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Homework Help: Mass Spectometry Problem

  1. Oct 31, 2007 #1
    1. The problem statement, all variables and given/known data

    I have to use mass spectometry to find out what chemical compound this is based on the positions and relative abundances of the peaks used in identifying it.

    mass/charge (amu/e-) (1st column) --->Relative Abundance (2nd column)

    2. Relevant equations

    Molar Mass of Cl=35,37; Carbon=12; Oxygen= 16, Hydrogen=1

    3. The attempt at a solution

    I know that this compound contains carbon, hydrogen, and probably oxygen (not sure though). It also contains chlorine because the molecular ion peak is at 98 and there's another small peak two amy/electric charges ahead at M+2=100. There's also a small peak at 35 and 37 which is the molar mass of chlorine. There isn't any sulfur or nitrogen in the compound. I don't know what the possible formulas would be for a compound that could add up to 98 or 100. Also, I hear that there's a book or reference which gives all the chemical formulas possible at a given mass/charge therefore it would give all the possible formulas for a peak at 50 or 51 or 63 or 65 etc.
    Last edited: Oct 31, 2007
  2. jcsd
  3. Nov 1, 2007 #2


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    It may help you to graph this data. You can do it as a histogram in Excel by plotting 0 through 102 on the X axis and use the intensities given for the Y axis. Make the values for which you have no data '0'. This will give you a graphical representation that will show you some interesting patterns. I've already done this for your dataset and I assure you that it will help.

    You have already noted that chlorine-containing fragments will have a particular pattern separated by 2 AMU. That is very important.

    The most easily formed ion appears to be AMU=62. What might this be? Does it contain chlorine? Oxygen?
  4. Nov 1, 2007 #3
    That's the problem. I can tell where it has peaks, etc and I've already graphed it on excel. The only problem is knowing all final possible compounds. I know that the final compound must have a mm of 98;100 but I don't know what compounds can contain Cl, H, O, and C and also form a mm of 98. I found one of them to be C1H3O3Cl but the hydroxide fragments (OH) from this don't show any peak at 17 which OH fragments are supposed to. So I need the possible compounds with a 98 MM, I know there are sites that provide this but I can't find them. Once I have alist, I use fragmentation to figure out which one it is.
  5. Nov 1, 2007 #4


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    Which of the fragments on your graph are likely to contain Cl? When you have identified them, subtract off 35 (or 37, depending on the peak) and look at what is left.

    For example, if there was a mass at 92 and 94 you might deduce that this fragment contained chlorine. One peak for the fragment that contained Cl 35 (92) and one peak for the fragment that contained Cl 37 (94). Subtraction of the chlorine mass from either of these:

    92-35 = 57
    94-37= 57

    gives you a fragment that must weigh 57 AMU. A likely candidate for that would be one that contained 4 carbons and 9 protons. You would then look on the graph for that fragment.

    Any help? You won't find the answer in any book. You need to do this type of analysis.
  6. Nov 1, 2007 #5
    Problem is that the next closest peak below 98 is at 65. Isn't there any mass spectometry reference book which is in the form of a website?

    Also, is it likely that one of the fragments is at 62 because it has an intensity of 100?
  7. Nov 1, 2007 #6
    Ok, I'm confused, would this compound have an M+ peak at 98 and then an M+2 peak and an M+4 peak at 102?

    I get the compound with just chlorine but if there's an M+4 peak then that means there's another haloge so what would that halogen be?
  8. Nov 2, 2007 #7


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    Could it have 2 chlorines?
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