prop15N <- c(0.00395, 0.02222, 0.00462, 0.00753, NA, 0.00422, 0.00492)
# Rstandard can be specified with a string for some preset references
d15N <- prop2delta(prop15N, "d15N")
d15N
# Rstandard can also be specified manually for non-preset references
d15N_manual <- prop2delta(prop15N, 0.0036765)
d15N_manual
# Call delta2prop() to get the detail of available references
delta2prop()
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