INSTRUMENT MAINTENANCE
PRACTICAL TASK P10. IR CELL PATHLENGTH Outline You would expect that the spectrum of an empty cell will be a straight line. Instead it exhibits a wave pattern, due to interference effects between reflected parts of the beam off each wall of the cell and the transmitted beam. This waveform can be used to measure the pathlength of the cell, by determining the frequency difference between two points on the spectrum and counting the number of complete waves included. The equation used is: 5n
pathlength 
 2  1
where the pathlength is in millimetres, n is the number of waves between frequencies 1 and 2. Procedure 1. Obtain a “sandwich” infrared cell, and blow dry air through it to remove any solvent traces. 2.
Record the spectrum of the empty cell. 3.
Find a region of the spectrum covering at least 200 cm‐1 where there is only a waveform pattern. 4.
Mark the frequencies of two wave peaks which cover at least ten cycles. Repeat this for two other intervals in the spectrum. They can overlap. 5.
For each interval, calculate the pathlength using the equation above. Average the values. Reporting Requirements Use the standard Performance Check reporting sheet. There is no specified tolerance/standard value – simply report the average value.