Operating the Isotope Ratio Mass Spectrometer (IRMS) in a Stable Isotope Geology Lab - Pro, Lab Reports of Geology

Download Operating the Isotope Ratio Mass Spectrometer (IRMS) in a Stable Isotope Geology Lab - Pro and more Lab Reports Geology in PDF only on Docsity! Stable Isotope Geology Fall, 2007 GEOL 648 E. Grossman OPERATION OF THE ISOTOPE RATIO MASS SPECTROMETER (IRMS) – LAB 1 The objectives of this lab are to demonstrate (1) the theory and operation of the DeltaPlus isotope ratio mass spectrometer, (2) how to optimize the signal, (3) scan the peak shape, background, and linearity, and (4) and run a CO2 sample in the dual inlet. ISODAT 2.0 software. ISODAT 2.0 is the software that runs the DeltaPlus mass spectrometer and its peripherals (Gas Bench, ConFlo, EA, TC/EA, GC PAL). The software consists of: • INSTRUMENT CONTROL - manual valve switching, scanning, focusing, and many other operations • ACQUISITION – for setting up Methods and Acquisition tables, and for running samples • DIAGNOSIS - for running tests such as Absolute Sensitivity, Linearity, and Resolution • WORKSPACE - for reviewing data. This can be done while a sample is being analyzed. In practice, many features and operations are common to all four programs. A. Instrument Startup Procedure 1. Make sure the green lights (Power, Host connection, High Vacuum OK, Turbo Pump >80%, Accel Voltage and Emission) on the DeltaPlus are lit, and that the three error lights (Main pump error, Second pump error and Inlet pump error) are not lit. 2. In the Excel document DeltaPlusLog.xls (lies in MY DOCUMENTS and there’s a shortcut on Desktop), RECORD: Date Time T (°C) Hum (%) He tank He (psi) Flush He (psi) Flush Rate (SCCM) Ref gas P (bar) Ion Source NV GB(O/X) NV CF(O/X) Vac (mb) FV (mb) a. T is temperature and Hum is humidity in the lab, both of which can be read from the electronic hydro- thermometer on top of the computer desk. b. He tank (ignore for this lab) c. Flush He (ignore for this lab) d. Enter whether the Ion source (filament) is on or off. Check by looking at the two green lights (ACCEL VOLTAGE and EMISSION) at the right side of the DeltaPlus. These two lights may be off when the computer is rebooted. Also check the MS panel in the INSTRUMENT CONTROL module. Box reading should be around 0.8 mA and Trap 0.66 mA when Ion Source is on. e. NV GB is Gas Bench needle valve, and NV CF is ConFLo needle valve, which are both inside the Delta plus XP Mass Spectrometer. NV GB is facing the GC PAL while NV CF is 90° clockwise. O is open and X is close. It can be shown in the Vac in the INSTRUMENT CONTROL module which stands for vacuum inside the mass spec. High vacuum (10-8 mbar) indicates the needle valve is closed and low vacuum (10-6 mbar) indicates the needle valve is open. This is because the carrier He gas input will decrease the vacuum (increase the pressure). f. FV means the fore vacuum in the dual inlet. Read it on the DUAL INLET panel in the INSTRUMENT CONTROL module. g. Ref gas P is the pressure of reference gas which can be read on the gauge of reference gas tank. It’s consumed very slowly during a run. Record it every 5 days of running. B. Background checks (jump to mass) Background checks are performed routinely to test for a leak or contamination in the system. Th procedure involves jumping to the appropriate mass, doing a peakcenter, and recording the appropriate value. We typically check the background for 18 (H2O), 28 (CO, N2), 40 (Ar), and 44 (CO2). 3. Scroll to the right side of DeltaPlusLog.xls. It reads: Bkgd MS (NV X) 18(3) Bkgd load (NV O) 18(3) Bkgd inj. 18 (3) Bkgd 18 load->inj Bkgd 28 (2) Bkgd load 40 (3) Bkgd inj. 40 (3) Bkgd 40 load->inj Bkgd 44 (3) 1 4. Open the Instrument Control module. In the MS panel you will see three readings (usually 44, 45, 46) representing three cups (from top to bottom, cup 2, 3, 4). Right-click on cup 3 and select jump to mass. Input the mass number (i.e. 18 for water, 28 for N2, 40 for Ar, 44 for CO2, etc.) in the pop up window and click ok button. Do a peak center before recording the readings. 5. The acceptable background values for masses 18, 28, 40, and 44 are <300, 170, 30, and 50 mV. 6. When all the background scans are done, remember to change cup 2 back to 44. You can do it either by inputting 44 on the pop-up or by choosing CO2 in the pull-down menu in the bottom left corner. C. Loading reference gas into reference (ST) bellows and Peak Center. 7. To pump away air or CO2 from the previous days run, OPEN VALVES 39, 13, 11, 12, 14, and 15 (Fig. 1). When a low Fore Vacuum pressure is reached (e.g., < 5 x 10-3 mbar), CLOSE 11, 13, and 39 and OPEN the valves on the top of the reference gas cylinder. 8. A signal should be seen for 44, 45, and 46. After the signal is about 2000 mV (roughly 20 mbar pressure in bellows), CLOSE the valves on the gas cylinder and valves 12 and 14. OPEN valves 13 and 39 and pump to minimum, then CLOSE 13 and 39. 12 34 24 39 32 3331 15 16 11 13 40 26 25 22 21 23 3 14 Reference gas Left port Right port Ion source Fore- vacuum pump Turbo pump Figure 1. View of the panel for the Dual Inlet in the ISODAT 2.0 software with added labels. D. Background scan (Magnetic field) To check for contamination of a sample or reference gas, introduce the gas into the mass spectrometer and perform the scan as described below. 9. Open INSTRUMENT CONTROL if it is not open already. In the right part of the window, to the left of the left pointing arrow (pointing to Y axis), select Intensity. The pull down menu to the right is the X axis. This should be set on Magnetic Field. The next pull down menu is for type of scan. Select Slow Magnet. This will provide the mass scan with the best resolution. 10. Background scan. Make sure valve 31 is closed. If it is not, close it and wait 1 minute. Click on right- pointing green arrow (►) to the left of Y-axis to start scan. The Magnet setting will start at 218 steps and progress upward. This is equivalent to scanning up in mass. 11. We can choose to display the signal in all three cups (2, 3, and 4), or select one that provides the best range for the expected signal. For this laboratory, select cup 3, the cup with intermediate sensitivity. 2