ALL ABOUT CHEMISTRY PANEL NEWS, ACTIVITIES OR EVENTS
Monday, August 3, 2009
Saturday, June 6, 2009
How a mass spectrometer works
The basic principle
If something is moving and you subject it to a sideways force, instead of moving in a straight line, it will move in a curve - deflected out of its original path by the sideways force.
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.
But suppose instead, you tried to deflect a table tennis ball travelling at the same speed as the cannonball using the same jet of water. Because this ball is so light, you will get a huge deflection.
The amount of deflection you will get for a given sideways force depends on the mass of the ball. If you knew the speed of the ball and the size of the force, you could calculate the mass of the ball if you knew what sort of curved path it was deflected through. The less the deflection, the heavier the ball.
Note: I'm not suggesting that you personally would have to do the calculation, although the maths isn't actually very difficult - certainly no more than A'level standard!
You can apply exactly the same principle to atomic sized particles.
An outline of what happens in a mass spectrometer
Atoms can be deflected by magnetic fields - provided the atom is first turned into an ion. Electrically charged particles are affected by a magnetic field although electrically neutral ones aren't.
The sequence is :
Stage 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 you would normally expect 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.
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 - in other words, 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.
A full diagram of a mass spectrometer
Understanding what's going on
The need for a vacuum
It's important that the ions produced in the ionisation chamber have a free run through the machine without hitting air molecules.
The vaporised sample passes into the ionisation 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 therefore 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 persuaded out into the rest of the machine by the ion repeller which is another metal plate carrying a slight positive charge.
Note: As you will see in a moment, the whole ionisation chamber is held at a positive voltage of about 10,000 volts. Where we are talking about the two plates having positive charges, these charges are in addition to that 10,000 volts.
The positive ions are repelled away from the very positive ionisation chamber and pass through three slits, the final one of which is at 0 volts. The middle slit carries some intermediate voltage. All the ions are accelerated into a finely focused beam.
Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on:
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 last diagram, ion stream A is most deflected - it will contain ions with the smallest mass/charge ratio. Ion stream C is the least deflected - it contains ions with the greatest mass/charge ratio.
It makes it simpler to talk about this if we assume that the charge on all the ions is 1+. 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+ (etc) ions, but the vast majority of A'level questions will give you mass spectra which only involve 1+ ions. Unless there is some hint in the question, you can reasonably assume that the ions you are talking about will have a charge of 1+.
Assuming 1+ ions, stream A has the lightest ions, stream B the next lightest and stream C the heaviest. Lighter ions are going to be more deflected than heavy ones.
Only ion stream B makes it right through the machine 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 box, its charge is neutralised by an electron jumping from the metal on to the ion (right hand diagram). That leaves a space amongst the electrons in the metal, and the electrons in the wire shuffle along to fill it.
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.
Detecting the other ions
How might the other ions be detected - those in streams A and C which have been lost in the machine?
Remember that stream A 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, you would need to deflect them less - by using a smaller magnetic field (a smaller sideways force).
To bring those with a larger m/z value (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) against m/z directly. The mass is measured on the 12C scale.
Note: The 12C scale is a scale on which the 12C isotope weighs exactly 12 units.
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 molybdenum looks lilke this:
You may find diagrams in which the vertical axis is labelled as either "relative abundance" or "relative intensity". Whichever is used, it means the same thing. 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.
As you will see from the diagram, the commonest ion has a mass/charge ratio of 98. Other ions have mass/charge ratios of 92, 94, 95, 96, 97 and 100.
That means that molybdenum consists of 7 different isotopes. Assuming that the ions all have a charge of 1+, that means that the masses of the 7 isotopes on the carbon-12 scale are 92, 94, 95, 96, 97, 98 and 100.
Note: If there were also 2+ ions present, you would know because every one of the lines in the stick diagram would have another line at exactly half its m/z value (because, for example, 98/2 = 49). 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.
If you want to go straight on to how you use these mass spectra to calculate relative atomic masses you can jump straight to that page by following this link rather than going via the menus below.
Saturday, May 16, 2009
Friday, May 1, 2009
Thursday, April 16, 2009
Wednesday, April 15, 2009
- Empirical Formula of a compound shows the ratio of elements present in a compound.
- Molecular Formula of a compound shows how many atoms of each element are present in a molecule of the compound.
- The empirical formula mass of a compound refers to the sum of the atomic masses of the elements present in the empirical formula.
- The relative molecular mass (formula mass, formula weight or molecular weight) of a compound is a multiple of the empirical formula mass.
RMM = n x empirical formula mass
- Empirical Formula can be calculated from the percentage (or percent) composition of a compound.
- The molecular formula for water is H2O, a 2 to 1 ratio.
Since the subscripts will not reduce H2O is also water's empirical formula.
- The molecular formula for hydrogen peroxide is H2O2, a 1 to 1 ratio.
The formula is correct. Remember that peroxide is a polyatomic ion with a charge of -2.Reduce the subscripts to the lowest form.
The empirical formula for hydrogen peroxide is HO, a 1 to 1 ratio.
- Subscripts in a chemical formula are usually thought of as a ratio of atoms.
Subscripts can also be thought of as a ratio of moles . . . . .
a mole being the number of atoms needed
to equal the atomic mass number of an element.
To determine an empirical formula, one must determine the mole ratio.
Monday, April 13, 2009
- The scientific method is a way to ask and answer scientific questions by making observations and doing experiments.
- The steps of the scientific method are to:
- Ask a Question
- Do Background Research
- Construct a Hypothesis
- Test Your Hypothesis by Doing an Experiment
- Analyze Your Data and Draw a Conclusion
- Communicate Your Results
- It is important for your experiment to be a fair test. A "fair test" occurs when you change only one factor (variable) and keep all other conditions the same.
Sunday, April 12, 2009
Answer: If you look 'chemistry' up in Webster's Dictionary, you'll see:
"chem·is·try n., pl. -tries.
1. the science that systematically studies the composition, properties, and activity of organic and inorganic substances and various elementary forms of matter.
2. chemical properties, reactions, phenomena, etc.: the chemistry of carbon.
3. a. sympathetic understanding; rapport. b. sexual attraction.
4. the constituent elements of something; the chemistry of love. [1560-1600; earlier chymistry]."
My glossary definition is the short and sweet, "scientific study of matter, its properties, and interactions with other matter and with energy".
An important point to remember is that chemistry is a science, which means its procedures are systematic and reproducible and its hypotheses are tested using the scientific method. Chemists, scientists who study chemistry, examine the properties and composition of matter and the interactions between substances. Chemistry is closely related to physics and to biology. As is true for other sciences, mathematics is an essential tool for the study of chemistry.
1 mole contains the same number of particles as there are in 12g of carbon-12 atoms by definition.
This number is called Avogadro's number or Avogadro's constant (NA) and is equal to 6.022 x 10 23 particles.
1 mole of a pure substance has a mass in grams equal to its Relative Atomic Mass (RAM) or its Relative Molecular Mass (RMM) [also known as molecular weight (MW) or Relative Formula Mass (RFM) or formula weight (FW)].This is often referred to as the molar mass.
1 mole of an ideal gas has a volume of:
22.4 litres (22.4L) at S.T.P.
[Standard Temperature and Pressure, 0 oC (273K) and 101.3kPa (1 atm)]
24.47 litres (24.47L) at S.L.C
[Standard Laboratory Conditions, 25 oC (298K) and 101.3kPa (1atm)]
Avogadro's Number (NA)
1 mole of atoms or molecules contains 6.022 x 10 23 atoms or molecules
Saturday, April 11, 2009
* Analytical chemistry is concerned with the separation, identification, and composition of materials.
* Organic chemistry is the chemistry of most carbon compounds.
* Inorganic chemistry is (you guessed it) the chemistry of materials other than those classed as organic.
* Physical chemistry involves the study of the physical characteristics of materials and the mechanisms of their reactions.
* Biochemistry includes the study of materials and processes that occur in living things.
* Nuclear chemistry involves the study of subatomic particles and nuclear reactions.
SPM Chemistry Form 4
|Form 4 Chemistry|
|1. Introduction To Chemistry|
|2. The Structure Of The Atom|
|3. Chemical Formulae And Equations|
|4. Periodic Table of Elements|
|5. Chemical Bonds|
|7. Acids And Bases|
|9. Manufactured Substances in Industry|
SPM Chemistry Form 5
|Form 5 CHEMISTRY|
|1. Rate Of Reaction|
|2. Carbon Compounds|
|3. Oxidation And Reduction|
|5. Chemicals For Consumers|
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