Give brief answers (2-3 lines) to the following questions: [a] Explain why a vac

Question

Give brief answers (2-3 lines) to the following questions: [a] Explain why a vacuum is required in mass spectrometry [b] Why is it necessary to ionise our molecules before they can be analysed by mass spectrometry? [c] Chemical ionisation (CI) and matrix-assisted laser desorption (MALDI) are both soft ionisation methods. Indicate which method is useful for ionising proteins and large molecules, for example, synthetic polymer Explain why the other ionisation method cannot be used for these types of molecules. [d] Matrix assisted laser desorption ionisation (MALDI) mass spectrometry (MS) requires the sample being analysed to be mixed with a matrix such as 2, 5-dihydroxybenzoic acid. What purpose does the matrix serve in MALDI-MS analysis?

Explanation / Answer

1. a) The ions produced in the ionization chamber should not get collided by air molecules in their path. So, vacuum is required to reduce the background noise due to all the volatile components in air. This also increases the sensitivity of the instrument.

b) Magnetic and electric fields can only affect the charged particles(ions). So, atoms and molecules are first turned into ions, to be deflected by magnetic fields.

c) Matrix- assisted laser desorption ionization (MALDI) method is useful for ionizing proteins and large molecules, for example synthetic polymers. MALDI ionizes biomolecules without smashing them to pieces in the process. So, fragmentation is minimum. Being a soft ionization technique it does not decompose the sample in the process.

Proteins and other large molecules are thermally labile, they may decompose on heating and tend to fragment when ionized by more conventional ionization methods.

d) In MALDI, a sample is mixed with a UV-absorbing crystalline matrix material such as 2,5-dihydroxybenzoic acid and spotted onto a metal target plate. the plate is then placed in a vacuum and hit with a UV laser. The matrix absorbs the laser energy to create ions from large molecules with minimal fragmentation. The matrix acts as a proton donor and acceptor to ionize the analyte in both positive and negative ionization modes respectively.

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