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To achieve the
maximum benefit from using Klarite, samples should be deposited onto
the active surface and analysed with a standard Raman instrument.
Many sample deposition techniques are possible ranging from dip coating
to multi array spotting.
A pipette dropping method is described here for basic analysis, although
other techniques may be better suited to your applications. For illustration
purposes the amino acid L-Alanine is used as an example throughout.
However, the methodology described here can be applied to many other
compounds and solvent.
Preparation of the sample on the Klarite substrate is achieved in
the three steps described below.
1. Deposition
Using a pipette, a liquid sample of typically 1
microlitre or less is deposited onto the active area of the Klarite
chip shown. If the drop does not detach from the pipette, carefully
let the drop touch the substrate and then transfer the drop onto the
substrate.
Avoid touching the chip with the tip of the pipette – a scratch
to the gold surface on the chip will adversely affect the measurement
results.
The solution used in this example was obtained by diluting 7.5mg of
L-Alanine (Sigma-Aldrich A7469) in 15ml of ultra filtered water (Sigma-Aldrich
95289).
2. Evaporation
In general the evaporation process of any liquid on a surface produces
one of three main types of patches: coffee rings, crystalline capillaries
or spot-like stains. The evaporation result depends on the interplay
of adhesive forces of the substrate and cohesive forces of the fluid.
The L-Alanine used in this example was left at room temperature in
a clean lab environment for about 30 minutes for the water solvent
to evaporate. Typically the evaporated L-Alanine will appear
as shown.
3. Measurement
Once the sample is dry, Raman measurements can be carried out across
the deposited patch. The patch will appear formed by many concentric
rings of different colours. Crystalline islands can also form inside
the patch for concentration typically >1mM. In general the
size and characteristics of the film patch and crystalline islands
depend on the concentration of the solution, the purity of the solvent
and the polar nature of both the analytes and the solution.
Typical Raman spectra of L-Alanine across the film patch are shown
below. The spectra were acquired in 10 sec of CCD exposure time
with 15mW laser power at 785nm excitation with a standard micro Raman
instrument. No normalisation and baseline subtraction were performed
on the spectra. Quantitative data on film composition and molecular
interaction can be obtained by probing the Raman features across the
patch.
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