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Determination of Calcium in Milk Powder (Flame Atomic Absorption Spectrometry)

10월 10, 2025

1. Method Overview

After the sample is digested, a lanthanum solution is added as a releasing agent. The sample is then atomized by a flame in an atomic absorption spectrometer, and the absorbance is measured at a wavelength of 422.7 nm. Within a certain concentration range, the absorbance is proportional to the calcium content. Quantitative determination is performed by comparison with a standard series.

2. Instruments and Reagents

2.1 Instruments and Equipment

2.1.1 Testing Instruments

Serial No.	이름	수량	Technical Requirements	부속품
1	Flame Atomic Absorption Spectrophotometer	1 set	–	Calcium Hollow Cathode Lamp
2	Air Compressor	1 set	Rated discharge pressure: 0.3 MPa	–
3	Acetylene Gas	1 cylinder	Purity ≥ 99.99%	–

2.1.2 Sample Pretreatment Equipment

Serial No.	이름	수량	Technical Requirements	부속품
1	Electronic Balance	1 set	Sensitivity: 0.1 mg	–
2	Quartz Crucible	Several	Volume: 100 mL	–
3	Adjustable Electric Furnace	1 set	Rated power: 2000 W	–
4	Muffle Furnace	1 set	Rated temperature: 1000 ℃	–
5	Adjustable Hot Plate	1 set	Rated temperature range: Room temperature ~ 300 ℃	–
6	Micropipette	1 each	Ranges: 100 μL ~ 1000 μL, 1000 μL ~ 5000 μL	–
7	Volumetric Flask	Several	Volumes: 100 mL, 1000 mL	–
8	Colorimetric Tube	Several	Volume: 50 mL	–

2.2 Reagents

2.2.1 Raw Reagents

Serial No.	이름	Technical Requirements	발언
1	Nitric Acid	MOS Grade	–
2	Hydrochloric Acid	Guaranteed Reagent (GR)	–
3	Lanthanum Oxide	Guaranteed Reagent (GR)	–

2.2.2 Prepared Reagents

Serial No.	이름	Preparation Method	발언
1	Nitric Acid Solution (5+95)	Measure 50 mL of nitric acid and mix it with 950 mL of water uniformly.	–
2	Nitric Acid Solution (1+1)	Measure 50 mL of nitric acid and mix it with 50 mL of water uniformly.	–
3	Hydrochloric Acid Solution (1+1)	Measure 50 mL of hydrochloric acid and mix it with 50 mL of water uniformly.	–

2.3 Reference Standards

2.3.1 Stock Solution

Serial No.	No.	이름	Technical Requirements	발언
1	GBW(E)080261	Calcium Single-Element Standard Substance Solution	Concentration: 1000 μg/mL	Provided by National Institute of Metrology, China

3. Operational Procedures

3.1 Sample Preparation

3.1.1 Preparation of Test Solution

샘플 전처리

Accurately weigh 2 g of the sample (accurate to 0.0001 g) into a 100 mL quartz crucible. Place the crucible on an adjustable electric furnace, heat it gently to carbonize until no smoke is produced. Remove the crucible and transfer it to a muffle furnace, then ash at 550 ℃ for 3~4 hours. Take out the crucible after cooling. If the sample is not completely ashed, add a few drops of nitric acid, place it on an adjustable hot plate, heat at 150 ℃ to carefully evaporate to dryness, and then transfer it back to the muffle furnace at 550 ℃ for further ashing for 1~2 hours until the sample turns into a white ash. Cool and take out the crucible, add 2 mL of nitric acid solution (1+1), place it on the adjustable hot plate, heat at 150 ℃ to dissolve the sample. After cooling, dilute the solution to the marked volume in a 50 mL colorimetric tube with water. Conduct a reagent blank test simultaneously.
Preparation of Sample Test Solution

Pipette 1 mL of the digested solution into a 100 mL volumetric flask, add 5 mL of lanthanum solution (20 g/L), dilute to the marked volume with nitric acid solution (5+95), and mix well. Prepare a reagent blank in the same way. Except for not adding the sample, the type and amount of reagents added to the blank test solution are the same as those of the test solution.

3.1.2 Preparation of Standard Solutions

Preparation of Calcium Standard Series

Pipette 0 mL, 0.10 mL, 0.20 mL, 0.30 mL, and 0.40 mL of the calcium standard solution (1000 μg/mL) into separate 100 mL volumetric flasks respectively. Add 5 mL of lanthanum solution (20 g/L) to each flask, dilute to the marked volume with nitric acid solution (5+95), and mix well. The concentrations of this calcium standard series are 0 μg/mL, 1.00 μg/mL, 2.00 μg/mL, 3.00 μg/mL, and 4.00 μg/mL.

3.2 Sample Testing

1) Testing Conditions

Reference Conditions for Flame Atomic Absorption Spectrophotometer

Parameter	Specification
Wavelength	422.7 nm
Spectral Bandwidth	0.4 nm
Element Lamp Current	3.0 mA
Burner Height	10 mm
Fuel Flow Rate	1600 mL/min
Air Compressor Pressure	0.22 MPa

2) Sample Testing

Introduce the calcium standard series solutions into the flame atomizer in ascending order of concentration, and measure their absorbance. Plot a standard curve with the mass concentration of calcium in the standard series as the abscissa and the corresponding absorbance as the ordinate.

Under the same experimental conditions as those for determining the standard solutions, introduce the blank solution and the sample test solution into the atomizer respectively, measure their corresponding absorbance, and perform quantitative determination by comparison with the standard series.