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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.

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Presentation on theme: "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."— Presentation transcript:

1 AP Chemistry Big Idea 1 MASS SPECTROMETRY

2 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 SPECTROMETRY Francis Aston

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

4 MASS 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.

5 A mass spectrometer consists of... an ion source, an analyser and a detector. ION SOURCE ANALYSER DETECTOR A MASS SPECTROMETER PARTICLES MUST BE IONIZED SO THEY CAN BE ACCELERATED AND DEFLECTED

6 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

7 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

8 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

9 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

10 HOW DOES IT WORK? ION SOURCE ANALYSER DETECTOR IONIZATION ACCELERATION DEFLECTION DETECTION

11 HOW DOES IT WORK? - Deflection 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 20 Ne 21 Ne 22 Ne HEAVIER ISOTOPES ARE DEFLECTED LESS

12 HOW DOES IT WORK? - Deflection if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved 20 Ne 21 Ne 22 Ne HEAVIER ISOTOPES ARE DEFLECTED LESS m/z values ABUNDANCE 1+ ions2+ ions 20 Ne 22 Ne Doubling the charge, halves the m/z value Abundance stays the same

13 In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated that naturally occurring neon consisted of three isotopes Ne, 21 Ne and 22 Ne. 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 Ne 90.92% 21 Ne 0.26% 22 Ne 8.82% WHAT IS A MASS SPECTRUM?

14 Calculate the average relative atomic mass of neon using data on the previous page. CALCULATING AVERAGE ATOMIC MASS Out of every 100 atoms are 20 Ne, 0.26 are 21 Ne and 8.82 are 22 Ne 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

15 CALCULATING AVERAGE ATOMIC MASS Naturally occurring potassium consists of potassium-39 and potassium-41. Calculate the percentage of each isotope present if the average is Assume that there are n nuclei of 39 K in every 100; n there will then be (100- n ) of 41 K. So Answer: 95% 39 K and 5% 41 K

16 IDENTIFY THE ELEMENT First, calculate average atomic mass Then find the element with the closest mass on periodic table Average Atomic Mass = Closest Element: Zirconium (Zr, Z=40)


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