P.W.J.M. Boumans

An experimental study of a 1-kW, 50-MHz RF inductively coupled plasma with pneumatic nebulizer, and a discussion of experimental evidence for a non-thermal mechanism

Abstract This paper deals with results of experiments performed with a new version of a free-running 50-MHz RF generator for producing an inductively coupled plasma (ICP) with two gas flows (argon). Samples were introduced as wet aerosols, after nebulization with a cross-flow pneumatic nebulizer. The background at various wavelengths, and the net signals of spectral lines of widely different character were studied as functions of the power input to the ICP (0.8–1.9 kW). The results were linked with those of interference measurements with alkali matrices, and it was found that with the new facilities also, a relatively low power input (∼1.1 kW) combined with a 15 mm observation height and carrier gas flow rates in the range 1.3–1.6 l/min provide the best “compromise conditions” for simultaneous multielement analysis including trace analysis. Intensity ratios of ionic and atomic lines of various elements, and relative intensities of ionic lines of Mg and Al were determined under the stated “compromise conditions”. These results were connected with a measured effective temperature of 5850 K and a literature value of 10 16 cm −3 for the electron number density. On this basis, it was made plausible that the substantial departures from LTE found by other authors in rigorous experiments using radial resolution do also manifest themselves under analytical conditions and that a non-LTE mechanism must be the principal ground for some of the favourable analytical properties of argon ICPs, viz. the high sensitivity of ionic lines and the smallness of ionization interferences. A critical examination of the literature including that dealing with a special type d.c. plasma-jet, which shows similarities with a toroidal ICP, and the results found in this work led the authors to a working hypothesis in which the non-LTE mechanism of argon ICPs is attributed to an overpopulation of metastable argon levels and a dual role of metastable argon, that of an “ionizer” (Penning ionization) and that of an “ionizant” (easily ionizable constituent). To picture this mechanism, the authors set up a simple mathematical model, in which an LTE system is perturbed by an inflow of metastable argon atoms. It is shown that this model can qualitatively account for various features and trends, but it is at the same time pointed out which gaps in our knowledge of ICPs must be essentially filled by further experiments to give the model more substance than that of a working hypothesis.

Studies of an inductively-coupled high-frequency argon plasma for optical emisson spectrometry—II: Compromise conditions for simultaneous multielement analysis1

Abstract This paper deals with the experimental selection of conditions under which a low-power inductively-coupled plasma (ICP) can be operated so as to achieve a good compromise for simultaneous multi-element analysis. With the experimental facilities employed by the authors such conditions were found at a power of 0·7 kW, a carrier gas flow of 1·31 min of argon, and an observation height of 15 mm. An outstanding detection power with detection limits below 1 ng ml for 27 out of 32 representative elements and satisfactory suppression of ionization interference effects were simultaneously achieved. Modifications of a previously described ultrasonic nebulizer led to a higher rate of sample injection into the plasma, an improved overall reliability of the sample introduction device, and better reproducibility of spectral-line intensities (1·0–1·2% at concentrations 100 times the detection limits for 15-s integrations). Details of the construction of the ultrasonic nebulizer and its performance are provided and a simple theoretical treatment of the dependence of the aerosol ejection rate on the carrier gas flow and the size of the fog chamber is presented. Possible interelement interferences in the ICP are broadly classified. From the optimization experiments and application studies general conclusions regarding the usefulness of the ICP for both the accurate determination of a few elements and general overall analysis comprising a large number of elements are drawn.

Studies of a radio frequency inductively coupled argon plasma for optical emission spectrometry—III. Interference effects under compromise conditions for simultaneous multi-element analysis☆

Abstract This work concerns interference effects in a 0.7-kW, 50-MHz inductively coupled plasma (ICP) provided with an ultrasonic nebulizer (USN) and desolvation apparatus (DA). The observations were made under (ICP) conditions adopted previously as “compromise conditions for simultaneous multi-element analysis.” Various matrices and analytes were considered. An arrangement of two identical USNs with separate DAs was used to distinguish between interferences due to processes in the plasma (“plasma effects”) and the nebulizer—desolvation apparatus (“nebulizer—desolvation effects”). The latter were identified as “desolvation effects” and attributed to a variation in the loss of analyte in the DA. This desolvation effect, whose magnitude varies between ±10%, is related to the difference in volatility between matrix and analyte. The experiments revealed plasma effects that cannot be reconciled with the common pictures of ionization interference and are not due to incomplete volatilization or dissociation either. Possible explanations are considered. The overall interference level in the ICP studied is discussed and practical conclusions regarding the use of desolvation, “pure” aqueous solutions as standards, and spectroscopic buffers are drawn.

Studies of flame and plasma torch emission for simultaneous multi-element analysis—I: Preliminary investigations

Abstract The need for rapid and relatively simple techniques for simultaneous multi-element analysis of solutions focusses attention on emission techniques using a high-temperature flame, a radio-frequency plasma torch or a plasma jet. This paper deals with the premixed nitrous oxide-acetylene flame and the inductively-coupled argon plasma torch as excitation sources for trace analysis of solutions. Experimental facilities encompass various commercially available burners (including a burner for a gas-shielded flame), an improved plasma torch using a new type of HF generator (52 MHz, 2 kW) and a new arrangement of an ultrasonic nebulizer, less conventional accessory optics, a medium monochromator (1-m Czemy-Turner), and a double-beam, dual-channel arrangement using two monochromators. Detection limits of representative elements were determined in both sources and the conditions favourable for simultaneous multi-element trace analysis were investigated. In many instances, detection limits lower than those published previously were attained. With the plasma torch, the ng ml region—and for many elements even the sub- ng ml region—is easily accessible. The paper includes a discussion of some fundamentals of detection limits. This is to connect the present approach, which stems basically from emission spectrographic analysis, more smoothly with common usage in flame spectrometrio analysis.

Modification and optimization of a 50 MHz inductively coupled argon plasma with special reference to analyses using organic solvents

Abstract The torch and nebulizer of an existing argon ICP system were modified and the system was (re-) optimized for aqueous and organic liquids. The paper describes the design considerations and construction of (1) a new, streamlined torch including a torch base used in this study, where a demountable rather than a prealigned version of the torch was preferred; (2) a cross-flow pneumatic nebulizer with adjustable teflon capillaries including a spray chamber with flow spoiler, concentric aerosol pick-up tube, and “U” tube with unequal legs to smooth the flow of wasted liquid to the drain. The (re)-optimization of the ICP system for analysis of aqueous solutions with inorganic matter or with both inorganic and organic matter is discussed in the light of earlier work in this laboratory regarding the selection of “compromise conditions” and the choice of representative spectral lines and measurement criteria for establishing such compromise conditions. In this context the authors consider the concepts of norm temperature and “hard” and “soft” lines, as well as recent results of measurements of spatial distributions in ICPs. The authors further describe experiments aimed at the optimization of the operating conditions of an “organic ICP” using methyl isobutyl ketone (MIBK) as organic solvent. Trends of net line and background signals and signal-to-background ratios with the ICP parameters (power; outer, intermediate and carrier gas flow; observation height; liquid feed rate) are reported, and a rational choice of compromise conditions for the ICP is argued. Performance characteristics of the modified ICP system, such as detection limits, precision and interference level, achieved under compromise conditions, have been communicated in a previous report [Spectrochim. Acta 36B, 1031 (1981)] to demonstrate the capabilities of the system for analysis of aqueous solutions. Detection limits in MIBK and oil diluted in MIBK are reported in the present work as an illustration of the performance of the system when used for organic liquid analysis.

Detection limit including selectivity as a criterion for line selection in trace analysis using inductively coupled plasma-atomic emission spectrometry (ICP-AES)—a tutorial treatment of a fundamental problem of AES

Abstract This paper proposes the “true detection limit” as a quantitative criterion for line selection in inductively coupled plasma-atomic emission spectrometry. The “true detection limit” is denned as the sum of (1) a selectivity term that accounts for the additional noise resulting from line overlap and (2) the “conventional detection limit”, which covers the common noise sources for the interference-free situation and, additionally, the effect of line wings. The theoretical part discusses the various arguments as well as the links with the previous work from which the present approach evolved. The theoretical discussion is concluded with the formulation of the mathematical expressions to be subsequently used in the experimental part, where the approach is applied to a case study dealing with the selection of the best analysis line(s) for the determination of traces of In in binary mixtures of W and Mo, the composition of which is assumed to vary from pure W to pure Mo. Both high and medium spectral resolution are covered, as well as the effects of changing the ICP operating parameters or modifying the transport rate of the metal sample to the plasma, as achieved by varying the concentration in the solution or using an ultrasonic instead of a pneumatic nebulizer. It is shown that the approach permits straightforward and unambiguous decisions on line choice. It is argued that the approach should be applicable to multi-component samples of whatever complexity and that it should prove useful not only for a priori line selection, as elaborated in this work, but also for a posteriori line selection in methods employing “multiple line analysis”. Although the experimental data were directly derived from digitized wavelength scans for pure analytes and interferents recorded with a particular spectroscopic apparatus, it is pointed out that the approach has every prospect of being applied in software systems for line selection using physically resolved spectral data in combination with a variable spectral instrumental function.

Inductively coupled plasma-atomic emission spectroscopy: Its present and future position in analytical chemistry

ZusammenfassungDiese Übersicht (mit 179 Literaturzitaten) beabsichtigt hauptsächlich, die gegenwärtige und zukünftige Stellung der atomaren Emissions-Spektrometrie unter Verwendung eines induktiv gekoppelten Hochfrequenz-Plasmas (ICP-AES) sowohl unter den verschiedenen herkömmlichen, »bewährten« spektroskopischen Methoden als auch unter den AES-Methoden mit neuen Plasmaquellen für Flüssigkeitsanalysen zu verdeutlichen. Nach gründlichem und kritischem Vergleich der Leistungsfähigkeit und Kosten der ICP-AES gegenüber schon bestehenden Laboreinrichtungen muß die Wirtschaftlichkeit der ICP-AES je nach Situation als mögliche Ergänzung oder als möglicher Ersatz zu den herkömmlichen Techniken bewertet werden. Im Hinblick darauf wird die ICP-AES als eine relativ neue Methode zur Analyse von Flüssigkeiten und gelösten Feststoffen untersucht.Das Prinzip der Methode und die Grundausrüstung werden kurz erklärt. Dabei wird auf den Unterschied eines Argon-ICPs niedriger Leistung gegenüber einem Stickstoff-Argon-ICP hoher Leistung hingewiesen und die Fähigkeit dieser beiden Arten für Analysen realer Proben besprochen. Die Analysenfähigkeit im allgemeinen wird anhand von Nachweisgrenzen, Genauigkeit, Richtigkeit und dynamischem Bereich diskutiert. Typische Anwendungsbeispiele werden erwähnt.Eine Zusammenstellung der besten Nachweisgrenzen von 67 Elementen in wäßrigen Lösungen wird für Argon-ICPs mit pneumatisch und mit Ultraschall betriebenem Zerstäuber gegeben. Nachweisgrenzen von 15 Elementen in Öl werden angeführt, um die Anwendungsmöglichkeiten von Argon-ICPs niedriger Leistung auch im Bereich der organischen Flüssigkeitsanalysen zu zeigen. Die Nachweisgrenzen von As, Sb, Bi, Se und Te, die durch eine Kombination von Hydridbildung und Argon-ICP erreichbar sind, werden dargestellt, um den neuesten Fortschritt in der Elementbestimmung zu zeigen, wo bisher das Nachweisvermögen noch nicht ausreichend war.Bei der Behandlung der Genauigkeit wird auf die Eigenschaft des ICPs als ein durch Schwankungsrauschen in der Lichtquelle begrenztes System hingewiesen, was durch die relative Standardabweichung (RSD) von ≤ 1 %, sowohl in den Untergrund- als auch Nettosignalen, wie sie die Quelle selbst bewirkt, deutlich wird. Die Abhängigkeit der RSD in den schließlich gemessenen Nettosignalen (Bruttosignal minus Untergrundsignal) vom Verhältnis Konzentration zu Nachweisgrenzen wird diskutiert.Eine ausführliche Besprechung der Richtigkeit umfaßt detaillierte Angaben über Faktoren wie spektrale Interferenzen, Reagensunreinheiten, Zerstäubungsund Transportinterferenzen, Verdampfungsinterferenzen und schließlich Ionisationsinterferenzen, die alle für die erreichte Richtigkeit ausschlaggebend sein dürften. Es wird gezeigt, daß die ICP-AES relativ frei von Interferenzen ist, so daß gute Richtigkeit in beträchtlichem Maß erreicht werden kann, wenn geeignete Vorsichtsmaßnahmen getroffen werden, die weniger aufwendig als diejenigen anderer AES-Methoden oder der AAS sind. Es wird jedoch ausgeführt, daß die zur Erzielung guter Richtigkeit erforderlichen Maßnahmen, immer aufwendiger werden, sobald die Analysenkonzentration sich mehr und mehr der Nachweisgrenze nähert oder die Proben komplexer werden. Dazu ist zu sagen, daß hier wieder die gleichen Probleme auftauchen, mit denen sich die Bogenspektroskopiker bei der Anwendung des Gleichstrombogens in der Spurenanalyse zu befassen hatten, besonders hinsichtlich der korrekten Abtrennung eines Nettosignals vom Untergrundspektrum, dessen Struktur von der Probenzusammensetzung abhängt.Der Vorteil des großen linearen dynamischen Bereichs über drei bis fünf Größenordnungen wird erwähnt.Die zahlreichen Anwendungen der ICP-AES werden in einer Liste nach denjenigen Matrialklassen aufgeführt, die in der neuesten Literatur beschrieben sind.Spektrometer werden als unentbehrlicher Teil einer ICP-Ausrüstung diskutiert, der zu einem vollständigen Analysengerät gehört und dessen Preis ein Vielfaches von dem der ICP-Quelle selbst betragen kann. Die unterschiedlichen Spektrometer für die ICP-AES werden in drei Gruppen eingeteilt, abhängig von der Art des Analysenproblems: 1. Allgemeine Übersichtsanalysen, 2. routinemäßige Multielementanalysen und nichtroutinemäßige Multielementanalysen mit beschränkter Flexibilität, und 3. flexible Einzelelementanalysen. Diese Klassifikation, angepaßt an die allgemeinen Kosten und Leistungsbetrachtungen, wird als nützliche Basis zur nachfolgenden Einschätzung der Stellung der ICPAES unter den herkömmlichen, »bewährten« spektroskopischen Methoden verwendet. Diese Einschätzung schließt Leistungsvergleiche zwischen ICP-AES, Flammen-AES, Flammen- und Ofenatomabsorptionsspektrometrie, Gleichstrombogen-AES, Funken-AES und Röntgenfluorescenzspektrometrie (XRFS) ein.Die Stellung des ICPs gegenüber anderen neuen Plasmaquellen für Flüssigkeitsanalysen durch AES wird im Hinblick auf die geschichtliche Entwicklung behandelt. Diese führte schließlich zu der heutigen Situation, wo mindestens 12 Hersteller von Spektrometerausrüstungen ICP-Geräte (einer mit einer Gleichstromplasmaquelle) anbieten, währenddem erneuter Handel mit einem kapazitiv gekoppelten Mikrowellenplasma (CMP) neue Interessen an CMPs weckt. Was Mikrowellen induzierte Plasmen (MIP) betrifft, wird die Aufmerksamkeit speziell auf den TM010-Hohlraum gerichtet, der die Erzeugung von Helium-Plasmen bei atmosphärischem Druck ermöglicht. Diese MIPs haben ausgezeichnete Eigenschaften als element-selektive Detektoren in der Gas-Chromatographie und bieten auch viele Verwendungsmöglichkeiten bei allgemeinen Multielement- und Einzelelementanalysen, speziell für Mikroproben, wenn diese vor der Eingabe in das MIP getrennt verdampft und atomisiert werden (z.B. elektrothermisch).SummaryThis review (with 179 references) is mainly intended to facilitate judgements about the present and future position of inductively coupled plasma-atomic emission spectrometry (ICP-AES) among both various “established” spectroscopic methods and AES methods based on novel plasma sources for liquid analysis. It is considered that a thorough and critical comparison of the capabilities and cost of ICP-AES with those of the existing outfit of a laboratory must be made in each individual situation separately to judge whether ICP-AES as a supplement to or a replacement of one or more established techniques is an economic proposition. From this point of view ICP-AES is reviewed as a relatively new method for the analysis of liquids and dissolved solids.The principle of the method and the basic instrumentation are briefly outlined. The distinction between low-power argon ICPs and high-power nitrogen-argon ICPs is pointed out and the viability of both approaches in the analysis of real samples is noted. Analytical performance is discussed in terms of detection limits, precision, accuracy and dynamic range. Applications of real-sample analysis are given as illustrative examples.A list is included of the best detection limits of 67 elements in aqueous solutions as reported for argon ICPs operated with pneumatic and ultrasonic nebulizers. Detection limits of 15 elements in oil are given to illustrate the potentials of low-power argon ICPs in the field of organic liquid analysis. The detection limits of As, Sb, Bi, Se and Te as achieved by combining hydride generation with an argon ICP, are presented to demonstrate recent progress in the determination of elements for which the detection power was hitherto less satisfactory than desired.With regard to precision, the behaviour of ICPs is pointed out as fluctuation-nose limited systems that are dominated by a relative standard deviation (RSD) of ≤ 1 % in both background and net signals as generated in the source. The dependence of the RSD in the eventually measured net signals (gross signal minus background signal) on the ratio of concentration to detection limit is discussed.An extensive discussion of accuracy incorporates detailed reference to factors such as spectral interferences and reagent impurities, nebulization and transport interferences, “solute vaporization” interferences, and ionization interferences, which may affect the accuracy attained in ICP-AES. It is shown that ICP-AES is relatively free from interferences so that a fair degree of accuracy can be reached, if proper precautions are taken, which, in comparison with AES methods using other excitation sources, or AAS, are not excessive. It is added, however, that the measures needed to ensure fair accuracy will become increasingly severe as the analyte concentration approaches the detection limit more closely or the composition of the sample becomes more complex. It is noted that under these conditions the problems that arc spectroscopists had to face in trace analysis using dc arc spectrography are again encountered, in particular as regards the correct isolation of a net line signal from a background spectrum whose structure depends on the sample composition.The advantages of the large linear dynamic range of three to five orders of magnitude are mentioned.The numerous applications of ICP-AES reported in literature are illustrated with a list of classes of materials for which analyses were described recently.Spectrometers enter into the discussion as indispensable parts of ICP equipment that are necessary for complete analytical instruments and the price of which may be a multiple of that of the ICP source itself. Alternative spectrometers for ICP-AES are grouped into three categories depending on the type of analysis problem: 1) general survey analysis, 2) routine multielement analysis and non-routine multielement analysis with limited flexibility, and 3) flexible single-element analysis.This classification, which fits in with general cost and performance considerations, is used as a convenient basis in the subsequent assessment of the position of ICP-AES among “established” spectroscopic methods. This assessment encompasses comparisons of the capabilities of ICP-AES,

Mutual spectral interferences of rare earth elements in inductively coupled plasma atomic emission spectrometry: I. Rational line selection and correction procedure

Abstract This paper is the first part of a series of three papers dealing with mutual spectral interferences of rare earth elements (REE). The present paper reports the measurement of the partial sensitivities of all REEs at the peak wavelengths of 30 prominent lines of Ce, La, Nd, Pr, Sm, and Yb, the most abundant REEs in geological samples. These sensitivities are split into line and wing contributions and are ratioed with respect to the analyte sensitivities. The “Q-values” thus obtained, along with the numerical values of the background equivalent concentrations for the pure solvent and the relative standard deviation of the background, permit a rational selection of analysis lines for REE mixtures of whatever composition using as a criterion the “true detection limit” as proposed by Boumans and Vrakking [ Spectrochim. Acta 42B , 819 (1987)]. The effectiveness of this approach is illustrated with examples. The data listed are likely to be also appropriate for other spectrometers having a spectral bandwidth of approximately the same magnitude as that used in the experiments, i.e. 17 pm. Therefore the paper includes brief instructions for applying the data. The article finally demonstrates the use of the same data for multielement interference corrections.

The widths and shapes of about 350 prominent lines of 65 elements emitted by an inductively coupled plasma

Abstract This work describes the measurement of the widths and shapes of about 350 prominent lines of 65 elements emitted by an inductively coupled plasma (ICP). The experimental procedure is an improved version of an earlier described approach using a 1.5-m echelle monochromator with predisperser. For most measurements the practical spectral bandwidth was smaller than 1.5 times the physical line width. Results are reported for both simple lines with chiefly Doppler broadening and complex structures with unresolved or partly resolved hyperfine structure (HFS). An atlas with the spectral scans of about 90 interesting line profiles is included and wavelengths of HFS components are tabulated. The latter were accurately determined in the case of well resolved structures and roughly estimated for poorly resolved or unresolved structures. The data were collected chiefly with a view to spectrochemical analysis, with a threefold aim: (a) to provide the basic data needed for comparing detection limits obtained with spectroscopic apparatus of different bandwidths; (b) to identify HFS components which, under high resolution conditions, may be useful as separate prominent lines in order to circumvent spectral interference and (c) to establish a basis for models that can be used in software for line selection such that the number of data needed in view of differences among line shapes and widths can be reduced to a minimum. It appears from the results that these targets are closely approached. Further work is required, however, for the full implementation of the results.

A comparative investigation of some analytical performance characteristics of an inductively-coupled radio frequency plasma and a capacitively-coupled microwave plasma for solution analysis by emission spectrometry

Abstract This paper reports results of an inter-laboratory investigation involving standardized experiments performed with standard measuring equipment to establish unambiguously the differences in analytical performance of an inductively-coupled radio frequency plasma (ICP) and a capacitively-coupled microwave plasma (CMP) for simultaneous multi-element analysis (SMEA) of solutions. The authors compared a laboratory-built, prototype ICP and a standard commercial CMP. The analytical characteristics covered were the following: (i) detection limits, (ii) matrix effects produced by cesium sulfate, cadmium sulfate, and di-amnioniumhydrogenphosphate, (iii) sensitivities, and (iv) precision. These characteristics were evaluated for 14 spectral lines of 12 representative elements. The results were corrected for differences in the performance (aerosol injection rate into the plasma) of the nebulizers used with the two excitation sources and for differences in transmission factor, optical conductance and photomultiplier responsivity between the two “identical” measuring equipments. It was definitively established that in all respects the ICP (investigated here) is superior to the CMP (considered here) as an excitation source for SMEA of solutions.