Exploratory Quantitative Fourier Transform Infrared Spectroscopy (FTIR) for Sodium Acetate

Wenfa Ng

doi:10.20944/preprints202609.1325.v1

Submitted:

15 September 2026

Posted:

16 September 2026

You are already at the latest version

Abstract

Transmission Fourier transform infrared spectroscopy (FTIR) is a very useful analytical technique for determining the types of functional groups present in a sample for various applications. However, the method remains qualitative, and lacks the ability to determine the relative abundance of different functional groups, or to quantify a specific compound in a sample. In this work, an attempt was made to develop quantitative FTIR as an analytical technique able to determine the relative amounts of an analyte in a sample, in this case, sodium acetate via the conventional KBr window transmission mode FTIR method. Results revealed that there was an infrared spectrum region (1700 to 2800 wavenumber) where there is response of percentage transmission with amount of sodium acetate added. Further analysis of this spectrum region revealed two distinct calibration windows of 1800 to 2000 wavenumber, and 2560 to 2680 wavenumber. Both calibration windows yielded good, linear calibration curves of good correlation coefficients of between 0.97 to 0.99. Hence, data in this work suggests that it may be possible to further develop a quantitative FTIR method to quantify the amount of sodium acetate in samples.

Keywords:

sodium acetate

;

calibration windows

;

linear calibration curve

;

correlation coefficient

;

transmission mode FTIR

Subject:

Chemistry and Materials Science - Analytical Chemistry

Introduction

Transmission mode Fourier transform infrared (FTIR) spectroscopy is a very useful tool for chemical analysis, particularly in identifying the functional groups present in the compound or surface.^{1 2 3} However, it remains a qualitative method where there is difficulty in quantifying the amount of compound present in the sample across the full FTIR spectrum.

This work attempts to explore the possibility of using part of the FTIR spectrum for quantifying the amount of compound present in a sample. The model compound here is sodium acetate, and conventional transmission mode KBr window FTIR was used. Results obtained suggests that across the full spectrum of sodium acetate, there may be possibility of quantifying the amount of sodium acetate present in the wavenumber range from 1700 to 2800 cm^-1.

By using an in-house MATLAB software implementing a ratio test algorithm, it is possible to identify two calibration windows within the full FTIR spectrum of sodium acetate. Both calibration windows: (i) 1800 to 2000 wavenumber, and (ii) 2560 to 2680 wavenumber, yielded linear and good calibration plots correlating percentage transmission and amount of sodium acetate added with good correlation coefficient of 0.97 to 0.99. Hence, there is possibility of the presented work here being further developed for quantitative analysis of sodium acetate in sample using the transmission quantitative FTIR approach.

Materials and Methods

Fourier Transform Infrared Spectroscopy (FTIR) Measurements

Transmission mode FTIR was used in this work to measure the infrared spectrum of sodium acetate (Sigma Aldrich, USA) To this end, the KBr window method was used, where 0.4, 3, or 5 mg of sodium acetate was mixed with different amount of KBr to constitute a KBr pellet after the two components were well mixed with the help of a mortar and pestle. The obtained pellet was placed in Bio-Rad FTS-3500ARX FTIR instrument, and 16 scans was done on the pellet with transmission mode.

Results and Discussion

Left panel of Figure 1 shows the overlay of FTIR spectrum of 0.4 mg, 3 mg, and 5 mg sodium acetate in a KBr window for transmission mode FTIR. Data obtained show good symmetry of the peaks and spectrum profile for wavenumber within the range of 1700 to 2800 cm^-1. But, there is a need to identify calibration windows within the obtained FTIR spectrum where we can obtain a linear response between percentage transmission versus amount of sodium acetate added to the KBr pellet.

To do this, an in-house MATLAB software (in software implementation section below) was written by me to process the FTIR spectrum from wavenumber 400 to 4000 cm^-1. Specifically, the computational test here was to calculate the ratio of percentage transmission at each wavenumber for 0.4 mg sodium acetate relative to 5 mg sodium acetate as ratio 1. Similarly, ratio 2 was calculated as the ratio of percentage transmission at 3 mg sodium acetate relative to 5 mg sodium acetate. For a good calibration window, plot of ratio 1 and ratio 2 should be parallel lines as this will translate mathematically to a linear calibration response between percentage transmission and amount of sodium acetate added to the KBr pellet. On the right panel of Figure 1, it can be seen that the ratio test did identify two calibration windows for sodium acetate. Specifically, calibration window 1 ranges from 1800 to 2000 wavenumber, and calibration window 2 ranges from 2560 to 2680 wavenumber.

Figure 2 shows the calibration plot for sodium acetate within the first calibration window from 1800 to 2000 wavenumber. In aggregate terms, good, linear calibration plot correlating percentage transmission with amount of sodium acetate added could be obtained across the entire calibration window with 6 test plots within the 1800 to 2000 wavenumber window. Good correlation coefficient of about 0.99 could also be obtained for the 6 calibration plots, indicative of good calibration performance, and a linear response between percentage transmission and amount of sodium acetate added.

Figure 3 shows the calibration plots for sodium acetate for the second calibration window that range between 2560 to 2680 wavenumber. Data shows good, linear calibration plots correlating percentage transmission and amount of sodium acetate added with good correlation coefficients of between 0.97 to 0.98. Hence, taking into account the calibration plots obtained for the two calibration windows, it may be possible to use transmission mode FTIR to quantify the amount of sodium acetate in samples.

Conclusions

Data in this work shows that it is possible to identify two calibration windows of different size within the infrared spectrum region of 1700 to 2800 wavenumber of sodium acetate. Each of the two calibration windows could yield linear calibration response between percentage transmission and amount of sodium acetate added with good correlation coefficient of 0.97 to 0.99. Data obtained in this work shows that it may be possible to use transmission mode FTIR to quantify the amount of sodium acetate in samples using either of the two calibration windows profiled here.

Software Implementation

Main programme loop

function main

[A, B, C] = xlsread("Input calibration data for qFTIR of Na acetate 13 Sep 2026.xlsx");

k1 = length(C);

j = 1;

for i = 2:k1

wavenumber = C{i,1};

intensity_std1 = C{i,2};

intensity_std2 = C{i,3};

intensity_std3 = C{i,4};

results(j).wavenumber = wavenumber;

results(j).sample1 = intensity_std1;

results(j).sample2 = intensity_std2;

results(j).sample3 = intensity_std3;

j = j + 1;

end

results = assess_calibration(results);

results_table = struct2table(results);

writetable(results_table, "Assessment of calibration for qFTIR of Na acetate.xlsx");

end

Dependent Function

function results = assess_calibration(results)

k2 = length(results);

for h = 1:k2

reference_intensity = results(h).sample3;

intensity_test1 = results(h).sample1;

intensity_test2 = results(h).sample2;

ratio_test1 = intensity_test1/reference_intensity;

ratio_test2 = intensity_test2/reference_intensity;

results(h).ratio_test1 = ratio_test1;

results(h).ratio_test2 = ratio_test2;

end

Funding

The author thank National University of Singapore for financial support.

Conflicts of interest

The author declares no conflicts of interest.

References

Mackie, D. M., Jahnke, J. P., Benyamin, M. S. & Sumner, J. J. Simple, fast, and accurate methodology for quantitative analysis using Fourier transform infrared spectroscopy, with bio-hybrid fuel cell examples. MethodsX 3, 128–138 (2016).
Bel’skaya, L. V., Sarf, E. A. & Solomatin, D. V. Application of FTIR Spectroscopy for Quantitative Analysis of Blood Serum: A Preliminary Study. Diagnostics 11, 2391 (2021).
Jordán Vidal, M. M. & Almendro-Candel, M. B. Semi-Quantitative Mineralogical Analysis of Ceramic Coatings and Their Raw Materials Using ATR-FTIR Spectroscopy. Coatings 16, 530 (2026).

Figure 1. Overlay of FTIR spectrum of sodium acetate at different input concentrations (left panel), and the use of ratio test to identify calibration windows of sodium acetate (right panel).

Figure 2. Panel of calibration plots correlating percentage transmission with amount of sodium acetate added for the first calibration window between 1800 to 2000 cm^-1.

Figure 3. Panel of calibration plots correlating percentage transmission versus amount of sodium acetate added for the second calibration window between 2560 to 2680 wavenumber.

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