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HI All-

And nice chapter too...

If you move out of the stable isotope arena, you'll have different types 
of detectors too, geared more for sensitivity, e.g. electron or photo 
multipliers, and they usually only have one detector with a fixed range. 
(Yes, I'm avoiding time of flight and ion traps).  If you start doing 
LC-MS (electrospray) and MALDI (matrix assisted laser desorption), 
you'll also have matrix affects in the source.  You can usually get an 
order of magnitude linear working range, using an internal 
standard/isotope dilution.  For organic chemists, the M+1 on a small 
molecule it used to determine carbon count, but you'll never get good 
natural abundance values on an HP 5800's ;)

FYI--some crude comparisons (I like Google (tm) searches):

Gasbench:  100ug calcium carbonate=1umol CO2 in ~10 mL, or 10 nmol in 
100 uL (sampling loop) which is ~100 pmol 13CO2.
GCC: ~30 ng alkane (160pmol C16 alkane)= 3 nmol CO2

A recent Analytical  Chem. paper reports quantifying proteins in the 10 
to 90 fmol/uL range, 2 uL injections, via isotope dilution: “Ultra 
Performance Liquid Chromatography Isotope Dilution Tandem Mass 
Spectrometry for the Absolute Quantification of Proteins and Peptides” 
Leah G. Luna, Tracie. L. Williams, James L. Pirkle, and John R. Barr* 
Anal. Chem., 2008, 80 (8), pp 2688–2693

For ICP ms, a recent report is looking at pg/g and fg/g levels of trace 
metals (20 mL sample sizes) and apparently the limit of detection tends 
to be contamination of the sample (very clean clean room needed). 
Analytica Chimica Acta  Volume 530, Issue 2, 14 February 2005, Pages 291-298
Analytical procedures for improved trace element detection limits in 
polar ice from Arctic Canada using ICP-SMS
Michael Krachlera, Corresponding Author Contact Information, E-mail The 
Corresponding Author, E-mail The Corresponding Author, James Zhenga, b, 
David Fisherb and William Shotyk

take care

gerry


On 7/26/10 11:41 AM, John Eiler wrote:
> 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.
>
>
> On Jul 26, 2010, at 2:12 AM, Willi A. Brand wrote:
>
>> 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 
>> <http://www.isotopems.info/IRMS%20presentations%20and%20publications/Brand_Chapter38_IRMS.pdf>, 
>> '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
>> .....................................................................
>