LECTURE 9: SEPARATION MATHODS // CHAPTER 9 Flashcards
Three major techniques used in forensic science labs
1) liquid phase extraction
2) solid phase extraction
3) chromatography
1) Liquid phase extraction
- Involves separation of two or more substances (analytes) in which two solvents that do not dissolve in each other compete for the analyte
- The distribution between the two solvents is called partitioning
Partitioning occurs according to chemical properties, mainly
polarity and ph
Polarity:
Tendency of a compound to behave like a magnet
Results from polarity of bonds and asymmetry of a compound
Excess of electron density on the side of a compound
Organic compounds tend to be non-polar
‘like-dissolves-like’; non-polar/ polar dissolves more readily and to a greater extend in non-polar/polar solvent
Mixture of polar and non-polar analytes can. Be separates using a polar (e.g. Water) and non-polar solvent (e.g. Methylene chloride)
pH
Measure of acidity or alkalinity of a fluid relative to water
pH = -log10 H+ concentration in mol/L
Negative logarithm of hydrogen ion concentration
Acidic = ph between 1&7 releases h+ or h3o+ into water upon dissolution
Neutral = ph 7
Alkaline = ph between 7&14 takes up h+, producing oh- upon dissolution
2) solid phase extraction
relies on adsorption
Adsorption
substance adheres to surface of solid phase
Absorption
substance drawn into solid phase
chromatography techniques
- Thin layer chromatography (TLC)
- Gas chromatography (GC)
- High performance liquid chromatography (HPLC)
All chromatograph systems consist of
- A stationary phase
- A mobile phase
- A way of getting the mixture into the system
- A away of telling if the compound in a mixture are coming out of the system (detector)
Analyte:
substance(s) being seprated
Solvent:
liquid solution used to dissolve all or part of an analyte
Solute:
describes substance dissolved in solvent, synonymous with analyte
Polarity (define)
- Tendency of a compound to behave like a immature magnet, with positive side and a negative side
organic
tend to be nonpolar or slightly polar
inorganic
range from nonpolar to very polar
symmetric compounds
tend to be nonpol;ar
polarity is caused by
an excess of electron density on one side of a molecule and therefore a deficiency on the other side
- Side with the excess electron density has a negative charge
- Side deficient in electrons has a net positive charge
like dissolves like
this means that polar compounds have a greater affinity for other polar compounds and in this case of solubility it means that polar solutes will dissolve more readily and to a greater degree in polar solvents
Acidic substance
one that releases hydrogen ions (they become hydrated in presence of water so they are in form of H2O H or H3O+ called hydronium ions) when does solved in water
Acids
have a ph value between 0-7
Alkaline or basic substances
one that releases hydroxide ions when dissolved in water and it’s ph is 7 to 14
Neutral substance
one that releases neither H3O+ no hydroxide ions (OH-) when dissolved
The word chromatography literally means
‘to write with colour’
Stationary phase
is a finely divided solid material or viscous liquid that is contained within a long column
Mobile phase
is a liquid or gas under pressure it moves through the stationary phase carrying the analyte mixture with it and depending on their affinity of the stationary phase components analyte move slowly or quickly through the column separating from other components along the way
