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Stable Isotope Ratio Mass Spectrometer

Isotopes are atoms of the same element that differ in atomic mass, due to differences in the number of neutrons contained in the nuclei. Having too few or too many neutrons compared to protons causes some isotopes, such as ¹⁴C, to be unstable. These unstable 'radioisotopes' will decay to stable products. Other isotopes, such as ¹²C and ¹³C do not decay, because their particular combinations of neutrons and protons are stable. These are referred to as stable isotopes.

Stable isotope abundances are expressed as the ratio of the two most abundant isotopes in the sample compared to the same ratio in an international standard, using the 'delta' (δ) notation.  Because the differences in ratios between the sample and standard are very small, they are expressed as parts per thousand or 'per mil' (‰) deviation from the standard.

Instrument: Thermofinnigan, DELTA^plusXP

The instrument is used for determination of ¹³C / ¹²C, ¹⁵N / ¹⁴N, ¹⁸O / ¹⁶O, and D/H in continuous flow and gas banch modes.

The Mass Spectrometer system utilizes a variety of instruments and accessories. These are:

1. Thermal Conversion Elemental Analyzer (TC/EA) for determination of D/H and ¹⁸O / ¹⁶O isotope ratios in solid samples through continuous flow system, reference gases being H₂ and CO,
2. Flash Elemental Analyzer for determination of ¹³C / ¹²C and ¹⁵N / ¹⁴N isotope ratios in solid samples through continuous flow system, reference gases being CO₂ and N₂,
3. Water Equilibration Unit with Autosampler for determination of ¹³C / ¹²C, ¹⁵N / ¹⁴N, ¹⁸O / ¹⁶O, and D/H in liquids through Gas Bench

Specifications

• Maximum Accelerating Voltage : 3 keV
• Maximum Field Strength : 0.75 T
• Maximum m/z : 70
• Mass setting : Achieved by varying magnetic field strength
• Vacuum : 10^-8 bar (inlet valve closed); 10^-6 bar (inlet valve open)
• Detectors : Universal Triple Collector system + 2 detectors for D and H
• Amplifier Dynamic Range : 50 V External
• Measurement Precision (‰) :
  • for H : 0.5
  • for C : 0.04
  • for N : 0.03
  • for O : 0.04

Sample Requirements

Currently, the instrument is operational in ¹³C/¹²C, D/H, and ¹⁸O/¹⁶O analyses of water and carbonate samples.

Water Samples: These samples should be placed in completely filled and tightly closed glass or polyethylene bottles. At least 10 mililiters of sample is required for analysis.

Carbonate Samples : Solid samples should be homogenous and preferably in the form of powder. At least 1 gram of sample is required for analysis.

Applications

Stable Isotope Ratio Analysis is a precise means of determination for specific research subjects in a wide array of scientific areas such as geochemistry, hydrogeology, environmental sciences, ecology and food engineering.