Mass spectrometry
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What information can be obtained from the m/e values in a mass spectrum?
The m/e values represent the mass-to-charge ratio of the ions detected. For singly charged ions (charge = +1), the m/e value directly corresponds to the mass of the ion, which can be used to identify the ion or molecule. It also shows the relative abundance of each detected ion.
How is relative atomic mass calculated from isotopic abundances in a mass spectrum?
The relative atomic mass is calculated as the weighted average of the masses of the isotopes. Multiply the mass of each isotope by its relative abundance, sum these values, and divide by the sum of the abundances (if not already scaled to 100).
What information does the molecular ion peak (M+) provide in a mass spectrum of an organic molecule?
The molecular ion peak (M+) represents the intact molecule that has lost one electron. Its m/e value corresponds to the molecular mass of the organic molecule, which helps in determining the compound's identity.
How can simple fragmentation patterns in a mass spectrum help identify unknown molecules?
Fragmentation patterns reveal information about the structure of the molecule. Specific fragments correspond to the loss of particular groups (
Describe how to determine the number of carbon atoms (n) in a compound using the [M+1]+ peak in a mass spectrum.
The [M+1]+ peak is due to the presence of carbon-13 (¹³C) isotope. The number of carbon atoms (n) can be estimated using the formula: n ≈ ([M+1]+ peak abundance / M+ peak abundance) x 100 / 1.1, where 1.1 is the natural abundance of ¹³C.
What does the presence of an [M+2]+ peak, with an approximate 1:1 ratio to the M+ peak, suggest about the compound?
An [M+2]+ peak with an abundance similar to the M+ peak indicates the presence of a bromine (Br) atom in the molecule. Bromine has two isotopes, ⁷⁹Br and ⁸¹Br, which exist in approximately equal abundance.
What does the presence of an [M+2]+ peak, with approximately one-third the intensity of the M+ peak, suggest about the compound?
An [M+2]+ peak with roughly one-third the intensity of the M+ peak indicates the presence of a chlorine (Cl) atom in the molecule. Chlorine has two isotopes, ³⁵Cl and ³⁷Cl, in an approximate 3:1 ratio.
A mass spectrum shows peaks at m/e = 78, 79 and 80 with relative abundances 100:97:33. What element does this indicate?
Given the high abundance of two peaks separated by 2 m/e values this indicates a compound containing bromine. The peaks at m/e 79 and 81 (100:97 ratio) relate to 79Br and 81Br.
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