PlasmQuad 3, ICP MS, Inductivity Coupled Plasma Mass Spectrometer

• Unit includes:
• PQ3 Instrument inc power leads
• Nenlab Chiller + leads
• Cetac ASX500 Autosampler (+ transformer)
• Dell Optiplex computer with pre loaded software, keyboard and mouse
• Hewlett Packard HP5 Printer
• Plasmalab Software
• All existing manuals
• Spray Chamber, Torch instrument and Cones
• Spare spray chamber, torches and cones. Cone extraction tool
• Peristaltic Pump

Hardware· Detection The detector is a unit, which is sensitive to charged species. Ions, which hit the surface of the detector, release electrons, which cascade down the body of the unit releasing more. The result is for a single ion hitting the top of the detector; a relatively large number of electrons are released from the bottom as a pulse. These pulses of electrons can be counted individually or can be integrated with time such that electrical currents created by the flow of electrons can be measured.· Filtering The mass filter, a quadrupole in the VG PlasmaQuad 3, sorts the ions according to mass to charge ratio, only allowing specific masses to be transmitted to detector.· Focusing Once in the high vacuum region of the mass spectrometer, the sample ions are accelerated and focused into mass filter by a set of charged plates or ion lenses. These lenses consist of stainless steel plates or tubes with precise and controllable DC voltage, which attract or repel the sample ion beam. Sampling The ions are introduced into a mass spectrometer via a pressure interface. The interface operates at a low pressure, usually maintained by a mechanical rotary pump, and is water-cooled to prevent the high temperature source damaging components. Ionisation Samples are sprayed into high temperature inductively coupled argon plasma. In the high temperature of the plasma, the samples are atomized and ionized.SoftwareThe PlasmaLab software suite has been designed in such a way that the user, whether experienced or inexperienced is presented with all the information required to make appropriate decisions on how to control the ICP-MS and collect or manipulate the data for reporting electronically or by printing. The instrument itself communicates to the software through the ICP Spectrometer Control Service, which is accessed through the icon on the start bar. All of the data is stored in a database. The database contains all information related to the experiment.

An Inductively Coupled Plasma (ICP) as used in the VG PlasmaQuad 3 is an exellent ion source. Its high operating temperature breaks down most matrices and will ionize most elements making it ideal for inorganic mass spectrometry. Also, as the iron source operates outside the vacuum system, sample introduction is more simply when compared to alternative methods of mass spectrometry.

The population of single charged species available in the plasma will differ according to the ionization potential of the element. The ICP is an excellent ionization source, and ever element with an ionization source, and every element with an ionization potential below 7.5 eV will be essentially 100% ionized. The degree to which various species are ionized is roughly described by the Saha equation.

The ICP consist of three distinct components:

§ The RF generator § The matching network (or torchbox) § The gas control

The degree to which various species are ionized is roughly described by the Saha equation. Bearing in mind the uncertainty in the values of temperature and electron density and the fact that thermodynamic equilibrium is not present, the degree of ionization can be estimated.

RF generator

The generator is a free-standing crystal controlled solid state unit located beneath the torch box. It can generate up to 2000 W at the industry standard frequency of 27.12 MHz, and the power is stabilized to better than 1%. The frequency is crystal controlled and the output power is achieved by connecting a number of driver stages in series. The final stage uses an array of high power FET transistors with forced air cooling, and the output circuits is tuned so that maximum efficiency is obtained operating into a 50 ohm resistive load. Switching the unit on and off is automatic, and software controls allow the RF output power level to be adjusted.

Gas Control

The gas panel provides the precise control and monitoring of the torch gas flows required for the ICP. The three mass flow controllers (MFC) are provided as standard for optimum stability. The longitudinal plasma position is established primarily by the load coil and the gas flows. The cool flow must be sufficient to prevent the outer tube of the torch from overheating, generally 121/min or greater. The auxiliary flow tends to push the plasma as a whole forwards.

The nebulizer flow rate has most effect on the analytical performance. For a given torch injector diameter, the gas velocity increases with flow rate, and hence the residence time for samples in the plasma decreases. Ion energy is observed to increase with nebuliser flow and the levels of refractory oxides, doubly charged ions and background plasma species are all affected. A valuable feature of the VG PlasmaQuad 3 (as its predecessors) is that a standard flow rate of around 0.701/min has been found optimum for a wide range of applications and sample introduction methods.

The standard spray chamber is a Scott double pass unit, made from quartz, however, if very corrosive solutions are being run a Teflon or Kel-F device (part of the optional Inert Sample Introduction System (ISIS)) is available. The spray chambers ensures that only the smallest solution droplets reach the plasma, so keeping the loading of the plasma is cooled significantly. Only about 1% of the sample is actually transported to plasma, and some improvement can be afforded with the optional Mistral or ultrasonic nebuliser.

Interface

The interface is crucial to the performance of the ICP-MS instrument. Its function is to transfer a representative sample of the plasma ion population to the high vacuum chamber where the ion focusing lenses and quadrupole can function.

The Cones

The standard cones supplied with the VG PlasmaQuad 3 are a sample cone with 1.0mm aperture and microskimmer with 0.7mm aperture.

The Lenses

The electrostatic lenses are mounted into the lens stack. The lens stack consists of a number of stainless steel elements on the system axis each connected to a separate stable DC voltage supply. The voltage on each lens element can be adjusted via the software.

Quadrupole

The is a mass filter, which can be controlled to allow only defined mass to charge ratios to pass through, and on to the detector. The quadrupole works by producing an electric field which allows stable trajectories for ions having a narrow range of charge to mass ratio. In normal use, the VG PlasmaQuad 3 mass filter needs no direct operator input, and there is generally no need to alter controls such as resolution or delta M. The quadrupole rod consists of 4 precisely aligned molybdenum rods, 220mm long and 12 mm in diameter. The VG PlasmaQuad 3 uses a discrete dynode detector. Discrete dynode detectors operate on the principle of electron multiplication; an ion striking the first dynode releases electrons which accelerate towards and into the dynode beneath which in turn releases more electrons. The electron cloud cascades down the body of the detector increasing at every dynode. The resulting electron cloud can be counted as a single pulse at the base of the detector.

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

§ A pulse counting and analogue electron 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 107 Hz) § Mulitiplier 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 an analogue device. To 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 arrival 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 amplifier is converted to a regular pulse train with a frequency of up to 2MHz by voltage to frequency converter (VFC)

Vacuum System

A good vacuum is essential in maintaining the high performance of an ICP-MS instrument. The high voltages in the quadrupole and detector dictate a low operating pressure in order to remove the risk of electrical discharges; but most importantly, the ion beam would be severely disrupted were it not in a vacuum. The VG PlasmaQuad 3 has a pair of 250 L/sec turbomolecular pumps, backed by a two stage 12m³/hr rotary vane unit. The interface is pumped by a 28m³/hr rotary pump. The three stage vacuum system is divided into the expansion (or interface), intermediate and analyzer regions.