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CHAPTER 14 509 
 
(M) (M+2) (M+4)+ + + (M) (M+2) (M+4)+ + +
 
 
Incidentally, we would arrive at the same characteristic pattern if we had started our analysis by first considering the 
effect of the chlorine atom and only then considering the effect of the bromine atom, as illustrated here: 
 
 
 
 
14.68. 
(a) The molecular formula for the dye is C21H15N2O6S-. Using the values in Table 14.5 for C, H, N and O, and the 
values given in the question for S, we can calculate the expected mass for the parent ion using the most abundant 
isotope of each element (1H, 12C, 14N, 16O, 32S) as follows. 
 
C21H15N2O6S¯ = 
(21 × 12.000) + (15 × 1.0078) + (2 × 14.0031) + (6 × 15.9949) + (1 × 31.9721) 
= 423.0647 
 
This is consistent with reported base peak of the spectrum [(M¯), 100% relative intensity]. 
Now we must consider the other two peaks – the peak at 424.0681 is nominally 1 amu higher than the parent molecular 
ion, while the peak at 425.0605 is nominally two amu higher. Considering the relative natural isotopic abundances of 
C, H, N, O and S, the two elements that have the highest percentage of a second isotope are carbon (12C = 98.93%, 13C 
= 1.07%) and sulfur (32S = 95.02%, 34S = 4.21%). With this in mind, the peak at m/z = 424.0681 is consistent with an 
ion where one of the 12C atoms is replaced with a 13C. 
 
12C20
13C1H15N2O6S¯ = 
(20 × 12.000) + (1 × 13.0034) + (15 × 1.0078) + (2 × 14.0031) + (6 × 15.9949) + (1 × 31.9721) 
= 424.0681 
 
The peak with a nominal mass of 425 is consistent with an ion with one 34S, as demonstrated by the calculation below: 
 
C21H15N2O6
34S¯ = 
(21 × 12.000) + (15 × 1.0078) + (2 × 14.0031) + (6 × 15.9949) + (1 × 33.9679) 
= 425.0605 
 
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