Determination of Heavy Metals Content in Goat Milk

Detection Target

Determination of Heavy Metals Content in Goat Milk

Overview

Atomic absorption spectrometry (AAS) offers the advantages of sensitivity, efficiency, and accuracy in determining heavy metals in goat milk. Depending on the type and concentration of the metal being measured, the testing requires the use of various AAS techniques, including graphite furnace, flame, and hydride generation, combined with appropriate pretreatment methods. Digestion equipment, modifiers, digestion reagents, and digestion temperature all directly impact the accuracy of the results. The application, refinement, and innovation of AAS will undoubtedly drive the regulation of food safety and the improvement of relevant testing standards. Atomic fluorescence spectrometry (AFS) offers the advantages of sensitivity, efficiency, and accuracy in determining heavy metals such as As, Hg, Pb, Sb, and Se in goat milk. An AAS coupled with a graphite furnace and an AFS can meet the requirements for heavy metal detection in goat milk.

Principle

1. Atomic Absorption Spectrophotometer (AAS)

The electromagnetic radiation of the characteristic wavelength emitted by the light source passes through the sample vapor generated by the atomization system (such as a flame or electrothermal device). This radiation is absorbed by the ground-state atoms of the target element in the vapor. Under specific test conditions, the absorbance value follows the Beer-Lambert law in relation to the concentration of the target element in the sample. The digested sample solution is directly aspirated into the flame, where the atoms formed in the flame absorb the characteristic electromagnetic radiation. By comparing the measured absorbance of the sample with that of standard solutions, the concentration of the target element in the sample is determined.

2. Atomic Fluorescence Spectrophotometer (AFS)

The pretreated sample solution is introduced into the atomic fluorescence spectrophotometer (AFS). Under acidic conditions with potassium borohydride (KBH₄) reduction, arsine (AsH₃), bismuthine (BiH₃), stibine (SbH₃), hydrogen selenide (H₂Se) gases, and mercury atoms (Hg⁰) are generated. These hydrides form ground-state atoms in the argon-hydrogen flame. Both the ground-state atoms and mercury atoms are excited by the radiation from the element-specific lamp, producing atomic fluorescence. The intensity of this atomic fluorescence is directly proportional to the concentration of the target elements in the sample solution within a certain range.

Requirements for Water

When using atomic absorption spectrometry for macro analysis, the water used must meet the specifications for Grade II water; for trace analysis, Grade I water should be used. Inorganic acids are commonly used reagents, but they often contain trace amounts of metal elements and must be rigorously inspected before use.

Requirements for solutions

1. A suitable solvent should be selected, and the prepared solution should not have any insoluble precipitates. The solution should be stored in a clean and appropriate container.

2. Reagents used for preparing standard solutions should be of high purity, with a composition that accurately matches the chemical formula, and should be stable in nature.

3. The mass concentration of standard stock solutions is generally 1 mg/ml. For some elemental standard solutions, a small amount of inorganic acid should be added to ensure stability for several days or longer. The storage duration varies depending on the element. These solutions should be stored in polytetrafluoroethylene (PTFE) or polyethylene containers.

Instruments and Reagents

1. Instrument: HKL-999.10 AAS for Determination of heavy metal element in goat milk (equipped with hollow cathode lamps for Cu, Cd, Pb, Zn, Ni, Fe, Mn, etc., as required) The instrument integrates flame, graphite furnace, and hydride generation systems, with configurable accessories to provide flexible solutions meeting diverse customer requirements. The HKL-999.10 AAS for Determination of heavy metal element in goat milk enables sophisticated sample analysis through automatic switching between multiple analytical methods, achieving complete unattended operation.

2. Reagent: Hydrochloric acid (GR grade), Nitric acid (GR grade), Standard solutions for each element, Acetylene gas (99.0%).

1. Instrument: HKL-AFS for Determination of heavy metal element in goat milk (equipped with hollow cathode lamps for As, Hg, Pb, Sb, Se, etc., as required)

2. Reagent: Nitric acid (GR grade), Hydrochloric acid (GR grade), Potassium borohydride (GR grade), Sodium hydroxide (GR grade), Thiourea (AR grade), Potassium dichromate (AR grade), Hydroxylamine hydrochloride (AR grade), Potassium permanganate (AR grade), Standard solutions for each element, High-purity argon gas (99.99%).

Pretreatment

1. Sample

Weigh 1.0 g to 4.0 g (accurate to 0.001 g) of milk or dairy product into a conical flask of the digestion apparatus. Add several glass beads and 30 ml of nitric acid. For milk, add 10 ml of sulfuric acid; for dairy products, add 5 ml of sulfuric acid. Swirl the flask to prevent localized charring. Attach a condenser and heat gently while conducting a blank test in parallel.

After pretreatment, in an acidic medium, arsenic (As) and mercury (Hg) in the sample are reduced by potassium borohydride (KBH₄) to their volatile species, which are then carried by the carrier gas (argon) into the atomizer. In the hydrogen flame, the elements are atomized. Under irradiation from a specially designed hollow cathode lamp, ground-state atoms are excited to higher energy states. Upon returning to the ground state, they emit fluorescence at characteristic wavelengths. The fluorescence intensity is proportional to the elemental concentration and is quantified by comparison with a standard series.

2. Instrument Setup

Adjust the instrument to optimal working conditions according to the user manual. Key parameters:

Photomultiplier tube voltage: 240 V

Mercury hollow cathode lamp current: 30 mA

Atomizer temperature: 300 °C

Carrier gas flow rate: 500 ml/min

Shield gas flow rate: 1000 ml/min

Interference and Elimination

In acidic media, elements capable of reacting with potassium borohydride (KBH₄) to form hydrides may interfere with each other. The addition of a thiourea-ascorbic acid mixed solution can effectively eliminate such interference. Additionally, transition metals such as copper (Cu) at concentrations above a certain threshold may also cause interference. However, the thiourea-ascorbic acid solution can remove the majority of these interferences.

To mitigate physical interference, a double-layered quartz tube atomizer is utilized. Both the inner and outer layers are purged with argon (Ar), creating a protective shield that isolates the sample from atmospheric oxygen (O₂) and nitrogen (N₂). This prevents collisions between ground-state atoms of the target elements and air molecules, thereby reducing fluorescence quenching and improving measurement accuracy.

Test Result