Gas Chromatography Flashcards
What is gas chromatography used for?
Separation techniques used to analyze volatile substances in the gas phase
In GC, the components of a sample are dissolved in a solvent and vaporized in order to separate the analytes by distributing the sample between two phases.
Describe the mobile and stationary phases in gas chromatography.
- Mobile phase: chemically inert gas that serves to carry the molecules of the analyte through the heated column.
-
Stationary phase:
- Gas-solid chromatography: a solid adsorbant
- Gas-solid chromatography: a liquid on an inert support
What is the basic theory of gas chromatography? [4]
- A pressurized gas pushes a sample through a narrow tube – column
- Column contains some sort of stationary phase that is temperature controlled
- Volatile analytes will leave the column as a function of temperature/time,
allowing for separation of compounds - Compounds can be detected in various ways
Describe a basic GC chromatogram.
Y-axis: Some sort of detector response
X-axis: Time; Parallels the LC chromatogram – response vs.
time/volume)
In GC, molecules can be separated by: [5]
- Differences in:
- boiling point
- vapor pressure
- polarity
- size
- “solubility” in the carrier gas
And more!
What are the two separation modes of GC?
- Adsorption mode
- Partition (absorption) mode
Describe the adsorption mode of gas chromatography.
Primarily uses a packed solid-phase and porous-layer open tube (“PLOT”) columns.
Describe partition mode of GC.
a.k.a. absorption mode
Mostly with capillary (liquid) film wall-coated open tube (“WCOT”)
columns or support-coated open tube (“SCOT”)
What types of columns are used in GC?
Packed and capillary
Describe packed columns used in GC. [4]
- Constructed from glass, stainless steel, copper, or aluminum
- Typically, 2–6 m in length with internal diameters of 2–4 mm
- The column is filled with a particulate solid support, with particle diameters ranging from 37–44 μm to 250–354 μm
- Packing is a porous solid absorbent material (e.g., clay; diatomaceous earth); must be inert!
Adsorption mode
Describe capillary columns used in GC.
- Constructed from fused silica and is coated with a protective polymer coating
- Columns range from 15–100 m in length with an internal diameter of approximately 150–300 μm
Open tubular column
Which GC columns are partition (absorption) mode and which are adsorption mode?
Describe adsorption gas chromatography. [4]
- The stationary phase is a solid high temperature polymer or mineral
- Separations are due to a series of adsorption/desorption events
- Variations in the polarity of the packing material determine the degree of interaction between column and sample
- Pore size of the packing and the packing density of the support determine the resolution (plate count/N) and loading capacity
What determines the degree of interaction between the column and the sample in adsorption GC?
Variations in the polarity of the packing material
What determines the resolution (plate count/N) and loading capacity of adsorption GC?
Pore size of the packing and the packing density of the support
Describe partition gas chromatography. [4]
a.k.a. absorption
- Separations in this mode are based on the gas-phase solutes
partitioning between the liquid phase of the capillary film coating the column tube - This mode of chromatographic separations allows for a greater range of interactions and can be better optimized for separating closely-related molecules
- More highly-retained molecular species have a greater affinity (“solubility”) for the film layer compared to its affinity for carrier gas
- Separation of samples is based on the differences in polar/non-polar interactions and their relative solubility with the liquid phase
Which mode of GC separations allows for a greater range of
interactions and can be better optimized for separating closely-related molecules?
Partition (absorption)
In partition GC, what is separation of samples based on?
a.k.a. absorption GC
The differences in polar/non-polar interactions and their relative solubility with the liquid phase
More highly-retained molecular species have a greater affinity (“solubility”) for the film layer compared to its affinity for carrier gas.
Describe porous layer open tube columns.
- Stationary phase is a porous solid material:
- Alumina (Al2O3)
- Molecular sieves (zeolites or other aluminosilicate
minerals) - Polymers
Capillary column
Describe wall-coated open tube and support-coated open tube columns.
- Stationary phase is a liquid film that is adsorbed onto:
- The column itself (WCOT)
- The solid support (SCOT)
- Liquid is non-extractable
- Column can be flushed with pure solvents to remove
contaminants; film remains (if manufacturer
instructions are followed)
- Column can be flushed with pure solvents to remove
Capillary columns
Give examples of liquid stationary phases used in WCOT and SCOT. [2]
Capillary columns
- Siloxane and its derivatives
- Polyethylene glycol (degree of polymerization can vary to tailor properties)
Compounds can be measured via GC if: [2]
- They have a boiling point up to 400C (generally upper operating limit of GC)
- They do not thermally decompose upon vaporization (no point in measuring pieces of molecules)
GC cannot separate compounds that: [2]
- Do not vaporize (e.g., inorganic metals; ions; salts)
- Are highly reactive compounds or chemically unstable (e.g., strong acids; ozone; NOx; etc.)
Compounds that may be potentially difficult to analyze via GC include: [2]
- Highly adsorptive compounds such as those containing carboxyl, hydroxyl, amino, or sulfur-containing groups (e.g., proteins; polysaccharides)
- Compounds for which standards are difficult to obtain (need standard to compare peaks to for quantitative analysis)
Where GC has a gap, HPLC can often fill it (e.g., proteins; polysaccharides)
Describe how sample and column properties must be matched.
- Need to balance column and analyte properties to optimize separation (don’t want elution to be too fast or too slow).
Separation in GC depends on […]
What dictates separation mode?
- the reversible adsorption OR absorption of organic
molecules primarily based on their boiling point and polarity under conditions where they have some affinity for the column’s stationary (liquid or solid) phase
How can the affinity between the sample and column be altered in GC? [2]
What other factors influence separation? [5]
- Temperature
- Carrier gas rate
Impacts separation
Describe the effect of temperature on separation time.
- Higher temperature = shorter retention time
- Too high = no separation
- Too low = takes too long, uses a lot of gas
Why does higher temperature shorten the retention time in GC?
- It reduces the strength of the interaction between the analytes and the stationary phase of the column
- It increases the kinetic energy of the gas molecules, causing the analytes to move through the column more quickly.
What are factors to consider with regard to temperature in GC analysis? [6]
- Must ensure temperatures stay within recommended operating temperatures of column (Tmin and Tmax; columns are expensive)
- Consider effect of temperature on:
- Variations in solubility of solutes
- Changes in volatility
- Stability of solutes (avoid thermal decomposition)
- Changes in gas flow rate as a function of temperature
- Stability of the stationary phase
Describe the mobile phase for GC.
Give some examples.
- Carrier gas; pushes solutes (as gas) through column to/through detector
When the carrier gas flow rate is increased, the retention time of analytes […]
Decreases
What is the concern if the carrier gas flow rate is too high?
If the carrier gas flow rate is too high, it can cause analyte peaks to become broader and less well-resolved: column temperature to become less uniform, resulting in peak distortion and broader peak widths.
What is the principle of GC?
- Similar to LC:
- Solutes interact differentially with a stationary phase
- Separation occurs due to differential rates of adsorption or absorption of the solutes
Sample molecules will partition (SCOT, WCOT) or adsorb (packed, PLOT) repeatedly on and off the stationary phase based on: [2]
- Physical properties; boiling point, vapour pressure, viscosity, etc.
- Cheimcal properties; predominantly polarity
How is resolution of GC optimized?
By controlling two variables: (1) carrier gas flow and (2) column oven temperature.
Describe how column selection impacts resolution.
Describe the effects of temperature on separation in GC.
- As temperature increases, retention time and R goes down; peaks too close together; k is being influenced
- As temperature goes up, polarity changes (peak order changes); alpha is being influenced
Describe detection in GC.
- Separated organic molecules are carried to an appropriate detector (or series of detectors) to provide a signal to be evaluated for, e.g., retention times or area under the peaks for the injected sample
- The best separation is useless without a detector
- The detector needs standards
- Without them you have just a signal and nothing to compare it against
Describe GC system design. [6]
All analytical GC systems have several common design components:
* Carrier gas with flow control
* Injection port and/or valve
* Column
* Oven
* Detector(s)
* Output (analog and/or digital)
