Reduction of spectral interferences in flame emission spectrometry by selective spectral-line modulation

Spectral interferences in flame emission spectrometry can be significantly reduced through the use of selective spectral-line modulation (s.l.m.). In this method, a mirrored, rotating chopper is used to direct the emission from a sample flame alternately through and around a second (modulating) flame; selective modulation is achieved when the modulating flame contains absorbing atoms identical to emitting analyte atoms in the sample flame. The effects of optical configuration and modulating conditions on working curve slope, linearity, and signal-to-noise ratio are examined. Also, the ability of s.l.m. to minimize broad-band and narrow-line spectral interferences is demonstrated. In particular, it has been shown that the interference of the 554-nm CaOH band on barium determinations can be largely overcome. Also, interference of several palladium lines on nickel in the 350-nm region can be reduced. Finally, it is shown how the interference of flame background itself can be restricted in s.l.m. procedures.     

Combination of flow-injection techniques with atomic spectrometry in agricultural and environmental analysis

Combinations of flow-injection techniques with flame atomic absorption spectrometry (a.a.s.) and inductively-coupled plasma/atomic emission spectrometry (i.c.p./a.e.s.) are reviewed in the general context of agricultural and environmental analysis. The flow-injection systems are valuable for sample introduction; appropriate dispersion control allows the analysis of solutions containing as much as 40% (w/v) urea or phosphate in fertilizers. A study on the determination of cadmium in soil extracts by on-line ion-exchange preconcentration and flame a.a.s. detection is described. The interpolative standard-addition method with i.c.p./a.e.s. detection is outlined. Improvements in the determinations of selenium in environmental samples by hydride-generation a.a.s. and of mercury by cold-vapour a.a.s. are reported.     

Improvements in repetitive scanning techniques for reducing spectral interferences in flame emission spectrometry

This paper describes several improvements in the use of the repetitive optical scanning (wavelength modulation) technique applied to the correction for spectral interferences due to line, band, and continuum radiation in flame emission spectrometry (FES). A simple and easily constructed mechanical device for performing the repetitive scan is described, and the procedures for optimizing experimental conditions are discussed. A new technique for the correction for spectral line interferences is introduced. Repetitive scanning FES is used for the analysis of several trace elements in NBS SRM-1633 (fly ash), and the precision and accuracy of results are compared to analysis by normal flame emission techniques.     

Evaluation of sequential and simultaneous detection in inductively-coupled plasma emission spectrometry of complex samples

A commercially available inductively-coupled plasma (i.c.p.) polychromator system and a laboratory-constructed i.c.p. sequential detection system are evaluated. Some previous studies are reviewed and theoretical expectations are compared to experimental data. The precision and detection limits of the two systems are approximately equivalent. Difficulties commonly encountered with complex samples, including background interferences, spectral overlap, and determinations beyond the linear range of an element are studied. The abilities of each instrument to circumvent these difficulties at different interference levels are evaluated. The wavelength flexibility of the sequential system is demonstrated to be advantageous in dealing with these interferences. Each instrument was evaluated for NBS spinach, NBS river sediment, volcanic ash, and refractory brick. The complementary nature of the sequential system and polychromator with respect to method development and quantitative work is discussed.     

The determination of sulphur and phosphorus by atomic emission spectrometry with an induction-coupled high-frequency plasma source

An instrumental assembly is described which permits detection of the atomic emission at their resonance lines below 200 nm from sulphur and phosphorus introduced into a high-frequency induction-coupled plasma source as aqueous solutions of their anions. Optimal conditions for the determination of sulphur at 182.04 nm and phosphorus at 213.62 nm have been established, and detection limits of 1.7 and 0.08 p.p.m. have been obtained for these elements, respectively. The determinations are relatively free from chemical and physical interferences, although spectral interferences from metal ions are observed in both cases. The method established for phosphorus has been applied to its determination in soil extracts.     

Determination of tin and methyltin species by hydride generation and detection with graphite-furnace atomic absorption or flame emission spectrometry

A method for the determination of tin and organotin species by hydride generation is described. Tin hydrides are detected by graphite-furnace atomic absorption, quartz-cuvette atomic absorption, or flame emission spectrometry with detection limits of 50, 50, and 20 pg as tin, respectively. Germanium interferences are eliminated with a dual-detector flame photometer with an electronic cancellation circuit.     

Interferences in hydride atomization studied by atomic absorption and atomic fluorescence spectrometry

A hydride atomizer able to operate in the flame-in-tube mode and in the miniature diffusion flame mode was used to investigate interferences of arsenic in selenium atomization. A twin-channel continuous flow hydride generator was utilized to eliminate liquid phase interferences. Both atomic absorption and atomic fluorescence detectors (EDL sources) were employed. The miniature diffusion flame can tolerate interferent concentrations up to 70 μg ml⁻¹. The magnitude of interferences in the flame-in-tube atomizer is controlled by the distance between the atomization and detection zones. The best tolerance to interferents, comparable with that in the miniature diffusion flame, was obtained for the minimum distance of the zones. The figures were deteriorated by two orders of magnitude when increasing the distance between the observation and the atomization zones to 50 mm. Also a curvature and rollover of calibration graphs was observed when increasing the distance. The presence of the interferent enhanced substantially the curvature and rollover, so that the magnitude of observed interferences was dependent on the analyte concentration. All the observed interferences and the calibration graph curvature are due to the decay of free analyte atoms by reactions in the free space. The nature of the species formed is discussed. No significant depletion of hydrogen radicals was observed. As demonstrated by measurements in the miniature diffusion flame, the species formed can be reatomized by interaction with hydrogen radicals with an efficiency better than 90%.     

The determination of sodium in pure limestones by atomic absorption spectrophotometry

The determination of sodium in pure limestones by atomic absorption spectrophotometry is described. The interferences of chloride and calcium ions are discussed; chloride affects the dissociation equilibrium, and calcium lowers the evaporation rate of sodium, thus lowering the concentration of free sodium atoms in the flame. The sensitivity achieved with a rather rough apparatus is 0.15 p.p.m. sodium.     

The atomic absorption spectroscopy of lead

Lead can be determined by atomic absorption spectroscopy at 3 wavelengths. The relative sensitivities are 1:1.5:300. No interferences were found from the cations studied. Anionic interferences were numerous and extensive, but were removed by adding EDTA. The use of a “T” -piece increased the sensitivity of atomic absorption when flame atomizers were used. However, extreme care was necessary in controlling flame conditions both with respect to oxygen-fuel ratio and the type of solvent used. The absorption by combustion products in the flame was high, and in many cases, much greater than that of the lead itself.The most sensitive conditions for the determination of lead appeared to be as follows: wavelength, 2170 A; solvent, aqueous or organic; flame, oxy-hydrogen, with the hydrogen atomizing the sample (reversed from normal). Aflame adapter enabled detection limits of 0.013 p.p.m. to be reached.     

Determination of total chromium in sediments by FAAS

The determination of chromium by flame atomic absorption spectroscopy (FAAS), e.g. as performed for environmental matrices, is still a controversial matter as can be concluded from a literature review. Most controversy is due to the measurement conditions or the effect of the interferences; other conditions such as digestion seem to be less critical. Consequently, this paper reports a systematic study of the instrumental settings for the two flame modes that are most widely used: air-acetylene and nitrous oxide-acetylene. In connection with the study of experimental conditions, possible interferences are investigated involving nineteen cations and some anions in different ratios Cr:interferent. Different releasers to overcome interferences are considered. The results are critically compared with current literature. For the operational conditions the quality parameters such as the linear range, detection limits, precision and accuracy are established. Although the nitrous oxide-acetylene flame is mostly recommended, this study reveals that a better choice can be the use of a fuel-rich air-acetylene flame with the addition of 1% 8-hydroxyquinoline (oxine) as protective agent. The method is applied to and validated by four sediment certified reference materials.     

Studies in atomic fluorescence spectrometry: Part II. Interference effects in the determination of tin with various premixed flames

The interference effects are reported for 27 elements, 6 acids and 4 organic liquids on the atomic fluorescence determination of tin with argon-hydrogen, argon-oxygen-hydrogen and argon-separated air-acetylene flames. The addition of1000 p.p.m. iron (III) eliminates most interferences from the elements but not from the acids. The basic trends in the interference effects in the argon-hydrogen flame for the atomic absorption and atomic fluorescence determinations of tin are similar. The detection limit, for an 18.2-s time constant, in the argon-oxygen-hydrogen and argon-hydrogen flames is 0.006 p.p.m. and in the air-acetylene flame it is 0.05 p.p.m. These detection limits are significantly better than previously reported limits. Analytical curves in all three flames studied are linear between the detection limits and 250 p.p.m.     

硅胶活性碳柱净化-气相色谱法测定茶叶中多种有机磷农药残留量

介绍了应用配有FPD检测器的气相色谱仪测定茶叶中有机磷农药残留量的新方法。磨碎的试样加入少量氯化钠，用水浸泡，再用乙腈提取。活性炭和硅胶混合小柱净化，乙酸乙酯洗脱。以GC-FPD测定，外标法定量，测定结果用不同极性的毛细管柱验证。方法准确，重现性、精密度好，杂质干扰少。有机磷农药检出限小于欧盟对茶叶的MRL值。     

Studies in atomic-fluorescence spectroscopy-VII Fluorescence and analytical characteristics of arsenic, with a microwave excited electrodeless discharge tube as source

The construction of an electrodeless arsenic discharge tube and its use for atomic-fluorescence studies is described. Cool nitrogen-hydrogen and argon-hydrogen diffusion flames as well as normal premixed flames are considered as atom reservoirs and the atomic-fluorescence emission at 15 different wavelengths is evaluated. The diffusion flames give the largest emission signals at arsenic concentrations below 200 ppm, but show a premature curvature at higher concentrations because of the presence of an abnormally high density of arsenic atoms. Above 200 ppm of arsenic, the premixed air-acetylene flame is superior. The limit of detection at 1890 A is 0.2 ppm of arsenic in the nitrogen-hydrogen diffusion flame and 1.0 ppm in the airacetylene flame. A long path-length diffusion flame is also particularly useful in atomic-absorption measurements because it absorbs very little radiation in the far ultraviolet region and gives an abundance of arsenic atoms.     

Physical phenomena and analytical applications of helium microwave discharges

The intensity of atomic emission from a microwave-induced helium gas discharge as a function of pressure in the range 13–130 mbar is described. Two of the spectra studied were given by excited helium atoms, and one by (a species of) an excited triplet helium molecule (He₂^*). The pressure dependence of the concentration of helium atoms in the triplet metastable state was studied by absorption spectroscopy. After introduction of known quantities of mercury, chlorine, and iodine into the helium plasma, the emission was measured for some intense lines in the visible and near-u.v. Comparison of the data suggests that the atoms of these elements can be excited by helium atoms and molecules to levels at which they emit light in the visible and near-u.v. The use of a helium discharge tube for the detection of single elements in gas chromatographic fractions is described. Selectivity is greatly improved by wavelength modulation. The method allows a highly sensitive and selective determination of nanogram amounts of organic compounds which contain the elements sought, including stable isotopes such as deuterium, carbon-13 and nitrogen-15.     

Evaluation of tungsten coil electrothermal vaporization-Ar/H2 flame atomic fluorescence spectrometry for determination of eight traditional hydride-forming elements and cadmium without chemical vapor generation

A tungsten coil electrothermal vaporizer (W-coil ETV) was coupled to an Ar/H(2) flame atomic fluorescence spectrometer for the determination of eight traditional hydride-forming elements (i.e., As, Bi, Ge, Pb, Sb, Se, Sn, and Te) as well as cadmium without chemical vapor generation. A small sample volume, typically 20muL, was manually pipetted onto the W-coil and followed by a fixed electric heating program. During the vaporization step, analyte was vaporized off the coil surface and swept into the quartz tube atomizer of AFS for further atomization and excitation of atomic fluorescence by a flow of Ar/H(2) gas, which was ignited to produce the Ar/H(2) flame. The tungsten coil electrothermal vaporizer and Ar/H(2) flame formed a tandem atomizer to produce reliable atomic fluorescence signals. Under the optimal instrumental conditions, limits of detection (LODs) were found to be better than those by flame atomic absorption spectrometry (FAAS) or inductively coupled plasma optical emission spectrometry (ICP-OES) for all the nine elements investigated. The absolute LODs are better or equivalent to those by hydride generation atomic fluorescence spectrometry (HG-AFS). Possible scattering interferences were studied and preliminary application of the proposed method was also reported.     

Calculation of non-linearities of the calibration curves in atomic absorption flame spectroscopy

The deviations from the linear relationship between absorbance and concentration in atomic absorption measurements are calculated. Besides the chemical reactions there are four groups of interferences: (a) profiles of emission- and absorption lines (b) hyperfine structure (c) optics around the flame (d) non resonant light detected by the amplifier due to the optical arrangement and/or resolution of the monochromator.

The calculations show that curvature of experimental curves as obtained by apparatus for atomic absorption spectroscopy is caused partly by optical compromises and partly by the emission profile of the source. With a narrow simple emission line profile of the source and a precision monochromator a linear relationship within a few percent is possible up to 95 per cent absorption in a flame at atmospheric pressure.     

The determination of uranium in rocks by inductively coupled plasma-optical emission spectrometry

A method for the determination of uranium in rock samples by emission spectrometry is presented. The rock is dissolved and the uranium content determined by nebulizing the solution into an inductively coupled-plasma optical excitation source. Various spectral lines were investigated. The uranium emission at 378.28 nm -was chosen because of its relative freedom from matrix element spectral interferences. For this emission, a practical detection limit of 0.1 p.p.m. in solution was achieved by optimizing source parameters (power, flow-rate, observation height). Results are compared with those obtained by a number of other techniques.     

Flame photometric detection inside of a capillary gas chromatography column

A novel micro-flame photometric detector (FPD) employing a miniature counter-current flame is described. The micro-FPD flame, encompassing a volume of about 30 nL, is operated inside the end of a capillary gas chromatography column (i.e. on-column) or inside of a quartz capillary after the column (i.e. post-column). Either air or oxygen can support a hydrogen flame in the device, although oxygen is far preferable. The detector can be operated for several hours without any observed degradation in performance or flame stability. The optimal gas flows established for the detection of sulfur and phosphorus are in the range of 4 mL min(-1) of oxygen and 9 to 13 mL min(-1) of hydrogen. The fuel-rich micro-FPD flame generates chemiluminescent blue S2* emission for sulfur and green HPO* emission for phosphorus, similar to a conventional FPD. Sulfur response in the micro-FPD is quadratic over nearly 3 orders of magnitude while that of phosphorus is linear over nearly 5 orders of magnitude. The micro-FPD detection limit for sulfur is 1 x 10(-9) g S s(-1), and that of phosphorus is 2 x 10(-10) g P s(-1). The properties established for the initial prototype of the micro-FPD make this counter-current flame method potentially suitable for integration with on-chip gas chromatography or other micro-analytical devices where flame-based detection methods are desirable.     

New method for removal of spectral interferences for beryllium assay using inductively coupled plasma atomic emission spectrometry

Beryllium (Be) has been used widely in specific areas of nuclear technology. Frequent monitoring of air and possible contaminated surfaces in U.S. Department of Energy (DOE) facilities is required to identify potential health risks and to protect U.S. DOE workers from beryllium-contaminated dust. A new method has been developed to rapidly remove spectral interferences prior to beryllium measurement by inductively coupled plasma atomic emission spectrometry (ICP-AES) that allows lower detection limits. The ion exchange separation removes uranium (U), plutonium (Pu), thorium (Th), niobium (Nb), vanadium (V), molybdenum (Mo), zirconium (Zr), tungsten (W), iron (Fe), chromium (Cr), cerium (Ce), erbium (Er) and titanium (Ti). A stacked column consisting of Diphonix Resin and TEVA Resin reduces the levels of the spectral interferences so that low level Be measurements can be performed accurately. If necessary, an additional anion exchange separation can be used for further removal of interferences, particularly chromium. The method has been tested using spiked filters, spiked wipe samples and certified reference material (CRM) standards with high levels of interferences added. The method provides very efficient removal of spectral interferences with very good accuracy and precision for beryllium on filters or wipes. This new method offers improvements over other separation methods that have been used by removing large amounts of all the significant spectral interferences with greater simplicity and effectiveness. The effective removal of spectral interferences allows lower method detection limits (MDL) using inductively coupled atomic emission spectrometry. A vacuum box system is employed to reduce analytical time and reduce labor costs.     

Theoretical model of the alkali metals interferences in flame emission spectrometry

The theoretical relationship between the analytical emission signal and the concentrations of the alkali metals in a sample has been formulated. The determination of potassium in the presence of sodium in the air-acetylene flame has been considered as an example. The following processes proceeding in the flame, proved to be important in the examination of interferences: ionization of K, Na and the flame components, as well as the formation of compounds between K and the species of the flame gas. The results of the calculations are in good agreement with the results obtained experimentally.A formulation of the simplified mathematical models of the interferences (matrix effect), which are of practical importance, is suggested. Only the most significant processes are taken into account in the formulation of the model. Some parameters in the model are obtained empirically by the fitting procedure.