An integrator for measuring the “dry mass” of cells and isolated components

Abstract

In general the total “dry mass” of a cell may be determined by measuring first the average optical path difference of the cell in water and then multiplying this value by the cell projected area. It is shown that this product can be measured directly provided that the maximum optical path difference of the cell in water is less than about 0.2 wavelengths. The method may be extended to cells with higher optical path difference by immersing them in media, of refractive index greater than water. No measurement of cell area is required and the method is applicable to cells of irregular shape. The method is not readily applicable to measuring the dry mass of the components of intact cells but may be used to measure the dry mass of isolated cell components, e.g. nuclei, nucleoli, chromosomes etc. The apparatus described for making the measurements employs a Cooke-Dyson interference microscope, but can be constructed with any two-beam interference microscope. A feature of the apparatus is that it measures small changes in light energy with high accuracy. Hence it has possible applications to the measurement of faintly stained objects, e.g. single Feulgen stained chromosomes produced by the cell crushing method [5]. The apparatus can also be used to make “point” measurements of optical path differences, sensitive to about 1 1000 wavelenghts. The method of measuring dry mass has been tested experimentally on model spheres. The results obtained were in good agreement with those predicted by theory. Possible errors due to light scattering by the object, out of focus errors, and the variation in optical path in plate-like objects with illuminating aperture have been examined.