The design of photometer instruments has a significant impact on their applicability. Recent design improvements provide better overall measurement performance and long-term measurement stability. So, when evaluating photometers, it helps to understand how certain features affect the validation and verification methods.
In our new tech brief, Validation of Inline Photometers for Increased Measurement Confidence, we explore the designs more in-depth as well as discuss their pros and cons. For a quick introduction to a few aspects of photometer instrument design, keep reading this post.
Simply put, an industrial photometer is used to measure light after it has passed through an industrial process stream. Photometers can be configured for a variety of purposes, with the configuration and technique used depending upon the specific measurements needed. This makes them applicable to a wide range of applications in process analytical chemistry.
Understanding the impact of Beer’s law
The science at the core of photometer instrument design is Beer’s law. Also known as the Beer-Lambert law, it relates the attenuation of light to the properties of the material through which light is traveling. Since different chemicals absorb light at different wavelengths, the application of Beer’s law helps determine the accurate concentration of a chemical solution.
For the majority of applications where absorbance characteristics obey Beer’s law, a simple zero-plus-one sample point will be sufficient to calibrate an online photometer. The key to obtaining a signal that is proportional to concentration is that the light reaching the photodetector must be monochromatic (single wavelength).
But there are exceptions that require non-linear calibration: Beer’s law does not describe the behavior of concentrated solutions (greater than 10-2 M) due to interactions between the absorbing molecules. It also does not apply when measurement is made at a wavelength where the slope of the sample absorption curve is very steep. In such cases, a non-linear calibration should be used. Industrial photometers typically include non-linear and piecewise linear calibration models.
Considering a few photometer instrument designs
Let’s take a look at a few design variations:
Spectrophotometers produce monochromatic light by using either a single photodetector with mechanically moving grating or prism, or a diode array detector if the instrument is more modern. They use almost exclusively broadband light sources such as halogen for visible (VIS, 400-700nm) and near-infrared (NIR, 700-2000nm) and deuterium for ultra violet (UV, 200-400nm).
Traditional process filter photometers
Traditional process filter photometers use narrow bandpass optical filters in conjunction with broad spectrum, high energy incandescent, halogen (VIS, NIR) and mercury vapor (UV) light sources to generate a facsimile of monochromatic light. In traditional designs, the light source is hot and a filter in close proximity to it is subject to potential damage over a relatively short period of time.
Modern filter photometers
Modern filter photometer designs utilize high performance specific peak wavelength light emitting diodes (LEDs) instead of broadband light sources and narrow band filters to produce tight, narrow band light emission. Unlike traditional process filter photometers, the LED source is cold and and does not damage or degrade other sensitive optical components in close proximity. A further advantage is that the process itself is no longer exposed to high energy light that can be potentially damaging to the process itself. This is particularly true for applications such as protein detection in bioprocessing that utilize UV light for the measurement
Addressing the issue of stray light
A problem with process filter photometers is the issue of stray light. It is the effects of stray light that often lead to dissatisfaction with photometer performance when deployed into a process environment. Typical bandpass filters only block to approximately 3 AU (0.1% of the total transmitted light) outside of the pass band and all remaining light below this level passes freely through the optical filter, creating non-linearity in absorbance measurement and limiting instrument range.
Narrow bandpass filters
Narrow bandpass filters are constructed to allow light to pass through a very small region or slot, typically 10nm wide, around a specific peak wavelength (the wavelength of interest), while blocking light from passing at all other wavelengths. When narrow bandpass filters are used in combination with process filter photometers that utilize LED light sources, there is virtually no stray light outside of the pass band region around the wavelength of interest.
As a result, LED light source photometers adhere to Beer’s law and provide native units of absorption as their baseline measurement without the need for additional absorption calibration. This means that any engineering unit can be correlated to absorbance while leaving a possibility for confident validation of the instrument’s performance.
Watch for decay
But a word of caution is warranted. Bandpass filters, while generally stable, can exhibit signs of decay over time with changes in peak transmission and stray light blocking capability caused by environmental factors e.g., moisture or erosion from the light source itself. This is particularly true in traditional design filter photometers. Since bandpass filters essentially define the operating wavelength and performance of a process photometer, it is important to ensure they meet or exceed the original specification from the manufacturer.
Impact of broad spectrum light sources
As mentioned above, when the light reaching the detector is not monochromatic, Beer’s law does not apply. Photometers that use broad spectrum light sources necessarily produce a significant amount of stray light passing beneath the out-of-band blocking capability of the filter and onto the detector. Such photometers must, consequently, be calibrated at multiple points for the instrument to work in units of absorption, noting that absorption is proportional to concentration. This requirement for signal correction presents some specific challenges when validating instrument performance.
In sum, an understanding of the stray light issues surrounding filter-based photometers can greatly improve the approach to field validation and can enhance measurement accuracy. Using multiple validation points can help describe non-linearity exhibited by a photometer and allow action to be taken to correct it. Provided stray light impact is understood, highly reliable validation measurements can be obtained with a resulting high confidence in the measurement results of the instrument itself.
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