Dear Sir I do not understand your question ? The 13C value of NBS 22 OIL is: -29.8 relative to PDB

Where did you find this equation? Send me the articl if possible !!

The conversion can be done as follows:

δVPDB(13C, X) = δVPDB(13C, NBS22) + δNBS22(13C, X) * [1 + δVPDB(13C, NBS22)]

where δVPDB(13C, NBS22) = -30.03 permille = -0.03003 and δVPDB(13C, X) is the isotope delta for sample X relative to VPDB (output), δNBS22(13C, X) is the isotope delta of sample X relative to NBS22 (input).

ps. The scale is VPDB, not PDB

First of all, the scale reference point for d13C is VPDB (as Juris has pointed out already).

Second, the up-to-date d13C value (v. VPDB) for NBS22 is -30.03 (see CIAW's website http://www.ciaaw.org/carbon-references.htm).

If your question is how to bring a measured d13C value fo NBS22 of -29.8 in line with or on the VPDB scale (i.e. a scale normalized value of -30.03), you should have analysed two 13C reference materials as scale anchors with your samples including NBS22. Proper scale normalization of measured d13C values (e.g. by EA-IRMS) requires contemporaneous analysis of two reference materials acting as end-point scale anchors. For example, one could use IAEA-CH-7 (-32.15 v. VPDB) and USGS72 (-1.54 v. VPDB) as scale anchors. By setting their accepted d13C values as Y1 and Y2 and their measured d13C values as X1 and X2, one constructs a linear regression equation of the form:

Y(d13C scale corrected) = s × X(d13C measured) + b.

This equation can then be used to scale normalize (scale realize) any measured sample d13C values in this batch run to VPDB.

On a well set-up and maintained EA-IRMS system in combination with a working gas CO2 cylinder whose d13C value has been determined, the parameter s (stretch factor) will typically be of the order of 1.01 (give or take).