Photothermal densitometer for reading electrophoresis gels

Abstract

A densitometer apparatus for evaluating electrophoresis gel samples based on photothermal techniques. In accordance with this invention, electrophoresis gels are characterized by passing a heating light beam through the gel at a particular location. Light absorbed by the presence of staining dyes in that area causes heat evolution which generates a local index of refraction variation or a 'thermal lens'. A probe beam is passed through the sample in the area of the thermal lens a predetermined period of time after it is generated and the modification to the beam caused by the thermal lens is evaluated. For example, defocusing of the probe beam can be sensed by a detector which receives transmitted light through a limiting aperture. Various means of separating the heating and probe beams are disclosed, including use of separate lasers, crossed beams, modulation by plane of polarization, etc. One embodiment of this invention is particularly adapted for characterizing dry gels in which the heating beam is absorbed by the sample and the probe beam passes across the sample and is modified by a thermal lens generated in the air above the sample. Several embodiments are related to means for offsetting the probe beam from the heating beam for use with samples that are swept by the photothermal techniques in accordance with this invention offer advantages in terms of sensitivity over conventional transmission-type densitometers. These advantages enable increased sensitivity and facilitate the use of simplified staining techniques.