Selecting Calibration Standards For Optical Instruments
Optical instrumentation needs to be calibrated with reliable standards to ensure accurate measurements. Before selecting standards for an application, review these basics of primary standards and polymer-based standards from APS Analytical Standards Inc. (APS Analytical Standards Inc.; Redwood City, CA; 800-827-9283). Polymer-based standards were originally created for turbidimeters—instruments that measure relative sample clarity—but can now be used in many other applications that involve scattering and absorbence optical instrumentation.
Basics of Standards
Standards can be defined in a variety of ways. The Environmental Protection Agency (EPA), Standard Methods (SM), American Society for Testing Materials (ASTM), National Institute of Standards and Technology (NIST), and U.S. Pharmacopeia define standards according to characteristics that they create within their agencies. Most standards are NIST traceable.
To ensure exact measurements, analysts from these agencies test the standards in environmentally controlled labs using up-to-date equipment and procedures. These labs operate documented quality-management systems and are involved in many internal and external audits. Primary standards are synthesized from raw materials and are usually tested for suitability by academic and industrial laboratories.
The primary standard parameters that are most thoroughly defined are: stability, solubility, reproducibility, and photosensitivity. In addition to using primary standards, many companies use a subclass of standards known as secondary standards. These are referenced to primary standards and are usually used for verification purposes.
Although precautions are taken by testing agencies to ensure that their standards are pure, problems still occur in four main areas:
- First, raw material purity depends on the quality of the source. If an agency uses a sub-standard source, it is unlikely to produce a pure standard.
- Second, standards often require correct storage conditions as they may deteriorate over time. Without these conditions, the standards may not be stable for long. Standard stability is critical because fast-settling precipitates or photosensitive solutions are poor choices for standards.
- Third, user techniques are important because both the dilution of the solutions and the use of measuring instruments can lead to considerable error.
- Finally, availability is an issue, as researchers do not want to place their projects on hold for weeks while waiting for standards to arrive.
Polymer-based standards are an alternative to standards made of traditional materials, as the polymer materials mimic or exceed the properties of many existing standards. Companies such as APS supply polymer-based standards which are submicron, non-surface charged, solid spheres in matrixes of ultrapure water. The standards’ small bead size coupled with Brownian motion ensures that the polymer spheres stay in a homogeneous suspension. This homogeneity allows linear dilutions to be made.
Standards that have long-term suspensions are ready-to-use at their stated values. Lot-to-lot variance for APS standards is 1%; they are traceable to NIST particle size standards 1690 and 1691. APS and most other vendors keep retention samples for extended periods so that they can be used as a consistent reference.
A standard is a reference. Its credibility is based on its performance, reproducibility, and stability. Polymer-based standards provide a great deal of flexibility in this area. This is attractive to OEMs seeking alternative calibration standards, as application-specific standards can be developed in a short time frame.
Spectrophotometric Calibration Certification Standards
Companies are now replacing standards such as McFarland standards (barium sulphate) and potassium dichromate, as well as haze standards for the beer industry and formazin for turbidity calibrations with styrene divinylbenzene polymer standards. In-Spec is the trade name used for APS spectrophotometric standards. Amco Clear is the trade name used for APS turbidity standards.
Polymer standards can be used in a wide array of instrumentation including all scattering and absorbence optical instrumentation. The biomedical and pharmaceutical industries are the industries most interested in polymer standards. However, many other industries such as the food and beverage, petrochemical, semiconductor, and paper and pulp industries are also using the standards because of their ease-of-use and precision.
Using Standards In Turbidimeters
Polymer standards were originally manufactured solely for use in turbidimeters. Although, polymer standards are now used in many other fields, turbidimeters are still an important application area for them.
Turbidimeters are instruments that are used primarily in the water-treatment industry to monitor suspended particulates in water samples. Turbidity is defined as an "expression of the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample."
Simply stated, turbidity is the measure of relative sample clarity. It is caused primarily by un-dissolved, finely distributed particles known as suspended solids. When a light beam passes through a sample in a turbidimeter, the suspended solids scatter the light in all directions. The intensity of the light beam is reduced when the suspended solids scatter the light.
In the past, turbidity was considered a rather 'low-tech' evaluation, however the instrument design has since been modified in several ways. These alterations include simpler calibration procedures, self compensating light sources, LEDs and fiber optics instead of tungsten lamps, data storage devices, signal averaging, microprocessors, increased sensitivity, multiple photodetectors, and a more compact size.
Turbidimeter Standard Applications
In the food and beverage industry, polymer standards are used to calibrate instruments that monitor the clarity of liquids such as beer, wine, and juices. In the semiconductor industry, ultra-pure water and other process fluids must be closely monitored for contamination. Turbidity measurements can help ensure against the presence of contaminating particles. Turbidity can be an inexpensive tool in verifying other analytical techniques that fall beyond the scope of the water-treatment industry.
Within the water-treatment industry, turbidimeters are very important because the EPA requires water-treatment facilities to report turbidity data in surface-water plants. Failure to do so can result in heavy fines. If high turbidity are reported, treatment facilities can be closed. Since 1972, these regulations have gradually reduced the turbidity threshold for drinking water so that it is safer for the public.
Turbidimeters are configured as bench top, on-line (continuous), or portable instruments. Because they cost from $800 to $2500, they are affordable for most laboratories.
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