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Modern analytical techniques
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- 1Mass Spectrometry stages:Injection
Vaporisation
Ionisation
Acceleration
Deflection
Detection - 2Injectiona sample is introduced into the spectrometer
- 3VaporisationLow M Wt samples vapourise in the vacuum or higher M Wt samples are heated
- 4Ionisation (including equation)M(g) + e- ----> M+(g) + e-
High energy electrons are fired at sample from electron gun. Usually knock off 1 electron forming 1+ ions.
1+ Ions are attracted to negative late where they are accelerated - 5Accelerationthe ions are accelerated in an electric field so they have similar kinetic energy.
- 6Deflectionthe ions of mass m are deviated by a magnetic field such that ions of low or high m/z value strike the sides of the spectrometer, but ions of one particular m/z value continue along the spectrometer body
by varying the magnetic field strength all ions are sequentially focused into the detector - 7Detectionthe ions are detected and the mass spectrum plotted
- 8molecular substances form individual molecules when injected into a vacuum. The molecules pass into the ionisation chamber where they are....bombarded with electrons to form cations
- 9What is the peak that results from this M+ ion called?Molecular ion or Parent ion.
always the one furthest to the right on mass spectrometer. - 10Fragmentation bombarding molecules with electrons not only ionises they but also....?splits them into fragments
only the charged fragments are accelerated, deflected and detected so only m+ will appear on the resulting mass spectrum - 11The most abundant peak is called theBase Peak
- 12The pattern of peaks is called thefragmentation pattern and is always the same so can be used to identify molecules.
- 13Isotopes Common isotopes seen in mass spectra:- Complete the ratios
- 14Why does the M+1 peak appear?Spectra of organic compounds are usually ’cleaned up’ to remove any contribution of 13C, however in larger organic molecules a small peak about 10% of the height of the molecular ion and 1 mass unit heavier is probably due to one of the carbon atoms being 13C.
- 15High Resolution Mass Spectroscopymeasures the M/Z values to enough accuracy to find out the molecular formula. Each molecular formula has a different Mr if measured to enough precision
However more than one compound can have the same molecular formula. so you need to look at the fragmentation pattern. - 16Infrared Spectroscopy-All bonds vibrate at a characteristic frequency (stretching and contracting as well as bending vibrations are the commonest types).
-The frequency depends on the mass of the atoms in the bond, the bond strength, and the type of vibration.
-The frequencies at which they vibrate are in the infra-red region of the electromagnetic spectrum.
- If infra-red light is passed through the compound, it will absorb some or all of the light at the frequencies at which its bonds vibrate.
-Rather than using the actual values of the wavelength or frequency, the IR light is measured in wavenumbers [1/frequency (in cm)] because it gives convenient numbers in the range 4000 – 400 cm-1. - 17fingerprint region-complex absorption pattern due to a variety of vibrations and molecular distortions
- used for identification purposes and determining the purity - 18Within the band region-Position of absorption suggests particular bond type – see Data Book
-Strong absorption suggests polar bond.
-H bonding broadens the absorption peak in alcohols, even more so in carboxylic acids. - 19important signals O-H (alcohols)Narrower than acid
- 20important signals O-H (acid)Wider and not as smooth as Alcohol OH
- 21Chromatographyencompasses a broad range of physical methods used to separate and / or to analyse complex mixtures. The components to be separated are distributed between two phases: a stationary phase bed and a mobile phase (the eluent) which percolates through the stationary bed
- 22paper chromatographystationary phase is the paper and the mobile phase is the solvent used.
- 23Liquid or column chromatography-This method is very useful for separating large quantities of soluble substances.
-The stationary phase is a powder (such as silicon dioxide or aluminium oxide) often packed into a glass column.
-The mobile phase is a solvent.
-The mixture is dissolved in the solvent and placed at the top of the column.
-As the solvent (eluent) moves down through the powder in the column, the substances in the mixture move through the column at different rates allowing them to be collected separately.
-The time it takes to move down the column depends on the balance between the solubility of each substance in the mobile phase (the solvent) and its retention in the stationary phase (the powder). - 24Liquid or column chromatography allowsseparation of ions or molecules that are dissolved in a solvent.
In column chromatography there is an equilibrium between molecules adsorbed on the surface of the stationary phase and molecules dissolved in the solvent - 25High performance liquid chromatography HPLC- column consists of a steel tube 10 – 30 cm long but only 4 mm. in diameter which is tightly packed with very small (to increase SA of stationary phase) particle of solid
- use a pump to force the mobile phase through the column and provide higher resolution and faster analysis time.
can be carried out at room temperature so can be used to separate mixtures of molecules that would decompose on heating.
eluent passes through a detector, typically UV absorption is monitored as many molecules absorb in the UV range. The detector indicates the presence of the substance of interest but not the concentration.
Early normal phase HPLC instruments used a polar stationary phase and a non-polar solvent. In this process more polar molecules are adsorbed more strongly to the stationary phase and take longer to appear in the eluent.
Reversed phase HPLC machines have a column containing silica to which long chain hydrocarbons have been attached making them non-polar. A polar solvent is used and in this case the polar molecules pass through the column more quickly. - 26High performance liquid chromatography HPLCThis type of column is used in the pharmaceutical industry to separate a drug from a mixture and to check the purity of its products.
- 27column characteristics depend on:-column dimension (size)
particle size and pore size
nature of the stationary phase - 28Gas Liquid ChromatographyThis method is used to separate mixtures of volatile liquids.
The stationary phase is a powder coated with with a film of a non volatile liquid. This coated powder is packed into a long, narrow, steel tube. The tube is coiled so that it takes up less space.
The tube is kept inside an oven to heat it up (so vaporising the volatile liquids).
The mobile phase is a unreactive gas such as nitrogen (a carrier gas).
continuous flow of the carrier gas passes through the column.
The sample is injected into the column.
The substances are separated as they travel through the column depending on their affinity (attraction) for the stationary and mobile phases.
The chromatogram shows the time it takes for each substance to pass through the column. The size of the signal is proportional to the amount of that substance in the mixture.
It is very common for a mass spectroscopy machine to be connected to a GLC machine. As each separated substance reaches the end of the column, it can be analysed by mass spectroscopy. - 29Mobile phase is aninert carrier gas e.g. N2 He CO2 Ar
- 30Describe the two main types of columnPacked columns contain a finely divided, inert, solid support material coated with liquid stationary phase.
Capillary columns have an internal diameter of a few tenths of a millimeter. They can be one of two types; wall-coated open tubular (WCOT) or support-coated open tubular (SCOT) - 31Detectors vary according to the application of the machine. Suggest two different types and what they measure.flame ionisation detector
Mass spectrometer. - 32explain why there is a big time difference between peaks A and B:Stationary phase is a thin layer of solid or a liquid coated on inside of tubing
Mobile phase, the carrier gas, is an unreactive gas such as N2 or He
Volatile components swept down the column by carrier gas
If stationary phase is a solid, separation is achieved by differing adsorptions onto solid surface, if a liquid by differing solubility of A and B.
- 33If the temperature is increased what effect would that have on the resulting chromatogram?The retention times of A and B, and the other components would both be shorter if the temperature were increased.
- 34Rf =distance travelled by spot
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distance travelled by solvent - 35Do the Rf values bear any relationship to the polarity of an amino acid in this procedurewould expect polar amino acids to have higher Rf values – more soluble in eluent
- 36What will affect the Rf value of an amino acid?Interaction with the stationary phase (silica gel on plate) and solubility in eluent.
- 37Chromatography summary
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- A level Chemistry Karteikarten am Modern analytical techniques, erstellt von megan trent am 02/11/2018.
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