Understanding FTIR Spectroscopy

Fourier Transform Infrared Spectroscopy (FTIR) is a versatile tool widely used in scientific fields for analyzing gases, liquids, and solids with minimal sample preparation. This guide explores the principles, applications, and challenges of FTIR, emphasizing its role in detecting phthalates and the importance of advanced sample preparation techniques for accurate results. The discussion covers molecular detection ranges, sampling techniques like ATR and Transmission Mode, and considerations in FTIR instrumentation for detecting phthalates under EU RoHS and IEC 62321.

What is FTIR?

The FTIR spectrometer, also known as the FT-IR spectrometer, plays a crucial role in various scientific fields, including organic synthesis, polymer science, pharmaceuticals, and analytical chemistry. Its versatility makes it indispensable for monitoring final products and intermediate reaction products and conducting pre-screening for analytical testing.

Its capability to analyze various samples, including gases, liquids, and solids, sets infrared absorption spectroscopy apart. This analytical method offers significant advantages, such as rapid analysis completion within seconds and the minimal need for sample preparation in most cases. The efficiency and versatility of FTIR have led to its widespread adoption, particularly in replacing dispersive IR spectrometers due to its higher signal-to-noise ratio.

Which Molecules Can be Detected Using FTIR?

The infrared region spans from 12800 to 10 cm⁻¹ which includes distinct segments:

The near-infrared region (12800 ~ 4000 cm⁻¹)
The mid-infrared region (4000 ~ 200 cm⁻¹)
The far-infrared region (50 ~ 1000 cm⁻¹)

Within this method, the absorption of infrared radiation can characterize molecular structures.

The absorbed energy changes the dipole moment of the sample molecule, rendering it infrared active. Notably, most molecules exhibit infrared activity, except for diatomic molecules like O₂ and N₂, which possess a zero dipole charge.

Does FTIR Require Sample Preparation?

As previously discussed, FTIR is a popular chemical analysis and screening method due to its minimal or no requirement for sample preparation. This section will delve into two standard sampling techniques employed in the FTIR method.

1. ATR (Attenuated Total Reflectance)

ATR measures the infrared radiation reflected at the interface between the sample and a high-refractive index crystal. This technique proves particularly advantageous for analyzing solids and liquids that lack transparency in the infrared region. Diamond, germanium, and zinc selenide are the most commonly used crystals. The method involves placing a small sample amount on the crystal for analysis.

2. Transmission Mode

In transmission mode FTIR, the technician places a sample in the path of an infrared beam. The beam traverses the sample, and the absorption of infrared light at various wavelengths is measured. For this method, prepared samples must be thin and transparent to infrared light. KBr pellets are helpful when analyzing solid samples, while NaCl or CaF₂ windows are better for liquid analysis through transmission. However, achieving a thickness of around 500 microns, especially for materials like plastics, can be challenging. In such cases, a practical solution involves utilizing a thin filmmaker equipped with heated platens and a hydraulic press to prepare thin plastic samples.

Challenges and Considerations in FTIR

Despite minimal maintenance costs, FTIR requires careful consideration of certain aspects. One crucial component is the beamsplitter (compensator), whose selection depends on the specific application. The chosen beamsplitter must not interfere with the analysis window range. Commonly used materials for beamsplitters include CaF₂, BaF₂, ZnSe, and KBr.

KBr is a prevalent choice due to its extensive analysis window (5000 cm⁻¹ – 400 cm⁻¹). However, KBr’s susceptibility to water solubility makes it prone to damage over time, particularly in humid conditions. Some instruments incorporate internal heaters and adsorptive agents to mitigate this impact, preventing water condensation inside the system and avoiding spectral interference from water absorption. In environments with high humidity, attaching a vacuum to the instrument or purging it with dry air or nitrogen becomes a viable option.

Conversely, ZnSe exhibits more excellent stability in moisture, as it is virtually insoluble in water. Although ZnSe provides a smaller analysis window (650 to 4000 cm⁻¹) compared to KBr, its resistance to water solubility makes it a more durable choice in certain conditions.

Detection of Phthalates Using FTIR

FTIR proves effective in detecting various polymers and structures, including phthalates. Many companies develop spectral libraries that utilize certified reference materials in the method and development phase. Users can purchase these libraries for applications like the food industry, forensic analysis, and polymer studies. These libraries work seamlessly for pure samples and can detect the chemical group of the sample in a few seconds. Analyzing mixtures of polymers and chemicals poses challenges, necessitating sample preparation tailored to the specific chemical composition. If the analyte of interest is not in the available libraries, the technician should use a certified referenced material to prepare a calibration curve and add the data.

Detection Limit Challenges

Under RoHS regulation, butyl benzyl phthalate (BBP), di-n-butyl phthalate (DBP), di(2-ethylhexyl) phthalate (DEHP), and diisobutyl phthalate (DIBP) are subject to a limited threshold of 1000 ppm (0.1%). FTIR demonstrates capability in detecting these phthalates. Many instrument providers claim their FTIR instruments can detect these phthalates, considering the concentration of these phthalates is 10% and above in most commercial products. However, this claim lacks solid research and scientific backing, as the risk of lower concentrations close to 0.1% remains.

The ATR technique can achieve a detection limit between 5-10%, depending on the sample and the instrument. According to the Beer–Lambert law, increasing the light path length enhances accuracy, allowing transmission mode to reach levels close to 0.1%. Through this method, the plastic sample must undergo preparation using a thin filmmaker to create a thin sample with a thickness of around 500 microns. Considering the standard error in analytical measurements, a detection limit of at least 0.03% should be attainable to confirm these phthalates’ presence or absence confidently.

How to Improve the Detection Limit

Advanced sample preparation methods, such as those involving thin filmmakers and meticulous calibration, can help provide more reliable and accurate results. Following a comparison among top industries providing FTIR, it becomes evident that achieving this detection limit is not possible using basic sample preparation techniques. Therefore, analytical testing standards such as IEC 62321 and CPSC-CH-C1001-09.3 suggest using FTIR only as a pre-screening technique.

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