Spectrophotometers

When a lot of energy is applied to molecules as in a flame or electrical discharge, chemical bonds are broken and the molecules are fragmented into individual excited atoms. The energy levels in these atoms are narrow and well defined.

These excited atoms produce extremely sharp spectral lines when electrons fall from higher to lower atomic energy levels. High resolution and wide dynamic range are required of the spectrometer to resolve spectral lines that are close together and differ greatly in intensity.

A good spectrometer has three characteristics:

• It will resolve emission lines that are very close together in wavelength. Most educational diode array spectrophotometers spread their active spectrum over 100 to 340 pixels in their detector. MicroLab's 211C spreads its spectrum over 2048 pixels, giving much better resolution between adjacent spectral lines.

• It will measure accurately spectral line intensity over a wide range of signal levels. Educational spectrometers have a 12-bit analog to digital converter, which separates the incoming signal into 4096 "bins". The MicroLab/Avantes 211C has a 14-bit ADC, with 16,384 "bins". Its dynamic range and electronic resolution is four times better.

• It will integrate spectral measurements over a long period of time to pick up very weak atomic emission lines of fluorescence spectra. Charge coupled detectors (CCD's) used in diode array spectrometers integrate photons of light like film. The longer the exposure, the fainter the light that can be measured. MicroLab's 211C will integrate signals from 1.2 mS to about ten minutes. It is about 40 times more sensitive than other educational spectrophotometers.