DOP tests require the same equipment as the HEPA filter tests described earlier: a smoke generator and a photometer. The isolator is filled with smoke and the photometer is used to trace leakage. This is a method of high sensitivity and one which reportedly defines leaks clearly, unlike gas tests, which give readings that are more difficult to interpret.
This test has a noticeable advantage over most others in that the isolator need not necessarily be sealed and pressurised; it can be used on negative-pressure isolators and on some of the unidirectional flow isolators that have panel filters as the air inlet and exhaust. It is still helpful if the isolator can be pressurised, since this will provide a greater challenge, but smoke will nonetheless diffuse to a measurable extent through leaks in neutral pressure. Some workers test negative-pressure isolators by placing the photometer head inside the isolator and wafting smoke along the outside of the seals and joints to be tested (Lumsden 1996).
A significant point in favour of the test is that the smoke particles are of similar dimensions to the microorganisms that we wish to exclude from sterile isolators; thus, the challenge is a representative one. If the isolator is leak-tight to DOP smoke, then it is arguably leak-tight to microorganisms, whereas an isolator that leaks significantly to a pressure hold test might yet be microbiologically sound.
It is also possible to put some sort of figures to this test, making it less subjective and more repeatable than others. Indeed, we can set up the smoke challenge and the photometer response (full-scale deflection at 100 p-g/l or mg/m3) in the same way as we have done for the HEPA filter test, and thus quantify any leaks with a penetration value. However, we are still left with the question as to what is an acceptable, overall leakage value.
A minor disadvantage of the smoke test is that it coats the internal surfaces of the isolator with oil that must eventually be cleaned off; it also puts a burden on the HEPA filters that may shorten their working life. It is still a convenient test to run if the HEPA filters are being checked at the same time. This test is widely used and accepted, particularly in the pharmaceutical industry.
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