Gas Chromatography (GC) | CHEP7003 & CHEA7004

• This module focuses on Chemistry for Chemical Engineers and Analytical Practice & Protocols
• It will cover Gas Chromatography (GC)
• The lecturer is Dr. Ambrose Furey, a Senior Lecturer

Module Details

• Module Code: CHEP7003 / CHEA7004
• ECTS Credits: 5.0
• Relevant for Bachelor of Engineering (Honours) in Chemical and Biopharmaceutical Engineering (CR_ECHBI_8)
• Also relevant for Bachelor of Science (Honours) in Analytical Chemistry with Quality Assurance (MT 874)

Goals

• Develop familiarity with basic methods of quantitative analysis using gas chromatography

Specific Objectives

• Apply the internal standard technique to determine the identities and concentrations within a mixture of volatile organic compounds

Gas Chromatography System Sections

• Carrier gas supply and controls
• Sample introduction/injector system
• Chromatographic column and oven
• Detector
• Amplifier, signal processing, and control electronics
• Integrator and chromatogram printout, often with computer-controlled software

Carrier Gas

• A chemically inert carrier gas acts as the mobile phase
• Transports sample components through the column to the detector
• Facilitates analyte separation via interaction with the stationary phase
• Components' adsorption and desorption properties determine their movement rate

Carrier Gas types

• Chemically inert gases, such as helium, argon, nitrogen, carbon dioxide, and hydrogen, must have a purity of 99.99%
• Detector type often dictates the choice of carrier gas

Carrier Gas System Components

• Pressure regulators, gauges, and flow meters
• Often includes a molecular sieve to eliminate water and other impurities

Inlet Pressure and Flow Rates

• Inlet pressures typically range from 10 to 50 psi above room pressure
• Results in flow rates of 25 to 150 mL/min for packed columns
• Results in flow rates of 1 to 25 mL/min for open-tubular capillary columns
• Constant inlet pressure maintains consistent flow rates
• Flow rates can be set using a rotometer at the column head
• Alternatively, a more precise soap-bubble meter can be used

GC Inlet System Features

• Rapid, clean sample switching or injection into the mobile phase without tailing or dispersion
• Accurate inlet temperatures that vaporize all sample components instantly without decomposition or condensation
• Minimal dead volumes to prevent sample diffusion in the mobile phase
• Well-designed inlet system for good precision (better than ±1 %)
• Prevents sample contamination, catalytic effects, retention loss, and septum bleed or leak

GC Oven Importance

• The oven is a fundamental component of the GC system
• The Oven temperature must be precisely controlled across a wide range
• Assures accurate isothermal temperature settings and temperature programming

GC Oven Requirements

• Temperature range: 5 - 450°C
• Temperature stability: approximately 0.1 degrees
• Programming rate: 0.1 - 50°C/minute
• Reproducibility: < 1%
• Cool down time: 350 to 50°C in under 10 minutes

Oven Temperature's Role

• Sufficient temperature to evaporate all sample components
• Affects the partition coefficient of analytes between stationary and mobile phases
• Temperature increase reduces analyte retention and vice versa
• Temperature stability and reproducibility are crucial

Isothermal Analysis Definition

• Using one specific oven temperature achieves adequate separation with a short analysis time if a sample has components with similar boiling points

Isothermal Analysis Considerations

• Optimum column temperature depends on the sample components' boiling points
• Challenges arise with samples containing numerous compounds with widely different boiling points

Complex Mixture Analysis at Low Temperatures

• Permits the resolution of low boiling compounds for analysis
• High boiling fraction bands will broaden due to diffusion
• Can lead to a loss of detector response, especially in dilute samples
• May result in quantitative measurement inaccuracies

Complex Mixture Analysis at High Temperatures

• Enables elution of high boiling compounds as sharp peaks for analysis
• Volatile components elute rapidly with severe resolution loss

Temperature Programming Use Case

• Temperature-programmed analysis is preferred when sample components have a wide range of boiling points
• It ensures efficient, sharp peak separation of both early and late-eluting analytes within reasonable analysis times

Column Temperature Adjustment

• Column temperature increases at a predetermined rate during chromatographic measurement
• Permits less strongly retained components to be resolved at a low temperature
• Results in later high boiling compounds eluting as sharp peaks at a higher temperature

Temperature Increase Impact

• Components' velocity increases, approximately doubling with each 30 °C rise
• Linear temperature program conditions lead to components eluting when column temperature is 30 °C below their elution point

Optimizing Heating Rate

• Achieves consistent peak widths
• Produces peaks that appear as if they were chromatographed isothermally at the ideal column temperature
• Often achieves analysis times shorter than isothermal conditions

Temperature Programming Steps

• Typically comprises a progression of isothermal and temperature-rise segments
• Initial temperature must be set low enough so that the most volatile components are adequately separated
• Ramp rate control the rate at which the temperature is raised
• Final temperature enables the proper separation of the highest boiling components
• Final time is the period for which the final temperature is maintained

Key Factors Guiding Ramp Rate

• Nature of the compounds being separated
• Desired degree of separation
• Necessary time scale for the chromatographic run
• Temperature rise profile

Temperature Rise Profile options

• Linear
• Multi-linear
• Convex
• Concave

Temperature Programming Issues

• Stationary phase volatilization due to high oven temperatures
• Carrier gas sweeps the vapor into the detector causing baseline drift

Temperature Programming Summary

• Refers to the controlled increase of the liquid temperature

Temperature Programming - Key Characteristics

• Oven temperature gradually increases over time
• Begins at a low initial temperature, ramping up to a higher final temperature
• Can implement multiple ramp rates and hold times

Temperature Programming - Advantages

• Produces sharper peaks, particularly for late-eluting compounds
• Allows for increased peak capacity, which means a higher number of separable analytes
• Shortens overall analysis time for complex mixtures
• Improves the sensitivity to less volatile components

Temperature Programming – Applications

• Suited to samples with wildly varying volatilities
• Used in modern capillary GC separations

Isothermal Analysis - Summary

• Consists of maintaining a constant column temperature through the entire GC separation

Isothermal Analysis - Key Characteristics

• Constant oven temperature the entire time
• Simpler to implement and understand theoretically

Isothermal Analysis - Advantages

• Simpler method development and instrumentation
• Provides better resolution for closely eluting peaks in some cases
• Useful for samples with components of similar volatilities

Isothermal Analysis - Limitations

• Later-eluting peaks become broader and shorter, reducing sensitivity
• Can result in very long analyses for complex mixtures

Analysis Time Comparison

• Temperature programming is generally shorter
• Isothermal analysis can be prohibitively long for samples with a wide range of component volatilities

Peak Capacity in Temperature

• Programming typically offers higher peak capacity
• Allows for the separation of more components at a time

Method Development in Temperature

• Isothermal methods are generally easier to develop and optimise
• Programming requires more complex optimization but offers greater flexibility

Applicability in Isothermal Analysis

• Suitable when peaks all elute over less than 25% of scouting gradient time
• Temperature programming is best for most other scenarios

Conclusion

• Temperature programming is the more widely method due to how well it can handle complex mixtures and provide sharper peaks across a wide volatility range
• Selection rests on the specific analytical requirements and the characteristics of the sample analyzed

Sample Preparation Considerations

• Solvent choice requires care to avoid chromatography issues

Sample Types

• Gaseous samples can be pumped into sampling valve from pressurized container or by using a gas tight syringe
• Liquid and solid samples can be directly extracted, or prepared directly.

Solvent Requirements

• No reaction with the sample or stationary phase
• Complete sample dissolution and miscibility
• No co-elution of solvent and sample peaks
• Absence of non-volatile material remaining in the column (solvent purity)
• Avoid column overloading with large solvent volumes

GC Detector Purpose

• To monitor the carrier gas as it exits the column
• To generate a signal that corresponds to shifts in composition because of the eluted components

GC Detector Function

• Chromatography is a separation technique
• Detectors like flame ionization, electron capture, and thermal conductivity can’t identify the components
• Additional techniques such as mass spectrometry can assist in identification of eluted components
• The chromatogram tracks detector signal against time so that it shows a concentration over time

GC Detector Types

• Flame ionization detector (FID)
• Electron capture detector (ECD)
• Thermal conductivity detector, TCD (katherometer)
• Nitrogen-phosphorous detector, NPD
• Flame photometric detector, FPD
• Photoionization detector, PID
• GC-MS (Gas Chromatography-Mass Spectrometry) detectors

Detector Selectivity Types

• Non-selective detectors respond to all components apart from the carrier gas
• Selective detectors respond to components within a common physical/chemical set
• Specific detector is used for a single chemical compound

Detector Groupings

• Concentration dependant detectors- Signal corresponds to the concentration of solute in the detector
• Not normally destructive to sample, and make up gas will lower detector response
• Mass flow dependant detectors - Signal is related to the rate at which solute molecules enter detector.
• Usually destroys the sample. Make up gas does not affect its response

Flame Ionization Detector (FID)

• Effluent mixed with ignited hydrogen and air
• Organic compounds burn and produce ions to conduct electricity through the flame
• Measures current resulting from pyrolysis of organic compounds

FID Properties

• It's a mass-sensitive rather than concentrations-sensitive detector. Mobile phase flow rate doesn't affect response.
• Useful general detector for organic compounds;
• High selectivity to virtually all organic compounds;

Materials not detected by FID

• H2
• O2
• N2
• SiCI 4
• SiF4
• H2S
• SO2
• COS
• CS2
• NH3
• NO
• NO2
• N2O
• CO
• CO2, H2O
• Ar
• Kr
• Ne
• Xe; HCHO
• HCOOH

Flame Ionization Detector Attributes

• High sensitivity
• Large linear response range
• Low noise
• Robust, easy to use
• Destroys the sample

Electron Capture Detector (ECD)

• Detects electron absorbing compounds
• It detects compounds that can capture electrons
• It can lead to a decrease in the detector's background current

ECD - Components are:

1. A radioactive source (Nickel-63)
2. A chamber filled with makeup gas (usually nitrogen)
3. Two electrodes: an anode and a cathode
4. An electrometer to measure current

ECD - Selectivity

• Has a higher electron affinity
• Contains Nitro groups, halogens, or organometallic compounds

ECD - Operational Mechanism

1. Electron emission using radioactive source Nickel
2. Ionization creates a large number of free electrons
3. Free electrons are attracted and generate a constant background current
4. As sample elutes, it enters the chamber
5. Electron absorbing capture molecules
6. The electron reduces the free electrons available to anode, causing current decreases
7. Finally, it is recorded as the positive peak in the chromatogram

ECD - Considerations

• Unlike flame ionization detector, electron capture detector is reversible meaning it won’t destroy the analyte
• Electron Capture varies

ECD - Applications

• The ECD is used for environmental, pharmaceutical analysis
• High temperatures and an inert GC-MS column

Internal Standard

• Approach adds specific internal standard to sample, and it increases response of the volume and minor issues
• It increases ethyl acetate
• It increases n-propyl
• It increases iso-butyl, n-butyl alcohol
• The 3-methyl-butan-2-ol

Internal Standard Importance

• Known volume and concentration can be pipetted into the solutions
• Must have same analysis from extraction to preparation
• Dissolves in sample
• Must be Pure, thermally and chemically available for purposes

Internal Standard Requirements

1. Must dissolve in the sample solution.
2. Must not be thermally labile.
3. Must be pure.
4. Must be freely available.
5. Must chromatograph similarly to the analyte under the same temperature programming conditions.
6. Like the analyte must behave chemically similar for extraction from sample matrix and concentration purposes.

Response Factor

• Accounts for all the differences in the detector
• Expressed as Internal/ Component Area Ratios

Response Curve

• The Fixed Calibration curve standardizes at a constant mass
• It standardizes the sample at a specific standard analyte It will show a slope response showing its linear response