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Analytical Techniques Used in Questioned Document Analysis - Coggle Diagram
Analytical Techniques Used in Questioned Document Analysis
chromatography
principle
separate compound (with different polarity, ionic strength, solubility, size) by moving mobile phase (liquid or gas) through stationary phase
steps
sample with analyte is introduced onto the stationary phase, which interacts and retains the analytes differently
analytes eluted (washing components) through the column by adding mobile phase (eluent/solvent)
eluent carries components for which it has affinity
changing the eluent changes the analytes affinity for the stationary phase, resulting in separation of components.
The mobile phase that exit the column is called the eluate
types
adsorption (solute and solvent compete for active sites on the solid adsorbent)
partition (solutes are partitioned between two immiscible liquids)
size exclusion (separation of molecules is based on the difference in sizes of the molecules)
Thin layer chromatography (TLC)
Liquid chromatography (LC)
Gas chromatography (GC)
spectroscopy
principle
chemicals (such as those present in inks) can interact with visible light or light of other wavelengths
The wavelength absorbed depends on the atoms and the bonds present between them in the molecules being analysed
types
IR
non-destructive
characterization of ink samples as it provide chemical information
bonds that link the atoms together in the chemicals being analysed can vibrate in a variety of ways (symmetrical and asymmetrical vibrations, rocking and twisting)
able to probe in situ single or multiple layers of adsorbed/deposited species at a solid/liquid interface
raman
non-destructive
scattering of that light (Rayleigh scattering) is dependent on the vibration of the molecules present
problem that can occur in Raman spectroscopy is fluorescence caused by the laser
UV-Vis
destructive
molecules absorb light in the visible region of the electromagnetic spectrum
absorption of the light often extends into the ultraviolet region
relationship between concentration and absorbance as described by the Beer-Lambert law
LIBS
principle
able to detect and quantify elemental composition (both heavy and light elements) in a solid, liquid or even gas state using short laser pulse to create micro-plasma on the sample surface
mechanism
involves the creation of a high temperature plasma above the sample (created by laser pulse from a laser source)
when the micron-sized laser beam is focused onto the sample surface, a small volume (micrograms) of the sample mass is ablated
during cooling process of plasma, electrons of atoms and ions at the excited electronic states fall down into natural ground states, causing the plasma to emit light with characteristic spectral peaks
emitted light is collected and transmitted to the spectrometer/CCD package for LIBS analysis (each element in the periodic table has a number of unique LIBS’ spectral peaks)