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
On 7/24/2010 03:40, Gerard Olack wrote:
> Hi Robert-
> 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 ;)
> Take care,
> 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.
>> Any thoughts?
Willi A. Brand, Stable Isotope [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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