Mass Spectrometry

Mass Spectrometry

A mass spectrometer is a chain of electric fields, and the rails that make them differ widely in what they draw and what they tolerate. A time-of-flight liner or reflectron at 5 to 10 kV draws almost nothing and tolerates ripple in proportion to the resolving power asked of it. An electron multiplier or conversion dynode at 2 to 10 kV draws microamps and is sensitive to noise on its rail. Lens stacks and the DC offsets of a quadrupole are set in volts, to a fraction of a volt, at no current, and the quadrupole itself runs on RF. The high-voltage work around the instrument is different again: conditioning electrodes, qualifying feedthroughs and insulators, and burning in assemblies on a production line all draw real current and ask for a supply that can source it, survive a flashover and come back up at a controlled rate. Figure 1 draws the ripple floor against the peak widths.

Ripple reaches the spectrum through ion energy. In a linear time-of-flight stage the flight time varies as the inverse square root of the acceleration voltage, and the mass is computed from the square of the flight time, so a fractional ripple on the voltage appears one for one as a fractional spread in the computed mass. That turns the datasheet ripple specification, 0.01 percent of nominal voltage plus 100 mV peak to peak, into a mass budget: on the Berkeley Nucleonics HVX-10000-200 the floor is 1.1 V on 10 kV, or 110 ppm, which is one-sixth of the peak width of an instrument resolving m/Δm = 1,500 (670 ppm) and larger than the peak width at m/Δm = 10,000 (100 ppm). Table 1 gives the same figure for each voltage class. Drift enters the same way: stability of 0.01 percent of nominal over 8 hours and a temperature coefficient of 0.01 percent per kelvin are 100 ppm each on the mass axis.

The table says where the line is. An HVX-Series supply holds a time-of-flight acceleration or liner rail for an instrument whose resolving power is a few thousand, and it is not the supply for one resolving ten thousand or more, where the ripple floor alone is a full peak width. Lens stacks, quadrupole DC offsets and multiplier bias draw microamps, below the 0.01 percent of nominal at which the current loop engages (20 µA on the HVX-10000-200), and are set in steps the 16-bit resolution of a kilovolt-class supply cannot reach (153 mV on a 10 kV model); they are served by the instrument's own low-current bias modules. Where an analyzer rail needs 10 ppm stability, the sector and high-resolution case, the Berkeley Nucleonics PNChp precision supply is the right tool, with stability of 0.001 percent of nominal over 8 hours and ripple of 0.001 percent of nominal plus 50 mV.

Electrode conditioning. A new or vented analyzer is brought up in voltage until microdischarges stop, and the supply has to deliver current into each discharge, limit it, and recover. The current limit is set in 16-bit steps from 0.01 percent of nominal, Arc Detection reports each flashover and can switch the output off, and Ramp Control raises the voltage again at a set gradient from 1 V/s upward. The time-tagged event log records every arc, which is the conditioning record.

High-voltage qualification and burn-in. Feedthroughs, insulators and source assemblies are held at a test voltage above their working voltage, a 10 kV feedthrough at 20 kV for instance on the HVX-20000-25, with the current limit protecting the item and the actual current shown on the display. On a production line the same supply steps through the test sequence over Ethernet with SCPI, and the 2U enclosure (89 mm high, 534 mm deep overall) racks into the test station.

Rails referenced away from earth. The HVX-1500-1400 FLO and HVX-1500-2000 FLO are floating-output models for a stage that cannot share the instrument's ground reference. The datasheet does not state how far they may be referenced from earth; confirm that figure against the stage's potential before ordering.

Two options matter here. Arc Detection detects a flashover, reports it and can switch the output off. Ramp Control sets the gradient of every rise and fall from 1 V/s to ten times the nominal voltage per second. Both are field-retrofittable. Ethernet and RS232 with SCPI, the external interlock input and the time-tagged event log are standard, and each unit ships with a 3 m high-voltage cable.

Limitations. The ripple floor of 0.25 V to 3.1 V peak to peak by model is fixed by the specification and is filtered at the load where a quieter rail is needed. The smallest current the supply regulates is 0.01 percent of nominal, so a microamp load runs in constant-voltage mode with the current limit as protection only. A polarity reversal waits for the output to discharge, 60 s typical and up to 120 s at no load.

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