OH vs. Water

The 1400 nm region contains the 1st overtone of the O-H stretch in water and hydroxyl, as shown above for water and ethylene glycol. Mixtures of water and alcohols, therefore, have over lapping features due to water and hydroxyl in this region. Water additionally has a unique peak near 1900 nm due to a combination of its O-H stretch and its H-O-H bending. The large peak near 2100 nm is a combination of O-H and C-H in the glycol. Notice that this pure glycol lacks an appreciable peak near 1900 nm where molecular water absorbs. With equivalent optical paths, you can see how much stronger water is compared to the C-H features.

Precision

The water peak at 1900 nm is about five times as large as its peak at 1430 nm. For example, 1 mAU in the 1900 nm region for n-propanol corresponds to about 20 ppm water for a 1 cm optical path, and about 90 ppm near 1430 nm. For water in methylisobutyl ketone (MIBK) or tetrahydrofuran (THF) these values are about 12 and 85 ppm/mAU, respectively. By doubling the optical path to 2 cm, if possible, the precision is also doubled. Take a conservative 3s drift of 1.5 mAU for a ClearView db, and we arrive at ±20 ppm at 1900 nm and ±125 ppm at 1430 nm with a 1 cm pathlength in situ probe.

The absorbance from an organic liquid near 1900 nm is highly variable. CCl4 has no appreciable absorbance in this region. Optical paths from 10 to 20 cm can be used to achieve <1 ppm precision. In contrast, many alcohols have significant absorbance. For example, ethylene glycol at 1900 nm has about 1.4 AU for a 1 cm optical path. Therefore, we could use a 5 mm optical path in a fiber optic probe to provide a 0.7 AU “baseline” absorbance in ethylene glycol. At 5 mm, the precision for water in ethylene glycol is about 24 ppm/mAU, so 1% water would add about 0.4 AU to the 0.7 AU resulting in an absorbance near 1.1 AU. You can see that adding >1% water to ethylene glycol, even in a 5 mm probe, would lead to an absorbance larger than our 1.2 AU desired limit. Thus, 2% water in ethylene glycol is measured with a shorter path, such as 2 or 3 mm, with some sacrifice in precision to achieve the greater concentration range.

Sample Clarity

A photometer computes the absorbance between the water wavelength and a “reference” wavelength (= -log Water/Reference) and converts that into water concentration. The “reference” wavelength is typically chosen in a “valley” which has little absorbance change, such as 1300 nm. The presence of light scattering entities entrained in the sample stream, such as bubbles, particles or immiscible phases, can “tilt” the entire spectrum between the water and “reference” wavelengths. Since we are already quantifying mAU changes at the water peak, it does not take much of a “tilt” to significantly impair a water calibration.  The greater the wavelength difference, the more the effect, such as at 1900 nm. We can always choose a “reference” wavelength on either side of the 1900 nm water to minimize this effect, but it is still important to install particle filters and sample temperature control.

By considering the issues raised above, we can provide you with a cost-effective solution to obtain the desired sensitivity for water over the concentration range of interest in most organic liquids.  Please contact us for questions or inquiries.

Our comprehensive GUIDED WAVE NIR UV-VIS process and lab analyzer offer optically matched components and a meticulously planned calibration approach, ensuring long-term efficiency and cost savings. Our advanced systems are engineered for continuous online operation, delivering real-time data of laboratory-grade quality, even in the harshest processing plant conditions.