Melody V. Smith

Intrinsic Wavelength Standard Absorption Bands in Holmium Oxide Solution for UV/visible Molecular Absorption Spectrophotometry

The transmittance minima of 18 absorption bands of a solution of 40 g/L holmium oxide in 10% (volume fraction) perchloric acid are certified as intrinsic traceable wavelength standards, by means of a multicenter measurement on material from a single source coupled with comparisons of a variety of preparations of the material evaluated on a single instrument. Fit-for-purpose artifact standards for the experimental calibration or validation of wavelength scales of chemical spectrophotometers can be carefully produced by end users themselves or by commercial standards producers. The intrinsic (data) standard confers traceability to the SI unit of length in place of costly transfer artifacts and repetitive calibration procedures. Certified values are provided for instrumental spectral bandwidths of 0.1–3.0 nm in 0.1 nm intervals, and information values are provided to a spectral bandwidth of 10 nm at wider intervals. Expanded uncertainties are typically less than ±0.1 nm for certified band positions.

Molecular Spectrometry Data Interchange Applications for NIST's SpectroML

SpectroML, a markup language for ultraviolet-visible spectroscopy data, has been developed as a “Web-aware” mechanism for instrument-to-instrument, instrument-to-application, and application-to-application data interchange and archiving. This article documents the application of SpectroML to the interchange and archiving of measurement data from three spectrophotometers that are used in the NIST optical filter standards program. It describes how result data from the NIST national reference spectrophotometer and two commercial spectrophotometers are converted into SpectroML format and how SpectroML-formatted data and metadata are imported into the optical filter standards database.

Challenges in providing standard reference materials for chemical and pharmaceutical process analysis

Abstract The NIST Analytical Chemistry Division has supplied transmittance verification and wavelength calibration standard reference materials (SRMs) specialized to the needs of chemical and pharmaceutical spectrophotometric analysis since 1970. Growing demand for UV/visible standards stems from the increasingly routine use of spectrophotometers for pharmaceutical quality control and from the escalation in the documented use of standards for regulatory and voluntary quality control purposes. To meet the demand, NIST is studying ways to accelerate standards production. Projects include studies of the origin of transmittance drift (which necessitates aging during production), investigations of solid UV filters and of sealed liquid standards, and the development of an NIST-traceable reference material (NTRM™) optical filters program to involve the private sector. Recently, we have revised the optical specifications of our solid filter standards to meet the requirements of reversed-geometry (post-dispersion) instruments. In the near-infrared, spectrochemical process control applications mandate the need for wavelength standards to support the stability and instrument-to-instrument transfer capability of multivariate analytical calibration models. We are presently producing one such standard and are investigating others. Finally, we are studying algorithms for locating peaks in wavelength standards to find a consistent means for making wavelength assignments from the UV through the mid-infrared.

Optical Wedge Effects in Instruments and Standards for Molecular Absorption Spectrophotometry

The optical wedge of some older, individual solid NIST absorbance filter standards has been found to cause bias in the indicated absorbance readings of certain instruments. In a collimated-beam spectrophotometer, the sample beam is deflected by about half of the wedge angle in the sample. For inverted geometry designs, small deflections can give rise to large changes in the field of view of the entrance slit of the spectrometer downstream from the sample. Beam deflection is also found to induce small apparent wavelength shifts in data taken through a wedged sample, resulting in spectral artifacts derived from spectral features of the system or of an absorbing sample. These spectral artifacts turn out to be robust, and simple cell-reversal difference spectra can provide useful diagnostic indicators of optical wedge. Solid photometric standards are found to be reliable, if manufactured to wedge angles of less than 0.1 mrad. Despite a wedge tolerance of 0.9 mrad in the sealed cuvettes of a wavelength standard, wavelength shifts are shown to be negligible when compared to the stated uncertainties. For normal use, accuracy may be achieved by utilizing the same cell in the same orientation for both background and sample spectra or by hand selection of cells.