Continuous flow thin layer headspace (TLHS) analysis

Summary A new method is presented for the continuous determination of volatile organic halogen (VOX) in water. It is based on isolation of the volatile analytes (VA) from the aqueous sample by means of the thin-layer headspace (TLHS) technique, mineralization of the VA in an empty quartz tube, and subsequent determination of the formed halogen ions absorbed in a liquid sorbent by means of an indirect spectrophotometric method using the iron thiocyanate complex. The method proved to be selective; no interferences of other elements at the levels encountered in practice have been found. Due to this fact and to the sensitivity of the method (of the order of 0.1 g/g) it is well suited for the determination of the VOX level in waste waters. The theory of the continuous flow TLHS analysis has been further developed. The results of mathematical analysis of the derived general equation for the concentration of the VA in the liquid sorbent are presented.Part I see [1]     

Continuous flow thin layer headspace (TLHS) analysis

Summary The theoretical basis of the optimization of the TLHS analysis has been further developed. A critical value of the enrichment factor (F_{cr, inf}) has been defined as the value, at which the relative systematic error and the relative precision error are equal. These errors have been defined with respect to the maximum and the inflexion point of the recovery curve of the volatile analyte in the liquid sorbent. Two methods of volatile organic halogen (VOX) determination in tap water, viz. the conductometric and the indirect potentiometric method, have been compared (with respect to their analytical characteristics) using natural samples. The accuracy of both methods proved to be similar, yet the conductometric method was more precise. The variances of the methods occurred to be homogeneous in the particular measuring series independently of the long time intervals between the particular measuring series and the differentiated VOX levels.Part IV see [4]     

Continuous flow thin-layer headspace (TLHS) analysis

Summary A new method of a continuous determination of volatile organohalogens in water is presented, based on the so-called thin layer headspace (TLHS) technique. Volatile analytes are isolated from the aqueous phase in a thermostatted spiral tube (TLHS column), where the sample flows in a form of a thin film countercurrently to a stream of purified air. The isolated compounds are mineralized in an empty quartz tube at 900°C, and the mineralization products are washed with triply distilled water. The conductivity of the wash solution is proportional to the VOX concent. The detection limit is of the order of a few ppb. The effect of the possible interfering substances has been examined and a method for the elimination of the effect of sulfur is presented. The results of the analysis of natural tap water samples are given. The theory of the THLS process is discussed and has been verified experimentally.     

Continuous flow thin-layer headspace (TLHS) analysis

Summary A comparison is presented of the accuracy of the conductometric and indirect potentiometric methods of VOX determination in tap water (using the TLHS isolation technique) with gas chromatographic results obtained by direct aqueous injection and electron capture detection (DAI-ECD). The agreement was very good. The conductometric method has been further examined by comparing the results of VOX determination obtained in the continuous mode with the gas-chromatographic results. Also in this case, the agreement was satisfactory, proving that the conductometric method can be successfully applied for monitoring the VOX level in tap waters. The theory of the TLHS technique has been further developed, enabling a simple determination of the critical values of some important parameters. The expression b_cr,inf(2·2)²·(3·3)^–3 approximates the critical value of the synthetic parameter b with an error not exceeding 8·10^–6. Direct practical utilization of the derived dependences for the optimization of the analysis is possible, provided that the values of the effective partition coefficients of the analytes and their mineralization products under given experimental conditions are known.Part I–V see [1–5]     

Continuous flow analysis of lead (II) and mercury (II) with substituted diazacrown ionophore membrane electrodes

A novel flow cell for use with ion-selective membrane electrodes is reported in which the carrier stream is drawn through a tube that suppresses the pump noise. PVC membrane electrodes based on 7,16-dithenoyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTODC), and 7,16-di-(2-thiopheneacetyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTAODC) for lead (II), and 7,16-dithenyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTDC) and 7,16-di-(2-methylquinolyl)-1,4,10,13-tertraoxa-7,16-diazacyclootadecane (DQDC), for mercury (II) were prepared and evaluated. The linear ranges were pPb: 5.5-3.0 (DTODC) and 6.0-2.0 (DTAODC); pHg: 5.5-3.0 (DTDC) and 4.5-2.5 (DQDC). With flow rate of 3 ml min(-1) the repeatability of measurements was less than 5% RSD (n = 3). The system was applied to the determination of lead (II) and mercury (II) in spiked natural water samples.     

Gas flow headspace liquid phase microextraction

There is a trend towards the use of enrichment techniques such as microextraction in the analysis of trace chemicals. Based on the theory of ideal gases, theory of gas chromatography and the original headspace liquid phase microextraction (HS-LPME) technique, a simple gas flow headspace liquid phase microextraction (GF-HS-LPME) technique has been developed, where the extracting gas phase volume is increased using a gas flow. The system is an open system, where an inert gas containing the target compounds flows continuously through a special gas outlet channel (D = 1.8 mm), and the target compounds are trapped on a solvent microdrop (2.4 μL) hanging on the microsyringe tip, as a result, a high enrichment factor is obtained. The parameters affecting the enrichment factor, such as the gas flow rate, the position of the microdrop, the diameter of the gas outlet channel, the temperatures of the extracting solvent and of the sample, and the extraction time, were systematically optimized for four types of polycyclic aromatic hydrocarbons. The results were compared with results obtained from HS-LPME. Under the optimized conditions (where the extraction time and the volume of the extracting sample vial were fixed at 20 min and 10 mL, respectively), detection limits (S/N = 3) were approximately a factor of 4 lower than those for the original HS-LPME technique. The method was validated by comparison of the GF-HS-LPME and HS-LPME techniques using data for PAHs from environmental sediment samples.     

Validation of thin layer and high performance thin layer chromatographic methods

Analytical validation is a key requirement to asses and to prove a method's reliability and suitability for an intended use. Planar chromatographic procedures are used in different applications ranging from simple screening tests to sophisticated instrumental quantitative assays of analytes in complex matrices. This paper intends to give guidance on how to adopt international accepted formal requirements and guidelines for validation of these different TLC/HPTLC procedures. In addition, some selected parameters for robustness testing and for on going quality assurance of analytical performance based on control charts are reported.     

Continuous flow analysis: Photometric determination of Cr(III) with chrome Azurol S using microwave treatment

The effect of microwave and ultraviolet radiation and ultrasonic treatment on the reaction of chromium(III) with Chrome Azurol S, Arsenazo I, Alizarin, and Thoron was studied in the batch and flow modes. It was found that the reaction of chromium with the above photometric reagents is most efficiently activated by microwave radiation of the power 500-200 Wt. The best analytical properties were found for Chrome Azurol S. A flow system was proposed for the photometric determination of chromium(III) with Chrome Azurol S using microwave treatment. The throughput of the system is 68 samples per hour, the analytical range for chromium(III) is 0.03-60 mg/L. Na, K, Mg, Ca, Zn, Cd, Pb, C1^-, SO ₄ ^2- , NO_3/-, CH₃COO^- in 1000-fold amounts; Cu(II) and F^- in 500-fold amounts; Fe(III) in a 10-fold amount; and Cr(VI) in a sevenfold amount do not interfere with the determination of Cr(III). Procedures for the photometric determination of chromium under batch conditions were developed. The accuracy of the developed procedures was verified in the analysis of tanning agents and dyes for leather. Deceased.     

Preparation of bismuth telluride thin film by electrochemical atomic layer epitaxy (ECALE)

Thin-layer electrochemical studies of the underpotential deposition (UPD) of Bi and Te on cold rolled silver substrate have been performed. The voltammetric analysis of underpotential shift demonstrates that the initial Te UPD on Bi-covered Ag and Bi UPD on Te-covered Ag fitted UPD dynamics mechanism. A thin film of bismuth telluride was formed by alternately depositing Te and Bi via an automated flow deposition system. X-ray diffraction indicated the deposits of Bi₂Te₃. Energy Dispersive X-ray Detector quantitative analysis gave a 2: 3 stoichiometric ratio of Bi to Te, which was consistent with X-ray Diffraction results. Electron probe microanalysis of the deposits showed a network structure that results from the surface defects of the cold rolled Ag substrate and the lattice mismatch between substrate and deposit. Translated from Chinese Journal of Applied Chemistry, 2005, 22 (11) (in Chinese)     

Porous layer flow injection analysis with potentiometric chloride detection

A porous paper stream with propulsion by a wider porous paper downstream 'pump' provided constant flow rate. Microliter size droplets touched to the surface served as sample injections. Dispersion was controlled by injection position. Potentiometric detection of chloride showed a range of 0.1 mM to 1M. Fifty or more samples per hour are possible.     

Thin layer distillation for matrix isolation in flow analysis

Analyte transfer from the matrix in a thin layer distillation (TLD) cell and its subsequent measurement were investigated in a flow injection configuration. We designed the cell such that the donor and acceptor streams flowed in parallel channels separated by a thin dividing wall. The matrix transfer process involved room-temperature distillation of the analyte into the headspace of the TLD cell and its subsequent condensation/uptake by a concurrently flowing acceptor stream. There are no membranes; hence there are no membrane-related problems. The TLD system design was optimized with respect to its dimensions and operational parameters. Throughput and sensitivity were compared with a conventional pervaporation flow injection (PFI) system for ammonia and five different amines. For the higher molecular mass amines, the TLD approach provided comparable or superior performance. The TLD technique should be an attractive approach for online analysis of volatile chemical species in ‘dirty’ samples, especially for volatile analytes of higher molecular mass.     

Nucleation kinetics of heterogeneous catalyst: Pd/thin layer of MgO (100)

The kinetics describing the formation of the system Pd/thin layer of MgO (100) surface was investigated by applying programs developed on the basis of Fortran Software. The simulation is based on studies related to nucleation, crystallite growth, coalescence and diffusion of clusters on the surface of thin films. The density of Pd islands is obtained by simulating the deposition of 1 × 10¹³ atoms cm^–2 s^–1 in 150 s. The density of clusters first increases to reach a plateau. Then, it either remains some time at a nearly constant value or decreases slowly after the saturation regime gives way to the coalescence stage. This phenomenon is explained via island migration process at the surface. The coalescence time strongly depends on the deposition temperature, the coalescence occurs when the substrate temperature is high. Also, the surface coverage decreases when the substrate temperature increases. The coalescence is pronounced at a low surface coverage. It involves the migration on the surface, defined as the dynamic coalescence and precedes the process of the formation of immobile islands that predominates at high extents of surface coverage.     

Smart thin layer chromatography plate

We present an innovative thin layer chromatography plate, which integrates a linear array of amorphous silicon photodiodes for real-time qualitative and quantitative chromatographic analysis.