Analytical chemistry
Ionization techniques (ESI, MALDI, EI)
An ion source converts neutral analyte molecules into gas-phase ions while retaining enough structure for measurement.
IntuitionIntuition: the measurement idea
A mass spectrometer sees only ions. Different ion sources transfer molecules into the gas phase in different ways: EI fragments, ESI transfers molecules without many fragments, MALDI gives mainly singly charged ions.
SchoolSchool level: signal and result
Ion sources differ in energy transfer and pressure regime: EI is hard and fragments organic molecules; ESI and MALDI are soft and preserve large molecules as protonated or charged clusters.
Definition:
An ion source converts neutral analyte molecules into gas-phase ions while retaining enough structure for measurement.
First identify the measurand, choose a signal that responds to it, and compare the sample with a calibrated standard or a validated model.
Example: Worked analytical example
Calculate the analyte result from the stated measurement and method relation.
Solution
A 12.36 kDa protein carrying 15 protons appears near m/z=(12360+15×1.0073)/15 ≈ 825.0 as a multiply charged ESI ion.
| Stage | Purpose |
|---|---|
| Prepare | Control matrix and contamination |
| Measure | Acquire a calibrated response |
| Validate | Check recovery and uncertainty |
UndergraduateUniversity: quantitative method
Electron ionization creates radical cations and reproducible fragments; electrospray creates multiply charged ions from solution; MALDI embeds analyte in matrix crystals and produces mainly singly charged ions after laser irradiation.
Calibration, selectivity, sample preparation and uncertainty belong to the method itself, not to afterthoughts. Report units, conditions and the calibration range.
References
- Mass Spectrometry: A Textbook, 3rd ed. · Jürgen H. Gross, 2017
- Fundamentals of Contemporary Mass Spectrometry · Chandra Dass, 2007