(1997; 248 pages) [French]
2.11 Instruments and their calibration
Instruments are subject to wear, corrosion, and mishandling. If they are not in good working order they may give rise to serious analytical errors that may remain undetected unless systematic checks are made.
Whenever possible, regular servicing of instruments by specialist maintenance teams should be arranged. Instruments exposed to high levels of humidity should be resistant to corrosion and adequately protected against mould and fungal growth. Where line voltage is variable, suitable voltage stabilizers should be installed.
Some instruments may need to be protected from extremes of humidity or temperature in a specially designed area. Otherwise, analytical instruments can be either grouped together or dispersed between the various units. The choice will depend on the types of instruments, their fragility, the extent to which they are used, and the skills required to operate them.
Regular calibration of all instruments used to measure the physical properties of substances is essential and specific schedules should be established for each type of instrument, having regard to the extent to which it is used. pH meters should be calibrated at least once a day. The reliability of the wavelength scale of melting-point instruments and spectrophotometers operating in the ultraviolet region should be checked once a week and a full calibration undertaken once a month. Infrared spectrophotometers require calibration every quarter, while refractometers and spectrofluorometers should be serviced half-yearly. Analytical balances should also be serviced at least half-yearly by a qualified balance specialist.
Volume 1 of the third edition of The international pharmacopoeia describes the procedure for calibrating refractometers, thermometers used for the determination of melting temperature, and potentiometers for pH determination (4). It also explains the methods for checking the reliability of the scales on ultraviolet and infrared spectrophotometers and spectrofluorometers. A clear description of the standard operating procedure should be placed beside each instrument together with a schedule of the dates on which it is due for calibration.
Whatever routine precautions are taken to ensure the calibration of instruments, they should also be checked whenever they are used in a test which indicates that a sample is not in compliance with specification. The results of the check should be attached to the analytical worksheet.