hope it\'s clear enough on the picture and yes that\'s one question it\'s long b

Question

hope it's clear enough on the picture and yes that's one question it's long but highly important

a) Discuss the functions of the main components of a mass spectrometer. (5) b) When a magnetic sector mass spegrometer was operated with an accelerating voltage of 300 x 10 v, a field of was required to focus the ciH. detector. What range of field strengths would be required to scan the mass range between 30 and 500 for singly charged ions, if the accelerating voltage is or held constant? c calculate the accelerating voltage that would be required to direct a singly charged ion (of MM e 120 g mol) through a magnetic sector mass analyser with a magnetic field strength of 0.250T and a radius of 15 cm. d) Considering that the natural abundance of the Cl isotope is 32.5% that of the LefNac isotope and the Br isotope is 98% that of the "Br isotope, how will it be possible to tell which of these two halogens is present in a compound by means of mass spectrometry, assuming that only one halogen atom is present in the compound?

Explanation / Answer

(a) There are four main compartments of the mass spectrometer

1. Inlet system through which the desired sample to be analyzed is introduced in the form of stream of molecules. The sample can be solid, liquid or gas, depending upon type of chromatography choosen.

2. Ion source, after the sample enters the system, they are converted in ions depending upon which system is employed like electron ionization, chemical ionization, Fast atom bombardment, Electron spray mass spectrometer etc.)

3. Mass analyzer, which helps analysis of ions formed in the second step via Quadrople, Ion trap, Magnetic sector, TOF (time of flight mass spectrometry)

4. Detector, which formulates the data in the form of chramatogram and mass spectra with the help of electron multiplier or Micro channel plates.

The kinetic energy of an accelerated ion is given by

1/2mv2 = zV              ……..(1)

Where m = mass of the ion

v is the velocity of ion

z is charge on the ion

V is the potential difference

In the presence of magnetic field, a charged ion describes a curved flight path, same can be determined using

r = mv/zB          ………(2)

where r is radius of curvature of the path

B is the strength of magnetic field

Thus equating both equation, by substituting value of v = rzB/m

We get

m/z = B2r2/2V      ……..(3)

(b) Accelerating voltage, V = 3.00 * 10 3 V

Magnetic field, B= 0.130 T

Mass of the cation C3H8+, m (as it is not clearly visible in image, so I think its H8 only and proceeding with calculation) = (3 * atomic mass of C) + (8* atomic mass of H)

= (3*12) + (8*1) = 36 + 8 = 44

Z, charge on cation = 1  

Thus by substituting these values we can get value of r from the above equation 3

44/1 = (0.130)2 * r2 / 2 * 3 * 103

r2 = 44 *2 * 3 * 103/ 0.0169

r = 3.96 * 103 m

Thus field range can be determined by using above determined radius and voltage for

m = 30

B = [30 *2* 3*103/ (3.96 * 103)2]1/2 = 0.107 T

When m = 500

B = [500 *2* 3*103/ (3.96 * 103)2]1/2 = 0.437 T

The required magnetic field range for determine mass from 30 to 500 is 0.107 T – 0.437 T

(c) Accelerating Voltage, V =?

M = 120 g/mol

B = 0.250 T

r = 15 cm = 0.15 m

Thus substituting these values in equation 3 we can determine accelerating voltage

V= B2r2z/2m = (0.250)2 (0.15)2 *1 / 2* 120 = 5.8 * 10-6 V

(d) When an organic compound contains one Chlorine atom, the ratio between M/M+2 peak equals 3:1, where M is molecular molar mass of compound. This due to the abundance of two Chlorine isotope 35Cl = 75.78% & 37 Cl = 24.22%

On the contrary, When an organic compound contains one bromine atom, the ratio between M/M+2 peak equals 1:1, where M is molecular molar mass of compound. This due to the abundance of two bromine isotope is nearly equal. ( 79Br = 50.69 & 81Br = 49.31)

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