Figure shows the mass spectrum of the same compound from an electron-impact ioni

Question

Figure shows the mass spectrum of the same compound from an electron-impact ionization source and an ionization source.

(A) Which mass spectrum would be best for determining the molecular mass of the compound? Why?

(B) Which mass spectrum would be best for determining the chemical structure? why?

(C) The electron-impact source was a pulsed source used with a TOF mass analyzer. If the fight tube were 1.0m long and the accelerating voltage were 3000V, what would the flight time be for the ion at m/z = 58?

(D) For two ions of m/z values m1/z and m2/z, derive an equation for the difference in flight times ?t as a function of the two masses, the charges, and the accelerating voltage.

(E) For the same TOF analyzer as in part (C), calculate the difference in fligt times between ions of m/z = 59 and m/z=58

Explanation / Answer

A)

Chemical ionization mass spectrum is used when the molecular ion is not visible in the electron impact ionization mass spectrum as the chemical ionization is a soft ionization technique.

B)

Electron impact ionization gives the fragments observed in a mass spectrum better than in any other techniques as it is a technique well suited for this. The fragmentation can be used to make decisions on the actual structure of the molecule.

C)

Time= d/(2U)0.5(m/q)0.5

Time= 1/(2*3000)0.5 ((58*1.66*10-27)/1.602*10-19)0.5

Time= 1*10-5sec

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