LOW-COST MICROFLUIDIC EMITTERS FOR NANO-ELECTROSPRAY IONIZATION-MASS SPECTROMETRY

We report a new microfluidic emitter for nano-electrospray ionization mass spectrometry (nanoESI-MS), made from polyimide tape. In contrast to the conventional pulled glass capillary emitters typically used for nanoESI-MS, these new polyimide emitters are simple and inexpensive to make, and are disposable after a single use. Thus, sample contamina- tion and clogging are never a problem when analyzing different samples. The new emitters have comparable performance to pulled glass emitters, and are applicable to a variety to sample types and volumes, with the advantages of low cost, ease of use, and disposability. described in the literature, many of which are formed by microfabrication and are often coupled with microfluidics. 5 Al- though these emitters have found utility in academic laboratories and constitute an active field of research, the majority suffer from the significant costs incurred in microfabrication, which inhibits widespread use. Here we report a new low-cost emitter for nanoESI-MS, formed by folding a thin film of polyimide tape into a cone with a micron-sized orifice at the apex. These new emitters are easy to make and do not require any specialized equip- ment to fabricate or use. The low cost of these emitters allows for single use, such that sample carry-over and clogging are not a problem. The new emitters show comparable performance to pulled glass emitters and are applicable to a varie- ty of sample types and volumes. These emitters are a low-cost alternative to glass capillary emitters for applications in- volving single use direct analysis by nanoESI-MS. EXPERIMENTAL Each emitter was formed by folding a piece of polyimide tape (5 cm x 5 cm) into a cone shape such that there was a small opening at the apex of the cone (Figure 1a). The diameter of the orifice from which the electrospray emanates is controlled by how tightly the tape is folded, and can be tuned from ~35 �m by inducing significant torque when folding, to greater than 200 �m if loosely formed into a cone. Smaller orifice diameters result in smaller charged droplets in the electrospray, thus increasing detection sensitivity. Therefore, polyimide emitters were generally folded with small orifice diameters, in the range of 35-55 �m (Figure 1a inset). Figure 1: (a) Picture of a polyimide nanoESI emitter formed by folding a piece of polyimide tape into a cone (main pan- el), and picture at 10x magnification showing the orifice from which the electrospray is formed (inset). (b) Taylor cone and electrospray generated by applying +2.40 kV to a solution of 4:1 acetonitrile (ACN)/water (H2O) (v/v) contained in the tip of a folded nanoESI emitter. The spray voltage is applied via a platinum wire in contact with the solution to be sprayed, and the tip is positioned ~3 mm from the grounded inlet of the mass spectrometer. Folded polyimide emitters were positioned ~3 mm from the grounded inlet of the mass spectrometer. Solutions to be analyzed were pipetted into the tip of the emitter through the rear (wide) side of the cone. For most emitters, spray vol- tage was applied via a platinum wire (100 �m diameter) inserted in the re ar of the emitter, making contact with the solu- tion. The Taylor cone and electrospray generated from an emitter with a ~50 m orifice operated in this mode is shown