The traditional methods
Dye Ingress Test
The most widely used in the past is certainly the Dye Ingress Test.
This method is based on the concept, common also to other different fields of technology (e.g. verification of presence of cracks or pinholes in metal welding), that a strongly coloured liquid can be pushed through the holes, cracks or defects present in a material, then detected visually.
If presence of the coloured liquid is visible inside the container, or in depth in a sealing or surface that is supposed to be well sealed, this fact gives evidence of a defective closure.
The liquid commonly used is an aqueous solution of Methylene Blue; alternatively, fluorescent colorants can be used (e.g. fluoresceine).
The test equipment consists in a liquid bath of suitable size (e.g. 50 cm x 40 cm) to accommodate one or more of the samples to be tested. The equipment should be provided of a grid or other device to force that samples to be submersed, avoiding buoyancy, but also to recover the samples at the end of the test.
The bath for Dye Ingress Test should normally be capable to be sealed in order to apply vacuum and/or pressure cycles, with the purpose of generating a driving force to the dye ingress into the container.
The test conditions must be controlled in order to make the test repeatable, and they consist in:
dye concentration, test duration, number, time and conditions of vacuum/pressure cycles, temperature, samples handling and manipulation before, during and after test cycle.
At the end of the test, the excess dye on the outside of the samples is carefully removed and each sample is inspected externally and internally searching for evidence of dye ingress. In order to perform internal examination, the containers must be opened, thus Dye Ingress Test is destructive and cannot be repeated.
The evaluation of the result (Pass or Fail) is based on the manipulation and visual observation by the operator, thus Dye Ingress Test is subjective, and it is probabilistic.
Normally the conclusion is made by operator by visual observation only; as an improvement, in some case it is possible to extract some liquid inside the container (if the container is filled with liquid) and test with a spectrophotometer. This variation of the method can provide some apparent level of metrology (the absorbance level measured), but it does not overcome all the doubts about the effective capability of the Dye to ingress the defect, if present.
As very well commented by Dana Guazzo (PDA Letter 2010), the spectrophotometric limit of detection of the dye in no way can be assumed as a proof of sensitivity of Dye Ingress Test.
Rather, ‘leak test method sensitivity is the demonstrated ability of the test to identify packages with specific defects among a random mix of no-leak and with-leak containers’.
The main advantage of the Dye Ingress Test consists in the low cost of equipment, but it is labour intensive and difficult to validate.
As a conclusion, Dye Ingress Test must be replaced, in Pharma industry, by modern Deterministic Method.
