Characterization goals phase identification

Figure 4.1. The flowchart illustrating common steps employed in a structural characterization of materials by using the powder diffraction method. It always begins with the sample preparation as a starting point, followed by a properly executed experiment both are considered in Chapter 3. Preliminary data processing and profile fitting are discussed in this chapter in addition to common issues related to phase identification and analysis. Unit cell determination, crystal structure solution and refinement are the subjects of Chapters 5,6, and 7, respectively. The flowchart shows the most typical applications for the three types of experiments, although any or all of the data processing steps may be applied to fast, overnight and weekend experiments when justified by their quality and characterization goals.

IR spectroscopy is one of the few analytical techniques that can be used for the characterization of solid, liquid, and gas samples. The choice of sampling technique depends upon the goal of the analysis, qualitative identification or quantitative measurement of specific analytes, upon the sample size available, and upon sample composition. Water content of the sample is a major concern, since the most common IR-transparent materials are soluble in water. Samples in different phases must be treated differently. Sampling techniques are available for transmission (absorption) measurements and, since the advent of FTIR, for several types of reflectance (reflection) measurements. The common reflectance measurements are attenuated total reflectance (ATR), diffuse reflectance or diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and specular reflectance. The term reflection may be used in place of reflectance and may be more accurate specular reflection is actually what occurs in that measurement, for example. However, the term reflectance is widely used in the literature and will be used here. 

Primary validation— the second part of discovery where a larger number of samples are used to verify or eliminate potential biomarkers. This type of experiment is normally focused on the best (statistical) biomarkers observed in the discovery or identified in the discovery experiment and may use different methodology to facilitate a larger number of samples to be used to increase the statistical confidence and eliminate weak or poor biomarkers from the discovery experiment. This is also the stage of the experiment where one starts using samples from related conditions, other geographical areas, etc. The goal of the validation experiment is to focus on the potential biomarkers that are most likely to answer the research question, and to reduce the number of possible biomarkers from the discovery phase before further time, effort, and money are invested in purification, identification, and characterization of the molecule (30-50 samples). 

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