Spectrophotometer design principles and working principle, allowing the absorbance changes within a certain range, that is, the instrument has a certain accuracy and accuracy. Such as the Eppendorf Biophotometer accuracy ≤ 1.0% (1A). The result of such multiple tests varies between about 1.0% of the mean, are normal. In addition, the physical and chemical properties of the nucleic acid need to be considered, as well as the pH value of the buffer solution for dissolving the nucleic acid, the ion concentration, and the like. During the test, the ion concentration is too high and the reading will drift. Therefore, Buffers, such as TE, greatly stabilize the readings. Dilution of the sample concentration is also a factor that can not be ignored: Due to the inevitable sample of some small particles, especially nucleic acid samples. The presence of these small particles interferes with the test results. In order to minimize the effects of particles on the test results, it is desirable that the absorbance of the nucleic acid be at least greater than 0.1 A and that the absorbance be from 0.1 to 1.5 A. Within this range, the particle interference is relatively small, the result is stable. This means that the sample concentration can not be too low or too high (beyond the photometer test range). Finally, the operating factors, such as mixing to be sufficient, or absorbance is too low, or even negative; mixed liquid bubble can not exist, the blank solution without suspended solids, or readings drift violently; the same cuvette must be used to test the blank solution and the sample , Otherwise the concentration difference is too big; The conversion factor and the sample concentration unit choice is consistent.