GC-MS Introduction

Study Notes

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

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

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

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

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

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

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

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

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