Improved Scan Speed with a High PowerMagnet Field Regulator

SummaryThis note describes the implementation and performanceof a high power magnet field regulator for the ThermoScientific ELEMENT 2 that enables significantly fastermagnet field jumping. For a ELEMENT 2 fitted with theoriginal magnet field regulator, the specification for the magnet jump m/z 23 - 240 - 23 was < 355 ms; with the new regulator the time for the magnet jumpm/z 7 - 238 - 7 is improved to < 150 ms - an increase in

speed of more than a factor of two. With this improve-ment in magnet jump speed, the scan duty cycle (the ratioof analysis time to total measurement time) for a full massrange analysis method can be increased to > 70 %. Thisnew magnet field regulator is now fitted as standard in theELEMENT 2 and Therno Scientific ELEMENT XR - allother instrument performance figures remain unchanged.

Introduction Double focusing magnetic sector mass analyzers arewidely accepted as the basis for high-performance ICP-MS systems as they offer several important advantages to theanalyst when compared to quadruple based systems:

• unambiguous removal of interferences through the useof high mass resolution

• high sensitivity

• low and uniform background noise

However, there has been a common perception thattheir magnet scan speed limits their use in applicationareas that require rapid full mass range analysis oftransient signals, for example when interfaced with eitherlaser ablation or chromatographic sample introductionsystems.

In order to improve the scan speed of the ELEMENT 2,a new high power magnet field regulator was developed[1],that is now introduced into the standard system.

There are two methods of varying the focused masswith the double-focusing sector field ELEMENT 2: witheither the Electric or Magnetic sectors. The principles ofthese two modes are outlined below.

Electric Mode In this scan mode, the magnet field strength is kept constant while the acceleration voltage is varied to 'jump'to points in a mass range. During the electric scan, themagnet remains stationary at a fixed mass called the Magnet Mass. In summary:

• It is extremely fast: the time required to change theacceleration voltage to move between masses - theelectric settling time - is 1 ms, independent of the massrange jumped.

• It is limited in mass range: the electric 'jump' range islimited to + 30% of the magnet mass, at which point the magnetic field has to be changed again. In normaloperation, only small areas across the top of each peakwithin the + 30% range need to be acquired, not theentire peak. Typically, only a 5% peak width is requiredin low resolution mode - equivalent to 'peak-jumping'.

Magnetic Mode The magnet 'jumps' between masses by changing thecurrent in the magnet's coils at a constant accelerationvoltage. In summary:

• It is slower than electric scan mode and the timerequired to magnet 'jump' from one mass to another - the magnet settling time - is dependent on mass range'jumped'.

• It is unlimited in mass range: the magnet can 'jump' the full mass range from e.g. 7Li to 238U directly.

In order to perform analysis over mass ranges widerthan that accessible by a single electric scan, a combi-nation of the two modes is routinely made. An example of such an analysis is shown in Figure 1.

Key Words

• ELEMENT 2

• Chromatography

• High ResolutionICP-MS

• Laser Ablation

• Magnet Scan Speed

ApplicationNote: 30074

Figure 1: Combination of electrical and magnetic scan modes used for theanalysis of twenty-one isotopes. An analysis of 25 ms was made at eachisotope. Note that the analysis of barium and the rare earth elements (138 to 175 a.m.u.) is achieved with a single electrical scan.

All analysis (across a peak) and mass selection (frompeak to peak) is preferentially made through changes inthe acceleration voltage (electric scan mode) while themagnet field remains fixed. When the measured massexceeds + 30 % of the initial magnet mass, the magnetfield changes. From this new magnet mass, a new series ofelectrical scans is made. This process is repeatedsuccessively across the required analysis mass range.

The absolute mass range accessible to an electricalscan increases with increasing magnet mass. In Figure 1,Ba and the rare earth elements (139La – 175Lu) aremeasured with a single electrical scan at a constantmagnet field.

Implementation of New Magnet Field RegulatorPower Stage The standard magnet field regulator provides a singlesupply voltage for both modes of magnet operation: thestatic and dynamic. In the static mode, the ohmicresistance of the magnet coils limits the current that canbe applied through them. In the dynamic mode, thecounter voltage generated by induction in the coils limitsthe increase of current through the coil. For the newmagnet field regulator in the Thermo Scientific ELEMENT 2, the supply voltages for the two magnetoperation modes have been optimized for their specificrequirements:

• In the static mode, the supply voltage is the same as inthe standard power regulator to minimize the power dissipation in the field regulator power stage.

• In the dynamic mode, the supply voltage is more thandoubled allowing a faster change of the magnetic fieldand therefore the mass measured.

Performance of New Magnet Field Regulator

A. Full Mass Range Jump 7Li - 238U - 7Li Figure 2 shows the variation of coil current (equivalent tomass) with time for two complete magnet cycles of themass range m/z 7 - 238 - 7. At each mass, an analysis of 5 ms duration is made. The time for the magnet jump m/z7 - 238 is 75 ms, while the return jump from m/z 238 - 7is 50 ms, for a complete cycle time of 125 ms. With thestandard Thermo Scientific ELEMENT 2 regulator, thetime required for the shorter magnet jump m/z 23 - 240 -23 was 355 ms. Therefore, the new high power magnetfield regulator leads to a factor of three improvement inmagnet scan speed over the full mass range relative to theprevious standard ELEMENT 2 configuration.

Figure 2: Variation of magnet coil current with time for two cycles of the mass range m/z 7 - 238 - 7. An analysis of 5 ms wasmade at each mass. The total magnet delay time with the high power magnet field regulator for this cycle is 125 ms.

B. Full Mass Range Multi-Elemental Analysis For the analysis of transient signals across the full massrange, the fastest magnet and electrical scan speeds arerequired in order to process accurately the time dependentsignal analyzed. This is particularly important for laserablation analysis where a low mass element (usually a major element, for example Ca) is often used as aninternal standard to correct for ablation efficiency between different samples.

An example of such a full mass range laser analysismethod is shown in Figure 3, a screenshot from theThermo Scientific ELEMENT 2 Method Editor. The scanduty cycle, the percentage of each scan acquiring datarelative to the total scan time (i.e. analysis plus magnetand electrical delay times), is calculated for this method to be 76%. Using this method, thirty full mass rangeanalyses can be made in twenty seconds.

Conclusion A new high power magnet field regulator is now standardon the Thermo Scientific ELEMENT 2 and ELEMENTXR. With this new regulator, magnet cycle times for thefull mass range analysis m/z 7 - 238 - 7 of < 150 ms areachievable - over twice as fast as previously possible withthe standard regulator. With this new regulator, samplemeasurements can now be made that were previouslylimited by magnetic scan speed. All other instrumentparameters are unaffected.

References[1] Wills J.D., Hamester M. and Pesch R., Full mass range multi-elementalanalysis with a rapid scanning single collector sector field ICP-MS-high speedhigh resolution 2000 Winter Plasma Conference on Plasma Spectrochemistry,Fort Lauderdale, FL, USA.

Full mass range: m/z 11 - m/z 238Twenty-one analytes; six magnet masses

= 168 ms = 525 msDuty cycle = % (525/(525 + 168))

= 76%

Figure 3: Screenshot from Element Method Editorshowing the isotopes analyzed and the associated measurement and delay times.

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