**3. Experimental processing**

**Figure 1.** Schematic diagram of a horizontal ATR sampling accessory illustrating the important parameters [16].

Diamond 2.4 45000–2500 1.35–1.66 [30][31][32] Germanium (Ge) 4 5500–870 0.65–0.73 [30][31][32] Zinc Selenide (ZnSe) 2.41 20000–650 1.22–1.66 [30][31][32] AMTIR (As/Ge/Se glass) 2.5 11000–750 1.46 [30] Silicon (Si) 3.4 - 0.84–1.17 [30][32] Thallium bromoiodide (KRS-5) 2.37 20000–250 1.22–1.73 [30][32] Cd telluride (CdTe) 2.67 10000–450 - [31] Saphire (Al2O3) 1.74 25000–1800 - [31] Zinc Sulfide (ZnS) 2.2 17000– 950 2.34 [31][32] Cubic Zirconia (ZrO2) 2.15 25000–1800 - [31]

Since the IR beam should penetrate the sample, the penetration depth (*dp*) is one of the important parameters in ATR-FTIR spectroscopy. The measure of the depth that the infrared

1

2 sin

p

( )

2 2 <sup>2</sup> <sup>1</sup> <sup>21</sup>

 q *Wn n* <sup>=</sup> é ù ê ú ë û

where *dp* is the depth of penetration, *W* the wavenumber, *n1* the refractive index of the ATR

Each of the parameters mentioned above has important messages to teach us about the ATR technique and its application. Readers interested in details of the theory of ATR should consult

1

*n*1 .

**Wavenumber range (cm-1)**

**dp (µm) References**

(2)

**index**

**Material Refractive**

206 Emerging Pollutants in the Environment - Current and Further Implications

**Table 1.** Relevant properties of some common ATR crystals.

beam enters into the sample is defined by equation (2):

crystal, *θ* the angle of incidence, and *n*21 the fraction *n*<sup>2</sup>

the respective literature [16, 25, 27].

*p d*

One of the advantages of the ATR-FTIR technique is that an experiment can be easily conducted to study the interactions between a chosen probe molecule and the surface of different metal oxides. The whole procedure consists in the preparation of a thin film of nanoparticles of the chosen metal oxide on the ATR crystal. This thin film should be stable, at least during the experiment, and its thickness should allow the penetration of the IR beam to reach the interface, e.g., the sample solution above the oxide layer. A thin homogeneous layer of the nanoparticles on the ATR crystal is generally produced from their suspension in an adequate solvent. This suspension is carefully drop-casted on the IRE material. Examples of the preparation of these thin layers, especially those made of TiO2, can be found elsewhere [18, 19, 23, 33].

It is worth noting that the contact between the probe molecule and the layer can lead to a change in some operational parameters such as pH, temperature, and ionic strength of the supernatant solution. Therefore, studies on adsorption phenomena are better carried out employing flow cell reactors either in the liquid or the gas phase (see Figure 2a) where the solution or the dispersant circulate continuously over the layer. This allows the control of the above-mentioned parameters and the monitoring of the evolution in time of the system under different conditions. As an alternative, a sample batch system can also be employed where the inlet and outlet are closed (Figure 2b) [23].

**Figure 2.** Flow cell reactor for ATR-FTIR spectroscopic studies (Reproduced from [23] with permission of the PCCP Owner Societies).

Prior to coating the ATR crystal, a spectrum of the blank ATR crystal is collected for spectral processing. Mainly, two different approaches can be used for the spectral processing. The first one is the normalization of the spectra of the ligand to that of the matrix (solvent at the pH of interest in the liquid phase or dispersant in the gas phase) from which a spectrum is collected. The probe molecule is then introduced and the corresponding spectrum is collected. The spectrum of the probe molecule is then referenced to the background spectrum (solvent/ dispersant). The second approach is as follows: after preparing the thin film, a spectrum of the solvent at the pH of interest (or of the dispersant in the gas phase) is collected; the probe molecule is introduced and a spectrum is collected; the single beam spectrum of both solvent/ dispersant and of the probe molecule in the solvent/dispersant is referenced to the blank ATR crystal to obtain the absorbance spectra of each. Subsequently, the absorbance spectrum of the solvent/dispersant is subtracted from the spectrum of the probe molecule. To collect spectra for the probe molecule alone the same experimental process is used but without the nanopar‐ ticle thin layer [30].
