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Mass Spectroscopy. 2.2 The mass spectrometer 1 hour Assessment statement 2.2.1 Describe and explain the operation of a mass spectrometer. 2.2.2 Describe how the mass spectrometer may be used to determine relative atomic mass using the 12 C scale.
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2.2 The mass spectrometer 1 hour • Assessment statement • 2.2.1 Describe and explain the operation of a mass spectrometer. • 2.2.2 Describe how the mass spectrometer may be used to determine relative atomic mass using the 12C scale. • 2.2.3 Calculate non-integer relative atomic masses and abundance of isotopes from given data.
A mass spectrometer is an instrument that separates particles into their masses and records the relative proportions of these. • In a mass spectrometer (five parts): • the substance is first converted to atoms or molecules in the vapor phase. [Vaporization] • These are then turned into positive ions [Ionization] and accelerated through a magnetic field [Acceleration]. • The fast moving ions are deflected (with a magnet or electromagnetic field) [Deflection]. • – the lighter the particle the greater the deflection (momentum) • Particles of a particular mass will be detected at the end of the spectrometer. [Detection] • The body of the instrument must be maintained at a high vacuum by a pump.
Applications The mass spectrometer has many applications, but one of the simplest is to determine the natural abundances of the isotopes of a particular element. This will allow for the calculation of its atomic mass. The element (magnesium) to the right would consist of approximately 80% 24Mg, 10% 25Mg, and 10% 26Mg. The relative atomic mass can be calculated from the data from the mass spectrometer.
Carbon-12 as a Standard • When 12C and 13C are analyzed in the mass spectrometer, the ratio of their masses is found to be • Since the atomic mass unit is defined against 12C (atomic mass is exactly 12), then on the same scale: Mass 13C = (1.0836)(12amu) = 13.0034 amu • The mass of other atoms can be determined in a similar way (compared to 12C) (12C scale).
Ionization • The vaporized sample passes into the ionization chamber. • The electrically heated metal coil gives off electrons which are attracted to the electron trap which is a positively charged plate. • The particles in the sample (atoms or molecules) are bombarded with a stream of electrons, and some of the collisions are energetic enough to knock one or more electrons out of the sample particles to make positive ions. • Most of the positive ions formed will carry a charge of +1 because it is much more difficult to remove further electrons from an already positive ion. • These positive ions are “influenced” out into the rest of the machine by the ion repeller (another metal plate carrying a slight positive charge). • Go to: http://www.chemguide.co.uk/analysis/masspec/howitworks.html for full article (and the pictures).
DeFLECTION • Only ion (green in the previous slide) (the one which is “just right”) makes it right through the machine to the ion detector. • The other ions (red and blue in the previous slide) collide with the walls where they will pick up electrons and be neutralized. Eventually, they get removed from the mass spectrometer by the vacuum pump. • Note: Red path barely moves compared to the other ion paths. This indicates those ions have greater mass than the other ions. • Blue path moves greater compared to the other two ion paths. This indicates that those ions have less mass than the other ions. • Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on: • the mass of the ion. Lighter ions are deflected more than heavier ones. • the charge on the ion. Ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge. • These two factors are combined into the mass/charge ratio. Mass/charge ratio is given the symbol m/z (or sometimes m/e). • For example, if an ion had a mass of 28 and a charge of 1+, its mass/charge ratio would be 28. An ion with a mass of 28 and a charge of 2+ would have a mass/charge ratio of 14.
Detection • When the positive ion hits the metal box (detector), its charge is neutralized by an electron jumping from the metal on to the ion. That leaves a space amongst the electrons in the metal, and the electrons in the wire shuffle along to fill it (movement of electrons = electricity). • A flow of electrons in the wire is detected as an electric current which can be amplified and recorded. • The more ions arriving, the greater the current. • Go to: http://www.chemguide.co.uk/analysis/masspec/howitworks.html for full article (and the pictures).
Detection Detecting the other ions • How might the other ions be detected - those in Red & Blue streams which have been lost in the machine? • Remember that Blue stream was most deflected - it has the smallest (the lightest ions if the charge is 1+). To bring them on to the detector, you would need to deflect them less - by using a smaller magnetic field (a smaller sideways force). • To bring those with a larger ions (Red, the heavier ions if the charge is +1) on to the detector you would have to deflect them more by using a larger magnetic field. • If you vary the magnetic field, you can bring each ion stream, in turn, on to the detector to produce a current which is proportional to the number of ions arriving. The mass of each ion being detected is related to the size of the magnetic field used to bring it on to the detector. • The machine can be calibrated to record current (which is a measure of the number of ions) directly. • Remember: The mass is measured on the 12C scale.
