 |
Electrospray ionization mass spectrometry (ESI-MS) is a technique that
uses high voltage to generate ions from an aerosol of charged liquid
droplets. Traditionally, commercial ESI mass spectrometers utilize flow
rates from tens of microliters per minute (10 µL/min) to milliliters per
minute (1 mL/min). Because of the relatively large volume of liquid
exiting the emitter, aerosol formation must be assisted by pneumatic
nebulization and/or by thermal heating in the effort to obtain a stable
spray. This requirement is especially pronounced for highly aqueous
liquids.
When the flow rate is reduced to nanoliters per minute (nL/min), droplet
formation occurs more readily, requiring only the applied voltage to
generate spray. No sheath gas or additional heat is required.
Consequently, the stability of spray, and therefore signal, at the lower
flow rates is typically improved for aqueous or "salty" mobile phases.
Working at the lower flow rates of nanoliters per minute is commonly
referred to as "nanospray" and has become a popular method employed in
protein analysis. Low flow ESI is especially tolerant to a wide range
of liquid compositions, and can even spray "pure" water with a high
degree of stability.
The efficiency of ionization improves as the flow rate is lowered
because less volume of mobile phase passes through the emitter,
producing smaller aerosol droplets. The lower flow rates in a nanospray
technique also allow for a longer length of analysis time. This
provides ample time to perform novel mass spectrometer scan functions to
obtain structural information of an analyte. Nanospray also provides
for the direct coupling of nanoscale chromatographic methods, thus
signal robbing dilution by a sheath or make-up liquid is eliminated.
If you are currently doing ESI-MS at flow rates of milliliters per
minute, we encourage you to try nanospray to improve your ion signal.
Take a look through our ESI Resources and Tech Support Sections for
information on getting started with nanospray.
Reference: Covey, T. R.; Devanand, P. Nanospray Electrospray Ionization
Development: LC/MS, CE/MS Application. Practical Spectroscopy Series,
Volume 32: Applied Electrospray Mass Spectrometry; Pramanik, B. N.;
Ganguly, A. K.; Gross, M. L., Eds.; Marcel Dekker: New York, NY, 2002.
 |
