Trace elements analysis
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Elemental Analysis
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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 interelement 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 0.53.0ng/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.