MALDI and ESI are now the most common ionization sources for biomolecular mass spectrometry, offering excellent mass range and sensitivity.
| Ionization Source | Ionization Event |
|---|---|
| ESI | Evaporation of charged droplets |
| nanoESI | Evaporation of charged droplets |
| MALDI | Photon absorption/proton transfer |
The most important considerations for both MALDI and ESI are the physical state of the analyte and the ionization energy. Both instruments can produce positive or negative ions.
Importance of vacuum
A good vacuum is needed to allow ions to reach the detector without undesirable collisions. Unwanted collisions would result in reduced resolution and sensitivity. A vulnerable spot is the point of sample insertion. ESI uses capillary column to maintain vacuum. MALDI evacuates the sample chamber with a vacuum lock.
| ESI | MALDI |
|---|---|
| Flow technique (LC,CE) | Pulse technique |
| Not very tolerant to salts (better off-axis) | More tolerant to impurities (wash) |
| Multiple charging: complex but useful | Generally singly charged |
| 1 fmol/ul possible with NanoSpray | Can consume less sample |
| Very high dynamic range | Lower dynamic range |
Actual results depend on sample, impurities and mass analyzer. Avoid or minimize salts, chaotropes, detergents, polymers, and non volatile compounds.