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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 Tech Support Section 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.