Mass Number & Isotopes

Question 1

What is mass spectroscopy used for?

Answer 1

Finding the abundance and relative atomic mass of each isotope in an element.

Question 2

Why do you ionise the sample?

Answer 2

For detection and acceleration of the sample.

Question 3

Why do we vaporise the sample?

Answer 3

So it can be ionised.

Question 4

What is the first step in mass spectroscopy?

Answer 4

1. Vaporise and ionise the substance.
   Electron impact:
   - used with small molecules/atoms to produce smaller fragments.
   - an electron gun fires high energy electrons and knock off 1 electron forming 1+ ions.
   X(g) -> X+(g) + e-

Electrospray ionisation:
- used with larger organic molecules eg. proteins, because it overcomes the destructive effects of an electron gun.
- the substance is dissolved in an inhert and volatile solvent.
- then it passes through a hypodermic needle attached to a high voltage power supply (4000V)
- This causes particles to gain a proton as they leave the needle
X(g) + H+ -> XH+(g)
- The substance comes out as a fine mist and the m/z ratio changes by 1

Question 5

What is the difference between electron impact and electrospray ionisation?

Answer 5

• In electron impact there is no addition of a proton whereas in electrospray ionisation there is an addition of a proton therefore the Ar or Mr increases by 1

Question 6

What is the second step in mass spectroscopy?

Answer 6

2. Acceleration
   - All of the ions formed from the ionisation stage are given the same kinetic energy so the velocity of the ions will depend on the mass.
   - The substance is accelerated into the tube by the negative electric plate.

Question 7

What is the third step in mass spectroscopy?

Answer 7

3. Drift
   - Heavier ions have a greater mass therefore a slower velocity so they will take longer to drift.
   - Air resistance is not a factor because the entire mass spectrometer tube is a vacuum.

Question 8

What is the fourth step in mass spectroscopy?

Answer 8

4. Detection
   - The 1+ ions become neutral because they hit the negative electric plate and gain an electron and induce a current attached to the detector plate.
   - The size of the current is directly proportional to the abundance of the isotope hitting the plate.

Question 9

How can the mass spectroscopy machine tell the difference between the heavier and lighter ions?

Answer 9

The heavier ions take longer and hit the plate later than the lighter ions which is how they can tell the difference and how abundant one isotope is to another.

Question 10

What is a mass spectrum?

Answer 10

Graph from mass spectroscopy that tells you the relative abundance and m/z

Question 11

What is the formula for KE?

Answer 11

KE=1/2mv^2

Question 12

What are the symbols for mass number and atomic (proton) number

Answer 12

mass number : A
atomic number: z

Question 13

What is the difference between mass number and relative atomic mass

Answer 13

The mass number is the number of protons and neutrons in one atom whereas the relative atomic mass is the weighted abundance of the entire sample

Question 14

Calculate the RAM of 75% Cl-35 and 25% Cl-37

Answer 14

35.5