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 
> .

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

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