TOF Mass Spectrometry

Question 1

State The two types of mass spectrometry

Answer 1

Electrospray Ionisation , Electron bombardment/impact

Question 2

Explain Electrosray Ionisation

Answer 2

Sample dissolved and passed through a hypodermic needle to make an aerosol. High voltage positive terminal attached to the tip of needle, Hᐩ ions shot at sample.

Question 3

Equations For Electrospray Ionisation and Electron impact

Answer 3

Electrospray Ionisation: X𝑔 + Hᐩ → XHᐩ𝑔
Electron Impact: X𝑔 → X𝑔ᐩ + e⁻

Question 4

Describe the process of acceleration in mass spectrometry

Answer 4

Positive ions accelerated by a negative electric field to have the same energy

Question 5

Which ion gets detected first in mass spectrometry?

Answer 5

The lightest ion; it has the least mass and therefore will have the most velocity as the kinetic energy given to each particle is a constant
Eₖ=1/2mv²

Question 6

Where do ions go through to get detected?

Answer 6

A flight tube which leads to the negatively charged plate

Question 7

How do the ions get detected?

Answer 7

The ions hit the negatively charged plate and the time taken to travel through the tube is recorded to calculate the mass of the ion
v=d/t

Question 8

How is abundance of a certain ion calculated?

Answer 8

The current induced by the potential difference of the plate and ions is directly proportional to the amount of ions.

Question 9

State any factors that could affect the mass/charge recording of the machine

Answer 9

Fragmentation of the molecule if it is large
Satellite Peak due to Carbon-13 isotopes
Subtract 1 from the value of the Mr if Electrospray ionisation is used

Question 10

A ³⁵Clᐩ ion travels through the flight tube of a TOF mass spectrometer with a kinetic energy of 3.65 ×10⁻¹⁶ J.
This ion takes 1.04 × 10⁻⁵ seconds to reach the detector.
Avogadro’s constant (L) = 6.022 × 10²³ mol⁻¹.
Calculate the length of the flight tube in metres.
Eₖ = 1/2mv²

Answer 10

mass of ³⁵Clᐩ ion = mass of 1 mole / avogadros constant
0.035kg/6.022E+23 = 5.81E-26 Kg
use equation Eₖ=1/2mv² → 2Eₖ/m = v²
(2x 3.65E-16)/5.81E-26 = 1.26E+10 = v²
v = √1.26E+10 = 112091 ms⁻¹
speed = distance/time → speed x time = distance
112091 x 1.04E-5 = 1.17 m

Question 11

What is Avogadros Constant?

Answer 11

6.022 x 10²³, The number of atoms in a mol of a substance

Question 12

State the equation that links mass, moles and mass number

Answer 12

Mr x Mol = grams of substance
Mass number x Moles of substance = grams of substance