Introduction to Microwave Digester

Microwave is an electromagnetic wave with a frequency range of 300MHz to 300GHz, that is, an electromagnetic wave with a wavelength range of 100cm to 1mm, which means the wavelength is between far-infrared and radio waves. In the microwave band, the wavelength range of 1-25cm is specifically used for radar, while the rest is used for telecommunications transmission. In order to prevent interference from civil microwave power on radio communication, broadcasting, television, radar, etc., the international standard stipulates that the frequency of civil microwave for industrial, scientific research, medical, and household use is 2450 to 50MHz. Therefore, the frequency used by microwave digestion instruments is basically 2450MHz, and this is also true for household microwave ovens.

Microwave characteristics

(1) Metal materials do not absorb microwaves and can only reflect microwaves. Such as copper, iron, aluminum, etc. Using metal (stainless steel plate) as the furnace of a microwave, it reflects back and forth on the heating material. Metal containers should not be placed in microwave ovens as the reflected microwaves can damage the magnetron.

(2) Insulators can pass through microwaves and hardly absorb their energy. Glass, ceramics, plastics (polyethylene, polystyrene), polytetrafluoroethylene, quartz, paper, etc. are transparent to microwaves, and microwaves can penetrate them and propagate forward. These substances do not absorb the energy of microwaves, or absorb microwaves very little. The strength of a substance's absorption of microwaves is essentially related to its complex dielectric constant, that is, the larger the loss factor, the stronger its ability to absorb microwaves [2]. Most household microwave oven containers are made of plastic products. The materials used for microwave enclosed digestion and dissolution tanks are polytetrafluoroethylene, engineering plastics, etc.

(3) Substances with polar molecules can absorb microwaves (belonging to substances with high loss factors), such as water, acid, etc. Their molecules have a permanent dipole moment (i.e. the centers of the positive and negative charges of the molecules do not coincide). Polar molecules rapidly change their orientation in the microwave field with the frequency of the microwave, rotating back and forth, causing collisions and friction between molecules, absorbing the energy of the microwave and raising the temperature. The food we eat contains water, which is a highly polar molecule and can be heated in a microwave. Below, we can further understand the principle of microwave digestion of samples.

Fast orientation change, 2450MHz microwave, molecules change direction 2.45 per second × 109 times, the molecules rotate back and forth, colliding and rubbing with surrounding molecules, increasing the total energy of the molecules and causing a sharp increase in the temperature of the sample.