There have been many attempts to define mass spectrometry. As I see it no one particular definition really points down all the benefits attained by mass spectrometry. Mass spectrometry is a diverse technique which can be applied to most any purpose required by both analytical and organic chemists as well as astronomists,biologists, health professionals to name a few.. As far as chemists are concerned, mass spectrometry is a powerful analytical technique for identifying unknowns, studying molecular structure, and probing the fundamental principles of chemistry. It is essentially a technique for "weighing" molecules. It has been described as "the smallest scale in the world". Detection of compounds can be accomplished with very minute samples (as little as 10-12g). This means that compounds can be identified at very low concentrations (one part in as small as 1012in chemically complex mixtures).
In order to acheive the above mentioned task, one puts charge on the molecules of interest, i.e., the analyte, then measures how the trajectories of the resulting ions respond in vacuum to various combinations of electric and magnetic fields.
For small and simple species the ionization is carried by gas-phase encounters between the neutral molecules and electrons, photons, or other ions. In recent years, the efforts of many investigators have led to new techniques for producing ions of species too large and complex to be vaporized without substantial decomposition.
2-The mass spectrometer:
However, the mass spectrometer does not actually measure the molecular mass directly, but it measures the mass-to-charge ratio of the ions formed from the molecules. The charge on an ion is referred to by the letter z and is measured as the charge on an electron (the unit of charge), and the mass-to-charge ratio m/z therefore represents daltons per unit of charge. In many cases, the ions encountered in mass spectrometry have just one charge (z=1) so the m/z value is equal to the molecular (ionic) mass in Da. Mass spectrometrists often speak loosely of the "mass of an ion" when they really mean the m/z ratio, but this convenient way of speaking is useful only for the case of singly-charged ions.
An actual mass spectrometer ranges in size from about the size of a home microwave oven to large research instruments that dominate entire rooms as ahow in the previous clip.
How a mass spectrometer works
Suppose you had a cannonball travelling past you and you wanted to deflect it as it went by you. All you've got is a jet of water from a hose-pipe that you can squirt at it. Frankly, its not going to make a lot of difference! Because the cannonball is so heavy, it will hardly be deflected at all from its original course.
The sequence is of what happens in the (basic magnetic) mass spectrometer is given below.
- 1: Ionisation: The atom is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which are normally expected to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Mass spectrometers always work with positive ions.
- Stage 2: Acceleration:The ions are accelerated so that they all have the same kinetic energy. so the only variable will be their masses.
- Stage 3: Deflection: The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected.The amount of deflection also depends on the number of positive charges on the ion (on how many electrons were knocked off in the first stage). The more the ion is charged, the more it gets deflected.
- Stage 4: Detection :The beam of ions passing through the machine is detected electrically.
I-The ion source
Figure 2:The Ion Source
The ion source is the heart of the spectrometer. The vaporised sample is allowed to passes into the ionisation chamber, non-volatile solids and liquids may be introduced directly on a direct probe . The electrically heated metal coil gives off electrons which are attracted to the electron trap (a positively charged plate located opposite the coil).The particles in the sample (atoms or molecules) are thus 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. This is called an EI (electron-impact) source. 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.
The ions thus formed usually carry high energy and thus break down into smaller fragments. Thus in the ion source we are left with a number of positively charged ions of vareing sizes (masses).
II-Acceleration:
Figure3 (The Accelerator)
A schematic depiction of the accelerator is shown above.
The positive ions Produced in the ion source are forced into the analyzer by the ion repeller which is a very positive plate and pass as a beam through three slits, of decreasing voltage the final one of which is at 0 volts. Some of these ions fragment into smaller cations and neutral fragments. All the ions are accelerated into a finely focused beam.
The clip below describes the process of acceleration.
The need for a vacuum: Because ions are very reactive and short-lived, their formation and manipulation must be conducted in a vacuum. Atmospheric pressure is around 760 torr (mm of mercury). The pressure under which ions may be handled is roughly 10-5 to 10-8torr (less than a billionth of an atmosphere). To reduce the chance of the produced ion being hit by air molecules.
III-Deflection :
A Schematic depiction of a mass spectrometer with magnetic analyzer
When the ion beam experiences a strong magnetic field perpendicular to its direction of motion (as shown above), the ions are deflected in an arc whose radius is directly proportional to the mass of the ion. Lighter ions are deflected more than heavier ions. By varying the strength of the magnetic field, ions of different mass can be focused progressively on a detector fixed at the end of a curved tube (also under a high vacuum).
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 56 and a charge of 2+ would also have a mass/charge ratio of 28.
In the above diagram, ion stream x is most deflected (ions with the smallest mass/charge ratio). Ion stream z is the least deflected (ions with the greatest mass/charge ratio).Most of the ions passing through the mass spectrometer will have a charge of 1+ , so that the mass/charge ratio will be the same as the mass of the ion. (Note: You must be aware of the possibility of 2+ions ).
Assuming that all the ions carry one positive charge(1+), stream x has the lightest ions, stream y the next lightest and stream z the heaviest. Lighter ions are going to be more deflected than heavy ones.
IV-Detection:
Only ion stream x makes it right through the analyzer to the ion detector. The other ions collide with the walls where they will pick up electrons and be neutralised. Eventually, they get removed from the mass spectrometer by the vacuum pump.
When an ion hits the metal plate of the detector, its charge is neutralised 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 move along to fill it.Thus creating an electric current proportional to the number of ions hitting the detector plate.In other words, the 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 produced current is .
Detecting the other ions :
The only ions detected at this particular magnetic feild are the ions of stream y which managed to arrive at the plate. The ions in streams x and z,which have been lost in the machine are not detected, since they were deflected outside the borders of the detector plate. Stream x was most deflected - it has the smallest value of m/z (the lightest ions if the charge is 1+). To bring them on to the detector, we would need to deflect them less - by using a smaller magnetic field (a smaller perpendicular force). To bring those with a larger m/z value (the heavier ions if the charge is +1) on to the detector we would have to deflect them more by using a larger magnetic field. If the magnetic field is varied, each ion stream is brought in turn will arrive on to the detector to produce a current which is proportional to the number of ions.
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 detector records current intensity (which is a measure of the number of ions) against m/z . The mass is measured on the 12C scale.
Note: The 12C Scale is a scale on which the 12C isotope weighs exactly 12 units.
V-Sample introduction:
Most ionization techniques are designed for gas phase molecules so the inlet must transfer the analyte into the source as gas phase molecule. If the analyte is sufficiently volatile and thermally stable, a variety of inlets are available. gases and samples with high vapour pressure are introduced directly into the source region. Liquids and solids are usually heated to increase the vapour pressure for analysis. If the analyte is thermally labile (it decomposes at high temperatures) or if it does not have sufficient vapour pressure, The sample must be directly ionized from the condensed phase. These direct techniques require special instrumentation and are more difficult to use.
Direct vapour inlet. ( the simplest sample introduction method)The gas phase analyte is introduced directly into the ion source region of the mass spectrometer through the needle valve. Pump out lines are usually included to remove air from the sample. This inlet works well for gases, liquids, or solids with high vapour pressure. Samples with low vapour pressure are heated to increase the vapour pressure. The inlet is limited to stable compounds and modest temperatures.
Gas Chromatography. The most common technique for introducing samples into a mass spectrometer. Complex mixtures are separated by gas chromatography and mass spectrometer is used to identify and quantitate the individual components. The most important characteristics of the inlet are the amount of GC carrier gas and the amount of analyte that enter the mass spectrometer. If a large amount of carrier gas enters the mass spectrometer it will increase the pressure in the ion source region and thus maintaining the required source pressure will require larger and more expensive vacume pumps. The amount of analyte that enters the mass spectrometer is important for improving the detection Limits of the instrument. Ideally all the analyte and none of the GC carrier gas would enter the source region.
The most common GC/MS interface now uses a capillary GC column. Since the carrier gas flow rate is very small for these columns, The end of the capillary is inserted directly into the Source region of the mass spectrometer. The entire flow from the GC enters the mass spectrometer. Since capillary columns are now very common, this inlet is widely used.
Liquid Chromatography.These are used to introduce thermally labile compounds which are not easily separated by gas chromatography. Because these inlets are used for temperature sensitive compounds, the sample is ionized directly from condensed phase.
Direct insertion Probe. (DIP) is widely used to introduce low vapour pressure liquids and solids into the mass spectrometer. The sample is loaded into a short capillary at the end of a heated sleeve. This sleeve is then inserted through a vacume lock so the sample is inside the source region. After the probe is positioned, the temperature of the capillary tube is increased to vaporize the sample. This probe is used at a higher temperatures than are possible with direct vapour inlets. In addition, the sample is under vacume and is located close to the source so that lower temperatures are required for the analysis. This is important for analyzing temperature sensitive compounds. It is useful for a wide range of samples.
Direct Ionization of samples. Unfortunately some compounds are either decomposed when heated or have no significant vapor pressure. These samples may be introduced to the mass spectrometer by direct ionization from the condensed phase These direct ionisation techniques are used for liquid chromatography/ mass spectrometry, fast atom bombardment and laser ablation etc...
What the mass spectrometer output looks like:
The output from the chart recorder is usually simplified into a "stick diagram". This shows the relative current produced by ions of varying mass/charge ratio.
The stick diagram for toluene is shown above. The vertical axis is labelled as either "relative abundance" or "relative intensity". The vertical scale is related to the current received by the chart recorder - and so to the number of ions arriving at the detector. The greater the current, the more abundant the ion. The intensities of all the ions are normalized to the most intense peak (which is given the value of 100%).
In the bove diagram, the commonest ion has a mass/charge ratio of 91. Other ions have mass/charge ratios of 92, 65, 47 and 29. Assuming that the ions all have a charge of 1+, that means that the masses of the of the ions on the carbon-12 scale are 92, 65, 47and 29.
Note: If there were also 2+ ions present we would have other lines at exactly half its m/z value (Those lines would be much less tall than the 1+ ion lines because the chances of forming 2+ions are much less than forming 1+ ions.
The quadupole Mass Spectrometer
To the left is a general schematic of aquadrupole mass spectrometer. The blue line illustrates ions of a particular mass/charge ratio which reach the detector at a certain voltage combination.As the name implies, it consists of 4 circular rods, set perfectly parallel to each other.In a quadrupole mass spectrometer Ions are separated in a quadrupole based on the stability of their trajectories in the oscillating electric fields that are applied to the rods.
The quadrupole consists of four parallel metal rods. Each opposing rod pair is connected together electrically and a radio frequency voltage is applied between one pair of
rods, and the other. A direct current voltage is then superimposed on the R.F. voltage. Ions travel down the quadrupole in between the rods. Only ions of a certain m/z will reach the detector for a given ratio of voltages: other ions have unstable trajectories and will collide with the rods. This allows selection of a particular ion, or scanning by varying the voltages.
The vedio clips shown below descibe how th quadrupole analyzer works
Related links
http://www.colorado.edu/chemistry/chem5181/#Links
http://http//www.scq.ubc.ca/
http://ael.gsfc.nasa.gov/saturnGCMSMass.shtml
http://www.physics.rutgers.edu/cyclotrone/12inchion.shtml/
http://en.wikipedia.org/wiki/Quadrupole_mass_analyzer
http://en.wikipedia.org/wiki/Mass_spectrometer
