A Comparison between the X-Ray Absorption and Optical Interference Methods for the Mass Determination of Biological Structures

IN histo- and cyto-chemistry there has been a trend towards the development of specific techniques for the identification and quantitative determination of biologically important compounds. However, there is a great need for a completely non-specific method which records the total mass of cell structures irrespective of the chemical nature of their constituents. Two such methods have been developed in recent years. One is based on the absorption of very soft X-rays by the biological tissue1. As the samples are investigated in vacuo, the total dry mass is obtained. The other method2,3 makes use of measurements of cell refractivity or optical path-length, which is related to the total mass of substances other than water, that is, the dry mass. Measurement of optical path-differences can be made by means of interference microscopy, and the application of the Dyson interference microscope4 in quantitative cytochemistry has been discussed5.