## 1 Introduction
### Sampling Methods in FTIR
#### 1.4.2 Reflection Methods  
All samples can be processed through transmission technique in which light waves is directly sent through the sample. Reflection methods, however, requires simple sample preparation and it can be either through internal reflection spectroscopy (attenuated transmission technique, ATR) or external reflection spectroscopy (specular reflection) for smooth and reflective surfaces. It can also be the combination of both internal and external reflections such as in diffusion reflection IR spectroscopy (DRIFTS) for rough surfaces [34]. * The Attenuate transmission reflection (ATR) technique:
* ATR was introduced in the 1960's but now being popular used due to its easier sample preparation. It can be used for qualitative and quantitative analyses of both liquid and solid gel coating samples in accordance with ASTM C494 Section 18.1.2 [35]. Its principle is based on internal reflection of light wave through air/sample, solid/sample or liquid/sample interfaces [36]. It was recently shown that attenuated total reflectance FTIR (ATR-FTIR) spectroscopy could be used for the detailed analysis of hardened fly ash GP [36] or to study pore characteristics or carbonation effect. On this premise, [PERSON] et al. [37] use this method to investigate the effect of natural carbonation on the pore structure and elastic modulus of the alkali-activated fly ash and slag pastes. The tested sample is placed in a reflective medium composed of a crystal (such as a diamond) with a high refractive index (usually greater than 2) [24]. Information is collected in internal reflection from the surface or thin layer by external reflection. Other materials could also be zinc selenide (ZnSe), germanium (Ge), and silicon (Si), with a higher refractive index and varying hardness and depth of penetration. ZnSe is not recommended for strong acid or base samples due to the possibility of surface erosion by the formation of zinc complexes. Rather, water insoluble AMTIR crystals made from germanium, arsenic, and selenium with similar refractive indices are preferred [38]. The crystal generates an exponentially decaying wave (evanescent wave) that projects orthogonally onto the sample [22]. IR is directed into a crystal or an internally reflected element (IRE) of a relatively higher refractive index and then absorbs a portion of the energy of an evanescent wave and transmits the rest to reach the detector [39]. The greater the depth of penetration (path length) onto the sample, the greater the wavelength of IR [38]. The advantage of this technique is that a spectrum can be generated with easy sample preparation by just placing the core sample on the holder and screwing it tightly to receive the radiation. Specular reflectance technique  
This requires the sample to be reflective or be on a reflective surface while the required information is collected through a thin layer as it involves externally reflection of incident light waves. Diffuse reflectance IR transmittance spectroscopy (DRIFTS) method  
The method has the advantage of higher resolution with no rigorous sample preparation, like the transmission method [24]. The sampling technique is through powder samples without any prior preparation. The sample is added to the sample holder and information is collected in internal reflection from the bulk of the matrix. The reflection of the light depends on the transmitted and reflected quantities by the sample and that also depends on the material bulk properties. The reflected rays are collected with an ellipsoid or paraboloid mirror, while KBR could be used to dilute any absorbent materials. The parameters being analyzed include compactness, shapes, reflectivity, refractive index, and absorption of the particles. The sample should be well-ground and fine to ensure that the wavelength of incident light is bigger than its particles. This is necessary to minimize Mie scattering [40]. They [40] reported that both transmission and ATR are closer in accuracy but at a disparity with the DRIFTS method [24].