What is Pathlength and Why is it Important? … When Selecting a Sample Interface

What is Pathlength?

Pathlength is traditionally the distance the light travels through the sample. For Guided Wave’s sample interfaces (insertion probe or flow cell) the pathlength is the term used to define the volume of the sample exposed to the analyzer’s light beam (or lamp). The light beam has a fixed diameter, so adjusting the length of the sample interface determines how much of the sample is measured.

Why is Pathlength Important?

Pathlength is selected to ensure absorbance values are within the dynamic range of the detector. The pathlength selected by Guided Wave is often the best compromise between long enough for the lowest expected concentration and short enough for the highest expected concentration. This compromise ensure absorbance values are within the dynamic range of the detector. Thus, providing optimal measurement results.

Determining the Correct Pathlength

As the concentration of the analyte of interest increases, there are more molecules to absorb, scatter the light, and otherwise attenuate the signal. Conversely, as the concentration decreases there is less interaction, so we must increase the pathlength to get enough attenuation.

The ideal target is 1 to 1.5 absorbance units (Au) for the absorbance peaks of interest, this gives the best signal to noise ratio. Of course, when dealing with multiple peaks compromises must be made, but we target to stay within the 0.5 to 2.5 Au range. Exceptions may be allowed depending upon the analyzer and application, contact Guided Wave for assistance.

Visual Tool for Understanding Pathlength

The photo above illustrates a laser beam being attenuated by soap. As the laser penetrates deeper into the sample less light is available for collection. If we apply this example to spectroscopy, the pathlength would be the physical distance between where the laser beam enters the sample and the location of the detector.

Three Examples of Pathlength Placement:

  1. If the detector is placed just below the surface of the liquid, a great deal of light can be collected. However very little of the sample will interact with the laser. This is not ideal because we likely will not have enough absorbance, signal, for good readings.
  2. If the detector is placed at the very bottom of the cylinder the opposite occurs, almost none of the laser light will reach the detector and we will have no signal. It would be difficult to distinguish between the laser being on or off.
  3. Ideally, the detector should be placed near the 4 mL on the cylinder. By locating the detector a distance of approximately 10 mm under the surface of the liquid, we provide a sufficient amount of the liquid to interact with the laser, while still collecting a large number of photons.

Another example Guided Wave recommends two different pathlengths for APHA color applications.

Available Pathlength Sizes

Typical pathlength sizes are 2, 5, 10, 20, 30, 50, and 100 mm. Custom pathlengths are available. For well-established applications Guided Wave typically knows what the pathlength should be. For new applications Guided Wave can make educated assumptions based on its 35+ years of experience. Occasionally test samples of the analyte are required to determine the best pathlength selection.

The Math and Science of Pathlength

Using Spectroscopy to measure physical properties and chemical concentrations depends on our ability to define a mathematical equation. The equation relates the variation in the chemical composition of our sample with the changes in how light interacts with the sample. This is defined by Beer-Lambert’s Law.

Beer-Lambert Law

  • The relationship between chemical concentration and spectral absorbance is defined by Beer-Lambert Law.
  • The Molar Absorptivity represents how sensitive the chemical is to a specific wavelength of light.
  • This is a constant/inherent property defined by physics. Pathlength and concentration are the remaining two variables that alter the absorbance levels measured by the spectrometer.
    • We cannot control the concentration, as it is the value we are attempting to measure.
    • This leaves pathlength as the variable to adjust to achieve absorbance values within the dynamic range of the detector.

Still Need Help Selecting a Pathlength?

Guided Wave selects a pathlength that provides the best solution from a technical and economic standpoint. Finding the balance between the signal-to-noise of the measurement and cost to manufacture, as well as accessibility for cleaning/maintenance by the user is always Guided Wave’s priority.  For more information contact us.