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Separation Techniques in Food Analysis > Unit 9 > Flashcards

Unit 9 Flashcards

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1
Q

define gas chromatography

A
  • a separation technique
  • the mobile phase is a gas
  • GC is carried out in a column
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2
Q

define analytes

A

gaseous and vaporizable (volatile or semi-volatile) molecules that are difficult to separate

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3
Q

what is the mobile phase

A

an inert gas

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4
Q

what is the stationary phase

A

liquid or solid phase immobilized in the column

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5
Q

define gas-liquid chromatography

A

liquid (non-volatile) stationary phase is adsorbed onto a solid inert packing or immobilized on the capillary tubing walls

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6
Q

what is the mechanism behind gas-liquid chromatography?

A

compounds separated based on their volatility and interactions with the stationary phase

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7
Q

what are important components of the GC instrument?

A
  • carrier ggas flow control equipment
  • injection port with heater
  • separation column with stationary phase
  • oven
  • detector
  • data recording system
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8
Q

what are examples of common mobile phases?

A

eg. helium, argon, nitrogen

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9
Q

what is the column flow rate in a packed column?

in an open tubular column?

A

packed column: 25-150mL/min

open tubular column: 1-25ml/min

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10
Q

what is the pressure of the inlet?

A

10-50psig

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11
Q

what are 2 most common injection modes?

A
  1. liquid samples are injected using a microsyringe manually or with autoinjection
  2. thermo-desorption unit: sample is heated for desorption of volatiles. Ca be solid or liquid
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12
Q

what are the 2 main parts of the injection port?

A
  1. rubber septum: syringe needle is inserted to inject sample
  2. glass insert: vaporization chamber
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13
Q

what temperature is the injection port usually maintained at?

A

high temp

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14
Q

what are 3 diff types of injections?

describe each.

A
  1. split injection:
    - only a small fraction will enter the column; remainder is sent to waste
    - used for routine analysis and high concentration samples
  2. splitless injection
    - all samples injected moves into the column
    - used for quantitative trace analysis
  3. on-column injection:
    - column is first at low temp to condense sample in narrow band
    - for examples that decompose above boiling point
    - no heated injection port used
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15
Q

what are packed columns?

A
  • glass or stainless steel coils
  • filled with the stationary phase or packed coated with the stationary phase

examples of phase

  • carbon based
  • zeolite
  • synthetic polymers
  • silica and alumina based
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16
Q

what are capillary columns?

A
  • thin fused silica (purified silicate glass) capillary

- has the stationary phase coated on inner surface, which provides fast equilibrium b/w mobile and stationary phase

17
Q

what are 2 types of GC capillary columns?

describe each

A

WCOT: wall coated open tubular: column coated w/ liquid

SCOT: support coated open tubular: porous support coated w/ liquid

18
Q

what are main differences between packed and capillary columns?

A

compared to packed columns, capillary columns have

  • larger: column size, theoretical plates, solute capacity
  • smaller: flow rates, head pressure, film thickness
19
Q

with a nonpolar PDMS coating on the stationary phase, how can the polarity of the polymer be increased?

A

by increasing percentage of phenyl groups on the polymer

20
Q

what are chiral GC columns?

A
  • function: enantiomer separation

- physiological activity may be diff for diff isomers

21
Q

common chiral stationary phases use derivatives of what?

A
  • cellulose
  • chiral silane
  • cyclodextrin (especially beta-cyclodetrin)
22
Q

in GC, what mechanism is very dependant on temperature?

A

the partitioning/separation mechanisms

23
Q

what compounds can’t be analyzed simply with GC? why?

what can be done about this?

A

compounds with functional groups w/ active hydrogens

they tend to form intermolecular hydrogen bonds which results in

  • lower inherent volatility
  • tendency to interact with column materials

for these compounds, derivatization (eg. alkylation, acylation and silylation) are used to modify properties of the compound to make them more amenable in GC

24
Q

what is an example of derivatization for GC?

A

conversion of FAs to fatty acid methyl esters (FAME)

Separation Techniques in Food Analysis (13 decks)

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