ESI-MS Concentration or Mass Flow Dependent

Other developments of the first nano-ESI experiments (Wilm 1994,1996) have improved on the overall absolute ionization efficiency e  

Thus 8 measures the product of the ionization efficiency (fraction of analyte molecules delivered as ions to the entrance from the API source into the MS) with the transmission efficiency of ions from the MS entrance aperture to the detector. The first experiments, conducted at flow rates of tens of nL.min, gave 8 values of about 1/1000 (Wilm 1994), later improved to 1/390 (Wilm 1996) using a quadrupole mass spectrometer operated with very high peak widths to improve transmission efficiency the sampling efficiency 

Most recently (Schneider 2006) an experimental source was used to conduct studies under conditions of total solvent consumption , with pneumatically assisted nebu-lization to stabilize the ESI process, a heated laminar flow chamber to enhance desolvation and ion production, and various atmosphere-to-vacuum aperture diameters to maximize ion transfer. The motivation for these experiments was to investigate the proposal that the reason for the much lower ionization sampling efficiencies at higher flow rates ( o,L.min and above) is that the electrosprayed droplets are much larger in view of the much larger ESI needle tip diameters required to maintain flow rates in this regime, and thus are much less efficiently evaporated down to the Rayleigh and/or ion evaporation limits than the droplets formed from the 1 (tm diameter tips nsed in nano-ESI (Juraschek 1999 Schmidt 2003). 

Under these ideal conditions, favorable compounds (e.g., a long chain quaternary ammonium compound) showed ionization sampling efficiencies of 70-85% at flow rates in the range 50-500 nL.min , although compounds with less favorable characteristics or solutions with high aqueous content gave lower efficiency values. However, all sampling efficiencies were far superior to 

Thus far the discussion has involved mainly constant infusion experiments rather than LC/MS. In the latter case, flow rate is generally varied via the diameter of the HPLC column or by post-column splitting of the effluent before transport to the ESI sprayer. In the case of conventional ESI sources and mobile phase flow rates, the apparent concentration dependent response will apply 

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