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Discover the intricate process of converting samples into gaseous phase, ionizing gas molecules, and counting stable isotopes with a mass spectrometer. Learn about the components, operational modes, and peripheral systems involved in obtaining accurate results.
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18O Isotope Lab D Samples In Numbers Out How do we get those numbers?OrWhat’s in the “Black Box”?
Mass Spectrometer Principles • Convert samples to gaseous phase • Ionize gas molecules • Separate molecules by mass • Count number of molecules • Stable isotopes (H and O)
MS Component Systems • Vacuum • Bellows and capillary tubing • Changeover valve • Ion Source and focusing • Flight Tube and Magnet • Ion collectors and amplifiers
Ion Source • Gas Inlet • Electron source (filament) • ~1 in 1000 molecules ionized • Collimating plates • Focus ion stream • Accelerate into flight tube
Operational Modes • Dual-Inlet • Compare sample to reference • Measure relative difference not absolute values • Continuous Flow • Reference and sample transported along same path by carrier gas
Peripheral Systems • Gas Bench II • Gas handling, water removal, GC separation • Continuous flow mode • CO2 gas from water equilibration • H Device • Dual inlet mode • H2 gas from Chromium reduction • PAL Autosampler • Temperature controlled block • Sampling arm • Offline Prep • Multiport & Microvolume
HDevice • Small sample size (1µl) • Cr reduction • ~66 samples/run • Dual Inlet mode Thermo Electron Corp.
Water Equilibration 0.3% CO2 in He • Flush vial with He + 0.3% CO2 • Load sample (0.5 mL) • Equilibrate >12 hours • Analyze
Gas Bench Schematic MAT 252 Thermo Electron Corp.
Valco Valve Open Splits Gas Bench GC Column Water Removal Gas Regulation
Getting Results I • Converting current into permil (‰) • Current generated in each collector is proportional to the number of ions. • Correct for 17O. • Correcting raw results to standard values. • Evaluate standard performance and reruns.
Rx - Rstd Rstd Isotope Ratios Delta Notation x = 1000 R = Isotope ratio in sample or standard and is determined by mass spectrometry. For an oxygen example: (18O/16O)sample - (18O/16O)standard 18O = 1000 * = 18O (‰) (18O/16O)standard
Getting Results II • Process standards alongside samples. • Periodically check house standards against “reference” standards. • Critically look at the results and understand what the instrument is telling you. • View results in the context of your experience.
