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Step 4: The Analysis

A. The pads are placed on a table and viewed in a dark room under a long wave ultra violet fluorescent light. All lights are turned out, doors closed, and all measures possible are taken to prevent ambient light from entering the analysis room. The darker the room is, the easier it will be to read the results.

U.V. light and Optical Brightener sampling pads in the analysis room

B. A non-exposed sampling pad is used as a control and compared to each pad as it is exposed to the U.V. light .

C. There are three Qualitative Results: Positive, Retest, and Negative.

A pad will very definitely glow (fluoresce) if it is positive. If it is negative it will be noticeably drab and similar to the control pad. All other samples are undetermined or retests. As each pad is read it is placed in either the positive, negative, or retest pile.

D. In some instances only a portion of the pad or simply the outer edge will fluoresce after being exposed to Optical Brightener. This can be caused by many factors but is usually the result of an uneven exposure to the dye in the watercourse due to sedimentation or the way the pad was placed in the water.

In these cases, one can always account for the unevenness by associating the pattern with the sedimentation distribution, folds in the pad, etc. Regardless, as long as a portion of the pad fluoresces and one can explain why the remainder was not, it should be considered positive.

Three piles of Optical Brightener sample results with control pads

When in doubt, call it a retest.

There is never a borderline positive or a negative call.

Samples can be left in for a longer period of time and/or placed closer to a suspected source in order to get a more definitive result.

E. Since paper and cotton dust is so pervasive, it is common to see specks or spots of fluorescence on the sample or control pads. These should be ignored and not used to indicate a positive result.

F. After all the pads have been read, lights are turned back on and the labels read as to the sampling location.

Next: Step 5