chemistry help Mass spectroscopy

need help in part b

for a number of molecular ions peak, I think there will be 4 (4 combinations)
for m/z value is it 320 (I have no idea)

can someone please explain me that

It's just statistics.

Four chlorines
Two isotopes

How many possible (unique) combinations

37, 37, 37, 37
37, 37, 37, 35
37, 37, 35, 35
37, 35, 35, 35
35, 35, 35, 35

Imagine you have 4 coins, how many different combinations of heads and tails can you get? All heads, 1H 3T, 2H 2T, 3H 1T, all tails. 5 different possibilities. It is the same principle in the TCDD. This compound has 5 possible molecular weights. The most abundant peak would be one with 3 Cl^37 and 1 Cl^35. Add up the Mr and this would give you 322g

The most abundant peak would be one with 3 Cl^37 and 1 Cl^35. Add up the Mr and this would give you 322g


I don't understand why 3 CL^37 and one cl^35?

The most abundant peak wold be the one wth the most likelihood.

The odds of a 35 are 75%, or 3/4 and the odds of a 37 Cl are 25% or 1/4

Hence the odds for each of the following are:

37, 37, 37, 37 = 1/256 = 0.39%
37, 37, 37, 35 = 3/256 = 1.17 and four different ways of doing it = 4.69%
37, 37, 35, 35 = 9/256 = 3.52% and six different ways of doing it = 21.09%
37, 35, 35, 35 = 27/256 = 10.55% and four different ways of doing it = 42.19%
35, 35, 35, 35 = 81/256 = 31.64%
--------------------------------------------------- total percentage checksum = 100%

So the most intense peak will be due to ³⁷Cl³⁵Cl³⁵Cl³⁵Cl
With a relative mass of 322

thanks
It makes sense