PHOTOLYSIS ASSISTED POLLUTION ANALYSIS (PAPA)
Photolysis Assisted Pollution Analysis (PAPA) is a new analytical technique developed by Infrared Analysis, Inc. that brings out the spectra of reactive gases while allowing the spectra of water and carbon dioxide to remain invisible. There are two advantageous aspects of PAPA: (1) difficult-to-measure compounds can be converted to easily measured forms. or (2) compounds whose spectra are hidden by interferences are seen by separating their spectra from the interferences. Three examples of compounds measured by PAPA are H2S, H2 and the NOx compounds.
The PAPA process is initiated by radiation from a quartz-mercury lamp mounted inside the absorption cell. When the lamp is turned on for just a few seconds, the ultraviolet radiation creates oxygen atoms, ozone molecules and hydroxyl radicals. These reactive species then transform most of the pollutant molecules into other more highly oxidized forms. The oxidizing conditions have no effect on water, carbon dioxide or other fully oxidized molecules.
In PAPA, spectra are recorded before and after the photo-oxidation. One spectrum is then divided by the other and the result is put into absorbance form. The infrared bands of compounds removed in PAPA appear downwards in the absorbance spectrum, while the bands of newly-created compounds appear upwards. Bands of pre-existing water and carbon dioxide do not
appear at all. The amounts of products and reactants may be measured by QASoft.
H2S has such a weak spectrum that it cannot be seen in the infrared except at very high concentrations. In a PAPA experiment the H2S is converted to SO2, which has a very strong infrared spectrum that allows measurement down to the parts-per-billion level. Similarly, molecular hydrogen has no infrared band, but PAPA converts it to the easily measured H2O2.
NO and NO2 pollutants in air are difficult to see by infrared because their bands are hidden by the spectrum of water. PAPA, however, removes these two compounds from a sample without changing the water. Then, a ratio of the spectra taken before and after photolysis clearly shows the NO and NO2 bands.
Molecular hydrogen is normally not measurable in the infrared, but the PAPA process converts the H2 to H2O2, which is readily measured. Many other examples could be given of compounds that are made measurable by the PAPA process.
The original paper on the PAPA technique, entitled Photolysis Assisted Pollution Analysis (PAPA), by Philip L. Hanst, was published in the Proceedings of the U.S. EPA/A&WMA International Symposium: Measurement of Toxic and Related Pollutants, pp 576-582. Reprints are available on request.
The PAPA cell has a path that is variable from 1 meter to 16 meters, in 1 meter steps. The cell has borosilicate glass walls and mirrors, anodized aluminum end-plates, protected gold mirror coatings and a quartz-mercury ultraviolet lamp.