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.

Headspace Microextraction. A Comprehensive Review on Method Application to the Analysis of Real Samples (from 2018 till Present).

This work describes current trends in the development of headspace microextraction methods. The main trends in the selection of detection techniques used in combination with microextraction and preferences in the selection of headspace liquid-phase microextraction (HS-LPME) or headspace solid-phase microextraction (HS-SPME) methods, depending on the analytes and their quantity, are also briefly presented. In the main part of the work, on the basis of current journal literature, headspace microextraction analytical methods used for the determination of various inorganic and organic analytes are classified and compared over the last five years. The work also reflects the current modifications of techniques and approaches proposed for these microextraction methods.

Trends in the development of headspace microextraction methods are discussed.Trends in choosing detector techniques after microextraction are also presented.Analytes determination with headspace microextraction is classified and compared.

Consecutive spectrophotometric determination of phosphate and silicate in a sequential injection lab-at-valve flow system.

A new highly sensitive and selective sequential injection lab-at-valve spectrophotometric method for the consecutive determination of silicate and phosphate is described. The proposed method is based on the formation of specific ion-association complexes (IAs) of 12-heteropolymolybdates of phosphorus and silicon (12-MSC) with Astra Phloxine. The addition of an external reaction chamber (RC) to the SIA manifold made it possible to significantly improve the conditions for the formation of the analytical form used. The formation of the IA took place in the RC; the solution is mixed by passing an air flow through it. The interfering effect on the determination of phosphate from silicate was completely eliminated by choosing an acidity at which the rate of 12-MSC formation is very low. The use of secondary acidification in the determination of silicate led to the complete exclusion of the influence of phosphate. The tolerable ratio of phosphate to silicate and vice versa is about 100-times, which allows the analysis of most real samples without the use of masking agents or complex separation steps. The determination ranges are 3.0-60 µg L-1 for phosphate as P(V) and 2.8-56 µg L-1 for silicate as Si(IV) at a throughput of 5 samples h-1. The detection limits are 5.0 and 3.8 µg L-1 for phosphate and silicate, respectively. Silicate and phosphate were determined in the tap water, river water, mineral water, main water of the Krivoy Rog (Ukraine) region, and the certified reference material of carbon steel.

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.