Electrospray

    The electrospray source consists of two major components: the needle containing the electrospray solution and the capillary that acts as the vacuum interface. Both metalized glass needles (PicoTip, New Objective, Cambridge, MA, 1.2 mm O.D., 2 mm tip) and hypodermic stainless steel needles (0.004 in. O.D., ~70 mm I.D.) glued into stainless steel tubes (1/16 in. O.D. x 0.010 in. I.D.) are used as spray tips. The needle is mounted on an x,y,z translation stage. The glass needles serve as nanospray tips (the needle is the liquid reservoir) and are typically operated without any backpressure. The hypodermic needles are fed by a syringe pump with typical flow rates of 20-50 mL/h. Nanospray appears to be ideal for quick experiments and for peptide work while the syringe pump setup appears to be the method of choice for proteins. The high voltage on the spray needle is typically +500-2000 V with respect to the capillary for nanospray and ~2 kV for positive ion electrospray. The capillary (0.010 in. I.D.) is 3.0 in. long and is inserted into an aluminum block (j) which can be heated to 250 °C. The aluminum block is mounted to a heat resistant PEEK insulator (k), which is in turn mounted to the source flange (d).

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Ion Funnel

    The ion funnel is the interface between the ESI source and the ion drift cell located in the high vacuum chamber. It is a high transmission RF ion guide device that has two functions. First, it compresses the divergent stream of ions leaving the capillary down to a small diameter. Second, it can move the ions from the source region to the drift cell without the use of high acceleration fields, thus avoiding high energy ion-neutral collisions.
    Our ion funnel was modeled after the original designs of Smith and coworkers [2]. In our design there are three individually tunable sections (l, m & n in the figure on top). The first two sections are stacks of 18 and 24 lenses (0.030 in. thick, 2.00 in. O.D.), respectively. The lens holes in the first stack decrease parabolically from 0.87 to 0.14 in. diameter; those in the second stack decrease down to 0.10 in. diameter. The third section consists of 25 lenses (0.030 in. thick, 1.60 in. O.D., 0.16 in. I.D.). The lenses in the first two sections are stacked on six ceramic tubes and mounted on the funnel entrance side to a plastic flange (o). Flange (o) is mounted to the source flange (d) allowing easy removal for cleaning.
    The first 18 lenses in the funnel (with the largest orifices) are spaced 0.100 in. apart using ceramic spacers. These lens hole diameters are nearly identical with those reported in ref. [2]. As the lens orifice diameter decreases to a value comparable to the lens spacing, however, our trajectory calculations carried out using SIMION [3] indicated that ion transmission drops sharply. In other words, the funnel works best when the space between the lenses is smaller than the lens orifices. Also, the orifice size must be larger than the lens thickness to avoid a field free region inside the lens. These effects were found independently in Smith's lab as well.[4] With this in mind, we decided to stack the lenses in section two (m) closer together and to avoid small orifice diameters altogether. Both of these goals were achieved by using 1/16 in. thick "O" rings to space lenses 18 through 42. Since the orifices of lenses 19 through 26 decrease linearly from 0.14 in. to 0.10 in. while lenses 27 through 42 all have 0.100 in. diameter orifices, the orifice diameters are always greater than the lens spacing of 0.06 in. The "O" ring spacers not only provide a closer lens spacing but also prevent radial pumping of gas out of the lens stack. This second effect has two consequences. First, conductance of the viscous gas flow into the next pumping stage (q in the figure on top) is greatly reduced because the flow is through a quasi tube rather than a single orifice. Second, the ions are embedded in a directed flow of gas into the next pumping stage, which should increase ion transmission.
    The final section of the funnel (n) is mounted to the hat flange (p) and is located in the next pumping stage (q). The first 16 lenses are again spaced by 0.100 in. ceramic spacers, while the last 9 are spaced by 3/32 in. "O" rings to decrease the pumping conductance into the following chamber.
    The effect of pressure on the funnel operation is somewhat uncertain. Previous reports [2,4] and our SIMION modeling both indicate that collisions are important in constraining the ions to the center of the funnel and reducing their kinetic energy. In the present instrument, the funnel operates in two pressure zones: the pressure in sections 1 & 2 (directly after the capillary) is ~0.2 Torr; section 3 is ~0.02 Torr. Efficient ion trapping clearly occurs in both sections, thus we can say that pressures between 0.02 and 0.2 Torr are adequate for funnel operation. Increasing the pressure in the first sections (above 0.2 Torr) with N₂ did not improve the funnel performance.
    Three 1 MHz RF generators with individually adjustable amplitudes are used for the three sections of the ion funnel. For each section two outputs are provided, 180° phase shifted from each other. In addition, three individually controllable DC drift voltages are provided as well, the voltage of each section floating on top of the previous section's potential. The appropriate drift voltage is applied to the first and last lens of each section, with the individual lenses being fed by a 1 MW resistor chain. The RF is applied to the lenses via 1000 pF capacitors, alternating the (+) and (-) outputs. The last two lenses in the funnel are independently tunable focusing lenses and do not carry any RF.
    The DC applied to section one (2.23 in. long) is typically 10-50 V, the peak-to-peak RF voltage 100-200 V. The corresponding values for section two (2.26 in. long) are 30-50 V DC and 100-200 V RF and for section three (3.11 in. long) 1-5 V DC and 50-150 V RF. The voltage between the last lens of section three and the drift cell entrance orifice determines the ion injection energy. It is adjustable from 0-200 V and is typically 20-60 V. Higher injection energies can be used to effect structural changes and/or fragmentation of the ions, if desired.
    The next to the last lens in the funnel can be pulsed to gate the continuous ion beam from the ESI source. A voltage high enough to stop the beam is applied to close the gate. Since ions cannot readily escape radially due to the RF trapping nor can they exit back into the source due to the DC ramp applied, they accumulate near the exit of the ion funnel. The gate is opened by pulsing the lens potential down to the normal voltage for approximately 10 μs. For cell drift times in the range of 100 μs to 1 ms a pulsing repetition rate of 1-10 kHz is typically used. The number of ions per second counted after the quadrupole mass filter is approximately the same in continuous and pulsed ion beam mode, indicating a near 100% trapping efficiency in the ion funnel.

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Drift Cell

    The drift cell used in this experiment is very similar to those described previously.[5] It consists of a near cubic copper body (3.5 x 3.5 x 2.0 in., "b" in the figure below), that can be heated by electrical heaters and cooled with a flow of liquid-nitrogen cooled nitrogen gas, a copper end cap (d) with separate temperature control, and a ceramic ring (h) that separates the end cap from the body. The drift field in the cylindrical interior of the cell body is provided by the entrance (f) and exit plates (i) (0.005 in. thick, 0.5 mm orifice) held in place by guard rings at the same potential (0.057 in. thick, 0.600 in. I.D.) and by four intermediate guard rings (e) (0.115 in. thick, 0.600 in. I.D.) equally spaced and mounted on six ceramic rods (j). A precision 1 MW resistor chain connects the rings. The total drift length from entrance to exit plate is 4.503 ± 0.002 cm. The cell temperature is variable from 80 K to above 800 K.
    Typical operating pressures are 4-5 Torr with drift voltages ranging from 2-20 V/cm yielding conditions within the low field limit.[6] Higher pressures and voltages (up to 1000 V across the cell) are possible, however this requires smaller cell orifice holes.

FIGURE: Perspective cross sectional view of the temperature controlled drift cell after Kemper and Bowers' design.[1] (a) cooling line, (b) cell body, (c) buffer gas inlet, (d) cell end cap, (e) drift guard ring, (f) ion entrance hole, (g) ion focusing lens, (h) ceramic ring, (i) ion exit hole, (j) ceramic rod holding guard rings, (k) ceramic rods holding cell assembly.

Mass Analyzer and Detector

    Ions exiting the drift cell enter a 0-4000 amu quadrupole mass filter ("g" in the figure on top, ABB Extrel, Pittsburgh, PA) followed by an off-axis conversion dynode (h). Particles leaving the conversion dynode are detected by a channel electron multiplier ("i", K & M Electronics, West Springfield MA). The TTL signal pulses from the preamp are collected with a multi-channel scaler board (MCSplus, EG&G Ortec, Oak Ridge, TN). The MCS is equipped with a voltage ramp generator, which is used to scan the quadrupole mass analyzer.

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Electronics

    The electronic units necessary to control the ESI source, the ion funnel, the drift cell, and the ion optics (v) in front of the drift cell (three lenses plus x/y-steering), (w) in front of the quadrupole (three lenses plus x/y-steering), and (x) in front of the detector (two lenses plus x/y-steering) were built in-house (the labels v, w, and x refer to the figure on top). The quadrupole mass filter employed here cannot be floated significantly and was thus fixed at ground potential. The voltage on the drift cell exit orifice then determines the ion energy along the quadrupole axis; the cell entrance voltage floats on top of the exit orifice potential and determines the drift potential; the end of the ion funnel floats on the cell entrance potential and determines the ion injection energy. Similarly, the three sections of the ion funnel float on top of each other and the ESI capillary on top of the funnel. The electrospray needle is the last element and is referenced to the capillary. This stacking arrangement is necessary since any change in voltage in the series (in the cell drift potential, e.g.) must be tracked by all the preceding potentials to maintain a constant ion formation and injection environment. If the individual lens supplies were individually referenced to ground, any change in one lens would require resetting all previous lens voltages.
    The potentially high voltages in the ion funnel with respect to ground and the relatively high pressure in the ion funnel make the system prone to discharging. To avoid discharges no grounded metal parts are present inside the ion funnel vacuum chamber and the vacuum flange, support rods, and the support flange for the ion funnel were made of plastic.

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