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Dear all

Willi Brand wrote:
> although the original delta as introduced by the Urey group in  
> Chicago by the end of the 1940ies (see for instance H. C. Urey,  
> Science 108, 602-603 (1948) or C. R. McKinney, J. M. McCrea, S.  
> Epstein, H. A. Allen, H. C. Urey, Review of Scientific Instruments  
> 21, 724-730 (1950).) does contain a factor of 1000 the pure relation  
> can written as
> Delta = Rsa / Rst -1.
That's correct. However, despite their delta definition containing the  
factor of 1000, the Urey group continued to talk about delta values in  
"permil" or "/mil". This practice has obviously misled people ever  
since. You can either use a factor of 1000, but then "per mill" (as it  
should be spelled according to ISO 31:2) should not be used. Or  
express everything in "per mill" units, but then don't incorporate a  
factor of 1000 into your definition of delta. The latter definition is  
recommended in Ty Coplen's manuscript.

> This is the eqn now recommended by IUPAC which in general  
> discourages the use of extraneous factors in basic relations (see  
> also Ty Coplen's manuscript http://old.iupac.org/reports/provisional/abstract08/coplen_prs.pdf) 
> .

A similar issue arose in nuclear magnetic resonance spectroscopy and  
was solved in a similar way. Older IUPAC recommendations (from 1972)  
on the expression of chemical shifts used to incorporate a factor of  
1,000,000 and no units. However, this definition never found wide  
acceptance in the community. A more recent IUPAC document consequently  
recommends expressing chemical shifts in units of ppm (Harris et al.  
Pure Appl.Chem., Vol. 73, No. 11, pp. 17951818, 2001).

Best regards
Jan


****************************************************************
Dr Jan Kaiser
School of Environmental Sciences
University of East Anglia
Norwich NR4 7TJ
United Kingdom
Tel. +44 (1603) 59-3393 (Office: 01.36)
Fax  +44 (1603) 59-1327