Question

why does double focusing mass analyser provide higher resolution than magnetic sector mass analyser?

calculate the ratio of (M+1)+ to M+ peak height for C4H10NO

Calculate the ratio of (M+2)+ and (M+4)+ to M+ peak height for CHBr3.

Answer 1

we know that in double focussing mass analyse we apply both magnetic and electrostatic analyzer.

It has higher resoltion than magnetic sector mass analyser as in magnetic sector mass analyzer the ions are spearted on the basis of mass / charge ratio, however when ions will be detected by detector the ions may have different velocities. So an improved resoltion may be obtained by introducing electric sector. the electric sector help ions to focus on the deterctor accroding to their velocity.

So ions are detected on the basis of both mass / charge ratio and velocity

a) calculate the ratio of (M+1)+ to M+ peak height for C4H10NO

The M+1 will be due to C¹³ isotope of carbon

The % of C¹³ = 1.11%

So there will be 1.11 X 4 = 4.44 C13 and 100-4.44 = 95.56 C12

So the M+1 / M+ peak ratio will be = 4.44 / 95.56 = 0.0465

b) Calculate the ratio of (M+2)+ and (M+4)+ to M+ peak height for CHBr3

Here the M+2 ion peak will be due to Bromine isotope , there are three bromine atoms So

97.7 = molecules with an ⁸¹Br molecule per 100 molecules

so ratio of M+2 / M+ =3 X 97.7 /100 = 293 / 100

M+4 / M+ = 286 / 100