Francis R. Preli

Direct solid sampling of nickel based alloys by graphite furnace atomic absorption spectrometry with aqueous calibration

Abstract The direct analysis of solid nickel based alloy samples was investigated using graphite furnace atomic absorption spectrometry (GFAAS) with Zeeman background correction and a Perkin-Elmer solid sampling cup accessory. Advantages of direct solid sampling include high sensitivity and reduced risk of sample contamination since no dissolution step is used. Using aqueous standards and STPF technology with Zeeman background correction, it has proved possible to analyze small chips of nickel based alloy directly. High temperature alloy standard reference materials (SRMs) were accurately analyzed for low and sub ppm concentrations of the trace metals thallium, bismuth, tellurium, selenium and lead in the alloys. Analyses were done on single alloy chips weighing between 0.5 and 4 mg with typical analytical RSDs of 6–14%.

Signal and noise considerations of non-dispersive laser-excited atomic fluorescence in a graphite tube atomiser with front-surface illumination

A commercial graphite tube electrothermal atomiser (ETA) was used as an atom cell for laser-excited atomic fluorescence spectrometry (LEAFS). From the point of view of signal to noise ratio (S/N), both transverse and front-surface illumination of the furnace were investigated, together with dispersive and non-dispersive fluorescence detection. Front-surface illumination offered better sensitivity, due to better illumination efficiency, than transverse detection. Non-dispersive detection had better light gathering power, hence better sensitivity, than dispersive detection. The effects of optical filter bandpass and slit-width on the S/N of non-dispersive detection were explored. Narrow bandpass filters cut down the noise and led to improvements in limits of detection (LOD). Sub-femtogram (sub-10–15 g) LODs for TI and Pb were obtained by using very narrow (1-nm) bandpass filters and non-dispersive detection of the fluorescence.

Laser-excited fluorescence spectrometry of phosphorus monoxide and phosphorus in an electrothermal atomizer: determination of phosphorus in plant and biological reference materials and in nickel alloys

Laser-excited molecular fluorescence spectrometry with electrothermal vaporization of phosphorus monoxide (PO ETV-LEMOFS) and laser-excited atomic fluorescence spectrometry of phosphorus in an electrothermal atomizer (P ETA-LEAFS) were studied. Phosphorus monoxide and phosphorus were excited at 246.291 and 213.618 nm, and their fluorescence was detected at 324.5 and 253.5 nm, respectively. Excitation spectra of phosphorus monoxide both in an air–acetylene flame and a graphite furnace were obtained. Detection limits of phosphorus by PO ETV-LEMOFS and P ETA-LEAFS were found to be 80 and 8 pg, respectively. The linear dynamic range of the calibration graph for phosphorus by PO ETV-LEMOFS and P ETA-LEAFS extended over three orders of magnitude, from 80 pg to 0.2 µg, and four orders of magnitude, from 8 pg to 0.1 µg, respectively. For phosphorus monoxide, significant interferences were observed from cations, such as nickel and calcium. Phosphorus was determined in National Institute of Standards and Technology plant and biological Standard Reference Materials by PO ETV-LEMOFS and P ETA-LEAFS. Good agreement with certified values was obtained, with an analytical precision of between 5 and 18%. Phosphorus ETA-LEAFS was applied successfully to the determination of phosphorus in high temperature nickel alloys, in which the concentrations of phosphorus were below the detection capabilities of electrothermal atomic absorption spectrometry. For P ETA-LEAFS a background signal from nitrogen monoxide, caused by the presence of nitric acid during dissolution, could be eliminated by a char step at temperatures above 600 °C.

Laser-excited atomic-fluorescence spectrometry in an electrothermal atomizer with Zeeman background correction.

A pulsed excimer-pumped dye laser was used to excite atomic flourescence in graphite tube electrothermal atomizer. A 60-Hz ac magnitude field was applied around the atomizer and parallel to the excitation beam, for Zeeman background correction. The correction system was found to degrade the detection limits for silver, cobalt, indium, manganese, lead, and thallium by a factor of between 1 and 10. An increase in magnetic field strength, or a decrease in laser linewidth, should improve the detection limits, but was not possible here. For copper, the application of Zeeman background correction was unsuccessfull because the instrumentation was unable to resolve the sigma components from the laser emission profile sufficiently during the background correction measurement. For elements that exhibit sufficient Zeeman splitting, the linear dynamic range was the same with or without background correction Zeeman background correction was used to correct for scatter, in the resonance flourescence determination of manganese in a zinc chloride matrix and in mouse brain tissue.

Diagnostic studies of the Zeeman effect for laser excited atomic fluorescence spectrometry in an electrothermal atomizer

Abstract Reported here are the effects of various theoretical and experimental parameters on the sensitivities and calibration ranges of Zeeman background corrected, electrothermal atomizer, laser excited atomic fluorescence spectrometry. The experimental arrangement included a pulsed excimer pumped dye laser, and a graphite tube electrothermal atomizer, with laser excitation occuring parallel to the field lines of an a.c. electromagnet. The influences of Zeeman splitting, applied field strength, laser excitation linewidth, and atomic spectral profile, on the detection limits and linear calibration ranges were studied for six elements (Ag, Co, Cu, In, Pb, Tl). At low analyte concentrations, when the magnetic field is on, overlap of the Zeeman sigma components with the laser emission profile can degrade the sensitivities. The linear ranges of calibration curves with, or without Zeeman background correction were similar for those elements that exhibited sufficient Zeeman splitting. At high concentrations (greater than 100 ng, or 10 μg ml ), broadening of the atomic spectral profile caused an increase in the sigma component overlap and some of the calibration curves became double valued (rollover).

Determination of Tin in Nickel-based Alloys by Electrothermal Laser-excited Atomic Fluorescence With Confirmation of Accuracy by Inductively Coupled Plasma Mass Spectrometry and Atomic Absorption Spectrometry

The determination of tin in nickel-based alloys by laser-excited atomic fluorescence in a graphite furnace was investigated. The concentrations of tin in four nickel-based alloys from Pratt & Whitney Aircraft were determined. The nickel in the alloys was found to behave as a permanent chemical modifier that remained in the graphite tube during analyses. The use of a mixture of hydrofluoric, hydrochloric and nitric acids proved to be the most efficient dissolution method. In order to confirm the accuracy of the results, Zeeman-effect background corrected ETAAS and ICP-MS methods were also used. The results from these different methods were compared by use of the Student’s t -test. The laser-excited atomic fluorescence method was found to be as accurate as the other techniques. The precisions of the techniques varied from 4 to 12% RSD. For the dissolution of 100 mg of nickel alloy in 100 ml of aqueous solution, the effective atomic fluorescence detection limit in the original nickel alloy samples was 1.5 ng g -1 . The atomic fluorescence method was simple to develop and did not need a sophisticated background correction technique to perform the analyses.

Nonlinearity of Calibration Graphs for Laser-Excited Atomic Fluorescence in Graphite-Tube Atomizers

Calibration graphs for Laser-Excited Atomic Fluorescence Spectrometry (LEAFS) were constructed for graphite-tube electrothermal atomizers (ETAs). Laser radiation was propagated through holes in the side of the tubes, and atomic fluorescence was detected at a right angle through the bore of the tube. The LEAFS calibration graphs, while linear with a relative slope of one for 4 to 6 orders of magnitude, became nonlinear at high analyte concentrations. Observations concerning this nonlinearity are presented here. The onset of nonlinearity was believed to be primarily caused by post-filter effects. For nonresonance LEAFS, long lifetimes for the terminating energy levels in the fluorescence processes were believed to increase the probability of post-filter effects and subsequently to increase curvature of calibration graphs. Those conditions that increased the residence time of atoms in the furnaces (i.e., no internal gas flow and platform atomization) appeared to also increase post-filter effects and thus increased the amount of curvature of the calibration graphs.