UV FAQ: Spectral Band Width

What is the Spectral Band Width (SBW)？

Spectral Band Width is an important specification for UV-VIS Spectrophotometer, it affects instrument’s resolution and test error.

The spectral bandwidth is defined as the band width of light at one-half the peak maximum, as shown in Fig.1. The spectral bandwidth of a spectrophotometer is related to the physical slit-width and optical dispersion of the monochromator system.

Fig.1 Spectral Band Width

What's the resolution?  What's the relation between SBW and Resolution?

Resolution is the ability of an instrument to separate light into finite, distinct wavelength regions and distinguish finite region from each other. Or said the ability to analyze adjacent peak. Resolution is primarily affected by the physical slit width of the instrument, in combination with the inherent optical dispersion of light from monochromator system to the detector of the instrument. Reducing the physical slit width will decrease the spectral band width, and improve the resolution.

How to affect sample test?

SBW affects data accuracy, repeatability, result accuracy, detection limit and method reliability.

Because most sample peaks are not very sharp, 2nm SBW can meet with most sample analysis, but for some samples, such as penicillin sodium, benzylpenicillin potassium, cytochrome C, their absorbance peaks are very sharp, if use 2nm SBW, some small and sharp peaks will disappear, big and sharp peaks will become lower, so choose a suitable SBW is very important for some sample analysis. Although a narrower spectral bandwidth does improve the resolution of closely spaced peaks, it also decreases the signal-to-noise ratio, reducing the bandwidth leads to a reduction in energy reaching the sample.

How to choose SBW?

Firstly, the UV-VIS’s slit must be changeable. Then choose the best resolution and the maximum absorption slit by spectrum scanning function, the operation could be easier if with the SBW scanning function. For some special accessory, such as integrating sphere, must set to 5nm SBW.

PERSEE’s T8DCS UV-VIS spectrophotometer is designed to accommodate the needs of a variety applications, it comes with continuously changeable slit from 0.1nm-5nm, minimum interval 0.1nm, and you can use SBW scanning function to find out the optimal SBW for different application.

How to test SBW or resolution?

A solution of toluene in hexane is the most widely used reference for qualifying the Spectral Bandwidth (SBW) of a UV spectrophotometer. It is accepted for this purpose by most Pharmacopoeias and Standardization Bodies, including: European Pharmacopoeia; Deutsches Arzneibuch, United States Pharmacopoeia, American Society for Testing and Materials, Therapeutic Goods Administration (Australia) , British Pharmacopoeia.

When scanning by an ultraviolet spectrophotometer, the spectrum shows a maximum absorbance at 269 nm and a minimum at 267 nm. The absorbance ratio of the peak maximum at 269 nm to the minimum at 267 nm gives a measure of the resolution (Spectral Band Width, SBW) of the instrument. As shown in Fig.2, this instrument resolution is 0.378/0.259=1.45 at 2nm slit.

Fig.2 Resolution spectrum