Final Flashcards
Relative Transmittance (T) definition
Fraction of incident light absorbed by the solution
Used as an index of analyte concentration
Relative Transmittance Equation
T = P/P0
P: radiant power of beam exiting cell
P0: radiant power of beam incident on absorption cell
Beer’s Law definition
Absorbance is directly proportional to concentration
Beer’s Law equation
A = abc
a: absorptivity
b: pathlength through solution
c: concentration of absorbing species
A = abc = ?
log(P/P0) = -logT = 2-log(%T)
Background Correction definition
processes other than analyte absorption (like reflection/scattering) result in significant decrease in power of incident beam
How to correct background?
Reference cell prepared by adding distilled water/solvent to an absorption cell
Placed in path of light beam, and the power of the radiation exiting the reference cell is measured and taken as P0 for sample cell
Absorbance equation using Background Correction:
A = log(Psolvent/Panalyte solution)
Psolvent: radiant power of beam exiting cell containing solvent (blank)
Deviation from Beer’s Law: (3)
- Only valid for diluted solution, up to mM (higher concentration, higher intermolecular distances, affect absorption of proton)
- Chemical processes such as the reversible association-dissociation of analyte molecules, or the ionization of a weak acid in an unbuffered solvent
- Instrumentation limitation (strictly applied to monochromatic radiation)
Reference solution in UV/Vis
Only solvent; chemically modify reference solution if sample compound is modified
Sample holder/cuvette in UV/Vis
Material doesn’t absorb any radiation in spectral region
Ex: quartz, silicate glass, plastic
How to choose appropriate wavelength in UV/Vis?
Choose wavelength at which the analyte has max absorbance and where the absorbance doesn’t change rapidly with changes in wavelength
How to Calibrate UV/Vis (0% and 100% Transmittance)
0%: shutter closed, set base level current, set 0%T
100%: shutter open, reference solution, set 100%T
Why would there be a nonlinear calibration curve in UV/Vis?
Due to concentration-dependent chemical change in the system or limitation of the instrument
Light Source in UV/Vis: (2)
Visible: tungsten filament lamp
UV: deuterium electrical discharge lamps; used with quartz holders,
Parts of UV/Vis spectrometer (5)
- Light source
- Monochromator
- Sample/reference holder
- Radiation detector
- Readout device
UV/Vis: Monochromator
To isolate the specific, narrow, continuous group of wavelengths to be used in the assay
UV/Vis: Radiation Detector
Produce an electric signal whn struck by photons
Signal is proportional to radiant power
UV/Vis: Readout Device
Analog meter or digital display showing transmittance or absorbance
Fluorescence Spectroscopy
Absorb energy from radiation in the UV/Vis range, then radiation simultaneously emitted when analyte relaxes
More sensitive than absorption spectroscopy
Key components of Fluorescence Spectroscopy (5):
- Light source
- Monochromator (emission and excitation)
- Sample/reference holder
- Radiation detector
- Readout device
Radiant power in fluorescence spectroscopy
Radiant power of the fluorescence beam emitted from a sample is proportional to the change in the radiant power of the source beam as it passed through the sample cell
Radiant power in Fluorescence equation
Pf = φ(P0-P)
Pf: radiant power
φ: quantum efficiency
Beer’s Law and Radiant Power
A = kP0c
(linear range; affected by pH, temp, solvent, impurity)
Infrared (IR) Spectroscopy definition
Measurement of the absorption of different frequencies of IR radiation by the matter
IR ranges (3):
Near: 800-2500 nm
Mid-IR: 2500-15000 nm
Far IR: 15000-100000 nm
(Near and Mid most used)
Energy equation in IR spectroscopy
E = hυ
h: Planck’s constant (6.626e-34 Js)
υ: frequency (Hz/s^-1)
T/F: The energy gap between each vibrational energy level is a different magnitude of energy of photons from IR radiation
False: it’s the same magnitude of energy
Major vibration in IR:
stretching (change bond length) and blending (change bond angle) produce a change in dipole moment
Molecular asymmetry in IR
Required for excitation by IR; fully symmetric molecules don’t display absorbance unless asymmetric stretching or bending transitions are possible
Mid-IR Spectroscopy
Absorb light in the 2.5-15 μm region
Fourier transform instrument (FTIR) components (3):
- Light source
- Interferometer
- Detector
Michelson interferometer in IR
Beam is split by a splitter and then recombined by reflecting back the split beams with mirrors
Detector in IR spectroscopy:
Output voltage varies with changes caused by varying levels of radiation striking the detector
Why is quantitative IR difficult to obtain? (3)
- Deviations from Beer’s Law (low intensity IR source, narrow bands, require calibration sources)
- Complex spectra
- Lack of reference cell
Other methods of IR Spectroscopy (3)
- Reflectance
- Photoacoustic
- Near-IR
ATR-FTIR (Reflectance IR) definition
To measure thick, solid, viscous liquid/paste; surface sensitive
Photoacoustic IR definition
Measures the effect of absorbed energy; can use tunable laser; gas/liquid/solid suitable for highly absorbing samples
Near-IR Spectroscopy definition
Directly measure the composition of solid food product by diffuse reflection technique (700-2500 nm)
Minimize the impact of size/shape of sample particles
Quantitative analysis of samples
Near IR: Transmission Mode
Liquid/solid samples at 700-1100 nm
Easier sample prep
Near IR: Reflection Mode
Solid/granular samples
Sample prep by packing food tightly into a cell against a quartz window
Reflectance equation (IR)
R =I/I0
I: intensity of radiation reflected from the sample at a given wavelength
I0: the intensity of radiation reflected from the reference at the same wavelength
Rheology definition
Science of deformation by rheological methods
Viscosity definition
internal resistance to flow
Stress definition (σ):
measurement of force, expressed with Pascals
Stress equation
σ = F/A
2 types of stress:
- Normal stress: force applied directly (perpendicular) to a surface-tension/compression
- Shear stress: force parallel to the sample surface
Strain definition (ε):
dimensionless quality representing the relative deformation of a material
determined by direction of stress: negative values for compression, positive for extension
Normal strain equation:
ε = ΔL / L
Hook’s Law for normal stress equation:
σ = E*ε
E; Young’s modulus (N/m^2 or Pa)
Shear strain equation:
γ = ΔL/h
Hook’s law for shear stress equation:
σ = G*γ
G = shear modulus (N/m^2)
Shear rate for liquid samples equation:
shear rate = U/h
Apparent viscosity (η) definition:
Shear stress/shear rate
If constant, then Newtonian fluid- otherwise non-Newtonian
Rheogram definition:
graphical representation of flow behavior showing the relationship between stress and strain or shear rate
Pseudoplastics fluid behavior
Shear thinning; shear rate increases, viscosity decreases
Dilatant fluid behavior
Shear thickening: shear rate increases, viscosity increases
Thixotropic definition
shear stress decreases with time (pseudoplastic, shear thinning)
Anti-thixotropic definition:
shear stress increases with time
(dilatant, shear thickening)
Rheological Fluid Models (4):
- Herschel-Bulkley
- Newtonian:
- Power Law
- Bingham Plastic
Herschel-Bulkley model equation:
σ = σ0 + kγ ^(n)
n: flow behavior index
σ0: yield stress (Pa)
k: consistency index (Pas)
Newtonian model equation:
σ = μ*y
Power law model equation:
dilatant/pseudoplastic
σ = k*y^(n)
Bingham Plastic model equation:
σ = σ0 + μpl*y
Rotational viscometry definition
Known test fixture in contact with a sample and through some mechanical, rotational means, the fluid is sheared by the fixture
2 types of steady shear mode viscometers:
- Concentric cylinder
- Cone and plate
Advantages/Disadvantages of Concentric Cylinders
Advantages:
1. Low-viscosity fluids
2. Suspensions
3. Large surface area increases sensitivity at low shear rates
Disadvantages:
1. Potential end effects
2. Large sample required
Advantages/Disadvantages of Cone and Plate:
Advantages:
1. Constant shear rate & stress in gap
2. High shear rates
3. Medium and high viscosity samples
4. Only small sample required
5. Quick n easy cleanup
Disadvantages:
1. Large particles interfere with sensitivity
2. Potential edge effects
3. Must maintain constant gap height
Experimental Procedures for Steady Shear Rotational Viscometry (5):
- Test fixture selection
- Speed (shear rate) selection
- Data collection
- Shear calculations
- Model parameter determination
Large strain testing for solids:
Compression and tension tests are used to determine large strain and fracture food properties
Texture Profile Analysis (TPA)
Double compression tests
Data related to hardness, cohesiveness and other sensory parameters
What kind of compounds is gas chromatography good for?
Thermally stable volatile compounds
Methods of Isolation for Gas Chromatography (4):
- Headspace methods
- Distillation methods
- Solvent extraction
- Solid-Phase Microextraction (SPME)
Headspace method summary definition
One of the simplest methods; direct injection of headspace vapors
2 headspace methods:
- Direct headspace sampling
- Purge and trap sampling
Direct Headspace sampling
Using a gas-tight syringe injected directly into GC
Rapid analysis
For volatiles in headspace at a detectable level for flame ionization detector
Purge and trap methods and system
Higher sensitivity for trace analysis
Valving system: Purge a gas into samples -> volatile compounds move into headspace vapors -> pass an adsorption trap -> trap is heated and carrier gas introduces the volatile compounds into the column
Headspace trap materials
synthetic porous polymers
Solvent extraction
Use of 2 immiscible phases (water and an organic solvent) by separatory funnel and continuous extractors
Preferred for the recovery of volatiles from food
Depends on solubility of solute in different solvents
SPME
Convenient, solventless extraction technique
Used for volatile aroma analysis of food and beverages
SPME Procedure (Extraction and Desorption)
Extraction:
1. Pierce sample septum
2. Expose fiber/extract
3. Retract fiber/remove
Desorption:
1. Pierce GC inlet septum
2. Expose fiber/desorb
3. Retract fiber/remove
SPME definition
Microextraction technique which employs a thin film of sorption polymers on a fine fused silica fiber
Advantages of SPME (4):
- Less solvent required, minimial solvent evaporation
- Fast, better precision and accuracy
- Less cost and contamination
- Automated
Components of GC (7):
- Gas supply and regulators
- Injection port
- Oven
- Column
- Detector
- Electrometer
- Recorder/data handling system
Gas Supply for GC:
Common carrier gas: nitrogen, helium, hydrogen
For detector: air and hydrogen
Injection port for GC
Place for sample introduction, vaporization, dilution/splitting
Variance is minimized by internal/external standards
Types of Injection Techniques (5):
- Split
- Splitless
- Temperature programmed
- On-column
- Thermal desorption
Factors to consider when direct injecting in GC (5):
- Thermal degradation
- Damage to GC column
- Effect of water vapor
- Contamination
- Vaporization speed
When is sample derivation used for GC?
If compounds are a) low in volatility, b) poorly separated due to polarity, or c) unstable
Oven in GC:
Controls the temp of the column
Determined by the interaction of the analyte with the stationary phase and the bp for the separation of compounds
Higher temp may cause faster elution but poorer resolution
2 types of column in GC:
Packed or capillary
Packed column for GC:
Solid support: silane treated diatomaceous earth
Liquid coating
Stationary phase: polysiloxane base
In general: Choose polar phase to separate polar compounds and phenyl-based column to separate aromatic compounds
Capillary column for GC:
Packing: hollow fused silica glass
Types: megabore, normal, microbore
Liquid coating: chemically bonded to the glass wall
Mobile phase - gas, stationary phase - liquid coating
Thicker film pros and cons in capillary column for GC
Pros: longer interaction w/stationary phase; better separation
Cons: Column bleeding, poor baseline
Thick vs Thin film in capillary column of GC:
Thick: for separation of very volatile compounds
Thin: good for large molecules separation
Capillary vs Packed Column in GC:
Capillary column has better resolution than packed column
Types of GC Detectors (5):
- TCD
- FID
- ECD
- FPD
- PID
Thermal Conductivity Detector (TCD)
- temp difference b/w carrier gas & mixed gas from column directly related to temp change of filament in detector
- carrier gas: hydrogen, helium, nitrogen
- universal & non-destructive; low sensitivity
-good for further analysis & separation of water
Flame Ionization Detector (FID)
-analytes from column burnt in hydrogen flame, forming ions detected by electrodes
-destructive
-good sensitivity and carbon bonds
-no response to h2o,no2, h2s
Electron Capture Detector (ECD)
- detecting decrease of a standing current generated from the flow of carrier gas
- good for halocarbons and double bonds
-high sensitivity, narrow linear range
Flame Photometric Detector (FPD)
- burn analyte, measuring wavelength of ligh
- when S/P present, characteristic wavelength is detected
- photo multiplier tube amplify current and transfer to recorder
- high sensitivity/specificity
- used for heavy metals and compounds containing S/P (pesticides and aroma)
Photoionization Detector (PID)
- UV light ionize analyte to form ions, ions detected by electrodes forming current
- Sensitive, non-destructive
-flavor analysis
Separation of GC Column: Carrier Gas
N2 most efficient carrier gas
Hydrogen is best choice
Concerns in Components: Gas Supply and Regulators (1)
Carrier gas: flow rate and type
Concerns in Components: Injection port (2)
Temperature, sample types (volume and concentration)
Concerns in Components: Oven (2)
Temp, sample types
Concerns in Components: Column (3)
Temp, sample types, packing material
Concerns in Components: Detector (3)
Temp, sample types, carrier gas
2, 4- Decadienal
Important aldehyde flavor, contributes deep fat characteristics
Key components in flavors like chicken, lamb, beef, and french fries
Mass Spectrometry definition
Analytical tool used for measuring the molecular mass of a molecule
Principles of Mass Spectrometry (3):
- Ionization of molecules: chemical/electron impact
- Separation of ions based on mass-to-charge ratio in mass analyzer
- Detection under electrostatic field
Steps of Mass Spectrometry (5):
- Sample introduction
- Ion source
- Mass analyzer
- Detector
- Data system
Routine in Analytical Labs (2):
- GC-MS: interface of mass spectrometer with GC
- LC-MS: interface of mass spectrometer with HPLC
Sample Introduction in MS: Pure Compounds (2)
- Direct Injection: for gases or volatile liquids (same as GC)
- Direct Insertion Probe: somewhat volatile solid
Sample Introduction in MS: Mixtures
GC-MS or LC-MS through an interface which removes excess GC carrier gas/HPLC solvent
MS: Electron Impact (EI) Ionization (3 steps):
- Beam of electrons emitted from a heated filament made of rhenium/tungsten metal
- Emitted electrons extract an electron from sample compound molecules, forming ionized molecules
- Ionized molecules further fragment into smaller molecular fragments due to high energy
Process of EI in MS:
- Molecule + electron from electron beam
- Molecular ion
- One electron from electron beam and one from the molecular ion
Role of repeller plate and quadrupole mass analyzer in EI in MS:
Repeller plate is positively charged, and positive fragments are thus repelled and move toward the quadrupole mass analyzer, where these positive fragments are analyzed
Acceleration in EI in MS:
Accelerating and focusing plates are used to increase the energy of the charged molecules and focus the ion beam
The max amount of ions with same kinetic energy reaches the mass analyzer
Types of Ionization in MS (2):
- Chemical Ionization (CI)
- Electron Impact (EI) ionization
Process of Chemical Ionization (CI) in MS: (2)
- Reagent gas subjected to electron impact
- Sample ions are formed by interaction of reagent gas ions and sample molecules
Chemical Ionization difference from EI:
Uses tight ion source and reagent gas
Generate fewer fragments and simpler spectrum
5 Types of Mass Analyzer in MS Separation:
- Magnetic sectors
- Quadrupoles
- Ion traps
- Time of flight (TOF)
- Fourier transform ion cyclotrons (FT-ICR)
Mass Analyzer: Separation
Heart of a MS, separating charged fragments based on their m/z, dictating the mass range, accuracy and sensitivity
Magnetic Sector in Mass Analyzer
Use magnetic field to separate ions based on their m/z
Smaller m/z ions are deflected more
Quadrupole Mass Analyzers
Four rods used to generate 2 equal but out of phase electric potentials
Used in quantitative analysis
Low resolution but cheap
How do Quadrupole Mass Analyzers work?
2 electric potentials: one is AC, the other DC
Creates an oscillating electric field b/w 2 of the opposite rods, w/ equal but opposite charges
By adjusting potentials on the rod, only selected ions can be detected
TOF Mass Analyzers
Separate ions based on time required to reach detector while traveling over known distance
Large applications: biopolymers and large molecules
Reflectrons increase path length and resolution
GC-MS definition
To identify unknown or determine purity of a compound
GC-MS mechanism:
GC column is connected directly to the MS via heated capillary transfer line, which is kept hot enough to avoid condensation of the volatile component eluting from GC column
LC-MS definition
Facilitates desolvation (removal of solvent), so compound integrity is maintained
The heat energy applied in evaporation doesn’t contribute to degradation of any thermally labile species
2 commonly used interface in LC-MS:
- Electrospray interface (ESI)
- Atmospheric pressure chemical ionization interface (APCI)
Electrospray Interface (ESI)
Most popular LC-MS interface
Generate multiple-charged ions and tolerate conventional HPLC flow rates
