AP Chemistry Big Idea 1

MASS

SPECTROMETRY

The first mass spectrometer was built in 1918 by Francis W Aston, a student of J J Thomson, the man who discovered the electron.

MASS SPECTROMETRYMASS SPECTROMETRY

Francis Aston

Aston used the instrument to show that there were different forms of the same element. We now call these isotopes.

MASS SPECTROMETRYMASS SPECTROMETRY

Francis Aston

MASS SPECTROMETRYMASS SPECTROMETRY

In a mass spectrometer, particles are turned into positive ions, accelerated and then deflected by an electric or magnetic field. The resulting path of ions depends on their ‘mass to charge’ ratio (m/z).

Particles with a large m/z value are deflected least

Particles with a low m/z value are deflected most.

A mass spectrometer consists of ... an ion source, an analyser and a detector.

ION SOURCE

ANALYSER

DETECTOR

A MASS SPECTROMETERA MASS SPECTROMETER

PARTICLES MUST BE IONIZED SO THEY CAN BE ACCELERATED AND

DEFLECTED

PARTICLES MUST BE IONIZED SO THEY CAN BE ACCELERATED AND

DEFLECTED

HOW DOES IT WORK?HOW DOES IT WORK?

ION SOURCE

ANALYSER

DETECTOR

Step 1: IONIZATION

•gaseous atoms are bombarded by electrons from an electron gun and are IONIZED• sufficient energy is given to form ions of 1+ charge

HOW DOES IT WORK?HOW DOES IT WORK?

ION SOURCE

ANALYSER

DETECTOR

Step 1: IONIZATION

Step 2: ACCELERATION• ions are charged so can be ACCELERATED by an electric field

HOW DOES IT WORK?HOW DOES IT WORK?

ION SOURCE

ANALYSER

DETECTOR

Step 1: IONIZATION

Step 2: ACCELERATION

Step 3: DEFLECTION•charged particles will be DEFLECTED by a magnetic or electric field

HOW DOES IT WORK?HOW DOES IT WORK?

ION SOURCE

ANALYSER

DETECTOR

Step 1: IONIZATION

Step 2: ACCELERATION

Step 3: DEFLECTION

Step 4: DETECTION•by electric or photographic methods

HOW DOES IT WORK?HOW DOES IT WORK?

ION SOURCE

ANALYSER

DETECTOR

IONIZATION

ACCELERATION

DEFLECTION

DETECTION

HOW DOES IT WORK? - HOW DOES IT WORK? - DeflectionDeflection

• the radius of the path depends on the value of the mass/charge ratio (m/z)

• ions of heavier isotopes have larger m/z values so follow a larger radius curve

• as most ions are 1+charged, the amount of separation depends on their mass

20Ne21Ne

22Ne

HEAVIER ISOTOPES ARE DEFLECTED

LESS

HOW DOES IT WORK? - DeflectionHOW DOES IT WORK? - Deflection

• if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved

20Ne21Ne

22Ne

HEAVIER ISOTOPES ARE DEFLECTED

LESS

0 4 8 12 16 20 m/z values

AB

UN

DA

NC

E

1+ ions2+ ions

20Ne

22Ne

Doubling the charge, halves the m/z value

Abundance stays the same

In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated that naturally

occurring neon consisted of three isotopes ... 20Ne, 21Ne and 22Ne.

• positions of the peaks gives atomic mass• peak intensity gives the relative abundance • highest abundance is scaled to 100% and other values are adjusted accordingly

MASS SPECTRUM OF NEON

19 20 21 22 23

20Ne 90.92%

21Ne 0.26%

22Ne 8.82%

WHAT IS A MASS WHAT IS A MASS SPECTRUM?SPECTRUM?

Calculate the average relative atomic mass of neon using data on the previous page.

CALCULATING AVERAGE ATOMIC CALCULATING AVERAGE ATOMIC MASSMASS

Out of every 100 atoms... 90.92 are 20Ne , 0.26 are 21Ne and 8.82 are 22Ne

TIP In calculations of this type... multiply each relative mass by its abundance add up the total of these values

divide the result by the sum of the abundances;

(100 in this case)

* if the question is based on percentage abundance, divide by 100 but if

it is based on heights of lines in a mass spectrum, add up the heights

of the lines and then divide by that number (see later).

A Answer: 20.18

CALCULATING AVERAGE ATOMIC CALCULATING AVERAGE ATOMIC MASSMASS

Naturally occurring potassium consists of potassium-39 and potassium-41. Calculate the percentage of each isotope present if the average is 39.1.Assume that there are n nuclei of 39K in every 100; there will then be (100-nn) of 41K.

So

Answer: 95% 39K and 5% 41K

IDENTIFY THE ELEMENTIDENTIFY THE ELEMENT

• First, calculate average atomic mass

• Then find the element with the closest mass on periodic table

Average Atomic Mass = 91.32Closest Element: Zirconium (Zr, Z=40)