Flat-top peak

So how does the IRMS get its stability Collector slits are several times the width of the ion beams. This gives a flat-topped peak shape (Fig 6) which makes the ion current intensive to drift. The main source of drift is temperature variation which both affects the electronic components used for mass selection and caused expansion and contraction of mechanical parts. Simultaneous measurement of ion beams using a double or triple collector is more precise than sequential measurement by mass scanning with a single detector. Finally, frequent comparison of sample gas under identical conditions also contributes to stability. Ion beam stability is more important than resolution for isotopic measurements. 

The MC-ICP-MS consists of four main parts 1) a sample introduction system that inlets the sample into the instrument as either a liquid (most common), gas, or solid (e.g., laser ablation), 2) an inductively coupled Ar plasma in which the sample is evaporated, vaporized, atomized, and ionized, 3) an ion transfer mechanism (the mass spectrometer interface) that separates the atmospheric pressure of the plasma from the vacuum of the analyzer, and 4) a mass analyzer that deals with the ion kinetic energy spread and produces a mass spectrum with flat topped peaks suitable for isotope ratio measurements. 

Fig. 2.14. Influence of the reverse activation energy on KER and thus, on peak shapes in metastable ion decompositions, suitable experimental setup as prerequisite. From left no or small reverse barrier causes Gaussian peak shape, whereas medium or yields flat-topped peaks and large For causes dish-shaped peaks.

Peak flatness only at m/Am = 300 flat top peak also at high resolution plateau flatness at high RP ... 

Required precision. This will lead you to the instrument you need. Quadru-pole ICP-MS is easy to use, robust and relatively inexpensive. In general, these instruments permit good precision of isotopes ratio measurements ranging from 0.1 to 0.5%. Applying high-resolution ICP-MS precision in isotope ratio measurements can generally be improved by a factor of 5-10 (mainly because of the flat-topped peak shapes and fewer spectral overlaps obtained with these high-resolution instruments). Multicollector ICP-MS systems increase precision due to the collection of all isotopes of interest simultaneously in a multicollector array and so they provide an opportunity to measure the isotopic composition of many elements more accurately than other ICP-MS instruments. 

The symmetry of each peak can provide information about the sample. Tailing (Fig. 2.9A) suggests some heterogeneity in the sample—either real or introduced by the chromatographic conditions. Flat-topped peaks (Fig. 2.9B) suggest that the capacity of the column has been exceeded. 

Fig. 19.3 Magnetic sector mass analyzer, with m/z 32 impinging on the detector note flat-topped peak profile. (Used with permission of ThermoOnix Inc.)...

Flat-topped peaks are also found for short separations in planar gaps, but there Wi/2 drops as 1/tres until the separation becomes long with Gaussian-shaped peaks, 4.2.1.)... 

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.

This sample preparation method is not instrument specific but was developed using a Thermo Fisher Element 2 MS-ICP-MS. Whatever instrument is used, the instrument should be mass calibrated. Although it is preferable for the sake of accuracy that all uranium isotopes be acquired in the same detector mode, cross-calibration should be up to date for single collector instruments, as should detector dead time. Since the Thermo Fisher magnetic sector instrument with a low-resolution ion slit in good condition produces data with flat top peaks, and automatically updates detector crosscalibration after acquisition of peak data with sufficient intensity, frequent recalibrations will not be necessary on this instrument or on the Thermo Fisher multicollector instruments. Detector dead time is relatively stable on this instrument but should be up to date. 

The absence of coupling for a-axis and Zt-axis superlattices can be understood within the RKKY model. The rapid rise and relatively flat-topped peak of the susceptibility fY( z)> as shown in fig. 1, give rise to long tails in the real space response. Figure 46a shows the exchange energy J q) in the a -c plane, assuming that the sharp peak in the... 

Example The metastable decay of the o-nitrophenol molecular ion, m/z 139, by loss of NO to yield the [M-NO] ion, m/z 109, has been studied on a single-focusing magnetic sector instrument (Fig. 9.13) [56]. The mass spectrum shows a flat-topped peak of low intensity expanding over three mass units. Some minor and narrow regular peaks corresponding to fragment ions formed within the ion... 

Figure 5.3 Mass resolution definitions referred to as the (a) single-ion method and (b) the double-ion method for flat-top peaks. The same definitions apply to round topped peaks.

---