6.3 Analysis Flashcards
What is an Rf value?
-a comparison between how far a component has moved compared to the solvent in thin layer chromatography
Define retention time ?
-the time taken for a component to travel from the inlet to the detector in a gas chromatograph
What is the stationary phase in chromatography?
-it is fixed in place
-it can be a liquid (e.g. in some gas chromatography systems where the mixture components have different attractions to each phase )
-it can also be a solid (e.g. in thin layer chromatography where the components of the mixture are absorbed by different amounts)
What is the mobile phase in chromatography ?
-moves in a definite direction
-e.g. an inert gas in gas chromatography (GC) or a liquid solvent in thin layer chromatography (TLC)
How does separation occur in chromatography?
-by each chemical in a mixture having a different attraction to each phase
-e.g. a chemical that is very attracted to the mobile phase but not the stationary phase will travel a long way up, but a chemical that is very attracted to the stationary phase will not travel far
What is thin layer chromatography used for ?
-to analyse organic compounds
E.g. to check the purity of a sample or to determine the extent of a chemical reaction
What is the stationary phase in TLC (thin layer chromatography )?
-a thin piece of inert material
-e.g. glass covered with an absorbent chemical such as aluminium oxide
What is the mobile phase in TLC (thin layer chromatography)?
-usually an organic solvent
-the solvent will move in one direction vertically up the plate and develop the chromatogram
Why is a lid sometimes used in TLC?
-as the lid reduces the rate at which the solvent is absorbed and allows for a better separation
What is the equation for an Rf value?
Rf value= distance moved by component
——————————————————-
distance moved by solvent
What do the number of spots on the plate in TLC tell you?
-how many chemicals make up the mixture
How can you use Rf values to identify substances in a sample when doing thin layer chromatography ?
By either:
-using data tables that list Rf values for a wide variety of chemicals
-or control spots can be run on the same TLC plate and a direct comparison can be made
When doing thin layer chromatography what should you be careful of when using data tables to identify a substance?
-the Rf values are different if the phases are changed (e.g if a different solvent is used) so it is important to use the correct table for the mobile and stationary phases in your experiment.
In TLC what will similar compounds have ?
-similar Rf values
What is a limitation of TLC (thin layer chromatography)?
-chemicals that a very attracted to the mobile phase may not be separated using this method
What is GC (gas chromatography ) used for?
-to separate volatile components in a mixture
Why is a gas chromatograph often connected to a mass spectrometer?
-so that the separate parts of the mixture can undergo further analysis
What is the stationary phase in gas chromatography?
-a solid or liquid coating on a coiled tube
-this coating is normally a hydrocarbon with a high boiling point
What is the mobile phase in gas chromatography ?
-an unreactive carrier gas such as helium or nitrogen
What will a GC produce?
-a chart of absorption against time
-where the x-axis is retention time
What can the retention time of a chemical tell you?
-this value can be compared to known values to identify the component as each component will take a different amount of time to travel through the column and reach the detector
-be careful as retention times for the same chemical will be different when different phases are used so use the correct data table
What do the different parts of a GC charts show?
-retention time-can be compared to known values to identify the component
-each peak represents a different substance each substance has a different retention time
-the area under the absorption peak tells us the amount of each substance or percentage of a component in a mixture, the larger the area the more substance .
i) Which compound is present in the greatest quantity?
ii) Which compounds were present in equal amounts?
iii) Which compound had the strongest interaction with the stationary phase?
i) D (the larger the relative size of the peak, the greater the quantity of that substance present)
ii) B and C (the peak sizes are equal)
iii) D (the larger the retention time, the greater the interaction of that component with the stationary phase)
i) Which compound is present in the greatest quantity?
ii) Which compounds were present in equal amounts?
iii) Which compound had the strongest interaction with the stationary phase?
i) D (the larger the relative size of the peak, the greater the quantity of that substance present)
ii) B and C (the peak sizes are equal)
iii) D (the larger the retention time, the greater the interaction of that component with the stationary phase)
What does the area under a peak in GC not tell you?
-it does not give you the exact concentration
What is the equation for % of component in mixture?
%of component in mixture= Area of peak
——————————-
Total area of all peaks
What is the equation for % of component in mixture?
%of component in mixture= Area of peak
——————————-
Total area of all peaks
What is a calibration curve used for?
-used in gas chromatography to confirm concentrations of components
How do you create a calibration curve?
-prepare standard solutions (known concentration) of compound under investigation
-obtain gas chromatogram for each standard solution and measure the area under the curve
-plot calibration curve (where the peak area is plotted against concentration)
-use this graph to find out the concentration of the sample
What is NMR?
-a non-destructive analytical technique
-can confirm the formula mass of an organic molecule and is used to determine the structure of a chemical
How does NMR work?
-The nucleus of some atoms, e.g. ¹H and ¹³C have a property known as nuclear spin
-This creates a weak magnetic field, NMR spectroscopy analyses how these small magnetic fields react when placed in a much larger magnetic field
-When these nuclei are placed in a much larger magnetic field they can either align themselves with a low -energy state or they can oppose it with a higher -energy state
-When electromagnetic radio waves are fired at them the energy from these waves are absorbed and causes the nuclear spins to flip and change direction e.g. nuclear spin changes from a low -energy state to a high-energy state
-These absorptions are called resonances and are recorded on the NMR spectrum
When does nuclear spin occur ?
-it occurs if an atomic nucleus has an odd number of nucleons (protons and neutrons)
What does the energy required to change the alignment of nucleons in NMR tell us?
-the energy required to change the alignment depends on the environment the atom is in and can be compared to a standard
How many carbon environments are in this diagram?
-here there are 3 carbon environments
-the green C (CH2Br) that is bonded to CH2CH3
-the purple C (CH2) that is bonded to CH2Br(CH3)
-the red C (CH3) that is bonded to CH2(CH2Br)
How many hydrogen environments are in this diagram?
-here there is 1 hydrogen environment, as the 4 red hydrogens are in the same environment and they are all equal distance from electronegative Br atoms
What is TMS ?
-also called tetramethylsilane
-used in both proton and carbon NMR
-it is used as a reference chemical to compare the values from the sample
-the spectrometer makes a relative scale (chemical shift) for the x-axis of the spectrum
-the TMS reference peak is given the value 0ppm and all other peaks are placed on the x-axis compared to it
What is the internal standard for carbon and proton NMR?
-TMS (tetramethylsilane)
What is the formula for TMS?
- (CH3)4Si
Why is TMS a good internal standard?
-contains both carbon and hydrogen so can be used for both proton and carbon NMR
-produces one sharp single as hydrogen atoms are in a single environment, also there is only one carbon atoms so it produces only one signal in carbon-13 NMR
-non toxic
-low boiling point and is volatile so can be easily removed from sample
-inert so unlikely to react with any chemical that is being investigated
What is chemical shift?
- is the x-axis variable on an NMR spectrum that compares the frequency of NMR absorption with the frequency of the reference peak of TMS
-has the symbol delta δ
-measured in ppm (parts per million)
-TMS (the internal standard) is given 0ppm and is on the far right of the axis
What effects chemical shift values?
-the more similar the protons or carbon atoms are to the symmetrical , non-polar TMS atoms the lower the chemical shift value
What effects chemical shift values?
-the more similar the protons or carbon atoms are to the symmetrical , non-polar TMS atoms the lower the chemical shift value
Why can you not use normal organic solvents in NMR?
-in NMR spectroscopic analysis the molecule is first dissolved in a solvent
-because this technique is used to investigate organic compounds organic solvents like trichloromethane are needed
-however these chemicals contain carbon and hydrogen which would produce signals that would swamp the spectrum and make it unreadable
What solvents are used in NMR?
-hydrogen has 3 isotopes but only ¹H has the property of spin and is active in NMR
-therefore the deuterium isotope 2H is used as it is not NMR active as it does not have the property of spin, it is shown by using a ‘D’ in the formula
-deuterium solvents are often called heavy as deuterium has a greater atomic mass then the ¹H isotope
-therefore trichloromethane CHCl3 (also called chloroform) would become heavy trichloromethane ,CDCl3
What is an example of a deuterated solvent used in NMR?
-CDCl3 , heavy trichlomethane
What does the number of peaks on a carbon-13 NMR tell us?
-peaks tell us about the number of different carbon environments in a molecule
What does the number of peaks on a carbon-13 NMR tell us?
-peaks tell us about the number of different carbon environments in a molecule
What information does carbon-13 NMR give us?
-gives information about the number of carbon environments in a molecule
-but not the ratio of atoms in each environment
In carbon-13 NMR carbon atoms in the same environment are known as …..
-equivalents
->they are bonded to the same atoms and feel the same magnetic field in the NMR spectrometer
How does electron shielding affect the chemical shift value?
-the lower the electron shielding (e.g. if the carbon is near an electronegative element) the higher the chemical shift
-the higher the electron shielding the lower the chemical shift
What should we look for when determining the structure of a cyclic compound from a carbon-13 NMR?
-look for symmetry to which atoms are in different environments
Define equivalent protons
-hydrogen atoms bonded to the same atoms that therefore experience the same magnetic field in the NMR spectrometer
What do the number of peaks on proton NMR’s tell you?
-the number of different hydrogen environments
What does the area under each signal tell you in proton NMR?
-it gives the ratio of atoms in each environment
-this is often shown as an integration trace
What does proton NMR spectroscopy tell you?
-how many different hydrogen environments there are
-how many hydrogens in each environment in the sample
How can you use the numbers above the peaks in proton NMR ?
-the number above the peaks tells you the ratio of the areas under the peak
-this allows you to work out the relative numbers of each type of proton present from the relative peak areas
E.g. if the numbers are 1 and 3 it will be a 3:1 ratio
How do you work out the relative numbers of each type of proton present from an integration trace?
-the height ratio of the trace corresponds to the area ratio
-so measure the vertical parts of the trace, then write the lengths down and use these numbers to come up with a ratio
What is a splitting pattern?
-peaks on the proton NMR that split into smaller peaks
-it is caused by the spin- spin coupling of neighbouring protons
What causes a splitting pattern?
-organic chemicals in the NMR spectrometer experience a large magnetic field
-the protons on neighbouring carbon atoms also affect the magnetic field
-this affects the alignment of the proton and causes a split in the signal
-different numbers of adjacent protons will cause different numbers of splits
What rule can be used to predict the number of peaks in the splitting pattern?
n+1 rule
-n is the number of equivalent adjacent hydrogen atoms on a neighbouring carbon
What rule can be used to predict the number of peaks in the splitting pattern?
n+1 rule
-n is the number of equivalent adjacent hydrogen atoms on a neighbouring carbon
Explain what a singlet peak, a doublet peak , a triplet peak and a quartet peak mean on a proton NMR spectrum?
-a SIGNLET peak means there are 0 hydrogens on neighbouring (adjacent) carbon
-a DOUBLET peak means there is 1 hydrogen on adjacent carbon
-a TRIPLET peak means there are 2 hydrogens on adjacent carbon
-a QUARTET peak means there are 3 hydrogens on adjacent carbon
Why can it be difficult to identify -OH and -NH protons?
-peaks can appear over a wide range of chemical shift values depending on the solvent used and the concentration of the sample
-the signals are often broad
-there usually no splitting pattern
Why does D2O not produce a signal in a proton NMR spectrum?
-it does not have an odd number of nucleons
How is D2O (heavy water) used to identify -OH and -NH protons?
-a proton NMR spectrum is run
-a small amount of D2O is added to the sample solution and is shaken vigorously as D2O and CDCl3 are immiscible
-a second proton NMR is run and any peak due to -OH or -NH protons disappear , as the deuterium in D2O exchanges with the H present in -OH and -NH (by proton exchange)
-the two spectra can be compared to show the absorptions due to -OH and -NH
Compare carbon-13 and proton NMR?
-both use TMS as an internal standard
-the number of peaks on a carbon-13 NMR tells you the number of different carbon environments, the number of peaks on a proton NMR tells you the number of different proton environments
-the chemical shift range is much smaller for proton NMR
-a much smaller sample size can be used for proton NMR
-unlike carbon NMR proton NMR gives information on the number of protons on adjacent carbons
-the peak area on a carbon-13 NMR gives information on the relative number of carbon atoms of each type , the peak area on a proton NMR gives information on the relative number of protons of each type
Compare carbon-13 and proton NMR?
-both use TMS as an internal standard
-the number of peaks on a carbon-13 NMR tells you the number of different carbon environments, the number of peaks on a proton NMR tells you the number of different proton environments
-the chemical shift range is much smaller for proton NMR
-a much smaller sample size can be used for proton NMR
-unlike carbon NMR proton NMR gives information on the number of protons on adjacent carbons
-the peak area on a carbon-13 NMR gives information on the relative number of carbon atoms of each type , the peak area on a proton NMR gives information on the relative number of protons of each type
