The major drawback of a potassium dichromate (PDC) reference solution intended for checking the accuracy of the photometric scale of an analytical absorption spectrophotometer is its lack of broad wavelength coverage. Two different PDC reference solutions are required to cover both the ultraviolet and visible regions of the spectrum. This application note describes a cobalt-nickel-nitrate (CNN) reference solution that is permanently sealed in a fused-silica cuvette. In contrast to PDC reference solutions, a single CNN solution/cuvette filter can be used in both the ultraviolet and visible spectral regions of the spectrum.
Standard mixtures of cobalt, nickel, and nitrate ions in aqueous perchloric acid were originally formulated in 1972 by Burke and Deardorff. The reference solutions were sealed in glass ampoules and issued in consumable form as SRM 931. Despite its exceptional popularity in today's modern analytical testing laboratories, SRM 931 is limited in its overall use because of the demanding requirements for careful handling, contamination control, and precise and skillful manipulations to ensure accurate transfer of a CNN reference solution into a separate cuvette for analysis. This restrictive feature of SRM 931 could be overcome by simply sealing the CNN solutions in fused-silica cuvettes.
The CRM 200 reference standard, first introduced in 1994, features many of the same positive attributes of SRM 931, with the added advantage that the reference solutions are permanently sealed in fused-silica cuvettes as shown in Figure 1. Further improvements in CNN solution/cuvette filters resulted with the introduction of the CA2M 706 reference standard in 2014 as part of the company's commemorative MAVRO series, and later again in 2016 with rebranding of the CRM 200 reference standard. The Stranaska® CRM 200 and CA2M 706 reference standards are distinguishable by their differing scopes of calibration and levels of scientific metrological rigor integrated into the calibration models that underpin their respective certifications. A comparison of the different CNN liquid absorbance standards is presented in Table I.
The major advantage of Stranaska® CRM 200 and CA2M 706 reference standards is that each solution/cuvette filter is individually certified so that it can be used repeatedly over time. However, it is recommended that the reference standards be returned periodically to Stranaska Scientific for science-based recertification in order to clean the cuvettes, and to update the certified reference values and uncertainties.
Figure 1: CNN solution/cuvette filters from Stranaska Scientific.
The Stranaska® CRM 200 and CA2M 706 reference standards, which are derived from the SRM 931 series, are proven alternatives to PDC solutions for performance evaluation and qualification of analytical absorption spectrophotometers in FDA-regulated laboratories. The CNN solution/cuvette filters are more convenient to use by metrologists, engineers, and calibration technicians that do not have meaningful experience in common analytical laboratory practices.
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