QuestionQuestion

1. Explain what is ESI, MALDI, EI and CI. Which one is considered as soft ionization? Which one is commonly coupled with LC separation? Which one with GC?

2. Explain how to do tandem mass spectrometry in triple quadrupole and ion trap instruments, respectively? Which one is capable of doing MSn?

3. One of the techniques in multiplexed quantification of peptides is using isobaric labeling tags, in which various reagents react with peptides from different samples. The quantification is carried out by tandem mass spectrometry using reporter ions generated from these tags. For iTRAQ-8 reagents, you can simultaneously quantify 8 samples. Two of the 8 reporter ions generated have the m/z of 118.1 and 119.1, respectively.
What is isobar? How much resolving power is needed to differentiate between these two ions? Which mass analyzer, quadrupole, ion trap, TOF, Orbitrap and FTICR, is capable of distinguishing these two ions?

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1. Explain what is ESI, MALDI, EI and CI.

EI and CI are methods of converting gaseous analyte molecules into ions (which can then be accelerated via and electric/magnetic field to the detector).
EI is a method where electrons are extracted via high electric field from one electrode and accelerated towards the other electrode. Gaseous analyte molecules are introduced between the plates and are struck by high energy electrons which “knock out” electrons from the molecule and turn it into ion. After ionization, molecule turned ion, still has a lot of energy and fragments into parts.
CI is a method where a gas (usually methane, isobutane, or ammonia) are introduced into similar chamber as above but “bombarded” by lower energy electrons. In a series of steps this results in protonation and ionization of the host gas, i.e. for methane (CH₄ to CH₅⁺). This species is reactive and transfers its H+ to a neutral analyte molecule (M) to form MH⁺ (CH₅⁺ reverts back to CH₄)....

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