Geoscience Reference
In-Depth Information
In most cases, cuvettes should be used with stoppers, especially if the solvent is corro-
sive or volatile. This will help to minimize the possibility of spills and to maintain sample
concentration. The optical windows that form the cuvette should not be touched with bare
hands (powder-free gloves are often used), as human fingerprints exhibit fluorescence. In
addition, careful and thorough cleaning of fused silica and glass cuvettes is fundamental
to accurate measurements. For this, it is common to soak cuvettes for several hours in a
nitric acid solution followed by a thorough rinse with deionized water. Before use, cuvettes
should be rinsed several times with the solvent to be used in the experiment. An alternate
method of cleaning glassware is to use a detergent solution but this has the risk that the
detergent may have some fluorophore in it. Therefore, it is sometimes appropriate to make
a fluorescence measurement on a dilute solution of any detergent that is used for clean-
ing purposes. At the same time, please note that there are whitening agents in many filter
papers, clothes, and tissues and these can introduce contaminant fluorophores.
5.4.15 Solvents and Contaminants
The fluorescence emission from a fluorophore can be strongly dependent on its local
environment. As such, choice of solvent and solvent purity are very important. Incorrect
choice of solvent can cause spectral shifts and changes in the peak shapes and reduce sam-
ples emission. As a matter of good practice, high-purity solvents should be used and then
checked before use by checking their absorption and fluorescence properties. Handling
techniques should be employed to minimize the possibility of stock solutions becoming
contaminated - even very small levels of contaminants may cause appreciable background
signals. As such, solvents should not be stored in plastic containers and should be reg-
ularly screened for contaminations before use. Regular screening of the solvent using a
fluorimeter will also indicate the position and magnitude of any Raman signal from the
solvent or contaminants in the analysis spectral range. Contamination opportunities exist
at each stage of sample preparation and handling. Trace levels or contaminants are enough
to destroy measurement quality. Insufficient cleaning of the cuvette and related glassware,
leaching from plastic parts in contact with samples, dirty pipettes, fingerprints, vacuum
grease from degassing stations, impure solvents and solutes, and old stock solutions are
just some of the opportunities for contaminated samples and all lead to erroneous fluores-
cence signals.
5.4.16 Background Signals: Rayleigh and Raman Scattering
Rayleigh and Raman scattering effects can be useful for providing reference signals to
enable comparison between samples; however, they can often be the cause of signal inter-
ference too. Rayleigh scattering is associated with
elastic
scattering from small mol-
ecules or particles and can be seen by scanning the emission monochromator over the
spectral region of the excitation wavelength. This invariably will produce intense opti-
cal signals and is normally to be avoided if possible, as such intense signals can “blind”
