Transcript Document

Basic Gas Chromatography
History
1850 - Separation of dyes by Runge
1906 - Separation of plant pigments by Tswett
1941 - Theoretical gc (Martin & Synge)
1952 - First gc
1954 - TC detector
Process
• Sample is vaporized (if it is not already a
vapor)
• Passes through a column where interaction
occurs - does analyte move with gas phase
or stay with stationary phase (column
coating)
• Separation occurs
• Detection - many types of detectors
• High purity!
• Source of mobile phase - He or H
• Detector gases - none or air/H (Flame
ionization detector)
Gas flow regulators
• Pressure regulators - stainless steel parts not welding quality!
• Flow regulators - Determine gas flow rates
through system (sensitive precision
instruments)
Injection port
• Introduce sample
• Vaporize sample
• Split sample (?)
Injection ports - many versions
• Split - only a portion of injection goes on
column
• Splitless - “all” material injected goes on
column
• On-Column - cold injection (sensitive
materials)
• Programmed temperature - sensitive
materials (more durable method than OC)
• Large volume - Can inject 1 ml - solvent
removal
Columns
• Packed (hard to find)
• Capillary (generally open tubular but can be
a wall coated PLOT type)
Columns
• Generally fused silica - strong and inert
• Inner diameters - 0.10 - 0.53 mm
• Length - 1 - 60 m
• Coatings - several - range in thickness from
0.1 - 5 um
Common Stationary Phase Coatings
Similar
Temperature
Composition
Polarity
1. 100% Dimethyl
Nonpolar
Polysiloxane (Gum)
Applicatiom
Phenol, hydroc.
amines, sulfur cpds
pesticides, PCBs
Reynolds Constants
OV-1
SE-30
Limits
60C to
325C
2. 100% DimethyL
Polysiloxane (Fluid)
Nonpolar
Amino Acid Der.
Essential oits
OV-101,
SP-2100
OC to
280C
3. 5% Diphenyl
to
95% Dimethyl
Nonpolar
Fatty Acids, Me Est
SE-52
-60C
Alkaloids, Drugs,
Halogenated Cpds
OV-23,
SE-54
325C
7% Cyanopropyl
Intermediate
7% Phenyl Polysiloxane
Drugs. steroids,
Pesticides
OV-1701
5.
50% Phenyl,
Intermediate
50% Me.Polysiloxane
Drugs, Steroids,
Pesticides, Glycols
OV-17
60C to
240C
6.
50% Cyanopro.
Intermediate
me 50% Phenyl
ethyl Polysiloxane
50% Trifluoropro Intermediate
Polysiloxane
Fatty Acids, Me Est
Alditol Acetates
OV-225
60C to
240C
Halogenated Cpds
Aromatics
OV-210
45 to
240C
8.
Polyethylene
Polar
Glycol-TPA modified
Acids, Alcohols,
Acrytates, Nitrites,
Aldehydes, Ketones
OV-351,
SP-1000
60 to
240C
9.
Polyethylene Gly
Free Acids, Alc
Carbowax 20M
Ethers,Essential Oils, Glycols, Solvents
4.
7.
Polar
-20 to
280C
60C to
220C
Phase selection
PUBLISHED INFORMATION
Kovats indices compilations
Journal articles
Internal work
INTUITION
like structures
NO IDEA?
Sample information
Nonpolar column
Change to polar if needed
Separation theory
1.Adsorption
2.Molecular exclusion
3. Partition
4. Vapor pressure
Adsorption chromatography
Interaction with a granular support e.g. Tenax,
charcoal, silica gel,
Molecular exclusion
Used for the separation of permanent gases e.g.
Zeolites, Linde molecular sieves
Partition chromatography
• Partitioning between mobile phase and
carrier gas vapor pressure
• SEPARATION BASED ON THE
BOILING PT
Column coatings (stationary
phases)
• Polar to nonpolar
• Polar - Carbowax
• Non Polar - silicone based phases
Column ovens
• Usually heat ovens to help in separations
• Ovens can be controlled from about -60 400C
Detectors
• Many types varying in sensitivity and
selectivity
• Discuss most common types
Thermal conductivity detector
Characteristics of TC detector
• Specificity - very little - will detect almost
anything including H2O - called the
universal detector.
• Sensitivity to 10-7 grams/sec - this is poor varies with thermal condition of the
compound.
• Linear dynamic range; 104 - this is poor response easily becomes nonlinear.
Flame
ionization
detector
Characteristics of a Flame
Ionization Detector (FID)
• Specificity - most organics.
• Sensitivity - 10-12 g/sec for most organics -this is quite good.
• Linear range 106 - 107 -- this is good.
•
A special type of FID is called an alkali flame
(AFID). Rubidium sulfate is burned in the flame
and the detector becomes specific for N and P.
Organics are not detected. Used for amines and
nitrosoamines. (more commonly called the NPD)
Electron
Capture
Detector
Characteristics of an ECD
• Specificity - sensitive to halogens,
conjugated carbonyls, nitriles, and a few
others - no response with ordinary organics
or H2O.
• Sensitivity 5 x 10-14 g/sec - excellent
• Linear range 104
• The radioactive detectors have definite
temperature limits.
Separation - terms
RESOLUTION
SELECTIVITY = relative interaction of column stationary
phase with both compounds to be separated
=
tr’2
tr’1
CAPACITY = retention “time” of compounds to be
separated
k = tr - tm = tr’
tm
tm
THEORETICAL PLATES = column EFFICIENCY
n = 5.545 (tr/Wh)2
Optimizing Gas Chromatography
Key factors influencing efficiency in gas
chromatography are column phase (nonpolar
are most efficient) and column diameter.
Carrier gas type and velocity
Phase thickness:
• Capacity and Efficiency – influenced by
column diameter and phase thickness
• Thick phase – capacity
• Thin Phase – less capacity
Column length
• Longer means better separations but longer
analysis times
• Time proportional to length
• Separation proportional to sq root of length
• Poor means of getting separation – costs too much
in time. Use diameter, phase thickness or phase
type
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