Carbonate Device - Dual Inlet Principles
The Carbonate Device and dual-inlet system allow for generation and cryogenic distillation of carbon dioxide gas from calcium carbonate and other inogranic materiles (e.g., bone apatite) under vacuum condtions.
Notes from Willi Brand on IRMS Linearity
Linearity in the stable isotope measurement world means that the ratio of two ion currents from one source is independent of the intensity of the ion beams.
I would like to widen this a bit: The ratio measured should be identical to the ratio in the original gas in the sample. Hence, I exclude from the discussion any effect related to the chemistry that converts the original whatever-nature-sample to the measurement gas.
- The first thing to realize is that the gas that is present in the ion source during ionization is not the same as the one in the inlet system. Due to the viscous leak, the ratio in the ion source differs from that in the reservoir by a factor close to sqrt(m2/m1); (see Halsted R. E. and Nier A. O. (1950) Gas Flow through the Mass Spectrometer Viscous Leak. Review of Scientific Instruments 21(12), 1019-1021.) In the paper, Figure 3 is a good illustration of the situation: For an original hydrogen 3/2 ratio of 0.042 one observes a measured ratio of 0.0515 when a viscous leak is used. For a molecular leak, the original ratio in the reservoir is observed. As shown in this Figure 3 there is a pressure component that can affect linearity.
- The second point is that the molecules must be ionized for analysis. The ionization probabilities for different isotopologues are close, but they are not identical. The ionization probablility largely depends on the ionization cross section, which is a geometrical relation where the molecular velocities play a role. The latter are different for different masses and may change with pressure. I think, however, that this has only a very minor effect on linearity.
- 3rd, and probably very important is the fact that the ions initially move at thermal velocities inside a magnetic field (generated by the source magnets, which are present to collimate the electron beam). Once the electric field has started to accelerate the ions, this influence becomes smaller. The overall effect is called predispersion, reflecting that the molecules with different masses (but the same energy) enter the mass spectrometer with different probabilities. They are discriminated against by the alpha slit. The time the ions spend in or near their origin depends on the number of ions produced. This space charge shields the external field, hence slower ions spend more time inside the space charge dominated area and are affected by the magnetic field slightly more.
- 4th, and probably most important in daily analysis practice are ion-molecule reactions in the ion source. These can produce ions with the same mass but different nature. The best known ion-molecule reaction is the formation of H3+ in the ion source, when H2+ reacts with H2 to form H3+ and a hydrogen radical. In general, ion source chemistry has not been studied very systematically so far, but I deem it responsible for most linearity effects as well as instrument drifts (provided it is not due to a poor electronic component). Typically, H+ is transferred to a neutral molecule, producing m+1 ions that interfere with the minor isotope. The most common reactions are protonation of CO2 with the proton originating from traces of H2O+, or ionized organics, mostly generated from pump oil.
The points 3 and 4 are the most common sources of non-linearity. They can be distingished from each other rather easily: Point 3 should scale with relative mass difference whereas point 4 is a mass specific isobaric interference. One often has good linearity for the 46/44 ratio while the 45/44 non-linearity is too large and variable. This means that the physical ion source conditions are good but that there still is water or other contaminants interfering.
For those who want to learn more, I have given some more insight in chapter 38 in Pier de Groot's Handbook (WA Brand, 'Mass Spectrometer Hardware for Analyzing Stable Isotope Ratios', Chapter 38 in 'Handbook of Stable Isotope Analytical Techniques', ed. P. deGroot, Elsevier Science ( ISBN: 0-444-51114-8) 2004 ). In addition, Magnus Wendeberg and I have contributed a chapter to a new Elsevier encyclopedia (Magnus Wendeberg and Willi A. Brand, Isotope ratio mass spectrometry (IRMS) of light elements (C, H, O, N, S): The principles and characteristics of the IRMS instrument in Encyclopedia of Mass Spectrometry Vol 5, ed. Dwight E. Matthews, Elsevier, Amsterdam) which is scheduled to be published in September.
Best regards Willi
Addendum from John Eiler: This is all quite well put; it is worth adding that a sufficiently precise measurement will show that the detectors and solid-state components of the counting boards are not strictly linear. E.g., the resistivity of the resistors through which we register ion currents can vary as a function of the voltage you put across them.