Photomultiplier tube detection limits

Air with radon is passed into a vessel coated internally with zinc sulphide. Alpha particles from radon in the chamber, and from decay products deposited on the walls, give scintillations which are counted by photomultiplier tubes viewing the chamber through windows (Lucas, 1957). With a chamber of volume 0.11, and a counting time of 1 h the detection limit of 222Rn in air was about 10 Bq m-3, but by concentrat-... 

In 1986, Foret et al.41 described an on-line UV absorbance detector that employed a commercial photometer and optical fibers in direct contact with the outer walls of the separation capillary. The optical fibers (200 (im I.D. fused silica core) conducted the light beam perpendicularly across the migrating zones one fiber was connected to a mercury lamp to serve as the illumination source, and the other directed light to a photomultiplier tube for detection. The detector was found to be linear in the range of 10"5 to 10 3 M (r = 0.994 for 10 measurements), with detection limits of 1 X 10 5 M for picric acid (S/N = 2). 

Radioisotope detection of P, 14C, and Tc was reported by Kaniansky et al. (7,8) for isotachophoresis. In their work, isotachophoretic separations were performed using fluorinated ethylene-propylene copolymer capillary tubing (300 pm internal diameter) and either a Geiger-Mueller tube or a plastic scintillator/photomultiplier tube combination to detect emitted fi particles. One of their reported detection schemes involved passing the radiolabeled sample components directly through a plastic scintillator. Detector efficiency for 14C-labeled molecules was reported to be 13-15%, and a minimum detection limit of 0.44 nCi was reported for a 212 nL cell volume. 

Because photomultiplier tubes can only detect events which occur on or near the surface of the scintillator column exposed to the tubes, the greater the surface to volume ratio, the more efficiently events will be detected. The literature contains a number of suggestions in which the simple U-tube is modified into coils and spirals which present larger surfaces per unit of volume to the photomultiplier tubes (3j. While this does increase sensitivity, a practical limit to minimum required sensitivity is set in most metabolism studies by the fact that we must separate enough compound to carry through identification studies. Since we have the option of selecting the specific activity of our original l c-compound, we have found the U-tubes described entirely adequate. 

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