Electroless ionization for MS
Ionization for mass spectrometry has never been so easy. The small disposable ELI-nozzle makes a fine spray that is automatically charged due to the interaction of the liquid with the nozzle. A mass spectrum is obtained by simply spraying towards the MS-inlet.
How it works
The ELI-nozzle exploits the electrokinetic interaction of the solvent with the nozzle surface for charging, and uses micrometer sized jets to disperse the sample. The mechanism of charging is well understood and gives excellent soft ionization with most commonly used solvents. By applying sufficient force to the syringe, a spray is formed that generates a cloud of ionized particles. A typical measurement only consists of: filling the syringe with the sample, fitting it to the ELI spray nozzle, and spraying the sample in the direction of the MS-inlet.
Download here a small manual how to use the ELI-nozzle.
The set-up and sample preparation
To spray your sample we recommend the use of 1 mL BD Plastipak syringes or an HPLC pump. In case of the use of a syringe, the sample can be sprayed manually or by using a syringe pump. The sample preparation is the same as for other MS applications. That is: the sample should not contain too much salt, for the ELI-nozzle the salt concentration should not exceed 10**-4.5 M. Similarly, the pH of the sample liquid should be between 4.5 and 9.5. Furthermore, samples should be free of particles, the nozzle contains a build-in filter, though poorly prepared samples could lead to clogging. Once your sample is ready, the spray should be directed towards the MS-inlet or sideways with a distance of 5-10 cm (See figure). Luer fit nozzles should be attached firmly to prevent the nozzle from coming off when the syringe is pressurized. A detaching nozzle can indicate clogging, check your sample and use a new nozzle.
Positive and negative charging, explained
The electroless charging of the ELI-nozzle relies on the interaction of the charge transfer layer (present inside the nozzle) with the sample liquid. To be able to change the polarity of the charging, from positive to negative or vice versa, for the same liquid, a different nozzle with a different charge transfer layer needs to be used. As for most solvents the charging polarity is the same with the same nozzle, the nozzle types are therefore designated as either “positive” or “negative”. In some cases however, such as anhydrous methanol or ethanol, the charging is opposite of the nozzle type label, that is negatively charged droplets when the nozzle type is positive. We therefore recommend to always use a water/ethanol mixture or water/methanol mixture with at least 25% water by volume. The following table shows the nozzle type and the solvent mixture with the charging polarity, and recommended solvents for optimal performance.
| Solvent/Solvent mix | Nozzle type | Spray polarity (+/-) | Recommended |
|---|---|---|---|
| Water (demineralized) | Positive | + | ✔ |
| Acetonitrile | Positive | + | ✔ |
| Water/Ethanol mixture (%Water ≥ 25) | Positive | + | ✔ |
| Water/Methanol mixture (%Water ≥ 25) | Positive | + | ✔ |
| DSMO | Positive | + | |
| Ethanol (anhydrous) | Positive | - | |
| Methanol (anhydrous) | Positive | - | |
| Water (demineralized) | Negative | - | ✔ |
| Acetonitrile | Negative | - | ✔ |
| Water/Ethanol mixture (%Water ≥ 25) | Negative | - | ✔ |
| Water/Methanol mixture (%Water ≥ 25) | Negative | - | |
| DSMO | Negative | - | |
| Ethanol (anhydrous) | Negative | + | |
| Methanol (anhydrous) | Negative | + |