Few factors are more important to operators than ensuring an instrument is working to specification and providing the correct measurement to an overall control scheme.
Finding the right method for process photometer validation is therefore a priority. Operators can realize significant gains by achieving that aim, including ease of equipment maintenance, replication of maintenance efforts, and increased confidence in the instrument’s measurement performance.
The most typical validation method that operators now use is verification of instantaneous reported instrument reading against a process sample measured offline. But the method comes with certain drawbacks:
- It is limited to only one sample concentration at a time
- It may not provide a true indication of how the instrument will perform under changing process conditions.
To more closely examine the pros and cons of various photometer validation and verification methods, we created a tech brief for download, “Validation of Inline Photometers for Increased Confidence”. It provides an in-depth look at key variables, including:
- Photometer Method & Design
- Validation of Wavelength and Blocking
- Validation of Photometric (Absorbance) Performance
- Optical Pathlength and Error Assessment
In this post, we will feature a sample of the various methods, but before we begin, let’s look at the definitions:
What is a photometer?
A photometer is an optical instrument designed to measure the absorption of light at a given wavelength after it passes through a fixed distance of sample. Different chemicals absorb light at different wavelengths, and through the application of Beer’s law, the concentration of a chemical in solution can be accurately determined. Photometers are highly applicable to a wide range of applications in process analytical chemistry.
What are different types of photometers?
Filter photometers are instruments designed to operate at one or more fixed wavelengths e.g. 280, 300, 330, 400nm.
- They are used as an online, real-time process instrument.
- They are a simpler and therefore more cost-efficient instrument configured to continuously monitor a fixed process stream as part of an overall plant control system.
- The expectation is that they will operate unattended over long periods of time without operator intervention or the need for maintenance and servicing.
- Process photometers use specialized industrial in-line measurement cells or immersion probes which are designed to withstand harsh process conditions e.g., extreme pressures and temperatures, hazardous/explosive and radioactive environments.
Spectrophotometers allow monitoring of a range of wavelengths within instrument specifications, e.g. 400-800nm. As they provide flexibility when working with a wide range of different samples, they are commonly used in chemical laboratories.
- They are precision instruments, designed to be manually configured and operated by a trained technician in the relative comfort of a laboratory.
- Spectrophotometers measure individual samples that are prepared and typically placed in a standard size 12.5 mm square glass cuvette before being inserted into the measurement path of the instrument.
- An offline sample-based measurement methodology operates, in nearly all cases, laboratory spectrophotometers. Feedback from measurements taken cannot be considered real-time.
Vary process concentration
The best way to validate a process photometer is to vary the process concentration itself between certain maxima and minima, taking samples at cardinal points and comparing the process photometer reading to laboratory data.
Potential con: This may not be practical for a continuously running production system.
Remove in-line process photometer measurement
An alternative method would be to remove the in-line process photometer measurement cell from the process line and introduce a series of verified samples or standard solutions in a controlled manner to confirm agreement.
Potential con: This method is not always desirable as the process must be interrupted and it may not be possible where line fluids are potentially hazardous to health or present disposal and other challenges. Both of these methods require process disruption and a considerable labor overhead.
Validation of the wavelength accuracy of a spectrophotometer is undertaken using a liquid sample containing a number of distinct peaks such as a solution of holmium perchlorate or a holmium oxide and/or didymium doped glass filter. Emission peaks enable some spectrophotometers to automatically calibrate the instrument, offer methods to validate the wavelength, and automatically generate a validation report.
Solid glass type validation filters provide a convenient and safe method to quickly validate wavelength accuracy. However, it is not possible to validate wavelength accuracy of a filter photometer using wavelength calibration standards. Most filter photometers operate at one or two fixed wavelengths and are therefore not able to resolve the distinct peaks that the wavelength validation standard provides.
For filter photometer designs that utilize fiber optic connections to the inline measurement cell, a portable fiber optic type spectrophotometer can validate wavelength accuracy.
Photometric Performance Validation
Let’s look briefly at three methods for validating photometric performance:
- Process samples
- Certified optical filters
- Certified standards
1. Process Samples
This is probably the most typical validation method employed in the field. A sample of the process stream is taken along with a simultaneous reading of the photometer output on the instrument display. The process sample is analyzed offline and the photometer reading is compared to the result of the analysis.
Potential con: This method is limited to only one sample concentration and may not provide a true indication of how the instrument will perform under changing process conditions
2. Certified Optical Filters
Certified filters are used to validate that a filter photometer is working to its absorbance specification. NIST have specified a range of robust glass type validation filters that are suitable for use with both laboratory spectrophotometers and field process photometers that use a standard 12.5mm type square cuvette filter holder.
This type of non-intrusive traceable reference standard verifies photometric accuracy and linearity, both of which are critical to measurement result quality.
Pros: The glass type validation filters specified by NIST exhibit long term stability and are simple and convenient to use. Metal on fused silica glass filters exhibit a relatively flat transmission profile over a wide wavelength range of 250nm-2200nm, compared to the neutral density type filters that are limited to 360-1100nm (do check the filter is suitable for the instrument under test).
Note: Process photometers that can be verified without the need to interfere with the process line allow regular trouble-free validation, assuring confidence while saving valuable time and resources.
3. Certified liquid standards
Liquid standards are commercially available to calibrate and validate a wide variety of optical instrument measurement units. They can be introduced into a photometer light path in one of two ways: Directly into the sample cell or using standard cuvettes or proprietary insertable liquid holding devices.
Pro: Although messy, using liquid solutions is specific
Con: Liquid Standards can exhibit quite wide variability in value, so care must be taken when using them..
Summing up: The pros of photometer validation and verification methods
- Reduced production costs: Filter type photometers provide valuable real-time measurements into an overall control scheme and assist in reducing production costs by improving product consistency and quality.
- Enhanced measurement accuracy: An understanding of the stray light issues surrounding filter-based photometers can greatly improve the approach to field validation and can enhance measurement accuracy.
- Improved confidence: The ability to use photometric standards certified by third parties to nationally maintained standards for measurement and performance validation allows a transparent correlation between offline and online measurements and therefore provides a greater confidence in the results provided by inline photometers.
- Enabling of corrective action: Using multiple validation points can help describe non-linearity exhibited by a photometer and allow action to be taken to correct it. The use of filters and absorbance standards to validate photometers removes the need to take photometers offline and handle potentially hazardous materials as test samples.
Do you have questions for us about photometers, or other measurement instrumentation and analyzer solutions? Please contact us. Our experts stand ready to assist.