Having it all and having it now

When it comes to improving productivity and precision when using an ICP-MS, real-time simultaneous detection of all isotopes is vital. Here we look at the design and applications of a fully simultaneous ICP-Mass Spectrometer

In most commerciallyavailable ICP-MS systems, the mass spectrometer is “scanning” to filter ions of different mass/charge ratios forcing them to arrive at the detector one at a time. With such instruments, be they quadrupole or magnetic sector designs, there is a small but finite time interval between the arrival of ions with different mass/charge ratios at the detector. Any fluctuations due to the sample introduction system or plasma “flicker” occurring during this interval can therefore introduce measurement errors or appear as “noise” on the signal. With sequential instruments, transient signals, as with hyphenated techniques such as LA-ICP-MS, may not be accurately tracked, a problem known as signal “skew”. These problems could be solved if all isotopes were measured simultaneously. Until recently, only limited simultaneity was available in mass spectrometry by either using so-called multi-collector instruments (capable of measuring a limited number of isotopes simultaneously) or time-of-flight (TOF) analysers (these have a limited duty cycle: only very short “time slices” of the continuous ion beam produced by the ICP source are measured). The only real solution is fully simultaneous and real-time continuous detection of all the isotopes of interest. One spectrometer configuration with the potential to overcome all the problems associated with “scanning” spectrometers is a double focussing sector field mass spectrometer in “Mattauch-Herzog” geometry (Figure 1).

This geometry uses an electrostatic analyser (ESA) followed by a magnetic sector. The ESA separates ions based on their kinetic energy and the magnetic sector on the basis of their mass/charge ratio. All the ions are focused simultaneously by the magnet onto the same focal plane, but resolved spatially. This is achieved without varying either the voltages applied to the ESA or the strength of the magnetic field, i.e. without scanning. Until recently, however, there has been no commercially available detector capable of detecting and measuring this spatially resolved mass spectrum. Photographic plates have been used, but these have obvious limitations. In a new spectrometer the Mattauch-Herzog geometry is combined with a novel array detector, positioned in the spectrometer focal plane that measures the entire mass spectrum simultaneously. This detector, the direct charge detector “Ion 120”, is a 12cm long CMOS (Complementary Metal Oxide Semiconductor) array that covers the whole mass range from Li to U with 4800 separate detector elements. With 4800 elements covering the mass range from~5 to 240 amu, every mass unit is on average covered by 20 separate detectors, resulting in a true mass spectrum rather than a single point for each amu. The basic principle of the detector is similar to that of a Faraday Cup: when a charged ion arrives at the detector, it is discharged by receiving an electron, generating a detector signal. This detector is described as a Direct Charge Detector (DCD) because every ion arriving at the detector contributes to the signal. Each detector element incorporates separate high- and low-gain areas, allowing each channel to independently handle a wide range of signal levels. This, combined with sophisticated readout electronics that keeps the detector working within its linear range, can give an overall linear dynamic range of more than 8 orders of magnitude.

Advantages of this new spectrometer/detector system include: •    Elimination of noise from the sample introduction system. •    Improved precision.

•    Improved productivity: the complete mass spectra of up to 100 samples per hour can be recorded with no limitations on the number of elements or isotopes measured. •    Element ratios (internal standards, isotope ratios) are calculated on measurements obtained fully simultaneously and therefore under identical conditions. •    Complete mass spectrum from every

measurement: method development can be conducted after the sample is measured to: -    See unexpected interferences -    Detect unexpected elements -    Make interference corrections after the measurement is completed -    Review spectra of samples that no longer exist

References:[1] M. Resano, K.S. McIntosh, F. Vanhaeche, J. Anal. At Spectrom 27, 165-173 (2012)