Instrumental Methods of separation and analysis Flashcards
List 5 methods of separation and analysis
- Mass Spectrometry
- Gas chromatography (CG)
-High performance liquid chromatography (HPLC) - Infra-red spectrometry (IR)
-Ultra-violet spectrometry (UV)
GC
Gas Chromatography
-In this technique gas is in mobile phase
- The stationary phase is a high boiling point liquid spread on silica gel packed into a long coiled tube of small diameter.
- The tube is kept in a temperature controlled oven (heat)
- The sample is injected into the column
-The column is heated to vaporise the sample and it is carried through by a gas such as nitrogen or helium as these are unreactive gases.
- The different components of the mixture separate out at different rates they are carried through the column.
- As each component leaves the column a detector records a signal and plots a chart called a gas chromatogram.
- Each component shows up as a peak.
What is Gas chromatography used for?
Gc is used to measure the level of alcohol in blood or urine and in drug test samples from athletes.
HPLC
High Performance liquid Chromatography
-Used to determine non-volatile components of a mixture.
-The particles of the solid phase are very small and so pressure is used to force the mixture (in a liquid solvent) through.
- The temperatures used is not as high as in GC so this prevents decomposition of substances at high temperatures.
-This column is not as long and is made of material that can withstand the high pressures used.
What is High performance liquid chromatography used for?
Used in food analysis - growth promoters in meat
IR
Infra-red spectrometry
-Infra-red radiation is invisible
-Qualitative analysis
- The bonds in organic compounds absorb infra-red radiation and begin to vibrate.
-Different bonds absorb different amounts and so vibrate differently.
-These vibrations produce a ‘fingerprint’ for each molecule and so the molecules can then be identified by comparing to those present in a database.
Infra-red spectrometry used for?
Infra-red spectrometry is used to identify plastics and drugs.
UV
Ultra-violet spectrometry
Quantitive analysis (what& amount)
-When a substance absorbs ultra-violet light, an electron may be promoted to a higher energy level and a spectrum is obtained.
- Maximum absorption tends to occur at a certain wavelength and this helps to identify the compound and its concentration, as absorbance is related to concentration.
What is Ultra-violet spectrometry used for?
It is used in the quantitative determination of organic compounds eg. drug metabolites and plant pigments.
Recrystallisation of benzoic acid and determination of its melting point,
principle & theory
Used to separate and purify organic solids.
- Principle: More solute is soluble in a hot solvent than in a cold solvent.
-Substance purified is dissolved in the minimum amount of hot solvent possible & then filtered to remove insoluble impurities.
- Done quickly to avoid solution from cooling.
- Filtered solution cools= crystals
- Crystals isolated by filtration and dried using a desiccator.
- Test for purity = measure melting point.
Benzoic acid
Apparatus & Materials
Apparatus :beaker, Büchner funnel, vacuum pump, filter paper, melting point tubes, thermometer , bunsen burner, clock glass.
Materials : benzoic acid, liquid paraffin
Recrystallisation of Benzoic Acid: Method
- 5g of benzoic acid crystals placed in a beaker.
- This was dissolved in the MINIMUM amount of hot water. Minimum so that the benzoic acid does not remain in solution on cooling to maximise the yield
- First filtration - This hot concentrated solution was filtered through a Büchner funnel (under suction). Insolubleimpurities remain on the filter paper.
- The filtrate was allowed to cool slowly and fully.(ice bath)
5 Second filtration- The excess solvent was filtered off and the crystals remain on the filter paper & were allowed to dry.
Method to determine the melting point of the benzoic acid crystals
- Some crystals were tapped into the open end of a melting point tube until there was about 0.5cm of the crystals at end of the tube.
- Using a rubber band the tube was attached to a thermometer as shown below.
- A boiling tube was half-filled with liquid paraffin and the tube and the thermometer were placed in the boiling tube.
- This was placed in a boiling tube and heated. the liquid paraffin was stirred thoroughly throughout.
- The temperature at which the crystals started to melt was noted and recorded.
-The paraffin was allowed to cool about 10C and a fresh melting point tube with crystals was put in the boiling tube .
6.The paraffin was heated again more slowly the time and the temperature at which the crystals melted again was noted.
Melting is indicated by the formation of a visible meniscus.
Results obtained from melting benzoic crystals
Pure Crystals = Sharp melting point obtained
Impure Crystals = A melting point range obtained.
Impure crystals have a lower melting point and broader melting point range whereas pure crystals have a narrow melting point range and higher melting point.
Theoretical yield calculations
Mass = Density X Volume
Use mass to find no. of moles
Multiply moles by C6H5COOH Mr (122)
