Time of flight (TOF) mass spectrometry can be used to analyse large molecules such as the pentapeptide, leucine enkephalin (P) - AQA - A-Level Chemistry - Question 2 - 2019 - Paper 1

The relative molecular mass of P is 556.

To find the mass of the ion Q, we first need to use the kinetic energy equation:

$KE = \frac{1}{2} mv^2$

Given that:

• $KE = 2.09 \times 10^{-18} \text{ J}$
• The time taken to reach the detector is $t = 1.23 \times 10^{-6} \text{ s}$
• The length of the flight tube is $d = 1.50 \text{ m}$

We can find the speed using: $v = \frac{d}{t} = \frac{1.50 \text{ m}}{1.23 \times 10^{-6} \text{ s}} \approx 1.22 \times 10^6 \text{ m s}^{-1}$

Plugging into the kinetic energy equation: $KE = \frac{1}{2} m v^2$

Rearranging gives: $m = \frac{2 \times KE}{v^2} \approx \frac{2 \times 2.09 \times 10^{-18} \text{ J}}{(1.22 \times 10^6)^2} \approx 2.81 \times 10^{-18} \text{ kg}$

To convert to relative molecular mass: $m_{relative} = m \times L$ where $L = 6.022 \times 10^{23} \text{ mol}^{-1}$.

Calculating gives: $m_{relative} = 2.81 \times 10^{-18} \text{ kg} \times 6.022 \times 10^{23} \text{ mol}^{-1} \approx 169 ext{ g mol}^{-1}$