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Final Flashcards

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1
Q

What is spectroscopy??

A

deals with the production, measurement, and interpretation of spectra arising from interaction of electromagnetic radiation with matter

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2
Q

Interference

A

Two waves cross one another, they result in an instantaneous wave, at the point of intersection, whose amplitude is the algebraic sum of the amplitudes of the individual waves at the point of intersection.

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3
Q

Types of interference

A

Maximum Constructive Interference (Wavelength doubled)

Maximum Destructive Interference (Wavelength is 0)

Interference

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4
Q

Energy and Wavelength of functional groups

A

Compounds with resonance have less energy and a higher wavelength (phenyl)

Compounds with no resonance have more energy and a lower wavelength (methyl)

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5
Q

Energy Level Transitions

A

Gaining Energy (E0 -> E1): Adsorption

Releasing Energy (E1 -> E0): Emission

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6
Q

Relative Transmittance

A

T = P/P0

Index of analyte concentration (fragment of incident light absorbed by solution)

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7
Q

Deviation from Beer’s Law

A
  1. Only valid for diluted solution (low concentration) up to 10 mM
  2. Chemical processes such as reversible association-dissociation of analyte molecules
  3. Instrumentation limitation
    Only applied to monochromatic radiation
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8
Q

What are the different energy levels

A

Largest: Electronic Energy Levels
Middle: Vibrational Energy Levels
Smallest: Rotational Energy Levels

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9
Q

UV/Vis components

A

Light Source, Monochromator, Sample/reference holder, radiation detector, readout device

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10
Q

Sample-holder or cuvette for UV/Vis

A

Material: not absorb any radiation in the spectral region being used
Quartz, glass, plastics

Dimension: path lengths ranging from 1 to 100 mm are commercially available

Narrow cells (4mm width) for limiting amount of sample solution

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11
Q

Choosing appropriate wavelength for UV/Vis

A

Expect to choose wavelength at which the analyte has maximum absorbance and where the absorbance does not change rapidly with changes in wavelength.

One where is there maximum sensitivity and better adherence to Beer’s Law

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12
Q

What range is UV/Vis in

A

200-700 nm

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13
Q

UV/Vis Light Source

A

Stable, sufficient energy for detection, cover the entire designated wavelength.

Visible: tungsten filament lamp

UV: deuterium electrical discharge lamps, used with quartz sample holders, glass absorb radiation below 350 nm

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14
Q

Monochromator

A

Isolates the specific, narrow, continuous group of wavelengths to be used in the assay.

Monochromatic: single frequency and wavelength of radiation

Has entrance and exit slits, concave mirrors, and a dispersing element

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15
Q

Radiation detector

A

Produce electrical signal when struck by photons to turn energy into an electrical current.

This signal is proportional to radiant power.

Examples: Phototube, photomultiplier tube, photodiode detector

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16
Q

Fluorescence spectroscopy

A

More sensitive than UV/Vis Spectroscopy (1-3 orders of magnitude)

Absorbs energy from radiation in the UV/Vis range, radiation is simultaneously emitted when analyte relaxes.

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17
Q

Fluorescence Spectroscopy Components

A

Same as UV/Vis but with 2 monochromators
Emission and Excitation Monochromator

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18
Q

Fluorescence

A

Moving from En -> E0

Emitting visible light after absorbing UV (non-visible) light

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19
Q

Phosphorescence

A

Intersystem crossing
Moving from the triplet exited state to the ground state
Longer Fluorescence Lifetime

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20
Q

Infared Spectroscopy

A

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

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21
Q

IR energy level

A

Stays in the ground state and only moves between rotational and vibrational levels

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22
Q

IR Molecular Vibrations

A

Bending and stretching of bonds

This molecular asymmetry is a requirement for excitation by IR because symmetric molecules do not display absorbance in IR regions.

Think of molecules like a spring (Hooke’s Law)

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23
Q

IR Regions

A

Near-IR (12,500-400 1/cm)
Mid Ir (400-650 1/cm)
far-IR (650-100 1/cm)

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24
Q

IR Functional groups

A

Identify functional groups by their frequency on the spectrum

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25
Q

Mid-IR Instrumentation

A

Fourier transform instrument (FTIR): all wavelengths arrive at detector simultaneously.

Nichrome wire light source

Michelson Interferometer: beam split by a splitter and recombined by reflecting back split beams with mirrors.

Thermocouple Detector: output voltage varies with changes caused by varying levels of radiation striking the detector.

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26
Q

Quantitative IR and deviations from Beer’s Law

A

Difficult to obtain reliable quantitative data based on IR

Deviations from Beer’s law: low intensity of IR source, narrow bands and wide slit required, require calibration sources

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27
Q

Attenuated Total Reflectance FTIR

A

Measuring thick solid, viscous liquid
Surface sensitive technique

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28
Q

Photoacoustic IR

A

Measure effect of absorbed energy (sound at different wavelength)

Used for Gas, liquid, solid, suitable for highly absorbing samples.

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29
Q

Near-IR

A

Quantitative analysis of sample that minimizes impact of size and shape on sample particles.

Can not use quartz (does not absorb radiation)

Range of 700nm to 2500nm

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30
Q

Near-IR Absorption Bands

A

Bands are broad and overlap which give a complex spectrum.

C-H, N-H, and O-H have sufficient intensity.

Determine constituents by where these bonds peak.

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31
Q

Near-IR Sample preparation

A

The food tightly packed into a cell against a quartz window, thereby providing a smooth, uniform surface from which reflection can occur.

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32
Q

Rheology

A

A science of deformation and flow of all materials

Measuring force/deformation as a function of time

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33
Q

Viscosity and Stress

A

Viscosity: define as the internal resistance to flow

Stress: the measurement of force divided by area

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34
Q

Normal Stress

A

Force directly perpendicular to a surface, tension or compression (chewing gum and kneading)

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35
Q

Shear Stress

A

Force parallel to the sample surface (spreading butter on a slice of toast)

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36
Q

Newtonian Fluid

A

Viscosity does not change as shear rate and time changes
Ex: water, air, oil, honey

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37
Q

Non-Newtonian Fluid

A

Viscosity changes as a function of shear rate

Stress = Newtonian Viscosity x Shear Rate

Most foods are Non-Newtonian

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38
Q

Apparent Viscosity

A

Shear-dependent viscosity.

Apparent Viscosity = Stress/Shear Rate

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39
Q

Shear thinning

A

As shear rates increases, viscosity decreases.

Pseudoplastic if time independent

Thixotropic if time dependent

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40
Q

Shear Thickening

A

As shear rate increases, viscosity increases

Dilatant if time independent

Anti-thixotropic is time dependent.

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41
Q

Herschel-Buckley Model

A

n: flow behavior index
σ0: yield stress (PA)
K: consistency index (Pa s)

σ = Kγ^n + σ0

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42
Q

Newtonian Model

A

n = 1
K = μ
σ0 = 0
σ = μγ^1

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43
Q

Power Law Model

A

σ0 = 0
σ = Kγ^n

Show no yield stress and a non-linear relationship between shear stress and shear rate

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44
Q

Bingham Plastic Model

A

n = 1
K = μpl

σ = σ0 + μplγ^1

Relationship between shear stress and shear rate is linear once flow is established.

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45
Q

Rotational Viscometry

A

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

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46
Q

Rheometer

A

Measure the viscosity, relationship between shear rate and shear stress

47
Q

Rheometry primary assumptions

A

Laminar flow: synonymous with streamline flow
Steady state: no net changes in system over time
no-slip boundary condition: when test fixture is immersed in fluid sample, walls of fixture and sample serve as boundaries for the fluid.

48
Q

Steady shear mode viscometer

A

Steady shear: the sheared fluid viscosity, contained between the boundaries, remains constant at any single position, the velocity gradient across the fluid is a constant

49
Q

Concentric Cylinder

A

Uses torque to generate rotation about an axis

Advantages: Good for low-viscosity fluids, suspensions, and dilute samples and large SA increase sensitivity at low shear rates

Disadvantage: Large sample is required

50
Q

Brookfield Viscometer

A

An example of a concentric cylinder rheometer

Records shear stress at different rpms

51
Q

Cone and Plate

A

Permits shear rate and shear stress to remain constant for any location of sample in fluid gap.

Advantages: Good for high shear rates, medium and high viscosity samples, small sample required, and cleanup is easy

Disadvantages: Large particles interfere with sensitivity, must maintain constant gap height

52
Q

Steady Shear Rotational Viscometry Procedure

A
  1. Test fixture selection
  2. Speed (Shear rate) selection.
  3. Data collection
    (Torque and speed)
  4. Shear calculations.
    (Shear stress and shear rate)
  5. Model parameter determination
    (Applicable rheological model and parameters: viscosity, yield stress, flow behavior index)
53
Q

Small strain testing

A

Goal is to apply minimal amount of strain or stress required to measure rheological behavior while preventing damage to sample.

Used to understand properties of a food network.

54
Q

Large strain testing

A

Used to determine the large strain and fracture food properties.

Used to give indication of sensory texture or product durability.

55
Q

Texture Profile Analysis

A

Data related to hardness, cohesiveness and other sensory parameters.

two-cycle compression test

56
Q

Texture Analyzer

A

Used to create correlation between sensory perception and physical properties.

Texture parameters: crispy, crunchy, chewy, sticky, elasticity, yield point, ripeness, gel strength,

57
Q

Gas Chromatography

A

Used for analysis of: FAs, triglycerides, sterols, gasses, solvent analysis, water etc.

Good for thermally stable volatile compounds

58
Q

GC Factors to consider

A

Temperature at injection point
Concentration of compounds
Enzymes
Microbial growth
Chemical reactions

59
Q

Direct headspace sampling

A

Using gas-tight syringe, injected directly GC

Good for rapid analysis of very low boiling point compounds

Has a low sensitivity

60
Q

Purge and trap headspace isolation

A

Has a higher sensitivity for trace analysis.

Purge a gas into samples -> volatile compounds move into headspace vapors -> pass a cryogenic or adsorption trap -> trap is heated, carrier gas introduced the volatile compounds into the column.

61
Q

Headspace trap materials

A

synthetic porous polymers

62
Q

Distillation methods of isolation

A

Effective at isolating volatile compounds from foods for GC analysis. Product moisture or outside steam is used to head and codistill volatiles from a food product.

63
Q

Solvent extraction for GC isolation

A

Preferred for recovery of volatiles from food.

Recovery depends on solubility of solute in different solvents.

Involves use of organic solvent and water (2 immiscible phases)

64
Q

Solid-phase microextraction (SPME)

A

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

Advantages: Less solvent required and minimal solvent evaporation, faster, more precise and more accurate, less cost and contamination, automated

65
Q

Solid-phase microextraction (SPME) method

A
  1. Phase is bound onto a fine fused silica filament.
  2. The fiber is immersed in a sample/headspace of sample.
  3. Fiber is pulled into a protective metal sheath, removed from sample, and forced through septum of GC after desired extraction.
  4. Adsorbed volatiles are thermally desorbed from the fiber
66
Q

SPME fiber

A

Having varying film thickness and coating of polymer

Thicker (100 micrometer) - volatiles, thinner - large molecules

Ex: polar phase (polyacrylate and carbowax coating)- polar compounds

67
Q

Components of GC

A

Gas supply and regulators
Injection port
Oven
Column
Detector
Electrometer
Recorder/data handling system

68
Q

Gas Supply and Regulators

A

Common carrier gas: nitrogen, helium, hydrogen

Detector Gas: air and hydrogen

Must use high purity gases

Gas lines, regulators, and fittings must be of good quality

69
Q

Injection port

A

A place for sample introduction, vaporization, some dilution/splitting.

Injection by syringe manually: might cause poor precision, or automatically (autosampler).

Variance minimized by internal/external sample

70
Q

Split injection

A

Splitting injection allows a portion of sample to go through column.

20 degrees Celsius above maximum column oven temperature.

71
Q

Splitless injection

A

Whole sample enters column.

Used to increase sensitivity.

72
Q

GC Direct Injection Factors to Consider

A

Thermal degradation
Damage to GC column
Effect of water vapor
Contamination
Vaporization speed

73
Q

When should sample derivatization occur?

A

Low in volatility
Poor separation due to polarity
Unstable

74
Q

GC Oven

A

Control temperature of column.

Determined by interaction of analyte with the stationary phase and boiling point for separation of compounds.

Higher Temp: may cause faster elution but poorer resolution.

75
Q

Packed GC Column

A

Solid support: silane-treated diatomaceous earth/

Liquid coating (loading) depends on analysis, impact retention time, resolution, baseline.

Liquid coating may be lost at high temperature which causes increase of baseline.

76
Q

Stationary phase of GC Column

A

Polysiloxane-basd

Choose polar phase to separate polar compounds and phenyl-based column to separate aromatic compounds.

77
Q

GC Capillary Column

A

Packing: hollow fused silica glass.

Types: Megabore column (0.53 mm i.d); normal (0.2- 0.32 mm i.d); microbore (0.1 mm i.d)

Liquid coating: chemically bonded to the glass wall.

Liquid phase (film) thickness and separation
Thicker film -> longer interaction with stationary phase and better separation
Thinner film -> column bleeding and a poor baseline

78
Q

Separation and resolution of GC Column

A

Capillary column has better resolution than packed.

Longer column gives better resolution but is not obvious for a capillary column.

Separation is achieved due to the high efficiency of capillary columns, even though the stationary phase is not optimal.

79
Q

Thermal Conductivity Detector

A

Temperature difference between carrier gas and mixed gas eluted from column directly related to temperature change of filament in the detector.

Carrier gas choice: hydrogen or helium, sometimes nitrogen.

Universal and non-destructive detector; low sensitivity.

Good for further analysis and separation of water, CO.

80
Q

Flame Ionization Detector

A

Analytes from column are burnt into hydrogen flame, forming ions which are detected by electrodes -> currents.

Good sensitivity, for quantitative study but destructs compounds.

Good for organic compounds containing C-H or C-C bonds, popular in food analysis.

No response to H20, NO2, H2S

81
Q

Electron Capture Detector

A

Detecting decrease of a standing current generated from the flowing of carrier gas only.

Good for Halocarbons, compounds with double bonds.

High sensitivity and specificity, but narrow linear range

82
Q

Flame Photometric Detector

A

Burn analyte, measure wavelength of illuminated light.

When S or P is present, characteristic wavelength of light is detected.

Photo multiplies tube (PMT) amplifies the current and transfers to recorder.

High sensitivity and specificity.

Used for heavy metals and compounds containing S and/ or P (pesticides and aroma)

83
Q

Photoionization Detector

A

UV light ionizes the analyte to form ions, ions are erected by the electrodes to form a current.

Small current is magnified by electrometer of the GC/

Sensitive, non-destructive

Common in flavor analysis

84
Q

Separation of GC Column

A

N2 is the most efficient carrier gas, but its minimum occurs in low u.

Hydrogen is the best choice.

85
Q

Degradation of 2,4-Decadienal

A

2,4-Decadienal is a key component in chicken, lamb, beef, and French fries

86
Q

Mass Spectrometry

A

An analytical tool used for measuring the molecular mass of a molecule.

Ionization of molecules
Separation of ions based on mass-to charge ratio.
Detected under electrostatic field.

87
Q

MS Sample Introduction for Pure Compounds

A

Direct Injection: for gasses or volatile liquids
Same as injection in GC

Direct Injertion Probe: for somewhat volatile solid
The sample is placed in a small cup on the tip of the prove.

88
Q

MS Sample Introduction for Mixtures

A

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

89
Q

Electron impact (EI) ionization

A

A beam of electrons emitted from a heated filament composed of rhenium or tungsten metal.

When a direct current is applied to the filament (~70 eV), it heats and emits electrons that move across the ion chamber toward a positive electrode.

Emitted electrons extract an electron from sample compound molecules, forming ionized molecules.

The ionized molecules can further fragment into smaller molecular fragments due to high energy

90
Q

MS Ionization Process

A

M + e -> M+ + 2e

91
Q

MS Ionization

A

Both negatively and positively charged molecules of various sizes unique to each compound are formed.

92
Q

MS Repeller plate

A

Repeller plate is positively charged: positive fragments are repelled and move toward the quadrupole mass analyzer and these fragments are analyzed

93
Q

MS Ionization Acceleration

A

Accelerating and focusing plates: to increase the energy of charged molecules and to focus the beam of ions.

A maximum number of ions with same kinetic energy reaches the mass analyzer.

94
Q

Chemical Ionization (CI)

A

Virtually the same ion source device as EI, except, CI uses tight ion source, and reagent gas.

Reagent gas (methane, ammonia, isobutane) is first subjected to electron impact. Then sample ions are formed by interaction of reagent gas ions and sample molecules. Process involved ion-molecule interaction.

95
Q

CI Advantages

A

Generate fewer fragments -> simpler spectrum.

Characteristic [MH]+ peak -> determine molecular weight.

Useful technique for Mw determination of unknown compounds, or when no molecule ion is observed in EI mass spectrum.

96
Q

CI Process

A

Depending on mode, MH+ (+ CI) or [M-H]- (- CI) are often observed. Unlike molecular ions obtained in EI method, MH+ and [M-H]- detection occurs in high yield and less fragment ions are observed.

97
Q

Mass Analyzer

A

Heart of MS
Separating charged fragments based on m/z
Dictating the mass range, accuracy, and sensitivity

98
Q

Types of Mass analyzers

A

Magnetic sectors
Quadrupoles
Ion traps
Time-of-flight (TOF)
Fourier-transform ion cyclotrons (FT-ICR)

99
Q

Magnetic Sensor

A

Use magnetic field to separate ions based on m/z.

Deflection depends on ion mass and charge, magnetic field strength and velocity of ions.

To detect multiple ions and produce the typical mass spectra, the magnet field strength is changed over the analysis time -> all possible m/z rations are seen.

Popular high-resolution instrument

100
Q

Quadrupole Mass Analyzer

A

Quadrupole: “four rods” are used to generate two equals but out-of-phase electric potentials.

One is alternating current (AC) frequency of applied voltage that falls in radiofrequency (RF) range, the other is direct current (DC)

The potential difference -> an oscillating electric field between two of the opposite rods, equal but opposite charges.

By adjusting the potentials on the rod, only selected ions can be detected.

Routinely used in quantitative analysis.

Low resolution but cheap

101
Q

GC-MS

A

To identify unknown or determine the purity.

GC column is connected directly to the MS via a heated capillary transfer line which is kept hot enough to avoid condensation of the volatile component eluting from GC column.

Ex: the separation of methyl esters from six long-chain FAs mixture

102
Q

LC-MS

A

LC-MS interface facilitates removal of solvent.

Applied energy is only for evaporation and compound integrity is maintained.

Heat energy applied in evaporation does not contribute to degradation of any thermally labeled species present in LC eluant.

103
Q

Electrospray Interface

A

Compressed nitrogen gas at high velocity is introduced to facilitate nebulization of the LC effluent, generating a high spray of highly charged droplets. Droplets go through desolvation to form analyte ions when they reach the sample cone.

Highly sensitive with limits of detection at femtogram (10^-15) level and a linear dynamic range between 3 and 4 orders of magnitude.

104
Q

Molecular absorption of radiation in the UV-Vis range results in transitions between what types of energy levels?

A

Electrical

105
Q

Molecular absorption of radiation in the IR range results in transitions between what types of energy levels?

A

Vibrational and Rotational

106
Q

Similarities and differences between UV-Vis and Fluorescence spectroscopy

A

Similarities: components are essentially the same and both are based on absorption of radiant energy by the analyte, and the detection of radiant energy at the detector.

Differences: Has 2 monochromators, fluorescence spectroscopy depends on measuring radiation emitted by the analyte as it relaxes from an excited electronic energy level to its corresponding ground state.

107
Q

Major ions expected in EI mass spectrum of ethanol?

A

CH3-CH2-OH+, CH2-0H+, CHO+, CH3 +

108
Q

Convert rpm to radians

A

(x revolution/min) (1 min/60 seconds) (2pi radians/revolution)

109
Q

Fluorometer

A

Instrument used for Fluorescence Spectroscopy

Light Source: Tungsten filament lamp

Has 2 monochromators (Emission and Excitation)

Sample/Reference Holder

Radiation detector

110
Q

Can IR Spectroscopy be used to find both qualitative and quantitative data?

A

Yes
Mid-IR is the most common.
Near-IR is possible.
Quantitative is more difficult to obtain than qualitative.

111
Q

Convert Pascals seconds to cp

A

Answer]: 1N = 1kg m/s2
1 poise=0.1 kg m-1 s^-1 = 100 cp
1 Pa s=1 N m^-2 s = 1 kg m^-1 s^-1 = 10 poise =1000 cp

112
Q

Components of Mass Spectrometer

A

Sample Introduction, Ion Source, Mass Analyzer, Detector, Data System

113
Q

Beer’s Law

A

A = abc
A: absorptivity (1/cm)
b: pathlength through solution(cm)
c: concentration of absorbing species (M, mM)

114
Q

Strain

A

ε=∆L/L

Food Analysis Final (1 decks)

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