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chem2006 - principles in analytical chemistry > Instrumentation > Flashcards

Instrumentation Flashcards

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1
Q

What is electromagnetic radiation?

A

A wave that consists of an oscillating magnetic field, perpendicular to an oscillating electric field.

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2
Q

What happens when two waves of light are in the same phase?

A

Constructive interference occurs and the amplitude of the waves are summed.

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3
Q

What happens when two waves of light are not in the same phase?

A

Destructive interference occurs and the amplitude of the waves are subtracted.

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4
Q

What are the wavelengths responsible for an electronic transition in molecules?

A

Visible (380-788 nm) and ultraviolet (180-380 nm).

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5
Q

What are the wavelengths responsible for vibrational and rotational transitions in molecules?

A

Near-infrared (0.78-2.5 μm) and mid-infrared (2.5-50 μm)

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6
Q

How does a solvent affect a spectrum?

A

The solvent interacts with the molecule and reduces molecular motion. This causes the peak to broaden.

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7
Q

What is a double-beam instrument?

A

An instrument that has two beams of light. One of the beams passes through a reference, the other passes through a sample.

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8
Q

What is the flowchart for a UV/Vis spectrometer?

A

Light Source → Wavelength Selector (Diffraction Grating) → Sample → Detector → Signal Processor and Readout

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9
Q

What are the features of the light source in UV/Vis spectroscopy?

A

Continuum light sources are used - these emit all wavelengths of light.

They must be reliable and powerful and have a stable output.

A tungsten lamp is usually used.

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10
Q

What are the features of the wavelength selector in UV/Vis spectroscopy?

A

A diffraction grating is used. This consists of grooves where the peak of each groove is highly reflective. There are around 2000 grooves per mm.

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11
Q

What is a modern alternative monochromater?

A

The Czerny-Turner monochromator.

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12
Q

What happens in a Czerny-Turner monochromator?

A

Light passes through an entrance slit and is reflected onto a reflection grating using a concave mirror. The light is reflected on the reflection grating and it hits another concave mirror which reflects the light to an exit slit.

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13
Q

What are the features of the sample in UV/Vis spectroscopy?

A

The sample is placed into a glass or synthetic quartz cuvette (quartz if UV is required).

There are ways that intensity can be lost such as reflection losses at the interface and scattering in solution.

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14
Q

How can the loss of light intensity be mitigated?

A

By using a reference sample.

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15
Q

What is an example of a detector in UV/Vis spectroscopy and how does it work?

A

A phototube. It is a glass tube under a vacuum containing a plate cathode (negative) and a wire anode (positive).

The UV/Vis light has enough energy to trigger electron emission from the cathode which causes a flow of current.

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16
Q

How is a calibration curve made?

A

Find the absorbance of known concentrations of the analyte.

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17
Q

What is absorbance?

A

Absorbance is a logarithmic term.

A = -log T

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18
Q

What is the molar absorption coefficient?

A

A measure of how strongly a chemical absorbs light at a particular wavelength.

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19
Q

What happens to the Beer Lambert law if A is very high?

A

The linear relationship breaks down and does not work.

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20
Q

What is the ideal range for absorbances?

A

0.1 to 2 (this covers transmittance of 1% to 80%).

If the concentration is too high, molecular interactions occur and this causes a deviation from linearity.

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21
Q

Where does light splitting in a UV/Vis dual beam instrument take place?

A

After the monochromator and before the sample/reference.

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22
Q

What are the benefits of using a dual beam instrument?

A

Removes variation due to the instrument and removes background signal from the sample.

Essentially, it removes some systematic error. It doesn’t remove random error.

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23
Q

What is the flowchart for a FTIR spectrometer?

A

Light Source → Wavelength Selector (Michelson Interferometer and Laser) → Sample → Detector → Signal Processor and Spectrum

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24
Q

What are the features of the IR source?

A

A globar light source is made of silicon carbide and is heated to 1500 °C.

A Nernst glower light source is made of zirconium yttrium oxides and is heated to 1100 °C.

Both of them provide broad band emissions.

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25
Q

What are the features of the Michelson Interferometer?

A

All wavelengths are introduced at once and they hit a beam splitter which causes two beams to be produced. One of the beams goes on to hit a stationary mirror and the other hits a moveable mirror. The difference in the path length causes constructive or destructive interference. The beams are then recombined and a Fourier transform is conducted on the complex interference pattern.

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26
Q

If the waves are in phase, by how much does the mirror have to move by to become out of phase in an interferometer?

A

A quarter of a wavelength.

This is because the signal goes there and back.

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27
Q

What are the features of the laser in FTIR?

A

The laser is of a known wavelength, and this calibrates the movement of the mirror.

As the distance the mirror needs to move to have constructive interference on the laser is known, the mirror position is known.

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28
Q

How is a sample typically prepared for FTIR?

A

The sample is mixed with potassium bromide and compressed into a pellet. KBr is used as it is transparent to IR.

Another method is using ATR.

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29
Q

What are the features of the detector for FTIR?

A

A photovoltaic detector is used.

Photoconductive transducers consist of a thin film of a semiconductor, such as mercury cadmium telluride (MCT) on a non-conducting glass surface. IR radiation promotes non-conducting valence electrons to a higher energy state which decreases the bulk resistance of the device. These are placed in series with a voltage source and a load resistor and the voltage drop measures the radiant power of the incident beam of radiation.

The MCR detector is used for mid-IR and is cooled by liquid nitrogen to reduce thermal noise.

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30
Q

What is an interferogram?

A

A graph of the light measured by the detector (wave amplitude) against mirror position.

As the mirror position is altered, the intensity of each wavelength varies.

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31
Q

In an interferogram, what is the large peak in the middle called?

A

The zero path difference and this represents maximum intensity for all wavelengths.

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32
Q

What does a Fourier Transform do?

A

Converts a graph of amplitude against time to a graph of power/amplitude against frequency.

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33
Q

What are the three steps for signal processing in FTIR?

A

1) Measure the background spectrum.

2) Measure the spectrum with the sample in place.

3) Subtract the background from the sample spectrum.

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34
Q

How can the resolution and intensity of an FTIR scan be increased?

A

Many scans can be taken and coadded together to improve the signal to noise ratio.

There can be a large optical path difference (the mirror moves further). This increases the accuracy of the Fourier Transform of the interferogram.

Increasing the electrical gain at the detector (more current) can enhance intensity but it will also increase signal to noise ratio.

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35
Q

Why doesn’t increasing the resolution in an FTIR scan always decrease the bandwidth?

A

The natural width of spectral bands in solids and liquids are several wavenumbers across so measurements at high resolutions are ineffective.

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36
Q

When should high spectral resolutions be used in FTIR?

A

When a sample requires it and in gases.

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37
Q

What are the benefits of using Fourier Transform in IR spectroscopy?

A

Multiplex advantage - simultaneously measured over a range of frequencies.

Fewer moving parts so very reliable.

Very fast.

High resolution and sensitivity. High precision and accuracy.

Well established technology.

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38
Q

Why is atomic spectroscopy less complex than molecular spectroscopy?

A

As there are no bonds, vibrations and rotations are not present and so only electronic transitions are measured.

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39
Q

What is the difference between atomic absorption and atomic emission spectroscopy?

A

Atomic absorption is concerned with the excitation of electrons to a higher energy state whereas atomic emission is concerned with the relaxation of electrons to a lower energy state.

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40
Q

What are the features of atomic emission spectra?

A

They consist of narrow lines of finite width and the lines are characteristic of specific elements.

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41
Q

What is atomic emission spectroscopy?

A

Measuring the emission from an atom in the excited state to identify the elemental composition.

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42
Q

What is the flowchart for atomic emission spectroscopy?

A

Nebulization → Atomization → Excitation → Wavelength Selector → Detector

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43
Q

What is nebulization and what are the features of a nebulizer?

A

A nebulizer breaks up the sample into very fine droplets (mist).

It works by having a high velocity stream of gas which is mixed with a flow of the liquid sample. This breaks the sample up into very fine droplets.

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44
Q

What are the steps for atomization and excitation?

A

1) Nebulized sample is passed into a flame.

2) Primary combustion zone - the solvent is evaporated.

3) Interzonal zone - the finely divided solid particles are carried into the inner cone where particles are vapourised. Excitation also happens here.

4) Secondary combustion zone - all material is ‘consumed’.

The excitation occurs at a very low efficiency and only a small number of atoms are excited.

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45
Q

What does a flow spoiler do in atomic emission spectroscopy?

A

Removes large droplets. If these are not removed, molecules will still be present.

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46
Q

What are the features of the burner head in atomic emission spectroscopy?

A

It is long and has a high path length.

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47
Q

What are the advantages of using a burner in atomic emission spectroscopy?

A

The fuel can change the flame temperature.

It is a low-cost solution.

48
Q

What are the disadvantages of using a burner in atomic emission spectroscopy?

A

Only a small amount of the atoms are excited - the unexcited atoms exceed the excited ones by 103 - 1010. This leads to poor sensitivity.

Non-atomized molecules can be present which interfere.

Emissions from the flame interfere.

49
Q

What is a plasma?

A

One of the four states of matter that is hot, partially ionized gas that contains high concentrations of electrons and ions.

50
Q

How can a plasma be generated?

A

A plasma is generated when a large voltage is applied to a non-conducting gas. The dielectric (polarizable insulator) gas breaks down into ions and electrons. This results in further collisions and formation of a plasma. Once formed, the plasma is capable of absorbing radio frequency power to keep the plasma indefinitely.

It can reach temperatures of 10,000 K.

51
Q

What are the features of a plasma torch?

A

1) Aerosol is fed into a stream of argon.

2) There is tangential flow of plasma gas.

3) Coils deliver an RF power supply to create the plasma.

52
Q

What are the advantages of using a plasma torch in atomic emission spectroscopy?

A

It is very stable.

Higher temperatures are achieved which increases the signal.

There isn’t interference from molecules as these can’t exist in the plasma.

No interference from the flame.

53
Q

What are the disadvantages of using a plasma torch in atomic emission spectroscopy?

A

More complex.

Higher cost.

54
Q

What needs to be conducted for every element, every time atomic emission spectroscopy is used?

A

A calibration plot needs to be made.

55
Q

What are the three types of peak broadening?

A

Natural broadening.

Pressure broadening.

Doppler broadening.

56
Q

What are the features of natural broadening?

A

The absolute width of the atomic spectral lines is determined by the lifespan of the excited state and Heisenberg’s uncertainty principle.

Δν = (2πτ)-1.

This means that the shorter the lifetime, the broader the line. Lifetimes are in the range of 10-8s which causes a natural line width of 10-5 nm. This is so small it is insignificant.

57
Q

What are the features of pressure broadening?

A

Collisions between atoms and molecules in the gas phase lead to deactivation of excited states and this reduces the lifetime and hence causes broadening.

The number of collisions is proportional to temperature and pressure. This gives a broadening of around 3 x 10-3 nm which cannot be disregarded.

58
Q

What are the features of Doppler broadening?

A

It is caused by the rapid motion of molecules. Atoms moving towards the detector emit wavelengths that are slightly shorter and atoms moving away from the detector emit wavelengths that are slightly longer.

The broadening of the lines are more pronounced at higher temperatures due to the greater velocities. This gives a broadening of around 5 x 10-3 nm which cannot be disregarded.

59
Q

What happens when the width of the absorption line is less than the width of the incident radiation?

A

A trough is formed in the transmission line but radiation on the edges still passes through. This will reduce the signal but it will not be a complete loss.

60
Q

What happens when the width of the absorption line is greater than the width of the incident radiation?

A

A flat transmission line is recorded. This is much more preferable as the signal detected is due to transmission rather than accidental transmission.

61
Q

Why is atomic absorption spectroscopy the most widely used method of atomic spectroscopy?

A

It is simple, reliable and cheap.

62
Q

What is the flowchart for atomic absorption spectroscopy?

A

Light Source → (Nebulizer) → Sample Atomizer → Monochromator → Detector

63
Q

What are the features of the light source in atomic absorption spectroscopy?

A

It is a line source that is caused by an element decaying after being excited. The element used is the element that is being looked at.

The lamp used is called a hollow-cathode lamp. It consists of an anode and a glass shielded cathode made of the desired element in an atmosphere of an inert gas (usually argon) at 1-5 torr. When a large voltage is applied, a plasma forms and the positive ions travel to the cathode. Upon collision, some atoms are knocked off the cathode (sputtering) and excited. When it relaxes, the emission is sent through a quartz or pyrex window.

64
Q

Why is the gas inside a hollow-cathode lamp kept at a low pressure?

A

To reduce the band broadening for the light source.

65
Q

Why can the monochromator in atomic absorption spectroscopy be low quality?

A

There are only discrete lines with a small bandwidth. The monochromator only needs to remove unwanted frequencies and due to the small bandwidth and lack of continuous nature, it doesn’t have to be very precise.

66
Q

How can the Beer-Lambert law be applied to atomic absorption spectroscopy?

A

As the Beer-Lambert law is only valid for monochromatic radiation, the graph needs to be integrated over a finite bandwidth.

If the molar absorption coefficient is constant over the range, then the Beer-Lambert Law can be applied.

67
Q

What do mass spectrometers measure?

A

The atomic mass per unit charge.

The m/z value.

68
Q

What are the basic steps in mass spectrometry?

A

An electron hits a molecule which results in ionisation. The ion is passed through a magnetic field and is attracted to the negative polarity. It hits the detector and based on how much it has curved, the m/z value is calculated.

69
Q

What are the features of a fragmentation pattern?

A

It can provide a fingerprint for the molecule.

70
Q

What is the flowchart for a mass spectrometer?

A

Sample inlet → (Vacuum system) → Ionisation method → Mass analyser → Detector

71
Q

What are the features of the sample inlet for mass spectrometry?

A

It is a leak valve (a thin capillary). This works for gases.

It adds the gas slowly so that the vacuum can be maintained.

72
Q

What are the features of ionisation for mass spectrometry?

A

A method is applied to generate an ion with a positive charge.

The ionisation can be either hard or soft.

73
Q

What are the features of hard ionization?

A

Energetic ionization that causes a lot of fragmentation.

74
Q

What are the features of soft ionization?

A

Lower energy which minimises fragmentation. This results in a simpler spectrum which can be used to identify the parent mass.

75
Q

What are the features of electron impact ionisation?

A

A heated (>1300 K) Tungsten filament is used to produce electrons. These are accelerated by 70 eV towards the anode (trap) due to a potential difference between the filament and the trap. These bombard the analyte molecules which causes ionisation through the loss of an electron.

M + e- → M+. + 2e-.

As a lot of energy is put in, the molecule is very likely to fragment.

76
Q

What are the features of a repeller in electron impact ionisation?

A

Once the sample has been ionised, the positive ions are repelled by the repeller. This is held at a 4 kV (4000 eV) positive potential. The ions gain kinetic energy and move towards the mass analyser.

Their velocity is dependent on the mass.

77
Q

What are the features of magnetic deflection and the magnetic sector?

A

The circular motion for the ion in the magnetic field is given by the equation: Br = mv/q.

This means that the field strength or the repeller voltage (which influences the velocity) can be changed to select the mass we want.

The accelerated ions are passed into the magnetic sector and the field strength is varied.

78
Q

Can different mass analysers be paired together?

A

Yes they can. This offers an increased resolution.

79
Q

What are the options of detector available for mass spectrometry?

A

Faraday cup

Electron mulitplier

80
Q

What are the features of a Faraday cup?

A

Charged ions pass into the collection cup and upon impact they are neutralised and pass a charge to the cup. This induces a current which is measured.

81
Q

What are the features of an electron multiplier?

A

Consists of a series of electrodes with a large voltage difference. The ions collide with an electrode which releases electrons. These cascade down a series of electrodes which amplifies the signal by a factor of 107 with 20 dynodes.

82
Q

What is alpha-cleavage?

A

Unpaired electron pairs up depending on electron donating ability.

83
Q

What is sigma-bond cleavage?

A

Carbocation is cleaved at the point of greatest substitution.

84
Q

What is inductive cleavage?

A

Electronegative hetero cation gains a pair of electrons to become neutral.

85
Q

What is the even electron rule?

A

Electron ionisation tends to form a radical cation. This typically splits into an even-electron cation and a neutral radical.

86
Q

What does a loss of m/z 14 represent?

A

Usually the loss of CH2 units.

87
Q

What does a loss of m/z 13 usually represent and what molecules is it found in?

A

Usually the loss of CH units and they are found in an aromatic ring structure.

88
Q

What is a McLafferty rearrangement?

A

Ketones undergoing beta-cleavage with gamma proton transfer.

89
Q

What are the features of chemical ionisation?

A

The ionisation source chamber is filled with a reagent gas (methane, isobutane, ammonia) at a pressure of 1 mbar. This is relatively high pressure so there are lots of collisions. Energetic electrons (100-200 eV) ionise the reagent gas. The ionised reagent is used to ionise the analyte in an adduct or proton transfer reaction.

90
Q

How are the ions formed in chemical ionisation?

A

Step 1 - Electron ionisation of the reagent gas, methane.

CH4 + e- → CH4+ + 2e-. Fragmentation can cause CH3+, CH2+ and CH+ to form.

Step 2 - Ion molecule reactions create stable reagent ions.

CH4+ + CH4 → CH5+ + CH3
CH3+ + CH4 → H2 + C2H5+

91
Q

Why are CH5+ and C2H5+ the dominant ions formed during chemical ionisation?

A

They are stable at low pressures.

92
Q

What are the four methods of reagent ions reacting with analyte in chemical ionisation?

A

Formation of pseudomolecular ions - CH5+ + M → CH4 + MH+.

Formation of adduct ions - C2H5+ + M → [M-C2H5]+.

Molecular ion by charge transfer - CH4+ + M → M+ + CH4.

Hydride abstraction - C3H5+ + M → C3H6 + [M]+.

93
Q

How does proton transfer mass spectrometry chemical ionisation work?

A

It is a very soft ionisation where the analyte is protonated by an acid.

R + H3O+ → RH+ + H2O

It is only good for species with a proton affinity greater than water. It is good for volatile organic compounds but doesn’t work for inorganics.

94
Q

What are the features of electrospray ionisation?

A

Electrospray ionisation passes a liquid sample through a fine nozzle to create a spray and ultimately a beam of ions. The analyte is ionised by the ions in the solution which leads to very soft ionisation. It is good for large non-volatile biomolecules as it takes the sample directly from solution.

95
Q

How does electrospray ionisation work?

A

The analyte solution contains ions and the solution of the analyte is passed through the spray nozzle. The tip has a very positive potential which drags solution out of the spray nozzle. This forms a Taylor cone. Charged droplets break away and due to their charge they move towards the cathode. As the droplets cross the space, they lose solvent molecules and get smaller which increase their charge density. At the Rayleigh limit, they explode and this process continues until only singly charged ions reach the mass spec. A backflow of nitrogen is used to encourage the desolvation process.

96
Q

How is the analyte ionised in electrospray ionisation?

A

If the molecule contains basic sites, protons can be used to ionise the molecule. This process can happen multiple times on the molecule which can affect the m/z value.

As pH has an effect on the ionisation efficiency, solutions are usually buffered.

97
Q

What are the features of a quadrupole mass analyser?

A

Consists of two pairs of rod electrodes through which a radio frequency current is passed through where the two are 180 degrees out of phase. The beam of analyte ions travel between the rods. The ions pass through the quadrupole with an oscillating trajectory.

At a specific RF frequency and power, only specific m/z units will reach the end of the quadrupole and therefore the detector.

98
Q

What is an RF current?

A

A high frequency AC voltage.

99
Q

What is the ion called that reaches the detector in a quadrupole mass analyser?

A

Resonant ion.

Ions that don’t are called non-resonant ions.

100
Q

What are the features in an ion trap mass analyser?

A

It uses a quadrupole where there is a positively charged plate at either end. Resonant ions will be passed back and forth which traps the ions that are the desired m/z value.

Different configurations are possible such as a Paul ion trap or the ring ion trap.

101
Q

What happens in a Paul ion trap?

A

Ions oscillate in a figure of eight pattern.

102
Q

What is potential energy in terms of an electric field?

A

When work is done against an electric field, the electric field stores that energy as potential energy.

103
Q

What happens when a negative potential is applied to a metal?

A

Extra negative charges are in the structure which repel each other. This destabilises the metal and it is higher in energy.

This means that work can be done on the surroundings.

104
Q

What happens when a positive potential is applied to a metal?

A

Electrons are removed from the structure which means that they are stabilised by greater access to positive charge.

This means that work must be done on the system.

105
Q

What is the definition of potential difference?

A

The work done to move a unit charge of zero mass between two locations. It has units of Joules/Coulomb.

As it is a difference, it must have a zero point which is the reference potential.

106
Q

What is the definition of Gibbs energy?

A

The maximum amount of work, other than pressure-volume work, that may be performed by a thermodynamically closed system at constant temperature and pressure.

107
Q

How is an electrochemical cell set up?

A

The voltage of the working electrode is measured against the reference electrode. The reference electrode has a specific, constant potential as given by the Nernst equation. The potentiostat measures the potential difference. The ions are connected by an ionically conducting phase.

108
Q

How does a pH meter work?

A

A saturated calomel electrode is used as a reference electrode. The other electrode is made of a silver wire that is coated in silver chloride. The voltage difference between the two electrodes are measured. The AgCl electrode is covered in 0.1M HCl.

Acid reacts with glass in the following reaction: SiO2 + H+ ↔ SiO2H+

This difference in charge is measured.

109
Q

What is the advanced flowchart for UV/Vis spectrometry?

A

Light source (Tungsten filament) → Wavelength selection (diffraction grating) → Sample (Cuvette) → Light Detector (Phototube) → Signal Processor and readout.

110
Q

What is the advanced flowchart for FTIR?

A

Light source (Globar or Nernst Glower) → Interferometer and laser (Michelson interferometer) → Sample (KBr Pellet) → Detector (photovoltaic detector) → Signal processor and readout

111
Q

What is the advanced flowchart for atomic emission spectroscopy?

A

Nebulisation → Atomizer (flame or ICP) → Excitation (flame or ICP) → Wavelength selector (diffraction grating) → Detector (phototube).

112
Q

How does a burner work for flame atomic emission spectroscopy?

A

The nebulizer mixes the fuel, sample, oxidant and generates an aerosol.

A flow spoiler is positioned at the exit of the nebulizer stream in the spray chamber and it removes the large droplets in the aerosol stream.

The burner head is a slit. This creates a long path length through the flame.

113
Q

What is the advanced flowchart for atomic absorption spectroscopy?

A

Light source (hollow-cathode lamp) → (Nebulizer) → Sample Atomizer → Monochromator (diffraction grating) → Detector (phototube)

114
Q

What is the advanced flowchart for mass spectrometry?

A

Sample inlet (leak valve) → (Vacuum system) → Ionisation method (electrospray, electron impact, chemical ionisation) → Mass analyser (magnetic sector, quadrupole, ion trap, time-of-flight) → Detector (Faraday cup, electron multiplier)

The pressure of the vacuum is 10-5 to 10-8 mbar.

115
Q

What is an equation for Gibbs Energy relating to electrochemistry?

A

ΔG = -zFE

116
Q

What is the Nernst Equation?

A

Ecell = Eocell - (RT)/(zF)lnQ

117
Q

What is entropy equal to?

A

ΔSo = zF(dEo/dT)

chem2006 - principles in analytical chemistry (2 decks)

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