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 chromatography 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. Selected reaction monitoring (m/z 304 from mass filter Q1 and m/z 182 from Q3 in Figure 21-26) gave a single chromatographic peak at 9.22 min for cocaine. The internal standard 2H5-cocaine gave a single peak at 9.19 min for m/z 309 (Q1) n 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 m/z 309 and 182, which atoms are labeled with deuterium?
(f) Explain how you would use 2H5-cocaine for measuring cocaine in blood.

Transcribed Image Text:

CH3 CO2CH3 ㄚ Cocaine 182 Mass spectrum of mz 304 after collisionalily activated dissociation 304 82 105 150 50 100 150 200 250 300 9.22 Cocaine m/z 304m/z 182 9.19 H-Cocaine m/z 309-mz 182 12 Time (min)