Monochromator wavelength calibration

Abstract

The essential components of a single beam fluorescence spectrometer without facilities for spectral corrections are shown in Fig. 2.1. The instrument has two monochromators or alternative wavelengthselection devices. The excitation monochromator (M1) selects the wavelength of the light with which the sample is to be irradiateci, and the emission monochromator (M2) selects the wavelength of fluorescence (or scattered light or phosphorescence, etc.) received by the detector. M1 and M2 may be coloured or interference filters in simple instruments, but in more advanced instruments grating monochromators are normal. Some instruments incorporate doublegrating monochromators to minimize stray light levels (see Chapter 3). The monochromators may normally be scanned using motorized drive units. Scanning M1 with M2 set at a fixed wavelength generates excitation spectra and scanning M2 with M1 fixed generates fluorescence emission spectra. In view of the increasing interest in synchronous scanning methods [1] the facility to scan both monochromators simultaneously is desirable (see also Section 7.1.3). The reflection gratings used in M1 and M2 are normally blazed to give maximum efficiencies (approximately 50%) at 250–350 nm and 350–500 nm, respectively.Holographic gratings, which yield low stray-light levels (see Chapter 3) have been introduced in some instruments.