Reference element technique

Yet, another feature that becomes available with the fast sequential mode of operation in HR-CS AAS is the use of the reference element technique, that is, the use of an internal standard. This technique has rarely been described in LS AAS [3] for a number of obvious reasons. Firstly, the technique requires a dual-or multi-channel spectrometer, and there have been only very few spectrometers of this type available commercially over the past decades. Secondly, the reference element technique is ideally suited to correct for nonspecific interferences, such as transport interferences, but it is notoriously difficult to find an appropriate reference element for element-specific interferences. Thirdly, the most successful multi-channel LS AAS equipment, the Perkin-Elmer Model SIMAA 6000, which has been available for a number of years, was designed for ET AAS only, a technique that does not typically exhibit nonspecific interferences. The number of publications using this technique is therefore very limited. 

In HR-CS AAS, although the simultaneous measurement of two elements or more is only possible within the simultaneous recorded wavelength interval, the reference element technique can be used essentially without compromises for FAAS, where nonspecific interferences are quite common. The fast-sequential... 

Another advantage of the reference element technique is that dilution errors can be recognized and eliminated when the reference element is added at an early stage of sample preparation before the final dilution. Moreover, using the reference element technique, quantitative analysis could actually be done without accurate dilution and without using volumetric flasks etc., resulting in additional timesaving and elimination of potential errors in sample preparation. 

The major anions and cations in seawater have a significant influence on most analytical protocols used to determine trace metals at low concentrations, so production of reference materials in seawater is absolutely essential. The major ions interfere strongly with metal analysis using graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma mass spectroscopy (ICP-MS) and must be eliminated. Consequently, preconcentration techniques used to lower detection limits must also exclude these elements. Techniques based on solvent extraction of hydrophobic chelates and column preconcentration using Chelex 100 achieve these objectives and have been widely used with GFAAS. 

As a second illustration of the finite-element technique, we proceed as follows. We assume a rectangular mesh of three rows and three columns with uniform step sizes h = k = along the x and y axes, respectively. Further, assume that potentials on the boundary have values f/oi, f/02, t/o3. t io. t i4. t 20. U24, U o, t/34. U41, f/42, f/43, where by f/ j we refer to the top and bottom or left and right rows or columns as i,j = 0 and i,j = 4, respectively. The five-point sampling Laplace equation has the algebraic form... 

The use of particles heavier than electrons, and especially proton-induced x-ray emission analysis, was developed under the supervison of Professor Sven Johansson at Lund University, Sweden, during the 1970s.9 Generally referred to as PIXE (particle- or proton-induced x-ray emission) analysis, it has proven to be a sensitive trace element technique. The initial response among medical researchers was a cautious one, most likely due to the fact that the problems of specimen preparation initially were... 

The direct determination of some major elements (Ca, K, Mg, Na, and P) and Zn by ICP-AES was performed in powdered milk [14]. Samples were diluted with a 5 or 10 percent (v/v) water-soluble, mixed tertiary amine reagent at pH 8. This reagent mixture dissociated casein micelles and stabilized liquid phase cations. No decrease in analyte emission intensities was observed. Reference solutions were prepared in 10 percent (v/v) mixed amine solution, and no internal reference element was needed for ICP-AES. The direct technique is as fast as slurry approaches, without particle size effects or sensitivity losses. 

Reference Elements Determined Technique Sample Pretreatment... 

We have employed two multi-elemental techniques (INAA and ICP-AES) to determine sulphur, halogens and 14 other trace elements in urban summer rainfall. Quality control was assured using NBS reference materials. The overall accuracy and precision of these two methods makes possible the routine analysis of many environmentally important trace elements in acid rain related investigations. Enrichment factor calculations showed that several elements including S, Cu, Zn and Cr were abnormally enriched in the urban atmosphere. A comparison of three separate sites showed a strong gradient of metal deposition from the industrial to the outlaying areas. 

Chemical interference is practically non existent as a result of the high temperature of the plasma. On the other hand, physical interference may be observed. This stems from variations in the sample atomisation speed which is usually due to changes in nebulisation efficiency caused by differences in the physical properties of the solutions. Such effects may be caused by differences in viscosity or vapour tension between the sample solutions and the standards due, for example, to differences in acidity or total salt content. The technique most commonly used to correct this physical interference is the use of internal standards. In this technique a reference element is added at an identical concentration level to all the solutions under analysis, standards, blank and samples. For each element, the ratio of simultaneous measurements of the lines of the element and the internal standard is then determined in order to compensate for any deviation in the response of the plasma. If the internal standard behaves in the same way as the element to be determined, this method can be used to improve the reliability of the result by a factor of 2 to 5. It can also, however, introduce significant errors because not all elements behave in the same way. It is thus necessary to take care when using it. Alternatives to the internal standard method include incorporating the matrix into the standards and the blank, sample dilution, and the standard addition method. 

Inductively coupled plasma-atomic emission spectrometry was investigated for simultaneous multielement determinations in human urine. Emission intensities of constant, added amounts of internal reference elements were used to compensate for variations in nebulization efficiency. Spectral background and stray-light contributions were measured, and their effects were eliminated with a minicomputer-con-trolled background correction scheme. Analyte concentrations were determined by the method of additions and by reference to analytical calibration curves. Internal reference and background correction techniques provided significant improvements in accuracy. However, with the simple sample preparation procedure that was used, lack of sufficient detecting power prevented quantitative determination of normal levels of many trace elements in urine. 

Elements other than Pb have been measured by TIMS in polar snow (43, 49, 50, 53), showing the high potential of the technique. Bismuth determination (43) is characterized an accuracy comparable to that of LEAFS and takes advantage of the simultaneous ionisation of Bi and Pb during the Pb isotopic abundance measurements. Determination of Ba was also carried out on some Antarctic samples (53) showing its possible use as a reference element in monitoring the contribution of terrestrial dust level. 

The purpose of this section is to provide a brief review of the methods and techniques commonly used in the elemental analysis of humic substances with special emphasis placed on areas that may cause difficulties in their analysis. There are few specific references to methods of elemental analysis of humic substances. A computerized search of Chemical Abstracts since 1966 revealed no references to techniques of elemental analysis when elemental analysis was cross-referenced with humic or fulvic acid materials. In general, the methods of analysis have been developed to be applicable to a wide range of organic materials. However, it should be pointed out that most methods have been validated on the basis of the analysis of stable, nonhy-groscopic, nonvolatile, pure compounds and not heterogeneous mixtures. 

To summarise, FAAS is very easy to use. Interferences are known and can be controlled. Extensive application information is also readily available. Its precision makes it an excellent technique for the determination of a number of commonly analysed elements at higher concentration in polluted soil samples. Its main drawback is its speed in relation to multi-element techniques such as ICP-AES and ICP-MS. Where direct-aspiration flame atomic absorption technique does not provide adequate sensitivity, reference is made to specialised techniques (in addition to graphite furnace procedure) such as the gaseous-hydride method for arsenic, antimony and selenium and the cold-vapour technique for mercury. 

Thermoelectric-, pyroelectric-, and thermoconductivity-based devices are other representatives of thermometric gas sensors (Korotcenkov 2011). In particular the thermal conductivity technique for detecting gas is suitable for the measurement of high (vol. %) concentrations of binary gas mixes. The heated sensing element is exposed to the sample and the reference element is enclosed in a sealed compartment (see Fig. 1.14). If the thermal conductivity of the sample gas is higher than that of the reference, then the temperature of the sensing element decreases. The higher their thermal conductivity, the lower the concentration which can be measured (Table 1.14). Power loss of a single filament thermistor by heat conduction via the ambient gas can be expressed as... 

Again refer to Plates B. 1 and B.2 finite-element technique to determine buckling and instability has been briefly formulated in the text. The European have given an improved design recommendations. 

The discussions of various flame analysis techniques in section 2.2 are equally applicable to the determination of rare earth elements in a complex mixture of these elements./Net intensity or absorbance measurements usually provide adequate precision, so that an added reference element is not needed. In addition, there is no evidence of inter-element effects, and line interferences, even in small monochromators, are rarely a serious problem. This contrasts sharply with the selective enhancement and absorption effects observed in X-ray fluorescent spectrometric measurements. Analyses of rare earth mixtures by AAS have been described by Jaworowsk et al. (1967), Kriege and Welcher (1968) and many others. 

The wavelengths of the analytical lines and the estimated limits of detection for different phosphor hosts are summarized in table 37E.2. The table is not exhaustive as several investigators who have utilized the XEOL technique for the determination of trace level rare earths have not tabulated the limits of detection. Typical analytical curves that have been obtained in our studies utilizing the internal reference element principle are shown in fig. 37E.12. 

The ISFET, developed from the fabrication techniques of semiconductor devices, is an important sensor device used in potentiometry. The main advantages are the extremely small size, solid-state structure and the ability to fabricate multi-ion sensors. More than 30 years ago, methods have been proposed to work with a differential arrangement, i.e. the integration of an ion-sensitive and an ion-insensitive structure, the later one working as the reference element (R(E)FET). The main problem is that semiconductor-modified surfaces required for R(E)FET are also not always in thermodynamical equilibrium with the test solution and can be sensitive to aggressive or interfering dissolved species or not well characterised aging phenomena. 

Written by a field insider with over 20 years experience in product development, application support, and field marketing for an ICP-MS manufacturer, the third edition of Practical Guide to ICP MS A Tutorial for Beginners provides an updated reference that was written specif ically with the novice in mind. It presents a compelling story about ICP-MS and what it has to offer, showing this powerful ultra trace-element technique in the way it was intended—a practical solution to real-world problems. 

This reference describes the principles and application benefits of ICP-MS in a clear manner for laboratory managers, analytical chemists, and technicians who have limited knowledge of the technique. In addition, it offers much-needed guidance on howbestto evaluate capabilities andcompare with other trace element techniques when looking to purchase commercial ICP-MS instrumentation. 

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