Metrohm NIRS XDS Transmission OptiProbe Analyzer - Manual (Page 32)

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The spectra show pure water and
99.5% pure 2-propanol, taken at
1mm pathlength. Water absorbs
strongly between 1350-1600nm and
1850-2150nm. 2-propanol has
absorbances between 1650-1800nm,
2050-2150nm, and 2250-2500nm. 2-
propanol is a weaker absorber than
water; the spectral intensity is lower.
The 2050-2150nm absorbance in 2-
propanol would probably be masked
by the 1850-2150nm absorbance of
water. 1mm pathlength is not a good
choice.
Spectra of 25% 2-propanol in water,
taken at 1mm and 2mm pathlengths,
show the main absorber is water, with
some absorbance due to 2-propanol.
Vertical lines highlight the 2-propanol
absorbances at 1690nm and 2310nm.
Absorbance at 1690mm using a 1mm
cuvette appears too low to be of
much use.
Absorbance at 1690nm is about 0.55
absorbance units using a 2mm
cuvette. If we work at that
wavelength, a 4mm cuvette would
provide a higher absorbance level.
The absorbance at 2310nm is about 2.6 absorbance units, using a 2mm pathlength, which might be
fine on this instrument, if the sample gives linear absorbance. Some work should be done at each
pathlength to determine which combination of wavelength area and pathlength gives the best
analytical result.
Many factors may come into consideration, including mixing accuracy, lab error for samples analyzed
by other methods, and operator differences. Contact your Metrohm NIRSystems Product Specialist
for more information.
5.4 Probe Cleaning and Maintenance
Cleaning the Probe End:
Following analysis, and before using the system on another sample type, the probe ends should be
cleaned to prevent sample carryover, which could affect analysis. Cleaning depends upon the sample
material. Always follow the precautions on the MSDS when performing cleaning.
Pharmaceutical applications may require a “Cleaning validation” which describes each step of the
cleaning process, solvents to be used, and specific verification tests to determine whether cleanliness