PerkinElmer SPECTRUM TWO N, User Manual

Page: 78 / 125

78 . Spectrum Two N User's Guide
Changing the Resolution
The Resolution setting is on the Setup Instrument Basic tab in Spectrum software (Figure 27).
The resolution achievable in the instrument is determined by both the optical path difference
in the interferometer and the beam divergence in the interferometer as seen by the detector.
The Spectrum Two N spectrometer has a slot in the double-sided slide holder that enables
you to insert a J-stop card before the sample (Figure 66).
Figure 68 The double-sided slide holder
J-stop cards set the upper limit to the resolving power (wavenumber/resolution) of the
instrument by limiting the beam divergence. Hence, to achieve a resolution defined by the
optical path difference (for example, 2 cm
− 1
), as specified by the Resolution on the Setup
Instrument Basic tab, a certain beam divergence is required and this is dictated by a given
J-stop aperture diameter. J-stop cards are provided with Spectrum Two to enable 8, 4, 2 and
1 cm
− 1
resolutions at 7200 cm
− 1
.
At a given wavenumber, the smaller the J-stop aperture, the finer the resolving power
(assuming we are not limited by the optical path difference scanned by the interferometer).
Hence, at 7200 cm
− 1
the 4 cm
− 1
J-stop card has a given diameter to deliver a resolving
power of 1800. However, this same diameter may be used achieve 2 cm
− 1
at 3600 cm
− 1
or
1 cm
− 1
at 1800 cm
− 1
, providing that the optical path difference (set by the Resolution
setting in the software) is set up appropriately. This can be useful where higher resolution is
needed for low-energy situations, where it is only required to achieve resolutions up to lower
wavenumber ranges. For example, using a 4 cm
− 1
J-stop to deliver 2 cm
− 1
resolution to
3600 cm
− 1
only can enable higher signal-to-noise to be achieved (or permit shorter scan
times) than using the 2 cm
− 1
J-stop card which would deliver 2 cm
− 1
resolution to 7200 cm
− 1
.
For normal operation at 4 cm
− 1
resolution as defined in the software, the Spectrum Two
optics are designed to deliver an approximately 4 cm
− 1
instrument lineshape and specified
wavenumber calibration without the use of the 4 cm
− 1
J-stop card (assuming the sample is
not acting to restrict the beam). However, when highest wavenumber accuracy in direct
transmission measurements is required, we recommend use of the optional AVI function with
the 4 cm
− 1
J-stop card installed. This ensures the purest lineshapes at higher wavenumbers.
NOTE: This assumes the sample is not restricting the beam and itself forming an effective
J-stop. For highest accuracy work, we recommend to always ensure the sample is
larger than the J-stop diameter.
Sample slide holder
J-stop card holder