Offline Methodology Static Nanospray

In comparison to the sophisticated engineering associated with ESI-MS at conventional (mL/min) flow rates, static nanospray is a method of elegant simplicity. Wilm and Mann [8] first observed that the reduced diameter of the ESI emitter to the micrometer scale reduces sample consumption to a flow rate typically within 20-40 nL/min and generates signal-to-noise (S/N) levels comparable to conventional ESI. Furthermore, the electric field that is generated by the high voltage within the source creates sufficient electrostatic pressure to pull the mobile phase through the emitter orifice and obviates the need for any additional pumping. In this pure electrospray mode, sample consumption is self-regulating; the volume consumed is near optimal for ESI source operation.

Interestingly, the implementation of nanospray was strikingly similar to the first published observations of electrostatic spraying of liquids by Zeleny in 1914 [18]. This self-limiting characteristic gives static nanospray its principal analytical advantage: reduced sample consumption (by 100:1 or more) without any significant loss or compromise in MS ion intensity. For example, at 20 nL/min a sample will yield a signal for roughly one hour before expiration. The extended run time yields sufficient signal for even the most ambitious tandem mass spectrometry (MS/MS) or MS" experiments. Extended run time gives the static format nearly universal application across a broad range of MS analyzers, which include triple-quadrupole, time-of-flight (TOF), ion-trap, Fourier-transform (FT), and hybrid instruments. The key feature of static nanospray, an ultralow flow rate, illustrates the behavior of ESI-MS as a (primarily) concentration sensitive detector [19]. Concentration-dependant sensitivity is the foundation for the success of ESI miniaturization, since sample volumes can be reduced by orders of magnitude (to the nanoliter/picoliter scale) without concurrent loss of S/N.

A schematic of a typical nanospray source and photomicrograph of a nanospray emitter are shown in Figure 1.1. The commonly accepted scheme for static nanospray employs a glass needle or nanovial. The typical nanospray emitter is fabricated from borosilicate, aluminosilicate, or quartz tubing with a large internal bore (0.5-1 mm) that is finely tapered at one end to a 1-4^m inside-diameter (ID) opening. Electrical contact with the mobile phase, a necessary requirement for operation, is usually made through a conductive coating that is applied to the outside of the emitter or via a platinum or gold wire inserted within. MS data from a typical glass nanospray tip of a peptide standard solution is shown in Figure 1.2. The sample loading requirement is typically 0.1 to 2 |^L, and the needles are normally used once per analysis, since adequate rinsing of the small orifice is not practical. Flow

Picture Nanospray Needle

FIGURE 1.1 (A) Schematic of a typical static nanospray apparatus that uses a glass needle tapered to a 1-4-|im ID tip. The high voltage for ESI is either applied to a conductive coating on the tip, or placed inside the tip with a metal wire. (B) Reflected light photomicrograph of a spray plume from a 4-|im ID nanospray tip at approximately 60 nL/min. The ESI spray plume is visible on the right-hand side of the image. (Photograph courtesy of New Objective, Inc.)

FIGURE 1.1 (A) Schematic of a typical static nanospray apparatus that uses a glass needle tapered to a 1-4-|im ID tip. The high voltage for ESI is either applied to a conductive coating on the tip, or placed inside the tip with a metal wire. (B) Reflected light photomicrograph of a spray plume from a 4-|im ID nanospray tip at approximately 60 nL/min. The ESI spray plume is visible on the right-hand side of the image. (Photograph courtesy of New Objective, Inc.)

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