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18 O. Isotope Lab. D. Samples In. Numbers Out. How do we get those numbers? Or What’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).
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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.
