Laser-induced breakdown spectroscopy: Time-integrated applications

Spectral analysis of the plasma produced by laser-induced breakdown was demonstrated to be an effective real-time technique for the detection of atomic constituents in gases and gas-entrained particulates. The time-integrated technique, LIBS (laser-induced breakdown spectroscopy), was applied to the detection of sodium and potassium in a coal gasifier product stream, of airborne beryllium, and of phosphorus, sulfur, and chlorine in various organic molecules. In a companion paper (following) the time-resolved technique will be discussed.     

Determination of bromine, chlorine, sulphur and phosphorus in peat by X-ray fluorescence spectrometry combined with single-element and multi-element standard addition

Bromine (20-40 ppm), chlorine (200-500 ppm), sulphur (0.2-3%) and phosphorus (300-1000 ppm) in peat have been determined by X-ray fluorescence spectrometry (XRFS) combined with the standard-addition method. Chlorine, sulphur and phosphorus have also been determined by other methods and agreement between the results is good. Theoretical calculations based on the Sherman equation were made to validate the linearity of the standard-addition curves. A multi-element standard-addition technique has been tested with addition of all elements at the same time. The results for chlorine were high but after correction for the difference in attenuation coefficient between the sample and added compound the results agreed with those from single-element standard addition.     

Development of a monitoring tape for phosgene in air

A porous cellulose tape impregnated with a processing solution that includes 4-p-nitroben-zylpyridine, N-benzylaniline and methanol is a highly sensitive means of detecting phosgene and maintains stable sensitivity for at least three months in air in a desiccator. When the sample including phosgene was passed through the tape, the color of tape changed to red. The degree of color change was proportional to the concentration of phosgene at a constant sampling time and flow rate. The degree of color change could be recorded by measuring the intensity of reflecting light (555 nm). The detection limit was 6 ppb for phosgene with a sampling time of 60 sec and a flow rate of 400 ml/min. Reproducibility tests showed that the relative standard deviation of response (n = 10) was 2.6% for 0.2 ppm phosgene. No interference was observed from ethanol (1 vol.%), trichloroethylene (1 vol.%), acetone (1 vol.%), carbon dioxide (4.9 vol.%), carbon monoxide (100 ppm), nitrogen dioxide (100 ppm), sulfur dioxide (50 ppm), hydrogen chloride gas (5 ppm), chlorine (3 ppm), acetic acid gas (24 ppm), ammonia (40 ppm), or benzyl chloride (20 ppm).     

The design of electrochemical probes for oxygen and chlorine and their applications in process control analysis

This paper describes the design of a membrane type polarographic probe for the determination of oxygen and chlorine in gaseous and liquid streams and of a membrane-less probe for the determination of chlorine in gaseous streams. The membrane type has limits of detection of 0.002% v/v and 0.02% v/v for oxygen and chlorine respectively and times for 80% response to step changes of 3 s and 30 s respectively. The membrane-less probe has a limit of detection of about 0.01 ppm v/v chlorine and the time taken for 80% response to a step change at the 10 ppm level is about 5 s. They can be made in a range of materials to suit particular applications. The paper gives six applications of their use in the chemical industry.     

ROC-curve approach for determining the detection limit of a field chemical sensor

The detection limit of a field chemical sensor under realistic operating conditions is determined by receiver operator characteristic (ROC) curves. The chemical sensor is an ion mobility spectrometry (IMS) device used to detect a chemical marker in diesel fuel. The detection limit is the lowest concentration of the marker in diesel fuel that obtains the desired true-positive probability (TPP) and false-positive probability (FPP). A TPP of 0.90 and a FPP of 0.10 were selected as acceptable levels for the field sensor in this study. The detection limit under realistic operating conditions is found to be between 2 to 4 ppm (w/w). The upper value is the detection limit under challenging conditions. The ROC-based detection limit is very reliable because it is determined from multiple and repetitive sensor analyses under realistic circumstances. ROC curves also clearly illustrate and gauge the effects data preprocessing and sampling environments have on the sensor's detection limit.     

A rapid and efficient mass spectrometric method for the analysis of explosives

The high explosives trinitrotoluene, nitroglycerine, pentaerythritol tetranitrate and hexahydro-1,3,5-trinitro-1,3,5-triazine are efficiently ionised under negative ion atmospheric pressure chemical ionisation (APCI) conditions. The limit of detection is improved, in some cases by several orders of magnitude, by complexation with chlorine demonstrating this to be a highly suitable method for enhancing the detection capabilities for explosives. The spectra produced from introduction of the analytes in a liquid matrix, with and without chlorine present, contain a number of ions that arise through secondary processes including breakdown and adduct formation. Sample introduction into an APCI source in air, via a heated-plate inlet with a supplementary feed of dichloromethane, produces improved response for the chloride adducts of the analytes and minimises their decomposition during analysis. The tandem mass spectra produced from the chloride adducts are simple. Optimisation of the trapping parameters of the ion trap detector enhances selected transitions, yields highly reproducible spectra and improves the limits of detection for MS/MS analysis.     

Laser-induced fluorescence detection of lead atoms in a laser-induced plasma: An experimental analytical optimization study

The combination of the laser-induced breakdown spectroscopy (LIBS) and laser-induced fluorescence (LIF) techniques was investigated to improve the limit of detection (LoD) of trace elements in solid matrices. The influence of the main experimental parameters on the LIF signal, namely the ablation fluence, the excitation energy, and the inter-pulse delay, was studied experimentally and a discussion of the results was presented. For illustrative purpose we considered detection of lead in brass samples. The plasma was produced by a Q-switched Nd:YAG laser and then re-excited by a nanosecond Optical Parametric Oscillator (OPO) laser. The experiments were performed in air at atmospheric pressure. We found out that the optimal conditions were obtained for our experimental set-up using relatively weak ablation fluence of 2–3 J/cm² and an inter-pulse delay of about 5–10 μs. Also, a few tens of microjoules was typically required to maximize the LIF signal. Using the LIBS–LIFS technique, a single-shot LoD for lead of about 1.5 part per million (ppm) was obtained while a value of 0.2 ppm was obtained after accumulating over 100 shots. These values represent an improvement of about two orders of magnitude with respect to LIBS.     

Determination of nitrogen trichloride (NCl3) levels in the air of indoor chlorinated swimming pools: an impinger method proposal

The quality of the environmental air from indoor swimming pools has been associated with various health risks. Particular attention has focused on the effects of chronic lung exposure to chlorine and its by-products, especially in young children. We developed a simple, non-toxic approach to detect and monitor nitrogen trichloride air levels in the indoor swimming pool environment. The proposed Impinger Method (IM) was used to measure the environmental levels of nitrogen trichloride (NCl₃) in 17 indoor swimming pools located in Northern Italy. This new analytical protocol is based on a colorimetric reaction commonly employed to detect the total and free chlorine levels in water. Specifically, IM allows the entrapment of NCl₃ into a water solution containing diethyl-p-phenylenediamine (DPD 1) and Potassium Iodide (DPD 3). NCl₃ from the air environment reacts with DPD 3 releasing iodine, which reacts with DPD 1 and produces a coloration proportional to the amount of NCl₃ from the sampled indoor swimming pool air. Our sampling of the monitored swimming pool environments evidenced a mean NCl₃ level (637?±?220?µg/m³) higher than the recommended WHO value (500?µg/m³). The IM was validated in terms of linearity (R ^2?=?0.996), limit of detection (3.6?µg/m³) and repeatability (CV?=?1.7%), demonstrating easy-to-use characteristics, good efficiency and low cost.     

A sensitive and rapid assay for 4-aminophenol in paracetamol drug and tablet formulation, by flow injection analysis with spectrophotometric detection

4-Aminophenol (4AP) is the primary degradation product of paracetamol which is limited at a low level (50 ppm or 0.005% w/w) in the drug substance by the European, United States, British and German Pharmacopoeias, employing a manual colourimetric limit test. The 4AP limit is widened to 1000 ppm or 0.1% w/w for the tablet product monographs, which quote the use of a less sensitive automated HPLC method. The lower drug substance specification limit is applied to our products, (50 ppm, equivalent to 25 μg 4AP in a tablet containing 500-mg paracetamol) and the pharmacopoeial HPLC assay was not suitable at this low level due to matrix interference. For routine analysis a rapid, automated assay was required. This paper presents a highly sensitive, precise and automated method employing the technique of Flow Injection (FI) analysis to quantitatively assay low levels of this degradant. A solution of the drug substance, or an extract of the tablets, containing 4AP and paracetamol is injected into a solvent carrier stream and merged on-line with alkaline sodium nitroprusside reagent, to form a specific blue derivative which is detected spectrophotometrically at 710 nm. Standard HPLC equipment is used throughout. The procedure is fully quantitative and has been optimised for sensitivity and robustness using a multivariate experimental design (multi-level ‘Central Composite’ response surface) model. The method has been fully validated and is linear down to 0.01 μg ml⁻¹. The approach should be applicable to a range of paracetamol products.     

Chlorine detection in cement with laser-induced breakdown spectroscopy in the infrared and ultraviolet spectral range

A significant parameter to monitor the status of concrete buildings like bridges or parking garages is the determination of the depth profile of the chlorine concentration below the exposed concrete surface. This information is required to define the needed volume of restoration for a construction. Conventional methods like wet chemical analysis are time- and cost-intensive so an alternative method is developed using laser-induced breakdown spectroscopy (LIBS). The idea is to deploy LIBS to analyze drill cores by scanning the sample surface with laser pulses. Chlorine spectral lines in the infrared (IR) and ultraviolet (UV)-range were studied for chlorine detection in hydrated cement samples. The excitation energies of these spectral lines are above 9.2 eV. Hence high plasma temperatures and pulse energies in the range of some hundred millijoules are needed to induce sufficient line intensity levels at the required working distance. To further increase the line intensity and to lower the detection limit (LOD) of chlorine a measuring chamber is used where different ambient pressures and gases can be chosen for the measurements. The influences on the line intensity for pressures between 5 mbar and 400 mbar using helium as process gas and the influence of different laser burst modi like single and collinear double pulses are investigated. For the first time a LOD according to DIN 32 645 of 0.1 mass% was achieved for chlorine in hydrated cement using the UV line 134.72 nm.     

A simple microfluidic chlorine gas sensor based on gas-liquid chemiluminescence of luminol-chlorine system

In this work, a microfluidic chlorine gas sensor based on gas-liquid interface absorption and chemiluminescence detection was described. The liquid chemiluminescence reagent-alkaline luminol solution can be stably sandwiched between two convex halves of a microchannel by surface tension. When chlorine gas was introduced into the micro device, it was dissolved into the interfacial luminol solution and transferred to ClO(-), and simultaneously luminol was excited and chemiluminescence emitted. The emitted chemiluminescence light was perpendicularly detected by a photomultiplier tube on a certain detection region. The remarkable advantage of the detection system is that both adsorption and detection were carried out at the gas-liquid interface, which avoids the appearance of bubbles. The whole analytical cycle including filling CL reagent, sample injection, CL detection and emptying the device was as short as 30 s. The linear concentration range of chlorine gas detection with direct introduction of sample method is from 0.5 to 478 ppm. The detection limit of this method is 0.2 ppm for standard chlorine gas and the relative standard deviation of five determinations of 3.19 ppm spiked chlorine sample was 5.2%.     

Determination of low chlorine concentrations in air using semiconductor chemical sensors

Comparative studies of electophysical gas-sensitive properties of semiconductor metal oxides (NiO, WO₃, and In₂O₃) in detecting trace concentrations of chlorine in air at 250–300°C were performed. WO₃ and In₂O₃ film sensors were found to have the best sensitivity, selectivity, and stability. However, WO₃ films are characterized by a longer relaxation time (3 min) compared to In₂O₃ films, for which it is no longer than 30 s. The kinetic and steady-state relative conductivity values of In₂O₃ films as functions of the chlorine concentration in air fall on the same curve in the range 0.01–0.7 ppm. This suggests that the concentration of chlorine in air can be determined from the initial rates of the variation of the relative conductivity of films, which significantly decreases the time of analysis (from 40 to 5 s at a sensor working temperature of 300°C). Changes in air humidity in the range from 40 to 80% have no effect on the initial rates of the variation of the relative conductivity of In₂O₃ films under kinetic conditions. The mechanism of the variation of In₂O₃ film conductivity in detecting chlorine in air was discussed.     

Continuous Flow Determination of Residual Aqueous Ozone With Membrane Separation-Chemiluminescent Detection

Abstract

A method for the determination of ultra-trace aqueous ozone utilizing a membrane-separation process and a chemiluminescent detector is proposed. The microporous PTFE tube was used as separator to transfer aqueous ozone into a gas phase. Air bubbling was used to enhance the separation. Chemilumi nescent signals produced from the reaction of ozone with ethylene in air were linearly proportional to concentration of aqueous ozone from 8 ppb to 9.5 ppm. The relative standard deviation (n=4) was 3.5% at 0.2 ppm. The time it took from starting the sample flow until the signal to reach a stable level was 1.5 min. The interference from chromium(VI), manganese(VII), chlorine, bromine, phthalate, and sulfophthalate was completely eliminated.     

A Photoluminescent Ag10Cu6 Cluster Stablized by a PNNP Ligand and Phenylacetylides Selectively and Reversibly Senses Ammonia in Air and Water

A photoluminescent bimetallic cluster [Ag₁₀Cu₆(bdppthi)₂(C≡CPh)₁₂(MeOH)₂(H₂O)](ClO₄)₄ ( 1 , bdppthi=N,N’-bis(diphenylphosphanylmethyl)-tetrahydroimidazole} was synthesized from the PNNP type ligand bdppthi generated in-situ. Upon excitation at 365 nm, 1 exhibited strong phosphorescent emission at 630 nm, which was selectively quenched by NH₃ in air or water. The sensing of NH₃ was rapid and recoverable, with detection limits of 53 ppm (v/v) in N₂ and 21 μmol/L (0.36 ppm, w/w) for NH₃ ⋅ H₂O in water. Cluster 1 could potentially serve as a bifunctional chemical sensor for the efficient detection of ammonia in waste-gas and waste-water.     

Nachweisgrenzen bei der R?ntgen-Emissionsspektralanalyse von Mineral?len

Zusammenfassung Die Nachweisgrenzen verschiedener Elemente in Mineralölen, mit Ordnungszahlen zwischen 82 und 15, wurden experimentell nach der Eichkurvenmethode nahe der Konzentration Null bestimmt. Unter Heranziehung aller instrumenteilen Möglichkeiten wurden Ergebnisse erhalten, die bis einschlie\lich Chlor unter 1 ppm, für Schwefel und Phosphor zwischen 1 und 3 ppm liegen. Die AbhÄngigkeit der Nachweisgrenze von WellenlÄnge und Me\zeit wurde ermittelt. Matrix-Elemente beeinflussen die Nachweisgrenze selbst in sehr hohen Konzentrationen nur unwesentlich. StÄrker wird durch sie die Nachweisempfindlichkeit gestört. Es empfiehlt sich deshalb auch bei Analysen im Spuren-Bereich die Verwendung innerer Standards.

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Summary The limits of detection of various elements in mineral oils, with atomic numbers between 82 and 15, were experimentally determined by the calibration curve method near zero concentration. By application of all given instrumental possibilities results were obtained which showed limits of detection well below 1 ppm for all elements down to chlorine, and between 1 and 3 ppm for sulphur and phosphorus. The dependence of the limit of detection on wavelength and test time has been investigated. Matrix elements affect the limit of detection only unessentially, even in very high concentrations. In contrary to that the sensitivity of the determination is lowered by them. Therefore the use of internal standards is highly recommendable even in analyses of trace amounts.

     

A highly selective colorimetric aqueous sensor for mercury

A new colorimetric mercury sensor is reported based on binding to terpyridine derivatives. It is able to selectively detect Hg II ions over a number of environmentally relevant ions including Ca II, Pb II, Zn II, Cd II, Ni II, Cu II, and others. The response time upon exposure to Hg II is instantaneous. By the "naked eye," the detection limit of Hg II is 2 ppm (25 microM) in solution. With a spectrometer, this detection limit is increased down to 2 ppb (25 nM), which is the current EPA standard for drinking water. The significant problem of mercury poisoning requires new methods of detection that are sensitive and selective. Here we report a new simple system that takes advantage of the unique optical properties generated by terpyiridine-Hg complexes.     

Flame atomic absorption analysis for selenium after electrochemical preconcentration

Selenium(IV) is determined in the presence of 3.5% sodium chloride by electrochemical preconcentration on a platinum spiral, prior to flame atomic absorption analysis. The electrodeposition is carried out in the presence of hydrazine dihydrochloride to prevent the generation of chlorine at the counter electrode; chlorine oxidizes selenium(IV) to the non-reducible selenium(VI) ion. A detection limit of 5 ppb and an electrolysis efficiency of 10% were obtained for a 25-ml sample and a 5-min electrolysis time. The absolute detection limit was 10 ng.     

Determination of lead in water using laser ablation–laser induced fluorescence

The detection sensitivity of laser-induced breakdown spectroscopy (LIBS) is improved by coupling it with a laser-induced fluorescence method. A waterjet sample containing 500 ppm of Pb as an analyte was ablated by a 266 nm, frequency-quadrupled Q-switchedNd:YAG laser at an energy of ~ 260 μJ. After a short delay the resulting plume was re-excited with a 283.306 nm, nanosecond pulse dye laser at energies ranging from 45 to 100 nJ. The limit of detection (LOD) of lead in water was determined both by the single-pulse LIBS technique and Laser Ablation coupled with Laser-Induced Fluorecence (LA–LIF) method. It was found to be 75 ppm in the case of single-pulse LIBS and 4.3 ppm for LA–LIF. When the resonant pulse was detuned from the transition wavelength the LA–LIF signal disappeared demonstrating the resonant selectivity of this technique.     

Pesticide residue analysis in foodstuffs applying capillary gas chromatography with atomic emission detection State-of-the-art use of modified multimethod S19 of the Deutsche Forschungsgemeinschaft and automated large-volume injection with programmed-temperature vaporization and solvent venting

Atomic emission detection (AED) provides high element-specific detection of all compounds amenable to gas chromatography (GC). The heteroatoms nitrogen, chlorine, phosphorus, sulfur, bromine and fluorine, which are important elements in pesticide residue analysis, are of major interest. A main drawback of AED is its lower sensitivity with respect to other selective detection methods used in pesticide residue analysis such as electron-capture and nitrogen-phosphorus detection. This holds true especially for the important nitrogen trace. For this reason, more sensitive detection can be achieved by injection of larger volumes or higher concentrations of sample extracts, because matrix compounds were usually registered only in the carbon, hydrogen and oxygen traces. This paper focuses on recent developments from the authors' laboratory in order to demonstrate the feasibility of screening analyses with the identification of pesticide residues down to the 0.01 ppm concentration level in plant foodstuffs. This has been achieved by means of automated large volume injection with programmed-temperature vaporization and solvent venting as well as careful optimization of make-up and reactant gases with AED. Clean up follows the principle of multimethod S19 of the Deutsche Forschungsgemeinschaft in a reduced procedure. After elimination of lipids and waxes by gel permeation chromatography, extracts from 10 g of the food samples were concentrated to 200 μl, of which 12.5 μl were introduced into the GC-AED system. Two analyses were usually performed with the element traces of sulfur, phosphorus, nitrogen and carbon in the first run and chlorine and bromine in the second run. Fluorine and oxygen were not detected in any screening analyses. The method has proved to be of great value especially with “problem foodstuffs”. The limits of detection were determined for 385 pesticides and are presented together with their retention data.     

Neutron activation analysis for chlorine in Zr-2.5 wt% Nb coolant tube material

Zr-2.5 wt% Nb has been found to be better compared to Zircaloy-2 as coolant tube material in Canadu-PHW reactors but has a stringent specification of less than 0.5 mg/kg of chlorine. Instrumental neutron activation analysis (INAA) for the determination of chlorine in Zr-2.5 wt% Nb is not possible because of the high activity produced due to the determination of the matrix. Hence a radiochemical neutron activation analysis (RNAA) procedure has been developed for the determination of chlorine in this material. For the first time the chlorine determination at less than a ppm level by NAA is being reported in this paper, in a number of Zr-2.5 wt% Nb samples ranging from 0.1 to 2 mg/kg.