Parylene electrospray tips

Microfabricated Parylene Electrospray Tips Integrated with Cyclo-Olefin Microchips for ESI-MS... 

The integration of a single parylene electrospray tip with the cyclo-olefin polymer chip is illustrated in Figure 6.3B. First, a microfluidic channel (20 pm wide, 10 pm deep) was formed in one cyclo-olefin polymer substrate by the... 

The ability of the parylene electrospray tip to confine the liquid enables the production of high-density ESI tip arrays without cross-channel contamination. 

We further investigated the utilization and performance of parylene electrospray tips integrated with cyclo-olefin microchips for quantitative mass spectro-metric determination of the small drug molecule methylphenidate (Ritalin). 

The microchip design is shown in Figure 6.11 A. It contained two independent microfluidic channels and corresponding parylene electrospray tips. The two channels on the microchip are closer at the tip end with a 1 mm separation, while they are separated by 2 cm at the inlet end. The purpose of this design was to make alignment of parylene tips to the channel exit easier while the Nanoport connectors can still be accommodated at the input. The total length of the channel is 2.5 cm and the depth 20 to 25 pm. The width of the channel at the sprayer end is 20 pm while it is 60 pm at the reservoir end. The purpose for different microchannel widths was due to the need for different requirements from each end. Manual drilling was used to make the reservoir holes after... 

Parylene C was chosen to fabricate the triangular electrosprayer tip because it was easy to deposit and could be patterned using photolithography and plasma etching. In addition, its resistance to most organic solvent makes it suitable for electrospray applications. Other materials of similar structure should have comparable properties. 

Electrospray tips from SU-8 photoepoxy have also been fabricated. Nevertheless, solvent resistance of polymer-based substrates is a critical issue, with poor resistance resulting in the swelling and distortion of the channels. A comparison between the performance of fused silica, parylene and borosilicate glass is discussed in Licklider etal. [10]. 

We also studied12 the relationship between the thickness of the tip (apex angle = 90°) and the electrospray performance and found that the parylene thickness does not affect the spray performance. Spray performance from three tips was compared with different thicknesses (3, 5 and 7 pm) whose sizes were the same as the triangular tip in Figure 6.4. The monitored total ion currents for these three tips were 35.5 1.2 nA for the 3 pm thick tip, 35.3 1.5 nA for the 5pm thick tip and 35.8 1.4nA for the 7pm tip. Liquid droplets were confined to the tips in all cases. This indicates clearly that only the top surface of the triangular tip is involved in the ESI activity. 

Polymeric microfluidic systems coupled to a microfabricated planar polymer tip can be used as a stable ion source for ESI-MS. A parylene tip at the end of the microchannel delivers fluid which easily produces a stable Taylor cone at the tip via an applied voltage. The described device appears to facilitate the formation of a stable spray current for the electrospray process and hence offers an attractive alternative to previously reported electrospray emitters. When this interface was employed for the quantification of methylphenidate in urine extracts via direct infusion MS analysis, this system demonstrated stable electrospray performance, good reproducibility, a wide linear dynamic range, a relatively low limit of quantification, good precision and accuracy, and negligible system carryover. We believe polymeric devices such as described in this report merit further investigation for chip-based sample analysis employing electrospray MS in the future. 

---