Firma:              VG

Modell:            Autospec

Kommentar:   Dokumente engl.

INTRODUCTION

The VG Analytical AutoSpec is a fully integrated mass spectrometer using the VG Opus data system to complement the automated, fast scanning, high sensitivity mass spectrometer. The system utilises the VG SIOS inteligent interface.

OPUS

Opus is based on a DEC VAXstation high performance desktop workstation making full use of the powerful VMS operating system and having the following attributes:

The full functionality of the present DS 11-250 with an increased user friendliness.

Total control of all aspects of MS tuning and data acquisition, including real time changing of acquisition mode.

The capability to share and/or pass data between appropriate processing facilities within the system.

An easily learned command file and batch processing facility.

The 32 bit VAX architecture, combined with the latest dedicated graphics Chip-set, produces unrivalled power for mass spectrometer control, data acquisition, and flexible data processing.

A large Winchester disk drive provides an excellent medium for bulk storage, and the streaming cartridge tape unit is a convenient vehicle for data archiving.

SIOS

SIOS embodies in one compact unit, more than sufficient computing power for the most arduous mass spectrometry operations. An excepionally high dynamic range combined with super fast analogue to digital conversion, ensures data integrity under all operating conditions.

Dedicated microprocessors, controlled by an integral supermicrocomputer, have been assembled to produce an expandable parallel processing system. The tasks of interactively, or automatically, controlling the instrument, and data acquisition are all performed optimally.

The acquisition processors handle time-critical operations, such as peak detection and mass assignment, in real time, whilst the multitasking supermicrocomputer performs data evaluation and interpretation operations.

ANALYSER

The Analyser has a three sector, double focussing geometry of the type ESA, Magnet, ESA. This geometry enables a compact machine to be constructed with a dispersion to magnification ratio of 0.82m, and a transmission factor of 0.42 about two times better than comparable instrumentation. The high resolution of the analyser has been achieved by geometric design with essentially zero image aberrations and manufacture to very high mechanical standards.

By using specially designed lenses in the analyser AutoSpec achieves high energy focussing throughout the mass range of the instrument without recourse to mechanically moving the magnet as in conventional instruments. This factor is important for accurate mass measurement.

The source and analyser are mounted on an optical bench which is isolated from the main frame of the mass spectrometer by air pads, thus ensuring a vibration-free environment. The electric sector plates are precision mounted onto a plinth an the bench, and from this the various lens elements are mounted, giving good optical alignment with respect to the electric sector plates. This arrangement enables the lenses to be positioned at precisely the correct place along the optical axis so that the performance of each lens is optimal. Also the lens elements are large with large apertures to reduce the possibility of contamination and transmission loss. The overall transmission of the instrument is enhanced by the addition of Z-focussing elements before and alter the magnetic sector.

The Z length through the flight tube is also large, ensuring maximum transmission through the analyser. Image curvature and rotation lenses are also used to maximise sensitivity at high resolution.

The vacuum housings are of an 'open plan" design and the analyser is pumped by two 280 I sec ^-1 diffusion pumps situated adjacent to each of the electric sectors which gives unrivalled performance for the abundance sensitivity of the instrument.

The three sector geometry offers further advantages in addition to the excellent sensitivity and resolution capabilities. For normal mass spectra, metastable peaks are not observed.

These metastable ions, which have a lower energy than the Ion transmitted from the source are rejected by the second electric sector. Linked scans, an forward or reverse geometry instruments are prone to extraneous artifact peaks formed from further decomposition within the field free regions of the instrument. Scanning the two electric sectors together eliminates these artifact peaks.

The instrument has the capability of MIKES using an optional floatable gas cell situated before the second electric sector. This in conjunction with the optional hybrid accessory allows high energy (E0) scans to be studied.

MAGNET

The magnet has a sector angle of 35°, and a radius of 381 mm. The mass range at full accelerating voltage (8kV) is 3000 Daltons. The laminated design enables the magnet to achieve a reproducible cycle time of five decade scans/sec under GC conditions. Also the defocussing effects are eliminated even when scanning at 0.1 sec/decade.

The fast response time of the magnet enables rapid positive/negative switching an alternate scans even under capillary column GCMS.conditions.

The focussing properties of all magnets change with scanning. This has been compensated for in the design of an electrostatic Y lens system which is programmed from the Hall probe and maintains the beam in focus whilst the magnetic field is changed.

ION SOURCES

The ion source incorporates two assemblies. An inner, easily removable volume which comprises all the normally cleanable or replaceable parts, the filament, trap and repeller for an EI/CI source. This inner volume may be removed and replaced within 30 seconds. The outer block comprises the source heater, thermocouple and other generally non-replaceable items. This block is located and electrically connected via simple spring contacts, and may be removed and replaced within minutes.

For routine techniques, including EI, Cl, and alternating EI and CI (ACE) a simple inner volume exchange enables these Ionisation techniques to be realised. For other techniques such as FAB and Thermospray, the outer block is changed. The inlets to the source comprise three ports. The eluent from the G.C. is carried co-axially with the CI line and reference inlet to a single re-entrant jet. Opposite this is a vacuum lock for introducing the solids probe. At the front of the source housing is a further lock for introducing the source volume.

The source ion exit plate is situated sufficiently distant from the object slit to eliminate contamination of this slit due to the sample. The source is pumped using a 700 I sec ^-1 diffusion pump. This high pumping capability, coupled with the differential pumping across the source slit, make AutoSpec ideally suited for techniques requiring high gas loads, e.g. Thermospray or Dynamic FAB. An Isolation valve between source and analyser, and a pneumatically operated butterfly valve above the source diffusion pump allow source block changeover to be simple, quick and straightforward.

DETECTOR

The detection system is the (Patented) VG photomultiplier dual detector. This combines two optimised non switchable conversion dynodes with an ultra long life photomultiplier to provide excellent positive and negative ion performance, with the most reliable permanent high gain collector commercially available. Mass discrimination effects encountered with conventional post acceleration dynode detectors are eliminated with this device.

BAKING

For continued ultimate performance, mass spectrometers need to be occasionally baked, the electric sector plates and flight tube especially needing this treatment. However, it is inconvenient to move the magnet to bake the flight tube, since this also necessitates considerable instrument re-tune after baking. The AutoSpec flight tube is baked with the magnet in place, heat being generated through the flight tube via a resistive heater. The electric sector plates are baked using internal heaters. The mass spectrometer can be programmed from the data system terminal to bake at regular intervals, e.g., overnight when the instrument is in standby.

ELECTRONICS

The electronics for the control of AutoSpec are situated in the mass spectrometer bench and under the GC table. Electronic component layout is an large printed circuit boards (280mm x 230mm), which are accessible from the front of the instrument. Comprehensive diagnostic circuitry has been designed into the electronics.

The Mains Distribution Unit provides power to all of the other units, as well as supplying mains power to the vacuum pumping system. This unit also controls the "Standby" and "Operate" functions of each unit.

The High Voltage unit supplies the source (Focus -2) and the Y-focus and Z-focus lenses. The source control unit, which has its own microprocessor, supplies the source accelerating voltag, Focus -1 voltage and controls the vacuum system. Voltages for the photomultiplier detector are provided by a separate supply. Situated an the main bench, the slave and control units for the magnet provide a maximum of 110 amps to drive the magnet, control the positive/negative ion switching capability and incorporate the magnetic field measuring circuitry.