Question

Identify the ions responsible for the four peaks having greater mass-to-charge ratios than the ${\mathrm{M}}^{+}$peak in Figure 20-4a.

Short answer

The ions responsible for the four peaks having greater mass-to-charge ratios than the${\mathrm{M}}^{+}$peak in Figure 20-4a are :

$\mathrm{m}/\mathrm{z}=85$is due to localid="1645698073981" ${}^{37}\mathrm{Cl}_{213}{\mathrm{CH}}^{2+}$

$\mathrm{m}/\mathrm{z}=86$is due to localid="1645698088230" ${{}^{37}\mathrm{Cl}}^{35}{\mathrm{Cl}}^{12}{\mathrm{CH}}^{2+}$

$\mathrm{m}/\mathrm{z}=87$is due to localid="1645698102915" ${{}^{37}\mathrm{Cl}}^{35}{\mathrm{Cl}}^{13}{\mathrm{CH}}^{2+}$

$\mathrm{m}/\mathrm{z}=88$is due to localid="1645698125124" ${}^{37}\mathrm{Cl}_{212}{\mathrm{CH}}^{{}_{2+}}$

Step-by-step solution

Concept Introduction

Mass spectrometry is a powerful analytical technique which is used to elucidate structure of compounds, quantify materials, identify unknown compounds etc. In mass spectrophotometer the molecule is ionized by giving it a positive charge by removing an electron or by adding a proton. The molecule may break into several positively charged fragments according to the ionization method used. Then in a mass analyzer these cationic molecular fragments are separated based on their mass and detected by the detector. Only positively charged molecular fragments are detected by the detector.

Observing the figure

The figure 20-4a is as follows -

The molecular ion peak, $M$+ occurs at the mass corresponding to the molecular mass of the analyte molecule. The $\mathrm{m}/\mathrm{z}$ratio of methylene chloride molecular ion is $84.$Other cation fragment peaks occur at $\mathrm{m}/\mathrm{z}$ratios below $84$. But in methylene chloride mass spectrum there are peaks with $\mathrm{m}/\mathrm{z}$ratio greater than that of molecular ion. This is due to the composition of molecule with different isotopes of atoms. C have two stable isotopes as ${}^{13}\mathrm{C}$and ${}^{12}\mathrm{C}$. Chlorine has two stable isotopes as ${}^{35}\mathrm{Cl}$and ${}^{37}\mathrm{Cl}$.

$\mathrm{m}/\mathrm{z}=85$is due to localid="1645698159336" ${}^{37}\mathrm{Cl}_{213}{\mathrm{CH}}^{2+}$

$\mathrm{m}/\mathrm{z}=86$is due to localid="1645698193465" ${{}^{37}\mathrm{Cl}}^{35}{\mathrm{Cl}}^{12}{\mathrm{CH}}^{2+}$

$\mathrm{m}/\mathrm{z}=87$is due to localid="1645698204930" ${{}^{37}\mathrm{Cl}}^{35}{\mathrm{Cl}}^{13}{\mathrm{CH}}^{2+}$

$\mathrm{m}/\mathrm{z}=88$is due to localid="1645698230022" ${}^{37}\mathrm{Cl}_{212}{\mathrm{CH}}^{2+}$