Final Flashcards
1
Q
Relative Transmittance (T) definition
A
Fraction of incident light absorbed by the solution
Used as an index of analyte concentration
2
Q
Relative Transmittance Equation
A
T = P/P0
P: radiant power of beam exiting cell
P0: radiant power of beam incident on absorption cell
3
Q
Beer’s Law definition
A
Absorbance is directly proportional to concentration
4
Q
Beer’s Law equation
A
A = abc
a: absorptivity
b: pathlength through solution
c: concentration of absorbing species
5
Q
A = abc = ?
A
log(P/P0) = -logT = 2-log(%T)
6
Q
Background Correction definition
A
processes other than analyte absorption (like reflection/scattering) result in significant decrease in power of incident beam
7
Q
How to correct background?
A
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
8
Q
Absorbance equation using Background Correction:
A
A = log(Psolvent/Panalyte solution)
Psolvent: radiant power of beam exiting cell containing solvent (blank)
9
Q
Deviation from Beer’s Law: (3)
A
- 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)
10
Q
Reference solution in UV/Vis
A
Only solvent; chemically modify reference solution if sample compound is modified
11
Q
Sample holder/cuvette in UV/Vis
A
Material doesn’t absorb any radiation in spectral region
Ex: quartz, silicate glass, plastic
12
Q
How to choose appropriate wavelength in UV/Vis?
A
Choose wavelength at which the analyte has max absorbance and where the absorbance doesn’t change rapidly with changes in wavelength
13
Q
How to Calibrate UV/Vis (0% and 100% Transmittance)
A
0%: shutter closed, set base level current, set 0%T
100%: shutter open, reference solution, set 100%T
14
Q
Why would there be a nonlinear calibration curve in UV/Vis?
A
Due to concentration-dependent chemical change in the system or limitation of the instrument
15
Q
Light Source in UV/Vis: (2)
A
Visible: tungsten filament lamp
UV: deuterium electrical discharge lamps; used with quartz holders,
16
Q
Parts of UV/Vis spectrometer (5)
A
- Light source
- Monochromator
- Sample/reference holder
- Radiation detector
- Readout device
16
Q
UV/Vis: Monochromator
A
To isolate the specific, narrow, continuous group of wavelengths to be used in the assay
17
Q
UV/Vis: Radiation Detector
A
Produce an electric signal whn struck by photons
Signal is proportional to radiant power
18
Q
UV/Vis: Readout Device
A
Analog meter or digital display showing transmittance or absorbance
19
Q
Fluorescence Spectroscopy
A
Absorb energy from radiation in the UV/Vis range, then radiation simultaneously emitted when analyte relaxes
More sensitive than absorption spectroscopy
20
Q
Key components of Fluorescence Spectroscopy (5):
A
- Light source
- Monochromator (emission and excitation)
- Sample/reference holder
- Radiation detector
- Readout device
21
Q
Radiant power in fluorescence spectroscopy
A
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
22
Q
Radiant power in Fluorescence equation
A
Pf = φ(P0-P)
Pf: radiant power
φ: quantum efficiency
23
Q
Beer’s Law and Radiant Power
A
A = kP0c
(linear range; affected by pH, temp, solvent, impurity)
24
Infrared (IR) Spectroscopy definition
Measurement of the absorption of different frequencies of IR radiation by the matter
25
IR ranges (3):
Near: 800-2500 nm
Mid-IR: 2500-15000 nm
Far IR: 15000-100000 nm
(Near and Mid most used)
26
Energy equation in IR spectroscopy
E = hυ
h: Planck's constant (6.626e-34 Js)
υ: frequency (Hz/s^-1)
27
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
28
Major vibration in IR:
stretching (change bond length) and blending (change bond angle) produce a change in dipole moment
29
Molecular asymmetry in IR
Required for excitation by IR; fully symmetric molecules don't display absorbance unless asymmetric stretching or bending transitions are possible
30
Mid-IR Spectroscopy
Absorb light in the 2.5-15 μm region
31
Fourier transform instrument (FTIR) components (3):
1. Light source
2. Interferometer
3. Detector
32
Michelson interferometer in IR
Beam is split by a splitter and then recombined by reflecting back the split beams with mirrors
33
Detector in IR spectroscopy:
Output voltage varies with changes caused by varying levels of radiation striking the detector
34
Why is quantitative IR difficult to obtain? (3)
1. Deviations from Beer's Law (low intensity IR source, narrow bands, require calibration sources)
2. Complex spectra
3. Lack of reference cell
35
Other methods of IR Spectroscopy (3)
1. Reflectance
2. Photoacoustic
3. Near-IR
36
ATR-FTIR (Reflectance IR) definition
To measure thick, solid, viscous liquid/paste; surface sensitive
37
Photoacoustic IR definition
Measures the effect of absorbed energy; can use tunable laser; gas/liquid/solid suitable for highly absorbing samples
38
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
39
Near IR: Transmission Mode
Liquid/solid samples at 700-1100 nm
Easier sample prep
40
Near IR: Reflection Mode
Solid/granular samples
Sample prep by packing food tightly into a cell against a quartz window
41
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
42
Rheology definition
Science of deformation by rheological methods
43
Viscosity definition
internal resistance to flow
44
Stress definition (σ):
measurement of force, expressed with Pascals
45
Stress equation
σ = F/A
46
2 types of stress:
1. Normal stress: force applied directly (perpendicular) to a surface-tension/compression
2. Shear stress: force parallel to the sample surface
47
Strain definition (ε):
dimensionless quality representing the relative deformation of a material
determined by direction of stress: negative values for compression, positive for extension
48
Normal strain equation:
ε = ΔL / L
49
Hook's Law for normal stress equation:
σ = E*ε
E; Young's modulus (N/m^2 or Pa)
50
Shear strain equation:
γ = ΔL/h
51
Hook's law for shear stress equation:
σ = G*γ
G = shear modulus (N/m^2)
52
Shear rate for liquid samples equation:
shear rate = U/h
53
Apparent viscosity (η) definition:
Shear stress/shear rate
If constant, then Newtonian fluid- otherwise non-Newtonian
54
Rheogram definition:
graphical representation of flow behavior showing the relationship between stress and strain or shear rate
55
Pseudoplastics fluid behavior
Shear thinning; shear rate increases, viscosity decreases
56
Dilatant fluid behavior
Shear thickening: shear rate increases, viscosity increases
57
Thixotropic definition
shear stress decreases with time (pseudoplastic, shear thinning)
58
Anti-thixotropic definition:
shear stress increases with time
(dilatant, shear thickening)
59
Rheological Fluid Models (4):
1. Herschel-Bulkley
2. Newtonian:
3. Power Law
4. Bingham Plastic
60
Herschel-Bulkley model equation:
σ = σ0 + k*γ ^(n)
n: flow behavior index
σ0: yield stress (Pa)
k: consistency index (Pa*s)
61
Newtonian model equation:
σ = μ*y
62
Power law model equation:
dilatant/pseudoplastic
σ = k*y^(n)
63
Bingham Plastic model equation:
σ = σ0 + μpl*y
64
Rotational viscometry definition
Known test fixture in contact with a sample and through some mechanical, rotational means, the fluid is sheared by the fixture
65
2 types of steady shear mode viscometers:
1. Concentric cylinder
2. Cone and plate
66
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
67
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
68
Experimental Procedures for Steady Shear Rotational Viscometry (5):
1. Test fixture selection
2. Speed (shear rate) selection
3. Data collection
4. Shear calculations
5. Model parameter determination
69
Large strain testing for solids:
Compression and tension tests are used to determine large strain and fracture food properties
70
Texture Profile Analysis (TPA)
Double compression tests
Data related to hardness, cohesiveness and other sensory parameters
71
What kind of compounds is gas chromatography good for?
Thermally stable volatile compounds
72
Methods of Isolation for Gas Chromatography (4):
1. Headspace methods
2. Distillation methods
3. Solvent extraction
4. Solid-Phase Microextraction (SPME)
73
Headspace method summary definition
One of the simplest methods; direct injection of headspace vapors
74
2 headspace methods:
1. Direct headspace sampling
2. Purge and trap sampling
75
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
76
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
77
Headspace trap materials
synthetic porous polymers
78
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
79
SPME
Convenient, solventless extraction technique
Used for volatile aroma analysis of food and beverages
80
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
81
SPME definition
Microextraction technique which employs a thin film of sorption polymers on a fine fused silica fiber
82
Advantages of SPME (4):
1. Less solvent required, minimial solvent evaporation
2. Fast, better precision and accuracy
3. Less cost and contamination
4. Automated
83
Components of GC (7):
1. Gas supply and regulators
2. Injection port
3. Oven
4. Column
5. Detector
6. Electrometer
7. Recorder/data handling system
84
Gas Supply for GC:
Common carrier gas: nitrogen, helium, hydrogen
For detector: air and hydrogen
85
Injection port for GC
Place for sample introduction, vaporization, dilution/splitting
Variance is minimized by internal/external standards
86
Types of Injection Techniques (5):
1. Split
2. Splitless
3. Temperature programmed
4. On-column
5. Thermal desorption
87
Factors to consider when direct injecting in GC (5):
1. Thermal degradation
2. Damage to GC column
3. Effect of water vapor
4. Contamination
5. Vaporization speed
88
When is sample derivation used for GC?
If compounds are a) low in volatility, b) poorly separated due to polarity, or c) unstable
89
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
90
2 types of column in GC:
Packed or capillary
91
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
92
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
93
Thicker film pros and cons in capillary column for GC
Pros: longer interaction w/stationary phase; better separation
Cons: Column bleeding, poor baseline
94
Thick vs Thin film in capillary column of GC:
Thick: for separation of very volatile compounds
Thin: good for large molecules separation
95
Capillary vs Packed Column in GC:
Capillary column has better resolution than packed column
96
Types of GC Detectors (5):
1. TCD
2. FID
3. ECD
4. FPD
5. PID
97
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
98
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
99
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
100
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)
101
Photoionization Detector (PID)
- UV light ionize analyte to form ions, ions detected by electrodes forming current
- Sensitive, non-destructive
-flavor analysis
102
Separation of GC Column: Carrier Gas
N2 most efficient carrier gas
Hydrogen is best choice
103
Concerns in Components: Gas Supply and Regulators (1)
Carrier gas: flow rate and type
104
Concerns in Components: Injection port (2)
Temperature, sample types (volume and concentration)
105
Concerns in Components: Oven (2)
Temp, sample types
106
Concerns in Components: Column (3)
Temp, sample types, packing material
107
Concerns in Components: Detector (3)
Temp, sample types, carrier gas
108
2, 4- Decadienal
Important aldehyde flavor, contributes deep fat characteristics
Key components in flavors like chicken, lamb, beef, and french fries
109
Mass Spectrometry definition
Analytical tool used for measuring the molecular mass of a molecule
110
Principles of Mass Spectrometry (3):
1. Ionization of molecules: chemical/electron impact
2. Separation of ions based on mass-to-charge ratio in mass analyzer
3. Detection under electrostatic field
111
Steps of Mass Spectrometry (5):
1. Sample introduction
2. Ion source
3. Mass analyzer
4. Detector
5. Data system
112
Routine in Analytical Labs (2):
1. GC-MS: interface of mass spectrometer with GC
2. LC-MS: interface of mass spectrometer with HPLC
113
Sample Introduction in MS: Pure Compounds (2)
1. Direct Injection: for gases or volatile liquids (same as GC)
2. Direct Insertion Probe: somewhat volatile solid
114
Sample Introduction in MS: Mixtures
GC-MS or LC-MS through an interface which removes excess GC carrier gas/HPLC solvent
115
MS: Electron Impact (EI) Ionization (3 steps):
1. Beam of electrons emitted from a heated filament made of rhenium/tungsten metal
2. Emitted electrons extract an electron from sample compound molecules, forming ionized molecules
3. Ionized molecules further fragment into smaller molecular fragments due to high energy
116
Process of EI in MS:
1. Molecule + electron from electron beam
2. Molecular ion
3. One electron from electron beam and one from the molecular ion
117
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
118
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
119
Types of Ionization in MS (2):
1. Chemical Ionization (CI)
2. Electron Impact (EI) ionization
120
Process of Chemical Ionization (CI) in MS: (2)
1. Reagent gas subjected to electron impact
2. Sample ions are formed by interaction of reagent gas ions and sample molecules
121
Chemical Ionization difference from EI:
Uses tight ion source and reagent gas
Generate fewer fragments and simpler spectrum
122
5 Types of Mass Analyzer in MS Separation:
1. Magnetic sectors
2. Quadrupoles
3. Ion traps
4. Time of flight (TOF)
5. Fourier transform ion cyclotrons (FT-ICR)
123
Mass Analyzer: Separation
Heart of a MS, separating charged fragments based on their m/z, dictating the mass range, accuracy and sensitivity
124
Magnetic Sector in Mass Analyzer
Use magnetic field to separate ions based on their m/z
Smaller m/z ions are deflected more
125
Quadrupole Mass Analyzers
Four rods used to generate 2 equal but out of phase electric potentials
Used in quantitative analysis
Low resolution but cheap
126
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
127
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
128
GC-MS definition
To identify unknown or determine purity of a compound
129
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
130
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
131
2 commonly used interface in LC-MS:
1. Electrospray interface (ESI)
2. Atmospheric pressure chemical ionization interface (APCI)
132
Electrospray Interface (ESI)
Most popular LC-MS interface
Generate multiple-charged ions and tolerate conventional HPLC flow rates
133
