Linewidth Expansion
Physical phenomena cause the measured width of a spectral emission or absorption line to increase beyond its natural limit. This spectral broadening limits the resolution of optical and laser based sensing instruments by spreading the signal energy across a wider frequency band.
Physical Causation
Collisions between gas molecules or high thermal velocities create Doppler shifts that widen the observed transition profiles of the sample. These interactions broaden the spectral lines, turning sharp peaks into wider, less defined shapes. As pressure and temperature within the measuring chamber increase, these collisional and thermal effects become more dominant, causing further spreading of the signal.
The instrument must use low pressure sample cells or rapid scan techniques to minimize these physical interactions.
Sensing Resolution
Overlapping spectral lines from different compounds reduce the selectivity of the instrument, leading to cross sensitivity errors. When spectral broadening occurs, the broadened wings of neighboring lines can merge, making it difficult for the analytical software to distinguish between the target gas and interfering species. This merging of signals degrades the signal to noise ratio and increases the detection limit of the sensor.
Instrument Qualification
Testing protocols verify the spectral resolution of the sensor by measuring the linewidth of a certified reference gas under controlled pressure conditions. Technicians compare the measured full width at half maximum of the peak against the calculated theoretical profile. The final calibration certificate specifies the maximum operating pressure at which the sensor can successfully resolve the target compounds without exceeding the drift limits.
