从 R 中给定的 X 轴值中查找 Y 轴的值

Question

Fairly new to R and I'm trying to run analysis of FTIR spectra for my dissertation through the ChemoSpec package. In specialist software like Spectragryph (can't access on my own computer, hence using R) it's possible to locate peak values very easily but I can't seem to work out the right way to do it here.

This is the formula I'm hoping to perform on all of my spectra:

Carbonyl Index (CI) = Absorbance at 1740cm-1 (the maximum of carbonyl peak)/ Absorbance at 1460cm-1 x (the maximum of carbonyl peak)

Here is an example of the plot code for the spectra:

## ChemoSpec plot
plotSpectra(HDPE_samples,
main = "48 hr exposure",
which = c(8, 9, 10, 11, 12, 13, 14, 15, 16,
39, 40, 41, 42, 43, 44, 60, 61),
## y axis shows absorbance (%)
yrange = c(0, 0.9),
offset = 0.005,
lab.pos = 2450,
## x axis shows wave numbers (cm-1)
xlim = c(1300, 3000))

For now I'd be happy just to retrieve the absorbance values associated with the wave numbers in the formula if anyone could give me pointers on which functions/packages to look at

Answer 1

Here is an example of reading data at a specific frequency.

library(ChemoSpec)
#> Loading required package: ChemoSpecUtils

data(metMUD1)
plotSpectra(metMUD1)

# Where is the maximum of signal 1?
which.max(metMUD1$data[1,])
#> [1] 1098

# What is the frequency and intensity at the max value?
metMUD1$freq[1098]
#> [1] 1.340894
metMUD1$data[1, 1098]
#> [1] 0.0680055

^{Created on 2020-01-15 by the reprex package (v0.3.0)}