Table 4.
– The measured and calculated carbon isotopic composition of amino acid α‐carboxyl groups. Values in bold are for internationally available RMs developed for inter‐laboratory calibration R
| Reference material | Nature | δ 13CCARBOXYL ×1000, VPDB | |||
|---|---|---|---|---|---|
| expecteda | measured | MU (k = 2)b | |||
| 99% | 96% | ||||
| USGS40 | L‐glutamic acid | −30.0 | ± 0.9 | ||
| USGS41a | L‐glutamic acid | +284.9 | +275.2 | +276.8 | ± 0.9 |
| USGS64 | glycine G1 | −37.7 | ± 0.3 | ||
| USGS65 | glycine G2 | −23.9 | −24.3 | −24.3 | ± 0.3 |
| USGS66 | glycine G3 | +16.6 | +14.9 | +14.7 | ± 0.3 |
| D01 | L‐valine | −5.6 | ± 0.6 | ||
| USGS73 | L‐valine V1 | −18.5 | ± 0.5 | ||
| USGS74 | L‐valine V2 | −2.8 | −2.9 | −2.7 | ± 0.6 |
| USGS75 | L‐valine V3 | +33.3 | +32.1 | +31.6 | ± 0.3 |
| TTA | L‐alanine | −31.60 ± 0.28 c | −31.58 ± 0.3 d | ||
| TTS | L‐alanine | −27.98 ± 0.08 c | −28.06 ± 0.1 d | ||
| TTW | L‐alanine | −28.85 ± 0.03 c | −28.80 ± 0.2 d | ||
a
Calculated from information supplied by Qi36 or Schimmelmann37 assuming either 99 or 96% purity of the α‐carboxyl labelled materials used.
b
Measurement uncertainty (MU) was determined from three sources, as explained in the text, using a coverage factor (k) of two. The reported MU is an error‐propagated SD, and multiplied by 2 to approximate a 95% confidence range for the reported mean value.
c
Values determined by off‐line reaction with ninhydrin.
d
Uncertainty for these materials is reported as ± one standard deviation for FIA/NR/IRMS anslysis.