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Chromatography > 5-7 > Flashcards

5-7 Flashcards

1
Q

What precautions would you take if you want tr as a means of peak characterization?

A

1) Inject and Hit Run button as reproducibly as possible

2) Column T and Flow Rate should remain constant

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2
Q

What is spiking?

A

increasing the concentration of one component in the mixture (used when 2 or 3 components have very close retention times

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3
Q

Why are tr’ and k’ considered unreliable variables?

A

they can change from day to day and they change with particle size, film thickness and SP packing

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4
Q

What is the Kovats Retention Index?

A

used to characterize SP with retention properties of normal alkane (differing by one methylene group (CH2) (e.g. butane, pentane, and hexane

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5
Q

Why is there a linear increase in tr’ value with increase in the number of carbon atoms?

A

Separation of alkane is controlled by difference in vapor pressure of boiling point of alkanes

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6
Q

Why alkane is used as Standards to determine KRI of unknown and to characterize SP?

A

low polarity and freedom from H-bonding with SP

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7
Q

What does the higher Index value mean?

A

greater retention and hence greater affinity for SP

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8
Q

Which are exceptions to the Index trend?

A

benzene and pyridine

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9
Q

Why are benzene and pyridine the exception to the I trend?

A

more pi-pi bonding characteristics

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10
Q

What are the most important characteristics of the chromatographic system if it is to produce accurate quantitative results?

A

1) Linearity of response over wide dynamic range (Dr)
2) Baseline separation of components to be separated
3) Universal response of analytes
4) Peak area is used to quantify chromatographic bands

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11
Q

Why do different compounds respond differently when they pass through the GC detector?

A

solutes respond differently to GC/HPLC detector of their inherent property
TC of one compound > TC of another compound
one way to resolve this problem is to use the Peak Normalization Method

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12
Q

What criteria must the Internal Standard meet?

A

1) Structure must be similar to the analytes of interest
2) Should elute near the peak of interest
3) There should be a well resolved peak
4) Available in the pure form

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13
Q

Why are calibrators used in the External Standard Method?

A

Because they are perpared and analyzed in separate chromatograms from unknown samples

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14
Q

Why is the external standard method more popular in HPLC?

A

Injection is more precise in HPLC than GC. The multiport injection allows more precision in injection volume

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15
Q

When is area normalization used?

A

Both GC and HPLC

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16
Q

When is the external standard used?

A

HPLC only

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17
Q

When is the internal standard used?

A

Both GC and HPLC

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18
Q

An increase in temperature will lead to what in GC?

A

1) decrease in tr
2) decrease in Vr
3) decrease in capacity factor
4) decrease in selectivity factor
5) increase in efficiency

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19
Q

What is temperature programming used for?

A

it is useful for separating complicated mixtures of boiling points or polarities.

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20
Q

How is temperature programming used?

A

the T of the column is increased in a systematic fashion so that at later times the column is at a high temperature. Hence, sample components that would otherwise slowly elute will emerge quickly from the column.

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21
Q

What is the trade off with T programming?

A

the time involved in recycling the oven

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22
Q

Why is there a decrease in tr and Vr with an increase in temperature?

A

an increase in T decreases retention time because solute vapor pressure(P) is increased, which is in accordance with the Clausis-Claperyron equation

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23
Q

What does the C-C equation tell us?

A

As the absolute T decreases, vapor pressure of the solute decreases logarithmically in the equation. There is a direct relationship b/w vapor pressure of the solute and temperature

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24
Q

How does a decrease in VP affect tr and k’?

A

the decrease in VP increases the relative amount of the solute in SP, which in turn increases both both k’ and tr

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25
Q

What experiments need to be done in order to determine the enthalpy?

A
  • measure tr and Vr of selected compounds
  • measure tm and calculate Vm of selected compounds
  • calculate Vr’
  • plot ln(Vr’) vs 1/T; H = mR
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26
Q

What does a plot of ln(Vr’) vs 1/T show?

A
  • many lines in the plot diverge slightly at low T

- GC separations are slightly better at low T (Vr

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27
Q

Why does an increase in T sharpen the peaks in GC?

A

as T increases, so does Ds and Dm

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28
Q

What does an increase in Ds mean?

A

means improvement in mass transfer of solute to and from SP

Adsorption Desorption

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29
Q

Why does a temperature increase lead to an overall increase in efficiency?

A

Since both Csu and Cmu are greater than B/u, the plate height decreases, which leads to an increase in N

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30
Q

What are some of fundamental concepts of GC?

A
  • little association b/w gaseous MP and vaporized solute particles
  • All solute spend same time in gas MP. Therefore, difference in tr observed primarily due to retention by SP
  • analyte is retained by partitioning or adsorption
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31
Q

What type of samples are analyzed by GC?

A

volatile and thermally stable

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32
Q

What is a gas chromatograph?

A

a chemical analysis instrument for separating chemicals in a complex sample

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33
Q

How does a gas chromatograph work?

A

uses a carrier gas to push injected sample from injector to a flow-through narrow tube known as the column. as the chemicals exit the column, they are detected and identified electronically

34
Q

What are the types of carrier gases?

A

He, Ar, N2, CO2, H2

35
Q

Why type of injector is used for a pack column?

A

a simple septum (on column) injector is used

36
Q

Where is the column located and what types are available for GC?

A

the column is located in the oven
a) Capillary column —-0.1-0.5 mm, 10-60 m in length

b) Packed column —–2-4 mm, 5-10 m in length

37
Q

What is the detector and where is it located?

A

detects the presence of each solute individually in the gas stream as it leaves the column

located in separately controlled heated zone

38
Q

What types of detectors are there?

A

Thermal Conductivity —(TCD) most economical
Flame Ionization —-(FID) workhorse for GC
Electron Capture —-(ECD) used for trace analysis of toxic chemical compounds

39
Q

What is the data system?

A

Collects the signal from the detector

reduces the need for manual calculations of chromatographic parameters

40
Q

Why is it important to heat injector and detector?

A

to vaporize sample rapidly so that no loss in N results from injection technique

must also be low enough to avoid “thermal decomposition”

41
Q

What is special about the carrier gas?

A

it is inert and does not interact chemically with the injected solute

42
Q

What is the choice of carrier gas dictated by?

A

cost, safety, dryness, freedom from O2, inertness, availability, type of detector used

43
Q

What is the secondary purpose of the carrier gas?

A

it is a suitable matrix for the detector to measure the component signal

44
Q

What are the choices for carrier gases based on the detector type?

A

(TCD) - He
(FID) - He or N2
(ECD) - Very dry N2, Ar, 5% CH4

45
Q

How does the molecular weight of the carrier gas affect the the optimum gas velocity or HETP?

A

increase in mw shifts v(opt) to lower velocity of MP

increase in mw decreases the H(min) at v(opt)

46
Q

Which carrier gas is the best compromise?

A

H2 and He provide equally good N. However, due to safety concerns, He is recommended

47
Q

Why are smaller injection volumes used for Open Tubular Capillary (OTC)?

A

less stationary used for column packing, lower injection volume had to be used to prevent column overloading

48
Q

What is another function of the GC injector?

A

provides means of maintaining pressure flow through the column during injection procedure

prevents ingress of air into the column system

49
Q

Why must different injection systems be used for OTC?

A
  • works on much smaller scale
  • peaks are very narrow
  • fast injector is required to reduce broadening
50
Q

Why does a sample need to be split?

A

prevents overloading

peak efficiency increases

51
Q

What is a neat sample?

A

no solvent

52
Q

What levels of concentration should be used for split injections?

A

concentrated neat samples

53
Q

What is the split ratio?

A

ratio of the amount of material entering the column versus the amount lost via the split
S = Fc/[Fs+Fc]

54
Q

What are the advantages of the split injection?

A
  • sample is small so high resolution is obtained
  • use large split ratio, samples can be neat
  • dirty samples can be used
55
Q

What are the disadvantages to split injection?

A
  • trace analysis is limited (only fraction enters column)

- discrimination against high mw solutes

56
Q

What are the advantages of using splitless injection?

A
  • improved sensitivity of detection of analytes
  • typically 20-50 fold more samples enters the column
  • high resolution of high boiling compounds is observed
  • improved trace analysis
57
Q

What are the disadvantages to splitless injection?

A

–time consuming

due to temp programming, and must dilute sample with a volatile solvent

58
Q

Is efficiency lost more slow or fast in OTC vs packed column?

A

N is lost more slowly because the plate height curve remains flat upon increasing the flow rate in OTC

59
Q

What is the van Deemter equation for OTC?

A

H = B/u +Cu

A depends on the stationary phase packing, which is non-existent in OTC

60
Q

What does N or H depend on?

A

particle diameter(dp) and factor factor (lambda)

  • -plate height decreases and efficiency increases with a decrease in dp (we want small range)
  • -if particles are too small, N decreases (
61
Q

What does the solid support in a packed column do?

A

serves to hold the liquid SP in place so that a large surface area of SP is exposed to to the mobile phase

62
Q

What compounds are nonpolar?

A
  • saturated hydrocarbons
  • aromatic hydrocarbons
  • halocarbons
  • mercaptans
  • sulfides
63
Q

What compounds are “weak intermediate polar?”

A
  • ethers
  • ketones
  • aldehydes
  • esters
  • tertiary amines
64
Q

What compounds are “strong intermediate polar?”

A
  • alcohols
  • carboxylic acids
  • phenols
  • primary and secondary amines
  • oximes
65
Q

What compounds are “strong polar?”

A
  • polyhyroxyalcohols
  • amino alcohols
  • hydroxy acids
  • polyprotic acids
  • polyphenols
66
Q

What is ideal support material?

A

small, uniform spherical particles with good mechanical strength and surface area of ~ 1m^2/g

67
Q

What is normally used to block the residual Si-OH groups?

A

dimethylchlorosilane, or HMDS,

blocks Si-OH with methylated siloxane bonds, Si-O-Si-Me

68
Q

What is Chromsorb P?

A
  • pink earth derived from crushed fire brick
  • high surface area(4.0m^2/g)
  • 35% w/w load of liquid SP
  • suitable for separation of alkanes, but needs deactivation when polar compounds are injected
69
Q

What is Chromsorb W?

A
  • prepared from calcined diatomile
  • low surface area(1m^2/g)
  • 12% w/w load
  • suitable for polar compounds
70
Q

What is Chromsorb G?

A
  • hardest and twice as dense as Chromsorb W

- maximum load is 5% w/w due to high density

71
Q

Selectivity is _______ for nonpolar solutes on a nonpolar column.

A

higher

72
Q

Compounds within the same class elutes according to _________ on both columns.

A

boiling point

73
Q

On a ________ column volatility dominates.

A

nonpolar

74
Q

On a ________ column polarity dominates.

A

polar

75
Q

What is WCOT?

A

liquid SP is directly coated onto the column wall, blowing the column dry with inert gas

76
Q

What is SCOT?

A

liquid is coated on a solid support or solid particles

77
Q

What is PLOT?

A

solid particles are active SP, a thin layer of solid absorbent is fixed to the inner wall of the capillary.

78
Q

What is the major purpose of bonding and crosslinking?

A
  • -prevent peak tailing due to unreacted silanols

- -prevent column bleed at high T

79
Q

Why are gases more likely to be analyzed by GSC than by GLC?

A

gases are not sufficiently soluble in most liquid SP at T>50C
gases have very little difference in polarity
Solid SP (molecular sieve) have rigid geometry
higher resolution but lower sample capacity

80
Q

Flow rate is ___________ in OTC than in packed column.

A

lower

81
Q

The length of the column in packed column is __________ compared to OTC.

A

shorter

82
Q

The diameter of the OTC is much __________ than packed column.

A

smaller

Chromatography (14 decks)

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