Tips for Electrospray Ionization LC–MS

Here are some great tips for optimizing liquid chromatography (LC)–electrospray ionization (ESI)–MS so that you achieve the best possible results every time. These tips and tricks have been collated by my colleagues to form a beginner’s guide to LC-ESI–MS, and as a primer for those who are already using the technique.

Optimize the Sprayer Voltage

This is often a parameter that is set and never adjusted again. That may work the majority of the time, and can certainly be a good approach if a variety of users with a myriad of samples are using an instrument; however, it is not going to be ideal for all analytes. If you are setting a voltage for a walk-up and use an open-access instrument, it is a good idea to use lower electrospray voltages. However, taking time to adjust the sprayer voltage can lead to vast improvements in MS sensitivity.

Optimize the Sprayer Position

The position of the sprayer is a parameter that can and should be optimized, but its optimal setting is likely to be one that is viable for providing high signal quality across a wide range of analytes. Analytes with varying surface activity (sensitivity constant K) will exhibit different responses depending on the position of the sprayer relative to the sampling cone. Primarily, this is due to the time taken for the analyte to be liberated into the gas phase under the atmospheric pressure ionization (API) conditions (primarily the drying gas temperature and flow rate) that will differ for differing analytes depending upon their ability to migrate to the droplet surface, and be liberated into the gas phase as well as the eluent composition at time of elution. At lower concentrations, any change in the sprayer position will cause the relative response of analytes to change.

Reduce or Eliminate Salts

The formation of metal adduct ions (such as [M+Na]⁺ and[M+K]⁺)in positive ion mode is a common phenomenon that can result in only the metal adduct ion being present in the MS spectrum. Therefore, we need to be aware of, and remember to look for, these species. It can also be advantageous to try to avoid the formation of adducts altogether, as their formation can lead to unusable mass spectra.

These types of adducts can be avoided by using plastic instead of glass vials. The glass manufacturing process uses a variety of metal salts that can be leached from the glassware by aqueous solvents. However, the use of plastic vials can also lead to the presence of plasticizer peaks within spectra, although these tend to be less problematic, as they appear at fixed m/z values and can be discounted when interpreting spectra. Also, note that high pressure liquid chromatography (HPLC) solvents, such as acetonitrile, can often contain surprisingly high amounts of metal ions such as sodium—choose your solvent grade carefully!

Select an Appropriate Solvent

For ESI, reversed-phase solvents (water, acetonitrile, methanol) are preferable, as they favor the formation and transfer of ions from the liquid to the gas phase. Normal-phase solvents (hexane, toluene, dichloromethane) cannot support ions in solution, and are, therefore, not suitable for use with ESI (they can, however, be used with atmospheric pressure chemical ionization [APCI]).

When optimizing ESI applications, the surface tension of the solvents should be considered. Solvents with low surface tension (such as methanol and isopropanol) allow for stable Taylor cone formation, and, hence, a stable and reproducible electrospray. The Rayleigh limit will be overcome at lower potential, and this will tend to lead, on average, to smaller droplets being produced, which aids in the ion formation process, and can lead to an increase in instrument sensitivity.

If a Little Bit Works, a Little Bit Less Will Probably Work Better!

While so much about modern API-MS has been developed to the point of “load and forget,” and, in most cases, this approach will enable us to generate a signal for many analytes, we sometimes forget that a certain degree of optimization will generate data that is far superior to that obtained under “generic” conditions.

This is an excerpt of a blog entry, “10 Great Tips for Electrospray Ionization LC–MS.” See the full blog entry on our website (www.chromatographyonline.com) for additional tips and more detailed explanations.

Tony Taylor is the Chief Science Officer of Arch Sciences Group and the Technical Director of CHROMacademy. Direct correspondence to: LCGCedit@mmhgroup.com.