Question

25-25. Chromatography-mass spectrometry. Cocaine metabolism in rats can be studied by injecting the drug and periodically withdrawing blood to measure levels of metabolites by HPLC-mass spectrometry. For quantitative analysis, isotopically labeled internal standards are mixed with the blood sample. Blood was analyzed by reversed-phase chro- matography with an acidic eluent and atmospheric pressure chemical ionization mass spectrometry for detection. The mass spectrum of the collisionally activated dissociation products from the m/z 304 positive ion is shown in the figure on the next page. Selected reaction monitoring (m/z 304 from mass filter Q1 and m/z 182 from Q3 in Figure 22-33) gave a single chromatographic peak at 9.22 min for cocaine. The inter- nal standard 2H5-cocaine gave a single peak at 9.19 min for m/z 309 (Q1)* 182 (Q3). (a) Draw the structure of the ion at m/z 304. (b) Suggest a structure for the ion at m/z 182. (c) The intense peaks at m/z 182 and 304 do not have 13C isotopic partners at m/z 183 and 305. Explain why. (d) Rat plasma is exceedingly complex. Why does the chromatogram show just one clean peak? (e) Given that 2H5-cocaine has only two major mass spectral peaks at miz 309 and 182, which atoms are labeled with deuterium? (f) Explain how you would use 2H5-cocaine for measuring cocaine in blood.

Answer 1

since abundance of 13 carbon is very less , hence it does not shows any spectra peak for that , the fragmentation of molecule also occur .

Srnture ion m/z 304 m/z 162

Inspite of complex rat plasma cocaine give sharp peak because blood was analyzed by reversed phase chromatography with acidic eefluent and atmospheric chemical ionization mass spectroscopy.