The assessment of surface quality plays a significant role in the clean process. Contact and technical surfaces near a product must be monitored to ensure an optimum clean process and a range of relevant measurement techniques and principles are available. The assessment of surface purity is a challenge.
A number of parameters need to be measured and monitored in order to control and understand clean processes.
Many processes in pharmaceutical production require the measurement of the concentration of airborne particles and airborne bioburden (colony-forming units). The limits for the monitored micro-contamination can be found in relevant standards and regulations such as ISO 14644.
An important aspect of controlling a production area is the cleanliness of technical surfaces. Contaminated surfaces pose a high risk of cross-contaminating the product. Not just bacterial contamination, but any contamination is a risk factor for the cleanliness/sterility of the end product. It is certainly true that only clean surfaces should be disinfected.
Detecting surface contamination is not easy
Unlike with airborne contamination, which is typically monitored using a particle counter, there is a broad range of procedures available to monitor surface cleanliness. Measuring contamination on technical surfaces presents greater technical challenges than, for example, determining airborne contaminants with an optical measurement system.
The detection of surface contamination has its own challenges which to date cannot be fully achieved by any one measurement method. The measuring principle for detecting contaminants in gases or liquids is based on different refractive indices for the measured medium and the contaminants. The medium to be tested is passed through a measurement cell, and stray light is scattered as the sample passes through a laser. The light intensity of the scattered light and the number of light impulses provide the measurement for size and number of contaminants.
It is not possible to take such samples from a surface and therefore this procedure cannot be used to detect contamination on surfaces. Basically two methods of contaminant detection on technical surfaces are available:
* Indirect detection
* Direct detection
One method looks at contamination directly on the technical surface, while the other passes contaminants to a substrate or to one of a number of measuring systems.
Challenges with sampling
The challenges with indirect detection methods are less to do with the downstream measuring system and more to do with the sampling of contaminants (i.e. lifting and feeding into the measuring system). The reason why it is difficult to lift contaminants off a measuring surface lies primarily in the surface-to-volume ratio of surface contamination. With increasing volume, the surface-to-volume ratio decreases for all contaminants because the surface increases by square, while volume increases by cubic factor. This is an important point as very small contaminants on surfaces (micron size) are monitored in clean processes. Some of the ever present van der Waals forces and electrostatic loads – which exist in varying intensities – generate such a high force of attraction that adherent surface contamination can only be lifted or removed by combining mechanical and chemical methods.
A practical example is pollination. Commonly, the majority of pollen has a diameter of 20μm to 50μm.
If pollen has landed on a vehicle, they will not get lifted off by wind even at high speeds on the motorway. A fuel cap that is left on top of the car, however, will be thrown off after just a few metres. The lower detection limit for monitoring technical surfaces in clean areas is approximately smaller by a factor of 10 compared to the average pollen diameter.
Detection through surface roughness is difficult
Another factor which makes it difficult to lift contaminants off surfaces and to obtain direct proof is the surface roughness of the measured technical surface. Having to determine contamination in clean areas in the micron range therefore presents a sizeable challenge. The surface roughness may well be within the same depth range as the size of the contaminants. The technical surface has ‘valleys’ into which contaminants may disappear entirely or in part.
For indirect detection methods which lift contaminants off the surface through contact and pass them to a substrate, surface roughness is a constraint in addition to the particle size. Direct detection methods based on optical effects or the application of liquids also experience restrictions caused by surface roughness: for optical methods in the visualisation of surface contamination and for the application of liquids in the influence on drop geometry.
Two indirect and two direct established methods are described below in order to make the properties of individual methods comparable for users. There are a number of additional methods on which the above characteristics of surface contamination have an equally restrictive effect.
A frequently used indirect method involves lifting surface contaminants off the surface with the help of a tape lift. Here a substrate – which will later be passed to a measuring system – is pressed against the measuring surface (using procedures with defined contact pressure). When the sample has been transferred to the tape lift, the substrates are examined with a microscope and both number and size range of contaminants are determined.
Drawback: limited reproducibility
The tape lift method makes sampling particularly easy and cost-effective. Many quality laboratories already have a microscope, ensuring low investment cost for this method. Another advantage of this approach is its high mobility. Tape lifts can be carried easily and used even at difficult-to-access measurement sites. Modern optical microscopes make it easy to detect surface contamination in the micron range. Optical microscopes can also be used to conduct morphological observations of surface contamination. Determining the presence of surface contaminants using the tape lift method has, however, limited validity as the detachment rate for the individual sampling points cannot be reproducibly demonstrated. In addition, significantly varying ratios between separation rate and surface properties must be expected.
Another indirect method includes a sampler and an optical particle counter. The sampler is designed in such a way that surface contaminants are first removed through air blasting the particles from the surface, then sucking them in, and finally fed to an optical particle counter. Optical particle counters have the advantage that they can detect and count contaminants of down to 0.1μm. This is another mobile method which generates measurement results directly at the measuring site and is less time-consuming than microscopy. However, this method also has limited reproducibility, since it cannot be verified what percentage of the particles on the surface were actually removed. In addition, there is a risk of further electrostatic charge caused by the air blast, resulting in far greater adhesion of the particles on the surface or their agglomeration.
Detection by water droplets
A method that involves less equipment is the direct detection of surface contamination through liquid droplets. A drop of a technical liquid is placed on the relevant surface. Based on certain geometric parameters in water droplets, a conclusion can be made about the presence of contaminants. The contaminants on the surface influence the surface tension of the droplet and thereby have an effect on the droplet geometry. It is not possible, however, to conclude the number and size of the contaminants.
Particle counters that are based on the scattered light principle represent another direct method. Here, the measuring surface is illuminated by laser light and any contaminants on the surface will reflect the light. Based on the incident light, the number and size of contaminants can be determined with a CCD camera. The measurement instrument reports the concentration of contaminants which are present on the surface and displays their size and shape in an image file.
The angle of incidence at which the light can be directed onto the surface is directly dependent on the existing surface roughness. So here the lower limit of detection is directly influenced by the surface roughness. Another limitation of the sidelight principle is the lower detection limit of 2μm for surfaces with very little roughness, such as polished stainless steel. As the roughness of the measurement surface increases, the lower limit of detection may well increase to up to 5μm.
Monitoring surface purity is an important component of process control in clean production areas. In particular, surfaces which need to be disinfected after cleaning require a defined surface purity level. Unlike other measurements in clean processes, such as the detection of airborne particles, no one measurement method for surface particle counting has prevailed to date. The main reason lies primarily in the challenges with regards to the detection of surface contamination. These are in particular the great adhesive forces on contaminants and the varying levels of surface roughness in the technical surfaces to be measured.
The currently available measurement systems for assessing surface cleanliness can be divided into two groups: direct and indirect detection methods. All available measurement methods and systems have in common, the issue that they cannot fully meet the technical challenges. The only option that users or anybody interested in such measurement systems has, is to evaluate the available systems for their individual requirements and purposes and to correlate the test results.
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