cell (L-cell), and low-cost CCD spectrometer is described in this work. Experimental configuration can be equipped with two different LEDs for absorbance measurement and other two LEDs for fluorescence excitation. This setup is capable of simultaneous multi-wavelength monitoring of absorbance and fluorescence when light produced by the individual LEDs and light emitted by the fluorescent analytes is resolved in the spectrum outputted by the CCD spectrometer. Effective optical path of the 0.25 μm I. D. L-cell is 1 mm. Absorbance baseline noise is 1 mAU due to use of low-cost and relatively noisy CCD spectrometer and LED drivers. Nevertheless, the setup can detect adenosine 5´-monophosphate down to micromolar concentration. Performance of fluorescence monitoring allows detection of 5·10⁻¹⁰ M fluorescein when 23 mW 470 nm LED is used for excitation. The dynamic range of absorbance and fluorescence measurement is 867:1 and 1622:1, respectively. Separation of test mixture (alkylbenzenes and polyaromatic hydrocarbons) demonstrate the effective use of the detector for simultaneous absorbance and fluorescence detection with 0.2 × 150 mm packed capillary column. The benefits of the setup are relative simplicity, compact design and the fact that it can be operated without any optical filters, slits, and extremely precise positioning of the optical elements.
A sensing platform for the in situ, real-time analysis of phosphate in natural waters has been realised using a combination of microfluidics, colorimetric reagent chemistries, low-cost LED-based optical detection and wireless communications. Prior to field deployment, the platform was tested over a period of 55 days in the laboratory during which a total of 2682 autonomous measurements were performed (854 each of sample, high standard and baseline, and 40 × 3 spiked solution measurements). The platform was subsequently field-deployed in a freshwater stream at Lough Rea, Co., Galway, Ireland, to track changes in phosphate over a five day period. During this deployment, 165 autonomous measurements (55 each of sample, high standard, and baseline) were performed and transmitted via general packet radio service (GPRS) to a web interface for remote access. Increases in phosphate levels at the sampling location coincident with rainfall events (min 1.45 µM to max 10.24 µM) were detected during the deployment. The response was found to be linear up to 50 µM PO 4 3− , with a lower limit of detection (LOD) of 0.09 µM. Laboratory and field data suggest that despite the complexity of reagent-based analysers, they are reasonably reliable in remote operation, and offer the best opportunity to provide enhanced in situ chemical sensing capabilities. Modifications that could further improve the reliability and scalability of these platforms while simultaneously reducing the unit cost are discussed.
Microfluidic technology permits the miniaturization of chemical analytical methods that are traditionally undertaken using benchtop equipment in the laboratory environment. When applied to environmental monitoring, these "lab-on-chip" systems could allow high-performance chemical analysis methods to be performed in situ over distributed sensor networks with large numbers of measurement nodes. Here we present the first of a new generation of microfluidic chemical analysis systems with sufficient analytical performance and robustness for deployment in natural waters. The system detects nitrate and nitrite (up to 350 μM, 21.7 mg/L as NO(3)(-)) with a limit of detection (LOD) of 0.025 μM for nitrate (0.0016 mg/L as NO(3)(-)) and 0.02 μM for nitrite (0.00092 mg/L as NO(2)(-)). This performance is suitable for almost all natural waters (apart from the oligotrophic open ocean), and the device was deployed in an estuarine environment (Southampton Water) to monitor nitrate+nitrite concentrations in waters of varying salinity. The system was able to track changes in the nitrate-salinity relationship of estuarine waters due to increased river flow after a period of high rainfall. Laboratory characterization and deployment data are presented, demonstrating the ability of the system to acquire data with high temporal resolution.
Compact solid-state deep-ultraviolet (DUV) light-emitting diodes (LEDs) go far beyond replacing conventional DUV sources such as mercury lamps. DUV LEDs enable new applications for air, water, and surface sterilization and decontamination, bioagent detection and identification, UV curing, and biomedical and analytical instrumentation. We review materials growth, device physics, design, fabrication, and performance of DUV LEDs with wavelength ranging from 210 to 365 nm, describe prototype systems for water purification and sterilization, and discuss other emerging applications and systems using DUV LEDs.
We provide a global assessment, with detailed multi-scale data, of the ecological and toxicological effects generated by inorganic nitrogen pollution in aquatic ecosystems. Our synthesis of the published scientific literature shows three major environmental problems: (1) it can increase the concentration of hydrogen ions in freshwater ecosystems without much acid-neutralizing capacity, resulting in acidification of those systems; (2) it can stimulate or enhance the development, maintenance and proliferation of primary producers, resulting in eutrophication of aquatic ecosystems; (3) it can reach toxic levels that impair the ability of aquatic animals to survive, grow and reproduce. Inorganic nitrogen pollution of ground and surface waters can also induce adverse effects on human health and economy.
A miniaturised, flexible and low‐cost capillary ion chromatography system has been developed for anion analysis in water. The ion chromatography has an open platform, modular design and allows for ease of modification. The assembled platform weighs ca. 0.6 kg and is 25 × 25 cm in size. Isocratic separation of common anions (F–, Cl–, NO2–, Br– and NO3–) could be achieved in under 15 min using sodium benzoate eluent at a flow rate of 3 μL/min, a packed capillary column (0.150 × 150 mm) containing Waters IC‐Pak 10 μm anion exchange resin, and light‐emitting diode based indirect UV detection. Several low UV light‐emitting diodes were assessed in terms of sensitivity, including a new 235 nm light‐emitting diode, however, the highest sensitivity was demonstrated using a 255 nm light‐emitting diode. Linear calibration ranges applicable to typical natural water analysis were obtained. For retention time and peak area repeatability, relative standard deviation values ranged from 0.60–0.95 and 1.95–3.53%, respectively. Several water samples were analysed and accuracy (recovery) was demonstrated through analysis of a prepared mixed anion standard. Relative errors of –0.36, –1.25, –0.80 and –0.76% were obtained for fluoride, chloride, nitrite and nitrate respectively.
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A new high sensitivity deep-UV LED photometric detector with a z-type flow cell (45 nL or 180 nL) for miniaturised and portable capillary liquid chromatography (LC) was designed and fabricated to overcome sensitivity limitations due to short pathlength in on-capillary detectors. The new detector has a 10 mm geometric pathlength and uses high intensity light-emitting diodes (LED) as light sources in the deep-UV range (254 nm and 280 nm). No optical reference was necessary due to the low drift in the signal. Stray light was minimized by the use of an adjustable slit with a 0.5 mm pinhole. The direct relationship between absorbance and concentration was obtained using dichromate to evaluate the sensitivity and the linearity range of the detector. Performance of the miniaturised version was compared with that obtained from a commercial benchtop detector for capillary LC under the same conditions using the same optical z-cell. The miniaturised version exhibited a superior performance across all parameters, including 3 times higher effective pathlength, 4 times higher upper limit of detector linearity, and 2–5 times lower stray light levels. An application of the new detector was shown with the detection of l-dopa, l-tyrosine, norfenefrine, phenylephrine and tyramine, separated using capillary LC. The baseline noise level recorded was as low as 3.9 μAU. Further, the detector was applied in a miniaturised capillary LC for the indirect detection of common inorganic anions. In comparison to an on-capillary LED detector applied under similar chromatographic conditions, there was a 50 times higher signal to noise (S/N) ratio.
In this communication, we describe a flow-through optical absorption detector for HPLC using for the first time a deep-UV light-emitting diode with an emission band at 235 nm as light source. The detector is also comprised of a UV-sensitive photodiode positioned to enable measurement of radiation through a flow-through cuvette with round aperture of 1 mm diameter and optical path length of 10 mm, and a second one positioned as reference photodiode; a beam splitter and a power supply. The absorbance was measured and related to the analyte concentration by emulating the Lambert-Beer law with a log-ratio amplifier circuitry. This detector showed noise levels of 0.30 mAU, which is comparable with our previous LED-based detectors employing LEDs at 280 and 255 nm. The detector was coupled to a HPLC system and successfully evaluated for the determination of the anti-diabetic drugs pioglitazone and glimepiride in an isocratic separation and the benzodiazepines flurazepam, oxazepam and clobazam in a gradient elution. Good linearities (r > 0.99), a precision better than 0.85% and limits of detection at sub-ppm levels were achieved.
Natural freshwater systems have been severely affected by excess loading of macronutrients (e.g., nitrogen and phosphorous) from fertilizers, fossil fuels, and human and livestock waste. In the USA, impacts to drinking water quality, biogeochemical cycles, and aquatic ecosystems are estimated to cost US$210 billion annually. Field-deployable nutrient sensors (FDS) offer potential to support research and resource management efforts by acquiring higher resolution data than are currently supported by expensive conventional sampling methods. Following nearly forty years of research and development, FDS instruments are now starting to penetrate commercial markets. However, instrument uncertainty factors (high cost, reliability, accuracy and precision) are key drivers impeding the uptake of FDS by the majority of users. Using nitrite sensors as a case study, we review the trends, opportunities, and challenges in producing and implementing FDS from a perspective of innovation and impact. We characterize the user community and consumer needs, identify trends in research approaches, tabulate state-of-the-art examples and specifications, and discuss data life cycle considerations. With further development of FDS through prototyping and testing in real-world applications, these tools can deliver information for protecting and restoring natural waters, enhancing process control for industrial operations and water treatment, and providing novel research insights.
Various techniques for the determination of nitrite and/or nitrate developed during the past 15 years were reviewed in this article. 169 references were covered. The detection principles and analytical parameters such as matrix, detection limits and detection range of each method were tabulated. The advantages and disadvantages of various methods were evaluated. In comparison to other methods, spectrofluorimetric methods have become more attractive due to its facility availability, high sensitivity and selectivity, low limits of detection and low-cost.
In this work for the first time a sub-250 nm light-emitting diode (LED) is investigated as a light source for optical detection in chemical analysis. A 235 nm deep UV-LED is employed within an on-capillary photometric detector and applied in capillary ion-exchange chromatography (IEC) to the detection of common UV absorbing anions, here iodide, nitrate and nitrite. This investigation focused on fundamental properties of UV-LEDs, in particular emission spectra, radiometric power, effective heat dissipation with a passive heat sink, and energy conversion. The detection showed excellent linearity with stray light down to 0.6%, and effective pathlength at 92% of the used capillary inner diameter. The analytical performance parameters were demonstrated by detection of chromatographic separation of iodide in simulated seawater, showing limit of detection (LOD) of 1.30 µM, linear range 7.9 to 3,937 µmol L-1, and reproducibility as relative standard deviation (RSD) of peak height 0.6%, and peak area 0.7%. In addition, nitrite and nitrate were selected to study the potential of using deep UV-LEDs as light source in photometric detection for even lower wavelength absorbing analytes (λmaxnitrite 209 nm and λmaxnitrate 200 nm), showing reproducibility as RSD of peak height 1.2% and 3.6%, and peak area 0.9% and 2.9% respectively, and limit of detection 7 and 26 µmol L 1.
A 260 nm deep UV LED-based absorption detector with low detection limits was developed and integrated with a small nano-flow pumping system. The detector is small in size (5.2 × 3.0 cm) and weighs only 85 g (without electronics). This detector was specifically designed and optimized for on-column detection to minimize extra-column band broadening. No optical reference was included due to the low drift in the signal. Two ball lenses, one of which was integrated with the LED, were used to increase light throughput through the capillary column. Stray light was minimized by the use of a band-pass filter and an adjustable slit. Signals down to the ppb level (nM) were easily detected with a short-term noise level of 4.4 µAU, confirming a low limit of detection and low noise. The detection limit for adenosine-5'-monophosphate was 230 times lower than any previously reported values. Good linearities (three orders of magnitude) were obtained using sodium anthraquinone-2-sulfonate, adenosine-5'-monophosphate, DL-tryptophan and phenol. The LC system was demonstrated by performing isocratic separation of phenolic compounds using a monolithic capillary column (16.5 cm × 150 µm i.d.) synthesized from poly(ethylene glycol) diacrylate.
Light-emitting diodes (LEDs) are playing increasingly important roles in analytical chemistry, from the final analysis stage to photoreactors for analyte conversion to actual fabrication of and incorporation in microdevices for analytical use. The extremely fast turn-on/off rates of LEDs have made possible simple approaches to fluorescence lifetime measurement. Although they are increasingly being used as detectors, their wavelength selectivity as detectors has rarely been exploited. From their first proposed use for absorbance measurement in 1970, LEDs have been used in analytical chemistry in too many ways to make a comprehensive review possible. Hence, we critically review here the more recent literature on their use in optical detection and measurement systems. Cloudy as our crystal ball may be, we express our views on the future applications of LEDs in analytical chemistry: The horizon will certainly become wider as LEDs in the deep UV with sufficient intensity become available. Expected final online publication date for the Annual Review of Analytical Chemistry Volume 7 is June 15, 2014. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
A detector for miniaturized HPLC based on deep-UV emitting diodes and UV photodiodes was constructed. The measurement is accomplished by the transverse passage of the radiation from the light-emitting diode through fused-silica tubing with an internal diameter of 250 μm. The optical cell allows flexible alignment of the light-emitting diode, tubing and photodiode for optimization of the light throughput and has an aperture to block stray light. A beam splitter was employed to direct part of the emitted light to a reference photodiode and the Lambert-Beer law was emulated with a log ratio amplifier circuitry. The detector was tested with two light-emitting diodes with emission bands at 280 and 255 nm and showed noise levels as low as 0.25 and 0.22 mAU, respectively. The photometric device was employed successfully in separations using a column of 1 mm inner diameter in isocratic as well as gradient elution. Good linearities over three orders of magnitude in concentration were achieved, and the precision of the measurements was better than 1% in all cases. Detection down to the low μM range was possible. This article is protected by copyright. All rights reserved.
In this study, we describe a novel platform based on centrifugal microfluidics for simultaneous determination of nitrite, nitrate+nitrite, ammonium, orthophosphate, and silicate in water samples. All processes from sample metering to detection were integrated and automatically conducted on a rotating disc-shaped device. Fluid transfer was controlled by laser irradiation on ferrowax-based microvalves. Liquid samples and reagents were pumped by centrifugal force in the rotating disc, and their positions and movements were controlled through a programmable light from a laser diode. This novel water analysis platform required only 500 μL of sample (100 μL for each nutrient) and 10~30 μL of reagents for colorimetric detection. In addition, the fully automated parallel processes and efficient mixing in the rotating disc allowed for a significant reduction in total analysis time (~7 min 40 s) and increased accuracy. Validation with seawater-certified reference material indicated that the platform accurately measured nutrient concentrations in water samples. In addition, we show that the nutrient concentrations in seawater collected from Chunsoo Bay in Korea measured by the proposed lab-on-a-disc and by a commercialized auto-analyzer are comparable.
A rapid and low-cost technique is presented for the fabrication of optical quality microfluidic devices in poly(methyl methacrylate) (PMMA) or cyclic olefin copolymer (COC). When polymer microfluidic devices are manufactured by rapid prototyping techniques, such as micromilling, the surface roughness is typically in the region of hundreds of nanometres reducing the overall optical efficiency of many microfluidic-based systems. Here we demonstrate a novel solvent vapour treatment that is used to irreversibly bond microfluidic chips while simultaneously reducing the channel surface roughness, yielding optical grade (less than 15 nm surface roughness) channel walls. We characterize this vapour bonding method and optimize the process parameters to avoid channel collapse, while achieving reflow of polymer and uniformity of bonding. The reflow of polymer is the key to enabling a fabrication process that takes less than a day and produces optical quality surfaces with low-cost rapid prototyping tools.
The optical characteristics of on-capillary photometric detectors for capillary electrophoresis were evaluated and five commercial detectors were compared. Plots of sensitivity (absorbance/concentration) versus absorbance obtained with a suitable testing solution yield both the linear range and the effective path length of the detector. The detector linearity is a crucial parameter when using absorbing electrolytes, such as for indirect photometric detection, and especially for highly absorbing electrolyte probe ions. The upper limits of the linear ranges (determined as 5% decline in sensitivity) for five commercial detectors ranged from 0.175 to 1.2 AU. The effective pathlength reflects the quality of the optical design of the detector and is equal to the capillary internal diameter only for a light beam passing exactly through the capillary centre, but becomes progressively shorter for imperfect optical designs. The determined effective pathlength for the five investigated detectors ranged from 49.7 to 64.6 μm for a 75 μm I.D. capillary.
Ion chromatography (IC) is now a well established methodology for the analysis of ionic species. The technique is applicable to the determination of a wide range of solutes in many sample types, although the determination of inorganic ions in drinking water continues to be the most widely used application of ion chromatography. Many regulatory and standard organizations, such as ASTM, AOAC, ISO, and US EPA, have approved methods of analysis based upon IC, most of which have been published within the last 10 years. Recent developments in the field of IC, such as the use of higher capacity columns, larger loop injections, more complex sample preparation and detection schemes, have been incorporated into new approved methods to allow the determination of inorganic contaminants, such as bromate, perchlorate, and chromate, at low μg/l levels in drinking waters. IC appears certain to remain an important technique for drinking water analysis and new methods based on IC will continue to be developed as more inorganic contaminants become regulated at lower limits in the future.
A review of the strategies employed to facilitate the detection, determination and monitoring of nitrate and/or nitrite is presented. A concise survey of the literature covering 180 reports submitted over the past decade has been compiled and the relevant analytical parameters (methodology, matrix, detection limits, range, etc.) tabulated. The various advantages/disadvantages and limitations of the various techniques have been exposed such that the applicability of a technique developed for one type of matrix can be meaningfully assessed before attempting to transfer the technology to another.
It has long been known that the photolysis of nitrite and nitrate solutions results in the formation of OH radicals. The mechanism of NO3− photolysis has been the subject of considerable controversy in the literature, however. This review summarizes the experimental work on NO2− and NO3− photolysis in the context of recent advances in the understanding of the chemistry of the peroxynitrite anion (ONOO−) in biological experiments. ONOO− has been found to play a far more significant role in the overall reaction mechanism of NO3− photolysis than had previously been suspected. Research on NO2− and NO3− photolysis, as a pathway to the destruction of organic contaminants in natural waters, is summarized. The possible impact of NO2− and NO3− on Advanced Oxidation Technologies (AOTs), in which OH radicals are used to initiate the destruction of hazardous organic pollutants in drinking water and industrial waste streams, is explored.
A 255 nm deep-UV-light-emitting diode (deep-UV-LED) is investigated as a novel light source for photometric detection in view of fundamental properties of UV-LEDs, in particular emission spectra and energy conversion. Its performance in on-capillary photometric detection in capillary electrophoresis (CE) is determined and the potential of deep-UV-LEDs in optical detection is discussed.
Miniaturization of analytical devices through the advent of microfluidics and micro total analysis systems is an important step forward for applications such as medical diagnostics and environmental monitoring. The development of field-deployable instruments requires that the entire system, including all necessary peripheral components, be miniaturized and packaged in a portable device. A sensor for long-term monitoring of phosphate levels has been developed that incorporates sampling, reagent and waste storage, detection, and wireless communication into a complete, miniaturized system. The device employs a low-power detection and communication system, so the entire instrument can operate autonomously for 7 days on a single rechargeable, 12V battery. In addition, integration of a wireless communication device allows the instrument to be controlled and results to be downloaded remotely. This autonomous system has a limit of detection of 0.3mg/L and a linear dynamic range between 0 and 20mg/L.
The concept of micro total analysis systems (muTAS) or Lab-on-a-chip is based on the twin strategies of integration and miniaturisation that have been so successful in the electronics industry. This paper will look at the materials issues, particularly with respect to the new polymeric materials that are becoming available, and strategies for integrating optical (colorimetric) detection. The influence of breakthroughs in apparently unrelated areas on the range of chemistries that can be applied will be illustrated. For environmental monitoring, the further integration of wireless communications with micro-dimensioned analytical instruments and sensors will become the ultimate driving force. The emergence of these compact, self-sustaining, networked instruments will have enormous impact on all field-based environmental measurements.
A model which takes into account both stray light and polychromatic light was used to predict and evaluate linearity in on-capillary detection in capillary electrophoresis (CE). According to the model the stray light is the major factor which determines linearity under typical CE operating conditions. By calculating theoretical absorbance versus concentration plots, the influence of different levels of stray light and polychromatic light on linearity is demonstrated. Experimentally, six light-emitting diodes (LEDs) in the range from 563 to 654 nm were examined as light sources for on-capillary detection in CE. Fitting theoretical curves to measured linearity plots enabled determination of the values of both effective path length and stray light for a particular detection system. The detector linearity for the four LEDs was compared to mercury and tungsten lamps used with interference filters. For potassium permanganate as the test compound, the linear range for a 563 nm LED was two times greater than that for a mercury lamp operated at 546 nm. The relatively poor linearity of the mercury lamp detector is explained by its high level of stray light. The noise of the LED563-based detector was the same as for the mercury lamp, whereas the other LEDs of higher light intensity gave approximately half the noise of the mercury lamp. The lowest noise level of 3 x 10(-5) AU was obtained for the LED at 554 nm (determined at a detector time constant of 0.1 s).
A flow-through optical absorption detector for HPLC was constructed using a novel deep-UV light-emitting diode as radiation source with a peak emission wavelength of 255 nm. For measuring the transmitted intensity (a property correlated to Transmittance) a special UV-sensitive photodiode was employed. Besides the power source, no optical or electronic components other than an inexpensive operational amplifier and a few passive components were necessary. The performance of the detector was tested with three substances, namely nitrobenzene, benzoic acid and methyl benzoate, which were separated by gradient elution using an acetonitrile/water mixture and tetrabutylammonium hydrogensulfate as pH-buffer. Calibration curves for concentrations between 1.6 microg.mL(-1) and 400 microg.mL(-1) (nitrobenzene) and 8 microg.mL(-1) and 2.5 mg.mL(-1) (benzoic acid and methyl benzoate) were determined and coefficients of determination, r(2), of 0.9945, 0.9972 and 0.9996 were obtained for quadratic curve fits for the 3 compounds respectively. Relative standard deviations (n = 7) for peak areas were determined as 0.35% (nitrobenzene, 80 microg.mL(-1)), 0.27% (benzoic acid, 400 microg.mL(-1)) and 0.83% (methyl benzoate, 200 microg.mL(-1)). The lower limits of detection were found to be 750 ng.mL(-1), 5.8 microg.mL(-1) and 12 microg.mL(-1) for nitrobenzene, benzoic acid and methyl benzoate respectively.
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