Continuum source tungsten coil atomic fluorescence spectrometry

A simple continuum source tungsten coil atomic fluorescence spectrometer is constructed and evaluated. The heart of the system is the atomizer: a low-cost tungsten filament extracted from a 150 W light bulb. The filament is resistively heated with a small, solid-state, constant-current power supply. The atomizer is housed in a glass chamber and purged with a 1 L/min flow of a conventional welding gas mixture: 10% H(2)/Ar. A 25 μL sample aliquot is pipetted onto the tungsten coil, the liquid is dried at low current, and then the atomic vapor is produced by applying a current in the range 3.5-5.5 A. The atomization current does not produce temperatures high enough to excite atomic emission. Radiation from a 300 W xenon lamp is focused through the atomic vapor, exciting atomic fluorescence. Fluorescence signals are collected using a hand-held charge-coupled device (CCD) spectrometer. Simultaneous determination of ten elements (Ag, Bi, Cr, Cu, Ga, In, Mg, Mn, and Tl) results in detection limits in the range 0.3 to 10 ng. The application of higher atomization currents (10 A) leads to straightforward detection of atomic emission signals with no modifications to the instrument.     

Continuum-source atomic absorption spectroscopy with an echelle spectrometer adapted to a charge injection device

An instrumental system for continuum-source atomic absorption spectroscopy has been developed for simultaneous multielement determinations. The system consists of an electrothermal atomizer and a charge injection device adapted to an echelle spectrometer to achieve multiplex detection. A continuous 40-nm spectral range in the two-dimensional echelle spectrum was acquired simultaneously through the capability of the charge injection device to integrate signals in its MOS capacitors. Novel methods were developed to compute absorbances by "scanning" through all orders in the entire echelle spectrum or selecting absorption lines randomly. In the range 300-430 nm, characteristic concentrations (1% absorption) were 1.6, 2.6, 2.9, and 3.8 ng mL-1 respectively for Cu, Mn, and two Cr lines; these values are similar to those (1.3, 2.2, 1.2, and 3.6 ng mL-1) obtained for single-element detection with an image-dissector system.     

Continuum source atomic absorption spectrometry in a graphite furnace with photodiode array detection

A graphite furnace continuum source atomic absorption spectrometer using a photodiode array detector is described that provides high-resolution wavelength versus absorbance spectra over a 2.5-nm range for a single atomization step. The multiwavelength detection power allows the simultaneous determination of several elements, reduces problems caused by spectral interferences, and automatically corrects for nonzero background absorbance. Each spectrum is acquired in 0.33 s, and several successive spectra can be obtained during a single run. Three-dimensional wavelength-absorbance-furnace temperature spectra can be obtained by using ramped heating steps to provide a rough separation of elements in a mixture. Limits of detection calculated for 19 elements range from 0.1 pg for magnesium to 700 pg for arsenic. The sampling precision was found to be better than 10% relative standard deviation in all cases, with the precision for a single atomization being greatly increased when multiple absorption lines for a single element are observed in the spectrum. The error found for the measurement of the iron concentration in an NBS standard bronze was 8.5%, with the calculated concentration agreeing with the certified concentration within 95% confidence limits.     

Fraunhofer-type absorption lines in double-pulse laser-induced plasma

We studied the confocal double-pulse laser-induced plasma in the very beginning of its life. It was found that the second laser pulse fired 0.7 to 5 μs after the first pulse produces plasma which, during the first 0 to 20 ns, resembles solar configuration. There is a very hot and compact plasma core that radiates a broad continuum spectrum and a much larger and cooler outer shell. The light from the hot core passes through the cold outer shell and is partly absorbed by atoms and ions that are in ground (or close to ground) states. This produces absorption lines that are similar to Fraunhofer lines observed in the sun spectrum. The possibility to use these absorption lines for new direct and calibration free laser-induced breakdown spectroscopy analytical applications, both in laboratory and industrial conditions, is proved.     

Fourier transform atomic absorption flame spectrometry with continuum source excitation

The design and performance of a Fourier transform atomic absorption flame spectrometer (FT-AAS) is presented. A 300-W xenon arc continuum source and a Michelson interferometer are used. A signal to noise disadvantage arising from the multiplex feature of FT-AAS is demonstrated by varying the photon flux at the detector without changing the exciting radiation. A grating is used for dispersion of the radiation before the interferometer to reduce the spectral window at the photomultiplier tube. Detection limits for several elements are generally an order of magnitude poorer than those obtained by continuum atomic absorption methods using echelle-grating spectrometers. Line profiles and absorption spectra, within the region of the spectral window selected by the grating, can be obtained with this method. Standard curves for sodium were constructed to extend the linear calibration range, by using absorbances measured at the absorption maximum and 0.022 nm off-line.     

New developments in atomic absorption spectrometry

We consider some new methods and measuring instruments used in atomic absorption spectrometry: flame atomizer/annular-slot burner, rapidly heated graphite furnace with ballast, high resolution spectrometers with continuum source, and also the new standard GOST R 8.649-2008 of January 1, 2010.     

Oxygen 630.0- and 557.7-nm line source for thermospheric dynamics studies

A novel design of a rf afterglow source of the oxygen forbidden lines at 630.0 and 557.7 nm for use in thermospheric dynamics studies is presented. With a repetitive discharge-afterglow excitation cycle, the source yields adequate afterglow intensities of the OI lines that are sufficiently free of background continuum for use in Fabry-Perot interferometer measurements of spectral line profiles. These lines provide accurate zero-velocity references in Fabry-Perot determinations of Doppler shifts in the OI thermospheric lines. The design considerations for the rf source are described, together with its preparation and filling by an ultrahigh-vacuum gas-handling system. Examples are given of the source's output spectrum, as measured by a grating spectrometer, and its spectral line profiles, as determined by a Fabry-Perot interferometer. Comparative interferometry between the OI afterglow source lines and nearby (within approximately 0.01-0.02-nm) lines from hollow-cathode sources illustrates the means of establishing secondary reference sources in cases in which the primary OI afterglow source is not available.     

Application of multifactor experimental design for optimizing the conditions of atomic emission determination of noble metals using a double-jet arc plasmatron

Conditions for atomic emission analysis using a Grand spectrometer equipped with a double-jet arc plasmatron as the excitation source are optimized. The impact of the plasma-forming and carrier gas flow rate on the analytic signal of precious metals is studied. On the basis of the mathematical method of multifactor experimental design, the influence of various factors on the intensity of the spectral lines of analytes are ascertained, and optimally compromise conditions for the determination of noble metals are chosen. An evaluation of the analytical capabilities of the Grand spectrometer is performed, and the detection limits for precious metals (PM) under chosen optimum conditions are calculated.     

Detection of Sodium in Highly Pure Graphite via High-Resolution Electrothermal Atomic Absorption Spectrometry with a Continuous Spectrum Source

This study is dedicated to elucidating the analytical abilities of electrothermal atomic absorption spectrometry with a continuous spectrum source (ETAAS-CSS) to detect the presence of sodium in highly pure graphite powder. In order to plot the calibration dependence for sodium detection, a special technique based on argon dilution of the atomic vapor obtained by introducing the aqueous reference solution in the atomizer is applied. The conditions for thermal pretreatment of highly pure graphite powder and atomization of sodium are established, and the sodium detection threshold is evaluated via ETAAS-CSS (2.6 × 10^–4 ng) as well. The validity of the results is confirmed via the sample variation method.     

Determination of optical coefficients of biological tissue from a single integrating-sphere

The detection of interactions between light and tissue can be used to characterize the optical properties of the tissue. The development is described of a method that determines optical coefficients of biological tissue from a single optical reflectance spectrum measured with an integrating-sphere. The experimental system incorporated a DH-2000 deuterium tungsten halogen light source, a USB4000-VIS-NIR miniature fiber optic spectrometer and an integrating-sphere. Fat emulsion and ink were used to mimic the scattering and absorbing properties of tissue in the tested sample. The measured optical reflectance spectrums with different scattering and absorbing properties were used to train a back-propagation neural network (BPNN). Then the neural network (BPNN) was used to determine the optical coefficients of biological tissue from a single optical reflectance spectrum measured with an integrating-sphere. Tests on tissue-simulation phantoms showed the relative errors of this technique to be 7% for the reduced scattering coefficient and 15% for the absorption coefficients. The optical properties of human skin were also measured in vivo.     

Light-emitting-diode-based light source for calibration of an intensified charge-coupled device detection system intended for galvanoluminescence measurements

A spectrally tunable light source utilizing three light-emitting diodes (LEDs) for calibration of a highly sensitive intensified charge-coupled device (ICCD) optical detection system intended for time-resolved galvanoluminescence (GL) measurements is described. The source has been conceived as a low-cost substitute for standard tungsten lamps usually used for relative and absolute calibration of optical detection systems. Three LEDs with different spectral characteristics in conjunction with a system of two integrating spheres as light mixers and light reducers are used. This construction provides control over the source spectrum by changing individual LED contributions. The use of integration spheres eliminated angular distribution of light intensities of LEDs as well as angular dependence of their spectral contributions. Moreover, by using the source we have avoided the problem of stray and diffuse light of higher wavelengths, as well as different light intensities for different wavelengths (up to three orders of magnitude in the range from 400 nm to 750 nm), which we have with standard tungsten lamps. A complete calibration procedure for the LED source and ICCD detection system is described. Finally, for the first time, we have performed time-resolved spectral GL measurements during aluminum anodization in porous film-forming electrolyte phosphoric acid in a transient regime. Two peaks at 425 nm and 595 nm are recognized, confirming the same mechanism of GL in both transient and steady-state regimes of anodization.     

Rugged, portable tungsten coil atomic emission spectrometer

Tungsten coil atomic emission spectrometry is an ideal technique for field applications because of its simplicity, low cost, low power requirement, and independence from cooling systems. A new, portable, compact design is reported here. The tungsten coil is extracted from an inexpensive 24 V, 250 W commercial light bulb. The coil is housed in a small, aluminum cell. The emission signal exits from a small aperture in the cell, while the bulk of the blackbody emission from the tungsten coil is blocked. The resulting spectra exhibit extremely low background signals. The atomization cell, a single lens, and a hand-held charge coupled device (CCD) spectrometer are fixed on a 1 × 6 × 30 cm ceramic base. The resulting system is robust and easily transported. A programmable, miniature 400 W solid-state constant current power supply controls the temperature of the coil. Fifteen elements are determined with the system (Ba, Cs, Li, Rb, Cr, Sr, Eu, Yb, Mn, Fe, Cu, Mg, V, Al, and Ga). The precision ranges from 4.3% to 8.4% relative standard deviation for repetitive measurements of the same solution. Detection limits are in the 0.04 to 1500 μg/L range. Accuracy is tested using standard reference materials for polluted water, peach leaves, and tomato leaves. For those elements present above the detection limit, recoveries range from 72% to 147%.     

Accurate and precise wavelength calibration for wide bandwidth array spectrometers

Calibrating the wavelength scale of an array spectrometer typically involves measurements of lines at well-known wavelengths from a calibration lamp such as a mercury-argon source. This process is relatively straightforward when the lines are well separated, relative to the bandwidth of the spectrometer. When the spectrometer's bandwidth is large, compared with the distance between calibration wavelengths, it becomes increasingly difficult to accurately locate lines in the calibration spectrum. Even calibrations for instruments with a modest bandwidth of 12 nm can be difficult. Here we present results from a simple approach to improve the accuracy of wavelength calibration for an instrument with a large bandwidth (12 nm, center-to-center pixel spacing 3.3 nm). A monochromator has been used to filter the source so that each calibration line can be measured separately. For ten spectrometers, we were able to achieve accuracy better than 0.12 nm, or 0.09 nm on average; this is less than 3% of the pixel spacing. We anticipate this approach will be useful for improving the accuracy of measurements on array spectrometers and particularly in transferring multivariate calibrations between instruments.     

Optimization of dual-band continuum light source for ultrahigh-resolution optical coherence tomography

We demonstrate a dual-band continuum light source centered at 830 and 1300 nm for optical coherence tomography (OCT) generated by pumping a photonic crystal fiber having two closely spaced zero-dispersion wavelengths with a femtosecond laser at 1059 nm. By use of polarization control, sidelobe suppression can be improved up to approximately 7.7 dB. By employing compression of the pump pulses, the generated spectrum is smooth and near-Gaussian, resulting in a point-spread function with negligible sidelobes. We demonstrate ultrahigh-resolution OCT imaging of biological tissue in vivo and in vitro using this light source and compare it with conventional-resolution OCT imaging at 1300 nm.     

Microchip laser-induced breakdown spectroscopy: a preliminary feasibility investigation

A commercial, 7 microJ/pulse, 550 ps microchip laser is used to induce plasma on Pb, Si, Cu, Fe, Ni, Ti, Zn, Ta, and Mo foils and a Si wafer. The measured plasma lifetime is comparable with the duration of the laser pulse (a few ns). The plasma continuum radiation is low, while some of the strong resonance lines (e.g., Zn 213.86 nm) show self-reversal. Quantitative analysis is possible using non-gated detectors but analytical lines should be chosen with care to avoid reduction in the linear dynamic range. The mass removed (0.5-20 ng/pulse) is sufficient to yield spectra that are detectable with portable grating spectrometers equipped with non-gated, non-intensified detector arrays. The spectrum of Cd is detected with a broadband portable spectrometer (200-950 nm). The combination of the broadband spectrometer and the microchip laser is very promising for material identification, especially in field applications.     

Detection of benzene and other gases with an open-path, static Fourier-transform UV spectrometer

Releases of benzene and other gases have been detected and quantified using a novel optical, open-path instrument based on a deuterium light source and a static Fourier-transform spectrometer. The spectrometer uses Wollaston prisms to form an interferogram in the spatial domain that is recorded by use of a detector array. The instrument is designed to operate in the ultraviolet region of the spectrum between 200 and 270 nm, which coincides with strong absorption features in the spectra of many gases of environmental and health interest. Using the instrument with a 5-s measurement period provides a path-integrated concentration sensitivity to benzene of 2 parts in 10(6) times meter, which corresponds to a 20-parts in 10(9) detection limit over a typical path length of 100 m.     

Quantitative analysis of liquids from aerosols and microdrops using laser induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) is shown to be capable of low volume (90 pL) quantitative elemental analysis of picogram amounts of dissolved metals in solutions. Single-pulse and collinear double-pulse LIBS were investigated using a 532 nm dual head laser coupled to a spectrometer with an intensified charge coupled device (CCD) detector. Aerosols were produced using a micronebulizer, conditioned inside a concentric spray chamber, and released through an injector tube with a diameter of 1 mm such that a LIBS plasma could be formed ~2 mm from the exit of the tube. The emissions from both the aerosols and a single microdrop were then collected with a broadband high resolution spectrometer. Multielement calibration solutions were prepared, and continuing calibration verification (CCV) standards were analyzed for both aerosol and microdrop systems to calculate the precision, accuracy, and limits of detection for each system. The calibration curves produced correlation coefficients with R(2) values > 0.99 for both systems. The precision, accuracy, and limit of detection (LOD) determined for aerosol LIBS were averaged and determined for the emission lines of Sr II (421.55 nm), Mg II (279.80 nm), Ba II (493.41 nm), and Ca II (396.84 nm) to be ~3.8% RSD, 3.1% bias, 0.7 μg/mL, respectively. A microdrop dispenser was used to deliver single drops containing 90 pL into the space where a LIBS plasma was generated with a focused laser pulse. In the single drop microdrop LIBS experiment, the analysis of a single drop, containing a total mass of 45 pg, resulted in a precision of 13% RSD and a bias of 1% for the Al I (394.40 nm) emission line. The absolute limits of detection of single drop microdrop LIBS for the emission lines Al I (394.40 nm) and Sr II (421.5 nm) were approximately 1 pg, and Ba II (493.41 nm) produced an absolute detection limit of approximately 3 pg. Overall, the precision, accuracy, and absolute LOD determined for single microdrop LIBS resulted in a typical performance of ~14% RSD, 6% bias, and 1 pg for the elements Sr II (421.55 nm), Al I(394.40 nm), Mg II (279.80), and Ba II(493.41 nm).     

Doppler width limited near-infrared Raman spectrometer

A new ultra high resolution spontaneous Raman spectrometer with a single mode tunable laser diode as an excitation source and a 0.275 m, f/#4 spectrograph is presented. The spectrometer is expanded by an atomic vapor (Rb) absorption filter. One of the rubidium resonance lines serves as the frequency marker by absorbing selected Raman lines during the frequency scan of the excitation laser. High resolution was achieved while preserving the speed of the spectrometer. The extended spectrometer's capability was tested by differentiating overlapping Raman lines from molecular hydrogen isotopomers: H2, D2, and HD. Experiments were carried out at 300 K and pressures near 10(4) Pa. Spectral lines separated by approximately 10 Ghz (0.3 cm-1) can be resolved with this instrument with data collection times of minutes.     

Solid-state digital micro-mirror array spectrometer for Hadamard transform measurements of glucose and lactate in aqueous solutions

A novel solid-state near-infrared spectrometer is presented based on a digital micro-mirror array device (DMD) that is well designed for Hadamard transform spectroscopy. This spectrometer is designed for the collection of transmission spectra over the C-H first overtone region of the near-infrared spectrum (6500-5500 cm(-1)). A spectral resolution of 2.2 nm (~11 cm(-1)) is realized by using a 25 μm diameter linear tungsten filament as the source. Such a thin filament reduces imaging aberrations into the micro-mirror array, thereby enhancing spectral resolution. After passing through the sample, the transmitted radiation is dispersed with a grating before being imaged onto the surface of the DMD. Hadamard transform masks are implemented through the DMD and the reflected light is monitored by a single-element photodiode detector. The analytical utility of this approach is demonstrated through the multivariate quantification of glucose and lactate in binary mixtures composed in an aqueous buffer solution. A signal-to-noise ratio of 35,000 is achieved through these aqueous samples, and the resulting quantitative measurements provide a standard error of prediction of 1.4 and 0.9 mM for glucose and lactate, respectively. The selectivity of the resulting calibration models is established by using both a pure component selectivity analysis as well as analysis of the net analyte signal for each component. These quantitative results from the DMD Hadamard transform spectrometer compare favorably to similar measurements performed with a commercial Fourier transform spectrometer.     

Determination of subnanogram per cubic meter concentrations of metals in the air of a trace metal clean room by impaction graphite furnace atomic absorption and laser excited atomic fluorescence spectrometry

Air, drawn by vacuum through a jet, was impacted against the inside surface of an atomic absorption graphite electrothermal atomizer (ETA). The amounts of the particles thus collected were determined at the ng m-3 level by graphite furnace atomic absorption or at the pg m-3 level by laser excited atomic fluorescence. The overall reproducibility of two sets of measurements, made 7 months apart, was 23%, with no significant difference between the two sets of data, based on Student's "t" test at the 95% confidence level. Short-term reproducibility varied from 13% to 34% depending upon the air concentration of the metal. The method shows promise for monitoring long-term effectiveness of the filtering systems in trace metal clean rooms. It was not possible to test for accuracy, due to the low concentrations involved, but accuracy was expected to be within a factor of 2 or 3 of the actual value, based on theoretical aspects of impaction.