Hard questions from exam

Question 1

To detect a drug by LC-MS, when do you choose the positive and negative mode based on their molecular structure?

Answer 1

Ibuprofen is a weak acid HA, the corresponding base A-, [M-H]-, should be easily formed in the ion source, in particular in the presence of a volatile basic buffer. Thus negative mode is preferred. Weak bases is the opposite with positive mode.

Question 2

Predict the m/z value for the molecular ion (singly charged) if the molecular structure is C13H18O2 and molecular mass 206 kDa

Answer 2

Molecular mass of ibuprofen is 206 Da. [M-1]-1

Question 3

Predict the m/z value for the molecular ion (singly charged) if the molecular structure is C13H18O2 and molecular mass 206 kDa

c. What would be the m/z value for the doubly charged molecular ion? (1p)

Answer 3

c. [M-2]-2 → m/z=102

Question 4

d. Which ion source is compatible with LC? Explain shortly how it works. (3p)

Answer 4

Electrospray ionization (ESI) is compatible with LC. See Harris p 605-607 and Fig. 22-30. ESI applies a high voltage at the outlet of a capillary, and the high electric field generated atomizes the liquid flowing out of the capillary into tiny charged droplets. As the solvent evaporates, the charge intensity on the surface of the droplet gradually increases, and finally the droplet splits into one or a plurality of charged ions, allowing the analyte to enter the gas phase in the form of a singly charged or multiply charged ion.

Question 5

a. Below you see the EI mass spectra of two compounds with composition C5H10O. The compounds contain one carbonyl carbon. Propose structures for these compounds! Confirm these by explaining the mechanisms to significant fragment ions. (5p)

Question 6

What is molecular mass vs nominal mass?

Answer 6

Molecular mass is the mass listed in the periodic table and nominal mass is the masses of the most common isotopes so it will be whole numbers vs decimals.