This article is excerpted from a whitepaper recently produced by Tintometer about the performances of its PTV Series of process turbidimeters, which use three different incident light sources. These different light sources are needed to meet different regulatory requirements. Click here to download the full whitepaper.
By Michael J. Sadar
Turbidity is simply based on the scatter of an incident light beam by materials that are contained within a fluid matrix. There are two basic types of light scatter to consider.
Mie scatter is caused by the scattering of light off particles that are at least as big, or bigger, as the wavelength(s) that make up the incident light beam. The scattering is non-uniform around the particle and is disproportionally in the forward direction (versus the 90° direction).
Smaller particles, whose size is approximately 0.10 the wavelength of the incident light, will scatter more symmetrically. This is known as Rayleigh scattering. It is Rayleigh scattering that occurs when light scatters off the air molecules in our atmosphere that causes the sky to appear blue. Rayleigh scattering is more dependent on wavelength.
A general rule with Rayleigh scattering is that shorter wavelengths are more effectively scattered relative to longer wavelengths of light. This means that a turbidimeter with a longer wavelength of incident light will read slightly lower (because it is less sensitive) than a turbidimeter with a shorter wavelength of incident light.
So what does all this light scatter mean? When looking at real-world samples, they are composed of particles that are a mix in sizes. This size mixture will generate light scatter in all directions, including the regulatory scatter angle of 90° (for turbidimeter compliance). The detection angle of 90° is sensitive to both Mie and Rayleigh particle scatter for turbidimeters that use light in the 400-900 nm range.
When a calibration is performed, any differences in light scatter between instrument types are further normalized by the calibration standard. The important characteristic of the turbidity calibrant is that it is composed of particles that cause both Mie and Rayleigh scatter. Formazin turbidity standards (or stabilized versions of formazin) contain a broad range of particles that cover both Mie and Rayleigh scatter principles and help to normalize the detector response, regardless of the wavelength of incident light used.
Thus, the detector angle, detector view area, and the use of a turbidity calibrant that causes both Mie and Rayleigh scatter will normalize the effects of light scatter from these different light sources.
Colour in samples can be caused by dissolved materials or by particles if they are capable of absorbing light. If the incident light beam contains wavelengths that are absorbed by these components in a sample, the quantity of light available to be scattered decreases. This will cause a negative interference in the turbidity response.
Stray Light is defined as light that reaches the scattered light detector in a turbidimeter that is not caused by particle scattering in the sample. Stray light is a positive interference. This can be caused by internal reflections of incident light after it passes through the view volume of the turbidimeter’s sample chamber. Light that is not columnated (it diverges) after it leaves its source can often miss an internal light trap (if designed into a turbidimeter) and eventually reflect back into the detector.
Light that is polychromatic (has many different wavelengths) will more readily diverge from parallelism than will a light source that is monochromatic (typically is composed of one wavelength). Thus, a white light source will have more stray light than a single wavelength light source.
Stray light is very difficult to quantify because there is always some true light scatter in the purist of samples. What can be performed is to compare measurements between technologies on a sample stream of essentially particle-free water. Particle-free water can be prepared through the filtration of tap water that is passed through a series of sub-micron filters to remove virtually all the insoluble and some soluble materials.
When measuring very low turbidity levels, the selection of the turbidimeter can have an impact on the reporting results. Aside from regulatory design requirements, the spectral output of the turbidimeters can impact the measurement floor of the instrument. It is important to understand the intended application and its respective measurement goals prior to selection of the turbidimeter.
Michael J. Sadar is with Tintometer Inc. They are represented in Canada by Cleartech. www.cleartech.ca. This article appears in ES&E Magazine’s April 2018 issue.