Posted by Bun on Tuesday, December 29, 2009 at 11:59pm.
1. The mass spectrum can be observed for all forms: pos. ion, neg.ion, molecules, and atoms, can't it?
2. Is there a formular to calculate how many spectrum of the substances we need to observe? Please give me some examples, and calculation.
3. I knew the examples of H2S+, and how to draw the table to find the peak, but I don't know the formula to calculate how many peak without using the table, so I need other examples, and how to calculate peaks.
Thanks a lot

chemistry  drwls, Wednesday, December 30, 2009 at 1:04am
1. A mass spectromter analyzes ions, or ions formed when a neutral atom or molecule is ionized and broken up before entering the mass analyzer. Since you are analyzing fragments of an original molecuile, it is often difficult to say what the original molecule was. It might have been a different molecule that produces the same ionic fragments.
2. The short answer is no, except that the OPTICAL spectra of oneelectron atoms (hydrogen) and ions can be easily calculated. Are you talking about mass spectra or optical spectra?
I am not clear why you ask how many spectra you NEED TO observe. In order to do what? Is this a lab assignment?
3. I have no idea what kind of spectra you are talking about here 
chemistry  Bun, Wednesday, December 30, 2009 at 2:33am
Thank you drwls
i am reviewing my chemistry and doing the problems from my textbook. Because my teacher did not have the lecture about this part, so I am reading my text book and trying to solve problems. My textbook only have two problems of this theory:
1/ find how many peaks you would observe in the mass spectrum of the positive ion of CF4^+.( C has 2 stable isotopes, F has 1 isotope)
and
2/ Hydrogen has two isotope H1 and H2, and Sulfur has four isotopes S(32), S(33) S (34) and S (35). How many peaks you observed in the mass spectrum of positive ion H2S+
I solved these problems and and checked my answer with the answer key, but I still don't understand a lot. I would like you to explain for me what the optical spectra and mass spectra are,and the difference between them as well as some other examples, please. 
chemistry  drwls, Wednesday, December 30, 2009 at 4:44am
Optical spectra are the wavelengths at which an element or compound emit and absorb light or infrared radiation. That subject used to be a specialty of mine.
Your questions are apparently concerned with mass spectra, not optical spectra. A mass spectrum shows the different chargetomass ratios of ions that pass through a device that separate them, based upon the value of that ratio.
Ions for which different elements have different isotopes will exhibit more than one "mass" peak for each ion. The spectrum looks like a series of "peaks" or "spikes" of different intensity. The strongest ones will correspond to the most abundant mass values.
1. CF4+ will have two possible masses (in amu units) of 12 + 4*19 = 88 or 13 + 4*19 = 89. The 88 peak will be about 99 times higher because C12 is 99 times more abumndant than C13.
2. H2S can have three possible mass values for the H2 part (1+1 = 2, 1+2 = 3, or 2+2 = 4) and 4 possible values for the S atom (32, 33, 34 or 35). The number of possible masses of the combined H2S+ ion are:
2+32 = 34
2+33 OR 3+32= 35
2+34 OR 3+33 OR 4+34 = 36
2+35 OR 3+34 OR 4+33 = 37
3+35 OR 4+34 = 38
4+35 = 39
That is a total of 6 possible lines.
Just construct a table with all the possible mass combinations, and count them, as I did above. There is probably a formula one could write, but it will differ depending upon what the different possible isotope mass values are, and how many atoms of each type there are in the molecular ion. 
chemistry  Bun, Wednesday, December 30, 2009 at 10:29am
Thank you so much drwls