Spectral interpretation and interference correction in inductively coupled plasma mass spectrometry

To tackle the several problematic polyatomic interferences in inductively coupled plasma mass spectrometry (ICP-MS), we have developed a software approach based on data reduction of the measured total mass spectrum through multicomponent analysis (MCA). The approach leans on a working knowledge of interferents that are likely to be encountered in a sample matrix, which composition is known by virtue of the total mass spectrum and knowledge of applied solvents. The full isotopic patterns for all elements and expected interferents are used in the modelling MCA matrix of 250 masses × 105 species at maximum. Polyatomic abundances are calculated by the software. Since all species are modelled fundamentally through their known natural abundances, the MCA matrix can be manipulated and reprocessed until interpretation of the mass spectrum and, hence, interference correction are optimal. The optimum is attained by use of the bar graph and calculation modes of the PC software and criteria for properly found isotopic patterns. With optimized models stored in the data base, the user may routinely process samples in one go, and operate the ICP-MS system in a true all-element mode. Use of elemental equations or measurement of large multivariate calibration sets and pure component solutions are superfluous. Data reduction is solely based on the information about the isotopic patterns, present in the measured mass spectrum itself. As a result, in the case of interferences, detection limits may be lowered by one to two orders of magnitude. The approach is illustrated with an industrial example of Hf determined in NdFeB, and with an environmental example. Here, a suite of elements over the 50–82 amu mass range has been determined in different salt matrices in ground water.     

Molecular hydrogen emission in the vacuum ultraviolet from an inductively coupled plasma

Vacuum ultraviolet radiation is observed through a sampling orifice in a metal cone inserted directly into an Ar inductively coupled plasma (ICP). The many-lined spectrum of the H₂ Lyman bands is observed from an argon ICP with either H₂ gas or nebulized water added. The most prominent bands are from the ν = 3, 4, 5 or 6 vibrational levels of the B ¹Σ⁺_u electronic state of H₂. The energies of the ν = 3, 4 and 5 levels are close to those of the Ar metastable levels and of photons from the Ar resonance lines (11.5 to 11.8 eV). These H₂ levels are probably populated at least in part by selective energy transfer reactions between Ar metastable atoms and H₂ and/or by absorption of Ar resonance photons by H₂. The H₂ emission emanates only from the upstream reaches of the axial channel, i.e. the part of the axial channel that is inside the load coil and induction region.     

Influence of hydrogen gas over the interference of acids in inductively coupled plasma atomic emission spectrometry

The effect of hydrogen gas on the plasma and its influence on acid interferences in plasma atomic emission spectrometry was studied. The study was performed with HCl and HNO₃ in the concentration range of 0–2 mol l⁻¹. Vanadium and magnesium were used as test elements, the study was extended to other several elements. The effects of hydrogen gas on the plasma were studied by measuring excitation temperature, electron number density and the ionic-to-atomic line intensity ratio. The net effect of hydrogen was an increase in electron density and ionic to atomic line intensity ratio. A small increase in the excitation temperature was observed. The signal suppression for ionic lines causes by mineral acid was reduced when small amounts of hydrogen was introduced into the plasma as sheathing gas. This effect was attributed to the increase in plasma electron density.     

Spectral diagnostics of inductively coupled RF discharge plasma

Results of UV spectral measurements in low-pressure (30–65 Pa) and atmospheric-pressure inductively coupled radiofrequency discharges at different power inputs and gas compositions typical for that of industrial processing regimes are reported. Isolated molecular bands have been observed in the spectra; by analyzing the unresolved rotational temperature of these bands, the rotational temperature of the upper emission state can be found.     

Spectral and non-spectral interferences in inductively coupled plasma mass-spectrometry

Inductively coupled plasma mass spectrometry of environmental and biological samples is often hampered by spectral and non-spectral interferences. Spectral interferences, caused by the limited resolution of the quadrupole mass spectrometer, can be eliminated in a variety of ways. For their identification inspection of a signal versus carrier gas flowrate is useful. Anion exchange allows the removal of most S and Cl containing compounds, which are at the origin of the majority of spectral interferences. Matrix modification, for example the addition of ethanol and subsequent optimization of the gas flow rates in a number of cases enables the reduction of the interferences to insignificant values. Often a mathematical correction based on isotopic signal ratios can be applied. Non-spectral interferences can be divided in reversible, that is occurring while the sample is being measured, and irreversible matrix effects, that is clogging of the nebulizer and sampling orifices or deposition on the torch or in the ion lens stack. The errors associated with non-spectral interferences can be eliminated by appropriate calibration procedures, adapted sample preparation or limitation of the amount of sample delivered to nebulizer, plasma and sampling devices, for example by the application of flow injection. Applications of all the elimination procedures are described for the analysis of sea-water, estuarine water, soil and sewage extracts, percolate water, urine, serum and wine.     

Effects of matrix on spatial profiles of emission from an inductively coupled plasma

Effects of easily ionizable elements and other concomitants on spatial profiles of analyte emission were studied in an inductively coupled plasma. The measuring system consists of a self-scanning linear photodiode array mounted vertically in the focal plane of a monochromator and a microcomputer for data acquisition and processing. Easily ionizable elements had complex effects on spatial profiles of analyte emission depending on the excitation characteristics of the spectral lines and the operating parameters of the plasma. On the other hand, sulfuric acid simply depressed the emission over the whole observation height independent of the operating parameters. These data are useful for a better understanding of the complex interference effects found in optimisation experiments and may add to an understanding of the excitation mechanisms responsible for them.     

Spatial characterization of analyte emission and excitation temperature in an inductively coupled plasma

Vertical, lateral and radial profiles of analyte emission in an inductively coupled plasma have been measured using photodiode array spatial profiling spectrometers. These profiles have been measured for both neutral atom and ionic lines of several elements. Neutral atom lines can be sub-divided into two basic groups on the basis of their vertical spatial emission characteristics. One group in which the peak vertical position of emission correlates positively with normal temperature and a second group in which the peak vertical position of emission correlates negatively with normal temperature. Utilizing radially resolved emission intensities and vertical and radial profiles of neutral atom excitation temperature, the caracteristic emission patterns of both groups of neutral atom lines can be explained. Analyte ionic line spatial emission characteristics, both vertically and radially, are shown to be relatively species independent and radial emission intensity ratio maps of ionic and neutral atom lines of the same element are presented that indicate the potential importance of plasma boundary regions as regions of major non-LTE behavior.     

Spectral interferences in the determination of trace elements in environmental materials by inductively coupled plasma atomic emission spectrometry

This paper deals with spectral interferences in inductively coupled plasma atomic emission spectrometry (ICP-AES) encountered with environmental materials. These samples normally contain high concentrations of aluminium, calcium, magnesium, iron, titanium, potassium and sodium. The investigations cover: (a) spectral data for Al, Ca, Mg, Fe, Ti, K and Na as interferents for 200 pm wide windows centred (±100 pm) around the prominent lines of As, B, Ba, Be, Cd, Cr, Cu, Hg, Mn, P, Pb, Sb, Se, Sn, Tl, U and Zn; (b) a data base of Q-values for line interference [Q_Ij(λ_a)] and Q-values for wing background interference [Q_Wj(Δλ_a)] for two values of the excitation temperature 6200 K and 7200 K. The lines free or negligibly influenced by line interference were selected for analyte determination. Q-values were used for calculation of correction factors under a spectral line without the measurement of a reference blank at the wavelength of the prominent analysis lines. The accuracy of ICP-AES with the Q-concept as a basic methodology is checked by the analysis of a certified reference material IAEA/Sediment SD-N-1/2/. The precision of the method is characterised by an RSD of 0.6–1.7%. Extraction of trace elements soluble in aqua regia was used as a decomposition method. This article is an electronic publication in Spectrochimica Acta Electronica (SAE), a section of Spectrochimica Acta, Part B (SAB). The hardcopy text is accompanied by an electronic archive, stored on the SAE homepage at http://www.elsevier.nl/locate/sabe. The archive contains the tabular material of this article in electronic form.     

Some hydride generation inter-element interference studies utilizing atomic absorption and inductively coupled plasma emission spectrometry

Three commercially available hydride generation devices have been used in conjunction with atomic absorption spectrometry and inductively coupled plasma emission spectrometry to study the possible inter-element interferences from fifty elements. Nineteen of these elements caused signal reductions of at least 10%. These interferences were reduced (drastically in some cases) by an appropriate choice of hydride generation system and by manipulation of the acid strength.     

Spectral interference in the determination of Ho,Tm and Ga as dopants in single crystals of potassium titanylphosphate by inductively coupled plasma atomic emission spectrometry

The present article discusses the spectral interferences affecting the determination of Ga, Ho and Tm as dopants in single crystals of potassium titanylphosphate (KTiOPO₄) by inductively coupled plasma atomic emission spectrometry (ICP-AES). The Q concept, as proposed by Boumans and Vrakking [Spectrochim. Acta Part B 43 (1988) 69] was used for this study, which covers: (a) spectral data for potassium, titanium and phosphorous as interferents (at a concentration of 4 mg/ml) in 400 pm wide spectral windows around the prominent lines of the analytes; (b) a database of Q values for line interferences (Q_I) and for wing interferences (Q_W); and (c) the interelement interferences in doubly doped crystals of KTP.     

Axial distribution of analyte emission in inductively coupled argon plasma

Summary Using a spectrometer, ICP axial profiles of analyte emission for a large number of atom and ion lines with different excitation and ionization potential have been studied. Obtained experimental results for axial distribution of atom and ion lines can be correlated with some of the proposed models for excitation mechanisms of analytes in argon ICP. On the basis of experimental data it can be concluded that in the case of simultaneous multielement analysis the observation heights between 14 and 17 mm above the load coil can be recommended. Experimental data show that there is no simple correlation between maximum intensity of lines and their excitation and ionization potential. Experimental results for axial distribution for ionic lines show that there is agreement between measured axial profile and calculated Boltzmann distribution of excited ionic state population.

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Axialverteilung der Analytemission im induktiv gekoppelten Argonplasma

Zusammenfassung Die ICP-Axialprofile der Analytemission wurden mit Hilfe eines Spektrometers für eine große Anzahl von Atom- und Ionenlinien mit verschiedenem Anregungs- und Ionisierungspotential untersucht. Die erhaltenen Ergebnisse können zu einigen Modellen für den Anregungsmechanismus im Argon-ICP in bezug gebracht werden. Aus den experimentellen Daten wird geschlossen, daß bei der simultanen Multielementanalyse Beobachtungshöhen zwischen 14 und 17 mm über der geladenen Wendel zu empfehlen sind. Es ergab sich, daß zwischen der Maximalintensität der Linien und ihrem Anregungs- bzw. Ionisierungspotential keine einfache Beziehung besteht. Die Untersuchung der Axialverteilung bei den Ionenlinien zeigte, daß Übereinstimmung besteht zwischen den gemessenen Axialprofilen und der berechneten Boltzmann-Verteilung.

     

Quality of calibration in inductively coupled plasma atomic emission spectrometry

Calibration is a crucial step during the whole analytical process. A procedure is suggested to assess the quality of linear regression used for the calibration graph, based on the use of confidence limits for the concentration, as the regression coefficient is not appropriate for this purpose. This procedure has been applied to inductively coupled plasma atomic emission spectrometry. It indicates that the use of only three standards should be discouraged because of unacceptably high confidence limits for the concentration. Moreover, use of weighted regression is more adequate in performing the least squares method and provides more constant confidence limits over the concentration range used to construct the calibration graph. This procedure could be easily added to any commercially available ICP system software.     

Application of prominent spectral lines in the 125-180 nm range for inductively coupled plasma optical emission spectrometry

A new axially viewed ICP optical emission spectrometer featuring an argon-filled optic and CCD detectors was evaluated for the application of prominent spectral lines in the 125-180 nm range. This wavelength range was investigated for several analytical applications of inductively coupled plasma optical emission spectrometry (ICP-OES). There are different advantages for the application of spectral lines below 180 nm. A number of elements, such as Al, Br, Cl, Ga, Ge, I, In, N, P, Pb, Pt, S and Te, were found to have the most intense spectral lines in the wavelength range from 125-180 nm. Compared with lines above 180 nm higher signal-to-background ratios were found. Low limits of detection using pneumatic nebulization of aqueous solutions for sample introduction were calculated for Al II 167.080 nm (0.04 microg L(-1)), Br I 154.065 nm (9 microg L(-1)), Cl I 134.724 nm (19 microg L(-1)), Ga II 141.444 nm (0.8 microg L(-1)), Ge II 164.919 nm (1.3 microg L(-1)), II 142.549 nm (13 microg L(-1)), In II 158.583 nm (0.2 microg L(-1)), P I 177.500 nm (0.9 microg L(-1)), Pb II 168.215 nm (1.5 microg L(-1)), Pt II 177.709 nm (2.6 microg L(-1)), S I 180.731 nm (1.9 microg L(-1)) and Te I 170.00 nm (4.6 microg L(-1)). Numerous application examples for the use of those lines and other important spectral lines below 180 nm are given. Because of fewer emission lines from transition elements, such as Fe, Co, Cr, lines below 180 nm often offer freedom from spectral interferences. Additional lines of lower intensity for the determination of higher elemental concentrations are also available in the vacuum ultraviolet spectral range. This is specially useful when the concentrations are not in the linear range of calibration curves obtained with commonly used lines.     

Diagnostics in an air inductively coupled plasma

Radial temperature distributions in an air inductively coupled plasma discharge, operated at atmospheric pressure, are calculated from measurements of the absolute intensities of two atomic nitrogen lines (746.9 and 493.5 nm), the first negative band system of the nitrogen molecular ion at 391.4 nm, and the air continuum at 560.0 nm. The radial intensity distribution of the Mg I 285.2 and Mg II 279.6 nm lines are employed with the determined radial temperature distribution to calculate the radial electron number density throughout the normal analytical zone. The temperatures ranged from about 6000 to 10,000 K, and the electron number density varied from 5 × 10¹³ to 2 × 10¹⁶ cm^?3 in the regions above the induction coil where differences of less than 3 fold were observed between experimental and calculated Ca II to Ca I intensity ratios. On the basis of agreement among the measured temperatures and calcium ion-to-atom intensity ratios, the extent of local thermodynamic equilibrium is evaluated.     

Iron spectrum in the 200–300 nm range emitted by an inductively coupled argon plasma

In order to make a comparison with the available wavelength tables, the iron spectrum has been studied in the 200–300 nm range in ICP-AES. About a hundred lines have been found that have, so far, been unreported. Examples of spectral interferences and conclusions on the field covered by the various tables are given.     

Ethanol loading in an inductively coupled plasma

Changes to the fundamental and analytical parameters of a plasma have been investigated when ethanol has been added to aqueous or organic solutions. Excitation temperature, electron number density, and intensity of the H line increased when ethanol has been added to aqueous solutions, while an electron density decrease and signal reduction have been found when ethanol has been added to xylene. The sensitivity has been improved for all ethanol concentrations when water has been the solvent, but the reverse has been found for xylene solutions.     

Ethanol loading in an inductively coupled plasma

Changes to the fundamental and analytical parameters of a plasma have been investigated when ethanol has been added to aqueous or organic solutions. Excitation temperature, electron number density, and intensity of the H(alpha) line increased when ethanol has been added to aqueous solutions, while an electron density decrease and signal reduction have been found when ethanol has been added to xylene. The sensitivity has been improved for all ethanol concentrations when water has been the solvent, but the reverse has been found for xylene solutions.     

Noise-power spectra of optical and acoustic emission signals from an inductively coupled plasma

Noise power spectra of emission signals from an ICP discharge have been measured. Below 5 Hz the noise power spectra show a marked dependence on the type of nebulizer used. While a strict 1/f character was not observed the spectra were clearly dominated by low frequency components. Above 5 Hz the noise power spectra were, broadly speaking, independent of the nebulizer. Distinct peaks were observed in the noise power spectra in the 200–400 Hz region. The exact position and intensity of these peaks were dependent on R.F. power, coolant gas flow rate and torch design. In addition the intensity of these peaks is dependent on the exact spatial region viewed by the spectrometer. The position of these peaks correspond exactly to similar peaks found in the acoustic noise power spectrum of the ICP. It is believed these peaks are the result of plasma rotation.