Laser-induced Atomic Fluorescence Research Articles

Continuous in situ measurements of alkali species in the gasification of biomass

This work describes the first on-line, in situ measurement of alkali species in biomass gasification using excimer laser induced fragmentation fluorescence (ELIF). Three pelletised biomass fuels were gasified under different operating conditions in a bubbling fluidized-bed reactor, using steam as the gasifying medium. Concentrations of potassium and sodium were measured in real time for several hours per measurement. Average concentrations obtained for potassium ranged from 140 to 350ppb, and those for sodium ranged from 1.7 to 60ppb. Also observed was a slow release of alkali after shutting off the fuel feed, which suggests that alkali species had been taken up in the bed material (olivine); this was confirmed by analysing the bed material before and after use. Consideration of the olivine structure/composition and possible interactions with alkali and other elements during gasification suggests that alkali-containing layers could have formed on the bed particles but appear not to form crystalline phases. This may allow for re-release of alkali while the bed is still hot.

An in situ optical gas phase analysis approach for TGA: Its assessment and application

Thermogravimetric analyzers (TGA) are widely applied for studies of coal conversion as a function of process parameters. However, weight change of fuel samples as a function of temperature is not always sufficient for detailed identification of volatilized or reacted species. Coupling TGA with various gas analyzers could provide useful additional information for understanding mechanisms of trace element release and reaction kinetics. Until now, coupling of TGA with gas analysis has mostly been carried out by extraction of gas samples from the hot reaction zone followed by time-delayed and external analysis in a separate measurement cell. Changes of sample composition during gas analysis often cannot be avoided due to temperature and pressure reduction and filtering of hot reaction gas. Additionally, effects of local gas phase change on sample surfaces due to displacement of purge gas by evolved components cannot be determined. Compared to external analysis, in situ measurement of gas composition immediately in the hot reaction zone would provide inherent advantages. In this paper, the application of three optical in situ measurement techniques for a high temperature TGA is shown. Therefore, a TGA standard setup was redesigned and equipped with sapphire windows and optical ports. Transmission behavior of sapphire against wavelength and temperature is presented. Coal and char gasification and alkali release were examined with thermogravimetric methods and complemented with optical techniques. The release of gaseous sodium compounds was investigated with Excimer Laser Induced Fragmentation Fluorescence Spectroscopy (ELIF). The main gasification products (H 2, CO 2, CO, H 2O) were detected with Raman and Fourier Transformation Infrared Spectroscopy (FTIR). Various experiments demonstrate that in situ spectroscopy is applicable up to 1300 °C even in an optically difficult accessible TGA and results give useful additional input to further evaluation of TGA data.

Sub-Part-per-Billion Analysis of Aqueous Lead Colloids by ArF Laser Induced Atomic Fluorescence

Highly sensitive analysis of aqueous lead carbonate colloids was demonstrated by two-pulse laser-induced atomic fluorescence. The first laser pulse at 1064 nm ablated the sample solution to create an expanding plume. The colloids, being heavier, trailed behind and became concentrated. They were then intercepted by an ArF laser pulse that induced prompt atomic fluorescence at 405.8 nm from the lead atoms. The detection limit for lead was 0.24 ppb. Tap water was analyzed, and lead emissions were clearly observed. Time-resolved spectroscopy revealed that the efficient 193-nm excitation of the analytes was more universal than expected. That was confirmed by the successful application of the technique to colloids and alloys other than lead.

Simultaneous detection of nickel and potassium in the flue gas of a fluidised bed coal combustor by excimer laser-induced fragmentation fluorescence

Photofragmentation fluorescence is now well known as a sensitive, real-time technique for detection of metals and other species of environmental and industrial interest. This work aims to demonstrate the feasibility of real-time monitoring of nickel species in a realistic industrial flue gas using excimer laser-induced fragmentation fluorescence (ELIF), but at the same time, potassium species were monitored in a second detection channel. An ArF-excimer laser (193 nm) was used for photofragmentation and the fluorescence of Ni and K atom fragments was detected at 440 and 769 nm, respectively. Actual flue gas measurements were performed at the atmospheric pressure fluidised bed combustor (FBC) of DMT and first observations of the behaviour of these two species under flue gas conditions were made. Results suggest that the release of both metal species is affected by both the amount of chlorine and by the amount of clay minerals in the system. Estimating from present signal–noise levels, a sensitivity of ca. 50 ppb is achievable for Ni at the present wavelength.

Phase discrimination of alkali species in PCFB combustion flue gas using simultaneous monitoring by surface ionisation and photofragmentation fluorescence ☆

This paper is concerned with the on-line determination of alkali traces in solid fuel combustion using simultaneous surface ionisation (SI) and excimer laser induced fragmentation fluorescence (ELIF). Both methods measure in real-time but are complementary in that ELIF can discriminate towards gas phase alkali species, whereas SI detects alkali both in the gas phase and on aerosol particles. Measurements were made in the flue gas of a 10 MW PCFB combustor at 10 bar total pressure, whereby the measuring equipment was mounted onto the flue gas pipe downstream of the hot gas filter. The combustor was fed with a bituminous coal and operated in the temperature range 850–900 °C. Results show that the general response of the measured signals to parameters such as fuel load and to bed and flue gas temperatures was very similar for the two methods and reflected variations in these parameters. Concentrations determined by ELIF ranged from 6 to 63 ppb (Na) and 1 to 54 ppb (K), depending on conditions. In contrast, concentrations obtained by the SI technique were a factor of 1–10 higher than for ELIF, depending on the flue gas temperature and the total alkali (gas+particle phase) present in the flue gas. In this way it can be shown that information about the physical state of the alkali species can be obtained by direct comparison of the two methods.

In-situ study of the effect of operating conditions and additives on alkali emissions in fluidised bed combustion

In this work, alkali concentrations were determined in a realistic industrial flue gas by an in-situ laser spectroscopic method, excimer laser induced fragmentation fluorescence (ELIF). An excitation wavelength of 193 nm (ArF-excimer laser) was used to photofragment the alkali species and fluorescence signals from the atomic metal fragments were detected at 589 (Na) and 766 nm (K), respectively. These signals provide a measure of the alkali compounds in the flue gas. In the actual flue gas measurements, which were performed at the atmospheric pressure fluidised bed combustor of DMT, the flue gas alkali concentrations were observed to vary, e.g., with bed temperature and with the rate of fuel supply. Data taken following a hot gas filter in the flue gas line gave significantly reduced levels of alkali species compared to those before the filter. The addition of sodium or potassium acetate to the coal leads to increases of up to a factor of two in flue gas alkali concentrations. In some experiments, small quantities of heavy metal chlorides were added to the coal fired and this resulted in very strong increases in flue gas alkali concentrations. A possible explanation for this effect may lie in reactions with the mineral matter to form heavy metal aluminosilicates and consequent release of alkali.

Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production

Time resolved atomic emission, atomic absorbance, and laser-induced atomic fluorescence measurements of a millisecond pulsed glow discharge, made perpendicular to the insertion probe, provide temporal profiles of 1s5 (3P2) and 1s3 (3P0) metastable argon atom populations. Acquisition of these profiles at different spatial positions in the plasma provides data from which two-dimensional spatial plots of relative populations are constructed. Each map, the result of 368 individual pulse profiles, provides insight into the production of metastable argon atoms as a function of time and position within the plasma. During power application, intensities plateau after 3 ms as the plasma reaches a steady state condition. Metastable argon atoms are most abundant 1–2 mm above the cathode surface during this time. Excitation mechanisms such as electron excitation and fast atom/ion impact appear to dominate in this temporal regime. In contrast, argon ion–electron recombination dominates metastable formation after pulse termination. The relative population maximum for metastable argon atoms in the afterpeak shifts to 5–9 mm above the cathode surface. This shift should impact signals for analyte species generated by Penning processes in the plasma. Absorption and fluorescence measurements of the 3P2 (11.55 eV) and the 3P0 (11.72 eV) metastable argon atom states indicate possible differences in the populations of these two states between the plateau and afterpeak time regimes.

Laser-induced fragmentation fluorescence detection of the vinyl radical and acetylene

When excited at λ=230 nm , the vinyl radical and acetylene may be detected via fluorescence from C 2( d 3Π g → a 3Π u ) using laser-induced fragmentation fluorescence (LIFF). Vinyl photodissociation creates C 2( d 3Π g ) more efficiently than acetylene photodissociation by a factor of 1300–200 depending on the internal excitation of the vinyl radical. The key difference in LIFF efficiency between the two molecules depends on the mechanism for loss of the first H atom. This LIFF process may provide a method for probing the elusive vinyl radical in methane flames.

The influence of fuel additives on the behaviour of gaseous alkali-metal compounds during pulverised coal combustion.

The alkali-metal vapour release during pulverised hard (bituminous) coal combustion was investigated in a semi-technical drop flow reactor in the temperature range 1100-1400 degrees C. Absolute concentrations of total gas-phase sodium and potassium species were determined using the in situ/on-line excimer laser induced fragmentation fluorescence technique (ELIF). Alkali-metal concentrations measured for the untreated coals were found to be in the range 0.1 to 4.7 ppm, depending on the temperature. As well as observing the temperature dependence, the effect of co-feeding defined amounts of silica and clay minerals was studied. In addition, to assist interpretation of ELIF measurements, ash samples were taken and analysed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The additives lead to a pronounced binding of the alkali-metal species and suppression of the sharp temperature dependence observed without co-feeding. Therefore, the use of such getter materials can be confirmed as an effective way to remove corrosive alkali-metal species from the flue gas in pulverised coal combustion.

Laser-enhanced ionization and laser-induced atomic fluorescence as element-specific detection methods for gas chromatography: Application to organotin analysis

We have demonstrated that flame laser-enhanced ionization (LEI) and flame laser-induced atomic fluorescence (LIAF) techniques can be used as, alternative sensitive detectors for gas chromatographic (GC) analysis of organotin compounds. These two element-specific detection methods are free from interferences from the organic solvent. Two types of LEI schemes for Sn detection are employed. For the two-step LEI scheme (TLEI), the tin atoms in the flame were stepwise excited and then ionized collisionally. In contrast, in detection with the single-step LEI scheme (SLEI), only one dye laser is used. For the analysis of tetramethyltin and tetraethyltin, the GC–TLEI, GC–SLEI, and GC–LIAF systems yield linear dynamic ranges of 0.015–400, 0.39–600 and 0.5–600 ng, respectively. The corresponding detection limits reach 0.15, 3.9 and 5.0 mg/l, with absolute quantities corresponding to 15, 390 and 500 pg, respectively, for an injection volume of 0.1 μl. These detection methods prove to be more sensitive and selective than the conventional flame ionization detection, which achieves absolute detection limits of 800 and 1667 pg for tetraethyltin and tetramethyltin, respectively, under identical GC conditions.

Ultratrace determination of thulium by laser-excited atomic fluorescence spectrometry with atomization in a rhenium lined tube furnace

In the present work, a method for the determination of ultratrace levels of thulium by laser induced atomic fluorescence (LEAFS) was developed. High sensitivity was achieved by using a dye laser pumped by a high repetition copper vapor laser and electrothermal atomization (ETA) in a tube furnace. Four different atomizer surfaces (pyrolytic graphite, tungsten, tantalum and rhenium) as well as coating treatments of the graphite with noble metal solutions were evaluated with the objective of improving the atomization efficiency of thulium. The best results in terms of intensity, reproducibility and lifetime were obtained with rhenium-lined graphite tubes. Since the rhenium surface was more resistant to oxidation under the operational conditions, the sample was atomized under a stop flow regime rather than in an argon miniflow regime, which was required when tantalum and tungsten foils were employed. Atomization under the stop flow regime avoided dilution of the analyte in the analytical zone, thereby improving sensitivity. In addition, the lifetime of the rhenium foil lined atomizer was longer than those observed with tantalum or tungsten linings. Furnace parameters were optimized to maximize the fluorescence signal–background ratio. The limit of detection for the rhenium lined atomizer was 200 fg with a precision of 6.0% at the 10 ng ml −1 level. Thulium was determined with excellent results in urine and coal fly ash samples using the new method.

Blank problems in the detection of gold by electrothermal atomisation and laser-induced fluorescence

The limit of detection of gold by electrothermal atomisation and double-resonance laser-induced atomic fluorescence can reach the low femtogram level as absolute amount of gold deposited in the furnace. At these high sensitivities, even dry firings of the graphite tube may result in a signal, which is unequivocally attributed to atomic fluorescence of the gold content of the tube released during atomisation. The occurrence and the analytical relevance of such blank signals are presented and discussed in this note.

98/02280 In situ alkali concentration measurements in a pressurized, fluidized-bed coal combustor by excimer laser induced fragmentation fluorescence

98/02280 In situ alkali concentration measurements in a pressurized, fluidized-bed coal combustor by excimer laser induced fragmentation fluorescence

In situ alkali concentration measurements in a pressuried, fluidized-bed coal combustor by excimer laser induced fragmentation fluorescence

Absolute concentrations of total gas-phase sodium and potassium compounds were determined in the flue gas of a fluidized bed coal combustor at 10 bar using excimer laser-induced fragmentation fluorescence (ELIF). In this in situ and on-line technique, the alkali compound molecules are photodissociated by ArF-excimer laser light at 193 nm, whereby excited Na(32P) and K(42P) atoms are formed. Fluorescence from these atoms is simultaneously detected in two separate channels at 589 nm and 768 nm, respectively. The calibration system and data evaluation procedure to convert measured signals to absolute concentrations is described. Studies on lignite and hard coal under several different combustion conditions are reported. The measured concentrations span a dynamic range of more than three orders of magnitude, with a detection limit of <0.2 ppb for Na and K. Large differences between the coals in the release of alkalis were observed. For the lignite investigated, potassium-compound concentrations up to 800 ppb and sodium-compound concentrations up to 500 ppb were determined, whereas the concentrations for the hard coal were less than 10 ppb. These large variations in alkali release reflect the very different structures of the respective coals, emphasizing the influence of mineral coal components on release and recapturing of gas-phase alkali species. It is also clear that more detailed, systematic studies are needed for a proper understanding of alkali release in PFBC.

Determination of spatial temperature distributions in a laminar premixed ethylene/air flame by laser-induced atomic fluorescence

A quick and simple detection system for spatially resolved temperature measurements in flames based on laser-induced thermally assisted atomic line fluorescence of seeded rubidium atoms is described. The fluorescence light from two atomic states is dispersed and simultaneously recorded by a CCD camera. The fluorescence ratio distributions lead directly to absolute temperature distributions. The practical use, the spatial and temperature resolution and error limits of the method are discussed and compared with other procedures for temperature measurements.

Determination of Alkali Traces in Coal Combustion by Excimer Laser Induced Fragmentation Fluorescence

AbstractGas phase sodium concentrations were measured in situ in the flue gas of a fluidised bed reactor using the excimer laser induced fragmentation fluorescence (ELIF) technique. Several different coals were investigated for a fixed set of reactor conditions at ∼ 1 bar total pressure. Measured signals were converted to absolute concentrations using a calibration system that monitors alkali compounds under well‐defined conditions. Concentrations ranging from the sub‐ppb region to 25 ppb were obtained. Contrasting concentration histories were observed for the brown and hard coals studied.

Sensitive detection of trace elements in pure water by laser-induced atomic fluorescence spectroscopy in microwave discharge atomization

Sensitive detection of trace elements in pure water using laser-induced fluorescence (LIF) spectroscopy with a new flameless atomizer is reported. The atomizer is a combination of a microwaveinduced plasma in atmospheric helium and a filament vaporization system. A newly developed pulsed dye laser, which was rapidly and automatically accessible over a wide spectral range from 220 to 740 nm, was used as the laser source. Sixteen elements were analyzed by this system, and for most of these, the detection limit was less than 1 ng/mL, which was better than those obtained by flame LIF analysis. The analytical curve and the saturation curve were measured for each element, and various factors which affect the detection sensitivity are theoretically discussed

Effect of some organic solvents and acids on the laser-induced atomic fluorescence of tin in air-hydrogen flames

The effects of some organic solvents and acids on the atomic fluorescence of tin in air-hydrogen flames have been examined. Ketones and alcohols greatly reduced the florescence sensitivity in fuel rich air-hydrogen flame whereas organic acids generally enhanced the fluorescence signal. The depressive effect of organic solvents was found to be highly dependent on the fuel to oxidant ratio in the flame. An attempt has been made to explain these effects, on the basis of possible reactions occurring in the flame.

Laser-induced fluorescence in a furnace: A viable approach to absolute analysis?

The possibility of performing absolute analysis by laser-induced atomic fluorescence in a graphite tube atomizer is discussed. At the sensitivities which are typical of these measurements, i.e., at absolute levels of the order of femtograms, it is argued that the lack of certified reference materials and the practical limitations in preparing standard solutions constitute the major incentives to the development of absolute analysis. The various theoretical and experimental parameters needed in the procedure of converting a measured fluorescence signal, e.g., a voltage, into the corresponding number of emitting atoms are discussed and evaluated with respect to a typical determination of a toxic element like thallium.

Laser-excited atomic fluorescence in a pulsed hollow-cathode glow discharge

A pulsed glow discharge with low background noise is evaluated as an atom reservoir for laser-induced atomic fluorescence spectroscopy. Aqueous solutions are dried in graphite electrodes that are used as disposable hollow cathodes. Atomic vapor is produced in a flowing-gas glow discharge with a water-cooled cathode holder. A dye laser triggers a switching circuit that pulses that glow discharge, and fluorescence measurements are made 100 microseconds after the discharge is extinguished. The atomic population is large during this dark period, while the background emission is negligible. Linear calibration curves are obtained by nonresonance fluorescence for two elements, Pb and Ir. The detection limits for Pb are 100 pg/mL and 500 fg, and for Ir are 6 ng/mL and 20 pg. The experimental detection limit for Pb is compared to the intrinsic detection limit.