SOP: Estimation of Minerals by Atomic Absorption Spectroscopy

GIFSQ — Global Institute for Food Safety & Quality

Principle

Atomic absorption spectroscopy and flame photometry are used for the detection and determination of metals in foods. When a test solution is aspirated into a flame, the metal atoms remain largely in the ground state; they can be made to absorb radiation of the correct wavelength from an external source (a hollow cathode lamp), reaching a higher energy level. The amount of absorbed energy is equivalent to the number of electrons available — in other words, to the concentration of the element in the sample — and forms the basis of atomic absorption spectroscopy: the measurement of the amount of energy of the correct wavelength needed to elevate the metal atoms from the ground state to the excited state, subject to calibration of the system.

Apparatus required

• Atomic absorption spectrophotometer with hollow cathode lamp of the specific metal to be estimated, giving the specified resonant wavelength
• Hollow cathode or electrode-less discharge lamp, chopper, atomiser, monochromator, detector and electronics

Reagents

a. 0.3 N, 3.0 N and 6.0 N HCl

b. 10% w/v lanthanum chloride solution

c. Standard solutions of metals: weigh analytical-grade chemicals (see table below) to give standard solutions containing 1000 mg/litre of the metal; dissolve the chemical in 25 ml of 3 N HCl and dilute to 250 ml with water; dilute the prepared standard solution with water (if the sample was prepared by wet digestion) or with 0.3 N HCl (if prepared by dry ashing) to prepare a series of standards within the working range given in the conditions table; add other salts as indicated in the conditions table

Weight of material for preparation of standard solutions (1000 mg/litre):

(Dry the potassium and sodium salts for 2 h at 105 °C.)

Recommended conditions for metal analyses using an air–acetylene flame:

(The maximum level indicates the limit of the linear working range.)

Working conditions of lead and copper (examples):

Sample preparation — ashing

Dry ashing: unless otherwise stated, destroy the organic matter by dry ashing in a muffle furnace at a temperature not exceeding 550 °C (see the procedure described in SOP: Destruction of Organic Matter); to the ash add 5–10 ml of 6 N HCl to wet it completely and heat to dryness at low temperature on a hot plate; add 15 ml of 3 N HCl and heat on a hot plate until the solution just boils; cool and filter through a hardened ashless filter paper (Whatman 541) previously washed with 3 N HCl; collect the filtrate in a volumetric flask; add 10 ml of 3 N HCl to the dish and heat until the solution just begins to boil; cool and filter through the same filter paper into the flask; wash the dish thrice with water and filter the washings through the filter paper; wash the filter paper and collect the washings in the flask. If calcium is to be determined, add 5 ml of lanthanum chloride solution per 100 ml of solution. Cool and make up to volume with water. Prepare a blank with the same amounts of reagents as for the sample.

Wet digestion: for dried products (less than 10% moisture), take 2 g of sample; if the sample contains more than 10% moisture, take 5 g or more into a 100-ml Kjeldahl flask; add 10 ml concentrated H₂SO₄ and shake vigorously, ensuring no dry lumps remain; add 5 ml of concentrated HNO₃ and mix; heat gently in a fume cupboard until the initial vigorous reaction subsides; then heat vigorously until most of the nitrous fume ceases to evolve; add HNO₃ dropwise and continue heating until all organic matter is destroyed and white fumes of H₂SO₄ evolve; cool the digest, add water, cool again, transfer to a volumetric flask and make up to volume with water. Carry a blank through the same operations with the same amounts of reagents.

Thoroughly wash all glassware used in the determination of metals by AAS with dilute HNO₃; keep the glassware used for trace metals separately.

Procedure — measurement

Set the apparatus according to the manufacturer’s instructions

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Use water (if wet digestion was used) or 0.3 N HCl (if dry ashing was used) as the blank; note the readings of the standard solutions and draw a calibration graph of concentration (μg/ml of metal) against absorption or emission

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Measure the sample solutions and the reagent blank, ensuring periodically while running samples that the calibration values remain constant.

Calculation

Metal content, ppm (or mg/1000 g) = (a − b) × V ÷ W

Metal content, mg/100 g = (a − b) × V × 100 ÷ (W × 1000)

Where,

• a = concentration of the metal in the sample solution (μg/ml)
• b = concentration of the metal in the sample blank (μg/ml)
• V = volume made up of the ash solution or digest of the sample (ml)
• W = weight of the sample (g)

Note: the AOAC recommended procedure for zinc is this general atomic absorption procedure.

References

1. Official Methods of Analysis, Association of Official Analytical Chemists, 13th edn. (1980).