Metrohm Vision – Diagnostics - Manual (Page 19)

▪▪▪▪▪▪▪ 17
bandwidth. With respect to peak position, the specification on S/D is 0.015 nm or less, depending
upon instrument model and type.
The instrument bandpass (bandwidth) is calculated using the ratio of the absorbance at the
polystyrene peak at 2167 nm and the valley at approximately 2154 nm (after an offset correction.)
The S/D test is also run in the 400-1100 area of the spectrum on models 5500 and 6500. In this area,
didymium/polystyrene is used for two of the peaks, at 806.10 and 878.85 nm. Bandwidth is not
calculated in the 400-1100 nm area.
Process instruments (and certain laboratory instruments) cannot use the polystyrene peak at 2306.10
nm due to attenuation of the signal in the fiber-optics, or limitations in wavelength range. On such
instruments the tests will not report values for the 2306.10 peak, and will perform wavelength tests
on peaks within the wavelength response of the instrument.
Note: Wavelength positions are empirically determined, and have been set based upon the best
measurements possible given the technology available. As instrumental or standardization
breakthroughs lead to greater precision in setting nominal peak positions, Metrohm reserves the right
to issue revised nominals.
2.5 Wavelength Accuracy / Precision Measurement on the XDS
The XDS instrument, like the model 5000 and 6500, uses an internal wavelength reference material.
When Performance Test is run, the relative wavelength positions and repeatability of these
wavelength materials are monitored and reported. The internal standard for the XDS is comprised of
polystyrene, erbium oxide and samarium oxide and six peaks across the range from 500-2310nm are
used to determine the internal wavelength performance of the instrument.
Note that these internal wavelength materials need not be precisely on the assigned nominals. These
nominals are arbitrary. The internal wavelength materials are a method to assure stable readings on
the external wavelength standard, measured at the sample plane during Instrument Calibration and
Wavelength Certification.