Uses for a holmium oxide filter in spectrophotometry

The use of a holmium oxide calibration filter is discussed for checking wavelength, trouble-shooting, and adjusting sensitivity of spectrophotometers. Correction tables may be prepared to permit setting the instrument to true wavelength. One spec-trophotometer was found to have a wavelength error large enough to impair its use-fulness in direct spectrophotometric methods. THE TECHNIC of direct spectrophotOhlietry has elicited much imiterest because of the constant introduction of new procedures for the analy-sis of constituents of blood aiid urine. These imiclude methods for bili-rubin (1-3), blood oxygen (4, 5), uric acid (6), porphyrin (7), bar-biturates (8), and many others. The miature of the technic imposes strict requirememits on the instrument beimmg used. If the instrument is known to be reliable and meets relatively high specifications, most workers will accept it as being without error. However, if the wave-length calibration is incorrect, direct spectrophotometry will lead to erroneous results, or they may not even be practical. This is particu-larly true if the error places the ol)served absorption peak n the as-cending or descendimig slope of the true peak. Thus it is imperative that the wavelength calibratiomi of an imistruniemit be checked not only once, but at frequent intervals thereafter. A calibration staiidard made of holmiunu oxide glass hmas recently been introduced for checking wavelemigth calibratiomus of spectropho-tometers. * It is imiserted in a metal block which in use replaces the 10-mm. square cell of the instrument. Imicluded with the standard is a spectral transmittance curve determuimied on a Bausch & Lomb 505 spectrophotometer using a 0.5-m hand pass. A spectral transmit