HPLC Mobile Phase Composition and LC-MS Electrospray Voltage

I am often asked about the importance of selecting and optimizing the LC-MS Electrospray Ionization Interface (ESI) voltage. To better understand why it is necessary to do so and how it effects the results obtained, let us review some key facts about ESI first.

• While a gas sheathed flow of volatile mobile phase is directed into the MS source, a strong positive or negative electric field (KV) is applied across the MS inlet. The effluent is atomized and evaporated to form ions (voltage polarity determines positive/negative mode).
• Too high of a capillary voltage may produce electrical arcing resulting in damage to the system (e.g. PEEK needle may melt, burn and/or clog).
• Too low of a capillary voltage and ion evaporation will not occur.
• The voltage needed to produce efficient desolvation and ion evaporation are directly related to the sheath gas flow rate, the mobile phase composition and the flow rate.

What Can You Do To Insure Finding A Suitable ESI Capillary Voltage?

1. High quality HPLC methods which utilize fully volatile mobile phases and first retain, hold, then elute all samples are needed to generate LC-MS or LC/MS-MS methods. Optimize the HPLC column type, dimensions, MS compatible mobile phase composition and flow rate before optimizing the MS settings. If you have enough sample available, use an infusion method (continuous flow injection) to establish the initial MS settings needed to detect the sample before continuing with the LC/MS method development optimization. Infusion (with a syringe pump) provides the needed time to makes changes, observe how they change the signal for fastest optimization.
2. The HPLC mobile phase and any dissolved additives or buffers used for LC/MS analysis must be of high purity and fully volatile.
3. Make sure your sample is fully dissolved in the mobile phase and filtered (0.22 u filter) before injecting into the system.
4. Basic samples can be protonated to form [M+H]+ clusters in acidic mobile phases.
5. Acidic samples can be deprotonated to form [M-H]- clusters in basic mobile phases.
6. The electrospray ionization (ESI) process used in LC/MS or LC/MS-MS analysis is affected by the surface tension of the HPLC mobile phase used. Water has a higher surface tension than most organic solvents (i.e. Methanol, Acetonitrile, Ethanol, IPA). Using conventional flow rates with highly aqueous mobile phases requires a higher initial voltage for ion evaporation to occur. IOW: Mobile phase mixtures high in water content will require a higher capillary voltage.
7. Higher organic solvent content usually leads to better atomization / droplet formation and require less capillary voltage to maintain.
8. Lower HPLC flow rates usually lead to better atomization / droplet formation and require less capillary voltage to maintain.
9. To optimize the ESI capillary voltage it is necessary to carry out experiments trying different voltages and monitoring the signal (S/N of a standard or sample) to find the best voltage which results in good signal quality and low noise. This process requires experience to know which settings are likely to enhance the signal and a great deal of skill operating the Mass Spectrometer.

Optionally, ESI signal output may be enhanced using: Adducts or changing the solution chemistry with other mobile phase additives.