Tips of FTIR measurement (easy sampling for NIR measurement)

July 28, 2021


Since near-IR light is transmissive to glass, it is widely used as the quantitative and qualitative analysis method to measure powder sample in test tube or sample bottle. On the other hand, generally the liquid sample is measured in transmittance method using thin cell with 1-3 mm cell length.

The commercial cell is too expensive to use as disposable, and it is difficult to clean the cell due to its thin cell length.

Measurement method by using hematocrit capillary cell is good solution to overcome it, and has some notices that the cell cap cannot be used for strong smell sample such as fragrance, or high viscosity sample cannot be inserted to the cell. This article shows the other good solutions by using the test tube with cell cap or easy washable laboratory dish.

Versatile FTIR spectrometer

Measurement example 1: Mini test tube

Figure 1 shows measurement example by mini test tube mounted on test tube holder. Cell length can be controlled by setting the flat surface cylindrical stainless spacer in the test tube. The light is irradiated from the bottom of the tube, transmit the solution, reflect on the surface of spacer, and then pass back to the bottom. Since the cell length (thickness) can be controlled to optimum length for near-IR, the good spectrum as the same as current transmittance angular cell can be obtained.

Figure 1. Measurement example of solution using mini test tube

Figure 2 and 3 show the solution spectrum obtained by this method, and good repeatability.

Figure 2. Measurement example using test tube

Figure 3. Reproducibility using Ethanol

Figure 4 show the reference data of the spectrum obtained by using hematocrit capillary cell. It can be confirmed that the saturated absorbance peak in the measurement method using hematocrit capillary cell is obtained without saturating in the test tube method.

Figure 4. Measurement example using hematocrit capillary cell

Measurement example 2: Laboratory dish

When the sample has strong flavor and the Amount is enough, it is more suitable to use the easy washable wide mouth case such as Laboratory dish. Figure 5 shows the mounted laboratory dish. Since the incident light is directly reflected on the surface of the flat bottom case such as laboratory dish, sample absorbance spectrum might not be measured. Therefore, the reflectance face is tilted to avoid it.

Figure 5. Measurement example of solution sample using laboratory dish

The cell thickness can be controlled by using metal reflectance plate with spacer on the laboratory dish. Figure 6 shows the measurement example of solution sample using laboratory dish. The optimum size of the spacer used with metal reflectance plate was approx. 400 µm (please note that the thickness of the spacer depends on the sample).

Figure 6. Measurement example using laboratory dish

About the Author

Spectroscopy Group