analytical techniques II Flashcards
define isotope
atoms of an element w the same number of protons and different number of neutrons
-have identical chemical properties to each other
define ionisation
the process whereby chemical species lose or gain an electrical charge
define dissocation
the process whereby a molecule separates into smaller fragments = fragmentation
what does a molecule need for MS
a POSITIVE CHARGE
what happens in mass spectrometer
-electrons bombard the sample and ‘knock’ electrons off the sample, leaving it with a positive charge
-sample then accelerated through w pos charge so can be detected at detector and deflected
what is the parent peak
this is the molecular ion peak = furthest to the right
the Mr of the molecule is the parent peak
what is the base peak
most abundant/highest up peak
fragmentation
fragmentation of the molecule is represented in the mass spectrum by peaks with a smaller m/z than the molecular ion
fragments can be produced by the breaking of almost any bond in the molecular ion
chromatography…
is a technique used to separate the substances present in a mixture
also used widely to identify a substance
used mainly in mixture of organic compounds
what is the stationary phase
-it is a solid or liquid the sample is applied to
-the substance ADSORBS to the stationary phase - stuck to the surface of and will not move when most strongly ADSORBED
what is the mobile phase
-a liquid or gaseous phase that is passed through or over the sample
-the substance is carried along by the mobile phase depending upon degree of solubility
thin layer chromatography principles
-stationary phase is plastic coated in solid material
-enables solvents that would damage paper to be used
-allows greater range of substance to be sep.
-faster
-w UV light - colourless compounds
Rf
-retention factor
-higher value = greater attraction + solubility in mobile phase
-lower value = stronger adsorbance to stationary `
Rf formula
distance spot moved/distance solvent moved
min=0
max=1
Rt - GC
retention time is the amount of time that a sample stays in the tube. they have a greater retention time if:
-adsorb more strongly to stationary phase
-are bigger so carried more slowly by the mobile phase
gas chromatography function
used for analysis of volatile, easily combustible compounds eg fuels and gases
HPLC
high performance liquid chromatography
used for analysis of large molecules and compounds that cannot be easily vaporised eg. proteins, fats, vitamins
-measures retention time
IR
infrared spectroscopy
helpful bc gives information about the functional groups present
energy from infrared…
is not enough to promote electrons to high energy levels BUT is enough to change the vibration of the bonds in molecules
atoms in the molecule can change position due to the bending or stretching of the covalent bonds and molecule vibrations
descriptions for IR absorption bands
strong, medium or weak
broad or narrow
NMR
nuclear magnetic resonance
fast method of analysis
qualitative only - only structure of compound can be determined, not structures or anything
v. expensive + hazardous bc of strong magnetic field
principles of NMR
the nuclei of the atoms have a property called nuclear spin - in magnetic field, they line up parallel or antiparallel to field
what happens inside the NMR spectrometer
the nuclei of the atoms are usually in a low energy state and aligned with a strong magnet. a radio transmitter = provides the energy to ‘flip’ nuclei into high energy state. over time, nuclei = flip back into lower energy spin and release pulse of energy that is detected
factors that affect retention times of components in the column
-length/distance of column
-molecular structure or size of molecule
-temperature of the gases over separation
-nature and chemical struc of stationary phase
-flow rate of carrier gas
