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:

1. dairy (grated cheese)
   
   
2. meats (beef, pork, liver, poultry, and fish)
   
   
3. cereals (white flour, whole wheat flour, oatmeal, cornmeal and rice)
   
   
4. 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:

1. Drying oven
   
   
2. Muffle furnace
   
   
3. Hot plate
   
   
4. Silica crucibles or quartz beakers (45-80 mL)
   
   
5. Watch glass of appropriate diameter

Reagents:

1. Deionized water (resistivity of 1 megohm or better), referred to as H₂O.
   
   
2. Reagent grade concentrated nitric acid (HNO₃), 70%.
   
   
3. Reagent grade concentrated hydrochloric acid (HCl), 37%.
   
   
4. Diluted HCl (approximately 9.25% HCl prepared from reagent grade HCl by diluting concentrated HCl (37%) 25 mL to 100 mL with H₂O.

Procedure:

1. With each set of samples, prepare a blank in the same manner.
   
   
2. 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.
   
   
3. Dry the sample in an oven for about 16 hours (overnight) at 105^o. Note: crucible plus dry sample weight.
   
   
4. Place the samples in the furnace set to 200^o. Keep the doors slightly ajar to allow the smoke to escape. Allow the samples to reach 200^o.
   
   
5. If there is little or no smoking, raise the temperature (50o increment).
   
   
6. Repeat step 5 until the temperature reaches 350^o, then close the furnace door, set the temperature to 450^o, and ash about 16 hours (overnight).
   
   
7. 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 375^o for about 1 hour. At higher temperatures potassium nitrate decomposes violently.
   
   
8. Repeat step 7, if necessary, until a white ash is obtained.
   
   
9. 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.
   
   
10. Determine elements by flame atomic absorption spectrophotometry. (See notes 1 and 2.)

Note 1. Ca, Mg

Prepare 5% (w/v) sol'n of Lanthanum

1. weigh 58.65 g of La2O3 in a 1 liter beaker.
   
   
2. wet the powder with deionized water.
   
   
3. add slowly and in small amounts 250 mL concentrated HCl.
   
   
4. 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

1. weigh 25.335 g CsCl into a 1 liter volumetric flask.
   
   
2. add 86 mL of concentrated HCl.
   
   
3. 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