chromatography and spectroscopy Flashcards
1
Q
what do all forms of chromatography have
A
- stationary phase
- mobile phase
2
Q
what is chromatography used for
A
seperate individual components from a mixture of substances
3
Q
what is the stationary phase
A
- does not move normally a solid or a liquid supported on a solid
4
Q
what is the mobile phase
A
- does move
- normally a liquid or a gas
5
Q
what are the uses for chromatography
A
- analysis of drugs
- plasttics
- flavourings
- air samples
- applications to forensic science
6
Q
what does TLC stand for
A
thin layer chromatography
7
Q
what is think layer chromatography
A
- quick inexpensive analytical technique that indicates how many components are in a mixture
8
Q
what does TLC technique show
A
- plastic sheet or glass coated with a thin layer of a solid adsorbent substance - usually silica
9
Q
what is the adsorbent in TLC
A
- stationary phase
10
Q
describe how tlc works to seperate differnet substances
A
- different component in a mixture have different affinities for the absorbent and bind with differing strenths to its surface
11
Q
what is adsorption
A
- process by which the solid silica holds the different substances in the mixture to its surface
- seperation achieed by relative adsorptions of substances with the stationary phase
12
Q
describe the process for carrying out TLC
A
- take a TLC plate using a pencil draw a line across the plate about 1 cm from one end of the plate this is the base line
- using a capillary tube spot a small amount of solution of the sample onto the base line on the plate
- prepare a chromatography tank fro the TLC plate this can be made from a small beaker with a watch glass placed on top pour some solvent into the beaker to the depth of about 0.5c,
- place the prepared TLC plate in the beaker making sure solvent does not cover the spot cover the breaker with the water glass and leave it undisturbed on the bench the solvent will rise up the TLC plate
- allow the solvent to rise up the plate until it is about 1cm below the top of the plate remove the plate from the beaker and immediatley mark the solvent front with a paper allow the plate to dry
- if there are any visible spots circle them with a pencil alternativley hold a UV lamp over the plate and circle any spots that you can see sometimes the plate is sprayed with a chemical or a locating agent such as iodine to show the position of the spots that may be invisible to the naked eye
13
Q
describe how you would itnerpret a TLC plate
A
- calculating the value for retention factor Rf
- each component can be identified by comparing its Rf value wit hknown values recorded using the same solvent system and absorbent
- or run a TLC of a sample alongside pure samples of compounds that may be present
- easy to identify amino acids in unknown sample visually without needing to calculate Rf values
14
Q
what is the formula to calculate Rf value
A
distance moved by component/distance moved by solvent front
15
Q
what is gas chromatography used for
A
seperating and identifying volatile organic compounds present in a mixture
16
Q
what is the stationary phase and mobile phase of gas chromatogrpahy
A
- stationary phase - high boiling liquid adsorbed onto an inert solid support
- mobile phase - inert carrier gas such as helium or neon
17
Q
describe how a gas chromatogram works
A
- small amount of volatile mixture is injected into the apparatus called a gas chromatograph
- mobile carrier gas carries the components in the sample through the capillary column which contains the liquid stationary phase absorbed onto the solid support
- the components slow down as they interact with the liquid stationary phase inside the column
- more soluble the component is in the liquid stationary phase hte slower it moves through the capillary column
- components of the mixture are seperated depending on their solubility in the liquid stationary phase
- compounds in the mixtyre reach the detectore at differnt times depending on their interactions with the stationary phase in the column
- the compound reatined in the column for the shortest time has the lowest retention time and is detected first
18
Q
what is retention time
A
time taken for each component to travel through the column
19
Q
describe the two pieces of information obtained from a gas chromatogram
A
- retention times can be used to identify the components present in the sample by comparing these to retention times for known components
- peak integration (areas under each peak) used to determine the concentrations of components in the sample
20
Q
describe how concentration of components are determines from a gas chromatogram
A
- the concentrations of a component in a sample is determined by comparing its peak integration (peak area) with values obtained from standard solutions of the component
21
Q
describe the procedure for how concentration of components are found in a gas chromatogram
A
- prepare standard solutions of known concentrations of the compound being investigates
- obtain gas chromatograms for each standard soltution
- plot a calibatation curve of peak are against concentration this is called external calibration and offers a method for converting a peak area into a concentration
- obtain a gas chromatogram of the compound being investigated under same conditions
- use the calibration curve to measure the concentration of a compound
22
Q
describe the test for an alkene
A
- add bromine water drop wise
- bromine water decolourised form orange to colourless
23
Q
describe the test for a haloalkane
A
- add silver nitrate and ethanol and warm to 50 degrees in a water bath
- chloroalkane - white precipitate
- bromoalkane- cream precipitate
- iodoalkane - yellow preciptate
24
Q
describe the test for a carboyl
A
- add 2,4 dinitrophenylhydrazine
- orange precipitate will form
25
Q
describe the test for an aldehyde
A
- add tollens reagent and warm
- silver mirror will form
26
Q
describe how to test for a primary and secondary alcohol and an aldehyde
A
- add acidified potassium dichromate (VI) and warm in a water bath
- colour change from orange to green
27
Q
describe how to test for a carboxylic acid
A
- add aqueous sodium carbonate
- effervscence
28
Q
what is NMR
A
- nuclear magnetic resonance
- uses a strong magnetic fiel and radio frequency radiation
- with right combination of magnetic field strenght and frequency the nuclei of some atoms absorb this radiation
- the energy for absorption can be measured and recorded as an NMR spectrum
29
Q
how is nuclear spin a factor of NMR
A
- nucleus also has a nuclear spin
- signigicant if there is an odd number of nucleons (protons and neutrons)
- NMR is relevant for 1H and 13C the isotopes with an odd number of nucleons
- NMR spectroscopy can be used to detect isotopes of other elements with odd numbers of nucleons such as 19F and 31P
30
Q
What is 1H NMR
A
- nucleus consists of just a proton
- 1H NMR usually referred to as proton NMR
31
Q
how does resonance play a factor in NMR
A
- nucleus has two difffernt spin states and these have different energies
- with right combination of strong magnetic field and radio ffrequency radiation the nucleus can absorb energy and rapidly flips between the two spin states
- this is called resonance
32
Q
what is energy used for an NMR spectrometer
A
- radio frequency radiation has much less energy than the infrared rations used in IR spectroscopy
- frequency required for this resonancy is proportional to the magnetic field strength and it is only strong and uniform magnetic fields that is a small quantity of energy that can be detected
- a very strong super conducting electromagnet is used cooled to 4K by liquid helium
33
Q
what is a chemical shift
A
- every carbon and hydrogen atom is bonded to other atoms
- all atoms have electrons surrounding the nucleus which shifts the energy and radio frequency needed for nuclear magnetic resonancy to take place
- frequency shift measured on a scaled called chemical shift
34
Q
what is TMS used for in NMR
A
- tetramethylsilane (CH3)4Si is used as the standard regernce chemical against which all chemical shifts are measures
- given a chemical shift calue of 0ppm
35
Q
what is the chemical shift determines by
A
- chemical environment espeically in the prescence of nearby electronegative atoms
36
Q
describe how the spectrum is run
A
- sample dissolved in a solvent and placed in a narrow NMR sample tube together with a small amount of TMS
- the tube is placed inside the NMR spectrometer where it is spun to even out any imperfections in the magnetic field within the sample
- spectrometer is zeroed against the TMS standard and the sample is given a pulse of radiaition containing a range of radio frequencies whilst mantaining a constant magnetic field
- any absorptions or energy resulting from resonance are detected and displayed on a computer screen
- after analysis ample can be recovered by evaporation of the solvent
37
Q
what are deutarated solvents
A
- usually used in which the 1H atoms replaced by 2H atom (deuterium)D
- deuterium produces no NMR singla in the frequency ranges used in 1H and 13C NMR spectroscopy
38
Q
what is the most common solvent used in NMR spectroscopy
A
- deuterated trichloromethane CDCl3 is commonly used as a solvent in NMR spectroscopy
- but will still produce a peak in carbon -13 spectrum
- computer usually filters out this peak before displaying a spectrum
39
Q
what two pieces of information does a carbon-13 NMR spectroscopy show
A
- number of different carbon environments - number of peaks
- types of carbon environment present - from chemical shift
40
Q
describe how the chemical environment of a carbon atom is determined by the position of the atom within a molecule
A
- carbon atoms that are bonded to different atoms or groups of atoms have different environment and will also absorb at different chemical shifts
- if two carbon atoms are positioned symmetircally within a molecule then they are equivalent and have the same chemical environments they then will absorb radiation at the same chemical shift and contribute to the same peak
41
Q
what are the four pieces of information provided by proton NMR
A
- number of different proton environments - number of peaks
- the types of proton environments pressent - from chemical shift
- relative numbers of each type of proton - ingration traces or ration numbers of relative peak area
- number of non equivaent protons adjacent to a given proton from the spin - spin splitting pattern
42
Q
what may cause the chemical shift value to go out of range
A
- solvent
- concentration
- substituentss
43
Q
describe the principles of chemical shift for proton NMR
A
- if two or more protons are equivalent they will aborb at the same chemical shift increasing the size of the peak
- protons of different types have different chemical environments and are non equibalent they absor at different chemical shifts
44
Q
describe how peak area is related in NMR
A
- ratio of relative areas under each peak gives the number of protons responsible for each peak
- NMR spectrometer measures the area under each peak as an integration trace
45
Q
describe spin to spin coupling in proton NMR
A
- proton NMR peak split into sub peaks or splititng patters
- caused by protons spin interacting with the spin states of nearby protons that are in different environments
- provide information about number of protons bonded to adjacent carbon atoms
46
Q
describe the n+1 rule for NMR
A
- splitting of a main peak into sub peaks is called spin-spin coupling or spin-spin splitting and number of sub peaks is one greater than the number of adjacent protons causing the splitting
- for each proton with n protons attatched to an adjacent carbon atom the number of sub peaks in a splitting pattern
47
Q
describe singlet pattern
A
- 1 peak
- no H on adjacent atoms
48
Q
describe a doublet
A
- 1:1 ratio
- adjaacent CH
49
Q
describe the pattern of a triplet
A
- 1:2:1
- adjacent CH2
50
Q
describe the splitting pattern of a quartet
A
- 1:3:3:1
- adjacent CH3
51
Q
describe the spin-spin coupling that occurs in pairs
A
- in an NMR spectrum if you see one splitting pattern there must alwaysbe another
- splitting patterns occur in pairs because each proton splits the signal of other
- they can be worked out using n+1 rule
52
Q
describe the chemical shift of NH and OH in proton NMR
A
- in solution NH and OH protons may be involved in hydrogen bonding and the NMR peaks are often broad and of variable chemical shift
- OH and NH occur at almost any chemical shift
- COOK more predictable absorbing at 10-12ppm
- boradening of peaks also means OH and NH protons are not usually invovled in spin-spin coupling
53
Q
what is proton exchange
A
- identify OH and NH protons
- proton NMR spectrum is run as normal
- a small volume of deuterium oxide D2O is added the mixture is shaken and second spectrum run
- deuterium exchanges and replaces Oh and NH protons in the sample with deuterium atoms
- CH3OH+D2O reversible sign Ch3OD=HOD
- so second spectrum run on Ch3OD as deuterium does not absorb chemical shift range OH pek disappears
