Water concentration is perhaps the most common measurement made in the near-infrared (NIR) spectral range. This is due to its strong effect on product properties and chemical reactivity of the starting materials. From an analytical perspective, water is easy to measure due to its relatively strong signal compared to the hydrocarbon background.

Moreover, because water is commonly analyzed with a single wavelength, filter photometers are the instrument of choice. Guided Wave’s application note, “A Word (or Two) About Online NIR Water Measurements in Liquid Samples”, explains how we arrive at recommending a system, that is, a photometer with the proper wavelengths and a fiber optic probe with an appropriate sample path length. The following considerations affect the choice (and price) of the appropriate photometer and probe system:

Factors to Consider
• Background hydrocarbon spectral characteristics
• Concentration range of water and desired analytical precision
• Potential interference from hydroxyl species
• Sample temperature variations and clarity Analytical Goals
• Provide maximum sensitivity
• Select wavelength(s) to stay within linear range
• Minimize interference due to background hydrocarbon variations and sample temperature changes
• Use an optical path of >1 mm in the fiber optic probe for ease of cleaning and minimal entrapment of bubbles and particles

Cost Effective Solution
The application note explains these factors and analytical goals in detail. Thus illustrating a cost-effective solution for obtaining the desired sensitivity for water over the concentration range of interest in most organic liquids.