PART III: LABORATORY METHODS OF ANALYSIS
A. Extraction and analysis of sugars
3.3.2 MINERALS
68 Reference
Association of Official Analytical Chemists (1984). Official Methods of Analysis. 14th Edition (Williams, S. ed.), AOAC, Virginia.
69 METHOD B: WET DIGESTION
Principle
The method consists of decomposition of an organic material into an ash by treatment with nitric acid.
Chemicals/Reagents
1. Nitric acid (HNO3), 65% Suprapur (Merck) 2. Hydrogen Peroxide (H2O2)
Apparatus/Instruments
1. Balance: Analytical sensitivity ±0.1 mg 2. Hotplate
3. Conical flask: 250 mL 4. Volumetric flask: 100 mL 5. Measuring cylinders: 10 mL 6. Pipette
7. Pasteur pipette 8. Watch glass
Procedures
1. Weigh accurately 2-3 g of food samples.
2. Add 10 ml of HNO3, cover with watch glass and soak sample overnight.
3. Heat the sample on a hotplate.
4. Add few ml of HNO3 if there is still brown fume and the level of solution is reduced.
Cool the sample first before adding the HNO3.
5. Heat until no brown fume given off.
6. Stop the heating when white fume is released.
7. Heat gently the sample again.
8. Allow the sample to cool down approximately 10-15 minutes.
9. Mix 2 ml of H2O2 and deionized water (H2O2:H2O) in a beaker. Add (H2O2:H2O) mixture dropwise into the sample to prevent reaction from becoming too vigorous. If upon addition brown fumes are given off, go back to step 4. If the H2O2:H2O mixture has been used up, use concentrated H2O2 for the dropwise addition, but do not exceed 10 mL. Add till there are no further colour changes of the sample.
10. Cool the sample and filtrate.
11. Make up to final solution of 100ml with 2% HNO3. Reference
Sim SF, Devagi K, Sung CL (2006). Evaluation of the acid digestion method with different solvent combination for the determination of iron, zinc and led in canned sardines.
Malaysian Journal of Chemistry.8 (1):010-015.
70 METHOD C: MICROWAVE DIGESTION
Principles
This digestion method is facilitated by the application of microwave power and elements volatilization is avoided by using closed digestion system. Microwave digestion method possess advantages such as less time consumption, easy handling, reduce consumption of high purity reagents and minimize contamination compared to conventional digestion method.
Chemicals/Reagents
1. Concentrated nitric acid (HNO3), 65%
2. Hydrogen peroxide (H2O2), 30%
3. Ultra-pure water Apparatus/Instruments
1. Microwave oven: Multiwave 3000, Anton Paar.
2. 8-position digestion rotor: 8SXQ80, Anton Paar.
3. Quartz digestion vessel: XQ80, Anton Paar.1 4. Grinder
5. Balance: Analytical sensitivity ±0.1 mg.
6. Pipette: 1 &10 mL
7. Volumetric flask: 100 mL 8. PTFE bottle
Procedures
1. Dry sample with oven at 105°C until a constant weight obtained.
2. Grind sample into powder form with conventional grinder.
3. Weigh accurately 0.1-0.5 g of food samples into digestion vessel.
4. Add 6 ml of HNO3 and 1 ml of H2O2 into the vessel.2
5. Make sure sample in the vessel completely wetted by the reagent and seal every vessel using vessel seal.
6. Allow pre-digestion of sample for 30 minutes.
7. Prepare a blank by adding same amount of HNO3 and H2O2 into a blank vessel.
8. After 30 minutes, place vessel with protective jacket and cap into the digestion rotor, make sure the venting screw of the vessel screw cap is tighten.
9. Start digestion in microwave oven by using the setting below:3
71 Power
(Watts)
Ramp (min) Hold (min) Fan
600 5:00 5:00 1
1200 5:00 25:00 1
0 15:00 3
10. After digestion is completed, remove rotor from microwave oven and cool them in fume hood.
11. Vent the vessel by rotating the release venting screw one half revolution, until the vessel is completely depressurized.
12. Remove the vessel seal cap and rinse it with deionized water.
13. Transfer the digested sample quantitatively into a volumetric flask and make up to volume of 100 ml.
14. Store the diluted sample in PTFE bottle and keep it in a cool place prior to analysis.
1Rotor of 8XF100, with PTFE-TFM vessel also applicable for food analysis.
2Different sample might require different ratio of HNO3 and H2O2. Some samples might need the aid of other reagent like hydrochloric acid for better digestion result.
3The digestion program use in microwave oven is sample dependent.
References
Michael Zischka. Microwave assisted digestion for food analysis. Anton Paar Application Note. B83lA18A.
Miller, R. O. 1998. Microwave digestion of plant tissue in a closed vessel. In Kalra Y. P.
(ed.). Methods for plant analysis, pp 69-73. Boca Raton: CRC Press.
72 3.3.2.2 DETERMINATION OF MAGNESIUM, COPPER, ZINC, CALCIUM,
IRON, SODIUM AND POTASSIUM IN FOOD
METHOD A: ATOMIC ABSORPTION METHOD Principles
The atomic absorption spectrophotometric method can be used satisfactorily for the determination of several minerals. Sample preparation is similar to that used for preparing samples for determination by the colorimetric methods. The organic matter is first destroyed either by dry ashing, wet ashing or microwave digestion method. The ashing sample is diluted to volume and read directly on the atomic absorption spectrophotometer. The quantity of food material taken for analysis depends upon the amount available and expected mineral content. In general, representative sample of 5-10 g are weighed.
Chemicals/Reagents
1. Deionized water. Use for preparing standards and dilutions.
2. Magnesium standard solutions.
a. Stock solution - 1000 ppm (magnesium nitrate standard solution for atomic absorption spectrophotometry)
b. Working standard – 0.1, 0.2 and 0.3 ppm.
3. Copper standard solutions.
a. Stock solution - 1000 ppm (copper (II) nitrate standard solution for atomic absorption spectrophotometry)
b. Working standard – 0.5, 1.0 and 1.5 ppm.
4. Zinc standard solutions.
a. Stock solution - 1000 ppm (zinc nitrate standard solution for atomic absorption spectrophotometry)
b. Working standard - 0.25, 0.5, 0.75, 1.0 ppm.
5. Sodium standard solutions.
a. Stock solution - 1000 ppm (sodium nitrate standard solution for atomic absorption spectrophotometry)
b. Working standard - 0.1, 0.2, 0.3, 0.4 and 0.5 ppm.
6. Potassium standard solutions.
a. Stock solution - 1000 ppm (potassium nitrate standard solution for atomic absorption spectrophotometry)
b. Working standard – 0.3, 0.6 and 0.9 ppm.
7. Calcium standard solutions.
a. Stock solution - 1000 ppm (calcium nitrate standard solution for atomic absorption spectrophotometry)
b. Working standard - 1,2,3, 4, 5 and 6 ppm.
73 8. Iron standard solutions
a. Stock solution - 1000 ppm (ferric nitrate standard solution for atomic absorption spectrophotometry)
b. Working standard – 1, 2 and 3 ppm.
Apparatus/Instruments
1. Perkin Elmer atomic absorption spectrophotometer: model AAnalyst 400 2. Micropipette
3. Volumetric flasks: 100 mL Procedures