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Food Analysis > Exam 3 > Flashcards

Exam 3 Flashcards

1
Q

What is chromatography

A

*general term applied to wide variety of separation techniques based on patitioning of sample (solute) between moving (mobile) phase and a fixed (stantionary) phase
*relative interaction of the solute with these two phases is described by the partition coefficient

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2
Q

what is the mobile phase (ex.)

A

gas (GC), liquid (LC), or supercritical fluid (SFC)

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3
Q

what is the stationary phase

A

liquid or solid

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4
Q

what determined the elution time

A

interaction of the solute with the stationary phase

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5
Q

Thin-Layer Chromotography

A

*separation based on polarities
*TLC plates with thin layer of stationary phase
*sorbents used: silica gel, alumina, diatomaceous earth, cellulose
*TLC put into solvent (mobile phase)
*subst. with polar prop. will interact strongly with stationary phase (move slower)
*subst. with nonpolar prop. will interact strongly with mobile phase (move up & faster)

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6
Q

How can separation occur in TLC

A

*adsorptive
*partition (normal/reversed phase)
*ion exchange
*size-exclusion

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7
Q

Advantages & applications to TLC

A

ADVANTAGES:
*better resolution, faster, more reproducible, easier to handle
APPLICATIONS:
*screen corn/peanuts for mycotoxins before processing, lipids, carbs, vitamins, amino acids, pigments

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8
Q

Visualization/ Quantitation of TLC

A

VISUALIZATION:
*colorimetric, fluorescence, UV light
QUANTITATIVE:
*scraping off the zone, eluting the compound and then analyze the resulting solution

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9
Q

Column Chromotography (basics)

A

*separation to isolate/separate solutions
*mobile phase = liquid
*stationary phase = solid or liquid supported by inert solid
*stationary phase is packed in column and equilibrated with mobile phase
*mobile phase moves through column by a pump
*components will separate as they move down column

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10
Q

What are the essential components of a column chromotagraphy system

A

*magnetic stirrer
*peristatic pump
*column
*spectrophotometer
*fraction collector

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11
Q

Adsorption

A

*solute is adsorbed on surface of stationary phase (silica, alumina)
*elution order of solutes depends on relative polarities (polar = last, nonpolar = first)
*Van der waals, electrostatic, H bonding, hydrophobic interactions
*uses: separates aromatic or aliphatic nonpolar compounds, base primarily on type of # of functional groups present

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12
Q

External Standards

A

standard solutions are chromatographed and peak area (or height or mass) are plotted vs. concentration on standard curve

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13
Q

Internal Standards

A

*amount of each component in sample is determined by comparing area (or height or mass) of component peak to that of the internal standard peak
*need to run set of standards at varying concentrations, with each solution containing constant amount of internal standard

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14
Q

Partition Chromatography

A

*liquid-liquid chromatography
*hydrophobic remains in organic phase
*hydrophilic remains in aqueous phase
*column with particle of solid support coated with liquid stationary phase
*pour mixture into column and mixture dissolves into stationary phase
*mobile phase will be poured in and one comp. will interact strongly with mobile while other interacts with stationary phase
*collected as fractions

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15
Q

Normal Partition Chromatography

A

*polar stationary phase vs. nonpolar mobile phase (nonpolar solutes elute first)
*common stationary phases: silica, organic moieties w/ cyano and amino funct. groups
*uses: separate and quantitate polar hydrophilic substances, ex. carbohydrates, water-soluble plant pigments

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16
Q

Reversed Partition Chromatography

A

*nonpolar stationary phase vs. polar mobile phase
*polar solutes first to elute
*common stationary phase: alkyl hydrocarbons
*uses: separate and quantitate lipophilic compounds ex. lipids and fat soluble pigments, separate and quantitate polyphenols

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17
Q

Ion-Exchange Chromatography

A

*solid-liquid chromatogrpahy
*based on attraction between opposing charges
*stationary phase contains fixed funcitonal groups with either positive or negative charge
*cation exchnager (has neg. charge)
*anion exchanger (has pos charge)
*elution: change mobile-phase pH or inc ionic strength so solute no longer binds
*uses: amino acids, sugars (HPAEC), alkaloids, proteins, drugs, fattya cids, acids of fruit

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18
Q

Size-Exclusion Chromatography

A

*solid-liquid chromatography
*molecules are separated based on size
*stationary phase is porous
*molecules too large to enter the pores travel with the mobile phase in the interstitial phase at elute first
*solutes of low molecular weight get slowed down by entering the pores

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19
Q

Affinity Chromatography

A

*solid-liquid chromatography
*isolates biomolecule from sample
*separation based on reversible interaction between a solute and an immobilized ligand (antibodies, enzyme inhbitiors, lectins)
*pour mobile phase into column w/stationary phase
*biomolecule will attach to ligand
*everything else will be washed away
*then another solvent will be poured in that competes with biomolecule to bind to ligand
*biomolecule will be separated/isolated
*uses: separate and purify enzymes or enzyme inhibitors, separate and purify glycoproteins

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20
Q

what is special about HPLC

A

*it can separate parts of a compound, tell you how much of each compound is in a mixture, and help identify what each compound is

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21
Q

what are the components of an HPLC system

A

*eluent reservoir
*proportioning valve
*pump
*injector
*column
*detector
*computer/data system

22
Q

How do piston pumps function in one-way flow

A

*they are reciprocating, piston-type pumps
*pump will reciprocate and will open up the suction valve and allow eluent to flow into pump
*pump will come back which will cause the suction valve to close but the delivery valve to open up allowing the eluent to flow out
*eluent will flow into column

23
Q

How is a gradient formed?

A

*gradient elution system is used to vary the mobile phase concentration during the run (mixes from two or more reservoirs)
*the multi-valve is what varies mobile phase conc.
*the valves are open or closed to a certian degree by an electrical current and low-pressure will mix them
*the flow of eluent draws out each eluent proportionally; they then mix into one system and enter the high pressure pump

24
Q

How does the injector work (HPLC)

A

*places sample into flowing mobile phase, so it can enter the column
*valve injectors that separate sample introduction from high-pressure eluent system (after the pump)

25
Q

Which detectors are common in HPLC

A

*fluorescence
*refractive index (RI)
*UV-Vis absorption
*electrochemical

26
Q

HPLC Detector (RI)

A

*detects solutes by monitoring the refractive index of the column eluent relative to a reference cell containing air, mobile phase, or a transparent material with a specified RI

27
Q

HPLC Detector (UV-VIS)

A

*measures the intensity of light that passes through a sample
*measures how much light a sample absorbs

28
Q

What are applications of HPLC

A

*normal phase; analysis rice bran oil
*reversed phase; analysis of vitamin B6 compounds
*anion exchange; analysis of isoamylase-treated waxy corn starch
*size exclusion; analysis of tomato cell wall pectin from hot/cold break tomato prep

29
Q

Why would someone use a fraction collector (HPLC)

A

it allows for the isolation of single peaks to obtain enough of chemical to be used as reference standard

30
Q

How does GC work

A

*used to separate volatile compounds
*components: mobile/stationary phase, molecular sieve, injector, column (tight coil), detector (FID)
*stationary phase packed into inner wall of column
*molecular sieve separates unwanted hydrocarbons, water vapor, O from mobile phase
*sample is injected into the column
*sample is transported through column by flow of an inert-gas mobile phase (often Helium)
*less volatile comp interact with stationary phase and more volatile interact with mobile phase (helium, nitrogen)
*the column is in an oven to control temp; ex. a gradient of the oven temp
*volatiles are separated based on properties of: boiling point, molecular size, polarity

31
Q

What characteristics of a sample are necessary for GC

A

thermally stable volatile substances

32
Q

GC Applications

A

*carbohydrate analysis
*lipid analysis
*analysis of contaminants, residues, and constituents of concern

33
Q

What should the gas supply system have

A

a gas line with traps to remove moisture and contaminants

34
Q

injection port- hardware GC

A

*functions: sample introduction, sample vaporization, possibility for sample dilution and splitting
*contains soft septum with gas-tight seal that can be penetrated by syringe needle
*samples introduced either by manual syringe technique or automated sampling system

35
Q

Why is there an oven GC

A

*oven controls the temperature of the column
*higher temp of oven causes sample to elute faster, but at cost of resolution
*capillary column can be heated directly with insulated heating wire based on low thermal mass technology (makes total heating/cooling cycle much shorter)

36
Q

Capillary Columns GC

A

*hollow infused silica glass
*5-100m long; very thin walls

37
Q

Preparation for GC- SPME

A

*Solid-Phase Microextraction
*stationary phase is bound onto fine fused silica filament or metal filament (fiber)
*fiber immersed in sample, or headspace above sample
*at end of extraction time, fiber is removed from sample and forced through septum of a GC
*adsorbed volatiles are thermally desrobed from fiber

38
Q

Preparation for GC- Headspace

A

*direct injection of headspace vapors above a food product is a simple method to isolate volatile compounds
*direct; headspace of sample taken using gas-tight syringe
*dynamic: involves passing large volume of headspace vapors through an adsorbent trap

39
Q

Why are some samples subjected to a Derivatization

A

*they are thermally unstable
*are too low in volatility (sugars, amino acids)
*yield poor chromatography separation due to polarity (phenols, acids)

40
Q

What happens upon injection with either a syringe or a SPE device?

A

it is vaporized to be able to pass through the column for separation

41
Q

Detector GC (FID)

A

*samples flow from coulmn and an igniter will ignite hydrogen and oxygen to make flame
*sample will flow into flame and be ionized
*there are electrodes that measure the current
*this signal is then detected

42
Q

Ionization, separation (resolution), detection

A

*Ionization: occurs in ion source by electron impact, matrix-assisted-laser-desorption
*Separation: charged molecular ion and/or fragments are separated according to their m/z (occurs in mass analyzer), higher mass = deflected more, lower mass = deflected less
*separated, charged fragments are then monitored by detector

43
Q

Types of ionization systems

A

*electron impact (EI) ionization process
*electrospray ionization (ESI-MS)
*matrix assisted laser desorption ionization (MALDI)

44
Q

Electron impact (EI) ionization process

A

*compound is exposed to beam of electrons emitted from filament
*when a direct current is applied to the filament, it heats and emits electrons that move across ion chamber toward pos electrode
*e- will pass through region and extract e- from sample molecules
*this forms an ionized molecule
*ionized molecule contains high enough energy to fragments into even smaller fragments
*then moves through electric plates

45
Q

Matrix assisted laser desorption ionization (MALDI)

A

*sample is dissolved in a matrix and ionized using UV laser (soft ionization)
*good for ionization of large biopolymers and other fragile molecules

46
Q

Types of resolution systems

A

*time of flight
*magnetic sector
*quadrupole mass analyzers

47
Q

Time of flight

A

*separates ions according to time required to reach detector
*ions are pulsed from source with same kinetic energy, so ions of different m/z ratios acquire different velocities, which determines time to reach detector
*useful for analysis of biopolymers and large molecules
*enables high resolution accurate mass applications

48
Q

Quadrupole mass analyzers

A

*dervied from “fourfold” and “pole” to describe the four rods used to generate two equal, but out-of-phase electric potentials: one is alternating current (AC) freq, second is direct current (DC) freq
*creates an oscillating electric field between two of the opposite rods
*it filters ions that achieve stability from those that do not

49
Q

Types of computer calculations for mass detection

A

*time of flight
*magnet current
*quadrupole voltage

50
Q

What can we learn from the GC-MS of fatty acids

A

*fatty acids are not volatile in native form, so they must be derivatized
*fatty acids must be methylated on the carboxyl group = fatty acid methyl esters, this eliminated the polar nature of the group

51
Q

How does one get carbohydrates linkage data from GC-MS

A

*begins with methylation of all free OH groups in the intact polysaccharide
*hydrolysis of polymer
*sample is then reduced and acetylated; the methyl ethers are stable to the acetylation conditions
*derivatives are analyzed by GC-MS

Food Analysis (4 decks)

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