trace elements analysis

7/30/2019 Trace Elements Analysis

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Trace elements analysis


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LOGOElemental Analysis

This chapter includes no detailed description of methods to

determine individual mineral components. Such procedures aredescribed in general textbooks of inorganic analysis, standardreference books on food analysis, and specialized textbooks on thedetermination of minerals in biological materials. The principles ofinstrumental methods used in the determination of mineralcomponents and trace elements also were descried in previous

chapters of this book. This chapter is primarily concerned with theapplications of those principles to food analysis.

Developments in the measurement of trace metal components infoods were described by LaFluer (1976), Winefordner (1976),Bratter and Schramel (1980), Das (1983), Schwedt (1984), andBenton-Jones (1984). Tshopel and Tolg (1982) reviewed the basicrules that have to be followed in trace analyses to obtain precise

and accurate results at the nanogram and pictogram levels. Theserules are as follows:

1. All materials used for apparatus and tools must be as pure andinert as possible. These requirements are only approximately metby quartz, platinum, glassy carbon, and, to a lesser degree,


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polypropylene.

2. Cleaning of the apparatus and vessels by steaming is veryimportant to lower blanks as well as element losses by adsorption.

3. To minimize systematic errors, microchemical techniqueswith small apparatus and vessels with an optimal ratio of surfaceto volume are recommended. All steps of the analytical procedure,such as composition, separation, preconcentration, and

determination, are best done in one vessel (single-vesselprinciple). If volatile elements or compounds have to bedetermined, the system should be closed off and the temperatureshould be as low as possible.

4. Reagents, carrier gases, and auxiliary materials should be aspure as possible. Reagents that can be purified by subboiling

point distillation are preferred.5. Contamination from laboratory air should be avoided by usingclean benched and clean rooms. By this, the blanks caused bydust can be decreased by at least two or three orders ofmagnitude.


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6. Low and constant reaction temperature should be used.

7. Manipulations and different working steps should be restrictedto a minimum in order to reduce unavoidable contamination.

8. All steps of the combined procedure should be monitored; thiscan best be done with radiotracers.

9. All procedures have to be verified by a second independent

one or, even much better, by an interlaboratory comparativeanalysis.

Element Enrichment

The determination of trace element often requires enrichment ofthe elements, and/or the separation of many elements at the trace

level from large amounts of major elements. Ion exchange hasproved to be a valuable tool in the concentration, isolation, andrecovery of ionic materials present in a solution in trace amounts.Ion-exchange chromatography on an ion-exchange resin can bealso used for fractionation, separation, and the elimination ofinterfering ions.


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method of measuring the desired emission by a galvanometer, null

meter, or chart recorder (see Chapter 10 for details). Theinstruments are used primarily to determine calcium, sodium, andpotassium.

Atomic Absorption Spectroscopy

Within the last two decades atomic absorption spectroscopy hasfound enthusiastic acceptance by science and industry. Hundreds of

papers are published annually on basic research, instrumentation,specific analytical methods, and practical applications of atomicabsorption spectroscopy.

Atomic absorption spectroscopy is not quite as free from inter-element effects as was originally expected, but it is far better in thisrespect than any from of emission spectrography. It is quite sensitive;

the limit of detection ranges from 0.01 ppm for magnesium to 5.00ppm for barium; and the method is rapid (about 1000 determinationscan be made per week). The equipment is relatively inexpensive(about20000), only one-tenth the coast of X-ray fluorescenceequipment. The limiting factor is the need for cathode lamps foreach 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 acylindrical flame is used. The flame is irradiated by resonanceradiation from a powerful spectral source, and the fluorescence thatis generated in the flame is measured at right angles to the incidentbeam of radiation. This is done to minimize the contamination of thefluorescence 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 astandard that contains a known weight of the element sought isexposed 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 theradioisotopes of the element sought and to remove all other inducedradioactivity. The quantity of the element in the sample is thencalculated 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 nuclearbombardment or if the energies of the emitted radiations differwidely, a direct measurement of trace elements is possible. Also, ifthe trace element has a substantially longer half-life than the otherinduce activities, the interfering materials may be allowed to decayand the radioassay completed when the interference is insignificant.Results obtained by neutron activation generally are within 5% of thetrue value, and replicate analyses under favorable conditions arewithin 2-3% of the mean.

The attractive features of neutron activation analyses are its wideapplicability, high sensitivity, and satisfactory accuracy and precision.There have been numerous applications of activation analysis inbotany and agriculture.

X-Ray SpectroscopyThere are three uses of X-rays in chemical analysis. Absorption

methods are of limited practical because the adjustment ofwavelength is most critical. X-ray diffraction is useful incrystallography and in establishing the complicated structure


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high concentrations of proteins and amino acids.

Miscellaneous MethodsTrace elements are determined in many laboratories by specific

colorimetric and turbidimetric methods, by fluorescence analysis,and by polarography. The use of infrared spectroscopy indetermining 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 byDuffield and Coulson.

Impressive advances have been made in developing instrumentsthat permit an essentially complete elemental analysis to beperformed in situ on the structures observed in the tissues of thinsections prepared by standard histological methods. The electron

probe microanalyzer or electron probe X-ray scanning microscopecan perform nondestructive elemental chemical analyses onlocalized regions with diameters as small as 1m and volumes of afew cubic micrometers. The limit of delectability is about 0.1%, andmany 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 thisinstrument, a laser beam is flashed through the optics of a regularmicroscope set to analyze a very small arc. The instrument isattached to a sensitive spectrograph.

Finally, mention should be made of biological methods of traceanalysis.

Comparison of MethodsBowen described the results of elemental analyses of a standard

plant material analyzed for 40 elements by 29 laboratories. Thetechniques used were neutron activation analysis, atomic absorptionspectroscopy, 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 inresults were obtained by different techniques were found for Cu, K,Mg, Na, P, Se, Sr, and Zn. For example, flame photometry gavehigh results for sodium, activation analysis without chemicalseparation was unreliable for determining potassium andmagnesium, 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, andtitanium. The significance of databases and food compositioncompilations in trace element analyses was stressed by Southgate.

According to Wolf and Hamly, inorganic trace elements of interestin human health can be divided into those that are of nutritional andtoxic interest, those that are primarily of nutritional interest, those

that are primarily of toxic interest. The two techniques considered bythe authors as having the required sensitivity and greatest potentialfor accurate trace element analysis are atomic spectroscopy andneutron activation analysis.

Hocquellet determined cadmium, lead, arsenic, and tin invegetable and fish oils by atomic absorption with electrothermalatomization by an oven equipped with a graphite tube. Addition ofdithiocarbamate for cadmium or of dithiocarbamate for lead andarsenic decreased volatility of the elements. Detection limits of 0.5-3.0ng/g and satisfactory recoveries were obtained in the 20-ppbrange when samples of oil diluted in chloroform or in methylisobutylketone 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 oflarge numbers of samples.

Noller and Bloom described an integrated analytical scheme forthe determination of major (sodium, potassium, calcium, andmagnesium) 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 allelements expect mercury, for which flameless atomic absorptionwas recommended. In a collaborative study involving 13 Australianlaboratories, cadmium, copper, iron, lead, tin, and zinc weredetermined in spiked and unspiked samples of apple puree. Atomicabsorption 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 asinsoluble metastannic acid. Lead was determined by direct flameatomic absorption, by solvent extraction followed by flame atomicabsorption, and by electrothermal atomization. The methods yieldedcomparable 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 inroutine 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 foodsby graphite furnace-AAS was described by Seong Lee et al.

trace elements analysis

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