The Delta, 8, Notation

Because the natural variations in stable isotope abundances are usually very small (see above), and since routine measurements are usually made in an isotope ratio mass spectrometer which compares the relative intensities of the mass resolved beams of the sample with those of some standard material (Section 7.2.2), it is standard practice to report abundance ratios using the dimensionless 8-value notation. 

The R s refer to isotope ratios, D/H, 13C/12C, 180/160, etc. By convention R is the ratio of heavy to light isotope. The factor of 1000 converts to per mil (%o) units. In the case of hydrogen isotope effects (Rd - Rstd)/Rstd is sometimes multiplied by 100 instead of 1000 and reported in per cent (%) rather than per mil, but in this text we use per mil notation almost exclusively. 

Standards for isotope ratio measurements are chosen for convenience and assigned 8 = 0%o. Ideally, every standard would be homogeneous, easily obtained in the necessary quantity, have an isotope ratio similar to that in the samples being 

Standard SDvsmow 813CVpdb 818Ovsmow 815Nair Approx, natural 

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Isotope ratios are reported using the delta (8) notation in parts per thousand or per mil (%o) 8 X = (Rsample — Rstandard) x 1000/R... 

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