Determination of Rhodamine 6G with direct immersion single-drop microextraction combined with an optical probe

The combination of an optical probe and single-drop direct immersion microextraction (DI-SDME-OP) was used for the preconcentration and subsequent spectrophotometric determination of rhodamine 6G (Rh6G). The developed method is based on the formation of an ionic associate between Rh6G and picric acid at pH 3.0 and its extraction with amyl acetate. A microdrop of the organic phase was stably placed in the hole of an optical probe immersed in the sample solution. The absorbance of the extraction phase was monitored at 534 nm. The proposed method combines in a single step several stages of the analytical procedure, such as pre-concentration, phase separation, transfer of the extraction phase to the instrument and online measurement. The sensitivity of the proposed approach is not inferior to existing microextraction methods involving the combination of liquid-phase or solid-phase extraction with spectrophotometry or HPLC with a UV-Vis detector. The evaluation of the greenness of the developed method carried out by the AGREE method (0.58 points) showed that it outperforms other similar existing techniques using this parameter. The calibration plot for the determination of Rh6G by the DI-SDME-OP method was linear over the range of 10-500 nM with a correlation coefficient of 0.9956. The limit of detection was 3.4 nM. The accuracy and applicability of the method were evaluated by the determination of Rh6G in natural waters and lipstick.

Direct immersion single-drop microextraction combined with fluorescence detection using an optical probe. Application for highly sensitive determination of rhodamine 6G

The use of an optical probe for fluorescence detection combined with direct immersion single-drop microextraction has been demonstrated as an innovative approach. The optical probe served both as a drop holder for extractant and as a measuring device which made it possible to eliminate the use of cuvettes. A laser and a light emitting diode (LED) were tested as possible light sources. Both of them showed comparable results. However, given the much smaller half-band width of the laser radiation, its use has proven to be preferable since background correction can be eliminated. Direct immersion single-drop microextraction of an ionic association complex of rhodamine 6G with picric acid with subsequent fluorescent detection (λex was 532 nm and 525 nm for laser and LED, respectively; λem was 560 nm for both laser and LED) was used a model system to evaluate the new approach. The extractant phase was a 55 μL amyl acetate microdrop fixed in the optical part of the probe. LOD, LOQ and linear calibration range were found as 0.14, 0.48 and 0.5-10 nmol L-1, and 0.15, 0.50 and 0.5-5 nmol L-1 for laser and LED light sources, respectively. The accuracy of the method was assessed by analyzing real water samples.

A new approach for sulfite determination by headspace liquid-phase microextraction with an optical probe

A new approach of headspace liquid-phase microextraction with an optical probe (HS-LPME-OP), which solves the problem of the extraction phase retention in the hole of the optical probe and provides the possibility of simpler, more precise and reliable online processing of the analytical signal, was used for sulfite determination. A 1 × 10^-4 M 5,5'-dithiobis-(2-nitrobenzoic) acid (DTNB) solution was used as an acceptor phase. It was placed in a plastic vial fixed in the headspace above the analyte solution. An optical probe immersed in the acceptor phase was used to monitor the analytical signal. Sulfite determination is based on the release of sulfur dioxide from the sample after the addition of ortho-phosphoric acid, followed by its extraction with an aqueous solution of DTNB at pH 7.0. The absorbance was measured at 412 nm. The calibration graph was linear in the range from 32 to 320 μg L^-1 with a detection limit of 14 μg L^-1. The developed method is sensitive, highly selective and reproducible. It was successfully applied for the sulfite determination in juice, alcoholic beverages and jam.

Online determination of sulfide using an optical immersion probe combined with headspace liquid-phase microextraction

A new design for headspace liquid phase microextraction in combination with an optical immersion probe (HS-LPME-OIP) was proposed and successfully tested for the determination of sulfide in wine and water samples. The developed method is based on the release of hydrogen sulfide from the aqueous phase after the addition of orthophosphoric acid and its extraction with an aqueous solution of 5,5′-dithiobis-(2-nitrobenzoic) acid (DTNB). The analytical signal was recorded using an optical probe immersed in a vial containing 200 μL of 0.1 mM DTNB solution. Using the optical immersion probe in combination with HS-LPME allowed to register the analytical signal online and significantly improve the reproducibility of sulfide determination compared to known microextraction approaches. In the proposed approach, the problems with drop stability, limitations in mixing rate or extraction time, too small volume of the acceptor phase and stability of the holding the acceptor phase in the hole of the optical probe were also satisfactorily solved. The calibration graph was linear in the range of 16–256 μg L⁻¹ with a correlation coefficient of 0.9992. The limit of detection was 6 μg L⁻¹.

A new design for headspace liquid phase microextraction combined with an optical probe.

In-vessel headspace liquid-phase microextraction

A new mode of headspace liquid-phase microextraction termed in-vessel headspace liquid-phase microextraction (IV-HS-LPME) has been developed. A plastic vessel was used as a holder for an extraction phase. Problems with drop stability, limitations in the stirring speed, and too little volume of the acceptor phase have been completely eliminated. The proposed approach is fully compatible with ordinary instruments and microcuvettes used in spectrophotometry. The potential of the method was evaluated by determining the iodide concentration in various samples. Iodide in the donor phase was converted to volatile I₂ by oxidation with 1 mmol L^-1 K₂Cr₂O₇. The reaction mixture was agitated on a magnetic stirrer for 30 min at a stirring speed of 1200 rpm. A 1% (w/v) aqueous solution of KI was used as the acceptor phase. The absorbance of the I₃^- ion formed in the acceptor phase was measured in a 50 μL microcuvette at 350 nm. For the case of extraction from 10 mL donor solution into 50 μL of acceptor phase, the calibration graph is linear in the range of 20-400 μg L^-1 (as I^-) with a detection limit of 6 μg L^-1. The developed method has a high precision comparable to conventional spectrophotometric methods (0.6-1.5%). The extraction efficiency obtained in the optimal conditions was 10.5%. The distribution constants for equilibria between the donor solution and the headspace and between the headspace and the acceptor solution are 0.8 ± 0.1 and 16 ± 2, respectively. The developed method was successfully applied to determine the iodine content in natural waters, medicines and algae.

Surface nanodroplet-based nanoextraction from sub-milliliter volumes of dense suspensions

A greener analytical technique for quantifying compounds in dense suspensions is needed for wastewater and environmental analysis, chemical or bio-conversion process monitoring, biomedical diagnostics, and food quality control, among others. In this work, we introduce a green, fast, one-step method called nanoextraction for extraction and detection of target analytes from sub-milliliter dense suspensions using surface nanodroplets without toxic solvents and pre-removal of the solid contents. With nanoextraction, we achieve a limit of detection (LOD) of 10-9 M for a fluorescent model analyte obtained from a particle suspension sample. The LOD is lower than that in water without particles (10-8 M), potentially due to the interaction of particles and the analyte. The high particle concentration in the suspension sample, thus, does not reduce the extraction efficiency, although the extraction process was slowed down up to 5 min. As a proof of principle, we demonstrate the nanoextraction for the quantification of model compounds in wastewater slurry containing 30 wt% solids and oily components (i.e. heavy oils). The nanoextraction and detection technology developed in this work may be used in fast analytical technologies for complex slurry samples in the environment, industrial waste, or in biomedical diagnostics.

A fully automated system with an optical immersion probe (OIP) for high-precision spectrophotometric measurements

A new and fully automated system with the interconnection of an Optical Immersion Probe (OIP) - pH meter - peristaltic pump was used to study the spectral and protolytic properties of carbocyanine the dyes 1,1',3,3,3',3'-hexamethylindocarbocyanine chloride (HIC); 1,1',3,3,3',3'-hexamethylindodicarbocyanine iodide (HIDC); and 3,3'-diethyloxadicarbocyanine iodide (DODC). This system can measure a large number of experimental points in a short time period. The effect of 32 various organic solvents on the UV-ViS spectra of the dyes was studied. The solvatochromic behaviour of studied dyes was characterized by positive solvatochromism for HIDC and negative solvatochromism for HIC and DODC. Through the application of a large number of experimental points, the protonation and hydrolysis constants of dyes were determined with high precision, where the confidence interval of the рK values is ±(0.001-0.005), compared with a confidence interval of ±(0.04-0.10) for standard procedures. The fully automated system presented is accurate, fast, environmentally friendly and promising for multiple analytical applications.

Direct immersion single-drop microextraction of semi-volatile organic compounds in environmental samples: A review

Single-drop microextraction (SDME) techniques are efficient approaches to pretreatment of aqueous samples. The main advantage of SDME lies in the miniaturization of the solvent extraction process, minimizing the hazards associated with the use of toxic organic solvents. Thus, SDME techniques are cost-effective, and represent less harm to the environment, subscribing to green analytical chemistry principles. In practice, two main approaches can be used to perform SDME - direct immersion (DI)-SDME and headspace (HS)-SDME. Even though the DI-SDME has been shown to be quite effective for extraction and enrichment of various organic compounds, applications of DI-SDME are normally more suitable for moderately polar and non-polar semi-volatile organic compounds (SVOCs) using organic solvents which are immiscible with water. In this review, we present a historical overview and current advances in DI-SDME, including the common analytical tools which are usually coupled with DI-SDME. The review also focuses on applications concerning SVOCs in environmental samples. Currents trends in DI-SDME and possible future direction of the procedure are discussed.

Development of a New Kinetic Spectrophotometric Method for the Determination of Chromium with an Optical Probe

The aim of this work is the development, optimization, and validation of a new spectrophotometric kinetic method for the determination of dissolved chromium species in water samples with the use of the polymethine dye Astra Phloxine FF. The progress of the chemical reaction was simple, effective, and precisely monitored from the start of the reaction using an optical probe. The method is based on the impact of Cr(VI) concentration on the rate of decrease in the Astra Phloxine FF concentration. The experimental data were evaluated using four experimental data analysis methods, namely with the initial rate method, the average rate method, the fixed time method, and the absorption peak volume change method. Under the optimal reaction conditions, the best results were achieved using the method of the average rate constant for evaluating the experimental data. Using this data evaluation method for the determination of Cr(VI), the LoD was found to be 1.87 µg L^-1 and RSD ( n = 6; 0.2 mg L^-1 Cr) 3.59%. The presented work was used for the determination of chromium in model samples-CRM material and tap and waste water-and with the calibration line method and the standard additions method.

Miniaturized sample preparation methods for saliva analysis

Saliva, as the first body fluid encountering with the exogenous materials, has good correlation with blood and plays an important role in bioanalysis. However, saliva has not been studied as much as the other biological fluids mainly due to restricted access to its large volumes. In recent years, there is a growing interest for saliva analysis owing to the emergence of miniaturized sample preparation methods. The purpose of this paper is to review all microextraction methods and their principles of operation. In the following, we examine the methods used to analyze saliva up to now and discuss the potential of the other microextraction methods for saliva analysis to encourage research groups for more focus on this important subject area.