Microwave digestion technology achieves rapid and decomposition of samples in a closed high-pressure environment through the synergistic effect of microwaves and chemical reagents. Its core principles can be summarized into the following three aspects:

1、 Microwave Selective Heating: Penetrating Energy Transfer

Traditional thermal digestion relies on heat conduction and convection, where heat is transferred layer by layer from the outer wall to the inner layer of the container, which is time-consuming and prone to local overheating. Microwave digestion utilizes the penetrability of microwaves (2.45 GHz) to directly act on polar molecules (such as water and acid molecules) in the sample and digestion solution (such as nitric acid and hydrofluoric acid mixed system). Polarized molecules rotate at high speed (2.45 billion times per second) in an alternating electric field, converting electromagnetic energy into thermal energy through intermolecular friction, achieving uniform heating from the inside to the outside of the sample. For example, when soil samples are digested, microwaves can raise the system temperature to 200 ℃ within 5 minutes, while traditional electric heating plates require more than 2 hours.

2、 High pressure sealed environment: breaking through the limitations of reaction kinetics

The closed digestion tank adopts a polytetrafluoroethylene (PTFE) inner tank and a metal outer shell structure, which can withstand a maximum pressure of 6MPa (corresponding to a temperature of about 300 ℃). High pressure environment brings dual advantages:

Raising the boiling point of the digestion solution: Under normal pressure, the boiling point of nitric acid is 83 ℃, and under high pressure, it can rise to over 200 ℃, significantly enhancing the oxidation ability of the acid and accelerating the decomposition of insoluble substances such as metal oxides and silicates.

Inhibiting volatilization loss: A closed system prevents acidic gases (such as NOx, HF) from escaping, avoiding element loss, especially suitable for accurate determination of volatile elements such as mercury and arsenic. For example, in the detection of heavy metals in food, microwave digestion can ensure a recovery rate of over 95% for lead and cadmium.

3、 Optimization of Acid System: Synergistic Dissolution of Complex Matrix

Microwave digestion usually uses a mixed acid system (such as HNO ∝ - HCl-H ₂ O ₂), which achieves decomposition through multiple mechanisms such as oxidation, complexation, and acid hydrolysis

Nitric acid: a strong oxidant that decomposes organic matter to produce CO ₂ and H ₂ O.

Hydrochloric acid: dissolves metal sulfides and some oxides, forming a "aqua regia" effect with nitric acid.

Hydrofluoric acid: destroys the lattice of silicates and releases encapsulated metallic elements.

Hydrogen peroxide: auxiliary oxidation, reduces the amount of nitric acid used, and lowers the blank value.

For example, when digesting geological samples, HNO ∝ - HF HClO ₄ mixed acid can decompose complex matrices containing ilmenite and mica within 15 minutes, while traditional methods require several hours and residual particles.

4、 Summary of Technical Advantages

Microwave digestion, through three mechanisms of selective heating, high-pressure intensification, and acid system synergy, shortens the sample decomposition time from several hours to 10-30 minutes, reduces the amount of acid used by 50% -70%, and has no element volatilization loss. It has become a standard method for sample pretreatment in fields such as environmental monitoring, food safety, and geological exploration.