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
, '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
On 7/24/2010 03:40, Gerard Olack wrote:
[log in to unmask]"
Changes in pressure in the source? There is always some source chemistry going on, and more molecules, more chemistry. No, I've studied this directly-so I hope those who know chime in...but you did ask for any ideas ;)
Robert Panetta <[log in to unmask]> wrote:
On the heels of the discussion launched by Daniel, can anyone explain (or
provide a good reference) why there is a mass-dependent linearity effect in
the first place? I was quite surprised to learn a few months back that it is
not isolated to IRMS: I sat in on a seminar about analyzing isotope-labeled
compounds by other MS techniques (such as ion trap) and very clearly, the
enrichment of a compound increased linearly with the amount injected. The
presenter had no explanation for it and just tossed it as the quirks of mass
spec, but it's curious that this seems to be a universal trait.
Willi A. Brand, Stable Isotope Laboratory [log in to unmask]
Max-Planck-Institute for Biogeochemistry (Beutenberg Campus)
Hans-Knoell-Str. 10, 07745 Jena, Germany Tel: +49-3641-576400
P.O.Box 100164, 07701 Jena, Germany Fax: +49-3641-577400
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