Trace elements analysis

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Trace elements analysis
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Elemental Analysis

• This chapter includes no detailed
  description of methods to determine individual
  mineral components. Such procedures are described
  in general textbooks of inorganic analysis,
  standard reference books on food analysis, and
  specialized textbooks on the determination of
  minerals in biological materials. The principles
  of instrumental methods used in the determination
  of mineral components and trace elements also
  were descried in previous chapters of this book.
  This chapter is primarily concerned with the
  applications of those principles to food
  analysis.
• Developments in the measurement of trace
  metal components in foods were described by
  LaFluer (1976), Winefordner (1976), Bratter and
  Schramel (1980), Das (1983), Schwedt (1984), and
  Benton-Jones (1984). Tshopel and Tolg (1982)
  reviewed the basic rules that have to be followed
  in trace analyses to obtain precise and accurate
  results at the nanogram and pictogram levels.
  These rules are as follows
• 1. All materials used for apparatus and
  tools must be as pure and inert as possible.
  These requirements are only approximately met by
  quartz, platinum, glassy carbon, and, to a lesser
  degree,

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• polypropylene.
• 2. Cleaning of the apparatus and
  vessels by steaming is very important to lower
  blanks as well as element losses by adsorption.
• 3. To minimize systematic errors,
  microchemical techniques with small apparatus and
  vessels with an optimal ratio of surface to
  volume are recommended. All steps of the
  analytical procedure, such as composition,
  separation, preconcentration, and determination,
  are best done in one vessel (single-vessel
  principle). If volatile elements or compounds
  have to be determined, the system should be
  closed off and the temperature should be as low
  as possible.
• 4. Reagents, carrier gases, and
  auxiliary materials should be as pure as
  possible. Reagents that can be purified by
  subboiling point distillation are preferred.
• 5. Contamination from laboratory air
  should be avoided by using clean benched and
  clean rooms. By this, the blanks caused by dust
  can be decreased by at least two or three orders
  of magnitude.

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• 6. Low and constant reaction
  temperature should be used.
• 7. Manipulations and different working
  steps should be restricted to a minimum in order
  to reduce unavoidable contamination.
• 8. All steps of the combined procedure
  should be monitored this can best be done with
  radiotracers.
• 9. All procedures have to be verified
  by a second independent one or, even much better,
  by an interlaboratory comparative analysis.
• Element Enrichment
• The determination of trace element often
  requires enrichment of the elements, and/or the
  separation of many elements at the trace level
  from large amounts of major elements. Ion
  exchange has proved to be a valuable tool in the
  concentration, isolation, and recovery of ionic
  materials present in a solution in trace amounts.
  Ion-exchange chromatography on an ion-exchange
  resin can be also used for fractionation,
  separation, and the elimination of interfering
  ions.

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• Emission Spectroscopy
• Emission spectroscopy is the oldest
  instrumental method for trace analysis. It
  depends on observing and measuring the radiation
  emitted by atoms of the various means fall back
  to the original (or a lower) level. For each
  element there is a pattern of wavelengths
  characteristic of the element when excited in a
  particular way. By identifying the wavelengths in
  the spectrum, the sample can be analyzed.
  Emission spectroscopy is sensitive but the
  precision is rather low.
• Flame Photometry
• Early studies during the nineteenth
  century by J. F. Herschel, D. Alter, and G.
  kirchhoff and R. Bunsen laid the foundations for
  the qualitative differentiation of salts
  depending on their emission in a flame. Later
  researchers developed suitable techniques and
  instruments for quantitative analyses based on
  flame photometry. A modern flame photometer
  consists essentially of an atomizer, a burner,
  some means of isolating the desired part of the
  spectrum, a photosensitive detector, sometimes an
  amplifier, and finally a

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• method of measuring the desired emission by
  a galvanometer, null meter, or chart recorder
  (see Chapter 10 for details). The instruments are
  used primarily to determine calcium, sodium, and
  potassium.
• Atomic Absorption Spectroscopy
• Within the last two decades atomic
  absorption spectroscopy has found enthusiastic
  acceptance by science and industry. Hundreds of
  papers are published annually on basic research,
  instrumentation, specific analytical methods, and
  practical applications of atomic absorption
  spectroscopy.
• Atomic absorption spectroscopy is not
  quite as free from inter-element effects as was
  originally expected, but it is far better in this
  respect than any from of emission spectrography.
  It is quite sensitive the limit of detection
  ranges from 0.01 ppm for magnesium to 5.00 ppm
  for barium and the method is rapid (about 1000
  determinations can be made per week). The
  equipment is relatively inexpensive (about
  20000), only one-tenth the coast of X-ray
  fluorescence equipment. The limiting factor is
  the need for cathode lamps for each element or
  several combinations.

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• In atomic fluorescence spectroscopy,
  atoms are generated in the same way as in atomic
  absorption spectroscopy, expect that a
  cylindrical flame is used. The flame is
  irradiated by resonance radiation from a powerful
  spectral source, and the fluorescence that is
  generated in the flame is measured at right
  angles to the incident beam of radiation. This is
  done to minimize the contamination of the
  fluorescence signal by light from the source.
• Atomic absorption spectroscopy can be
  used in the ppm range atomic fluorescence
  spectroscopy in the ppb range.
• Neutron Activation Analysis
• In neutron activation analysis, a weighed
  sample together with a standard that contains a
  known weight of the element sought is exposed to
  unclear bombardment. The radioactivity of the
  element in the sample is then compared with the
  radioactivity in the standard. Generally, a
  chemical separation is required to purify the
  radioisotopes of the element sought and to remove
  all other induced radioactivity. The quantity of
  the element in the sample is then calculated from
  the ratio of the separated activities. In some

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• instance, the final measurement of activity
  can be made on the intact sample. If the
  background remains inactive during nuclear
  bombardment or if the energies of the emitted
  radiations differ widely, a direct measurement of
  trace elements is possible. Also, if the trace
  element has a substantially longer half-life than
  the other induce activities, the interfering
  materials may be allowed to decay and the
  radioassay completed when the interference is
  insignificant. Results obtained by neutron
  activation generally are within 5 of the true
  value, and replicate analyses under favorable
  conditions are within 2-3 of the mean.
• The attractive features of neutron
  activation analyses are its wide applicability,
  high sensitivity, and satisfactory accuracy and
  precision. There have been numerous applications
  of activation analysis in botany and agriculture.
• X-Ray Spectroscopy
• There are three uses of X-rays in chemical
  analysis. Absorption methods are of limited
  practical because the adjustment of wavelength is
  most critical. X-ray diffraction is useful in
  crystallography and in establishing the
  complicated structure

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• of biological molecules. The use of X-ray
  for the identification of chemical components is
  based on emission methods, involving secondary or
  fluorescent emission.
• Measurement of the intensity and wavelength
  of fluorescence radiation is a well-established
  method of analysis. Coefficients of variation of
  about 1 in the concentration range 5-100 and of
  5 in the range 0.1-1.0 can be obtained. In some
  instances determinations in the chemical
  combination of the element, and nondestructive in
  the sense that specimen examined is not
  destroyed, though some specimen preparation may
  be required. Instrumentation for X-ray
  spectroscopy is quite expensive.
• Glass Electrodes
• When a thin membrane of glass is interposed
  between two solutions, an electrical potential
  difference is observed across the glass. The
  potential depends on the ions present in the
  solutions. Depending on the composition of the
  glass, the response may be primarily to the
  hydrogen ion, to other cations, or even to
  organic cations. The electrodes are unaffected by
  oxidants and reducing agents, and only slightly
  affected by anions (except fluoride) or by

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• high concentrations of proteins and amino
  acids.
• Miscellaneous Methods
• Trace elements are determined in many
  laboratories by specific colorimetric and
  turbidimetric methods, by fluorescence analysis,
  and by polarography. The use of infrared
  spectroscopy in determining polyatomic ions was
  described by Miller and Wilkins. Relatively
  simple chromatographic methods for rapid routine
  evaluation of trace elements in crops and foods
  were described by Duffield and Coulson.
• Impressive advances have been made in
  developing instruments that permit an essentially
  complete elemental analysis to be performed in
  situ on the structures observed in the tissues of
  thin sections prepared by standard histological
  methods. The electron probe microanalyzer or
  electron probe X-ray scanning microscope can
  perform nondestructive elemental chemical
  analyses on localized regions with diameters as
  small as 1µm and volumes of a few cubic
  micrometers. The limit of delectability is about
  0.1, and many inorganic elements can be
  measured.

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• Another promising technique that has been
  adapted to microanalysis of inorganic elements is
  the laser microprobe. In this instrument, a laser
  beam is flashed through the optics of a regular
  microscope set to analyze a very small arc. The
  instrument is attached to a sensitive
  spectrograph.
• Finally, mention should be made of
  biological methods of trace analysis.
• Comparison of Methods
• Bowen described the results of elemental
  analyses of a standard plant material analyzed
  for 40 elements by 29 laboratories. The
  techniques used were neutron activation analysis,
  atomic absorption spectroscopy, a catalytic
  technique, colorimetry, flame photometry,
  turbidimetry, and titrimetric analysis.
  Consistent results were obtained by more than one
  laboratory for Au, B, Br, Ca, Cl, Co, Cr, Fe,
  Ga, I, Mn, Mo, N, P, Rb, S, Sc, and W. Small
  differences in results were obtained by different
  techniques were found for Cu, K, Mg, Na, P, Se,
  Sr, and Zn. For example, flame photometry gave
  high results for sodium, activation analysis
  without chemical separation was unreliable for
  determining potassium and magnesium, and atomic
  absorption spectrometry gave high

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• result for copper and strontium. Gross
  discrepancies were found in the result reported
  for aluminum, arsenic, mercury, nickel, and
  titanium. The significance of databases and food
  composition compilations in trace element
  analyses was stressed by Southgate.
• According to Wolf and Hamly, inorganic
  trace elements of interest in human health can be
  divided into those that are of nutritional and
  toxic interest, those that are primarily of
  nutritional interest, those that are primarily of
  toxic interest. The two techniques considered by
  the authors as having the required sensitivity
  and greatest potential for accurate trace element
  analysis are atomic spectroscopy and neutron
  activation analysis.
• Hocquellet determined cadmium, lead,
  arsenic, and tin in vegetable and fish oils by
  atomic absorption with electrothermal atomization
  by an oven equipped with a graphite tube.
  Addition of dithiocarbamate for cadmium or of
  dithiocarbamate for lead and arsenic decreased
  volatility of the elements. Detection limits of
  0.5-3.0ng/g and satisfactory recoveries were
  obtained in the 20-ppb range when samples of oil
  diluted in chloroform or in methylisobutyl ketone
  were injected into the atomizer. This rapid
  (minutes

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• compared to hours for methods with
  nitrosulfuric digestion) direct determination
  method was recommended for rapid routine testing
  of large numbers of samples.
• Noller and Bloom described an integrated
  analytical scheme for the determination of major
  (sodium, potassium, calcium, and magnesium) and
  minor (zinc, copper, nickel, iron, chromium,
  cesium, lead, tin, and mercury) elements in
  foods. The methods involved flame atomic
  absorption and flame emission spectrometry for
  all elements expect mercury, for which flameless
  atomic absorption was recommended. In a
  collaborative study involving 13 Australian
  laboratories, cadmium, copper, iron, lead, tin,
  and zinc were determined in spiked and unspiked
  samples of apple puree. Atomic absorption was
  used in the flame mode to determine copper, iron,
  and zinc it was efficient and accurate and
  yielded low interlaboratory coefficients of
  variation and good recoveries. However, tin many
  be lost in ashing as volatile stannic chloride or
  as insoluble metastannic acid. Lead was
  determined by direct flame atomic absorption, by
  solvent extraction followed by flame atomic
  absorption, and by electrothermal atomization.
  The methods yielded comparable results.

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• The graphite furnace as an alternative to
  the combustion flame in atomic absorption
  spectrometry (AAS) because available commercially
  about 1970. Electrothermal atomization offers
  many advantages in terms of sensitivity and
  smaller sample requirements. At the same time,
  the comparatively long analysis times, the need
  to optimize conditions for each element, and the
  occurrence of matrix interferences impaired the
  application of graphite furnace AAS in routine
  analysis. Recent developments in instrument
  design and methodology have reduced the severity
  of those problems, and graphite furnace-AAS is
  the most widely used method for determination of
  trace elements. A combination of wet charring and
  dry ashing suitable for the determination of
  trace metals in oily foods by graphite
  furnace-AAS was described by Seong Lee et al.

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