Peak jumping

V(Mt) represents the mass bias when all the masses are peak-jumped in the same cup, such as with the standard procedure still employed on all single-collector mass spectrometers. If we assume that mass i is collected in cup a and the bias V(M) follows the exponential law, we obtain the expression for the measured ratio of masses i to k as... 

Tests on copper SRMs show that a stable signal is obtained when each isotope is measured using the peak jumping mode and one point per peak, with a dwell time of 25,000 ps. The quadrupole mass spectrometer scans three times the mass range per replicate and accumulates 10 replicates for a total acquisition time of about 1 minute. For this application, 22 isotopes were selected (Table I). 

Figure 3 Flat-topped peaks obtained with magnetic sector instrument are advantageous for longer term reproducibility when analysis is performed by peak jumping because precise mass calibration is unnecessary. Also peak overlaps are simpler to deconvolute because the peak tops coincide over a more extensive mass.

VG Isotopes Ltd., is another leading manufacturer of ICP-MS equipment. The special features of their VG Plasmaquad PQ2 includes a multi-channel analyser which ensures rapid data acquisition over the whole mass range. The multi-channel analyser facilities include 4096 channels, 300 m facility for spectral analysis, user-definable number of measurements per peak in peak jumping mode, and the ability to monitor data as they are acquired. A multi-channel analyser is imperative for acquiring short-lived signals from accessories such as flow injection, electrothermal vaporisation, laser ablation, and so on, or for fast multi-element survey scans (typically 1 minute). 

Continuous scanning of (a part of) the mass spectrum or peak hopping or peak jumping is accomplished by changing these voltages in such a way that the U/V ratio remains constant. The spectral peak width is constant over the mass range (mass resolution, on the other hand, varies as a function of the mass considered). 

With no stable isotope pair within the U system or a suitable AME, a standard-sample bracketing protocol is usually employed to correct for mass bias. Human urine generally contains very low concentrations of U (generally 1-5 ng/L), so an isotope dilution strategy is required, together with ion-counting detection (ideally a Daly photomultiplier or discrete dynode secondary electron multiplier) and a multi-static (rather than multi-dynamic) peak-jumping routine, for precise measurement of the total U concentration and the minor isotopes of and even... 

Low level data acquisition is the same for peak jump and scan and only differs for continuous and TRA in the handling of the per sweep data. The first step is to convert the acquisition regions, which have been specified in terms of mass, into terms that the acquisition electronics can handle. The mass value of each chaimel in the specified regions is converted to a digital value that the quadrupole electronics can handle by using a mass calibration equation. Once this conversion has been done, the acquisition electronics can acquire a sweep of data. At the end of this sweep the data handling differs depending upon whether this is a continuous acquisition or TRA. 

After the conversion of the analog count data to equivalent pulse count rates, the data can be integrated. The integration technique for peak jump is slightly different from that used for scan. 

For peak jump data the integrated count rate for an analyte is the average count rate of all of the channels in the region defined for that anal3de. 

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