The spectrum of an organic compound was recorded on an LC-MS instrument, using e
ID: 1060131 • Letter: T
Question
The spectrum of an organic compound was recorded on an LC-MS instrument, using electrospray ionization (ESI) to produce negative ions (“negative ion mode”). These conditions produce a high yield of the fragment [M-H]-. Based on the spectrum shown, how many chlorine or bromine atoms does this compound have? Make sure to explain your answer!
x 105 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Negative ion mode IM-HIT 395.0073 397.0045 399.0018 396.0104 398.0073 400.0047 392 393 394 395 396 397 398 399 400 401 402 403 404 Mass-to-charge (m/zExplanation / Answer
M+2 peak in a mass spectrum arises from the presence of chlorine or bromine atoms in an organic compound. The presence of M+4 peak in compounds indicates the compound contains two chlorine or bromine atoms.
If the peak heights are in the ratio of 3 : 1, then the halogen is chlorine. Since the fact is chlorine contains 3 times as much of the 35Cl isotope as the 37Cl one.
Bromine has two isotopes, 79Br and 81Br in an approximately 1:1 ratio (50.5 : 49.5). Unlike compounds containing chlorine, though, the two peaks will be very similar in height.
Now the spectra has
m/z: 395 + 2 = 397
m/z: 395 + 4 = 399
The intensity of peaks at m/z = 395 and m/z = 397 are almost similar and it contains M+2 and M+4 peak. So, there are two bromine atoms.
Similarly,
396, 398 (M+2) and 400 (M+4).
Intensity of peaks at m/z = 396 and m/z = 398 are almost similar.
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