Aplicações

Determination of Calcium in Milk Powder – Flame Atomic Absorption Spectrometry

1. Method Overview

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

2. 2. Instruments and Reagents

2.1 Instruments and Equipment

2.1.1 Testing Instruments

Flame Atomic Absorption Spectrophotometer(include Calcium Hollow Cathode Lamp)

Air Compressor(Rated Discharge Pressure: 0.3MPa)

Acetylene Gas(cylinder Purity≥ 99.99%)

2.1.2 Pretreatment Equipment

Electronic Balance(Sensitivity: 0.1mg)

Quartz Crucible(100mL)

Adjustable Electric Furnace(Rated Power: 2000W)

Muffle Furnace(Rated Temperature: 1000℃)

Adjustable Electric Hot Plate(Rated Temperature: Room Temperature ~ 300℃)

Micropipette(100μL ~ 1000μL, 1000μL ~ 5000μL)

Volumetric Flask(100mL, 1000mL)

Colorimetric Tube(50mL)

2.2 Reagents

2.2.1 Reagents

Nitric Acid(MOS Grade)

Hydrochloric Acid(Ultra-Pure Grade)

Lanthanum Oxide(Ultra-Pure Grade)

2.2.2 Prepared Reagents

Nitric Acid Solution (5+95) Measure 50mL nitric acid and mix uniformly with 950mL water. Nitric Acid Solution (1+1) Measure 50mL nitric acid and mix uniformly with 50mL water. Hydrochloric Acid Solution (1+1) Measure 50mL hydrochloric acid and mix uniformly with 50mL water.

3. 3. Operational Procedures

3.1 Sample Preparation

3.1.1 Preparation of Test Solution

1. Amostra pré -tratamento

Accurately weigh 2g of the sample (accurate to 0.0001g) into a 100mL quartz crucible. Place it on an adjustable electric furnace and heat gently to char until no smoke is produced. Transfer to a muffle furnace and ash at 550℃ for 3–4 hours. Cool and take out. If the sample is not completely ashed, add a few drops of nitric acid, heat on an adjustable electric hot plate at 150℃ to evaporate carefully, then transfer back to the muffle furnace at 550℃ to continue ashing for 1–2 hours until the sample is white ash. Cool, take out, add 2mL of nitric acid solution (1+1), heat on an adjustable electric hot plate at 150℃ to dissolve the sample. Cool, dilute to 50mL in a colorimetric tube with water. Perform a reagent blank test simultaneously.

2. Preparation of Test Sample Solution

Transfer 1mL of the digested test solution to a 100mL volumetric flask, add 5mL of lanthanum solution (20g/L), dilute to the mark with nitric acid solution (5+95), and mix well. Perform a reagent blank test simultaneously. The blank test solution shall contain the same types and amounts of reagents as the test solution except for the sample.

3.1.2 Preparation of Standard Solutions

Preparation of Calcium Standard Series

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

3.2 Sample Testing

1. Testing Conditions

Reference testing conditions for the flame atomic absorption spectrophotometer:

Wavelength 422.7nm

Spectral Bandwidth 0.4nm

Element Lamp Current 3.0mA

Burner Height 10mm

Gas Flow Rate 1600mL/min

Air Compressor Pressure 0.22MPa

2. Teste de amostra

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

Under the same experimental conditions as those for the standard solutions, introduce the blank solution and the test sample solution into the atomizer, measure their respective absorbance values, and quantify by comparison with the standard series.

3.3 Result Calculation

The calcium content in the sample is calculated using the following formula:

Onde:

X: Calcium content in the sample (mg/kg);

ρ: Mass concentration of calcium in the test sample solution (μg/mL);

ρ0: Mass concentration of calcium in the blank solution (μg/mL);

V: Constant volume of the sample digestion solution (mL);

f: Dilution factor of the sample digestion solution;

m: Sample mass (g).