CAP survey to assess the extent of stray light problems in precision spectrophotometry

When the absorbance at 240 nm. of a solution of sodium iodide was measured by 159 laboratories using narrow-band-pass spectrophotometers, 15% reported results that suggested a problem caused by stray light. The laboratories were also requested to measure the absorbance at 240 nm. of solutions of acid potassium dichromate and alkaline potassium chromate. The laboratories that had stray-light problems reported absorbance results that were skewed in distribution and showed low mean values and wide instrument-to-instrument variation. The laboratories without stray light problems reported results with less instrument-to-instrument variation, the results followed a symmetrical distribution, and the mean of the results provided an accurate estimate of the absorbance of the solutions.