Scan speed is one of the settings specified in analytical conditions for UV-VIS spectrophotometers.
The scan speed setting choices include fast, medium, slow, and very slow speeds, which determine the number of times measurement data are acquired (number of scans) at each measurement wavelength point.
Fig. 1 LabSolutions UV-Vis method settings for Scan Speed using the UV-2600
The "Scan Speed" parameter indicated in the software is not actually the speed of analysis for the method. The Spectrometers operate using hardware components that all move at their continuous maximum rate. The monochromator turns to provide dispersion of wavelengths using a stepper motor under precise control which operates at a very fast rate (e.g. 24,000 nm/min for the UV-1900i). The parameters that affect the overall speed of analysis are how many times the monochromator has to stop (i.e. the data interval) and the scan range required. Additionally, the "Scan Speed" is actually how long in relative terms the detector spends looking at the wavelength whilst the scanning monochromator is stopped. This is the signal-averaging time for the data point. before the monochromator moves on to the next point in the spectrum.
Fig. 2 shows a baseline obtained using each "scan speed" (signal-averaging) setting using a Shimadzu UV-2600 UV-VIS spectrophotometer. As evident from Fig. 2, the slower the scan speed, the more scans are performed at each measurement wavelength, which provides spectra with lower noise. The medium speed setting is recommended for typical analysis. However, to obtain spectra with lower noise levels, select slow or very slow scan speed settings, in exchange for longer measurement times.
Fig. 2 Baselines Obtained at Each Scan Speed Setting Using the UV-2600
Note that total scan times for an analysis are indicated in the method after changing the chosen parameters. They can be quite lengthy if the data interval is very short and the signal averaging if high.
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