Not sure about the newer models, but for the Delta Plus XL the H3 
determination is not linear under 5V according to the manual

Harlow, Benjamin wrote:
>
> Effie,
>
> In the method you used, under “Peak detection @H2”, you will see the 
> starting value of the H3 factor. Here you can select overwrite and put 
> in a starting value of 5. You could try this using the reevaluate 
> option for the CF document in question. But I think you should use a 
> broader span of voltages to calculate the H3, like 1-7V for example. 
> That way you are giving isodat more of slope to work with, thus 
> reducing error in the estimate of the H3.
>
> Isodat will start with the last H3 calculated, so you need to force it 
> to start at a more realistic starting point. I have one method I only 
> use for calculating the H3 where I overwrite or define the H3 starting 
> value to be 5.
>
> Hope this helps,
>
> Ben Harlow
>
> Washington State University
>
> ------------------------------------------------------------------------
>
> *From:* Stable Isotope Geochemistry [mailto:[log in to unmask]] 
> *On Behalf Of *effie
> *Sent:* Monday, April 16, 2007 12:40 AM
> *To:* [log in to unmask]
> *Subject:* [ISOGEOCHEM] The "start value" of the H3 factor
>
> Hi all,
>
> I have started to look into dD analysis, and I have some problems 
> concerning the H3 factor. I hope you can enlighten me with some 
> explanations. The analysis is of water using the TC/EA and DeltaplusXP 
> MS. Back ground levels are of 10 for mass 3, and 30 for mass 2. During 
> the determination of the factor (that is preceded by the zero on off 
> measurement while increasing H2 ref gas pressure. Increasing being in 
> the intensity range of 1000mV to 2000mV), the software (Isodat 2.0) 
> determines a value from which it starts to calculate the H3 factor. On 
> screen it will show it as "start value". The start value is of 42, the 
> H3 factor which is determined is around that figure (with a standard 
> deviation 0.6).
>
> My questions:
>
> What is that "start value" and how it is determined?
>
> Assuming my factor should be less then 10, what may cause a larger figure?
>
> Thanks in advance,
>
> Effie
>
> *Effie Eliani
>
> **Stable isotopes lab
>
> ***Geological Survey of Israel
>
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-- 
Jonathan D. Karr, Ph.D.
Technical Director, Duke EnVironmental stable Isotope Laboratory (DEVIL)
Duke University Dept.of Biology & Nicholas School of the Environment and Earth Sciences
Phytotron Bldg, 71 Science Drive 
Durham, NC 27708-0340

(919) 660-7418
Fax: (919) 660-7425
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laboratory website: http://www.biology.duke.edu/jackson/devil/




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