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.
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.