Chapter 29 - Chromatography + Spectroscopy

Rf value equation

distance travelled by substance / distance travelled by solvent

thin layer chromatography (TLC)

adsorption = process by which silica holds substance to its surface by hydrogen bonds

stationary phase = solid adsorbant substance coated on glass sheet e.g. silica

mobile phase = suitable solvent

Rf value for TLC

-different substances have different adsorptions = stronger adsorption, substance moves slower, lower Rf value

e.g. phenylamine will travel further than tetrachloromethane as it can form hydrogen bonds

how to make colourless components visible?

-view under UV light

-dried chromatogram sprayed with ninhydrin which turns them purple

gas chromatography (GC)

-used to separate volatile organic compounds in a mixture

stationary phase = column containing high boiling point liquid adsorbed onto solid

mobile phase = inert gas that carries components

order that components leave column

from most volatile to least volatile

-most volatile = less soluble in liquid stationary phase so less interaction so faster travel in column so less time taken

info from gas chromatogram

-retention time = time taken to travel - identify components

-peak integration area = identify concentration of each component

calibration curves

-area under peak is proportional to concentration of substance in mixture

-peak area is compared with peak areas from different standard solutions (different known concentrations)

NMR spectroscopy

Nuclear Magnetic Resonance

-nucleus has nuclear spin that is significant if odd number of electrons

-electron + nucleus have 2 different spin states with different energies

-used in MRI body scanners

¹³C NMR

-number of peaks = number of different carbon environments (what carbon is bonded to)

-can have several carbons in same carbon environments = lines of symmetry

-chemical shift = type of carbon environment (in data sheet)

chemical shift + TMS

-chemical shift - units = ppm

-TMS = tetramethylsilane (CH₃)₄Si - used as standard reference - value = 0 ppm

deuterated solvents

-used in which ¹H atoms have been replaced by ²H atoms (deuterium D) so has neutron in nucleus

-produces no NMR signal as it has even number of electrons

2 solvents used = CDCl₃ used as solvent, D₂O used to identify -OH or -NH protons

proton NMR 

-number of peaks = number of proton environments (what hydrogen is bonded to)

-chemical shift = type of hydrogen environment

-relative peak area = number of protons in environment - given as ratio with numbers next to or as integration trace 

splitting patterns

-shows what is on adjacent carbon

n + 1 rule = n is number of hydrogens on adjacent carbon, + 1 is environment

singlet = no hydrogens on adjacent carbon

doublet = 1 hydrogen on adjacent carbon (CH)

triplet = 2 hydrogens on adjacent carbon (CH₂)

quartet = 3 hydrogens on adjacent carbon (CH₃)

spin-spin coupling pairs

-splitting patterns occur in pairs because each proton splits signal of other

-OH and -NH groups + use of D₂O

-D₂O used to identify -OH or -NH groups

process: proton NMR run as normal, D₂O added to mixture + shaken, D replaces OH and NH protons to form OD or ND

-so peak of OH or NH will disappear as deuterium does not absorb