A thermal cycling method and device is disclosed. The device comprises a sample chamber whose temperature can be rapidly and accurately modulated over a range of temperatures needed to carry out a number of biological procedures, such as the DNA polymerase chain reaction. Biological samples are placed in glass micro capillary tubes and then located inside the sample chamber. A programmable controller regulates the temperature of the sample inside the sample chamber. Monitoring of the DNA amplification is monitored by fluorescence once per cycle or many times per cycle. The present invention provides that fluorescence monitoring of PCR is a powerful tool for DNA quantification.