manufacturer :       VG Elemental

model :                   ICP/MS Plasma Quad ExCell

annotation :           Dokumente engl .

Technical Description

Instrument overview ( PlasmaQuad System)

The PlasmaQuad is a powerful, inductively-coupled mass spectrometer developed for the analysis of a wide variety of samples.

The high performance quadrupole analyser exhibits good transmission characteristics, combining optimal sensitivity and resolution with high abundance sensitivity, giving minimal interelement "cross-talk". Different parts of the spectrometer are held at different pressures to ensure correct operation of the plasma sampling interface and quadrupole mass filter. A combination of rotary and turbomolecular vacuum pumps is used to maintain these pressures.

The physical interface between the plasma and the mass spectrometer comprises two cones. The first cone, designated the sampling cone, is used to sample the ions created in the plasma; the second cone provides an ion skimming function. A series of cylindrical electrodes hehind the cones are designed to extract and focus Ions from the interface into the mass spectrometer. A range of alternative sample introduction facilities can be used with the PlasmaQuad , including laser ablation techniques.

Ions of a selected mass-to-charge ratio are transmitted by the quadrupole mass filter to a system used to detect the number of ions transmitted, and to measure and display this information in a number of ways. The ion detector comprises a channel electron multiplier with suitable amplification facilities. Pulses from the detection system are supplied to a microprocessor-controlled multi-channel scaler (MCS) data acquisition unit.

Overall control of instrument operation is provided by another microprocessor. The entire system is controlled via a standard communications link by a PC running special PQ Vision software. The accumulated data from the MCS is also transferred over the communications link for storage and farther processing.

Technical Details (VG PlasmaQuad )

The ions transmitted by the quadrupole are detected by the detector assembly. The full system is

• a pulse counting and analogue eIectron multiplier assembly
• a high voltage supply
• a head amplifier assembly comprising pulse counting amplifier and associated comparator
• an analogue amplifier and associated VFC (voltage to frequency converter)
• signal routing multichannel scaling cards in the control unit ratemeter with meter range selection (10² Hz to 10 ⁷ Hz)
• multiplier protection circuits including trip indicators and manual reset ion beam defocusing grid voltage supply for multiplier protection

The detector is a dual simultaneous pulse counting and analogue device. For the trace analysis which is one of the great strengths of the ICP-MS technique, the multiplier is operated in the pulse counting mode. This allows individual ion arrivals to be recorded. For a large working range the pulse counting mode is augmented by a less sensitive analogue gain mode. Data is collected simultaneously in both the pulse counting and analogue modes, into the MCA. In analogue mode the output of the analogue amplifier is converted to a regular pulse train with a frequency of up to 2MHz by a voltage to frequency converter (VFC).

The output is sent from die head amplifier assembly to the scaler in the VG PlasmaQuad 3 control unit as a differential pair via a twisted pair transmission line. This gives good noise immunity.

A positive ion striking the first dynode causes the ejection of one or more secondary electrons from the surface, which are accelerated down the tube. Collisions with other dynodes liberate further electrons and as the gain for this process is generally greater than unity, the avalanche builds up exponentially. Towards the bottom of the detector, the avalanche process reaches saturation. The space charge of electrons that builds up during the pulse becomes sufficient to inhibit further rnultiplication , so that the output pulses tend to be grouped around a standard size (about 15 picocoulombs ).

With these high electron fluxes, residual gas is likely to be ionised . High electron currents will ultimately damage individual dynodes, so a charged grid mounted within the body of the detector can stop the build up of large fluxes. If it were not for the orientation of the dynodes, these ions might be accelerated back to the entrance to start another avalanche and the process, called ion feedback would repeat, giving a very long output pulse. Even with this detector design, it is important not to operate the multiplier with a high residual gas pressure (such as would be the case if insufficient pumpdown time was allowed) because die ionic feedback mechanism will produce an undesirably high gain.

The cloud of about 10 ⁷ to 10 ⁸ electrons leaves the base of the channel and is collected by the collector electrode. A decoupling capacitor isolates the - 150V DC voltage at the exit of the multiplier from the pulse counting amplifier.

Ultimate detection limits are determined only by the counting statistics of the random background counts. These result when multiplier electron avalanches are initiated by species other than the analyte ions of interest. This background rate can be as low as 5 Hz, but does vary considerably from one individual multiplier to another. A discriminator threshold level may be set which eliminates electronic noise almost entirely, while passing most of the real pulses. This is achieved by a comparator which compares the output of the amplifier with a reference level sent from the VG PlasmaQuad 3 control unit.

The allowed discriminator levels 1 to 255 correspond to the range 0.08 to 20mV. The required value can be found by operating the instrument with all noise sources on (ICP, quadrupole , computer) and pulse counting mode selected, but with the high voltage off. As increasing discriminator settings are sent from the data system, the electronic noise level should be seen to drop to zero on the ratemeter . Typical values to achieve this lie between 10 and 30. A trial scan in pulse counting mode with the chosen discriminator level set and with the multiplier HT disabled should show no counts.

Having set the discriminator level, it is then possible to set the correct operating voltage for pulse counting operation. With a modest ion flux (a few tens of kHz) incident on the detector and the pulse counting mode selected, the voltage can be carefully increased and the signal observed. The choice of operating voltage is a compromise between achieving best multiplier lifetimes, which implies tower settings, and keeping the pulse height distribution well above the discriminator level so that excessive counts are not lost when the gain is suppressed by high count rates, which implies higher settings.

Technical Data (12000 ICP Power Supply Operator’s Manual)

Output Frequency:

27.12 MHz

Frequency Stability:

0.005% Short Term

RF Power Output

2000 Watts into a 50 Ohm load

Output Impedance:

50 Ohms +/- 5 Ohms

Metering Accuracy:

Forward: +/- 1% Full Scale, +/- 3% of reading

Temperature Coefficients:

0.025% per °C

Output Connector:

Type N female

Power Stability:

0.1% Long Term @ 25°C typical

Regulation:

-45dBc into 50 Ohm load

Line Regulation:

No change from 198-250VAC.

Load Tolerance:

No oscillation or failure into a mismatch

Noise, Hum, and Ripple:

-45dBc

Harmonie Distortion:

All harmonics less than -50dBc

Spurious Radiation:

Meets or exceeds FCC and VDE regulations

Operating Temperature/Humidity:

0 to +40°C, 10% to 90% relative humidity, non-condensing.

Power Required:

180-250VAC, 47-63Hz, Single Phase,

20Amps maximum, 3100 Watts maximum,

Power factor: 0.99

Over Current Protection:

Circuit breaker, 20 Amp over-current, 1000 Amp interrupt capacity

Overall Dimension, inch (cm):

8.75 (22.23) High x 19 (48.26) Wide x 19(48.26) Deep, including fan

Weight:

60 LBS / 27.2Kg

Cooling:

Air, 235 CFM fan below 11,000 feet

Power Cord:

Standard: 12 AWG 600V type SOW, length: 6 feet / 183crn (custom cord configurations available)

Power Cord Connector

Standard: 20 Amp 250V twist-lock, male, NEMA type L6-20P (custom connector configurations available)

Analog Interface Connector:

15-pin female D-subminiature connector.