This technique is called mass spectrometry and it was pioneered by British physicist Francis Aston in Let's take a closer look at how it works!

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1 Mass Spectrometry

2 Everyone loves a rainbow and most people understand, at least roughly, how they work: raindrops split a beam of white sunlight into rays of colored light, bending the bluish ones more than the reddish ones to make the well-known arc in the sky. Rain, then, is a brilliant method for separating sunlight. Chemists and physicists use a similar method for separating mixtures of substances into their components, turning them into beams of particles and then bending them with electricity and magnetism to make a kind of spectrum of different atoms that are easier to identify. This technique is called mass spectrometry and it was pioneered by British physicist Francis Aston in Let's take a closer look at how it works!

3 What is a mass spectrometer? Mass spectrometers are much simpler than they look or sound. Suppose someone gives you a bucketful of atoms of different chemical elements and asks you what's inside. You need to separate out the atoms quickly and efficiently, but how do you do it? Simple! Tip your bucket into a mass spectrometer. It turns the atoms into ions (electrically charged atoms with either too few or too many electrons). Then it separates the ions by passing them first through an electric field, then through a magnetic field, so they fan out into a spectrum. A computerized detector tallies the ions in different parts of the spectrum and you can use this information to figure out what kinds of atoms were originally in your bucket. That's the basic idea, anyway. In reality, it's a bit more complex than this there's no bucket, for a start!

4 How does a mass spectrometer work? There are numerous different kinds of mass spectrometers, all working in slightly different ways, but the basic process involves broadly the same stages. You place the substance you want to study in a vacuum chamber inside the machine. The substance is bombarded with a beam of electrons so the atoms or molecules it contains are turned into ions. This process is called ionization. The ions shoot out from the vacuum chamber into a powerful electric field (the region that develops between two metal plates charged to high voltages), which makes them accelerate. Ions of different atoms have different amounts of electric charge, and the more highly charged ones are accelerated most, so the ions separate out according to the amount of charge they have. (This stage is a bit like the way electrons are accelerated inside an old-style, cathode-ray television set.

5 The ion beam shoots into a magnetic field (the invisible, magnetically active region between the poles of a magnet). When moving particles with an electric charge enter a magnetic field, they bend into an arc, with lighter particles (and more positively charged ones) bending more than heavier ones (and more negatively charged ones). The ions split into a spectrum, with each different type of ion bent a different amount according to its mass and its electrical charge. A computerized, electrical detector records a spectrum pattern showing how many ions arrive for each mass/charge, or mass. This can be used to identify the atoms or molecules in the original sample. In early spectrometers, photographic detectors were used instead, producing a chart of peaked lines called a mass spectrograph. In modern spectrometers, you slowly vary the magnetic field so each separate ion beam hits the detector in turn.

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7 What is mass spectrometry used for? Like chromatography, with which it's often paired, mass spectrometry is an important method for identifying the atoms or molecules in complex chemical substances. The inventor of the spectrometer, Francis Aston ( ), used his machine to prove the existence of many naturally occurring isotopes (atoms of the same element with different numbers of neutrons and different mass). Mass spectrometry is also widely used by materials scientists (for example, to study impurities in steel) and with radio-carbon dating to calculate the approximate age of important deposits unearthed by archeologists.

8 Mass Spectrometry Molecular weight can be obtained from a very small sample. It does not involve the absorption or emission of light. A beam of high-energy electrons breaks the molecule apart. The masses of the fragments and their relative abundance reveal information about the structure of the molecule.

9 Mass Spectrometry THE MAIN USE OF MS IN ORGANIC CHEM IS: DETERMINE THE MOLECULAR MASS OF ORGANIC COMPOUNDS DETERMINE THE MOLECULAR FORMULA OF ORGANIC COMPOUNDS

10 HOW DO WE ACHIEVE THIS? CONVERT THE MOLECULE TO ITS GASEOUS STATE BOMBARD THE MOLECULE WITH HIGH ENERGY ELECTRONS TO PRODUCE IONS SEPARATE THE IONS ACCORDING TO THEIR MASS MEASURE AND RECORD THESE IONS

11 HERE WE SEE A METHANE MOLECULE GETTING HIT WITH A HIGH ENERGY ELECTRON, e-. THE CATION PRODUCED IS UNSTABLE AND BEGINS TO BREAK APART INTO FRAGMENTS. EACH FRAGMENT HAS A UNIQUE MASS. Mass = 16 H H-C:H H H + e H-C H H + 2e RADICAL CATION H H-C H H BOND-BREAKING H H-C H CATION + + H RADICAL Mass = 15 ONLY CATIONS ARE CARRIED TO DETECTOR

12 Separation of Ions Only the cations are deflected by the magnetic field. Amount of deflection depends on MASS. The detector signal is proportional to the number of ions hitting it.

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14 Mass Spectrometer =>

15 Here is the Mass Spectrum of Methane. The last peak in the spectrum is called the Molecular Ion or Parent Peak. The Parent Peak gives us the Molecular Mass of the molecule. The base peak is the largest peak in the spectrum. In this spectrum the base and parent peaks are the same. Molecular ion base peak fragments 1 m/z 15 16

16 Here is the Mass Spectrum of 2,4-dimethylpentane. Use the formula given below to find the molecular wt, and compare with the molecular ion peak. Masses are graphed or tabulated according to their relative abundance. =>

17 The GC-MS A mixture of compounds is separated by gas chromatography, then each separated component of the mixture is identified by mass spectrometry. Liquid chromatography can also be used for the initial separation, followed by mass spec. GC MS =>

18 Mass Spectra of 2-methylpentane More stable cations will be more abundant. Fragments =>

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21 Summary Mass spectrometry (MS) is an analytical technique that measures the MASS of charged particles. It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and other chemical compounds. The MS principle consists of ionizing chemical compounds to generate charged molecules or molecule fragments and measurement of their mass. In a typical MS procedure: A sample is loaded onto the MS instrument, and undergoes vaporization. The components of the sample are ionized by bombarding them with high energy electrons, which results in the formation of charged particles ions. The ions are separated according to their mass in an analyzer. The ions are detected by mass and abundance. The ion signal is processed into a graph called a mass spectrum.