Spectrometer Watch

LearningMath
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Right, im trying to understand the basic process here, need a few things explained.

Right, so we need evidence for the existance of isotopes. We choose a substance, in a gaseous form and bombard it with high energy electrons, which creates positive ions.

1. High 'Energy' electrons? I thought electrons were electrons end of, or does the textbook mean they are moving fast?
2. I know nothing of electrons reacting with atoms other than when they are within their energy levels/shells. Why does firing electrons at an atom make it a positive ion, electrons are negative, so surely they make the atom a negative ion. Or are they like a sticky ball... hitting the atom and picking up some of the atoms electrons before flying off?

Next, ions with 1+ charge are selected and accelerated by an electric field into a focused stream which is passed through a magnetic field.

3. How does an electric field accelerate the ions?
4. Ok, so ions are deflected just after the magnetic field, the degree of the deflection depends on the mass of the ion, lighter ions will bend more, heavier ones less. Right...so what purpose does the magnetic field serve? Is it simply that it pulls the ions in a slightly different direction... and hence lighter ions react to this force faster because of their smaller mass?

- The textbook uses the term 'evacuated', the context is the pump on the spectrometer. What is the meaning of this word in relation to chemistry?

- Lastly, how reliable is this evidence really? Because surely, working at this level, its impossible to make the atoms in the spectrometer purely from one element. If say, you were testing for the existence of isotopes of... nitrogen, then the nitrogen will not be completly pure, and is bound to have afew atoms from other elements mixed in...

Trying to bridge the gap :p: Really appreciate anyone who takes the time to explain some of the above
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JW GINGER
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1. High energy electrons are high kinetic energy so yes moving fast.

2. I try to think of it as being literally a gun, shooting off parts of a target, makes a +ve ion due to way the lectrons with high energy reacts with the certain parts of the atoms.

3. I think it it to do with E=ItV so the energy passed on by the charge per second. So as gaseous atoms enter the electric field they gain some of charge and move faster.

4. There is a detector, so at differnet magnetic field strengths differnet sized ions will go straight to it and you get a readin adn others will simply miss, too high or too low.

5. I think evacuated basically means removed, you do generally have to remove the ions and that does fit.

6. When you do mass spectrometry you see lots of lines, for each of the different ions. As you will always have differnet ions form the ionisation stage. In perfect world would have 3 lines for nitrogen ( I think ) but you dont, you would have many lines. Then i think you just read the largest oercentage one around the ion mass levels

However in my experience you really dont need to know any of this for as, though maybe a2 or a differnet course form ocr

This should help, if you dont like the typos dont read them haha
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zedliv
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(Original post by LearningMath)
Right, im trying to understand the basic process here, need a few things explained.

Right, so we need evidence for the existance of isotopes. We choose a substance, in a gaseous form and bombard it with high energy electrons, which creates positive ions.

1. High 'Energy' electrons? I thought electrons were electrons end of, or does the textbook mean they are moving fast?
2. I know nothing of electrons reacting with atoms other than when they are within their energy levels/shells. Why does firing electrons at an atom make it a positive ion, electrons are negative, so surely they make the atom a negative ion. Or are they like a sticky ball... hitting the atom and picking up some of the atoms electrons before flying off?

Next, ions with 1+ charge are selected and accelerated by an electric field into a focused stream which is passed through a magnetic field.

3. How does an electric field accelerate the ions?
4. Ok, so ions are deflected just after the magnetic field, the degree of the deflection depends on the mass of the ion, lighter ions will bend more, heavier ones less. Right...so what purpose does the magnetic field serve? Is it simply that it pulls the ions in a slightly different direction... and hence lighter ions react to this force faster because of their smaller mass?

- The textbook uses the term 'evacuated', the context is the pump on the spectrometer. What is the meaning of this word in relation to chemistry?

- Lastly, how reliable is this evidence really? Because surely, working at this level, its impossible to make the atoms in the spectrometer purely from one element. If say, you were testing for the existence of isotopes of... nitrogen, then the nitrogen will not be completly pure, and is bound to have afew atoms from other elements mixed in...

Trying to bridge the gap :p: Really appreciate anyone who takes the time to explain some of the above
Your questions are all just physics really (I covered it at A2)

1. High energy means that they have been accelerated through a high voltage. Their kinetic energy is equal to half times their mass times their speed squared. (1/2 mv^2 ) and this is equal to their charge, multiplied by their accelerating voltage. You don't need to know those formulae, but basically, they are made to move fast by using a high voltage, and this means they have high energy.

2. I've always just heard that described as the electron "knocks" other electrons out of the atom, and so ionises it. I don't think any more detail is needed in chemistry at this point.

3. An electric field is an area where a charge experiences a force. So, as an electron has a negative charge, when an electric field is applied a force acts on it. As the forces on it are unbalanced, it accelerates (Newton's first law). I'm explaining this in a physics-type way, which you would not normally need to put in an A level chemistry exam.

4. The point of a mass spectrometer is to show the proportion of different isotopes, and it does so by separating them according to mass. I can't remember any more on this point, sorry!

5. Evacuated means that it is a vacuum. This means that nothing will get in the way of the electrons, or ions, so no unwanted collisions will occur.

6. In industry much more complicated, advanced spectrometers are used. I don't think it's an issue.

Hope that helps! It's a while since I've done spectrometry
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LearningMath
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This is the new OCR AS chem textbook, from the way you both say A2, im starting to feel worried this course is gona be hard Interesting so far though!
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zedliv
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No, I did mass spectrometer at AS, just now that I've done A2 physics I understand some of it a little better. At AS chemistry, you tend not to need to understand the physics of it (which is what you were asking about) it's more about knowing a little about how it works and interpreting the spectra obtained.
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LearningMath
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Cheers Zed+Ginger. Got the theory sorted, and the calculations understood. Rep ya when i can.
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