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2
GC
Mechanism of separation is primarily volatility. Difference in boiling point, vapor pressure
etc. What controls this?
Molecule to molecule bonds Van der Waals, dipole dipole for example.
Molecular Weight
8
Block Items
Carrier gas - He, N2, or H2
Injector - usually septum introduction Column with Stationary phase – a
nonvolatile liquid – carbowax is a common example
Detector – converts chemical to electrical information.
Last three items are held at elevated temperatures, usually
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After volatility we can work with polarity differences.
Simple rule is that likes dissolve likes. We could determine log P or just use our chemical intuition.
There is not a big effect here so a short list of columns will usually get the job done.
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Packed Columns
Still find their uses. Can handle larger samples. Have a support coated with stationary
phase Support often diatomite. Issues with active sites.
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Retention Index.
A measure of retention compared to the n-alkanes.
The alkanes are assigned a number that is 100 times the number of carbons. There is related in a linear way to the log tr’
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Retention Index
)(log)(log
)(log)(log)([100
''
''
ntNt
ntunknowntnNnI
rr
rr
Where N is the number of carbons in the higher alkanen is the number of carbons in the lower alkanetr
’ is the adjusted retention time
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What if an analysis is too slow?
Temperature programming Increase temp as the run progresses
Pressure programming Increase pressure as the run progressesAdvantage is that pressure can be quickly
returned to original value where it takes time to reduce the heat.
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Sample Injection
Manual – syringe through the septum port
Automatic – syringe through the septum port.
38
Detectors
A transducer – converts chemical information to electrical signal. Most tell us no additional information other than there is a detector response. TCD FID ECD Others (Mass spec)
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Peak Identification / Quantification
Co-injection. Run on multiple columns of different
polarity.
Area of peak is proportional to amount of sample. Different samples can have different responses.
Area (Gaussian peak) = 1.064*peak ht*w1/2
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Internal Standard
A compound added that is close in nature to the compound being analyzed. Gets around a variety of problems.
][][ S
AF
X
A sx
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Less common detectors
Nitrogen Phosphorus - burner heats a glass bead that contains Rb2SO4 - 104 to 106 greater response to N and P over C.
Flame Photometric - P, S, Pb, Sn Photo ionization detector. Aromatics,
unsaturated compounds Sulfur (nitrogen) chemiluminescence
detector SO mixed with O3 from flame 107 over carbon
Atomic emission
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Mass Spectroscopy
Since full spectra are collected at each time point then we can selectively look for our analyte of interest.
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Resolution Improvement
Longer Column Narrower Column Thinner stationary phase Different Stationary phase
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Injection Comparison
Split Injection Concentrated sample High resolution Dirty samples Thermal decomposition issues