C3110 Final Flashcards
Mobile Phase
The phase in which the solute moves through the column
Eluent
Fluid entering column
Eluate
Fluid leaving the column
Stationary phase
Determines the amount of time the solute spends in the column
2 Main types of columns
- Packed
- Open Tubular
What are the 5 mechanisms of separation?
- Adsorption
- Partition
- Ion-exchange
- Size exclusion
- Affinity
Definition of retention time
Time between injection onto the column and when the component reaches the detector
Definition of retention volume
Volume of mobile phase reuired to elute a particular solute from the column
Dead time
Time required for an unretained solute to travel through the column
Volume flow rate
How many mL of solvent per minute travel through the column
Linear Flow rate
how many cm are travelled in 1 min by the solvent
What do we keep constant in terms of flow rate when changing column size?
Linear flow rate
Adjusted retention time
Time required for a solute to elute beyond the time for the solvent
Relative retention equation
a= tr2’ / tr1’
What value of alpha gives better separation?
Higher
Retention factor equation
k= (tr-tm) / tm
2 factors that contribute to how well compounds are separated
- Difference in elution time between peaks (farther apart = better separation)
- Peak broadness (wider peaks = poorer separation)
Column Resolution
how far apart two bands are relative to their widths (ability of the column to separate 2 solutes)
Plate Height
proportionality constant between variance of a band and distance travelled; length of column required for one equlibration
What is the plate height/number relation?
N=L/H
Does smaller or larger plate height give better resolution?
Smaller
Is a band narrow or wider for lower number of theoretical plates?
Wider
In the Purnell resolution equation, how are resolution and plate number related?
Resolution is proprtional to the square root of N
If the column length is increased by a factor of 4, how much does the resolution increase?
Factor of 2
What are the 2 main sources of extra column broadening?
- Variation due to injection or detection
- Variance due to connective tubing
What equation describes on column broadening?
Van Deemter Eqn
Describe what vD eqn terms are present in packed, OT, and capillary columns
Packed: A, B, C
OT: B and C
Capillary: B
Describe longitudinal diffusion
Solute molecules diffuse away from the concentrated center of a band in both directions due to a concentration gradient. Inversely proportional to flow rate. Greater for gases than liquids.
Describe finite equilibration time
The solute must diffuse from the mobile phase to the surface of the sp for equlibration to occur. The time required depends on the distance the solute must travel to reach sp (and inversely on how fast it diffuses)
Describe the multiple flow paths term
Some flow paths through a packed column are longer than others.
What are the advantages of OT columns? (3)
- Higher resolution
- Shorter analysis time
- Increased sensitivity
Disadvantages of OT columns (1)
Lower sample capacity
Fronting
Too much solute has been applited (or too concentrated) and the solute becomes more soluble in sp, eventually resembling sp.
Tailing
Small quantities of solute are retained more strongly than large quantities. Leads to a long tail of gradually decreasing concentration
Separation Process in GC
Gaseous analyte transported through a column by inert mp (carrier gas)
2 Main types of GC Separation
- Partition
- Adsorption
Common carrier gases
He, N2, or H2
Column Oven
Provides temperature control for the column
Importance of Temp Control in GC
Column must be hot enough to provide sufficient vapour P for analytes to be eluted in a reasonable time
In what type of chromatography are long, narrow OT columns made of fused silica coated with polyimide used?
GC
Wall Coated OT
Liquid sp on inner wall of the column
Support coated OT
Solid particles coated with liquid sp attached to inner wall of column
Kovats Retention Index for a linear alkane
100 times the number of carbon atoms
Temperature Programming
Temperature of the column is raised during separation to increase analyte vapor pressure and decrease RT of late-eluting compounds
Lower Temp Limits
Isothermal T limit at which column can be kept for a long time
Upper Temp Limit
Column should only be exposed for a few minutes at the end of a T run
What is a consequence of using really high temperature in GC?
Sp bleeding
3 Types of Injection into OT columns
- Split Injections
- Splitless Injections
- On-column Injections
Split Injection
Used for high concentrations of analyte. High T. Mixing chamber to promote vaporization and mixing of analytes. A large portion of the sample is vented (only 0.2-2% of sample goes to column). Can be inaccurate because split ratio is not reproducible.
Splitless Injection
Trace analysis. Same port used as split injection. No mixing chamber. A large volume of dilute solution is injected slowly. Around 80% of the sample is applied to the column.
Solvent Trapping
Initial column T is 40C below boiling point of solvent, which condenses. As solutes become trapped in a narrow band at the head of the column, detection occurs.
Cold Trapping
Column T is 150C lower than the boiling points of analytes. Solvent and low boiling solutes are quickly eluted. High boiling solutes trapped in a thin band at the head of the column. T is increased to initiate chromatography.
On Column Injection
Sensitive compounds that decompose above their boiling points. Solution injected directly to the column without entering a hot port. Initial column T is low enough to condense solutes in a narrow zone. Warming initiates chromatography.
List a couple common GC detectors (8)
- Thermal Conductivity
- Flame Ionization
- Electron capture
- Flame photometric
- Nitrogen-phosphorus
- Sulfur chemiluminescence
- Photoionization
- Mass Spec
Briefly describe thermal conductivity detection in GC
Common in the past, simple and universal. Not very sensitive.
Describe thermal conductivity detection (the process).
Eluate from column flows over a hot filament (tungsten-rhenium). H2 and He have thermal conductivities about 6-10 times that of organics. A drop in eluates thermal conductivity causes the filament to heat and electrical resistance to increase, changing the voltage across the filament. This is measured by the detector.
Advantages of Thermal Conductivity
Simple
Large linear range
Responds to organics and inorganics
Nondestructive to sample
Responds to everything but carrier gas
Disadvantages of Thermal Conductivity
Low sensitivity
Won’t work well for complex mixtures
Describe flame ionization detection (process)
Eluate is burned in a mixture of H2 and air. Carbon atoms produce CH radicals, which further produce CHO+ ions and electrons in the flame. Ion production is proportional to number of carbons entering the flame. Current changes in presence of analyte.
Advantages of FID (4)
- Insensitive toward noncombustible gases
- Good for samples contaminated with water/oxides
- Good detection limits (100 times better than thermal conductivity)
- High linear response
Disadvantages of FID (2)
- Destructive to sample
- Requires additional gases and controllers
Describe electron capture detection
Sample eluate from column is passed over a radioactive beta emitter, usually 63Ni. An electron from the emitter causes ionization of the carrier gas which produces a cloud of electrons. With no analyte, there is a constant current between electrode. Current decreases when analyte with EN FGs enter and capture electrons.
What is electron capture detection used for typically?
Environmental samples, anything with halogens
What detector is also called an alkali flame detector?
Nitrogen-phosphorus
Which detector measures optical emission from selected elemetns?
Flame photometric
Which detector uses a vacuum UV source to ionize aromatic and unsaturated compounds?
Photoionization
Sample Prep Processes (4)
- Extracting analyte from complex matrix
- Preconcentrating dilute analytes
- Removing or masking interfering species
- Derivatizing analytes into a more convenient or easily detectable form
Solid Phase Extraction
Sorbent is loaded into a cartridge and sample is loaded onto the phase, retaining analytes that strongly interact with the phase.
Solid-phase mixroextraction
Extracts compounds from liquids, air, or sludge without solvent.
Fused silica fiber coated with sp is attached to a syringe. A large volume of solution is extracted; mass extracted is proportional to concentration of analyte in solution.
Which sample prep method uses a magnetic stir bar to absorb hydrophobic analytes?
Stir-bar sorptive extraction
Purge and Trap
Method for removing volatile analytes from liquids or solids, concentrating the analytes, and introducing them into GC. Goal is to remove 100% analyte from sample.
Thermal Desorption
Method for relasing volatile compounds from solid samples.
Order of decisions in method development (GC), (5)
- Goal of analysis
- Sample Prep
- Detector
- Column
- Injection
Typical length and inner diameter of fused-silica capillary tubes
50cm
25-75um
Principle of CE
Different ions have different mobilities and move through the column at different speeds.
What is the only source of band broadening in CE?
Longitudinal diffusion
Describe electrophoresis
When an ion with charge q is placed in an electric field E, the force on the ion is qE. The ion reaches a steady speed when the accelerating force equals the frictional force.
Electrophoretic mobility
Proportionality constant between the speed of the ion and the electric field strength
Describe electroosmosis (EOF)
Inside wall of a fused-silica capillary is covered with silanol, which have negative charge above pH 3. A tightly adsorbed layer of cations partially neutralizes the negative layer. Remaining charge is neutralized by mobile cations in the diffuse part of the double layer in the solution near the wall. Excess cations in the diffuse part are attracted to the cathode and impart net momentum, called EOF
What is the contrast between EOF and hydrodynamic flow?
In hydrodynamic flow, the velocity flow is fastest at the center and slower at the walls due to friction. This is not the case in EOF.
Apparent mobility
Sum of electrophoretic mobility and electroosmotic mobility
True or false: smaller ions have smaller electrophoretic mobility
False
True or false: At neutral or high pH, electroosmosis transports anions to cathode because electroosmosis is faster than electrophoresis.
True
What is the number of theoretical plates proportional to in CE?
Apparent mobility, applied electric field, and distance to the detector.
At a fixed voltage, EMF is strongest in shorter or longer capillary?
Shorter
2 Methods of Sample Injection in CE
- Hydrodynamic injection (P)
- Electrokinetic Inject (electric field)
What is BGE and how does it change during CE runs?
Background electrolyte
pH will be altered during electrophoresis, potentially forming a gradient and effecting migration times. BGE must be replaced after every run to avoid this.
How do we reverse the direction of EOF?
Add a cationic surfactant
What is Micellar electrokinetic chromatography?
Used to separate neutral species. Uses a pseudostationary phase in the form of a micellar phase.
Describe the process of MEKC
In the absence of micelles, neutral analytes reach detector at the same time. Negative micelles, injected into sample, reach detector at a specific time. If a neutral molecule equilibrates between solution and micelle, the migration time is increased, since the micelles migrate slower.
What term does MEKC contain that CE does not in the vD eqn?
Mass transfer, C
How can we use MEKC to separate 2 chiral molecules?
ADD BGE modifiers, such as cyclodextrins
Name 5 common CE detectors (briefly describe each)
- UV; poor sensitivty and background must have low absorption
- Fluorescence; high dynamic range
- Contactless conductivity detection; electrodes outside capillary that measures conductivity differences
- Amperometric detection; sensitive to analytes that oxidize or reduce
- Electrospray MS; low detection and gives structural info
Advantages of CE compares to chromatography (4)
- High resolution
- Low waste (Green)
- Simple equipment
- Low sample consumption
Disadvantages of CE (4)
- Higher detection limits (insensitive)
- Irreproducibility
- Insolubility of analyes in BGE
- Challenging scale up
What is a real world advance of CE?
Lab on a chip devices
Zipchip
What are the 6 components of HPLC?
- Autosampler
- Solvent delivery system
- Sample injection valve
- High-P chromatography column
- Detector
- Computer
Why do smaller particles give better resolution in HPLC? (2)
- More uniform flow reduces A term
- Less distance the solute must diffuse reduces C term
What do smaller particles give in HPLC? (4)
- Higher plate number
- Shorter run time
- Lower detection limits
- Higher P
Describe HPLC column sp
Filled with sp of highly pure, spherical, microporous particles of silica.
Guard Column
Protects the main chromatographic phase in the column
Most common bonded phase
C18
RPLC
sp is nonpolar/weakly polar, solvent is polar. Less polar solvent is a stronger mp.
NPLC
Polar sp and less polar solvent. More polar solvent has higher strength.
Eluent strength
Measure of the solvent adsorption E on bare silica; value of pentane defined as 0
What is HILIC and what is it used for?
HILIC is hydrophilic interaction chromatography. Used to separate molecules too polar to be retained by RPLC.
Sparging System
Removes dissolved gases by sweeping them out of solution with fine bubbles of an inert gas such as He.
Why is sparging necessary in LC?
Dissolved oxygen can absorb UV and interfere with UV detection. Other gases can cause issues for the column, pumps, and detectors.
What is the process to clean a column before storage (LC)?
- Replace aqueous buffer with water and wash with 5-10 mobile phase volumes.
- Wash with 10-20 volumes of strong eluent.
- Store column with solvent to inhibit bacterial growth
Why would a material with a high UV cutoff not be used very often as a solvent with UV detection?
A high UV cutoff results in very many materials absorbing UV and thus is not very sensitive.
Describe the general elution problem and what is used as a solution.
The general elution problem is that one set of conditions is not suitable to separate a complex mixture of analytes in a reasonable amount of time. Gradient elution is a solution.
Gradient Elution
Uses a continuous change of solvent composition to increase the mobile phase strength.
How do spectrophotometric detectors work?
A photodiode array records the spectrum of each solute as it is eluted. Spectra can be matched with library spectra to identify the analyte.
Fluorescence Detection
Eluate is excited with a laser and fluorescence at longer wavelengths is measured. Up to 100 times more sensitive than UV detectors, but few analytes fluoresce.
Electrochemical Detector
Responds to analytes that can be reduced or oxidized. Potential is controlled with respect to a reference electrode, while current is measured between working electrode and auxillary electrode.
Refractive Index Detection
Responds to almost every solute. Detection limits 1000 times poorer than UV. When a solute with a different refractive index than the mobile phase enters the cell, a beam of light is deflected. Useless for trace analysis and gradient elution.
Common methods of LC-MS interface
- ESI
- APCI
What does a performance qualification sample (internal standard) tell us?
If there is a problem with the run, the internal standard indicates whether or not the issue is with the chromatography of the analyte or with the instrument.
Anion exchangers
Contain positively charged groups bound to the sp, that attract solute anions
Cation exchangers
Contain negatively charged groups bound to the sp, that attract solute cations
Resin
Amorphous particles of organic material
Which resins are commonly used in ion-exchange?
Polystyrene resins
When would it be ineffective to use a weakly acidic resin?
Above pH where they become protonated and lose their ion-exchange capacity
Selectivity coefficient
Equilibrium constant for ion-exchange reactions
What factors contribute to favoring of ions in ion-exchangers?
Higher charge
Decreased hydrated radius
Increased Polarizability
The binding of one divalent metal ion releases how many hydronium ions?
2
Equivalents
The amount of charge exchanged; amount of cation that will exchange with one mole of a monovalent ion
Ion-exchange capacity
The number of ionic sites on the resin that can participate in the exchange process
List the 3 classes of ion exchangers and when they are used.
- Resins, small molecules
- Gels, large molecules
- Inorganic, harsh chemical conditions
How is gradient elution used in ion-exchange (2 methods)
- Ionic strength
- Changing pH
List 3 applications of ion-exchange chromatography
- Convert one salt into another
- Preconcentration of trace components to obtain enough for analysis
- Purify water
What are 3 desired attributes of a new chromatographic method?
- Adequate resolution
- Short run time
- Rugged
What should k be for a good separation?
Between 0.5 and 20.
Too small = first peak distorted by solvent.
Too large = too long.
What are the 4 initial steps of method development?
- Determine goal
- Select method of sample prep
- Choose detector
- Test initial system
What are two methods to optimize a separation?
- Adjust mobile phase composition
- Add gradient elution
What starting conditions need to be considered for a method (6)?
- Sp
- Column
- Flow rate
- Mobile phase
- Temperature
- Sample size
What needs to be checked for efficiency during method development?
N, number of theoretical plates
What are 4 ways the separation factor can be adjusted in method development?
- Fine-tune the solvent composition
- Vary column T
- Change solvent polarity
- Change stationary phase
How are the column dimensions (and what dimensions) optimized in method development?
Length, particle size, flow rate.
Changing column length may increase separation time.
How does pH affect retention?
Depending on at what pH different forms of a compound are protonated/deprotonated, they may be similar in retention.
What is a supercritical fluid?
Formed above the critical T/P of a substance where a distinct liquid/gas phase cannot exist.
Densities, viscosities, and other properties are intermediate between gas and liquid.
What is an important property of SCFs?
Ability to dissolve large non-volatile molecules
What is one condition to use SCFs for LC?
Their critical T must lie within the operating conditions of the instrument.
Supercritical Fluid Chromatography (SFC)
Provides a system intermediate between GC and HPLC. Usefule for compounds that are non-volatile or thermally labile
Why is temperature control important in SFC?
Small changes in T have large changes in density of a SCF.
What is the effect of Pressure on SFC?
Density is also easily impacted by small pressure changes. Increasing P increases the solvent effectivity and shortens elution times.
Which term is better for SFC than HPLC in the van Deemter eqn?
C term because lower density and lower IM forces of SFCs leads to faster partitioning
What sp is used for SFC?
Packed columns, similar to LC and capillary, similar to GC
Which detectors can be used for SFC?
Any previous GC/LC detector
Comparison of SFC to GC and LC (5)
- SFC usually faster than LC
- Less band broadening than in GC
- Plate height lower at higher flow rates for SFC compared to LC
- Faster separations with no loss in selectivity or resolution
- Variation in mobile phase can be used to change selectivity factors
What are the advantages of using supercritical solvents? (3)
- Cheap
- Easily recovered or vented
- Green
Advantages of SFC (sample prep) (4)
- Fast extractions
- Solvent strength is altered by changing P
- Liquid easily removed
- Liquid is cheap, non-toxic, and inert
Explain liquid-liquid extraction
Usually used in prep scale work-ups to isolate and/or remove interferences. Used to extract a nonvolatile analyte from aqueous solution into an organic solvent.
Continuous liquid-liquid extraction
Solvent boils from flask and condenses into extraction vessel. Dense droplets of solvent falling through the liquid column extract the analyte. When the liquid level is high enough, extraction solvent is pushed through the return tube to the solvent reservoir. Analyte is slowly transferred from the light liquid at the left into the dense liquid in the reservoir.
Steps in dispersive liquid-liquid microextraction (4)
- Extraction solvent plus dispenser solvent in a syringe is transfer to aqueous sample containing analyte in centrifuge tube
- Liquid injected rapidly to create emulsion and mixed with vortex. Fine droplets of extration solvent form
- Centrifuge to separate extraction solvent and water. Analyte dissolved in dense extraction solvent
- Remove some solvent and inject to a column
Solid Phase Extraction
Used to concentrate and purify samples for analysis. An aqueous solution containing analyte passed through a small volume of sp. Analyte adheres to sp. Concentrated analyte is eluted using a small amount of eluent such as MeOH.
What does QuEChERS stand for
Quick, Easy, Cheap, Effective, Rugged, Safe
What is QuEChERS used for
To remove matrix components from extract prior to chromatography
Describe the QuEChERS process. (4)
- Analytes extracted from solid samples with acetonitrile and centrifugation.
- Aliquot of extract deposited into QuEChERS tube with MgSO4, ion exchangers and other sorbents
- Agitate and centrifuge
- Analyze aliquot by LC
Advantages of QuEChERS (2)
- Little organic solvent required
- Many samples can be handled at once
Mass Spectrometry
Technique for studying masses of atoms/molecules/fragments using behavior of their ions in the gas phase and under the influence of electric or magnetic fields
3 Essential Components of any Mass Spectrometer
- Ion source
- Mass Analyzer
- Detector
Most common ionization sources (gas) (2)
- EI
- CI
Most common ionization sources for liquids (2)
- ESI
- APCI
Most common ionization sources for solids (2)
- LDI
- MALDI
Explain how MS occurs
Ions are created or volatilized in the ion source through reactions with electrons (EI and ESI) or chemical reactions (proton transfer or adduct formation, CI and APCI). Ions are accelerated by an electric field, and then ions are separated by their mass-to-charge ratio in the mass analyzer. Electron multiplier detector converts each arriving ion into electrons that reach anode where current is measured.
Mass-to-charge ratio
Particles with the same m/z move down the same path in a vacuum when subjected to the same magnetic and electric fields
Molecular ion
m/z particle represents the intact molecule with only one electron removed
Protonated molecule
[M+H]+
Base Peak
Highest intensity m/z ion in the spectrum
Nominal Mass
Integer mass of the most abundant naturally occuring stable isotope
Exact Mass
Exact mass of the most abundant naturally occurring stable isotope
Isotope Ratio
Mass spectrum give peaks corresponding to the same molecule with different isotopes
Nitrogen Rule
For compounds with an odd nominal mass, the number of N atoms is odd. An even nominal mass indicates an even number of N atoms or 0.
Resolution in MS
Smallest difference in m/z values that can be detected as separate peaks
Resolving power
Ability of a mass spectrometer to separate 2 peaks with similar mass
For which compound is the base peak also the molecular ion?
Benzene
What are the 3 main types of fragmentation processes (note that 1 is specific to ketones)
- Homolytic cleavage
- Heterolytic cleavage
- McLafferty rearrangement
How can Cl and Br be specifically identified using MS?
The M+2 peak of Br is in a 1:1 ratio, whereas for Cl it is in a 1:3 ratio.
Types of Mass Spectrometers (6)
- Magnetic sector
- Transmission quadrupole
- Time of flight
- Quadrupole Ion Trap
- Linear Ion Trap
- Orbitrap
Magnetic Sector MS
separates gas ions by accelerating them in an electric field and deflecting ions of different m/z ratio through different arcs in a magnetic field. Detected by an electron multiplier.
Conversion dynode
Ensures all ions produce a similar electrical response at detector for magnetic sector MS
What is an electric sector used for in magnetic sector MS?
Reduce the spread of KE
Double-focusing MS
Attains high resolution by employing an electric sector with the magnetic sector to select ions with a narrow range of KE.
Go back and clarify differences between TOF and all other MS, write it out explicitly, might have to use textbook or another source cuz its not clear in notes
Fo
Most common TOF (2)
- Quadrupole
- Ion Trap
Why does MS require high vacuum?
Prevent molecular collisions during ion separation
Electron Ionization
Most common ionization source. Used with GC. Electrons emitted from a hot filament are accelerated through 70V before interacting with incoming molecules M. Resulting molecular ion can have so much extra E that it breaks into fragments. These fragments and the ion are accelerated to mass analyzers. The molecular ion may have a small or absent peak.
Chemical Ionization
Less fragmentation than EI. Ionization source is filled with a reagent gas and energetic electrons convert methane into a variety of reactive products, including CH5+ which reacts with analyte M to give MH+, the most abundant ion in the MS.
Describe APCI
APCI uses a corona discharge to create a variety of gas ions from aerosol droplets. Both APCI and ESI produce unfragmented ions.
Describe ESI
Employs high voltage at the exit of the column, with coaxial N2 flow, to create a fine aerosol of charged droplets containing ions. Analyte often associated with other ions to give new species. Controlling pH helps ensure selected analytes are in anionic or cationic form
Reconstructed Total Ion Chromatogram
Shows the signal from all ions above a chosen m/z emerging from chromatography as a function of time
Extracted Ion Chromatogram
Shows the signal for one ion taken from the complete MS (scans all, shows one)
Selected ion monitoring
One or a few values of m/z; improves signal-to-noise because all time spent measuring just that one or a few ions
Selected reaction monitoring (MS/MS)
A precursor ion is isolated by one mass falter and passes into a collision cell in which it breaks into products. One or more product ion is then selected by a second mass filter for passage to the detector. Extremely selective for one analye
What are 3 types of scans for MS/MS and briefly describe
- Product ion scan: precursor ion is focused in Q1 and transferred to collision cell where it interacts and fragments, and then the fragments are measured.
- Precursor ion scan: the fragments are selected and the original Q is measured to see what produces those fragments
- Neutral Loss Scan: everything is passed through, and the ions that are lost are measured
Atomic Spectroscopy (Overview)
Samples are vaporized and decompose into atoms and ions. Concentrations are measured based on their absorbance or emission of light. Gives the ability to distinguish one element from another in a complex mixture. Analyte can be measured at ppm or lower.
Atomic Absorption
A hollow-cathode lamp passes light through the sample in the flame and the intensity of the transmitted radiation is measured by a photon transducer. Bandwidths of 10-100nm. Spectra consist of sharp lines (so sharp that there is little overlap)
Atomic Fluorescence spectroscopy
Atoms in the flame are irradiated by a laser to promote them to an excited state from which they can fluoresce to return to the ground state.
Atomic Emission Spectroscopy
Radiation from hot atoms whose electrons have been promoted to an excited state in the flame or plasma is emitted
Flame atomizer
A liquid sample is aspirated through a plastic tube into the flame, where liquid evaporates and the remaining solid is atomized in the flame
Premix Burner
Fuel, oxidant, and sample are mixed prior to introduction into the flame.
Pneumatic Nebulizer
Sample solution is drawn in by rapid flow of oxidant past the tip of the sample capillary
How does a premix burner work to prepare sample for a flame?
The liquid breaks into a fine mist as it leaves the capillary. Spray is directed against a glass bead, upon which droplets break into smaller particles. Mist, oxidant, and fuel flow past baffles that promote mixing and block large droplets.
Nebulization
Formation of small droplets
Aerosol
Fine suspension of liquid (or solid) particles in a gas
In flame atomization, about how much of the initial sample is contained in the aerosol reaching the flame?
5%
List several of the processes that can occur in a flame (most important one first)
- Excitation/Emission
- Ionization/suppression
- Vaporization
- Oxidation
- Decomposition
What is the most common fuel used for flame atomization?
Air/acetylene, giving temps of 2400-2700K
Rich flame
Excess fuel; increases sensitivity because excess carbon reduces metal oxides/hydroxides
Lean flame
Excess oxidant; gives a hotter flame
In cyanogen/oxygen fuel, which component is the fuel and which is the oxidant?
Cyanogen - fuel
Oxygen - oxidant
What must be subtracted in flame atomization to get the analyte signal?
The light emitted by the flame must be subtracted from the total signal
Graphite furnace
Atomization source; more sensitive than a flame and requires less ample.
How does a graphite furnace work for atomization?
1-100uL of sample is injected into the furnace, through a hole at the center. Light from a HCL travels through the window at each end of the graphite tube. Sample injected onto platform, which is heated by radiant E from wall; sample vaporized when wall reaches a constant T.
Disadvantages of graphite furnace (3)
- Requires more operator skill
- Limited lifetime
- Memory effect causes interference from previous runs
Advantages of Furnace over flames (6)
- Less sample volume
- Furnace confines sample in light path for several seconds
- Pre-concentration possible; multiple aliquots can be deposited onto furnace
- Nebulization can dilute sample; no nebulization in graphite furnace
- Solids can be analyzed
- No combustible gases required
Matrix Modifier
Substance added to sample to reduce loss of analyte during charring by making matrix more volatile or analyte less volatile
Inductively Coupled Plasma
Twice as hot as flame; stable; inert Ar environment; simultaneous multi-elemental analysis by atomic emission or MS; costs more to purchase and operate.
How does ICP for atomization work?
High-purity argon is fed into the plasma gas inlet at 14-18L per minute. A spark from a tesla coil initiates ionization of Ar, forming Ar+ and electrons, accelerated by the radio frequence field. Electrons collide with Ar atoms and transfer E to the entire gas (T 6000-10000K). Quartz torch protected from overheating by coolant gas.
What instrument in ICP is used to reduce the concentration of analyte needed for an adequate signal?
Ultrasonic nebulizer
Piezoelectric crystal
One whose dimensions changed in an applied electric field. Sinusodial voltage applied btwn two faces of the crystal cause it to oscillate
How does Temperature affect Atomic Spectroscopy? (2)
- Deterines the degree to which a sample breaks down into atoms.
- Determines the extent to which an atom is found in its ground, excited, or ionized state. (Boltzmann distribution)
Due to the Boltzmann distribution, how does varying the temperature by small amounts impact the ground and excited state populations?
Ground state: negligible influence
Excited state: significant increase (10K could have up to 4% increase in population)
If the excited state population increases by 4%, how does the emission intensity change?
Increase by 4%
By what principle are linewidths determined?
Heisenberg uncertainty principle
Dopple Broadening in Atomic Spectroscopy
Atomic motion occurs in every direction. An atom moving toward radiation source sees a higher frequency light than one moving away. There is no shift for atoms moving perpendicular to the source.
Pressure broadening in atomic spectroscopy
Caused by collisions of the emitting or absorbing species with other atoms or ions in a hot environment. Collisions between atoms shorten the lifetime of the excited state. Collision frequency is proportional to P. This is similar in magnitude to Doppler broadening.
What do we use to produce narrow lines of correct frequency in atomic spec?
Hollow-cathode lamps
Hollow-Cathode Lamp
Consist of a tungsten anode and a cylindrical cathode, and a glass tube filled with neon or argon. Cathode is made of metal whose spectrum is desired. Potential difference is applied across the electrodes, generating a current and ionizing inert gas. Gas cations can acquire enough KE to dislodge some metal atoms from cathode producing an electron cloud (sputtering). A portion of sputtered atoms are in excited state and emit radiation as they return to ground state.
Background correction
Distinguishes analyte signal from absorption, emission, and optical scattering by/from a sample matrix (flame, plasma, or furnace).
Why is background correction critical for graphite furnaces?
Residual smoke from charring
List the 3 main background correction methods
- Beam chopping
- Deuterium lamp
- Zeeman
Describe beam chopping as a correction method
Signal reaching detector while beam is blocked must be from flame emission. Difference between two is analytical signal. Does not correct for scattering
Describe deuterium lamp background correction
Broad emission from a D2 lamp is passed through the flame in alternation with that from the hollow cathode. Light from hollow cathode is absorbed by analyte and background; but, light from D2 lamp is only absorbed and scattered by the background. Difference between the 2 is the absorbance of the analyte
Describe the Zeeman effect background correction for graphite
A magnetic field applied parallel to light path through the furnace is pulsed on and off, splitting analyte signal into 3 parts: Two shifted and one not. Unshifted signal does not absorb light. Sample and background observed when the field is off, thus, only background is observed when the field is on. Difference between them is the corrected signal.
Compare the sensitivities for flames, furnaces, and ICP
Detection limits for furnaces is 2 orders of magnitude lower than for flames, because the sample is confined in the small volume of the furnace for a long time.
ICP has intermediate sensitivity; close to the sensitivity of the graphite furnace
Spectral Interference
Overlap of the analyte signal with signals due to other elements or molecules in the sample or with signals due to the flame or furnace
Chemical Interferences
Caused by a component of the sample that decreases the extent of atomization of the analyte. Elements that form very stable diatomic oxides are not completely atomized. Molecular spectra are much broader than atomic spectra. This can be removed by extraction or Chromatography.
Releasing Agents
Chemicals added to a sample to decrease chemical interference
Ionization Interferences
Can be a problem in the analysis of alkali metals with low ionization potentials. Ionized atoms have diff E levels than neutral atoms, so desired signal is decreased.
Ionization Suppressor
Decreases the extent of ionization of the analyte
Standard Addition
Compensates for many types of interferences by adding known quantities of analyte to the unknown.
Describe laser ablation
A pulsed laser beam is focused onto a solid sample. An explosion occurs of particles, atoms, electrons, and ions in the gas phase
Which laser is commonly used for laser ablation?
Nd:YAG (neodymium-doped yttrium aluminum garnet crystal)
Laser-Induced Breakdown Spectroscopy
Analyzes the ablation product in a plasma by observing atomic emission. Suitable for any element that emits a photon with a measurable UV or visible wavelenth
What is the emission due to in laser ablation?
Emission is due to formation of a short-lived plasma under sample irradiation with a highly energetic laser pulse. Can be used for depth profiling
ICP-MS
When ICP plasma is directed into an MS, separating and measuring ions according to m/z ratios.
What is a downside of ICP-MS
Requires extremely pure water and trace-metal grade HNO3, very expensive
What is the ICP source rich in?
Ar+
Secondary ions ArH+, ArC+, ArN+, etc.
Isobaric interference in ICP-MS
They interfere with the measurement of analyte ions of the same m/z charge ratio
Dynamic Rxn Cell in ICP-MS
Uses thermodynamically favorable reactions to reduce isobaric interference
X-Ray Fluorescence
Emission of X-Rays following absorption of X-Rays by a material. Elements identified by peak energies and quantified by number of photons in each peak
What is the hump in XRF that makes it not good for quantification in this region?
Bremsstrahlung
Why is XRF semi-quantitative? (3)
- Higher uncertainty (RSD ~30%)
- Matrix effects (self-absorbance)
- Matrix components (absorbing/scattering X-rays)
Describe handheld XRFs
Can detect levels as low as 1ppm for As, Pb, and Hg. Should be mounted in a lead-lined test stand to prevent user exposure.
What is electric charge measured in?
Coulombs (C)
What is the charge of an electron?
1.602*10^-19C
What do electrons appear as in electrochemical reactions?
Reactants or products
Electric current
Measured in ampere (A); represents rate of electron flow. Provides a measure of rate of an electrochemical reaction
Electric potential
Measured in volts (V). The difference between two points exerts a force on electrons and causes a current to flow.
Potentiometry
A difference in concentration (chemical potential) generates a potential difference
Potential vs. Current
-Potential controls/measures E of electrons in a material. Electrons move from low potential to high potential.
-Potential is related to thermodynamics
-Current is related to kinetics
What are the 3 main electroanalytical techniques?
- Coulometry
- Potentiometry
- Amperometry
Potentiometry definition
Measurement of voltage at zero current using 2 electrodes to provide chemical information
Applications of potentiometry (2)
- Measuring activities of specific ions
- Detecting end-point for titrations
Indicator electrode
An electrode that responds to an analyte
Reference electrode
An electrode with known constant potential
Junction Potential
Develops at the interface btwn 2 dissimilar electrolyte solution. Typically small and are found at each end of the salt bridge connecting 2 half-cells.
How are junction potentials minimized?
By choosing ions with similar mobilities
Ion Selective Electrodes
Do not involve a redox process. Responds selectively to one ion through a thin membrane that bindings only a specific ion.
Components of Ion-selective electrodes (4)
- Hydrophobic organic polymer membrane
- Viscous organic solution
- Ion-exchanging ligand that binds the analyte cation
- A hydrophobic anion
How do ISE work?
The inner filling solution contains a constant concentration plus counter-ion. Outside analyte solution contains constant and counteranion. C+ can diffuse across the membrane but not R-, B-, or A-. Difference in potential across membrane is measured by 2 reference electrodes on the inside and outside. When the concentration of C+ in analyte solution changes, voltage changes
K+ ISE
Uses valinomycin as the ligand. Chosen because it has a high affinity for K+ and low affinity for other ions. Membrane is PVC and a liquid plasticizer
What is the most common ISE?
Glass electrode
Describe Glass Electrode
pH sensitive part is the thin glass bulb at the bottom of the electrode. 2 Reference electrodes measure potential across the membrane. Salt bridge is the porous plug
How do Glass Electrodes work?
Each side of the glass membrane is hydrated and separated by a dry glass layer. Metal cations can diffuse out of the glass in these regions and into the solution. H+ can diffuse into the membrane and replace the metal cations, in an ion-exchange equilibrium.
What are the 8 sources of errors in pH Measurements?
- Standards
- Junction potential
- Junction potential drift
- Sodium error
- Acid error
- Equilibration time
- Temperature
- Cleaning
What are the 4 classes of ISE?
- Glass Membrane
- Solid-state electrodes
- Liquid-based electrodes
- Compound electrodes
What are solid-state ISE based on?
An inorganic crystal
Describe the fluoride electrode
Used to monitor the fluoridation of municipal water supplies. Employs a crystal of LaF3 doped with Eu2+, creating anion vacancies, with a filling solution. F- migrates across the crystal. Adjacent F- can jump into the vacancy, leaving a new vacancy behind. F- diffuses from one side to the other, jumpint through the vacancies. The response is recorded
What is measured in coulometry
The number of moles of electrons
What is constant during coulometry?
Current
Advantages of Coulometry (3)
- Precision
- Sensitivity
- Generation of unstable reagents in situ
Amperometry
Current at the working electrode is proportional to analyte concentration
2 Common Amperometric techniques
- Clark electrode
- Glucose biosensor
How does the clark electrode in amperometry work?
Measures dissolved O2
How does the glucose biosensor work in amperometry?
Generates H2O2 by oxidation of glucose, which is measured. A mediator is employed to rapidly shuttle electrons between electrode and analyte.
Coulometric glucose monitor
Counts electrons released by oxidation of all glucose in a small blood sample
What is electrical wiring in amperometry?
Enzyme and mediator; increases signal from the desired reaction and decreases background current from mediator diffusing to the auxillary electrode
What is voltammetry?
A collection of methods in which the dependence of current on the applied potential of the working electrode is observed
How does voltammetry work?
Current from an oxidation or reduction is measured as the potential of the working electrode is changes
Residual current
Due to oxidation or reduction of species in solution other than analyte. Should be subtracted
Faradaic current
What is measured in voltammetry; due to reduction or oxidation of analyte at working electrode
Charging current
Comes from the flow of ions toward the electrode surface and interferes with detection of the reduction reaction
What happens to limiting current for quantitative analysis? (3)
- Is controlled by analyte diffusion rate
- Convection is minimized by using an unstirred solution
- A high concentration of supporting electrolyte is used to minimize migration
Microelectrodes
Low capacitance increases sensitivity by reducing charging current, permits rapid voltage scanning (short-lived species can be studied).
