Photometric Detection Research Articles

Group-Type Analysis of Hydrocarbons and Sulfur Compounds in Thermally Stressed Merox Jet Fuel Samples

Methods to investigate the response of hydrocarbons and sulfur compounds to different extents resulting from thermal oxidation of jet fuels refined via the mercaptan oxidation (Merox) process have been developed. Relative comparison of hydrocarbon contents including n-paraffins, isoparaffins, olefins, naphthenes, and aromatics (PIONA) was performed by using comprehensive two-dimensional gas chromatography hyphenated with a quadrupole-accurate mass time-of-flight mass spectrometer (GC × GC–Q-TOFMS). A simple approach for quantification of sulfur compound groups was then developed using both GC × GC and GC hyphenated with flame photometric detection (GC × GC–FPD; GC–FPD). The approach largely separated the sulfur compounds into two distinct regions comprising of lower polarity S1 (thiophenes, thiophenols, thiols, disulfides, sulfides, and alkyl thianaphthenes) and higher polarity S2 (sulfones and phenylsulfides) groups. The reliability of this approach was evaluated using higher resolution GC × GC–FPD, show...

Determination of tributyl phosphate in the air of workplace by gas chromatography

Objective: To establish a practical method forsampling and detectingtributyl phosphate intheworkplace. Methods: The samples were extracted by glass fiber membrane, eluted with ether, separated by gas chromatography, and detected by flame photometric detector. Results: There were good linear relationship in the minimum detection concentration was 7.2-720.0 μg/ml, and the correlation coefficient was 0.999 92. The detection limit was 0.86 μg/ml, and the minimum detection concentration was 0.14 mg/m(3) (sample volume was 30 L) . Recovery rates were 99.8%-100.2%. The with-in relative standard deviations were 4.0%-5.4% and the between relative standard deviations were 2.0%-5.5%, and average samplingefficiency was about 99.1%-100.0%. Conclusion: This method conforms with the requirements of "Standardization of Methods for Determination of Toxic Substance in Workplace" . Tributyl phosphate in air could be determined accurately using this method.

High power deep UV-LEDs for analytical optical instrumentation

Optical detection systems for portable analytical instrument require commercially available, miniaturized, robust and well performing light sources. Light emitting diodes (LEDs) have been extensively used in portable analytical devices as they are exceptionally stable light sources offering a number of advantages over traditional light sources including robustness, small size and low-cost. In the analytically more significant deep UV spectral range, LEDs suffer from low optical power and parasitic emissions in the visible wavelength range. Recently, newly developed deep UV-LEDs based on aluminium nitride (AlN) substrates have been introduced claiming improved performance, however there is little in terms of systematic evaluation of their performance parameters when used as light sources in chemical sciences. The aim of this work was to investigate the performance of these new generation deep UV‐LEDs, and assess their analytical applicability. Three AlN substrate based LEDs (OPTAN255H, OPTAN255J and OPTAN280J) were selected based on their maximum emission wavelength and maximum optical power. Their emission spectra, optical output power values and energy conversion efficiency (electrical/optical) were investigated. This work shows effects of forward current and voltages on the ratio of intensities of the visible parasitic emission to the desirable deep UV emission. Experimental results show that while the parasitic emission still exists with the new generation deep UV-LEDs, with increasing forward current the ratio of undesirable parasitic emission to the main deep UV emission rapidly decreased to values as low as 0.1% at the maximum forward current (100 mA). The new generation deep UV-LED (255 nm) was then applied as a light source for a photometric on-capillary detection, showing excellent linearity with stray light down to 0.8%, and effective pathlength above 92% of the used capillary inner diameter, and finally the performance was demonstrated by detection of four parabens separated by miniaturised capillary liquid chromatography.

Ionogel-Based Ionic Liquid Coating for Solid-Phase Microextraction of Organophosphorus Pesticides from Wine and Juice Samples

In this work, a novel fiber coating was prepared by using a hybrid material of silica-based ionogels and ionic liquid 1-hexadecyl-3-methylimidazolium bis(trifuluoromethylsulfonyl)imide ([C16MIM][NTf2]). Instead of chemical bonding to sol-gel matrix, high loadings of [C16MIM][NTf2] was confined into the network of silica-based ionogels and thus, the extraction efficiency was significantly improved. The prepared fiber was used for solid-phase microextraction (SPME) of four organophosphorus pesticides (OPPs) from wine and juice samples prior to gas chromatography coupled with flame photometric detection. Under the optimized conditions, the proposed method results in good linearity (R 2 > 0.9991), with low method detection limits (MDLs, S/N = 3) and method quantification limits (MQLs, S/N = 10) in the range of 0.57–4.13 and 1.89–13.76 ng L−1, respectively. The precisions of the method were assessed with repeatability and reproducibility of RSD% ˂28 and ˂23%, respectively. The spiked recoveries of OPPs from wine and juice samples were in the range of 54 ± 5–118 ± 20 and 56 ± 14–109±12%, at two different concentration levels. The expanded uncertainties at 1 μg L−1 were below 20% for target OPPs in all selected samples.

Molecular absorption measurements with an optical fibre coupled array of ultra-violet light-emitting diodes

A photometric detector based on eight different light-emitting diodes covering the ultraviolet range from 255 nm to 350 nm is described. These are coupled with fused silica optical fibres to a conventional cuvette with 1 cm optical path length or to a low volume flow through cell for detection in high-performance liquid chromatography. Photodiodes are employed for the measurement of the transmitted intensity as well as of a reference signal and the photocurrents are processed with a log-ratio amplifier to obtain a voltage proportional to absorbance values. The wavelength desired for the measurement at hand is selected by electronically switching on the requisite light-emitting diode. The detector was found to have a low noise level of 80 μAU. In batch-wise measurements as well as in detection for high-performance liquid chromatography dynamic ranges of 2–3 orders of magnitude were possible. Reproducibilities in peak areas for the latter application were better than 1%.

Optimalizace stanovení obsahu dimethylsulfidu v mladině a pivu

Sulphur compounds present in barley, malt and beer are mostly non-volatile substances (amino acids, proteins, inorganic sulphates) from which volatile sensory active substances can be produced under certain conditions. The presence of these substances in beer can adversely affect its organoleptic properties. Dimethyl sulfi de (DMS) is one of the most studied volatile sulphur compounds in beer; therefore a suitable analytical method for monitoring DMS content is necessary. The static headspace method coupled to gas chromatography with fl ame photometric detector (HS-GC-FPD) used for the determination of DMS content in wort and beer was optimized and validated. Optimal yield of HS extraction was achieved after 30 minutes at 50 °C. DMS contents moved from 9.6 to 59.6 μg.l-1 in the analysed worts, from 6.1 to 34.9 μg.l-1 in beers.

Up to 21 Different Sulfur‐Heterocyclic Fatty Acids in Rapeseed and Mustard Oil

AbstractThree sulfur‐heterocyclic fatty acids (SHFA) had been tentatively identified in rapeseed oil in the late 1980s. In this study we aimed to enrich and verify the presence of potential SHFA in one sample of native rapeseed oil, refined rapeseed oil and mustard seed oil. Fifty‐gram samples of the three oils were individually saponified and converted into methyl esters. The resulting samples were hydrogenated and subjected three times to urea complexation. The resulting extracts of native rapeseed oil and mustard oil contained 21 different SHFA with 18, 20, 22 or 24 carbons. The refined rapeseed oil contained only nine C18‐SHFA. Structure investigation of the SHFA was performed by gas chromatography with mass spectrometry (GC/MS) using methyl esters and also 3‐pyridylcarbinol esters. A direct screening of non‐enriched samples by GC/MS in the selected ion monitoring mode and by GC with flame photometric detector (sulfur‐selective) verified that the SHFA were native compounds of the oils and no artefacts of the sample preparation. Similar abundances of the four isomer groups of SHFA with monoenoic fatty acids of the same carbon number in these and five further rapeseed and mustard samples indicated that these could be the precursors of the SHFA.

A Sensitive Photometric Procedure for Cobalt Determination in Water Employing a Compact Multicommuted Flow Analysis System.

In this work, a multicommuted flow analysis procedure is proposed for the spectrophotometric determination of cobalt in fresh water, employing an instrument setup of downsized dimension and improved cost-effectiveness. The method is based on the catalytic effect of Co(II) on the Tiron oxidation by hydrogen peroxide in alkaline medium, forming a complex that absorbs radiation at 425 nm. The photometric detection was accomplished using a homemade light-emitting-diode (LED)-based photometer designed to use a flow cell with an optical path-length of 100 mm to improve sensitivity. After selecting adequate values for the flow system variables, adherence to the Beer-Lambert-Bouguer law was observed for standard solution concentrations in the range of 0.13-1.5 µg L-1 Co(II). Other useful features including a relative standard deviation of 2.0% (n = 11) for a sample with 0.49 µg L-1 Co(II), a detection limit of 0.06 µg L-1 Co(II) (n = 20), an analytical frequency of 42 sample determinations per hour, and waste generation of 1.5 mL per determination were achieved.

Investigation of 2-Sulfanylethyl Acetate Cysteine-S-Conjugate as a Potential Precursor of Free Thiols in Beer

The compounds 2-sulfanylethyl acetate (2SEA, roasted- or burnt-like flavor) and 3-sulfanylpropyl acetate (also with roasted- or burnt-like flavor), mainly issued from the Ehrlich pathway (in this case, the catabolism by yeast of cysteine and homocysteine, leading to the formation of 2-sulfanylethan-1-ol and 2-sulfanylpropan-1-ol, which are subsequently acetylated), are found in all fresh beers, at levels strongly depending on the yeast strain used in fermentation. Yet, even in the absence of yeast, their production is still seen to occur in the bottle during the first months of storage. As for other thiols, S-cysteine adducts have been suspected to be hydrolyzed during beer aging. The commercially unavailable S-cysteine conjugate of 2SEA (Cys-2SEA, 93% purity), never described before in the literature, was synthesized by acetylation of the S-cysteine conjugate of 2-sulfanylethan-1-ol (Cys-2SEol). Cys-2SEA was solubilized in model aqueous media (pH 4.2) and in beer before aging. 2SEA was further extracted and analyzed by gas chromatography coupled to a pulsed flame photometric detector (GC-PFPD), and high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (HPLC-ESI(+)-MS/MS) allowed us to quantify the undegraded Cys-2SEA (specific m/z ions at 120, 148, and 191). Cys-2SEA was shown to be relatively stable in both media, although some beer constituents were noted as being able to slightly facilitate the chemical degradation (molar conversion into free thiol up to 0.3% at room temperature). A modified “Strecker degradation” mechanism is proposed here, explaining the release of thiols from the Schiff bases created between the cysteine adducts and beer diketones. Up to now, tests have failed to reveal evidence for the presence of Cys-2SEol and Cys-2SEA in hops by direct HPLC-ESI(+)-MS/MS analyses, although 2SEol was released from cysteine-S-conjugate extracts in the presence of apotryptophanase.

Gas chromatographic determination of benzothiophenes in high-purity sulfur

A procedure is developed for the determination of benzothiophene, dibenzothiophene, and 4,6-dimethyldibenzothiophene in high-purity sulfur, including liquid-phase microextraction preconcentration, identification by gas chromatography−mass spectrometry, and gas chromatographic determination with flame photometric detection. The rate of microextraction recovery was from 16 to 78%. The limits of detection were 8 × 10–5 wt % for benzophiophene and 2 × 10–5 wt % for dibenzothiophene and 4,6-dimethyldibenzothiophene.

Indirect determination of zinc by thiol complexation and iodine coulometric titration with photocell detection

We have extended the capabilities of iodine coulometry for the determination of ionic zinc through its complexation with cysteine and subsequent titration of the thiol. The titration endpoint times of cysteine with zinc are proportionally longer as compared to cysteine itself. A homebuilt constant-current coulometer is employed for this method development. The instrument is composed of a constant-current source connected to nonisolated platinum electrodes contained in a 150-mL beaker (the coulometric cell) that sits on a photocell mounted in a styrofoam lid on top of a magnetic stirrer. Photometric endpoint detection is facilitated by connecting the photocell detector to an operational amplifier circuit; its output, recorded on a multifunctional chemical analysis system, resembled a sigmoidal curve. The titration conditions are optimized in terms of pH, temperature, stirring rate, and cysteine-to-zinc concentration ratios. Calibration curves generated as time versus increasing zinc concentrations in the presence of a constant cysteine concentration are used to determine zinc content in dietary supplements. The limit of detection is 5.5×10−5M Zn and the limit of quantification is 6.2×10−5M Zn. Percent relative standard deviations are at 4%. The recoveries of zinc from liquid zinc, enhanced zinc lozenges, and zinc tablets generally range from 80 to 120%. Titration of ascorbic acid and then zinc in the same solution is successfully performed with recoveries greater than 95% for both analytes. A comparison of the coulometry Zn data to atomic absorption results is also made for most samples.

An optrode for photometric detection of ammonia in air

A scheme of constructing an LED optrode for photometric detection of ammonia in air is considered. The components of the device are (1) a glass plate coated with a film of polydimethylsiloxane with an ion-coupled cation of brilliant-green dye, (2) an LED emitting at a wavelength of 655 nm, and (3) a metal housing. The nominal static conversion function, sensitivity, and relative measurement error of the device are analyzed on the basis of mathematical modeling. The obtained results allow one to design an LED optrode capable of carrying out control for automated technological processes, solving problems in the area of security, etc. The device provides the ability to create photometric gas analyzers of ammonia with small overall dimensions, power consumption, and cost.

Determination of organotin compounds in sediment samples by dispersive liquid-liquid microextraction followed by gas chromatography – Pulsed flame photometric detection (DLLME-GC-PFPD)

Dispersive liquid-liquid microextraction (DLLME) was evaluated for the simultaneous determination of butyl-, phenyl- and octyltin compounds in sediment samples. The most significant factors in DLLME procedure were type and volume of extraction and disperser solvents, and the optimal conditions were reached when 413 and 34μL of methanol and tetrachloroethylene, respectively, were used. Under optimal conditions, the limit of detection and the relative standard deviation ranged between 0.3 and 1.0ngL−1 and 2–6%, respectively. The developed method was validated for the analysis of a certified reference material and spiked seawater and marine sediment samples, with satisfactory results. Finally, harbor sediment samples collected from the Chilean coastal region were analyzed to demonstrate the applicability of the developed method in the determination of organotin compounds in the environment.

Influence of fumigants on sunflower seeds: Characteristics of fumigant desorption and changes in volatile profiles

Fumigation of transport containers is common practice to protect stored products from pests. Yet little is known about the desorption times and effects of the highly toxic gases used in this process. To shed light on the behavior of fumigants in real food, we treated sunflower seeds (Helianthus annuus L.) with 100ppm phosphine (PH3), methyl bromide (MeBr) or 1,2-dichloroethane (DCE) for 72h. The compound concentrations in the air were then analyzed by thermal desorption/2D gas chromatography coupled to mass spectrometry and flame photometric detection (TD-2D-GC–MS/FPD). A desorption time of several months was observed for DCE, whereas PH3 and MeBr were outgassed in a matter of days. To investigate possible interactions between gases and constituents of the seeds, non-fumigated, fumigated and outgassed samples were analyzed by headspace solid-phase microextraction GC–MS. We observed significantly different volatile profiles in fumigated and subsequently outgassed seeds compared to non-fumigated seeds. Whereas PH3-treated seeds released far more terpenoids, the volatile pattern of seeds exposed to DCE revealed significantly fewer terpenoids but more aldehydes. These changes are likely to affect food aroma characteristics.

Characterization of the key aroma compounds in different light aroma type Chinese liquors by GC-olfactometry, GC-FPD, quantitative measurements, and aroma recombination

The aroma profiles of five Chinese light aroma-type liquors were investigated in this study. Gas chromatography-olfactometry (GC-O) and GC-flame photometric detection (FPD) were employed to investigate aroma-active compounds. A total of 80 odorants were identified, including ten sulfur compounds. In addition, aroma-active compounds (FD≥16) were determined, according to quantitative study and odor activity values (OAVs). Finally 27 key aroma compounds, mainly possessing fruity and floral notes, were successfully used to simulate the aromas of Chinese light aroma-type liquors by dissolving them in 53% (v/v) hydroalcoholic solution in their natural concentrations. Moreover, the correlation between the key aroma compounds, seven sensory attributes and liquor samples was evaluated through partial least-squares regression analysis.

Capillary Electrophoresis in Analytical Control of Wine

Background/Objectives: Quality of grape wines and by-products in connection with the circumscribed resources of natural raw materials receive urgency. The standardize indexes are directed on control of production received from qualitative grape. Methods: For analytical control of grape wines the system of capillary electrophoresis with the photometric detector, the quartz capillary with an external polyimide coating was used. Results were treated with Multichrome for Windows, version 1.5 and statistical programs for Windows. The new methods with use of instrumentation of capillary electrophoresis have allowed dilating standardized indexes of grape wines and provided control principal organic acids, sulphates, sorbic acid, lysozyme. Findings: Appearance and introduction of new directives based on capillary electrophoresis testifies to dynamical development of standard provision of grape wine production quality. Procedures of determination by means of capillary electrophoresis of prioritise amino acids, major cations, inorganic anions, principal organic acids, total nitrogen in grape wines are modified and developed. The results received by means of new methods, have important and allow solve many questions bound to quality. Results: It is shown the influence of composition of electrolyte to separation of analysed components. It is estimated sensitivity, detection limits, linearity, and was calculated electrophoretic mobility determined components of grape wines. It is positioned that no more than 5 measurements of samples on one portion of electrolyte in the yielded conditions with the guaranteed error of a procedure. The further exploitation leads to growth of an error of quantitative calculations up to 8%. For determination of complex indexes may be used the temperature destruction of sample of grape wine and further determination by method of capillary electrophoresis.

Double-beam photometric direct-injection detector for multi-pumping flow system

A second generation of the photometric direct-injection detector for flow analysis is presented. In the detector, a log ratio amplifier to measure the absorbance in the system of double paired emission-detection LED diodes (double-PEDD) has been applied for the first time. The detector co-works with a solenoid pulse micro-pumps system and consists of two chambers: reaction-detection and reference. The sample and reagents are injected by solenoid micro-pumps directly into the reaction-detection chamber. The carrier solution flows consecutively through the reference and then reaction-detection chambers, cleaning them. Both chambers are irradiated by two identical emission LEDs. Two identical detection LEDs, situated at the opposite ends of both chambers, are connected with the integrated precise log ratio amplifier. Thus, the output voltage is a linear function of absorbance and can be measured properly, with high accuracy, over the entire range, up to 2.2. The software developed for controlling this system is implemented with the option of auto-zeroing in each measurement cycle. It results in a higher stability of the absorbance signal and possibility of working with solutions in which incidental gas bubbles are created.The performance of the detector is characterized using a series of methylene blue dye studies and analytical reactions of different kinetics. The potential applications to the Fe(III) determination using the thiocyanate method is discussed. The system is characterized by better repeatability (R.S.D. below 1%) and lower limit of detection (0.06mgL−1) than a similar, single-beam PEDD detector. The calibration graph proved to be linear in the range of up to 15mgL−1. It was found that the optical design of the detector minimizes the Schlieren effect.The chief advantages of the presented device are: very high stability and repeatability, simple and compact construction, high sampling frequency, low volume of the solutions used and low energy consumption. Therefore, the flow system can be especially recommended for the Green Chemistry monitoring stations working in a fully automatic mode.

Manuka honey (Leptospermum scoparium) inhibits jack bean urease activity due to methylglyoxal and dihydroxyacetone.

Manuka honey (Leptospermum scoparium) exerts a strong antibacterial effect. Bacterial enzymes are an important target for antibacterial compounds. The enzyme urease produces ammonia and enables bacteria to adapt to an acidic environment. A new enzymatic assay, based on photometric detection of ammonia with ninhydrin, was developed to study urease activity. Methylglyoxal (MGO) and its precursor dihydroxyacetone (DHA), which are naturally present in manuka honey, were identified as jack bean urease inhibitors with IC50 values of 2.8 and 5.0mM, respectively. Urease inhibition of manuka honey correlates with its MGO and DHA content. Non-manuka honeys, which lack MGO and DHA, showed significantly less urease inhibition. MGO depletion from manuka honey with glyoxalase reduced urease inhibition. Therefore, urease inhibition by manuka honey is mainly due to MGO and DHA. The results obtained with jack bean urease as a model urease, may contribute to the understanding of bacterial inhibition by manuka honey.

Validation of QuEChERS based method for determination of fenitrothion residues in tomatoes by gas chromatography–flame photometric detector: Decline pattern and risk assessment

A simple and rapid gas chromatography with flame photometric detector (GC-FPD) determination method was developed to detect residue levels and investigate the dissipation pattern and safe use of fenitrothion in tomatoes. A modified quick, easy, cheap, effective, rugged, and safe (QuEChERS) using an ethyl acetate-based extraction, followed by a dispersive solid-phase extraction (d-SPE) with primary-secondary amine (PSA) and graphite carbon black (GCB) for clean up, was applied prior to GC-FPD analysis. The method showed satisfactory linearity, recovery and precision. The limits of detection (LOD) and quantification (LOQ) were 0.005 and 0.01mg/kg, respectively. The residue levels of fenitrothion were best described by first order kinetics with a half-life of 2.2days in tomatoes. The potential health risks posed by fenitrothion were not significant, based on supervised residue trial data. The current findings could provide guidance for safe and reasonable use of fenitrothion in tomatoes and prevent health problems to consumers.

3D printed LED based on-capillary detector housing with integrated slit

A 3D printed photometric detector body with integrated slit was fabricated to position a LED and photodiode either side of capillary tubing using a fused deposition modelling (FDM) printer. To make this approach suitable for capillaries down to 50 μm i.d. the dimension of the in-built slit is the critical element of the printed housing. The spatial orientation of the model for printing was found to significantly impact on the resolution of the structures and voids that can be printed. By designing a housing with a slit positioned in the XY plane in parallel with the print direction, the narrowest void (slit) that could be printed was 70 μm. The potential use of the 3D printed slit for photometric detection was characterised using tubing and capillary from 500 down to 50 μm i.d, demonstrating a linear response from 632 to 40 mAU. The effective pathlength and stray light varied from 383 to 22 μm and 3.8% - 50% for 500- 50 μm i.d tubing and capillary. The use of a V-shaped alignment feature allowed for easy and reliable positioning of the tubing inside the detector, as demonstrated by a RSD of 1.9% (n = 10) in peak height when repositioning the tubing between measurements using flow injection analysis (FIA). The performance of the 3D printed housing and 70 μm slit was benchmarked against a commercially available interface using the CE separation of Zn2+ and Cu2+ complexes with PAR. The limit of detection with the 3D printed slit was 6.8 and 4.5 μM and is 2.8 and 1.6 μM with the commercial interface.