Fig 2 : Boxplots of the accuracy of one-point, two-point, and multipoint normalizations that are matrix matched and bounded (A) and matrix mixed/both mixed and extrapolated (B). Significance is shown in the compact letter display, where any two methods with the same letter are not significantly different as per Dunn’s post-hoc testing. Nitrogen (uppercase) and carbon (lowercase) letters are not compared with each other.

Effect of matrix and isotope range on normalization accuracy

To isolate the effect of matrix-matching on normalization errors, we present results from only three-point normalizations. When the results are constrained to bounded normalizations, the matrix of the standards relative to the quality controls has a significant effect for both N and C (Fig. 3). Nitrogen normalizations that have calibration standards composed of a different matrix than the quality controls (median error = 0.104‰, n = 688) exhibit significantly higher errors than matrixed-matched normalizations (median error = 0.064‰, n = 102,p < 0.0001) and normalizations where the standards and quality controls are both composed of a combination of matrixes (median error = 0.058‰, n = 190, p < 0.0001). Similarly, mixed-matrix C normalizations (median error = 0.165‰) exhibit significantly greater errors than normalizations composed of matched matrixes (median error = 0.056‰, p < 0.0001) and both-mixed matrixes (median error = 0.121‰, p < 0.0001). Furthermore, N normalizations with both-mixed matrixes have significantly greater errors than matched-matrix normalizations (p < 0.0001).