Acidic Potassium Dichromate Solutions as Ultraviolet Absorbance Standards.

The absorbances of five concentrations of potassium dichromate in 0.001 M perchloric acid have been determined at eight wavelengths in the ultraviolet on the National Bureau of Standards Institute for Materials Research high-accuracy spectrophotometer. Four of the wavelengths-235, 257, 313, and 350 nm-correspond to absorbance maxima or minima in the HCrO4 - spectrum and are useful wavelengths for checking the accuracy of the absorbance scale of narrow bandpass spectrophotometers. Although partial dimerization of HCrO4 - to Cr2O7 = produces small positive deviations from Beer's law at these wavelengths, the apparent absorptivities calculated for each concentration are reproducible to one part in a thousand. The estimated uncertainties in the absorptivity values are ± 0.7 percent at 0.1 absorbance (A) and ± 0.2 percent near A = 1. These uncertainties include all known sources of possible systematic error and the 95 percent confidence level for the mean. The remaining four wavelengths used for measurement are near two predicted isosbestic points in the HCr04 -/Cr2O7 = spectra. The absorptivities at 345 nm are sufficiently independent of concentration that this wavelength can be used for checking absorbance linearity to one part in a thousand over the range A = 0.2-1.

Considerations for the Use of Semi-Transparent Metallic Thin Films as Potential Transmittance Standards in Spectrophotometry.

Various characteristics of evaporated metal-on-fused silica filters are discussed in relation to their optical transmission properties. Special metal holders provided with shutters were designed to be used with these filters, and are described in detail. Transmittance measurements, performed in various conditions, are reported and indicate that the evaporated metal-on-fused silica filters might present an acceptable material as transfer standards in spectrophotometry.

Efficient averaging spheres for visible and ultraviolet wavelengths.

On the basis of a theoretical analysis that identifies the design criteria for optimization of efficiency, an averaging sphere with a barium-sulfate target was constructed that has an efficiency of 80-90% in the visible. In spite of the decreased reflectance of the sphere walls in the uv, this sphere is still 30% efficient at 300 nm and, thus, can be used for near-uv work. For applications below 300 nm, a cell containing a fluorescent dye is placed inside the sphere in order to shift the incident radiation into the longer wavelength region in which the reflectance of the sphere walls is higher. The use of this fluorescent wavelength converter, which was designed so that the sphere remains usable in the visible, resulted in a sphere efficiency of about 20% in the region between 200 nm and 300 nm and 50% in the visible. The averaging properties of these spheres were found to be adequate for high-accuracy photometric measurements.

Adaptation of a High-Accuracy Spectrophotometer for Ultraviolet Work.

A high-accuracy spectrophotometer, originally designed for work at visible wavelengths, was modified to permit measurements in the ultraviolet without degradation of its original performance. This was accomplished by equipping the spectrophotometer with a stable deuterium arc source, a highly efficient averaging sphere with fluorescent wavelength converter, a new grating, and achromatic sample-compartment optics. The modified spectrophotometer will be used for the development of new Standard Reference Materials, as well as for materials research, in the region between 200 and 300 nm.

Reflection Correction for High-Accuracy Transmittance Measurements on Filter Glasses.

Multiple reflections in the sample compartment of a spectrophotometer constitute a source of systematic bias in transmittance measurements on filter glasses. This bias may be removed by applying a numerical correction obtained from measurements on tilted samples in polarized light. For a high-accuracy spectrophotometer, this correction was found to be of the order of several 10-4 transmittance units, independent of polarization, but slightly wavelength dependent.

An Accurate Spectrophotometer for Measuring the Transmittance of Solid and Liquid Materials.

The optical transmittance of solids and liquids as well as the molar absorptivity of various chemical species are parameters of fundamental significance in characterizing these materials. Meaningful transmittance data can be obtained only when the measurements are performed with well-known accuracy and precision. To perform such measurements, a high accuracy spectrophotometer was designed and assembled at NBS, Analytical Chemistry Division, and will be described in this paper. This single-beam instrument is composed of a constant radiation source, a monochromator, a sample carriage, an integrating sphere-photomultiplier assembly followed by appropriate electronics, and a read out system consisting of a digital voltmeter and a computer data acquisition and handling provision. The accuracy of transmittance measurements is determined by the light-addition principle used in conjunction with a two-aperture arrangement. The spectrophotometer can be used in manual or automatic modes of operation. A detailed discussion of the data obtained with this instrument, used in both modes, will be presented together with its application to the certification of solid and liquid Standard Reference Materials for checking the photometric scales of conventional spectrophotometers.

A multichannel spectrometer for simultaneous atomic absorption and flame emission analysis.

The simultaneous determination of a number of elements by atomic absorption is based upon the combination of desired characteristic radiations from hollow cathode tubes into a single, collimated polychromatic beam that is passed through the absorbing medium, is then resolved into its components, and each component is brought onto a detector. The radiations are combined by locating each source behind a slit that is so positioned with respect to a diffraction or dispersion element to satisfy the diffraction or dispersion equations under conditions which bring each beam along a common path to a single exit slit. The combined beam is resolved into its components by conventional spectrometric principles. Methods by which the optics may be folded to accommodate all elements within a single enclosure are described. The sensitivity and precision of analysis obtained by this method were found to be equivalent to the results of conventional single element determinations.