Modern Analytical Techniques I and II

What happens when a molecules absorbs infrared radiation?

It makes the covalent bond vibrate more in a stretching or bending motion

What factors affect the amount of vibration of a bond

• bond strength

• bond length

• Mass of each atom in the bond

how does infrared spectroscopy work?

-every bond has a unique vibration frequency in the infrared region of Em spectrum

bonds absorb radiation that has the same frequency as their frequency of vibration

infrared radiation emerged from a sample is missing the frequencies that have been absorbed → this information can be used to identify the compounds’s function group

what do the peaks on an infrared spectrum represent

• absorbance of energy from the infrared radiation

what happens inside a mass spectrometer?

• the molecule is injected into the spectrometer

• molecule is vaporised into the gas-phase and passed through the spectrometer

• the gas-phase molecules are bombarded with high-energy electrons to cause the molecule to lose electrons and form a positively charged species - cations

• excess energy from the bombardment causes sample to split into fragments - so mass spectrum consists of a fragmentation pattern

• molecules break up more readily at weak bonds or at bonds which give rise to stable fragments

• the charged ions are accelerated

• they pass through a magnetic field which causes them to deflect. the amount of deflection depends on mass and charge, the smaller the mass to charge ratio the larger the deflection

• as ions reach the detector, a signal is generated and a mass spectrum is produced (relative ion intensity vs mass to charge ratio)

Which positive ions are more stable as fragments

• tertiary carbocations

• RCO⁺

• C6H5⁺ - from aromatic compounds related to benzene

is fragmentation predictable?

no, it can happen anywehere in the molecule

What are the advantages of using mass spectrometry? Disadvantage?

• cheap

• small quantities of samples required

disadvantage

• sample is completely destroyed

What do the troughs on an infrared spectrum show?

• the frequencies where radiation has been absorbed - match to table to find out which bonds they represent

what is the name given to the highest peak in a mass spectrum?

• base peak - produced by the fragment with the greatest abundance

• the most stable ion

fragment ions are often carbocations. What is the order of stability of carbocation?

tertiary most stable because of the inductive effect of alkyl group, lone pair of electrons more attracted to the carbon, less available for electrophiles

what is the peak that corresponds to the molecular ion called? Why is it not always visible?

• molecular ion peak

• unstable molecular ions and all molecular ions fragment to produce more stable ions

what is the relative intensity ratio of chlorine isotopes and bromine isotopes

• chlorine: 3:1

• bromine 1:1

what is special about aldehyde peaks in infrared spectrum

what about primary/secondary/tertiary N-H?

• there are 2 peaks appear around 2800

• if primary, 2N-H peak

• if secondary, 1N-H peak

• tertiary amines have no NH peak

What is chemical shift

What does it mean when C13 atom show a ifferent chemical shift value

The resonant frequency of the nucklei, compared to that o a H1 atom

having different chemical environment

When does splitting/spin-spin coupling occur

Neibouring hydrogen atom affect the magnetic field of H1 atom and causes their peaks to split

Chromatography can be used to:

• Separate and identify the components of a mixture of chemicals

• check the purity of a chemical

• identify the impurities in a chemical preparation

• purify a chemical product

What relationship between a sample and the mobile phase makes the sample move faster

more soluble components/components with more affinity to the solvent move faster

What is the relationship between a sample and the stationary phase that make the sample move solwer

moer affinity for the stationary phase means component moves solwer, often attracted by hydrogen bonding

How are substances separated by chromatography

the balance between affinity for mobile phase and stationary phase is different for each component of mixture, so they move at different rate so separate over time

Why will different substances show different Rf values

They are bonded differently and have different polarities - more polar bond means longer retention time, hydrogen bonding are attracted more strongly to the stationary phase (generally mention stationary phase, not affinity to mobile phase)

What is TLC

stationary phase/mobile phase?

Thin layer chromatography, the rate at which a sample moves up a TLC plate depends on the equilibrium between adsorption on the solid and solution in the solvent

silica

solvent

What are the advantages of TLC over paper chromatography

• runs faster

• smaller amounts of a mixture can be separated (need small sample only)

• TLC plates are more robust than paper

How can you observe colourless spots

• spray ninhydrin, and heat(100celcius), colourless to purple

• or shine UV light

How can you confirm the identity of a substance from its Rf value

• Rf = distance moved by chemical/distance moved by solvent front

• compare your Rf value to accepted values for that substance in the same solvent and set-up

• if match then identity is confirmed

Column chromatography

• stationary phase solids can adsorb chemicals onto their surfaces, liquid flowing through the column is the eluent, which washes the components of the mixture through the column

• more than one eluent can be used, leads to better separation

• fairly large amounts can be separated and collected after separation

gas chromatography

• used for compounds that vaporise on heating without decomposing

• can separate compounds in a sample and give a measure of how much is present - area of peak under the graph give how much is present, x-value tells you retention time

• gas chromatography can work with mass spectroscopy to overcome some limitation of gas chromatography such as those with similar retention time cannot be distinguish by gc alone, cannot identify new chemicals as there is no standards that can be used to determine retention time under standard conditions

• mobile phase: inert gas e.g. N2, which carries the mixture of volatile chemicals thorugh a long tube containing the stationary phase

• stationary phase is usually silica with an outer polymer coating in the inner surface of the column, may also be an inert solid rated with a thin film of a liquid with higher boiling point (GLC - chemicals then separate because they differ in solubility in the liquid of the stationary phase)

GC

very sensitive

use in testing blood and urine for drugs

not only separates the chemical sin a sample but also gives a measure of how much of each is present

The column is coiled inside an oven, heating column makes it possible to analyse any chemicals that turn to vapour at the temperature of the oven

Applications of gas chromatography

• trading down source of oil pollution from pattern of peaks

• monitoring the presence of chemicals in industrial processes

• measuring the level of alcohol in blood samples from drivers

• detecting pesticides in river water

What does retention time mean

the time it takes for a compound in a mixture to pass through a chromatographic column and reach the detector

Why does high performance liquid chromatography require a pump?

The use of fine particles increases surface area of the stationary phase, which makes the separation efficient but a high pressure pump is needed to force the solvent though the tightly packed column

advantage: carried out at room temperature, can analyse mixture that would decompose on heating, study urine sample to investigate what happens to drugs as they are metabolised in the body

Explain mass spectrometry, infrared spectroscopy and Nuclear magnetic resonance

• Mass spectrometry gives the relative molecular mass of a compound and can suggest a likely structure for a compound from fragmentation peaks

• infrared spectroscopy shows the presence of particular functional groups by detecting their characteristic vibration frequencies

• NMR help to detect groups with carbon atoms and hydrogen atoms in particular environments in molecules. This technique is used to identify useful compounds, check for impurities and to study shapes of molecules. But this is limited to nuclei which behave like tiny magnets because they have a property called spin

How can GC and mass spectrometry work together? Why is this beneficial?

• GC separates the chemicals in an unknown mixture, such as sample of urine, then mass spectrometry detects and identifies the components

• similar compounds often have similar retention times in GC, which means that they cannot be identified by chromatography alone, even if the conditions are carefully standardises.

• GC alone cannot identify any new chemicals because there are no standards that can be used to determine retention times under given conditions

What is an absorption spectrum

a plot showing how strongly a sample absorbs radiation over a range of frequencies

What is a standard reference compound?

add to solution of a substance being tested by NMR. The standard produces as single sharp absorption peak well away from other peaks and the position of other peaks is compared to this peak

Integration trace

ratio of areas below peakshy w

why is it that a peak of an atom is bonded to an atom with two protons splits into three lines?

There are three energy states available to the two protons:

when both aligned with the field, one aligned with and one against the field, or both aligned against the field

What is a labile atom

when an atom quickly and easily reacts or moves from one molecule to another

They do not couple with the protons linked to neighbouring atoms - NMR peak for a proton in OH group appears as a single peak

Medical benefits form NMR

• MRI uses NMR to detect the hydrogen nuclei in human body. A computer translates the information from a body scan into images of the soft issue and internal organs that are normally transparent to X-rays

Name the compound responsible for the peak at a chemical shift of 0 ppm, stating its purpose

• represent TMS (tetramethylsilane)

• as a reference, so shifts can be compared