Isotopic molecular ion

Ionization Method Type of Molecular Ion Formed Good Molecular Mass Information Abundant Fragment Ions MS/MS Needed for Structural Information Accurate Values for Isotope Ratios 

Molecular ion. An ion formed by the removal (positive ions) or addition (negative ions) of one or more electrons from a molecule without fragmentation of the molecular structure. The mass of this ion corresponds to the sum of the masses of the most abundant naturally occurring isotopes of the various atoms that make up the molecule (with a correction for the masses of the electrons lost or gained). For example, the mass of the molecular ion of the ethyl bromide CzHjBr will be 2 x 12 plus 5 x 1.0078246 plus 78.91839 minus the mass of the electron (m ). This is equal to 107.95751p -m, the unit of atomic mass based on the standard that the mass of the isotope = 12.000000 exactly. 

Molecular ion The presence of sulfur can be detected by the 34S isotope (4.4%) and the large mass defect of sulfur in accurate mass measurements. In primary aliphatic thiols, the molecular ion intensities range from 5-100% of the base peak. 

Isotopic molecular ion. A molecular ion containing one or more of the less abundant naturally occurring isotopes of the atoms that make up the molecular structure. Thus, for ethyl bromide there exist molecular isotope ions such as CCHjBr, C2H4DBr , C2H5 Br, CjHj Br, etc. 

Partial mass spectra showing the isotope patterns in the molecular ion regions for ions containing carbon and (a) only one chlorine atom, (b) only one bromine atom, and (c) one chlorine and one bromine atom. The isotope patterns are quite different from each other. Note how the halogen isotope ratios appear very clearly as 3 1 for chlorine in (a), 1 1 for bromine in (b), and 3 4 1 for chlorine and bromine in (c). If the numbers of halogens were not known, the pattern could be used in a reverse sense to decide their number. 

Dichloronitrotoluenes are indicated by the presence of an odd molecular ion with chlorine isotopes showing two chlorine atoms and losses of 30 and 46 Daltons. Again, when the chlorine atoms are on the benzene ring, the loss of chlorine from the molecular ion does not occur. An M - Cl ion indicates that at least one of the chlorines is on the alkyl group. 

The isotope patterns for two simple organometallic compounds in the molecular ion region (a) dimethylmercury and (b) dimethylplatinum. The seven isotopes of mercury show clearly and appear quite different from the six isotopes of platinum. Since there are only two carbon atoms, the contribution from C is negligible. 

A further point about mass spectrometry, noticeable in the spectrum of propane (Figure 12.2), is that the peak for the molecular ion is not at the highest m/z value. There is also a small peak at M + l because of the presence of different isotopes in the molecules. Although 12C is the most abundant carbon isotope, a small amount (1.10% natural abundance) of 13C is also present. Thus, a certain 

Not only the molecular ion peak but all the peaks m the mass spectrum of benzene are accompanied by a smaller peak one mass unit higher Indeed because all organic com pounds contain carbon and most contain hydrogen similar isotopic clusters will appear m the mass spectra of all organic compounds 

This example can be used in reverse to show the usefulness of looking for such isotopes. Suppose there were an unknown sample that had two molecular ion peaks in the ratio of 3 1 that were two mass units apart then it could reasonably be deduced that it was highly likely the unknown contained chlorine. In this case, the isotope ratio has been used to identify a chlorine-containing compound. This use of mass spectrometry is widespread in general analysis of materials, and it 

Look for characteristic isotopic abundances that show the presence of bromine, chlorine, sulfur, silicon, and so on. If the deduced molecular ion is of sufficient intensity, the probable molecular formula may be determined using the observed isotopic abundances of the molecular ion region. Set the deduced molecular ion, M, at 100% abundance, and then calculate the relative abundances of M + 1 and M + 2 either manually or using the data system. 

One example is the high-resolution ESI-FT-ICR MS analysis of a mixture of Verapamil and peptide A. The following MS data were obtained for the mono-isotopic molecular ion and the isotopic molecular ions of peptide A using an external calibration (calculated mass mass error in ppm) (829.5393, 0.1 ppm), (830.5423, 1.3ppm), (831.5450, l.Oppm), and (832.5476,1.2ppm). Similar results were obtained for the Verapamil (data not shown). 

Knowing what to look for with respect to isotopic clusters can aid in interpreting mass spectra How many peaks would you expect to see for the molecular ion in each of the following compounds At what m/z values would these peaks appear (Disregard the small peaks due to and ) 

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

In a process similar to that described in the previous item, the stored data can be used to identify not just a series of compounds but specific ones. For example, any compound containing a chlorine atom is obvious from its mass spectrum, since natural chlorine occurs as two isotopes, Cl and C1, in a ratio of 3 1. Thus its mass spectrum will have two molecular ions separated by two mass units (35 -b 2 = 37) in an abundance ratio of 3 1. It becomes a trivial exercise for the computer to print out only those scans in which two ions are found separated by two mass units in the abundance ratio of 3 1 (Figure 37.9). This selection of only certain ion mas.ses is similar to selected ion recording (SIR) or, sometimes, selected ion monitoring (SIM). 

Naturally occurring isotopes of any element are present in unequal amounts. For example, chlorine exists in two isotopic forms, one with 17 protons and 18 neutrons ( Cl) and the other with 17 protons and 20 neutrons ( Cl). The isotopes are not radioactive, and they occur, respectively, in a ratio of nearly 3 1. In a mass spectrum, any compound containing one chlorine atom will have two different molecular masses (m/z values). For example, methyl chloride (CH3CI) has masses of 15 (for the CH3) plus 35 (total = 50) for one isotope of chlorine and 15 plus 37 (total = 52) for the other isotope. Since the isotopes occur in the ratio of 3 1, molecular ions of methyl chloride will show two molecular-mass peaks at m/z values of 50 and 52, with the heights of the peaks in the ratio of 3 1 (Figure 46.4). 

For other elements that occur with major relative abundances of more than one isotope in the natural state, the isotope pattern becomes much more complex. For example, with chlorine and bromine, the presence of these elements is clearly apparent from the isotopes Cl and for chlorine and Br and Br for bromine. Figure 47.2a shows the molecular ion region for the compound chlorodecane. Now, there are new situations in that C, C, C1, and Cl isotopes all have probabilities of occurring together. Thus, there are molecular ion peaks for + Cl, C + Cl, + Cl, and so on. Even so, the isotopic ratio of 3 1 for Cl to Cl is very clear 

---