Reproducibility in fabrication and analytical performance of polyaniline-coated nanoelectrospray emitters

Nanoelectrospray ionization mass spectrometry is an ideal technique for analysis of biomolecules when sample quantities are limited. With the use of this technique, 1-2 microL of sample can be electrosprayed for long time periods (hours) because of the low flow rate (nanoliters per minute) attainable. However, the long-term durability of such emitters has been an impediment to the routine use of nanoelectrospray. The development of longer-lasting nanoelectrospray emitters has often resulted in increasingly complex and tedious fabrication processes. Furthermore, an easily produced, reproducible, and durable nanoelectrospray emitter is the ultimately desired goal. Here, the reproducibility of the inner diameters and geometry for nanoelectrospray emitter glass substrates is assessed using scanning electron microscopy (SEM). The results indicate that provided that glass pulling parameters remain constant, reproducible inner diameters can be produced from glass capillary tubing within the same batch; however, there are interbatch differences. In addition, SEM revealed reproducible taper geometry could also be obtained. Borosilicate and fused-silica nanoelectrospray emitters produced by these protocols were then coated with polyaniline, and their analytical figures of merit were determined using a triple quadrupole mass analyzer. Over a 1-h run, polyaniline-coated emitters showed fairly stable signal with coefficients of variation ranging from 8.92 to 27.6%. Single-scan detection limits below 1 amol were achieved for polyaniline-coated fused-silica emitters for flow rates averaging <10 nL/min. Linear mass spectrometric response with solution concentration was observed for the polyaniline-coated emitters over the range 10 nM-10 microM, with coefficients of variation ranging from 1.44 to 7.26%. This indicates that when nanelectrospray emitter inner diameters are made reproducibly, it is possible to achieve linear quantitative response for nanoelectrospray.