1. 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”?

2. Mass Spectrometer Principles
Convert samples to gaseous phase
Ionize gas molecules
Separate molecules by mass
Count number of molecules
Stable isotopes (H and O)

3. MS Component Systems Vacuum Bellows and capillary tubing
Changeover valve
Ion Source and focusing
Flight Tube and Magnet
Ion collectors and amplifiers

4. Ion Source Gas Inlet Electron source (filament) Collimating plates
~1 in 1000 molecules ionized
Collimating plates
Focus ion stream
Accelerate into flight tube

5. 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

6. 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

7. HDevice Small sample size (1µl) Cr reduction ~66 samples/run
Dual Inlet mode
Thermo Electron Corp.

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

10. 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

11. 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.

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

13. 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 ‰.

14. 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

15. 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

16. 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.

17. Water Equilibration Run Correction

18. Water Correction…

19. Field Trip Results

20. Precipitation Record at Ladue