Instrument to instrument variability: Unless they have been “standardized” all spectrometers will characterize a reference emission spectrum differently. This is a function of many issues including variations in detector efficiency, bandpass and various system optics
Instrument dependent spectral resolution: If a natural spectrum has features with a Full Width at Half Maximum (FWHM) that is less than the bandpass (resolution) of the spectrometer then the spectrum will be observed with the spectrometer’s FWHM, not the natural line width.
This is an example of an instrument constraint. The observed bandpass of the sample spectrum is a convolution of the instrumental bandpass and the natural line widths present in the sample spectrum.
For example, if the natural full width at half maximum (FWHM) of a low-pressure mercury (Hg/Ar) lamp is some hundredths of a nanometer (nm), and the bandpass of the spectrometer is set to say 1 nm, then all line widths will be 1 nm, not the natural line width.
Therefore, a Hg/Ar lamp such as the MIDL can be used to determine the actual spectral bandpass and wavelength accuracy of any spectrometer operating in the UV to near IR.
Uncertainties in wavelength ratios: If a sample spectrum covers an extended spectral range, wavelength ratios are very likely to vary between instruments, sometimes significantly. This is due to instrumental component variables. If measuring accurate wavelength ratios is necessary then the data delivered by an instrument needs to be “normalized” to a standard.
Without correction, data will be highly reproducible, yet wavelength ratios will be “precisely inaccurate.” The main wavelength dependent culprits include:
- The quantum efficiency (QE) of the spectrum detector typically a scientific camera or imaging photomultiplier tube (IPMT)
- The wavelength versus efficiency profile of a diffraction grating or prism
- Variations between electronic bandpass filters such as Acousto Optic Tunable Filters (AOTF), Liquid Crystal Tunable Filters (LCTF) or interferometers.
- Optical coatings, lenses, mirrors, filters, polarizers and any other optic that is located between the sample and the detector
- Aliasing when the system fails to accurately reconstruct an analog spectrum into a digital format.
The impact of all these influences are eliminated when a spectrum is acquired in %transmission, absorption, or %reflection. In these cases, the sample spectrum is divided by a spectrum of the illumination source that has also passed through the same system.
However, in emission spectroscopy including fluorescence, luminescence and Raman there is no illuminant spectrum to divide by. Therefore, in order to accurately reconstruct wavelength ratio intensities, it is necessary to apply a correction factor at each wavelength.