Ch' 24 Gas Chromatography GC

1
Ch. 24 Gas Chromatography (GC)
2
GC Process

• In gas chromatography, vapor-phase analyte is
  swept through the column by a gaseous mobile
  phase (carrier gas)
• Gas-liquid chrom (liquid stationary phase)
• Gas-solid chrom (solid stationary phase)
• The mobile phase is usually He, N2, or H2
  depending on the application
• The analyte is a volatile liquid or gas that is
  injected through a septum (rubber disk)

3
GC Schematic
Injector and oven are heated to make sure vapor
pressure of analytes remains high enough to
proceed through column
4
Open Tube Columns
Usually made of fused silica (SiO2) coated
with a polyimide that can withstand 350C.
Columns are 15-100 m in length. Open tube
columns give higher resolution, greater
sensitivity, and shorter analysis times compared
to packed columns.
5
Packed Columns

• Packed columns are columns filled with small
  particles of stationary phase. The are typically
  a few m in length.
• Packed columns generally have poorer resolution
  than open tube columns
• Not used frequently for analytical experiments
• Often used for synthetic purification
• Particle size is denoted by mesh size

6
Stationary Phases
Stationary phases are chosen based on the
rule that like dissolves like. The silica
backbone can be derivatized with with functional
groups to increase the polarity. The more
polar the column, the higher polarity analytes
that can be separated with that column. Over
time the stationary phase coating bakes off and
Si-OH groups become exposed. This causes
increased tailing during analyses.
7
Analyte Classifications
8
Retention Indices

• Relative retention times can be used to predict
  where a compound will elute
• This is called a retention index
• The retention times are a function of the column
  parameters

On the non-polar column, solutes elute
mostly based on boiling point. On the
polar compound, alcohols are much more strongly
retained.
9
The Kovats Index

• The Kovats retention index (I) is defined for
  alkanes where IC atoms100
• So, for butane, I400, but for octane I800
• Every other compound has a retention index I
  based on what alkanes it elutes between
• N carbon atoms in larger alkane
• n carbon atoms in smaller alkane

10
Retention Indices
The retention index for a compound is unique to
whatever mobile/stationary phase combination
being used. The retention index is obtained
using calibration compounds, and then used to
identify compounds when observed in the unknown.
11
Kovats Example

• If retention times for methane, octane, and
  nonane in a GC run are 0.5, 14.3, and 18.5
  minutes respectively, what is the retention index
  for an unknown that elutes at 16.7 minutes?

12
Temperature Programming

• During most GC analyses, the temperature of the
  column is raised during the analysis
• This is called temperature programming
• It decreases retention time and sharpens the peaks

Temp Prog (50-250C) 8/min
Isothermal (150C)
13
Choice of Carrier Gas

• The choice of what carrier gas to use is mostly
  dependent on what type of detector is used (more
  about those coming up)
• The most common carrier gas is He, N2 is next
• H2 provides the fastest separations and better
  resolution because it has a higher velocity and
  solutes diffuse more rapidly in it.
• But H2 has limited use because of its reactivity

14
Carrier Gas Velocities and Plate Height
15
Sample Injection
A syringe is usually used to inject the
sample through the septum. Its recommended to
take some air before and after the sample. The
air before it makes sure that the sample does not
volatilize in the injector oven before you inject
it. The air afterwards helps sweep the sample
into the injector. You can also take a plug of
pure solvent after the sample to make sure that
it washes the complete sample out of the
syringe. If you have many samples to run,
you can use an autosampler, a device where the
computer automatically injects multiple samples
contained in vials.
16
The Injector Port
17
Injection Techniques
Split injection is used if analytes are gt
0.1 of the sample. High resolution separations
work best with the smallest amount of sample that
can be detected. Split injection also makes sure
that impurities do not get onto the column in
large concentrations. Splitless injection is
appropriate for trace analyses lt (0.01 of the
sample). On-column injection is for samples that
thermally decompose. These go straight onto the
column rather than through an injector oven.
18
Temperature of Column

• Typically, the initial column temperature is set
  to 40C below the boiling point of the solvent
• This ensures that everything condenses at the
  beginning of the column (Solvent trapping)
• Alternatively cold trapping is when you set the
  initial column temperature below (usually 150C)
  the boiling point of the solute of interest
• Cryogenic trapping is a specific form of this for
  extremely volatile compounds using liquid
  cryogens
• Formaldehyde boils at -21C, so liquid N2 or CO2
  is used to cool the column much lower than room
  temp.

19
Detectors

• Many types of detectors can be used for GC
• Universal detectors have a response to all
  analytes
• Thermal conductivity detector (TCD)
• Flame ionization detector (FID)
• Other detectors can also be used
• Electron capture (ECD)
• Nitrogen-phosphorous (NPD)
• Mass spectrometer (MSD-Ch. 22)
• Infrared spectrometer (IRD)
• Atomic emission (AED)

20
TCD
The TCD was the most common detector for a
long time in GC because of its universal
detection. The TCD measures how much a
substance can transport heat from a hot to cold
region. Helium has a high thermal conductivity
(2nd only to H2), so if it is used as the carrier
gas, anytime an analyte emerges from the column
with it, conductivity will decrease.
Filament gets hotter in presence of analyte,
changing voltage
21
FID
The FID is now the most common detector for
GC because it is nearly universal but has greater
sensitivity than a TCD. In an FID, eluate is
burned in a mixture of H2 and air. Most carbon
atoms (except CO) produce radicals that produce
CHO in the flame Electrons are used to
neutralize the CHO atoms and the electron
current can be measured. This current is
proportional to the number of molecules present,
and exhibits a linear response over 7 orders of
magnitude. The detection limits are lower than
the TCD detector by a couple orders of magnitude.
22
ECD
The ECD is sensitive to compounds with high
electron affinities (halogen-containing,
conjugated carbonyls, nitriles, nitro-compounds).
Gas entering the detector is ionized by a high
energy radioactive source that gives off
electrons (often 63Ni). When analytes of high e-
affinity enter the detector, they capture some of
the electrons. A carrier or makeup gas
of N2, H2, or CH4/Ar must be used. The ECD is an
extremely sensitive detector for those compounds
that it detects.
23
Other Detectors

• The NPD is a specific type of FID that is
  particularly sensitive for compounds that contain
  N or P.
• Often used for detecting nerve agents or
  pesticides
• The MSD well talk more about soon
• It is the detector of choice, but
• IRD is not particularly sensitive or often used
• Very good for isomers
• The AED uses a plasma to excite compounds and
  obtain emission spectra
• Most often used for organometallic compounds

24
Detector Figures of Merit
25
Sample Preparation

• In the simplest case, a sample must be prepared
  for analysis by GC
• This involves converting it to a volatile form if
  necessary
• Steps in preparation
• Extraction
• Preconcentration
• Removing or masking interference(s)
• Derivatization
• For GC
• Solid phase microextraction (SPME)
• Purge and trap

26
SPME
SPME is used to extract from liquid, air, or
sludge without using any solvents. A silica
fiber coated with a stationary phase for a GC is
attached to a syringe. The fiber is exposed to
sample for a certain time to allow the phase to
become saturated with analyte. After
sampling, the fiber is retracted into the syringe
and the syringe gets injected into the inlet of
the GC. SPME does not remove all of the analyte
because it is an equilibrium reaction. It
usually can obtain 30-50 of the molecules.
Because the binding to the stationary phase of
the fiber is a partitioning reaction, there is an
equilibrium involved. Equilibration time for
analytes must be obtained using calibration
experiments.
27
Purge and Trap
The purge and trap method is used
specifically to remove and concentrate volatile
analytes from liquids or solids. The goal in
this case is to concentrate 100 of the analyte
Purge gas is swept through the heated sample
and volatilized components go with it into a
trap. The trap contains particles made of
adsorbent compounds that the analytes adsorb
onto. After trapping the analytes for a
specified procedure time, the flow through the
trap is reversed and analytes desorb back off of
the trap and into the injection port of the GC.
28
Steps in GC Analysis

• 1) Goal of the analysis
• Qualitative or Quantitative?
• Analysis time?
• Universal or specific detection?
• 2) Sample preparation
• 3) Type of detector
• Sensitivity
• Dynamic Range
• 4) Type of column
• 5) Injection method
• Split/splitless

29
Specifications for GC

• Advantages
• Can analyze very complex mixtures
• Robust for multiple samples
• Disadvantages
• Wont do heavy (non-volatile) compounds
• Potentially complex sample preparation