Fig. 4 : Boxplots of the accuracy of matrix-matched three-point normalizations based on whether at least one of the quality control standards falls within the isotope range of the standards (bounded) or if both quality control standards fall outside the isotope range of the standards (extrapolated). Significance is shown in the compact letter display, where any two extrapolation status 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.