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ISOGEOCHEM  July 2010

ISOGEOCHEM July 2010

Subject:

Re: Why is there a linearity effect?

From:

"Willi A. Brand" <[log in to unmask]>

Reply-To:

Stable Isotope Geochemistry <[log in to unmask]>

Date:

Mon, 26 Jul 2010 14:51:45 +0200

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (112 lines)

  Hi,

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,
>
> Gerry
>
> Robert Panetta<[log in to unmask]>  wrote:
>
>> Hi,
>> 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?
>> Thanks!
>> Robert

-- 
.....................................................................
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

http://www.bgc-jena.mpg.de/
http://www.bgc-jena.mpg.de/service/iso_gas_lab/
.....................................................................
WMO/IAEA Experts Workshop on Observations of Greenhouse Gases and Related Tracers:
http://www.bgc.mpg.de/service/iso_gas_lab/IAEA-WMO2009/index.shtml
.....................................................................

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