5-7 Flashcards
What precautions would you take if you want tr as a means of peak characterization?
1) Inject and Hit Run button as reproducibly as possible
2) Column T and Flow Rate should remain constant
What is spiking?
increasing the concentration of one component in the mixture (used when 2 or 3 components have very close retention times
Why are tr’ and k’ considered unreliable variables?
they can change from day to day and they change with particle size, film thickness and SP packing
What is the Kovats Retention Index?
used to characterize SP with retention properties of normal alkane (differing by one methylene group (CH2) (e.g. butane, pentane, and hexane
Why is there a linear increase in tr’ value with increase in the number of carbon atoms?
Separation of alkane is controlled by difference in vapor pressure of boiling point of alkanes
Why alkane is used as Standards to determine KRI of unknown and to characterize SP?
low polarity and freedom from H-bonding with SP
What does the higher Index value mean?
greater retention and hence greater affinity for SP
Which are exceptions to the Index trend?
benzene and pyridine
Why are benzene and pyridine the exception to the I trend?
more pi-pi bonding characteristics
What are the most important characteristics of the chromatographic system if it is to produce accurate quantitative results?
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
Why do different compounds respond differently when they pass through the GC detector?
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
What criteria must the Internal Standard meet?
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
Why are calibrators used in the External Standard Method?
Because they are perpared and analyzed in separate chromatograms from unknown samples
Why is the external standard method more popular in HPLC?
Injection is more precise in HPLC than GC. The multiport injection allows more precision in injection volume
When is area normalization used?
Both GC and HPLC
When is the external standard used?
HPLC only
When is the internal standard used?
Both GC and HPLC
An increase in temperature will lead to what in GC?
1) decrease in tr
2) decrease in Vr
3) decrease in capacity factor
4) decrease in selectivity factor
5) increase in efficiency
What is temperature programming used for?
it is useful for separating complicated mixtures of boiling points or polarities.
How is temperature programming used?
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.
What is the trade off with T programming?
the time involved in recycling the oven
Why is there a decrease in tr and Vr with an increase in temperature?
an increase in T decreases retention time because solute vapor pressure(P) is increased, which is in accordance with the Clausis-Claperyron equation
What does the C-C equation tell us?
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
How does a decrease in VP affect tr and k’?
the decrease in VP increases the relative amount of the solute in SP, which in turn increases both both k’ and tr
What experiments need to be done in order to determine the enthalpy?
- 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
What does a plot of ln(Vr’) vs 1/T show?
- many lines in the plot diverge slightly at low T
- GC separations are slightly better at low T (Vr
Why does an increase in T sharpen the peaks in GC?
as T increases, so does Ds and Dm
What does an increase in Ds mean?
means improvement in mass transfer of solute to and from SP
Adsorption Desorption
Why does a temperature increase lead to an overall increase in efficiency?
Since both Csu and Cmu are greater than B/u, the plate height decreases, which leads to an increase in N
What are some of fundamental concepts of GC?
- 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
What type of samples are analyzed by GC?
volatile and thermally stable
What is a gas chromatograph?
a chemical analysis instrument for separating chemicals in a complex sample
How does a gas chromatograph work?
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
What are the types of carrier gases?
He, Ar, N2, CO2, H2
Why type of injector is used for a pack column?
a simple septum (on column) injector is used
Where is the column located and what types are available for GC?
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
What is the detector and where is it located?
detects the presence of each solute individually in the gas stream as it leaves the column
located in separately controlled heated zone
What types of detectors are there?
Thermal Conductivity —(TCD) most economical
Flame Ionization —-(FID) workhorse for GC
Electron Capture —-(ECD) used for trace analysis of toxic chemical compounds
What is the data system?
Collects the signal from the detector
reduces the need for manual calculations of chromatographic parameters
Why is it important to heat injector and detector?
to vaporize sample rapidly so that no loss in N results from injection technique
must also be low enough to avoid “thermal decomposition”
What is special about the carrier gas?
it is inert and does not interact chemically with the injected solute
What is the choice of carrier gas dictated by?
cost, safety, dryness, freedom from O2, inertness, availability, type of detector used
What is the secondary purpose of the carrier gas?
it is a suitable matrix for the detector to measure the component signal
What are the choices for carrier gases based on the detector type?
(TCD) - He
(FID) - He or N2
(ECD) - Very dry N2, Ar, 5% CH4
How does the molecular weight of the carrier gas affect the the optimum gas velocity or HETP?
increase in mw shifts v(opt) to lower velocity of MP
increase in mw decreases the H(min) at v(opt)
Which carrier gas is the best compromise?
H2 and He provide equally good N. However, due to safety concerns, He is recommended
Why are smaller injection volumes used for Open Tubular Capillary (OTC)?
less stationary used for column packing, lower injection volume had to be used to prevent column overloading
What is another function of the GC injector?
provides means of maintaining pressure flow through the column during injection procedure
prevents ingress of air into the column system
Why must different injection systems be used for OTC?
- works on much smaller scale
- peaks are very narrow
- fast injector is required to reduce broadening
Why does a sample need to be split?
prevents overloading
peak efficiency increases
What is a neat sample?
no solvent
What levels of concentration should be used for split injections?
concentrated neat samples
What is the split ratio?
ratio of the amount of material entering the column versus the amount lost via the split
S = Fc/[Fs+Fc]
What are the advantages of the split injection?
- sample is small so high resolution is obtained
- use large split ratio, samples can be neat
- dirty samples can be used
What are the disadvantages to split injection?
- trace analysis is limited (only fraction enters column)
- discrimination against high mw solutes
What are the advantages of using splitless injection?
- 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
What are the disadvantages to splitless injection?
–time consuming
due to temp programming, and must dilute sample with a volatile solvent
Is efficiency lost more slow or fast in OTC vs packed column?
N is lost more slowly because the plate height curve remains flat upon increasing the flow rate in OTC
What is the van Deemter equation for OTC?
H = B/u +Cu
A depends on the stationary phase packing, which is non-existent in OTC
What does N or H depend on?
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 (
What does the solid support in a packed column do?
serves to hold the liquid SP in place so that a large surface area of SP is exposed to to the mobile phase
What compounds are nonpolar?
- saturated hydrocarbons
- aromatic hydrocarbons
- halocarbons
- mercaptans
- sulfides
What compounds are “weak intermediate polar?”
- ethers
- ketones
- aldehydes
- esters
- tertiary amines
What compounds are “strong intermediate polar?”
- alcohols
- carboxylic acids
- phenols
- primary and secondary amines
- oximes
What compounds are “strong polar?”
- polyhyroxyalcohols
- amino alcohols
- hydroxy acids
- polyprotic acids
- polyphenols
What is ideal support material?
small, uniform spherical particles with good mechanical strength and surface area of ~ 1m^2/g
What is normally used to block the residual Si-OH groups?
dimethylchlorosilane, or HMDS,
blocks Si-OH with methylated siloxane bonds, Si-O-Si-Me
What is Chromsorb P?
- 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
What is Chromsorb W?
- prepared from calcined diatomile
- low surface area(1m^2/g)
- 12% w/w load
- suitable for polar compounds
What is Chromsorb G?
- hardest and twice as dense as Chromsorb W
- maximum load is 5% w/w due to high density
Selectivity is _______ for nonpolar solutes on a nonpolar column.
higher
Compounds within the same class elutes according to _________ on both columns.
boiling point
On a ________ column volatility dominates.
nonpolar
On a ________ column polarity dominates.
polar
What is WCOT?
liquid SP is directly coated onto the column wall, blowing the column dry with inert gas
What is SCOT?
liquid is coated on a solid support or solid particles
What is PLOT?
solid particles are active SP, a thin layer of solid absorbent is fixed to the inner wall of the capillary.
What is the major purpose of bonding and crosslinking?
- -prevent peak tailing due to unreacted silanols
- -prevent column bleed at high T
Why are gases more likely to be analyzed by GSC than by GLC?
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
Flow rate is ___________ in OTC than in packed column.
lower
The length of the column in packed column is __________ compared to OTC.
shorter
The diameter of the OTC is much __________ than packed column.
smaller
