Single-beam signal reference

Time resolution is a strength of modern FTIR spectrometers that makes them powerful tools for investigations of fast processes. For the investigation of slow processes, the same single-beam FT spectrometers have a disadvantage sensitive IR measurements require the acquisition of a reference spectrum, which is usually done 

---

The laser hres at f = 0 and causes an increase in absorbance in the sample as a consequence the intensity of light reaching the detector decreases. While laser photolysis systems are normally single-beam spectrometers, in fact they behave as dual-beam instruments. The reference beam is separated from the sample beam in time, rather than space. Thus, the reference signal is acquired before laser excitation and leads to Iq. The absorbance at time t in Figure 18.3 is given by Eq. 1 ... 

Figure 11.9 Schematic of a Shimadzu F-4500 spectrofluorometer. A fraction of the incident beam, reflected by a semi-transparent mbror, reaches a reference PMT. A comparison of the signals from the two PMTs leads to the elimination of any drifting of the source. This procedure, for single beam instruments, gives approximately the same stability as with a double beam spectrometer. However, the spectrum of a given solution will often present minor differences when recorded upon different instruments (reproduced courtesy of Shimadzu).

To obtain a spectrum with a single-beam instrument, the signal for the sample is first stored. A reference signal with a good reflector, such as finely ground KBr or KCI, is then recorded in place of the sample. The ratio of these signals is then taken to give the reflectance. 

---