Laser-excited Atomic Fluorescence Spectrometry Research Articles

Direct Determination of Dissolved and Total Thallium in Lake Waters by Laser-Excited Atomic Fluorescence Spectrometry

Abstract Thallium is a highly toxic, under-studied priority element. However, it has recently created much interest due to afresh and rapid improvements in detection limit. It appears that there are no published T1 data for Great Lakes waters, likely due to the poor sensitivity of classical methods. An electrothermal Laser-Excited Atomic Fluorescence Spectrometer has been optimized to detect sub-femtogram of thallium and used to develop a method for direct determination of dissolved and total thallium in lake waters. The method voids the labor-intensive, contamination-prone tasks of filtration, centrifugation and acid digestion of collected particulates. Adequate precision and recoveries were achieved using several lake waters (undigested and digested) and a certified reference material. The concentration of thallium in the acidified (0.2% HNO3) Milli-Q-Water was monitored over a period of four months and averaged 0.02 ± 0.01 ng/1. The concentration in Hamilton Harbor ranged from 3 to 48 ng/1. The mean of...

Electrothermal atomizer laser-excited atomic fluorescence spectrometry for the determination of phosphorus in polymers by direct solid analysis and dissolution

Electrothermal atomizer laser-excited atomic fluorescence spectrometry (ETA-LEAFS) with direct solid analysis was used to measure phosphorus in polymers that consisted mainly of poly(ethylene terephthalate). The ETA-LEAFS detection limit for phosphorus was 8 pg, and the linear dynamic range extended over approximately five orders of magnitude. Electrothermal atomic absorption spectrometry (ETAAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES) were used to validate quantitative results obtained. A novel dissolution method that utilized trifluoroacetic acid and toluene was used for validation purposes. Of the three techniques investigated, only ETA-LEAFS, with either direct solid analysis or dissolution, could measure the phosphorus content of all the samples over the range 2–3000 µg g–1; ETAAS and ICP-AES were limited by a relatively poor linear dynamic range and/or poor detection limit. The relative standard deviation for the ETA-LEAFS analyses by dissolution and direct solid analysis was in the range 5–11% and 11–12%, respectively. A general comparison was made between the use of ETA-LEAFS and alternative techniques for the direct solid analysis of polymers.

Development of a dual-wavelength dye-laser system for the UV and its application to simultaneous multi-element detection

In this paper, the design and performance characteristics of a pulsed tunable dye laser system for the simultaneous generation of two UV wavelengths are presented. The system is composed of an oscillator and an amplifier stage, pumped by the second harmonic of a commercial Nd:YAG laser. Dual-wavelength operation is achieved with one additional tuning mirror introduced to the prism expanded grazing incidence oscillator. The two obtained wavelengths are independently tunable, their separation is only limited by the gain profile of the dye. Both wavelengths are frequency doubled by Second-Harmonic Generation (SHG) in two KDP or BBO crystals. Performance characteristics such as bandwidth, efficiency, tuning range and wavelength separation are reported. As application two such systems are used for the simultaneous detection of the four elements cadmium, nickel, manganese and lead by Laser-Excited Atomic Fluorescence Spectrometry in a graphite furnace (ETA-LEAFS).

Bismuth in recent snow from Central Greenland: Preliminary results

We present here the first data on the occurrence of bismuth (Bi) in recent snow from central Greenland. They were obtained by analysing surface snow and a shallow snow core covering the past twenty years, using the new ultrasensitive laser excited atomic fluorescence spectrometry technique. Measured concentrations range from 0.66 to < 0.05 pg g −1. These are the lowest concentrations ever reported for precipitation. No significant time trend is observed during the past two decades. Bi appears to be probably mainly derived from volcanic emissions to the atmosphere, opening the way to obtaining time series of the past global volcanic activity through the analysis of this heavy metal in deep Greenland or Antarctic ice cores.

Spatial discrimination against background with different optical systems for collection of fluorescence in laser-excited atomic fluorescence spectrometry with a graphite tube electrothermal atomizer.

A single 90 degrees off-axis ellipsoidal mirror fragment was used in a dispersive detection system for electrothermal atomization laser-excited atomic fluorescence spectrometry. The performance of the new optical arrangement was compared with those of optical arrangements that employed a plane mirror in combination with biconvex or plano-convex lenses. All the optical arrangements collected fluorescence in a scheme called front surface illustration. BEAM-4, an optical ray tracing program, was used for calculations of spatial ray distributions and optical collection efficiency for the various optical configurations. Experimentally, the best collection efficiency was obtained by use of the ellipsoidal mirror, in qualitative agreement with simulations done by use of the BEAM-4 software. The best detection limit for cobalt with the new optical arrangement was 20 fg, which was a factor of 5 better than that obtained with conventional optical arrangements with otherwise the same instrumentation. The signal-to-background ratio and the fluorescence collection efficiency were also studied as a function of position of the optical components for the various optical arrangements. For both cobalt and phosphorus, the signal-to-background ratio with the new optical arrangement remained stable within 10-20% during +/- 8 mm shifts in the position of the detection system from the focal plane of the optics. Overall, the new optical arrangement offered high collection efficiency, excellent sensitivity, and facile optical alignment due to efficient spatial separation between the fluorescence signal and the background radiation. The advantages of the new optical arrangement were particularly important during measurements in the presence of high levels of blackbody radiation.

Laser-induced fluorescence in a graphite furnace as a sensitive technique for assessment of traces in North Arctic atmospheric aerosol samples

An improved version of the highly sensitive laser-based spectroscopic trace-element detection technique, laser-induced fluorescence in a graphite furnace, LIF–GF (often also referred to as laser-excited atomic fluorescence spectrometry in electrothermal atomizer, ETA–LEAFS) has been used to assess the trace-element content of Al and Pb in size-fractionated aerosol samples from the Norwegian Arctic. The ordinary LIF–GF technique has been modified for improved selectivity by the incorporation of a multi-channel intensified CCD detector (ICCD) which makes constant monitoring of various background signals possible (scattered laser light, concomitant fluorescence light, and black body radiation). It is shown that the sensitivity and selectivity of the LIF–GF–ICCD technique is sufficient for efficient detection of the trace contents of Al and Pb in dissolved aerosol samples from the Norwegian Arctic (0–75 pg for each furnace heating). The Al and Pb concentrations in air from Ny Alesund, Svalbard, at the time of sampling (March–April 1992) were found to be 1–50 ng m–3.

Determination of mercury in microwave-digested soil by laser-excited atomic fluorescence spectrometry with electrothermal atomization

A sample digestion procedure was developed which employs microwave heating of soil and sediment in concentrated nitric acid in a high-pressure closed vessel. Complete dissolution of mercury into the sample solution occurs within 5 min at 59 W/vessel without loss of analyte through overpressurization. Laser-excited atomic fluorescence spectrometry with electrothermal atomization (LEAFS-ETA) was used as the detection method. The scheme uses a two-step excitation, with λ 1 = 253.7 nm and λ 2 = 435.8 nm. Direct line fluorescence was measured at 546.2 nm. The absolute instrumental limit of detection was 14 fg; 1.4 pg/ml with a 10 μl sample injection. The recoveries of mercury in two spiked samples were 94 and 98%. The SRM 8406 (Mercury in River Sediment) was digested and analyzed for mercury, and the results (58.4 ± 1.8 ng/g) agreed well with the reference value of 60 ng/g. The results obtained by LEAFS-ETA with microwave sample digestion are in good agreement with those found by cold vapor atomic absorption spectrometry with EPA Series Method 245.5 sample digestion, which is one of the most commonly used methods for the determination of mercury in soil.

Ultratrace determination of Bi in Greenland snow by laser excited atomic fluorescence spectrometry

The results of the first determination of Bi traces in snow samples from Greenland by laser excited atomic fluorescence spectrometry are presented. A limit of detection as low as 0.05 pg ml for one-color LEAFS technique is attained. Strong matrix interference in real snow samples was observed and some ways of reducing this are tested. The main source of background is molecular fluorescence of unidentified species. The measured Bi concentration in snow samples ranges within 0.07–0.6 pg ml .

Analytical characterization of laser excited atomic fluorescence of bismuth

The possibilities of direct determination of Bi traces by laser excited atomic fluorescence spectrometry were investigated. Practically all one-color excitation-detection schemes were examined. For the resonance fluorescence at 306.8 nm, the main source of background was a molecular fluorescence of residual water vapors. The background characteristic OH rotational lines of O-O band of 2Σ - 2Π transition were registered in the excitation spectra. From the relative intensities of the rotational components, the gas temperature of the analytical volume above the open graphite cup of the atomizer was estimated as 680 ± 120°C. A detection limit (LOD) of about 1 pg/ml was achieved for the resonance scheme. For the excitation wavelength shorter than 230 nm, broadband fluorescence of unidentified species in the 300-400 nm spectral range was the main source of background. The best LOD of 0.05 pg/ml was realized for the 223.1/ 299.3 nm excitation-detection scheme. This LOD is the best one for the one-color LEAFS technique. Possible ways of further increasing the LEAFS sensitivity of Bi traces detection are briefly discussed.

Argon Fluoride Laser-Excited Atomic Fluorescence of Arsenic in a H2/Air Flame and in an Ar ICP

Laser-excited atomic fluorescence spectrometry (LEAFS) can be an extremely sensitive method of trace analysis. However, some elements, such as As, have strong absorption lines only below 200 nm. Dye laser systems capable of resonance excitation of As are complicated and very expensive. By coincidence, a simple fixed-frequency argon fluoride (ArF) excimer laser produces a broad-band output centered at 193.0–193.2 nm overlapping the As absorption line at 193.7 nm. The use of an ArF excimer laser as a source for LEAFS detection of As in a H2/air flame and an Ar-ICP has been evaluated. Detection limits of about 20 ng/mL were obtained with both atomizers. The limiting noise was laser-induced scatter in both cases. Methods of improving the detection powers are discussed.

Determination of bismuth down to sub PG/G level in Greenland snow by laser excited atomic fluorescence spectrometry

We present here preliminary data on the first direct determination of Bi in Greenland recent snow down to the sub pg/g level by Laser Excited Atomic Fluorescence (LEAF) spectrometry in clean room conditions. Calibration of the spectrometer was achieved using ultralow concentration Bi standards (concentration range 0.05-50 pg/g). The limit of detection was found to be 2.5 fg Bi. Various Greenland samples were analysed, giving Bi concentration values in good agreement with these anticipated from the available volcanic emissions data

Determination of ultra-trace amounts of cobalt in ocean water by laser-excited atomic fluorescence spectrometry in a graphite electrothermal atomizer with semi on-line flow injection preconcentration

A method has been developed for the determination of trace and ultra-trace amounts of cobalt in sea-water. Samples of CASS-2 Nearshore Seawater and NASS-4 Open Ocean Water reference materials from the National Research Council of Canada were employed. Laser-excited atomic fluorescence spectrometry in an electrothermal atomizer (ET-LEAFS) was used, and integrated with semi on-line flow injection microcolumn preconcentration. For cobalt, the effects of pH on the preconcentration efficiency, the concentration of the chelating agent and the distribution of cobalt in the ethanol eluate were studied. A bonded silica, with octadecyl functional groups (C18) in a 10 µl column, was employed for preconcentration of cobalt in ocean water. Ocean water volumes of 0.40 and 1.00 ml were required for the determination of cobalt in CASS-2 and NASS-4, respectively. These volumes were almost two orders of magnitude smaller than those required by inductively coupled plasma mass spectrometry and some other competitive techniques. The preconcentration factors were 5- and 12.5-fold for CASS-2 and NASS-4, respectively. The detection limits (3s), based on 12.5-fold preconcentration, were 0.08 and 1.0 ng l–1 for cobalt in aqueous standard solutions and in Ocean Water Reference Materials, respectively. Results for the determination of cobalt in CASS-2 and NASS-4 showed that there were no significant differences between the certified values and the measured values, based on Student's t-test at the 95% confidence level. The relative standard deviations for the determinations of the concentrations of cobalt in CASS-2 and NASS-4 were 9 and 13%, respectively.

High-sensitivity detection of selenium and arsenic by laser-excited atomic fluorescence spectrometry using electrothermal atomization

High-sensitivity detection of the trace elements selenium and arsenic is reported. The method applied is laser-excited atomic fluorescence spectrometry using electrothermal atomization within a graphite furnace atomizer. For the production of tunable laser radiation in the vacuum ultraviolet (VUV) spectral region a laser system was developed that consists of two laboratory-built dye lasers pumped by a Nd:YAG laser. The laser radiations are subsequently frequency doubled and sum frequency mixed by non-linear optical KDP or BBO crystals, respectively. The system works with a repetition rate of 20 Hz and provides output energies of up to 100 µJ in the VUV at a pulse duration of 5 ns. The investigations focused on the detection of selenium and arsenic in aqueous solutions and in samples of human whole blood. From measurements on aqueous standards, detection limits of 1.5 ng l–1 for selenium and 5.4 ng l–1 for arsenic were obtained, with corresponding absolute detected masses of only 15 or 54 fg, respectively. The linear dynamic ranges spanned six orders of magnitude and good precision was achieved. In the case of human whole blood samples, the recovery was found to be within the range 96–104%. The determination of the selenium content yielded medians of 119.5 ± 17.3 µg l–1 for 200 frozen blood samples taken in 1988 and 109.1 ± 15.6 µg l–1 for 103 fresh blood samples.

Direct determination of lead in sea-waters by laser-excited atomic fluorescence spectrometry

This paper describes a laser-excited atomic fluorescence spectrometric method for direct determination of lead in sea-waters down to femtogram levels. No separation/concentration steps nor chemical modifier were used. A programmable, in situ, known addition technique was developed and used as an integral part of the method. The technique reduces sample preparation steps and compensates for spectral line drift better than a standards calibration technique. Four sea-water Certified Reference Materials from the National Research Council of Canada were analysed for Pb concentrations, which were found to be well within certified ranges. Spike recoveries of 100 ± 10% were achieved using a Certified Reference Material and an unknown sea-water sample. The practical detection limit was 3 fg of Pb absolute (or 1 ng l–1 relative), which, to the authors' knowledge, is the lowest absolute detection limit ever reported for sea-water analysis.

A Novel, Simple Method for Tuning Dye Lasers

A novel, simple and cost-effective method has been developed for tuning dye lasers used in laser-excited atomic fluorescence spectrometry. The method uses two common laboratory items-a pierced mirror and a commercial electrodeless discharge lamp (EDL). In contrast to the existing procedure using flame, there is no nebulizer-burner, hood, gas tanks, nor burner control unit involved. The EDL lamp method also facilitates the simultaneous monitoring of reference and analyte peaks. It allows the monitoring of laser stability during experiments. A practical minimum pulse energy required for tuning and LEAFS studies was found to be 200 nJ.

Capacitive Discharge Graphite Furnace Laser-Excited Atomic Fluorescence of Thallium

For the first time, an anisotropic graphite furnace heated by capacitive discharge was used for laser-excited atomic fluorescence spectrometry. A detection limit of 5 fg for thallium was obtained with a laser repetition rate of 500 Hz and a peak integration time of 80 ms. The use of a capacitive discharge furnace allows for a shorter integration time, which in turn should allow for integration of less background noise, and improved detection limits. Theoretically, the magnitude of the shot noise should be proportional to the square root of the integration time, and inversely proportional to the square root of the laser repetition rate. Experimental data illustrated the effect of laser repetition rate, but were inconclusive with respect to integration time. The linear dynamic range of the calibration curve was six orders of magnitude, which was comparable to that normally obtained for laser-excited atomic fluorescence in modern commercial graphite furnaces. Thallium was accurately determined in NIST biological samples at levels one to two orders of magnitude below the detection limit of electrothermal atomic absorption spectrometry, with an analytical precision between 8 and 20%. The interference effects of calcium, sodium chloride, and potassium chloride on the thallium signal were investigated and shown to be similar to both laser-excited atomic fluorescence in a conventional furnace and capacitive discharge furnace atomic absorption results reported in the literature.

Two-step laser excited atomic fluorescence spectrometry determination of mercury

A novel method for the determination of mercury by laser excited atomic fluorescence with electrothermal atomization (LEAFS-ETA) has been developed. The experimental set-up consisted of a dual dye-laser system pumped with a XeCl excimer laser operated at 10 Hz, and an electrothermal atomizer with platform atomization. The atomization program allowed time for the injection of Pd (as a matrix modifier) and used a drying step at 110°C and an atomization step at 1200°C. The collection is made at 90° using a pierced mirror, an achromat lens and a long-pass filter. The monochromator is fitted with a 1P28 PMT. The signal is processed by using a boxcar and an analog to digital interface. The excitation scheme is a two-step process, with λ 1 = 253.7 nm and λ 2 = 435.8 nm. Direct fluorescence is observed at 546.1 nm. The limit of detection (LOD) obtained is 90 fg (9 pptr with 10 μ1 injection). The linear dynamic range (LDR) is five orders of magnitude and is limited by the non-linearity of the co-operative processes occurring at higher concentrations. In order to extend the LDR to higher amounts of mercury, indirect fluorescence is collected with the less sensitive line at 407.8 nm, allowing concentrations of 1 ppm and up to be measured, extending the LDR of the technique to at least seven orders of magnitude.

Direct analysis of solid samples by laser excited atomic fluorescence spectrometry with sample atomization by ion sputtering in a planar magnetron discharge

A specially designed planar magnetron for direct atomization of solid materials by ion sputtering for laser excited atomic fluorescence spectrometry (LEAFS) was investigated. This atomizer provides a much higher atomic flux than the previously used hollow cathode type. The potential of the atomizer was examined through the analysis of Si traces in high-purity In and Ga. Theoretically simulated calibration curves for a particular geometry of the atomizer were compared with those that were experimentally obtained. For both matrices, the deviations of the experimental and simulated calibration curves were by less than a factor of three. The limits of detection (3σ) of 0.4 ng g for In and of 1 ng g for Ga were obtained.

Detection of vanadium by laser-excited atomic fluorescence spectrometry in a side-heated graphite furnace

Vanadium has been detected at picogram levels in a side-heated integrated contact graphite tube furnace (electrothermal atomizer) by laser-excited atomic fluorescence spectrometry (ETA-LEAFS). The detection limit of vanadium by this technique has been improved from the nanogram region (1.7 ng) to the picogram region by the use of a side-heated integrated contact graphite tube furnace together with double resonance excitation in the visible, followed by detection of fluorescence in the ultraviolet region by a solar-blind photomultiplier, which thus completely eliminates the influence of scattered laser light in the system. It was found that the limiting factor for the detection of vanadium was contamination from the graphite furnaces. The leakage of vanadium out of the graphite material thus limited detection to approximately 10 pg. The leakage rate depended only weakly on the number of firings of the tube after an initial decrease during the first 20 firings. The detection limit for vanadium with ETA-LEAFS that would have been obtained without contamination from this non-optimized equipment can be estimated to be 2–3 picograms.

Determination of tellurium and antimony in nickel alloys by laser excited atomic fluorescence spectrometry in a graphite furnace

Analytical laser excited atomic fluorescence of the metalloids tellurium and antimony in an electrothermal atomizer was studied. The detection limits were 20 fg and 10 fg for tellurium and antimony respectively, equivalent to about 0.01 ng g −1 in nickel based alloys by direct solid sample analysis, for a 1 mg solid sample, or 1 ng g −1 by the dissolution method, for a 100 mg solid sample in 100 ml solution. The detection limits were three orders of magnitude better than those obtained by graphite furnace atomic absorption spectrometry. They were also comparable to, or better than, those by inductively coupled plasma mass spectrometry. The linear dynamic ranges of the calibration curves were found to be six and seven orders of magnitude for antimony and tellurium respectively. By use of aqueous calibration, tellurium was accurately determined in NIST nickel alloy Standard Reference Materials by both a solid sample method, with a relative standard deviation (RSD) of about 13%, and a dissolution method with an RSD of about 9%. Antimony in Pratt and Whitney “A” series nickel alloy standards was successfully determined by the dissolution method, with an RSD of about 7%, but by solid sampling the antimony method gave incomplete recovery. Molecular fluorescence backgrounds from nitric oxide and silicon monoxide were observed and discussed.