GC-MS Introduction

GC-MS Introduction

• GC-MS is a tandem technique that combines two different instruments: Gas Chromatography (GC) and Mass Spectrometry (MS)
• GC separates an analyte based on boiling point and molecular weight, while MS measures the mass-to-charge ratio (m/z) of the analyte

Gas Chromatography

• Process of separating a mixture into its components
• Types of chromatography: Simple, Phases (Mobile and Stationary)
• Mobile phase: an inert gas such as nitrogen or helium gas that carries the analyte through the stationary phase
• Stationary phase: a thin layer inside a metal or glass column that affects retention time
• Retention time: the amount of time a molecule spends in the column, affected by column type, carrier gas flow rate, and injection temperature

Mass Spectrometry

• Technique that measures the mass-to-charge ratio (m/z) of an analyte
• Three parts: Ion source, Mass analyzer, and Detector
• Ion source: Electron Ionization (EI) is the most commonly used, where high-energy electrons interact with the analyte in a gaseous phase under vacuum, causing molecules to fragment into smaller pieces
• Mass analyzer: a quadrupole mass analyzer that uses an electric field created by four electromagnets to stabilize the m/z ratio at a time
• Detector: uses a computer program to convert the impulse created by molecules into two spectra: abundance vs retention time and abundance vs m/z ratio

Interpreting GC/MS Spectra

• GC/MS produces two different spectra: GC chromatogram and Fragmentation pattern
• GC chromatogram: plots relative abundance vs retention time, showing all components in the mixture
• Fragmentation pattern: produced by a mass spectrometer for each molecule in the mixture, used to identify the molecule

GC Chromatogram

• X-axis represents retention time, and Y-axis represents abundance
• Factors that affect retention time: column type, type of carrier gas, injection temperature, and column flow rate
• Peaks in the chromatogram represent individual compounds separated from the mixture

Fragmentation Patterns

• Parent molecule comes into contact with high-energy electrons and fragments into smaller charged molecules, creating a fingerprint of the molecule that can be used to identify it
• Remains fairly constant and can be used to help identify the molecule

Incompatible Samples for GCMS

• Samples with insufficient volatility, thermally unstable samples, and samples that could damage the instrument or column
• Samples with pH 9 or higher, and inorganic acid and base samples

Parts of GC

• Capillary Column: a long, thin fused silica column containing a thin interior coating, held in an oven that is programmed to increase the temperature gradually, which helps separation
• Detector: capable of creating an electronic signal whenever the presence of a compound is detected
• Injector: contains a heated chamber and a glass liner, which sample is injected through a septum

GC Modes

• Split Mode: a portion of the carrier gas and sample is directed to the column, while the rest is vented to waste
• Splitless Mode: the entire sample is vaporized in the injector and goes into the column
• Septum Purge: an outlet that prevents septum bleed components from entering the column