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 CoatingsSimilar

TemperatureComposition Polarity Applicatiom Reynolds Constants Limits1. 100% Dimethyl Nonpolar Phenol, hydroc. OV-1 60C toPolysiloxane (Gum) amines, sulfur cpds SE-30 325C

pesticides, PCBs

2. 100% DimethyL Nonpolar Amino Acid Der. OV-101, OC toPolysiloxane (Fluid) Essential oits SP-2100 280C

3. 5% Diphenyl Nonpolar Fatty Acids, Me Est SE-52 -60Cto95% Dimethyl Alkaloids, Drugs, OV-23, 325C

Halogenated Cpds SE-54

4. 7% Cyanopropyl Intermediate Drugs. steroids, -20 to 7% Phenyl Polysiloxane Pesticides OV-1701 280C

5. 50% Phenyl, Intermediate Drugs, Steroids, OV-17 60C to 50% Me.Polysiloxane Pesticides, Glycols 240C

6. 50% Cyanopro. Intermediate Fatty Acids, Me Est OV-225 60C to me 50% Phenyl Alditol Acetates 240C ethyl Polysiloxane7. 50% Trifluoropro Intermediate Halogenated Cpds OV-210 45 to Polysiloxane Aromatics 240C

8. Polyethylene Polar Acids, Alcohols, OV-351, 60 to Glycol-TPA modified Acrytates, Nitrites, SP-1000 240C

Aldehydes, Ketones

9. Polyethylene Gly Polar Free Acids, Alc Carbowax 20M 60C to Ethers,Essential Oils, Glycols, Solvents 220C

Phase selectionPUBLISHED INFORMATIONKovats indices compilationsJournal articlesInternal work

INTUITIONlike structuresNO IDEA?

Sample informationNonpolar columnChange 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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