INSTRUMENT MAINTENANCE
PRACTICAL TASK P12. UV/VIS STRAY LIGHT Outline Stray light is any radiation reaching the detector that has not come through the “correct” pathway of sample cell & wavelength selector. This is most likely to come from the exterior of the instrument and would mean that there was a problem with the slight seals, but it could also come from some internal reflection of the source beam. It would cause the detector to respond more than it should, causing an incorrect, most likely lower, absorbance. To check this, it is necessary to put something in the sample cell that blocks out all radiation, so that any anything that does reach the detector is clearly stray. Rather than put a piece of masking tape over the source input window or a piece of black plastic in the sample cell (these don’t simulate reflection under normal use), a solution of very high absorbance is used. Because no one solution absorbs at all wavelengths, a number are used in combination. Procedure (Cary 1/Cary 5000/Philips 8625) 1. Set the instrument to read transmittance at a single wavelength. 2.
Set to the measurement wavelength listed in the Table below. 3.
Zero with water. 4.
Fill a quartz cell with the specified solution. 5.
Measure the %transmittance. 6.
Repeat steps 2‐5 for the other solutions (a plastic cell can be used for the methylene blue). Stray light test solution parameters Solution Measurement wavelength (nm) Range covered (nm) 10 g/L KI (or NaI) 220 200‐250 50 g/L NaNO2 350 300‐400 100 mg/L methylene blue 650 600‐800 Maximum acceptance transmittance: 0.1% Reporting Requirements Use the standard Performance Check reporting sheet: include all three results – in the Standard Value column, write the Test Solution.