What is a monochromator? Resolution versus sensitivity.

A typical spectrometer will process  wavelength spectra to provide some measure of the analytical makeup of a sample.

Within that spectrometer will be some separating device such as a monochromator or polychromator to measure the strength of the wavelength of interest.

The diagram below shows a Czerny Turner Monocromator

grating schematic.jpg

Point A is the entry point into the monochromator of the multiwavelength signal.

B is a set of entry slits that perform a masking effect to remove extraneous light on either side of the entry point.

C  is a collimating mirror, whose angled surface makes the various wavelengthstravel parallel to each other. This parallel or collimated beam then impinges on the central component, D, a diffraction grating.

The diffraction grating  performs a dispersing action on the wavelengths, similar to a prism. The grating consists of an optic mirror with a large number of parallel fine lines that each act similar to a prism.

The density of these lines can be from 1,200 to 3,600 lines per millimeter.

The separated wavelengths then are reflected off mirror E, which focuses each wavelength at a unique point at the exit slits, F.

The grating is able to rotate so that a different angle of this grating will bring a different wavelength into focus at the exit slit,F.

The rotating grating is controlled by a stepper motor to provide a set of recall angles for different wavelengths.

The density of lines on the grating surface has a noticeable effect on the dispersion of the wavelengths.

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In the resulting spectrums above, the same light source falls on a 1,200 line grating in the top diagram and a 2,400 line grating on the bottom diagram.

The 1,200 line grating creates two distinct groupings of wavelengths, a doublet A1 and A2 at approx. 308nm and a triplet, B1,B2 and B3 around 316 nm. With this grating the peaks are not completely resolved at the baseline.

The 2,400 line grating, operating on the same light source, however is able to resolve all of these peaks back to the baseline.

The additional effect of the higher density grating is that the signal strength of each wavelength, is reduced. Compare the height of peak B3 under the influence of both gratings.

In summary, lower grating line densities give less wavelength resolution but greater light throughput whereas higher densities yield better resolution but poorer light throughput or signal strength.