Most spectroscopic techniques are based on the comparison of the signal which
passed the sample with a reference signal. If the difference between the two
signals becomes small because the concentration of the species generating the
signal alteration is low, the resulting signal becomes more and more unreliable.
In contrary, fluorescence spectroscopy is basicly an absolute methode, i.e. the
spectrum is not generated by the comparison with a reference. This is the reason
why fluorescence techniques can be extremely sensitive. This sensitivity calls
for a application in surface analysis. However, most of the surfaces which are
under investigation in our group do not emit fluorescence by themselfes. But we
are able to couple fluorescence dyes selectively to certain functional groups at
the surface of a polymer. With this kind of
derivatization the concentration of
hydroxyl, carboxyl, carbonyl, and amino groups can be determined.
With state of the art equipment one is able to record the fluorescence spectrum
of a femtomolar fluorescein solution. This value corresponds to a surface
concentration in the order of 1e-16 mol/cm2. In a practical experiment, this
limit is shifted toward higher concentration mostly due to light defraction in
the sample. But the concentration limit we can reach is about 1e-12 mol/cm2
which is well in the sub-monolayer range.
For further information on fluorescence labeling visit
www.polymer-analysis.com/surfaces/fluorescence.html