When you zoom into the M+1 or M+2 region of a high-resolution spectrum you see several peaks separated by a few millidaltons. The question is: which pair of isotope substitutions produces the gap you just measured?
This page precomputes every pairwise separation between isotopologues of common elements within the same nominal level. Enter the observed gap and a tolerance, click Search, and the table returns every element pair that could explain it.
Show code
## Build one row per element × isotopologue for M+1 and M+2
ref_raw <- map_dfr(REF_ELEMENTS, function(el) {
p <- tryCatch(isotope_fine_pattern(el), error = function(e) NULL)
if (is.null(p) || nrow(p) == 0) return(NULL)
base <- p %>% filter(label == "") %>% pull(mz)
if (length(base) == 0) return(NULL)
p %>%
filter(label != "") %>%
mutate(
element = el,
nominal = round(mz - base),
offset_M = round(mz - base, 6)
) %>%
filter(nominal %in% 1:2) %>%
select(element, level = nominal, isotopologue = label, offset_M, abundance)
})
## All pairwise separations within each nominal level
pairs <- map_dfr(split(ref_raw, ref_raw$level), function(d) {
if (nrow(d) < 2) return(NULL)
level_label <- paste0("M+", d$level[1])
idx <- combn(nrow(d), 2)
map_dfr(seq_len(ncol(idx)), function(i) {
ra <- d[idx[1, i], ]; rb <- d[idx[2, i], ]
tibble(
level = level_label,
`isotopologue A` = paste0(ra$element, ": ", ra$isotopologue),
`isotopologue B` = paste0(rb$element, ": ", rb$isotopologue),
`difference (Da)` = round(ra$offset_M - rb$offset_M, 6),
`abundance A (% of M)` = round(ra$abundance, 4),
`abundance B (% of M)` = round(rb$abundance, 4)
)
})
}) %>%
arrange(level, abs(`difference (Da)`)) %>%
mutate(`ppm error` = "")
Search by observed separation
Enter the gap you measured between two fine-structure peaks and a tolerance in millidaltons (mDa). A Da window makes more physical sense here than ppm — these separations are just a few tenths of a millidalton, so a 1 mDa window already covers the instrument’s measurement uncertainty regardless of the parent ion’s m/z. The reference m/z is used only to compute the ppm error display column, so you can judge the match in the units your instrument reports.
The table shows every pair of isotopologues within the same nominal level (M+1 or M+2). Search by entering the gap you measured between two peaks in the fine structure.
How to use this table
- Zoom into the M+1 or M+2 region of your spectrum and measure the separation between any two peaks you can resolve.
- Enter that gap in the observed gap field and a tolerance in mDa matching your instrument’s mass accuracy. Use the level dropdown to restrict results to M+1 or M+2 if you already know which region you are working in. Click Search.
- Each returned row names the pair of isotopologues whose exact-mass positions are that far apart. The ppm error column shows how far off the theoretical value is relative to the reference m/z — a useful cross-check against your instrument’s quoted mass accuracy. Check the abundance columns: if one member of the pair is far too small to see at your current resolution, rule it out.
For a formula-specific analysis — how large each peak should be, and what resolving power you need to separate them — see Isotope fine structure.