Determination of Cadmium in Fish Meal by AA-1800C Atomic Absorption Spectrometer - Master's thesis - Dissertation

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AA-1800C

Determination of Cadmium in Fish Meal by Atomic Absorption Spectrometry

Key words:

AA-1800C

Atomic absorption spectrometer; cadmium in fish meal;

1

Introduction to Cadmium

1.1

Chemical Properties of Cadmium

Cadmium (Cd) is a silvery-white metal with a slight blue tint, melting at 320.9°C and boiling at 767°C. It slowly oxidizes in humid air to form cadmium oxide (CdO). In nature, cadmium mainly exists in the +2 valence state. Common forms include water-soluble cadmium, adsorptive cadmium, and poorly soluble cadmium. Among common cadmium compounds, only cadmium sulfate (CdSO4), cadmium nitrate [Cd(NO3)2], and cadmium chloride (CdCl2) are soluble, while cadmium oxide is the most toxic.

1.2

Hazards of Cadmium

1.2.1

Cadmium accumulates in the renal cortex proximal convoluted tubules, binding to metallothionein and depleting it. This leads to mitochondrial swelling and degeneration in epithelial cells, causing reabsorption dysfunction, kidney damage, and increased urinary calcium and phosphorus. Long-term accumulation may lead to osteoporosis. Kidney function impairment also affects VD activity, disrupting calcium and phosphorus metabolism in bones.

1.2.2

Cadmium strongly interferes with the absorption of divalent metals like iron, copper, and zinc, leading to deficiencies. High cadmium intake increases iron in urine and can inhibit hemoglobin synthesis, causing anemia. Due to its higher affinity than sulfate, cadmium replaces zinc in enzymes, rendering them inactive.

1.2.3

Cadmium causes hemorrhage and necrosis in testicular epithelial and interstitial cells, shrinking testicles in males. This may be due to vascular damage and reduced blood supply. Long-term exposure reduces growth rates or even halts growth.

1.2.4

Another hazard is cadmium residual in animal products. The cadmium content in tissues correlates with feed levels. Feeding 2mg/kg cadmium for a month showed no residues, but at 48mg/kg, residues were detected. According to GB13078-2001, feed cadmium must not exceed 0.5mg/kg.

2

Method Principle

Sample is ashed dry. Under acidic conditions and with potassium iodide, cadmium ions form complexes with iodide, extracted by methyl isobutyl ketone. The organic phase is sprayed into an air-acetylene flame, where cadmium atoms emit resonance line at 228.8nm. Absorbance is compared to a standard series to determine cadmium content.

3

Reagents and Solutions

Unless otherwise specified, reagents are analytical grade, and water is distilled per GB/T 6682-1992.

3.1 Nitric acid (extra grade)

3.2 Hydrochloric acid (extra grade)

3.3 2mol/L Potassium Iodide Solution: Dissolve 332g KI in 1L distilled water.

3.4 5% Ascorbic Acid Solution: Dissolve 5g ascorbic acid in 100mL distilled water.

3.5 1mol/L Hydrochloric Acid: Mix 10mL HCl (3.2) with 110mL water.

3.6 Methyl Isobutyl Ketone

3.7 Cadmium Standard Stock Solution: 100μg/mL, stored at 4°C away from light.

3.8 Cadmium Working Solution: Dilute 1mL stock solution (3.7) in 100mL 1mol/L HCl (3.5), resulting in 1.0μg/mL.

4

Equipment

4.1 Electronic Balance: 0.0001g precision

4.2 Muffle Furnace

4.3 Adjustable Temperature Electric Furnace: 1000W

4.4 AA-1800C Atomic Absorption Spectrophotometer with Cadmium Hollow Cathode Lamp

4.5 Porcelain Crucible: 50mL

4.6 Volumetric Flasks: 50, 100mL

4.7 Pipettes: 1, 2, 5, 10, 15, 20mL

4.8 Colorimetric Tube: 25mL

5

Measurement Steps

5.1 Sample Preparation

Weigh 5g feed sample (0.0001g precision), place in a porcelain crucible (4.5), carbonize in an electric furnace (4.3) starting low and gradually increasing temperature until no smoke. Place in muffle furnace at 200°C for 1h, then 300°C for 1h, finally 500°C for 16h. Cool, add distilled water, 5mL nitric acid (3.1), heat on electric furnace until near dry. After cooling, add 10mL 1mol/L HCl (3.5), boil, cool, dilute to 50mL in volumetric flask, shake and filter. Prepare a reagent blank.

5.2 Standard Curve

Accurately pipette 0, 1.25, 2.50, 5.00, 7.50, 10.00mL of working solution (3.8) into 25mL colorimetric tubes, dilute to 15mL with 1mol/L HCl (3.5), add 2mL 2mol/L KI (3.3), shake, let stand 15min. Add 1mL 5% ascorbic acid (3.4), shake, let stand 5min. Add 5mL methyl isobutyl ketone (3.6), shake for 5min, let separate. Measure absorbance at 228.8nm using AA-1800C. Plot standard curve with concentration vs. absorbance.

5.3 Determination

Pipette 10mL sample solution (5.1) and same volume reagent blank into 25mL colorimetric tube. Add 2mL 2mol/L KI (3.3), shake, let stand 15min. Add 1mL 5% ascorbic acid (3.4), shake, let stand 5min. Add 5mL methyl isobutyl ketone (3.6), shake for 5min, let separate. Measure cadmium concentration using the standard curve.

6

Measurement Results and Analysis

6.1 Measurement Results

Figure 1: Standard Curve for Cadmium Concentration via Atomic Absorption Spectrophotometer

Table 1: Standard Curve Line

Table 2: Recovery Rate

Table 3: Feed Sample Test Data

In the formula:

X - the amount of cadmium in the sample, Mg/kg;

A1 - the quality of cadmium in the sample to be tested, Gg;

A0 - the quality of cadmium in the reagent blank solution, Gg;

m - sample quality, g;

V1 - the total volume of the sample processing solution, mL;

V2 - taking the sample processing solution volume, mL.

6.2 Data Analysis

According to GB 13078-2001, allowable cadmium in feed is ≤0.5mg/kg, in fishmeal ≤2.0mg/kg, and concentrated feed has no limit. Therefore, compound feed should comply with ≤0.5mg/kg. Table 3 shows that samples A are compound feed, B are concentrated feed, C are fishmeal, and all meet standards. The standard curve r value is 0.9994, indicating good instrument performance. Average recovery is 98.26%, with higher recovery when adding more cadmium, peaking at 7.5μg.

7

Precautions

7.1 Compound feed, concentrated feed, and fishmeal should be fully ashed to ensure data stability and reduce deviation between parallel samples. If black particles remain after ashing, cool, add two drops of water, and continue ashing at 550°C for 4h. The sample should be grayish white without visible black particles.

7.2 When boiling samples on an electric furnace, ensure yellow smoke from the crucible is fully exhausted. Otherwise, uncertainty risk and relative deviation increase. After adding nitric acid, boil to near dryness, but avoid boiling. Cool, then add 1mol/L HCl. Low concentration HCl decreases recovery and increases absorbance variability.

7.3 When using atomic absorption spectrophotometer, increasing slit width enhances absorbance but increases drift. Reducing slit width lowers drift but reduces standard curve linearity. Setting slit width at 8nm provides optimal balance.

7.4 After sample treatment, extract with methyl isobutyl ketone. Ensure sufficient extraction time. Avoid introducing aqueous solution from the colorimetric tube into the atomizer, as this may affect absorbance measurement.

Key words:

AA-1800C

Atomic absorption spectrometer; cadmium in fish meal;