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Analytical Chemistry Study Notes, Lecture notes of Analytical Chemistry

Study notes for Voltammetry, Chromatography, and more analytical topics

Typology: Lecture notes

2022/2023

Available from 01/17/2024

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Voltammetry
Methods:
1. Potentiometry:
a. Measures potential of electrochemical cell in absence of current
b. E.g. ion selective electrodes, glass electrode
2. Voltammetry:
a. Measures cell current as function of applied potential under conditions of
electrode polarization
b. Polarography: form of voltammetry using dropping mercury electrode
c. Amperometry: form of voltammetry where measured current is proportional to
analyte concentration at fixed potential
d. E.g. oxygen sensor, glucose sensor
3. Coulometry:
a. Based on quantitative conversion to new oxidation state
b. Constant potential methods, constant current methods, electrogravimetry
Linear Scan Voltammetry:
- Energy of electrode matches or exceeds redox potential of species, current flows as
electrons are exchanged between electrode and valence orbitals of electroactive species
of interest
- Max current is limited by ability of electroactive species to diffuse and react at electrode
- Voltage tells us about species
- Magnitude tells us about quantity of species in solution
Voltammogram:
- Limiting current (Id): info about rate of delivery of material to electrode surface, related
to concentration
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Voltammetry

Methods:

  1. Potentiometry: a. Measures potential of electrochemical cell in absence of current b. E.g. ion selective electrodes, glass electrode
  2. Voltammetry: a. Measures cell current as function of applied potential under conditions of electrode polarization b. Polarography: form of voltammetry using dropping mercury electrode c. Amperometry: form of voltammetry where measured current is proportional to analyte concentration at fixed potential d. E.g. oxygen sensor, glucose sensor
  3. Coulometry: a. Based on quantitative conversion to new oxidation state b. Constant potential methods, constant current methods, electrogravimetry Linear Scan Voltammetry:
  • Energy of electrode matches or exceeds redox potential of species, current flows as electrons are exchanged between electrode and valence orbitals of electroactive species of interest
  • Max current is limited by ability of electroactive species to diffuse and react at electrode
  • Voltage tells us about species
  • Magnitude tells us about quantity of species in solution Voltammogram:
  • Limiting current (Id): info about rate of delivery of material to electrode surface, related to concentration

o I = diffusion current o Contributions: § Charging current (non-faradaic) § Residual current § Kinetic current § Diffusion current (faradaic)

  • Charging Current/Migration Current: o Current flowing from external circuit to deposit/withdraw electrons necessary to provide desired potential o Slow increase before the shoot up from reaction o No redox occurring, build-up of EDL o Use strong buffered electrolyte to minimise effects on outcomes § Avoid pre-concentration of sample at electrode surface § Fixes pH § Eliminates large resistance and ohmic drop issues
  • Residual Current: o Current form of contaminations o Oxygen dissolved in solution o Must de-gas solutions before conducting experiment § Bubble N2 through solution
  • Kinetic Current: o Recombination of water to form H2 or O o Reaction is very slow o Deviation from potential predicted for Nernst equation
  • Polarisation effect: o Occurs in vicinity of electrodes (concentration polarisation) § Reduction current is limited by rate at which Cd2+ is brought to thin film region (near electrode where reduction occurs) by diffusion § Creates concentration gradient § Fick’s Law of Diffusion: o Occurs at electrode surface (kinetic polarisation) o Factors that effect: § Size and shape of electrodes

Reference Electrodes: Methods:

  1. Linear Scan Voltammetry: a. Voltage increases linearly with time
  2. Differential Pulse Voltammetry: a. Pulses superimposed on linear waveform b. Better resolution, higher sensitivity
  3. Square Wave Voltammetry: a. Pulses superimposed on staircase scan b. Rapid and highly sensitive
  4. Stripping Voltammetry: a. Deposition and stripping steps b. Quick and extremely sensitive
  5. Cyclic Voltammetry: a. Oxidation and reduction cycles Ohmic Drop:
  • Potential lost on its way from reference to working electrode
  • More potential must be applied than expected for expected drop Three Electrode Potentiostat:
  1. Working Electrode
  2. Reference Electrode a. Compensates for Ohmic drop

b. No current driven through

  1. Counter Electrode
  • Measured current is output to computer, amplified voltage via current-to-voltage converter Electric Double Layer:
  • Debye Length: distance at which potential has decreased to 1/e of its value at electrode surface o Highly dependent on ionic strength Cyclic Voltammetry:
  • Small stationary electrode
  • Forward and reverse scan
  • Include anodic and cathodic peak currents and peak potentials
  • Used to study electrochemical processes, mechanism, and rates of redox reactions o Organic and organometallic systems

Mass Spectrometry

Fragmentation:

  • High energy conditions to ionise sample and cause fragmentation o Fragmentation causes molecules to disrupt at the weakest bonds
  • Theories to predict fragmentation: o Molecular Orbital Theory o Quasi Equilibrium Theory Molecular Orbital Theory: Quasi-Equilibrium Theory:
  • Sample decomposition treated as a process following unimolecular reaction kinetics (no secondary reactions)
  • Assume frank-codon principle applies (absorption of energy leading to ionisation is a fast process)
  • Energy imparted is distributed throughout molecule by vibrational processes
  • Energy pools into different bonds with weakest and highest vibrational frequency bonds dissociating first
  • Each molecule displays a distinct fragmentation pattern, pattern algorithms used to identify compounds based on overlap with known fragmentation libraries Instrument Design: Inlet Systems: Batch Inlet Systems:
  • For liquids and gasses
  • Small amount of liquid/gas sample instructed via detachable tube and introduced to heated sample reservoir
  • Sample maintained in gas state with low pressure and elevated temperature (300C)
  • Conditions must be maintained
  • Steady stream of sample admitted to ionisation region via molecular leak inlet o Glass or metal diaphragm containing pinhole created by laser or electron drilling
  • Flow is usually molecular
  • Diagram:

Ionisation Sources: Electron Impact:

  • Creation of positive ions from small molecules
  • Ion repellers – placed after molecular leak, before ionisation source, eliminates interfering positive ions from gas samples
  • Neutral sample molecules are bombarded with electrons from electron gun: o Filament (tungsten, selenium) is heated to high temperatures and serves as cathode in electron gun system o Collector anode pulls free electrons from hot filament across large gap, by means of large potential applied
  • Generally, electron energy of 70eV
  • <10eV electron energy less than ionisation potential = no ionisation
  • 100eV relativistic effects become important, interaction time between impacting electron and sample molecule decrease

  • Can control fragmentation via electron impact energy
  • Accelerator System Diagram:
  • metal tube/horn with semiconductor internal coating
  • electric field exists within tube interior
  • ion strikes top of tube, secondary electrons released, and cascade effect follows to provide amplification Ion Collector:
  • well insulated electrode or electrically isolated through high load resistor
  • electrode charged by positive ions
  • measure amount of positive charge collected on electrode (voltage)
  • requires electrometer for read of electrical signals Resolution: 𝑅 =
  • M2 is m/z of heavier ion and delta M is difference in m/z of 2 ions
  • Resolution equation when peaks are not separated at 10% valet heigh condition or are of differing widths Ion Sources and Ionisation Methods: Field Ionisation:
  • 2 electrodes placed in close proximity; high potential applied to provide high electric field intensity
  • One electrode, anode, in form of sharp blades/tiny points o Radius of curvature of blade end or needle points serves to concentrate field
  • Quantum level distortion occurs such that conduction band energy of electrode brough to be on par with valence band electrons of sample – tunnelling occurs to provide ionisation of gas phase sample interacting with anode
  • Cathode is a slit, so cations are accelerated out of ionisation chamber
  • Gentle source of ionisation – not so much fragmentation, useful for molecular mass determinations Field Desorption:
  • Similar to field ionisation
  • Sample is not in gas phase, pre-coated onto plate or micro-needle electrode
  • Heated to assist with ionization
  • Useful not non-volatile samples
  • Extremely gentle ionisation

Chemical Ionisation:

  • Usually done with EI for best information
  • Modified EI with reagent as at high pressure
  • Creates many ions which are strong proton donors
  • Sample introduced into ionised reagent gas at lower concentrations than reagent gas
  • Sample ionisation by proton acceptance not electron impact
  • Peaks of M+1 produced, useful for molecular weight information, limited structural information
  • Minimal fragmentation, mild process
  • Useful in rapid drug analysis Quadrupole:
  • 4 short parallel metal rods symmetrically about the ion beam
  • Opposite rods are connected together electronically (one pair positive, one pair negative)
  • Radio frequency superimposed
  • Filters ions what experience an unstable trajectory through the quadrupole
  • Ions move down axis between rods and experience an oscillating electric field, charge field interaction induced
  • Ions accelerate and decelerate within quadrupole region
  • Only stable m/z (in resonance) will make it through, others will strike rods removing them from ion beam and detection
  • Three variables applied for mass selection:
  • Detection accomplished by electric device
  • Tube output is fed to digital oscilloscope – vertical input for detector signals while oscilloscope horizontal axis is time
  • Useful for studying fast reactions and short-lived species
  • Low resolution, poor performance when considering reproducibility, ease of mass detection
  • Instruments are more rugged, good for non-volatile samples, biomolecules, heat labile species, fast, small, mobile instrument

Chromatography

Separation:

  • Selective retardation of constituents of mobile phase mixture flowing past stationary phase
  • Results in band segregation
  • Length of chromatography column determines spatial distribution of such bands Types of Chromatography: Adsorption Chromatography:
  • Solute adsorbed onto surface of stationary phase particle Partition chromatography:
  • Solute dissolved in liquid phase/ bonded phase coating on solid support particle Ion Exchange Chromatography:
  • Anions are selectively retained near cationic moieties covalently bound to stationary phase Size Exclusion Chromatography:
  • Small molecules transit through porous particles
  • Large molecules excluded from porous particles Affinity Chromatography:
  • One type of molecule in complex sample mixture interacts with affinity capture moieties bound to stationary phase in highly selective manor
  • Other molecules wash through column un-retained Elution Development:
  • Eluent is unabsorbed by stationary phase, moves rapidly
  • Partition occurs where bands of A+E and B+E are completely separated Definitions: Partition Coefficient (K):
  • Measure of analyte solubility difference between stationary and mobile phase Capacity Factor (k’):
  • Weight of solute in stationary phase/ weight of solute in mobile phase Chromatographic Theory:
  • First chromatographic theory was ‘plate theory’
  • Do I need to know? Plate Theory:
  • Envisages a chromatographic column as being made up of series of plates
  • Each plate is defined as an equilibrium zone in terms of concentration distribution between mobile and stationary phase
  • Movement is assumed to occur stepwise increments between plates
  • More plates available, more equilibrium separation occurs, better separation
  • Number of theoretical plates is measure of column efficiency
  • Vair = retention volume of air or non-adsorbed components due to: o Interstitial spaces in packing o Injection volume o Detector dead volume Means of Describing Separations: Theoretical Plates:
  • Measure of column efficiency
  • Function of ratio of retention time to peak width HETP:
  • Height equivalent to theoretical plate, H = L/N Separation Factor:
  • Measure of the degree of separation per equilibration
  • Provides measure of relative solubility difference between species A and B in liquid stationary phase
  • LC more versatile
  • GC can only vary the stationary phase, better detection limits, sensitivity, more methods available Practicalities:
  • Carrier Gas: must be dry and free of impurities
  • Injector: must be temperature controlled, syringe based
  • Column: within temperature-controlled oven, temperature is programmable
  • Detector: universal or selective Injection Systems:
  • Syringe or sample loops used
  • Solvent and solute are vaporised into heated injection port and then flow into column
  • Manual syringe – quality of separation is dependent on injection technique – must be smooth and fast o Hold syringe in place for few seconds after completed so you don’t backflush your samples Columns: Packed Columns:
  • Particulate packing where particles are either liquid coated or used as is
  • Chemistry on stationary phase defines separation chemistry
  • Requires higher operating pressures
  • Can accommodate much larger sample sizes than capillary columns Capillary Columns:
  • Highly refined drawing (glass) and rolling (metal) methods
  • Hollow tube of narrow diameter where inside wall is coated with liquid or covalently bonded species to provide desired stationary phase chemistry
  • Carrier gas flows very quickly with little restriction
  • Cannot hold a lot of material – require high sensitivity detectors
  • Made from fused silica with polyimide outer coating
  • Very efficient separations for complex sample mixtures Detectors (GC): Universal (Non-selective) Detectors:
  • Respond to almost any type of compound
  • Response factor vary depending on detector
  • E.g. Thermal conductivity detector Selective Detectors:
  • Discriminate as to what compounds can be detected
  • Usually, a limited group of substances due to functional groups Information Detectors:
  • Provides molecular identity and structure

Thermal Conductivity Detector:

  • Measures difference in thermal conductivity between carrier gas and mixture of carrier gas and eluting component
  • Greatest sensitivity when carrier gas and component have different thermal conductivities
  • Gas effluent flow through cavity contained in metal block heated to constant temperature, contains a filament of tungsten
  • Heat transfer is function of thermal conductivity of gas filling cavity
  • As thermal conductivity changes, rate of filament heat loss changes, changing temperature and resistance of filament
  • Voltage output from bridge circuit is proportional to concentration of species traversing the TDC filament
  • Hydrogen and helium are preferred carrier gases because of relatively large thermal conductivity compared to organic compounds
  • Characteristics: o Non-selective o LOD: 10- 6 – 10 - 8 o Sensitivity varies o Simple, low cost o Non destructive o Good for compounds in mixtures Flame Ionisation Detector:
  • Universal
  • Detector base acts as one electrode (cathode), anode situated above flame tip
  • Flame destroys sample, releases ions into flame region
  • Potential applied across flame, drives flow of current within the flame
  • 3 gasses required: o Carrier gas o Hydrogen o Air § H2 and air for combustion
  • Flame ignited by ignitor coil, column effluent mixes with H2 and then burned in air, water produced provides little background
  • Electrical current measured by use of an electrometer
  • Characteristics: o Uniform molar response for given species o Insensitive to inorganic compounds o Sensitivity depends on size of analyte and proportional to number of carbons o No response for oxidised carbons o Most common o Good for trace analysis

HPLC:

Introduction:

  • Limited plates, large particles, slow flow
  • Efficiency can be enhanced by decreasing particle size, must be low pressure
  • For high MW and low vapour pressure (opposite GC is best)
  • Better than GC – chemistry of stationary and mobile phase can be tuned for best performance Bonded Phase: Partition Chromatography:
  • Popular for reverse phase applications
  • Organic phase bound to silica gel
  • Limited loading capacity Reverse Phase vs. Normal Phase Chromatography:
  • Check TB
  • Component A and B change in sample polarity Instrumentation: Injection Loops:
  • Reproducible in terms of injection volume and delivery
  • Made in variety of sizes
  • Used for manual injections in conjunction with autosamplers Pumps:
  • Reciprocating Pumps: o Continuous delivery of solvent, limited by size of reservoir o Some band broadening – because of pulsation – minimize by use of pulse dampers and multiple pumps working in sequence o Pneumatic or motorised Detectors:
  • 3 types: o Phase transport: removes solvent, uses GC type detectors o In-situ analysis: UV, refractive index, fluorescence o Colligative Properties: conductivity
  • Deflection Refractometer: o Universal o Light passes through interface between sample and reference cells, refracted as function of differences in refractive index o Configured such that difference in intensity of light is function of disparity in n between reference and sample channels
  • UV-Vis Detector: o Presumes analyte is capable of absorbing UV/Vis light o Detection of fixed or multiple wavelength o Beers law o Normally uses a Z-flow cell placed in detector path § Long path length – maximise sensitivity
  • Fluorometric Detectors: o Selective and sensitive o Can become universal by adding fluorophores prior to reaction
  • Electrochemical Detectors: o Dropping mercury electrode used to conduct voltametric investigations of eluent – provides species selective detection o Also, array of microelectrodes can be used to oxidise and reduce analyte as it travels through detection capillary o Signal from all electrodes is pooled together o Targets: