CHEM1053_Lecture+I_Week1_Mass+Spectroscopy_2025

Mass Spectrometry Overview

• Mass Spectrometry (MS): Analytical technique to identify unknown compounds, quantify known substances, and elucidate molecular structures.

• Measures molecular and atomic weights to determine molecular composition.

Spectroscopic Approach to Structure Determination

• Initial step for unknown organic compounds: Elemental Analysis.

  • Example: Determine %C, %H, %O to establish molecular formula.

  • Example of molecular formula: C3H6O can represent different structures (ketone, aldehyde, alcohol).

  • Determine actual molecular formula by multiplying empirical formula by a coefficient.

Structure Determination Techniques

• Advancement in spectroscopy has been critical for organic chemistry.

• Different techniques are required to differentiate between isomers based on provided information.

• Key instruments include NMR, MS, FTIR, and Elemental Analysis to determine molecular formulas and structures, including functional groups.

Basics of Mass Spectrometry

• Key Component: Mass spectrometer devices operate in high vacuum (10^-6 to 10^-9 mbar).

• Importance of High Vacuum: Minimizes ion interactions with air, allowing for accurate measurements of ions.

• Sample introduction methods include:

  • Chromatography (GC, LC)

  • Direct infusion via syringe

  • Probe or plate methods.

Ionization in Mass Spectrometry

• Mass spectrometry relies on the creation of charged particles (ions) for measurement.

• Ionization Techniques:

  • Electron Impact Ionization (EI) is common.

  • Ions can be positively or negatively charged.

Fragmentation in Mass Spectrometry

• Ionization can fragment molecules into smaller ions.

• Analysis of fragments provides insight into molecular weight and structure.

• Only charged species are detected; radicals do not form part of this analysis.

Mass Analysis of Ions

• Separation of ions based on mass-to-charge ratio (m/z).

• Different ions produced from fragmentation can be sorted in the mass spectrometer using magnetic deflection.

• Each ion's deflection corresponds to its mass to charge ratio.

Mass Spectrometer Detection

• Common detectors include:

  • Faraday Cup

  • Electron Multiplier

  • Array Detector (Multichannel Analyzer).

• Ion strikes anode, generating a current that is measured to determine abundance and identify compounds.

Mass Spectrum Overview

• Mass spectrum displays peaks for molecular and fragmented ions.

• Example compound: 2-Methylbutane (C5H12) with known molecular peaks to derive molecular weight.

Effects of Isotopes in Mass Spectrometry

• Natural isotopes can cause additional peaks in mass spectra (e.g., M+1 and M+2 peaks).

Resolution in Mass Spectrometry

• Resolution: Ability of a spectrometer to differentiate between closely spaced peaks in a spectrum.

Historical Perspective

• Key pioneers in mass spectrometry include:

  • Francis William Aston (Nobel laureate for mass spectrograph development).

  • John Joseph Thomson (discovery of the electron).

  • Others who contributed to technical advances in mass spectrometry.

Summary of Mass Spectrometry Principles

• MS involves ionizing molecules, separating ions based on mass-to-charge ratio, and measuring relative abundance, aiding in accurate molecular mass determination.

• Extremely sensitive, capable of identifying compounds in low concentrations and complex mixtures.

• Application extends beyond identification to quantification and relative abundance measurements of isotopes.

Key Learning Objectives of Mass Spectrometry

• Hands-on practical sessions like GC-MS and LC-MS.

• Understanding fundamental principles: ionization types, fragmentation mechanisms.

• Application of MS techniques in structural determination of organic compounds.