Analytical Chemistry

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Analytical Chemistry
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Chapter 24Gas Chromatography
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Gas Chromatograph
Mobile Phase gas (He, N2, H2)
Stationary Phase typically a nonvolatile liquid
Analyte gas or volatile liquid
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Packed Columns vs. Open Tubular
1. Higher resolution
2. Shorter analysis time
3. Greater sensitivity
4. Lower sample capacity
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GC Columns
Coiled GC Columns
Longitudinal Cross Section
Transverse Cross Section
Wall-Coated Open Tubular Column (WCOT)
Porous Layer Open Tubular Column (PLOT)
Support-Coated Open Tubular Column (SCOT)
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Open Tubular Columns
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Common Stationary Phases
Nonpolar
Intermediate Polarity
Strongly Polar
(Diphenyl)x(dimethyl)1-x polysiloxane
(Cyanopropylphenyl)0.14(dimethyl)0.86 polysiloxane
(Biscyanopropyl)0.9 (cyanopropylpheny)0.1
polysiloxane
x 0 nonpolar -60 to 320 C
-20 to 280 C
0 to 275 C
(Diphenyl)x(dimethyl)1-x polysiloxane
Polyethylene glycol
x 0.65 intermediate 50 to 370 C
40 to 250 C
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Order of Elution
Kovats Retention Index (I)
n number of carbon atoms in smaller alkane N
number of carbon atoms in larger alkane
Simply relates the retention time of a solute to
the retention times of linear alkanes.
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Order of Elution
Predict the order of elution for the following
compounds using the following stationary phases
1) Nonpolar (diphenyl)(dimethyl) polysiloxane
C B A
2) Polar (poly(ethylene glycol)
A B C
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Order of Elution
Predict the order of elution for the following
compounds using the following stationary phases
1) Nonpolar (diphenyl)(dimethyl) polysiloxane
I 672 664 691
B A C
2) Polar (poly(ethylene glycol)
I 956 1142 987
A C B
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Temperature Programming
Isothermal at 45 C
The column temperature is raised during the
separation to increase solute vapor pressure and
therefore decrease retention time of the slow
eluting analytes.
Isothermal at 145 C
Temperature Programmed 30-180 C
Raising Temperature
1) Decreases tr 2) Sharpens peaks
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Pressure Programming
Pressure can also be used to reduce retention
times.
This helps prolong the lifetime of the column.
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Carrier Gas
Cannot use H2 with mass spectrometers!
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Improving Resolution
1) Increase length of column
2) Decrease column diameter
3) Decrease stationary phase thickness
4) Change stationary phase
5) Temperature and/or Pressure programming
6) Optimize carrier gas and flow rate
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GC Detectors
1) Thermal Conductivity Detector
(Universal) 2) Flame Ionization Detector 3)
Electron Capture Detector 4) Mass Spectrometric
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Thermal Conductivity Detector (TCD)
Measures the ability of a substance to transfer
heat.
Table 24-4
104 linear response range
H2 and He give lowest detection limits
Sensitivity increases with
increasing filament current
decreasing flow rate
lower block temperature
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Flame Ionization Detector (FID)
Eluate is burned in a mixture of H2 and air.
Carbon atoms produce CH radicals.
1 in 105 carbon atoms produces an ion.
107 linear response range
100 times lower detection limit than TCD.
N2 gives best detection limits.
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Electron Capture Detector (ECD)
Measure the decrease in current caused when
compounds with high electron affinity capture
emitted electrons
Often use N2 or 5 CH4 in Ar make-up gas.
Selective for halogens and conjugate carbonyls
103 to 104 linear range
The effluent from the column passes over a
beta-emitter, such as 63Ni or tritium. An
electron from the emitter causes ionization of
the carrier gas and the production of a burst of
electrons. In the absence of organic species, a
constant current between a pair of electrodes
results from this ionization. The current
decreases in the presence of organic molecules
with high electron affinities that capture these
electrons.
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Comparison of Common GC Detectors
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Qualitative Quantitative Analysis
Qualitative
Co-chromatography add a known sample to your
unknown and see if it increases the relative area
of the unknown peak.
Mass spectrometry compare mass spectrum of
unknown to a compiled library of mass spectra or
that of a standard.
Quantitative
The area of the chromatographic peak is
proportional to the quantity of the analyte.
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Cocaine Lab
Everyone uses the same set of drug standards.
Calculate response factor for each standard.
Only one standard ran. F 1. Therefore
Example