Sample Preparation by Dry Ashing for the Determination of Various Elements
by Flame Atomic Absorption Spectroscopy
Definition:
This method describes a procedure for ashing a variety of foods for
subsequent determination of mineral nutrients by flame atomic absorption
spectroscopy.
Scope:
The method is applicable to food samples for the determination of various
mineral nutrients (Ca, Mg, Ne, K, Fe, Zn, Cu, and Mn). The sample
preparation method and subsequent analysis were evaluated by three
laboratories. The samples consisted of four composite food groups:
- dairy (grated cheese)
- meats (beef, pork, liver, poultry, and fish)
- cereals (white flour, whole wheat flour, oatmeal, cornmeal and rice)
- vegetables (potatoes, cabbage, peas, carrots, tomatoes) and a standard
reference material, NBS-1577 Bovine Liver Powder
Principle:
The method consists of preparing an ash by using heat and nitric acid
to decompose the organic matter, and dissolving the inorganic residue in
an appropriate volume of dilute hydrochloric acid.
Apparatus:
- Drying oven
- Muffle furnace
- Hot plate
- Silica crucibles or quartz beakers (45-80 mL)
- Watch glass of appropriate diameter
Reagents:
- Deionized water (resistivity of 1 megohm or better), referred to as
H2O.
- Reagent grade concentrated nitric acid (HNO3), 70%.
- Reagent grade concentrated hydrochloric acid (HCl), 37%.
- Diluted HCl (approximately 9.25% HCl prepared from reagent grade
HCl by diluting concentrated HCl (37%) 25 mL to 100 mL with H2O.
Procedure:
- With each set of samples, prepare a blank in the same manner.
- Weigh the amount of sample required (about 5 g of dry matter) into
a crucible. Note: crucible weight, and crucible plus
as is sample weight.
- Dry the sample in an oven for about 16 hours (overnight) at 105o.
Note: crucible plus dry sample weight.
- Place the samples in the furnace set to 200o. Keep the
doors slightly ajar to allow the smoke to escape. Allow the samples
to reach 200o.
- If there is little or no smoking, raise the temperature (50o increment).
- Repeat step 5 until the temperature reaches 350o, then
close the furnace door, set the temperature to 450o, and
ash about 16 hours (overnight).
- Remove the samples from the furnace and allow them to cool. Wet the
ash with H2O and add sufficient HNO3 to cover
the ash (typically 2-4 mL). Cover with a watch glass and reflux on a
hot plate for about 1 hour, then remove the watch glass and if necessary
reduce the heat to gently evaporate the acid. Failure
to remove all the acid can cause ignition of the organic residue.
Return the samples to the furnace and ash at 375o for about
1 hour. At higher temperatures potassium nitrate decomposes violently.
- Repeat step 7, if necessary, until a white ash is obtained.
- Add 2-5 mL diluted HCl and dissolve the ash by gently boiling the
solution, cool and make up to the appropriate volume with H2O.
- Determine elements by flame atomic absorption spectrophotometry.
(See notes 1 and 2.)
Note 1. Ca, Mg
Prepare 5% (w/v) sol'n of Lanthanum
- weigh 58.65 g of La2O3 in a 1 liter beaker.
- wet the powder with deionized water.
- add slowly and in small amounts 250 mL concentrated HCl.
- quantitatively transfer and make to 1 liter in a volumetric flask
with deionized water, then mix.
Add Lanthanum chloride solution in appropriate volume to the final
dilution so that the aspirate contains 0.5% Lanthanum. Standards must
also contain 0.5% Lanthanum, and provide a reagent blank for the
standards.
Note 2. Na, K
Prepare 2% (w/v) Cesium solution
- weigh 25.335 g CsCl into a 1 liter volumetric flask.
- add 86 mL of concentrated HCl.
- make to 1 liter using deionized water, then mix. Add appropriate
volume in the final dilution so that the aspirate contains 0.2% Cesium.
Prepare standards to contain 0.2% Cesium, and provide a reagent blank
for the standards.
Method Evaluation
This common method of dry ashing of different categories of foods for
the determination of Na, K, Ce, Mg, Fe, Zn, Cu and Mn (as opposed to different
ashing conditions for individual foods and mineral nutrients recommended
by the A.O.A.C.) was evaluated by analysing check samples including NBS
Bovine Liver Powder No. 1577, and samples representing the food groups:
dairy, meat, cereal and vegetables. The analyses of the check samples
and the NBS Bovine Liver Powder were performed at three laboratories.
Results of the analysis of the check samples are given in Table 1. They
indicated lack of agreement in some of the results for the dairy sample
and for sodium in the cereal sample. This was attributed to lack of homogeneity
of the dairy sample or the lower level of sodium in the cereal sample.
The results of the analysis of the NBS standard No. 1577, given in Table
2 were satisfactory except for the slightly low values for calcium. Further
work undertaken by 2 of the laboratories gave acceptable results.
The results for four infant formula powders given in Table 3 are based
on
analysis by two laboratories and, except for iron, all the results were
satisfactory. It was suspected that the samples were nonhomogeneous with
respect to iron. So iron determinations in some other foods, including
one infant formula powder, were performed again in both participating laboratories.
The results given in Table 4 were satisfactory and are based on the analysis
from the same two laboratories. Moreover, the slightly larger relative
standard deviation for the infant formula powder confirmed the nonhomogeneity
of the preparation.
The overall accuracy of this method, as judged by the results of the
NBS standard, is ±10% and the precision is ±10% relative standard
deviation.
Table 1 - Mineral Check Samples (dry weight basis) Analysed
by Three Laboratories
View
Mineral
Check Samples (dry weight basis) Analysed by Three Laboratories Table
Table 2 - NBS Bovine Liver (No. 1577) (dry wt basis) Analysed
by Three Laboratories
View
NBS
Bovine Liver (No. 1577) (dry wt basis) Analysed by Three Laboratories
Table
Table 3 - Infant Formula Powders, Analysed by Two Laboratories
View
Infant
Formula Powders, Analysed by Two Laboratories Table
Table 4 - Cereal and Grain Products Analysed by Two Laboratories
View
Cereal
and Grain Products Analysed by Two Laboratories Table
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