MECHANICAL

Comstock  miniTOF^® II is comprised of a vacuum vessel with a linear TOF spectrometer inside, a PC system meeting today’s standards, an external power supply box, and vacuum pumps. Vacuum seals are Viton O-rings.  (Seals of other materials may be used.  Contact factory.)

Location of Major Components    The spectrometer itself is attached to the inside of the front panel of the vacuum vessel for convenient removal, and a small satellite power supply is on the other (air) side past electrical feedthroughs.  The main power supply is in a metal box, normally placed underneath the vacuum vessel.  The Time-to-Digital Converter, control firmware, and digital as well as analog interfaces are on a single specially designed PC card and housed in the PC tower.  A turbomolecular drag vacuum pump(70 l/s)  is also attached to the vacuum vessel. An oil free diaphragm foreline pump is normally placed on the floor.  A port is on the vessel for an optional vacuum gauge.

Spectrometer dimensions are approximately 22” (D) x 12” (W) x 20” (H), excluding PC, CRT and pump.

ELECTRONICS

Power Supplies  Generates all the voltages and currents necessary to operate the spectrometer.  A number of these voltages are singly or doubly regulated for the precision operation of the system.

Time-to-Digital Converter    Resides on a PCI card housed in the PC.  It converts the time elapsed between repeller time and detection at the end of the flight into a digital form, when the information is transferred from the clock to a FIFO memory.  Multiple stops are allowed to each start.  One count is recognized for each of the stops regardless of the amplitude of the signal from the detector so long as it exceeds the discriminator level.  The time data stored in the FIFO memory is transferred to the PC memory on firmware command.  The  time resolution of the clock (bin width) is 10 nanosecond. Time count is 32 bit long.  The event count capacity used in the PC is 64 bits.

PC Interface/Controls    Digital and analog I/O interface ports are available on the PC card, most of which are used for the systems purposes.  The use of extra ports for additional functions are possible but requires special engineering and programming.  Contact factory.

Safety Interlocks    In order to safeguard the spectrometer components inside the vacuum vessel, a special sensor in the turbomolecular pump signals the pressure inside the vessel.  Voltages are disabled until a proper vacuum is reached.

SPECTROMETER

General    The spectrometer is a linear, orthogonal injection, electron impact ionization Time-of-Flight spectrometer. The entire instrument, from sample intake to ion detector and gas exhaust is carefully designed and carefully machined for optimum space and time focusing.  Standard sample inlet is an undeactivated 14 mil OD capillary in a Cajun connector mounted on the front panel of the vacuum vessel that provides a pressure of approx 5x10^-5 Torr to the TOF.  The capillary is a standard GC capillary.  (Other sample inlet arrangements are possible.  Contact factory.)  The flight path is about 40 cm from repeller to detector.  The detector is a discreet dynode type for sensitivity and ruggedness.

Operation    Operating parameters are set on the PC screen and communicated to the power supplies via the interface board.  Filament current and detector bias are monitored back on the PC screen so that the operator can be assured that there is no significant deviations.

Upkeep   The entire spectrometer is mounted on the vacuum side of the front panel to ensure easy access.  Electron emitting filament is positioned by two screws that are easily accessible.  It takes less than 30 minutes from the time the vacuum pumps are turned off until they are turned back on with a new filament inside.

PERFORMANCE*

Mass Resolution    Typical M/dM resolution as it is tested at the factory is about 1,200.

Mass Range    1 ~ 1200 nominal. 

Stability  Peak shift < 0.02% over 24 hours. Peak broadening not observed over 24 hours. 

Minimum Detection Limit  5 ppm without pre-concentration.  The limit is a function of measurement time (= number of sweeps) and the gas mixture injected.

Dynamic Range  The dynamic range is a function of measurement time and discriminator setting, among other parameters.  Due to the nature of TDC counting, the dynamic range, defined as the maximum count that can be stored in the system divided by the least significant count, can be an extremely large number.  The maximum event counts (the vertical scale on the spectrum display) is 2^64.

Mass Calibration    Carried out on quadratic model based on three or more data points.  Single set of coefficients over the mass range.  Calibration coefficients could change significantly when detector voltage is changed.  Operating software issues a warning in a pop up window when it sees an old mass calibration.  Operator defines how old is old. When the software does not see calibration performed for that voltage, it refuses to convert the time scale (horizontal) to mass scale.

SOFTWARE

Labview®    The programs provided with the system are written in LabView® language under Windows OS.  They cover set up of operational parameters, data acquisition, data handling between TDC and PC and inside PC, display, and mass calibration. A hook to the search engine for NIST’s MS library as well as access to the library data itself are available. If an additional function is needed, contact factory.