6.3 identifying the molecule Ion Peak

The molecular ion optimal is both crucial reference allude and is integral in identify an unknown compound. While it may seem the the molecular ion peak have to be the most abundant peak in the spectrum, this is not the instance for the bulk of compounds. Organic compounds the contain alcohols, nitrogens, carboxylic acids, esters, and also high amounts of branching may totally lack a visible molecular ion. In instance of an absent molecular ion peak, it is an essential that highest molecular load fragment peak is not mistakenly established as the molecule ion peak. This failure would an outcome in misidentification that the analyte of interest. Obtaining a chemistry ionization spectrum (CI) can correctly recognize the molecular ion (Section 5.2.1.2b).

Even without a CI spectrum that the compound, the fixed spectrum offers some information that can assist in judgment out potential masses together the molecular ion. The “nitrogen rule” is one beneficial tool for identifying the molecular ion. This rule indicates the if a molecular ion has actually an strange mass it must have actually an odd variety of nitrogens and also that a molecular ion through an even mass have to either lack nitrogen atoms or contain an even variety of them. Since the bulk of essential compounds analyzed through the GC-MS contain one of two rwandachamber.org zero or one nitrogen atom, the rule practically states an odd molecular ion is attributed come a solitary nitrogen and an even molecular ion shows the sample lacks nitrogen (Figure 6.2). This dominion only applies to compounds that contain carbon, hydrogen, nitrogen, oxygen, sulfur, halogens, and a couple of other less typical elements. Due to the fact that the bulk of organic compounds that room analyzed making use of the GC-MS are consisted of of these elements, this stipulation is almost ignored.

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Figure 6.2 The Nitrogen ascendancy - The massive spectrum the N,N-dimethyl-ethanamine illustrates the visibility of one odd molecular ion and even fragments. Spectra native the NIST/EPA/NIH fixed Spectral Library. Reprinted with permission indigenous NIST.

This rule is a result of nitrogen’s unique property. Nitrogen has actually an even atomic mass yet bonds v three various other atoms in its most stable form (an even amu plus an odd number of substitutents gives an overall odd molecular mass). Various other atoms that have even molecular weights prefer carbon, oxygen, and also sulfur bond v an even number of other atom (an also amu to add an even number of substitutents gives an overall even mass). Atoms that bond v an odd variety of other atoms favor hydrogen, chlorine, bromine, and also iodine have odd molecule weights (an weird amu plus and odd variety of substitutents gives an overall even mass). This rule is invaluable as soon as a chemist knows that a compound lacks nitrogen. This can occur if a sample is all set from a synthesis whose products and also solvents lack nitrogen atoms. In this case, any kind of odd peak cannot it is in attributed to the molecule ion the the analyzed compound.

Most fragments, excluding rearrangements (Section 6.6), outcomes from breaking a single bond. The nitrogen ascendancy indicates that as soon as a molecule v an also mass produces a fragment by break a solitary bond, the fragment will have an strange mass. Once the sample’s mass is odd, fragmentation via a comparable pathway will offer an also fragment as long as the nitrogen is still included in the it was observed fragment. Since this is the observed tendency (See Stevenson’s dominance Section 6.6), examining the significant fragments can assist determine if the molecule ion have to be even or odd. Practically, if the significant fragments are largely odd, the molecule ion is likely even and also contains no nitrogen. If the significant fragments room even, the molecule ion is most likely odd and contains one nitrogen atom as displayed in number 6.3.

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Figure 6.3. The usage of the Nitrogen dominance in identify the molecular Ion - need to the faint optimal at m/z 60 be attributed to instrumental noise or is it the molecule ion? The presence of the base optimal at 45 in mix with our knowledge around the nitrogen rule says that the height at m/z 60 is most likely the molecule ion due to the fact that even molecular ion usually create odd molecular fragments by breaking single bonds. Given this spectrum is of isopropil alcohol, our deduction is correct although chemistry ionization techniques can verify the molecular mass that the sample. Spectra indigenous the NIST/EPA/NIH fixed Spectral Library. Reprinted with permission native NIST.

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Since molecular ion fragment in predictable ways, the presence of details fragmentation peaks can suggest that a particular peak is the molecular ion. The observed pieces must have the ability to be attributed to logical losses. The visibility of a height from the ns of CH3, a top from the loss of H20, or a indigenous the ns of OCH3 are a few examples of this logical fragments.

The opposite is true for pieces that room not logical. These peaks suggest that a specific peak is no the molecule ion. Part illogical fragmentation peaks encompass peaks that is 3 to 14 mass devices away from the peak suggest that the figured out peak is most likely not the molecule ion peak. The ns of fragments of mass devices 1-3 can result from the loss of increase to 3 hydrogen atoms. From 14 come 18, lot of peaks have the right to be defined from the ns of CH3, oxygen, a hydroxide ion, or water. The loss of fragments from the 19-25 selection is also unlikely other than in the case of fluorinated compounds, which produce (loss the F) and also (loss of HF).

The molecular ion deserve to be an overwhelming to recognize without chemistry ionization due to the fact that there is no definitive test. When these patterns can greatly assist in identifying the molecular ion, they have to not be relied ~ above for absolute confirmation. Sometimes facility rearrangements can potentially an outcome in the misidentification the the molecular ion. As soon as in doubt, it is good practice to dual check with a soft ionization technique such together chemical ionization.