Automated nucleic acid extractors achieve nucleic acid extraction mainly based on the following principles:

I. Magnetic Bead Method

1. Nucleic Acid Adsorption
   Magnetic beads are tiny particles with specific functional groups (such as carboxyl, amino, hydroxyl, etc.) on their surface. These functional groups can bind specifically to nucleic acid molecules. During the extraction process, the sample (such as blood, tissue, cell lysate, etc.) is first mixed with a system containing magnetic beads and a specific binding buffer. The composition and conditions of the buffer (such as pH value, salt concentration, etc.) are optimized so that nucleic acids can interact with the functional groups on the surface of the magnetic beads and be adsorbed. For example, under appropriate pH and salt concentration, the negatively – charged phosphate backbone of nucleic acids can bind to the positively – charged functional groups on the surface of magnetic beads through electrostatic interaction.
2. Separation and Washing
   After the nucleic acids are adsorbed onto the magnetic beads, an external magnetic field is used to attract the magnetic beads to the wall of the container, separating the magnetic beads from other impurities (such as proteins, polysaccharides, cell debris, etc.) in the liquid sample. Then, the liquid is removed, and a washing buffer is added to wash the magnetic beads to remove unbound impurities. The composition of the washing buffer can disrupt the non – specific binding between impurities and the nucleic acid – magnetic bead complex without affecting the binding between nucleic acids and magnetic beads. This process may be repeated several times to ensure a high – purity nucleic acid – magnetic bead complex.
3. Nucleic Acid Elution
   Finally, an elution buffer is added to change the buffer conditions (such as changing the pH value or ionic strength), causing the nucleic acids to dissociate from the functional groups on the surface of the magnetic beads and enter the elution solution. In this way, a relatively pure nucleic acid solution is obtained, which can be used for subsequent experimental operations such as polymerase chain reaction (PCR), gene sequencing, etc.

II. Column Chromatography Method

1. Nucleic Acid Binding
   The column chromatography method uses a special chromatographic column filled with a medium that can bind nucleic acids specifically. When the processed sample (such as after cell lysis) passes through the chromatographic column, the nucleic acids bind to the medium in the column, while other impurities flow out of the column with the liquid. The surface of these media usually has some chemical groups that can interact with nucleic acids. For example, the surface of a silica – based medium has groups that can bind to the phosphate groups of nucleic acids. Under appropriate buffer conditions, nucleic acids can bind firmly to the medium.
2. Impurity Elution and Nucleic Acid Elution
   First, a washing buffer is used to elute the impurities bound to the column. The composition of this buffer is designed to remove impurities without affecting the binding of nucleic acids. Then, by changing to an elution buffer and altering the buffer conditions (such as changing the pH value or ionic strength), the nucleic acids are dissociated from the medium, and the eluate containing nucleic acids is collected. Similar to the magnetic bead method, the obtained nucleic acid solution can be used for subsequent detection and analysis.

III. Centrifugation Method

1. Density – Gradient Centrifugation
   This method separates nucleic acids based on the difference in sedimentation rates of different substances in a density – gradient medium. First, the sample is mixed with a density – gradient medium (such as sucrose solution, cesium chloride solution, etc.) to form a gradient with a gradually decreasing density from the bottom to the top. When high – speed centrifugation is carried out, different components (such as nucleic acids, proteins, cell debris, etc.) will migrate to different positions in the gradient according to their density and shape. Nucleic acids will concentrate in a specific density region, and then the part containing nucleic acids can be obtained by puncturing the tube wall or fraction – collection.
2. Differential Centrifugation
   Differential centrifugation separates components of different sizes and densities by gradually increasing the centrifugation speed. First, centrifuge at a lower speed to precipitate larger and heavier impurities (such as cell debris, organelles, etc.) to the bottom of the tube, and collect the supernatant. Then, centrifuge the supernatant at a higher speed to further separate smaller impurities and nucleic acids. After several centrifugation steps, the supernatant or precipitate containing nucleic acids can be obtained. Subsequent purification steps, such as precipitating nucleic acids with organic solvents to remove residual impurities, are carried out to finally obtain pure nucleic acids.