1948
Anal. Chem. 1988, 60,1948-1952
Improved Electrospray Ionization Interface for Capillary Zone Electrophoresis-Mass Spectrometry R i c h a r d D. Smith,* Charles J. Barinaga, a n d Harold R. Udseth
Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory, Richland, Washington 99352
A new electrospray lonlzatlon Interface for caplllary zone electrophoresis-mass spectrometry (CZE-MS) Is descrlbed. The Interface uses a sheath flow of liquid to make the electrlcal contact at the CZE terminus, thus defining both the CZE and electrospray fleld gradients. Thls allows the compodtlon of the electrosprayed llquld to be controlled Independently of the CZE buffer, providing operatlon wlth buffers that could not be used previously (e.g., aqueous and hlgh Ionic strength buffers). The Interface operation Is Independent of CZE flow rate; CZE capillarles are easily replaced and requlre no additional preparation. Slnce the electrospray occurs directly from the CZE caplllary termlnus, additional mlxlng volumes and metal surfaces are avolded and electrophoretlc separation eff lclency appears unperturbed. The dead volume a m clated wlth the electrospray interface is < I O nL, correspondlng to 30000 L/s. The analyzer quadrupole chamber was pumped a t 500 L/s with a turbomolecular pump. A single ion lens with a 0.64-em aperture separated the ion focusing and analysis quadrupole chambers. The pressures in the focusing and analysis chambers were -1 X 10-6and 2 X l(r7Torr, respectively. The countercurrent flow of N, (at -10 'C) for desolvation of the electrospray was in the range of 3-6 L/min. The mass spectrometer (Extrel Co., Pittsburgh, PA) had a range of m / z 2ooO. ESI Interface Design and Construction. Figure 1 shows a schematic illustration of one version of the liquid sheath electrode ESI interface developed in this work. The ESI probe body was machined from a polycarbonate rod and mounted on custom holders that are movable on a small optical bench rail. A 0.16 em 0.d. Teflon tube that contains the CZE fused silica capillary and the sheath electrode liquid was connected via a Teflon tee outside the probe body (not shown). The polycnrbnate "tip holder" carries the electrospray electrode fabricated from a %-gauge (0.25 mm i.d., 0.46 mm o.d.), 3.3 cm long stainless steel ( S S ) tube soldered into a 1.9 cm long, 21-gauge (0.51 mm i.d., 0.81 nun 0.d.) SS tube. The ESI end of the SS electrode was machined to an -45" taper and then electropolished. As the tip holder is screwed into the probe body, the S S electrode slides over the protruding fused silica CZE capillary, making contact with the spring-loaded high-voltage connector and snugly fitting into the central Teflon tube. The position of the CZE terminus capillary relative to the SS electrode is easily adjusted by sliding the capillary in the aforementioned Teflon tee. An auxiliary sheath gas flow capability was added, in part, to prevent any deleterious effectsdue to heating at either the sheath electrode (due to high CZE currents) or the CZE capillary (due
Flgure 2. Photograph of a stable electrospray from a fused
silica
capillary.
to the countercurrent flow of heated nitrogen) although under ordinary operating conditions this provision is not required. The central axial channel contains six 0.16 em 0.d. Teflon tubes that carry nitrogen or oxygen (at 0.1-1 L/min) for the probe gas sheath. Two 0.16 cm i.d. holes drilled into the back half of the tip holder are combined into a single coaxial channel that surrounds the ESI source a t the front half of the holder. These serve to direct approximatelyone half of the gas delivered by the six Teflon tubes through the holder and over the ESI tip. The rest of the gas flow hack through the body in the spaces between the tubes. An additional electrode, a 0.5 m long, 11-gauge (2.4 nun 1.d.. 3.2 mm 0.d.) SS tube, is mounted around the FSI tip in the central channel of the tip holder. It directs the coaxial gas flow over the tip and is held at the ESI potential by a spring connector touching the SS electrode (see Figure 1). RESULTS A N D DISCUSSION
ESI Interface Operation. The key improvement provided by the interface shown in Figure 1 is a much broader range of CZE operating conditions that may be adapted to mass spectrometric detection. For example, the interface allows operation at CZE flow rates previously too small for stable ESI performance (
