A Micro Photofluorometer

A need arose for a fluorometer sufficiently sensitive to measure the amounts of riboflavin and thiamine in a few hundredths of a ml. of blood or blood serum.There are approximately 1 and 0.25 millimicrograms of riboflavin and thiamine respectively in 20 c.mm. of blood serum, an amount conveniently obtainable from the finger.Conventional commercial fluorometers require lOO-to IOOO-fold greater quantities of these vitamins for adequate measurement.Friedemann and Frazier (1) have reported the use of a more sensitive fluorometer, but the instrument has not been described.Hinton (2) used a very sensitive galvanometer in conjunction with the Spekker fluorometer and was able to measure as little as 1 millimicrogram of thiamine with a reproducibility of 20 per cent.Special capillary cells were employed.It has been found easily possible to increase the sensitivity (and stability) of a commercial fluorometer lOOO-fold by substituting a "multiplier" phototube for the phototube originally present.With this modified instrument, 0.1 millimicrogram of riboflavin (lo-lo gm.) in 0.5 ml. of solution can be measured with a precision of 5 per cent.An instrument embodying the same principles is now available commercially.'Description will be given of the changes made to increase the sensitivity and stability of the instrument and of the alterations necessary to decrease the optical blank, the last being more difficult to accomplish.Increasing the Xensitivity-The phototube of the Coleman photofluorometer, model 12, was replaced by an RCA multiplier phototube, No. lP21.2A standard eleven prong tube socket was substituted for the original socket, and was carefully oriented to present the photo surface directly to the