3 Key excerpts on "Elemental Analysis"

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  eBook - ePub

  Soil Analysis Handbook of Reference Methods

  • Soil and Plant Analysis Council Inc., Jr. Jones(Authors)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  All but a few of these procedures involve some form of spectrophotometry, the utilization of a specific wavelength to measure either intensity or absorption to determine elemental or ion specie concentration. There are essentially four analytical techniques that have application for such elemental assays: procedures that are still in common use in soil analysis laboratories today; colorimetry (UV-VIS spectrophotometry); turbidity; and emission and atomic absorption spectrophotometry. The trend today is greater analytical sophistication—multi-element computer-controlled analytical instrumentation—resulting in a lessened understanding of the analytical principles involved, and “black box” concepts of instrument calibration, maintenance, and operation. Many technicians today are less knowledgeable about the analytical procedures they are using and more concerned as to which button to push in order to carry forward an analysis.

  Preparation of the analyte and suitable adaptation of the method of analysis require an understanding of the principles of operation of the method as well as its requirements and limitations. Adequate testing is usually required before putting a method into use, following procedures such as those that have been adapted by the Association of Official Analytical Chemists (AOAC) (McLain, 1982).

  Although the elemental sensitivity of the analytical technique (see Table 16.2 ) is a significant factor for some procedures and with some elements (particularly the heavy metals, see Chapter 9 ), the precision of the method is equally important. Normally the elemental content found in a soil extractant is considerably above the detection limit of most analytical procedures with long-term stability of the method over the assay time period being the most important for determining its acceptance and use. This involves both the characteristics and stability of the analytical instrument as well as the quality of reagents used. The positioning of “check samples” or “reference standards” in an array of sequential extractant unknowns is essential in order to monitor the stability of the analytical run (see Chapter 15

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  eBook - ePub

  Analytical Chemistry Refresher Manual

  • John Kenkel(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  Analytical Chemistry for Technicians, Lewis Publishers, Chelsea, MI, 1988. With permission.)

  1.3    TERMINOLOGY

  The basic terminology associated with analytical chemistry and analytical laboratory work is important and may be foreign to persons who have not been associated with such laboratory work in preparation for their jobs. We thus present a small glossary of terms in this section. Other important terms specific to particular analyses are given elsewhere in this book and can be found in the index.

  Chemical Analysis	This is the determination of the chemical composition or chemical makeup of a material sample.
  Qualitative Analysis	The determination of what substances are present in a material sample, usually without the need or desire to determine quantity of these substances.
  Quantitative Analysis	The determination of how much of a specified substance is present in a material sample.
  Quantitation	This is the determination of quantity, as in the quantitative analysis above.
  Quantification	This is another word for quantitation.
  Quantitative Transfer	A transfer of a chemical or solution from one container to another, making sure that every trace of this chemical is in fact transferred.
  Analyte	This is the substance being analyzed for in an analytical procedure. This can be an element, a compound, or an ion.
  Assay	This is another word for chemical analysis.

  1.4    FUNDAMENTALS OF MEASUREMENT

  In the analytical chemistry laboratory, many measurements are made, and the accuracy of these measurements obviously is a very important consideration. Different measuring devices give us different degrees of accuracy. A measurement of 0.1427 g is more accurate than a measurement of 0.14 g simply because it contains more digits. The former (0.1427 g) was made on an analytical balance, while the latter was make on an ordinary balance. A measurement recorded in a notebook should always reflect the accuracy of the measuring device. It does not make sense to use a very accurate measuring device and then record a number that is less accurate. For example, suppose a weight on an analytical balance was found to be 0.14g. It would be a mistake to record the weight as 0.14 g, even if you know personally that the weight is 0.14 g. Presumably, there are other people in the laboratory using the notebook, and your entry will be construed as to contain only two digits. The following example further illustrates this point.

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  eBook - ePub

  Materials Characterization

  Introduction to Microscopic and Spectroscopic Methods

  • Yang Leng(Author)
  • 2013(Publication Date)
  • Wiley-VCH

    (Publisher)

  (Reproduced with kind permission of Springer Science and Business Media from Ref. [1].© 1992 Springer Science.)

  Figure 6.16

  EDS maps of a polished area of an alloy specimen. The distributions and concentrations of chemical elements shown in the maps include: (a) Si; (b) Mo; (c) Cr; and (d) Co.

  (Reproduced with permission from Ref. [4]. © 1988 Taylor & Francis Group Ltd.)

  We may select the stationary mode to analyze the elements in microscopic features such as impurities, precipitates, and grain boundaries. We may select the scanning mode to examine the composition change in a certain dimension of the specimen or composition variations in a selected area of a specimen.

  6.4 Qualitative and Quantitative Analysis

  6.4.1 Qualitative Analysis

  Identifying the chemical elements in specimens by X-ray spectrometers is in many ways similar to identifying chemical compounds by X-ray diffractometry (XRD ). However, identifying elements is relatively easy because there are only about 100 elements, while there are several million chemical compounds. We may only find a major peak with a few minor peaks of a certain element in WDS and EDS, not several tens of peaks for a crystalline compound as in an XRD spectrum. For example, Figure 6.17 shows an XRF spectrum (EDS type) for a Ni–Ti alloy. The spectrum shows one major Kα peak and one minor Kβ peak for each of Ni and Ti. The Kα includes Kα1 and Kα2 , which are often overlapped in the spectrum due to the small difference in their energy levels. The relative positions and intensities of the α and β lines also can help us to identify chemical elements if there is a possibility that the Kα or Kβ line of one element overlaps another element's L lines.

  Figure 6.17

  EDS spectrum of a Ni–Ti alloy obtained in an XRF spectrometer.

  We should note that the primary X-ray source of XRF includes both continuous and characteristic radiation. The continuous X-rays generate the background of the spectrum. The primary characteristic X-rays will generate additional peaks in a spectrum. Thus, such background and additional peaks of a target material of the X-ray tube should be deducted from a spectrum. For XRF, an uncommon element such as rhodium (Rh) is often selected as the target material of X-ray tube in order to reduce confusion in identifying elements in samples. For example, Figure 6.18

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