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The water break test is a visual inspection method. The clean tile on the left demonstrates how blue-dyed water continues to wet the part in sheets. The tile on the right has residue, so the water breaks up in droplets and streaks.
The methylene blue test is a semi-quantitative, ingredient-specific method of checking for the lack of cleaner concentration.
A recent webinar presented by Thomas McGuckin, vice president of research at International Products Corporation (IPC) in Burlington, N.J., provided a clear understanding of the most widely used cleaning validation methods, their advantages and disadvantages, and why they are used. IPC manufactures specialty cleaners for a diverse range of critical cleaning applications.
Cleaning validation can take on a number of definitions, depending on the industry a company serves, as well as what the company is cleaning, how it is cleaning and its specific regulatory and customer requirements. Generally, cleaning operations can be classified into two broad categories—those regulated by the FDA and those that are not. The detection limits for these two groups can be vastly different. Typically, FDA-regulated companies have very low detection limit requirements. They need methods that quantify potential cleaner residue down to parts per million (or even parts per billion). Industrial cleaning operations generally do not have such stringent requirements; acceptable cleaner concentrations may be within several percent or considered unimportant as long as the part rinses clean and looks clean at the end of the cleaning process.
Another significant variable is what a company analyzes. Some are concerned with the specific concentration of the cleaner. Others are concerned with the lack of cleaner—they may check rinse tanks or rinse water to verify that cleaning solution is not left in those tanks. Others are not as concerned with the cleaner concentration in either the cleaner baths or the rinse water baths, but rather the lack of soil or cleaner left on the part that is being cleaned.
Because the variables differ significantly, the appropriate validation technique for the application can be difficult to determine. Essentially, the choice is between qualitative and quantitative techniques. A basic visual inspection of parts is sufficient for some customers, whereas others need to know the exact concentration of the cleaner in different solutions.
Below are overviews of 10 cleaning validation techniques that span many different industries.
Visual Inspection
The most basic, although quite reliable, validation technique is visual inspection of the part for soil. This method is performed systematically using what is called the water break test. After a part has been thoroughly rinsed, the surface is watched while the final rinse drains off. If the last trace of water continues to wet the part in a continuous sheet instead of breaking up in droplets and streaks, the part is said to have a “clean water break” and is considered clean.
This test provides immediate results. It can be done tank-side, the cost is very low, and it only requires the operator’s time. The results are qualitative, which is sufficient for some applications. In most cases, this test only detects gross contaminants. Another drawback is that this test is subjective and operator-specific.
Foam Test
Similar to the visual test, the results of the foam test can be achieved very quickly and require minimal cost—only operator evaluation. However, soils and hard water can potentially interfere with the results. Hard water will reduce the foam, and soils can either reduce or contribute to the foam. This method is a subjective test that is operator specific—one operator may consider the foam significant, believing that it is time to change the cleaner bath, while another may think differently.
Titrations
The titration evaluation method is more analytical than the previous two. Its purpose is to check for the cleaner concentration. Titrations are quantitative, and the detection limits are at about 0.1 percent. Titrations are quick, easy and inexpensive; however, they require a calibration curve to determine the concentration. Calibration curves are usually available from cleaner manufacturers, but they are also easy to develop.
Titrations work best on alkaline (pH above 8) or acidic (pH below 6) cleaners and do not work well with neutral cleaners. These tests are ideally suited for lab work and require glassware, reagents, solvents and indicators. They cannot be performed easily at the tank. Certain types of soils may interfere with titrations, and handling and waste disposal requirements must be considered for the solvents involved.
Refractive Index
The refractive index method provides quantitative results as it measures cleaner concentration in the cleaning baths or the rinse tanks. Faster than titrations, this method requires minimal technique and can be performed right at the tank. It also requires a calibration curve, but involves no disposal costs. A small quantity of the cleaning solution is applied to the refractometer to take a reading. Hand-held refractometers are fairly reasonable in price, and digital versions are available.
Drawbacks with this method typically involve the ability to achieve accurate readings. In general, detection limits range between 0.25 and 0.50%. Foam, excess soils and cloudy solutions can interfere with the reading, and sufficient lighting is necessary.
Conductivity
Conductivity is a popular cleaning validation technique and is, in many ways, similar to the refractive index. It provides quick results in checking for cleaning concentration, and it requires a minimal amount of technique. A calibration curve is used to achieve quantitative results. Unlike the refractive index method, the conductivity detection limits can be as low as 50 ppm. The costs are low to moderate, depending on what type of instrument is purchased. The hand-held conductivity meters are fairly reasonable in price. Inline meters are also common and provide readings in real-time, continuously.
This technique is popular with both FDA and non-FDA regulated companies. FDA regulated companies may use this technique as a primary evaluation of the cleaning concentration, and if this concentration exceeds specifications or control limits, the company may refer to another, more analytical cleaning validation technique.
Drawbacks include the need for higher quality water (ideally, distilled or deionized). Results will not be accurate with relatively hard tap water combined with a low concentration of cleaner. Since conductivity varies with temperature, the meter’s automatic temperature compensation (ATC) factor should be used. Each meter has a default ATC, but the accuracy of this factor should be verified for each cleaning application. And similar to titrations, conductivity is not suitable for pH-neutral cleaners.
Methylene Blue
The methylene blue test is predominantly used on rinse tanks to check for the lack of cleaner concentration. It is semi-quantitative, and the detection limits are between 10 and 50 ppm for the visual method. What is unique about this test is that it is ingredient-specific; it will not work on all cleaners. The tester must know certain information about the cleaner’s formulation to be assured that this test will work. The test requires solvents (chloroform or n-propyl bromide), so handling is an issue. It also requires sufficient lighting, so it might not be ideal for tank-side evaluations.
The test can be enhanced by a UV visible spectrophotometer, which will help to lower the detection limit (typically less than 10 ppm). This technique is commonly used by FDA-regulated companies. It is one of the least expensive choices for companies that need to have low detection levels.
In addition to the spectrophotometer, other requirements include lab equipment, a centrifuge and a calibration curve. Results take a little longer than the manual method, particularly if a calibration curve is being developed for the first time, but results can still be achieved within 30-60 minutes. Solvents are required, but the volume of the solvents is reduced significantly if using the analytical equipment.
Black Light
The black light cleaning validation technique can be used to check for soil or cleaning residue. Black lights are typically available with two different wavelengths—254 or 366 nanometers. When evaluating parts, users are typically looking for soil or cleaner residues. Black lights are also used to assess the coverage area of cleaners in new equipment.
Results are qualitative, and the costs are low to moderate, depending on the black light that is purchased. Low lighting is necessary to evaluate the parts. At times, cleaner residue or the soil may not be fluorescent, which is required for inspection. However, fluorescent pigments can be purchased that work at very low concentrations. A small amount (1 ppm) of a pigment can be added to a cleaning solution to check for coverage inside tanks or any residues remaining on parts. Potential hazards from black lights include skin and eye exposure problems.
Total Organic Carbon
Total organic carbon or TOC, is used to check for organic carbon, typically in rinse water from either the tanks or from the parts. It provides quantitative results and very low detection limits (parts-per-million concentrations), and the results are obtained quickly. This method is used predominantly by FDA-regulated companies. It is not ingredient-specific, so if a high result is obtained, the source is not necessarily evident (it could be from the cleaner, soil or contaminant). Instruments required for this method make it more expensive than any of the previous methods mentioned. This method is best suited for laboratory work and only works on water-based cleaners. When mentioned, solvents are used as a final rinse solution. In these cases, TOC is not an appropriate cleaning validation technique.
Gas, Liquid and Ion Chromatography
Gas, liquid and ion chromatography instruments check for cleaner concentrations, typically analyzing rinse water or swab residue. These methods are the most versatile techniques available providing quantitative results in parts-per-million or even parts-per-billion concentrations. They do evaluate specific cleaner ingredients, and once instrument methods are developed, results can be available in several minutes.
These validation methods are run predominantly by FDA-regulated companies. Costs are relatively high for both initial purchase and upkeep. High-purity solvents, reagents, and high-purity water are necessary to run these tests properly. Typically, companies have a full-time technician running the instruments, and method development can be time-consuming.
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