How to read the spectrum on a CEMS spectrometer

Reading a CEMS spectrometer spectrum involves identifying gas-specific absorption peaks, interpreting their intensity to determine concentrations, and correlating these with emission standards.

Understanding the CEMS Spectrum

A CEMS spectrometer measures the concentration of gases such as NOx, SOx, CO, CO2, O2, and particulate matter by detecting how these gases absorb light at specific wavelengths using techniques like Non-Dispersive Infrared (NDIR) or UV-Vis spectrophotometry . Each gas has a characteristic absorption wavelength, and the spectrometer produces a spectrum showing absorbance or signal intensity versus wavelength.

• Peaks in the spectrum correspond to the presence of specific gases. The height or area of a peak is proportional to the gas concentration.
• Baseline readings indicate zero or background levels. Deviations from the baseline reflect the amount of gas absorbed.
• Multiple peaks may appear if several gases are present; the spectrometer's software typically deconvolutes overlapping signals to quantify each gas.

Steps to Interpret the Spectrum

1. Check Calibration: Ensure the spectrometer is calibrated using standard gas mixtures. Calibration ensures that peak intensities accurately reflect gas concentrations .
2. Identify Peaks: Compare the observed peaks with known absorption wavelengths for the target gases. For example, CO absorbs strongly in the infrared region around 4.6 µm, while NOx has characteristic UV absorption bands .
3. Quantify Concentration: Use the spectrometer's software or calibration curve to convert peak height or area into concentration units (ppm or mg/Nm³). Most CEMS systems provide automated calculations for this step .
4. Check for Interferences: Ensure that overlapping absorption from other gases or particulate matter does not distort readings. Some CEMS analyzers include filters or scrubbers to minimize interference .
5. Analyze Trends: Continuous monitoring allows you to observe temporal changes in emissions. Look for spikes or gradual increases that may indicate process issues or regulatory non-compliance.

Practical Tips

• Maintain Sample Integrity: Ensure the probe and umbilical are heated and moisture-controlled to prevent condensation, which can affect spectral readings .
• Use Auto-Purge and Zero Checks: Periodically purge the probe and perform zero checks to maintain accuracy .
• Interpret Opacity for Particulates: For particulate matter, opacity readings correlate with light obstruction. Higher opacity indicates higher SPM concentration .
• Software Assistance: Modern CEMS spectrometers include software that automatically identifies peaks, calculates concentrations, and logs data for compliance reporting. By following these steps, you can accurately read a CEMS spectrometer spectrum, identify the gases present, and quantify their concentrations to ensure compliance with environmental regulations and effective emissions monitoring .