PerkinElmer Spotlight 200, User Manual

Page: 49 / 132

Getting Ready to Use the Spotlight 200 . 49
4. Select the Start and End values for the scan.
For mid-infrared analysis, it is normal to set the upper wavenumber limit to 4000 cm
− 1
and the lower limit to the low wavenumber specification of the detector, as shown in
the table below.
Detector type Low wavenumber limit (cm
− 1
)
MCT mid-band 580
MCT wide-band 450
DTGS 380
The Service engineer can advise you of the lower limit to the low wavenumber, at the
time of installation, or it will be recorded on the Service Installation test spectra.
For near infrared measurements, on a system fitted with an InGaAs detector, the upper
wavenumber value is typically set to between 15800 and 8000 cm
− 1
and the lower limit
to 4000 cm
− 1
.
NOTE: To work in the near infrared range of the spectrum, the Spotlight 200 must be
attached to a dedicated NIR spectrometer or a dual-range spectrometer which has
been set up to operate in the near infrared range.
The Setup Instrument Basic tab also provides an option for selecting the number of
accumulations. There are no firm rules about these selections since the number of
accumulations needed to generate acceptable spectra will depend on the nature of the
sample, the size of the area being analyzed and the requirements of the application.
For single point analyses where the analysis is relatively quick, it is probably better to “over-
scan”; 50 accumulations taken at 8 cm
− 1
resolution using an MCT or InGaAs detector
(20 accumulations for a DTGS detector) can be collected reasonably quickly.
For mapping or imaging experiments, the total time for the analysis is normally very
important and most analysts set the number of accumulations to be the very minimum
required to get a sufficiently useable spectrum. In mapping, this may be 1 to
5 accumulations per point and for imaging it may be 15 or 30 accumulations per pixel for the
background image and 2 or 4 accumulations per pixel for the sample image.
Optimizing Transmittance measurements
In most cases, samples for transmittance measurements are placed flat on top of an
IR-transmitting window such as NaCl, KBr, BaF
2
, ZnSe or diamond. In some cases, such as
flattened fibers, the sample is simply secured across an open aperture mounted in the
standard sample holder.
These samples should be prepared such that they are thin, ideally 10 to 25 microns, but
certainly thinner than 50 microns (see section on sample preparation) and also they should
sit as flat on the window as possible.
In transmittance measurements, the infrared beam passes through the window supporting
the sample and this will alter the beam characteristics (focus, etc.). This variation will be
different for different window materials depending on their refractive indices. To optimize the
infrared energy through the window (and sample), the lower cassegrain is moved up or
down using the Correction facility on the Setup Microscope Advanced tab to compensate
for this refractive index variation.