How do we get those numbers? Or What’s in the “Black Box”? Isotope Lab 18O 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)

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) Collimating plates ~1 in 1000 molecules ionized Collimating plates Focus ion stream Accelerate into flight tube

Operational Modes Dual-Inlet Continuous Flow 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 H Device PAL Autosampler Offline Prep 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.

Hydrogen Results 8 Comparisons of samples versus reference gas in the “Standard” bellows

Water Equilibration Flush vial with He + 0.3% CO2 Load sample (0.5 mL) 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 The Gas Bench is basically a peripheral device to manage gas flow and sample introduction to the MS. Sample gasses are obtained from the autosampler, pass through an initial water removal step, are discretely inserted into the continuous helium stream, run through a GC column to separate constituent molecules, pass through another water removal step, and are then introduced to the mass spectrometer source. Thermo Electron Corp.

Gas Bench Valco Open Splits Valve GC Column Water Removal Gas Regulation

Typical ISODAT result file for a CO2 run in continuous flow mode. Upper plot shows mass ratios against run time (15 minutes per run). Lower plot shows signal strength (millivolts) against run time. 5 peaks of reference gas establish the “zero” value 10 peaks of sample gas are averaged to determine the value for the sample. Declining voltage for each sample peak is due to loss of CO2 in sample vial, reference gas comes from large tank and shows no such decline. Accuracy target for sample standard deviation is +/- 0.1 ‰.

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. Converting current The instrument only measures the current produced when an ionized particle neutralizes against the collector wall. The MAT252 has 3 collectors positioned to measure the 44, 45, and 46 mass signals which correspond to the various combinations of stable carbon and oxygen isotopes that can make up CO2. 44: 12C + 16O + 16O 45: 13C + 16O + 16O 46: 12C + 16O + 18O 12C + 16O + 17O 13C + 16O + 17O 12C + 17O + 17O

Isotope Ratios Rx - Rstd Rstd x = 1000 Delta Notation R = Isotope ratio in sample or standard and is determined by mass spectrometry. For an oxygen example: In the isotope ratio, the heavy isotope is listed in the numerator and the lighter (usually more abundant) isotope appears in the denominator. Oxygen isotopes are measured using CO2 gas and the mass 44 (O-16) and 46 (O-18)signals (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.

Water Equilibration Run Correction

Water Correction…

Field Trip Results

Precipitation Record at Ladue