Bandpass and Slit Width
Definition
The wavelength interval the monochromator delivers to the sample. Routine forensic UV-Vis runs at 1 nm or 2 nm bandpass; pharmacopoeial methods sometimes drop to 0.5 nm.
- Field
- UV-Vis spectrophotometry
- Routine forensic setting
- 1 nm or 2 nm
- Pharmacopoeial minimum
- As low as 0.5 nm
Common questions
What happens to a spectrum if the bandpass is set too wide for the sample?+
A bandpass wider than a narrow absorption feature averages light over too broad a wavelength range, which flattens and broadens the observed peak, reduces the apparent absorbance at the peak maximum, and can cause deviation from Beer-Lambert linearity, so a wide bandpass can produce an inaccurate quantitative result on samples with sharp spectral features.
Why do pharmacopoeial methods sometimes require a narrower bandpass than routine casework?+
Pharmacopoeial assays are often validated to distinguish closely related compounds or resolve fine spectral shoulders relevant to identity and purity testing, so a narrower bandpass such as 0.5 nm preserves the resolution those methods were validated at, while routine forensic screening at 1 or 2 nm is adequate because it targets broader, well-separated absorption bands.
Is bandpass the same thing as slit width?+
They are related but not identical: slit width is the physical opening in the monochromator, while bandpass is the resulting wavelength interval of light that reaches the detector, and the relationship between the two depends on the instrument's dispersion, which is why instrument manuals specify bandpass directly rather than only a slit setting.
Related terms
- Absorbance (A)
- The base-10 logarithm of the ratio of incident to transmitted light intensity, A = log10(I0/I). Dimensionless. Linear with concentration in the working...
- Chromophore
- The molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the decolourised...
- Diode Array Detector (DAD)
- A UV-Vis detector that records the full 190 to 800 nm absorbance spectrum of every peak as it elutes, instead of a...
- Molar Absorptivity (ε)
- The proportionality constant in the Beer-Lambert law, in L mol⁻¹ cm⁻¹. A property of the analyte at a given wavelength, solvent and...
- Photomultiplier Tube (PMT)
- Cascaded electron-multiplication detector behind the monochromator slit on a dispersive scanning instrument. Reads one wavelength at a time at very high sensitivity.