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Final Flashcards

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1
Q

Relative Transmittance (T) definition

A

Fraction of incident light absorbed by the solution
Used as an index of analyte concentration

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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

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3
Q

Beer’s Law definition

A

Absorbance is directly proportional to concentration

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4
Q

Beer’s Law equation

A

A = abc
a: absorptivity
b: pathlength through solution
c: concentration of absorbing species

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5
Q

A = abc = ?

A

log(P/P0) = -logT = 2-log(%T)

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6
Q

Background Correction definition

A

processes other than analyte absorption (like reflection/scattering) result in significant decrease in power of incident beam

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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

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8
Q

Absorbance equation using Background Correction:

A

A = log(Psolvent/Panalyte solution)
Psolvent: radiant power of beam exiting cell containing solvent (blank)

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9
Q

Deviation from Beer’s Law: (3)

A
  1. Only valid for diluted solution, up to mM (higher concentration, higher intermolecular distances, affect absorption of proton)
  2. Chemical processes such as the reversible association-dissociation of analyte molecules, or the ionization of a weak acid in an unbuffered solvent
  3. Instrumentation limitation (strictly applied to monochromatic radiation)
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10
Q

Reference solution in UV/Vis

A

Only solvent; chemically modify reference solution if sample compound is modified

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11
Q

Sample holder/cuvette in UV/Vis

A

Material doesn’t absorb any radiation in spectral region
Ex: quartz, silicate glass, plastic

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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

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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

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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

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15
Q

Light Source in UV/Vis: (2)

A

Visible: tungsten filament lamp
UV: deuterium electrical discharge lamps; used with quartz holders,

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16
Q

Parts of UV/Vis spectrometer (5)

A
  1. Light source
  2. Monochromator
  3. Sample/reference holder
  4. Radiation detector
  5. Readout device
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16
Q

UV/Vis: Monochromator

A

To isolate the specific, narrow, continuous group of wavelengths to be used in the assay

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17
Q

UV/Vis: Radiation Detector

A

Produce an electric signal whn struck by photons
Signal is proportional to radiant power

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18
Q

UV/Vis: Readout Device

A

Analog meter or digital display showing transmittance or absorbance

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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

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20
Q

Key components of Fluorescence Spectroscopy (5):

A
  1. Light source
  2. Monochromator (emission and excitation)
  3. Sample/reference holder
  4. Radiation detector
  5. Readout device
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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

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22
Q

Radiant power in Fluorescence equation

A

Pf = φ(P0-P)
Pf: radiant power
φ: quantum efficiency

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23
Q

Beer’s Law and Radiant Power

A

A = kP0c
(linear range; affected by pH, temp, solvent, impurity)

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24
Q

Infrared (IR) Spectroscopy definition

A

Measurement of the absorption of different frequencies of IR radiation by the matter

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25
Q

IR ranges (3):

A

Near: 800-2500 nm
Mid-IR: 2500-15000 nm
Far IR: 15000-100000 nm
(Near and Mid most used)

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26
Q

Energy equation in IR spectroscopy

A

E = hυ
h: Planck’s constant (6.626e-34 Js)
υ: frequency (Hz/s^-1)

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27
Q

T/F: The energy gap between each vibrational energy level is a different magnitude of energy of photons from IR radiation

A

False: it’s the same magnitude of energy

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28
Q

Major vibration in IR:

A

stretching (change bond length) and blending (change bond angle) produce a change in dipole moment

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29
Q

Molecular asymmetry in IR

A

Required for excitation by IR; fully symmetric molecules don’t display absorbance unless asymmetric stretching or bending transitions are possible

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30
Q

Mid-IR Spectroscopy

A

Absorb light in the 2.5-15 μm region

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31
Q

Fourier transform instrument (FTIR) components (3):

A
  1. Light source
  2. Interferometer
  3. Detector
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32
Q

Michelson interferometer in IR

A

Beam is split by a splitter and then recombined by reflecting back the split beams with mirrors

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33
Q

Detector in IR spectroscopy:

A

Output voltage varies with changes caused by varying levels of radiation striking the detector

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34
Q

Why is quantitative IR difficult to obtain? (3)

A
  1. Deviations from Beer’s Law (low intensity IR source, narrow bands, require calibration sources)
  2. Complex spectra
  3. Lack of reference cell
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35
Q

Other methods of IR Spectroscopy (3)

A
  1. Reflectance
  2. Photoacoustic
  3. Near-IR
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36
Q

ATR-FTIR (Reflectance IR) definition

A

To measure thick, solid, viscous liquid/paste; surface sensitive

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37
Q

Photoacoustic IR definition

A

Measures the effect of absorbed energy; can use tunable laser; gas/liquid/solid suitable for highly absorbing samples

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38
Q

Near-IR Spectroscopy definition

A

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

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39
Q

Near IR: Transmission Mode

A

Liquid/solid samples at 700-1100 nm
Easier sample prep

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40
Q

Near IR: Reflection Mode

A

Solid/granular samples
Sample prep by packing food tightly into a cell against a quartz window

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41
Q

Reflectance equation (IR)

A

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

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42
Q

Rheology definition

A

Science of deformation by rheological methods

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43
Q

Viscosity definition

A

internal resistance to flow

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44
Q

Stress definition (σ):

A

measurement of force, expressed with Pascals

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45
Q

Stress equation

A

σ = F/A

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46
Q

2 types of stress:

A
  1. Normal stress: force applied directly (perpendicular) to a surface-tension/compression
  2. Shear stress: force parallel to the sample surface
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47
Q

Strain definition (ε):

A

dimensionless quality representing the relative deformation of a material
determined by direction of stress: negative values for compression, positive for extension

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48
Q

Normal strain equation:

A

ε = ΔL / L

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49
Q

Hook’s Law for normal stress equation:

A

σ = E*ε
E; Young’s modulus (N/m^2 or Pa)

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50
Q

Shear strain equation:

A

γ = ΔL/h

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51
Q

Hook’s law for shear stress equation:

A

σ = G*γ
G = shear modulus (N/m^2)

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52
Q

Shear rate for liquid samples equation:

A

shear rate = U/h

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53
Q

Apparent viscosity (η) definition:

A

Shear stress/shear rate
If constant, then Newtonian fluid- otherwise non-Newtonian

54
Q

Rheogram definition:

A

graphical representation of flow behavior showing the relationship between stress and strain or shear rate

55
Q

Pseudoplastics fluid behavior

A

Shear thinning; shear rate increases, viscosity decreases

56
Q

Dilatant fluid behavior

A

Shear thickening: shear rate increases, viscosity increases

57
Q

Thixotropic definition

A

shear stress decreases with time (pseudoplastic, shear thinning)

58
Q

Anti-thixotropic definition:

A

shear stress increases with time
(dilatant, shear thickening)

59
Q

Rheological Fluid Models (4):

A
  1. Herschel-Bulkley
  2. Newtonian:
  3. Power Law
  4. Bingham Plastic
60
Q

Herschel-Bulkley model equation:

A

σ = σ0 + kγ ^(n)
n: flow behavior index
σ0: yield stress (Pa)
k: consistency index (Pas)

61
Q

Newtonian model equation:

A

σ = μ*y

62
Q

Power law model equation:

A

dilatant/pseudoplastic
σ = k*y^(n)

63
Q

Bingham Plastic model equation:

A

σ = σ0 + μpl*y

64
Q

Rotational viscometry definition

A

Known test fixture in contact with a sample and through some mechanical, rotational means, the fluid is sheared by the fixture

65
Q

2 types of steady shear mode viscometers:

A
  1. Concentric cylinder
  2. Cone and plate
66
Q

Advantages/Disadvantages of Concentric Cylinders

A

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
Q

Advantages/Disadvantages of Cone and Plate:

A

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
Q

Experimental Procedures for Steady Shear Rotational Viscometry (5):

A
  1. Test fixture selection
  2. Speed (shear rate) selection
  3. Data collection
  4. Shear calculations
  5. Model parameter determination
69
Q

Large strain testing for solids:

A

Compression and tension tests are used to determine large strain and fracture food properties

70
Q

Texture Profile Analysis (TPA)

A

Double compression tests
Data related to hardness, cohesiveness and other sensory parameters

71
Q

What kind of compounds is gas chromatography good for?

A

Thermally stable volatile compounds

72
Q

Methods of Isolation for Gas Chromatography (4):

A
  1. Headspace methods
  2. Distillation methods
  3. Solvent extraction
  4. Solid-Phase Microextraction (SPME)
73
Q

Headspace method summary definition

A

One of the simplest methods; direct injection of headspace vapors

74
Q

2 headspace methods:

A
  1. Direct headspace sampling
  2. Purge and trap sampling
75
Q

Direct Headspace sampling

A

Using a gas-tight syringe injected directly into GC
Rapid analysis
For volatiles in headspace at a detectable level for flame ionization detector

76
Q

Purge and trap methods and system

A

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
Q

Headspace trap materials

A

synthetic porous polymers

78
Q

Solvent extraction

A

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
Q

SPME

A

Convenient, solventless extraction technique
Used for volatile aroma analysis of food and beverages

80
Q

SPME Procedure (Extraction and Desorption)

A

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
Q

SPME definition

A

Microextraction technique which employs a thin film of sorption polymers on a fine fused silica fiber

82
Q

Advantages of SPME (4):

A
  1. Less solvent required, minimial solvent evaporation
  2. Fast, better precision and accuracy
  3. Less cost and contamination
  4. Automated
83
Q

Components of GC (7):

A
  1. Gas supply and regulators
  2. Injection port
  3. Oven
  4. Column
  5. Detector
  6. Electrometer
  7. Recorder/data handling system
84
Q

Gas Supply for GC:

A

Common carrier gas: nitrogen, helium, hydrogen
For detector: air and hydrogen

85
Q

Injection port for GC

A

Place for sample introduction, vaporization, dilution/splitting
Variance is minimized by internal/external standards

86
Q

Types of Injection Techniques (5):

A
  1. Split
  2. Splitless
  3. Temperature programmed
  4. On-column
  5. Thermal desorption
87
Q

Factors to consider when direct injecting in GC (5):

A
  1. Thermal degradation
  2. Damage to GC column
  3. Effect of water vapor
  4. Contamination
  5. Vaporization speed
88
Q

When is sample derivation used for GC?

A

If compounds are a) low in volatility, b) poorly separated due to polarity, or c) unstable

89
Q

Oven in GC:

A

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
Q

2 types of column in GC:

A

Packed or capillary

91
Q

Packed column for GC:

A

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
Q

Capillary column for GC:

A

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
Q

Thicker film pros and cons in capillary column for GC

A

Pros: longer interaction w/stationary phase; better separation
Cons: Column bleeding, poor baseline

94
Q

Thick vs Thin film in capillary column of GC:

A

Thick: for separation of very volatile compounds
Thin: good for large molecules separation

95
Q

Capillary vs Packed Column in GC:

A

Capillary column has better resolution than packed column

96
Q

Types of GC Detectors (5):

A
  1. TCD
  2. FID
  3. ECD
  4. FPD
  5. PID
97
Q

Thermal Conductivity Detector (TCD)

A
  • 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
Q

Flame Ionization Detector (FID)

A

-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
Q

Electron Capture Detector (ECD)

A
  • 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
Q

Flame Photometric Detector (FPD)

A
  • 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
Q

Photoionization Detector (PID)

A
  • UV light ionize analyte to form ions, ions detected by electrodes forming current
  • Sensitive, non-destructive
    -flavor analysis
102
Q

Separation of GC Column: Carrier Gas

A

N2 most efficient carrier gas
Hydrogen is best choice

103
Q

Concerns in Components: Gas Supply and Regulators (1)

A

Carrier gas: flow rate and type

104
Q

Concerns in Components: Injection port (2)

A

Temperature, sample types (volume and concentration)

105
Q

Concerns in Components: Oven (2)

A

Temp, sample types

106
Q

Concerns in Components: Column (3)

A

Temp, sample types, packing material

107
Q

Concerns in Components: Detector (3)

A

Temp, sample types, carrier gas

108
Q

2, 4- Decadienal

A

Important aldehyde flavor, contributes deep fat characteristics
Key components in flavors like chicken, lamb, beef, and french fries

109
Q

Mass Spectrometry definition

A

Analytical tool used for measuring the molecular mass of a molecule

110
Q

Principles of Mass Spectrometry (3):

A
  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
Q

Steps of Mass Spectrometry (5):

A
  1. Sample introduction
  2. Ion source
  3. Mass analyzer
  4. Detector
  5. Data system
112
Q

Routine in Analytical Labs (2):

A
  1. GC-MS: interface of mass spectrometer with GC
  2. LC-MS: interface of mass spectrometer with HPLC
113
Q

Sample Introduction in MS: Pure Compounds (2)

A
  1. Direct Injection: for gases or volatile liquids (same as GC)
  2. Direct Insertion Probe: somewhat volatile solid
114
Q

Sample Introduction in MS: Mixtures

A

GC-MS or LC-MS through an interface which removes excess GC carrier gas/HPLC solvent

115
Q

MS: Electron Impact (EI) Ionization (3 steps):

A
  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
Q

Process of EI in MS:

A
  1. Molecule + electron from electron beam
  2. Molecular ion
  3. One electron from electron beam and one from the molecular ion
117
Q

Role of repeller plate and quadrupole mass analyzer in EI in MS:

A

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
Q

Acceleration in EI in MS:

A

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
Q

Types of Ionization in MS (2):

A
  1. Chemical Ionization (CI)
  2. Electron Impact (EI) ionization
120
Q

Process of Chemical Ionization (CI) in MS: (2)

A
  1. Reagent gas subjected to electron impact
  2. Sample ions are formed by interaction of reagent gas ions and sample molecules
121
Q

Chemical Ionization difference from EI:

A

Uses tight ion source and reagent gas
Generate fewer fragments and simpler spectrum

122
Q

5 Types of Mass Analyzer in MS Separation:

A
  1. Magnetic sectors
  2. Quadrupoles
  3. Ion traps
  4. Time of flight (TOF)
  5. Fourier transform ion cyclotrons (FT-ICR)
123
Q

Mass Analyzer: Separation

A

Heart of a MS, separating charged fragments based on their m/z, dictating the mass range, accuracy and sensitivity

124
Q

Magnetic Sector in Mass Analyzer

A

Use magnetic field to separate ions based on their m/z
Smaller m/z ions are deflected more

125
Q

Quadrupole Mass Analyzers

A

Four rods used to generate 2 equal but out of phase electric potentials
Used in quantitative analysis
Low resolution but cheap

126
Q

How do Quadrupole Mass Analyzers work?

A

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
Q

TOF Mass Analyzers

A

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
Q

GC-MS definition

A

To identify unknown or determine purity of a compound

129
Q

GC-MS mechanism:

A

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
Q

LC-MS definition

A

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
Q

2 commonly used interface in LC-MS:

A
  1. Electrospray interface (ESI)
  2. Atmospheric pressure chemical ionization interface (APCI)
132
Q

Electrospray Interface (ESI)

A

Most popular LC-MS interface
Generate multiple-charged ions and tolerate conventional HPLC flow rates

133
Q
A

Food Analysis (1 decks)

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