Single-detector micro-electro-mechanical scanning grating spectrometer

A compact, robust grating spectrometer based on an optimised micro-electro-mechanical grating mirror component has been developed, built, and characterised. The application of an oscillating reflection grating micro-mirror component as scanning dispersive element in a modified Czerny–Turner monochromator layout enables the design of compact grating spectrometers capable of acquiring full spectra using a single detector element. Designed for a wavelength range between 1200 and 1900 nm, the spectrometer features a spectral resolution of 10 nm with wavelength stability better than ±0.5 nm. One-hundred scan spectra can be acquired in less than one second, or spectral changes can be monitored at time a resolution of less than 10 ms. In combination with a fibre-optic interface and a typical weight of less than 1 kg, this makes this novel type of fully portable micro-electro-mechanical near-IR scanning spectrometer an interesting alternative to existing spectrometers and opens a range of new applications, in particular the detection of major and minor components in the near-IR. MEMS SG spectrometer prototype     

Analysis of laser-induced breakdown spectroscopy spectra: The case for extreme value statistics

In most instances, laser-induced breakdown spectroscopy (LIBS) spectra are obtained through analog accumulation of multiple shots in the spectrometer CCD. The average acquired in the CCD at a given wavelength is assumed to be a good representation of the population mean, which in turn is implicitly regarded to be the best estimator for the central value of the distribution of the spectrum at the same wavelength. Multiple analog accumulated spectra are taken and then in turn averaged wavelength-by-wavelength to represent the final spectrum. In this paper, the statistics of single-shot and analog accumulated LIBS spectra of both solids and liquids were examined to evaluate whether the spectrum averaging approach is statistically defensible. At a given wavelength, LIBS spectra are typically drawn from a Frechet extreme value distribution, and hence the mean of an ensemble of LIBS spectra is not necessarily an optimal summary statistic. Under circumstances that are broadly general, the sample mean for LIBS data is statistically inconsistent and the central limit theorem does not apply. This result appears to be due to very high shot-to-shot plasma variability in which a very small number of spectra are high in intensity while the majority are very weak, yielding the extreme value form of the distribution. The extreme value behavior persists when individual shots are analog accumulated. An optimal estimator in a well-defined sense for the spectral average at a given wavelength follows from the maximum likelihood method for the extreme value distribution. Example spectra taken with both an Echelle and a Czerny–Turner spectrometer are processed with this scheme to create smooth, high signal-to-noise summary spectra. Plasma imaging was used in an attempt to visually understand the observed variability and to validate the use of extreme value statistics. The data processing approach presented in this paper is statistically reliable and should be used for accurate comparisons of LIBS spectra instead of arithmetic averaging on either complete or censored data sets.     

Preliminary evaluation of laser induced breakdown spectroscopy for slurry samples

Laser-induced breakdown spectroscopy (LIBS) was applied to the analysis of simulant slurry samples used in the vitrification process of liquid radioactive wastes. A spectroscopic analysis was performed by two different detection systems: Czerny-Turner spectrometer coupled with intensified diode array detector (IDAD) and an Echelle spectrometer with intensified charge coupled device (ICCD). For the Czerny-Turner detection system, the normalized intensity method, which is the normalization of the atomic emission intensity by the released whole plasma emission area intensity, was employed to improve the reproducibility of LIBS signals. The Echelle detection system showed a high efficiency in simultaneous multi-element detection and determination of the physical quantities of the simulant.     

A computer program for the collection,reduction and analysis of echelle spectra

This article is an electronic publication in Spectrochimica Acta Electronica (SAE), the electronic section of Spectrochimica Acta Part B (SAB). The hardcopy text, comprising the main article and an appendix, is accompanied by a disk containing the compiled program, a manual, the source code and tutorial in ASCII format, and data files. The work presented is a result of the need to facilitate collection, calibration, and extraction of data from an echelle spectrometer employing charge coupled array detection (CCD). A computer program, written using Microsoft's BASIC Professional Development System version 7.1 under MS DOS, is explained and demonstrated. Wavelength calibration requires critical spectrometer dimensions, grating counter settings, and the identification of a single spectral line. Calibration accuracies are better than ± 1 pixel across a 576 × 384 pixel array. Extraction of intensity-normalized spectra for all detected orders requires less than 3 min on a 33 MHz 80386 personal computer with an 80387 math coprocessor.     

An evaluation of a commercial Échelle spectrometer with intensified charge-coupled device detector for materials analysis by laser-induced plasma spectroscopy

In this work we evaluate the performance of a commercial Échelle spectrometer coupled with an intensified charge-coupled device (ICCD) detector for the analysis of solid samples by laser-induced plasma spectroscopy (LIPS) in air at atmospheric pressure. We compare results obtained in aluminum alloy samples with this system and with a ‘conventional’ Czerny-Turner spectrometer coupled to an intensified photodiode array (IPDA). We used both systems to generate calibration curves and to determine the detection limit of minor elements, such as Mg, Cu, Si, etc. Our results indicate that no significant differences in terms of analytical figures of merit exist between the Échelle/ICCD system and a conventional Czerny-Turner spectrometer with IPDA. Moreover, measurements of plasma temperature and electron density using the two assemblies give, in general, very similar results. In the second part of this work, we aim to present a critical view of the Échelle spectrometer for LIPS applications, by drawing up the balance sheet of the advantages and limitations of the apparatus. The limitations are either inherent to the dispersion method, or result from the dynamic range of the detector. Moreover, the minimum ICCD readout time does not allow a fast data acquisition rate. On the other hand, the Échelle spectrometer allows complete elemental analysis in a single shot, as spectral lines of major, minor and trace constituents, as well as plasma parameters, are measured simultaneously. This enables a real-time identification of unknown matrices and an improvement in the analytical precision by selecting several lines for the same element.     

Laser induced breakdown spectrometry with an échelle spectrometer and intensified charge coupled device detection

The combination of an échelle spectrometer with a time-gated intensified charge coupled device (ICCD) camera for simultaneous measurements of spectral lines in wide ranges of the optical spectrum is presented. The analytical and diagnostic power of the échelle-ICCD combination is demonstrated by laser induced breakdown spectrometry (LIBS) of solid samples. The apparatus allows complete elemental analysis by single shots as the spectral lines of major, minor and trace constituents are measured simultaneously. The analytical precision and detection limits in single shot measurements are improved by the evaluation of many lines for each element. Further, plasma parameters, such as the plasma temperatures, can be determined from the same spectrum and, if necessary, used for normalization of the analyte line intensities.     

Microchip micellar electrokinetic chromatography separation of alkaloids with UV-absorbance spectral detection

A microchip device is demonstrated for the electrophoretic separation and UV-absorbance spectral detection of four toxic alkaloids: colchicine, aconitine, strychnine, and nicotine. A fused-silica (quartz) microchip containing a simple cross geometry is utilized to perform the separations, and a miniature, fiber-optic CCD spectrometer is coupled to the microchip for detection. Sensitive UV-absorbance detection is achieved via the application of online preconcentration techniques in combination with the quartz microchip substrate which contains an etched bubble-cell for increased pathlength. The miniature CCD spectrometer is configured to detect light between 190 and 645 nm and LabView programming written in-house enables absorbance spectra as well as separations to be monitored from 210 to 400 nm. Consequently, the configuration of this microchip device facilitates qualitative and quantitative separations via simultaneous spatial and spectral resolution of solutes. UV-absorbance limits of quantification for colchicine, 20 microM (8 mg/L); strychnine, 50 microM (17 mg/L); aconitine, 50 microM (32 mg/L); and nicotine, 100 microM (16 mg/L) are demonstrated on the microchip. With the exception of aconitine, these concentrations are > or =20-times more sensitive than lethal dose monitoring requirements. Finally, this device is demonstrated to successfully detect each toxin in water, skim milk, and apple juice samples spiked at sublethal dose concentrations after a simple, SPE procedure.     

Evaluation of laser induced breakdown spectroscopy for cadmium determination in soils

Cadmium is known to be a toxic agent that accumulates in the living organisms and present high toxicity potential over lifetime. Efforts towards the development of methods for microanalysis of environmental samples, including the determination of this element by graphite furnace atomic absorption spectrometry (GFAAS), inductively coupled plasma optical emission spectrometry (ICP OES), and inductively coupled plasma-mass spectrometry (ICP-MS) techniques, have been increasing. Laser induced breakdown spectroscopy (LIBS) is an emerging technique dedicated to microanalysis and there is a lack of information dealing with the determination of cadmium. The aim of this work is to demonstrate the feasibility of LIBS for cadmium detection in soils. The experimental setup was designed using a laser Q-switched (Nd:YAG, 10 Hz, λ = 1064 nm) and the emission signals were collimated by lenses into an optical fiber coupled to a high-resolution intensified charge-coupled device (ICCD)-echelle spectrometer. Samples were cryogenically ground and thereafter pelletized before LIBS analysis. Best results were achieved by exploring a test portion (i.e. sampling spots) with larger surface area, which contributes to diminish the uncertainty due to element specific microheterogeneity. Calibration curves for cadmium determination were achieved using certified reference materials. The metrological figures of merit indicate that LIBS can be recommended for screening of cadmium contamination in soils.     

Laser-induced breakdown spectroscopy—From research to industry,new frontiers for process control

This paper presents R&D activities to explore new laser parameter ranges in pulse energy, time and space for laser-induced breakdown spectroscopy. The collinear double pulse effect, which is well studied for pulses of typically several 100 mJ energy can also be observed for laser pulses having a pulse energy two orders of magnitude lower. In this case, maximum line emission intensity occurs at interpulse separations of a few 100 ns. Temporal pulse tailoring to improve the performance of LIBS is only a first step. A comprehensive approach includes spatial pulse shaping to generate craters with predefined shape or to improve spatial averaging for the analysis of inhomogeneous samples. High performance components for LIBS systems such as spectrometers, electronics and sample stands are required to enable industrial applications. Latest developments offer wide-band single spectra acquisition with a high spectral resolution at a measuring frequency of up to 500 Hz. The next generation of multi-channel integrator electronics for Paschen–Runge spectrometers equipped with PMT detectors will further push the measuring speed to up to 5 kHz, thus opening a new area of high-speed LIBS microanalysis. Novel LIBS devices for various industrial applications presented include analysis of metallic process control samples with scale layers, on-site analysis of slag samples in secondary metallurgy, high-speed identification of Al scrap, mix-up detection of pipe fittings as well as recent work towards in-process identification of hot coils in a rolling mill.     

Optimization of micro-Laser Induced Breakdown Spectroscopy analysis and signal processing

As a result of continuing instrumental development (Echelle spectrometer and ICCD detectors), micro-Laser Induced Breakdown Spectroscopy analysis may become an increasingly recognized analytical technique for determining elemental compositions of geologic materials. Best conditions of time resolution conditions (delay and time acquisition window) are estimated with respect to the collection geometry of optical plasma emission of our system. It turns out that the level of the Bremsstrahlung continuum emission is weak in the first tens of nanoseconds after the laser excitation pulse. The enlargement of the emission lines is identified in the first 100 ns but remains comparable to the spectral resolution of our system. Thus, results show that time-resolved conditions are not necessarily required to perform elemental analysis at the micrometric scale using LIBS, contrary to macro-LIBS. This suggests potential improvements of micro-LIBS analysis (sensitivity and spectral resolution) using non-intensified CCD connected with the laser pulse.     

An electron bombardment CCD-camera as detection system for an echelle spectrometer

The electron bombardment CCD (EB-CCD)-camera system is a very sensitive detector designed mainly for low-light imaging applications. The major advantage compared to a microchannel plate intensified CCD system is the superior spatial resolution due to the special amplification system. This property would make it an ideal detection system for a high-resolution echelle spectrometer, where high sensitivity in combination with high spatial resolution is required. However, we found that part of the light focused on the photocathode of the EB-CCD is transmitted through it and detected on the CCD chip as a diffuse spot. Thus, it lowers the signal to background ratio and can lead to ghost peaks. The problem becomes particularly serious when detecting emission lines of very different intensities.     

Evaluation of minor element concentrations in potatoes using laser-induced breakdown spectroscopy

We have performed spectroscopic analysis of the plasma generated by Nd:YAG laser irradiation of flesh and skin of fresh potatoes. From the spectra recorded with an Echelle spectrometer 11 minor elements have been identified. Their relative concentrations were estimated by comparing the measured spectra to the spectral radiance computed for a plasma in local thermal equilibrium. According the moderate plasma temperature of about 6500 K at the time of spectroscopic observation, the electrons are essentially generated by the ionization of the minor metal atoms, making plasma modeling possible although the organic elements may be out of equilibrium. Among the spectral lines selected for the analysis, the Na I 588.99 and 589.59 nm doublet was found to be partially self-absorbed allowing us to estimate the number density of sodium atoms. The value was found to agree with the number density predicted by the plasma model. As a result, the relative concentrations of the detected minor elements have been estimated for both the flesh and skin of the potatoes. Among these, aluminum and silicon were found to have relatively large mass fractions in the potato skin whereas their presence was not detected in the flesh. The present study shows that laser-induced breakdown spectroscopy is a promising tool to measure the elemental composition of fresh vegetables without any sample preparation.     

Development of a miniature simultaneous MPT spectrometer

An unique miniature simultaneous microwave plasma torch (MPT) atomic emission spectrometer employing linear UV intensified charge-coupled device (ICCD) array detector has been developed and studied preliminarily. The detection limits and precisions of the spectrometer for Ag, Al, Ba, Ca, Cr, Cu, Fe, Mg, Mn, Sr and V by using different CCD exposure times have been determined. An analysis of a practical sample has been carried out. The preliminary results demonstrate that such simultaneous spectrometer has advantages of saving sample and time, especially suitable for use as detector for chromatography and in combination with flow injection systems. Taking analytical figures of merit and portability into accounts, the miniature simultaneous MPT system will have extended application areas and greater competition potential as compared with commercialized scanning MPT spectrometers.     

Analysis of lead and sulfur in environmental samples by double pulse laser induced breakdown spectroscopy

In the present work, a model of double pulse laser-induced breakdown spectroscopy (LIBS) spectrometer has been developed and results from two different applications of double pulse LIBS for solving the problems of environmental interest are presented. In one case, laser induced breakdown spectroscopy has been applied to the determination of heavy and toxic metals (lead) in soil samples. In the second case, laser induced breakdown spectroscopy was used in preliminary experiments for the detection of sulfur content in coal, and on the basis of spectral features, ways to improve the sensitivity of laser induced breakdown spectroscopy detection of sulfur are proposed. The detection limit for lead in soil was estimated to be approximately 20 ppm that is lower than the regulatory standards for the presence of lead in soil.     

Development of a miniature dielectric barrier discharge–optical emission spectrometric system for bromide and bromate screening in environmental water samples

Dielectric barrier discharge (DBD) at atmospheric pressure provides an efficient radiation source for the excitation of bromine and it is used for the first time for optical emission spectrometric (OES) detection of bromide and bromate. A portable DBD–OES system is developed for screening potential pollution from bromide and bromate in environmental waters. Bromide is on-line oxidized to bromine for in-situ generation of volatile bromine. Meanwhile, a helium stream carries bromine into the DBD micro-plasma for its excitation at a discharging voltage of 3.7 kV and optical emission spectrometric detection with a QE65000 charge-coupled device (CCD) spectrometer in the near-infrared spectral region. Similarly, the quantification of bromate is performed by its pre-reduction into bromide and then oxidized to bromine. The spectral characteristics and configuration of the DBD micro-plasma excitation source in addition to the oxidation vapor generation of bromine have been thoroughly investigated. With a sampling volume of 1 mL, a linear range of 0.05–10.0 mg L⁻¹ is obtained with a detection limit of 0.014 mg L⁻¹ by measuring the emission at 827 nm. A precision of 2.3% is achieved at 3 mg L⁻¹ bromide. The system is validated by bromine detection in certified reference material of laver (GBW10023) at mg L⁻¹ level, giving rise to satisfactory agreement. In addition, it is further demonstrated by screening trace bromide and bromate as well as spiking recoveries in a series of environmental water samples.     

Preliminary studies of laser-induced breakdown spectrometry for the determination of Ba,Cd, Cr and Pb in toys

The performance of laser-induced breakdown spectrometry (LIBS) for the determination of Ba, Cd, Cr and Pb in toys has been evaluated by using a Nd:YAG laser operating at 1064 nm and an Echelle spectrometer with intensified charge-coupled device detector. Samples were purchased in different cities of São Paulo State market and analyzed directly without sample preparation. Laser-induced breakdown spectrometry experimental conditions (number of pulses, delay time, integration time gate and pulse energy) were optimized by using a Doehlert design. Laser-induced breakdown spectrometry signals correlated reasonably well with inductively coupled plasma optical emission spectrometry (ICP OES) concentrations after microwave-assisted acid digestion of selected samples. Thermal analysis was used for polymer identification and scanning electron microscopy to visualize differences in crater geometry of different polymers employed for toy fabrication. Results indicate that laser-induced breakdown spectrometry can be proposed as a rapid screening method for investigation of potentially toxic elements in toys. The unique application of laser-induced breakdown spectrometry for identification of contaminants in successive layers of ink and polymer is also demonstrated.     

Detector comparison for sulfur and chlorine detection with laser induced breakdown spectroscopy in the near-infrared-region

Laser-induced breakdown spectroscopy has been employed for the investigation of the sulfur and chlorine content of building materials. Both, chloride and sulfate ions are major damaging species affecting the stability and lifetime of a structure. Chlorine and sulfur are mostly detected in the VUV and the NIR. In case of building materials the main elements like calcium or iron have many strong spectral lines over the whole spectral range, so that trace elements can only be detected in spectral windows unaffected from these lines. With regard to a preferably simply, robust against dust and vibrations and portable setup only the NIR spectral features are used for civil engineering applications.

Most detectors, mainly CCD cameras have rapidly decreasing quantum efficiency in the NIR. Also the quantum efficiency of the photocathode of CCD-Detectors with image intensifier is decreasing in the NIR. Different CCD-detectors were tested with respect to high quantum efficiency and high dynamic range, which is necessary for simultaneous detection of weak spectral lines from trace elements and intense spectral lines from main elements.

The measurements are made on reference samples consisting of cement, hydrated cement, cement mortar and concrete with well-defined amounts of the trace elements. Experimental conditions are chosen for an optimum intensity of the trace element spectral lines. The detector systems are compared by limit of detections and the signal to noise ratio.     

Detection of trace amounts of Ni by laser-induced fluorescence in graphite furnace with intensified charge coupled device

A laser-induced fluorescence in graphite furnace (LIF-GF) set-up has been equipped with an intensified CCD detector (ICCD) that enables simultaneous multichannel detection of large wavelength regions. The main advantages of such a system in comparison with conventional photomultiplier detection are: simultaneous detection of several fluorescence wavelengths for easy characterization of excitation and fluorescence spectra and for an increase of the absolute sensitivity and spectral selectivity; simultaneous monitoring of background signals, such as those due to matrix interferences, blackbody radiation and scattered laser light; decrease of the susceptibility to radio-frequency pick-ups emitted from the pump laser due to the delayed read-out procedure; time-resolved studies of fluorescence spectra for improved elemental selectivity or for studies of atomization processes, and a possibility to perform two-dimensional imaging of height distributions of atomization and, in combination with an imaging spectrometer, diffusion processes in the furnace. The first work on LIF-GF with ICCD detection has been performed on Ni. The most sensitive and versatile excitation and detection wavelengths have been identified. Detection limits in water solutions by the LIF-GF technique have been improved by two orders of magnitude and are found to be 0.015 pg with ICCD and 0.01 pg using a photomultiplier at the most sensitive excitation and detection wavelengths. Nickel in concentrations has been detected in aqueous standard reference samples with sodium concentrations ranging from to % (riverine water and estuarine water) with good accuracy and precision. The Ni contents of standard riverine and estuarine water were determined to 1.00 ± 0.11 and 0.75 ± 0.07 ng/ml, respectively. The certified concentrations are 1.03 ± 0.10 and 0.743 ± 0.078 .     

An echelle polychromator for inductively coupled plasma optical emission spectroscopy with vacuum ultraviolet wavelength coverage and charge injection device detection

The development and evaluation of a simultaneous inductively coupled plasma optical emission spectrometer with vacuum ultraviolet (VUV) wavelength coverage and charge injection device (CID) detection are described. The spectrometer is an echelle polychromator with a magnesium fluoride prism as a cross-disperser. A thin coating of Lumogen, used as a wavelength conversion phosphor, is applied to the surface of the CID detector. The Lumogen coating increases sensitivity in the VUV region dramatically. Wavelengths between 130 and 800 nm can be measured by this spectrometer. A measured spectral bandwidth of 0.006 nm at 134.724 nm is achieved. The analytical performance in radial viewing mode is evaluated. The 3σ detection limit measured for Cl at 134.724 nm in aqueous solution is 100 ng ml⁻¹.     

Autonomous Portable Solar Ultraviolet Spectroradiometer (APSUS) – a New CCD Spectrometer System for Localized,Real‐Time Solar Ultraviolet (280–400 nm) Radiation Measurement

Terrestrial solar ultraviolet (UV) radiation has significant implications for human health and increasing levels are a key concern regarding the impact of climate change. Monitoring solar UV radiation at the earth's surface is therefore of increasing importance. A new prototype portable CCD (charge‐coupled device) spectrometer‐based system has been developed that monitors UV radiation (280–400 nm) levels at the earth's surface. It has the ability to deliver this information to the public in real time. Since the instrument can operate autonomously, it is called the Autonomous Portable Solar Ultraviolet Spectroradiometer (APSUS). This instrument incorporates an Ocean Optics QE65000 spectrometer which is contained within a robust environmental housing. The APSUS system can gather reliable solar UV spectral data from approximately April to October inclusive (depending on ambient temperature) in the UK. In this study the new APSUS unit and APSUS system are presented. Example solar UV spectra and diurnal UV Index values as measured by the APSUS system in London and Weymouth in the UK in summer 2012 are shown.