Time of flight (TOF) mass spectrometry can be used to analyse large molecules such as the pentapeptide, leucine enkephalin (P). P is ionised by electrospray ionisation and its mass spectrum is shown in Figure 2. **Figure 2** Abundance 0 556 m/z 1. Describe the process of electrospray ionisation. Give an equation to represent the ionisation of P in this process. 2. What is the relative molecular mass of P? Tick (✓) one box. 555 556 557 3. A molecule Q is ionised by electron impact in a TOF mass spectrometer. The Q+ ion has a kinetic energy of 2.09 × 10^-18 J. This ion takes 1.23 × 10^-6 s to reach the detector. The length of the flight tube is 1.50 m. Calculate the relative molecular mass of Q.

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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

Question 2

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Time of flight (TOF) mass spectrometry can be used to analyse large molecules such as the pentapeptide, leucine enkephalin (P). P is ionised by electrospray ionisat... show full transcript

Worked Solution & Example Answer: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

Step 1

Describe the process of electrospray ionisation.

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Answer

Electrospray ionisation is a technique where a sample solution is pushed through a fine needle at high voltage (typically around 4000 volts). This creates an aerosol of charged droplets, which evaporate, leaving charged ions in the gas phase. In this process, molecules gain protons, leading to ion formation.

Equation:

ightarrow ext{P}H^+ $$

Step 2

What is the relative molecular mass of P?

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Answer

The relative molecular mass of P is 556.

Step 3

Calculate the relative molecular mass of Q.

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Answer

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

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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

Worked Solution & Example Answer: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

Describe the process of electrospray ionisation.

What is the relative molecular mass of P?

Calculate the relative molecular mass of Q.

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