Flow Rate Determination of the Nanoflow Sheath Liquid CE–MS‐Coupling Applying the <i>nanoCEasy</i> Interface

In recent years, nanoflow sheath liquid (nanoSL) interfaces have been used more commonly for capillary electrophoresis-mass spectrometry (CE-MS) coupling due to their high sensitivity combined with flexibility in CE separation conditions. So far, the exact amount of sheath liquid (SL) and, thus, the dilution effect remain unknown due to the self-supplying type of the nanoSL interfaces. To quantify the SL flow rates, we present here an approach for the determination of the flow rate based on isotopically labeled standards using the nanoCEasy interface. SL and pumped CE flow are spiked with atrazine and d5-atrazine, respectively, enabling the determination of the total flow rate for different electrospray (ES) conditions. Generally, flow rates increase with higher ES voltages. Only at high ES voltages, larger emitter sizes lead to higher flow rates; for low voltages, flow rates are independent of emitter size. Additionally, smaller emitter sizes can generally be operated at lower ES voltages. Visualizing the ES, higher flow rates lead to a larger spray angle. Higher flow rates may decrease the signal intensity by diluting the CE-separated analytes. Indeed, this effect is observed for the flow-injected atrazine analytes and in the CE-MS analysis of an amino acid standard mix under aqueous acidic separation conditions. Therefore, lower flow rates are preferred for low electroosmotic flow (EOF) applications. However, in high EOF applications, higher voltages are required to prevent the accumulation of aqueous background electrolyte (BGE) at the emitter tip, which otherwise leads to peak broadening and inefficient ionization. Overall, the presented data enable the selection of stable and sensitive settings for nanoSL interfaces.