Photosensitive Samples; Stirring And Temperature Control - Agilent Technologies Cary 8454 Installation Manual

Photosensitive Samples

A few substances are very photosensitive. They degrade or undergo
photochemical reactions if exposed to light. This can be easily seen by a
decrease of sample absorbance over time.
Use of Filters
The shorter wavelength, higher-energy UV light is most likely to degrade
photosensitive samples. If you have a problem, you can selectively block
portions of the UV spectrum with a UV cut-off filter. An optical filter wheel
assembly with three cut-off filters is available for the spectrophotometer. The
cut-off wavelength of the filter you choose should be low enough that it does
not eliminate important spectral information but high enough that it blocks
the light that could degrade your sample. If you use a filter with your samples,
you must use the same filter when you make your blank measurement.
Turning the D2-Lamp off
The short wavelength radiation leading to photodegradation comes from the
light of the D2-Lamp. For application where readings are taken at wavelengths
above 400 nm, the D2-Lamp can be turned off. The light intensity supplied by
the Tungsten lamp is sufficient for a good signal to noise ratio over the
wavelength range 400 nm - 1100 nm.When using cells with small apertures,
check the signal to noise ratio by making sample measurements under
conditions of your application.

Stirring and Temperature Control

Solution homogeneity can be a problem, especially for viscous solutions. There
are cases where, due to convection induced gradients, rapid absorbance
changes may give irreproducible data. These changes can be observed
spectroscopically by taking measurements with short integration times. To
minimize convection effects keep the temperature of your sample the same as
the cell holder or environmental temperature. Problems like these can also be
minimized by using a thermostattable cell holder and/or a stirring module.
A similar effect can occur in cases of incomplete mixing. This is especially true
where the specific gravities or miscibilities of the solvent and analyte are quite
different. Again, stirring is a way to prevent this kind of problem.
Agilent Cary 8454 UV-Visible Spectroscopy System Installation Guide
Good Measurement Practices
General Considerations
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