User A: I tested the same sample, why do I get different spectra every time!

User B: The sample didn't move there, but the spectral intensity changed after repeated testing!

User C: Is there a problem with the instrument? The repeatability is poor!

用户 D/E/F：XXX…XXX…

Many users of Fourier transform infrared spectrometers may have similar doubts when conducting liquid testing. The professional technicians who provide Fourier transform infrared services must have also received such questions from users. In this issue, we will talk about why sometimes the repeatability of infrared liquid testing is not good, and how to solve it?

Figure 1: Continuous testing of multiple spectra using the same liquid sample, the same instrument (ATR attachment), and different testing personnel

As shown in Figure 1, different personnel testing the same sample will yield different results. The left spectrum has poor repeatability, while the right spectrum has good repeatability. What is the reason for this? Let's start analyzing from the spectrum. Firstly, we compare the first spectra measured by two different personnel together (as shown in the left figure of Figure 2), and we find that the difference in spectra is very small; Then we compared the last spectra measured by two different personnel together (as shown in the right figure of Figure 2), and at this point we found a significant difference in the intensity of the spectra. According to Lambert Beer's law, a decrease in spectral intensity indicates a decrease in the content of the component. By now, I believe everyone should have thought of the key to the problem——The sample has evaporated!

Figure 2 Comparison of the first and last spectra measured by different personnel

How to ensure accurate testing of volatile samples during testing? For the scenario of testing volatile liquids with ATR accessories, we only need a small component to solve the volatilization problem——Cover the liquid with a hollow lid to prevent rapid evaporation of the liquid.

Figure 3: Small Cover Plate for ATR Liquid Testing

If a liquid pool is used to measure liquids,It is necessary to ensure that the liquid pool window is sealed without leakage, the lid covers the inlet and outlet to prevent evaporation, and there should be no bubbles in the liquid pool.

In addition, some liquid samples are highly sensitive to temperature,During testing, it is necessary to maintain the stability of the sample at a certain temperature condition for testing，Especially when conducting quantitative calculations, it is necessary to use temperature control accessories during testing.

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