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Improve the quality of your
ICP-MS data with qualifier ions
By Ed McCurdy
Agilent ICP-MS Product Marketing Manager
Agilent’s 7700 Series ICP-MS can help you to improve the quality of your results with effective interference removal in ORS3 helium mode. This frees up the measurement of secondary isotopes allowing you to validate your ICP-MS data.
Qualifier ions in ICP-MS
The Agilent 7700x ICP-MS incorporates a 3rd generation Collision/Reaction Cell (CRC) – the Octopole Reaction System (ORS3), which operates effectively in helium (He) mode. In contrast to reactive cell gases which work only for specific reactive interferences, He mode is universal, as it effectively filters out all polyatomic ions regardless of their reactivity. The benefits of He mode for multi element analysis of complex, variable and unknown sample matrices have been well documented, but He mode has a further important benefit. He mode simultaneously removes all polyatomic interferences from all isotopes of each analyte, thereby making secondary ions (isotopes) available for many analytes (Figure 1).
The use of qualifier ions to confirm the identity of a target analyte is common practice in organic mass spectrometry, where the mass of the target ion does not provide unequivocal analyte identification. ICP-MS spectra are comparatively simple, which means that primary or preferred isotopes give much more certain identification of the target analyte; however, quantification of many elements can be affected by the presence of matrix-based polyatomic interferences. By quantifying an element independently using both the primary and secondary isotopes, the results can be compared; good agreement validates the data, indicating that the reported concentration was not affected by any interference.
Comparison of results for isotope pairs
In the data presented here, ten complex synthetic sample matrices were analyzed on the 7700x, using He, reaction (H2) and no gas modes. In each matrix, the relative % difference (RPD) was calculated, to compare the results from the primary and qualifier isotopes of several analytes; good agreement, i.e. an RPD close to zero, indicates effective removal of interferences from both isotopes.
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Figure 2. Comparison of 65Cu/63Cu results in 3 modes and 10 matrices (Enlarge image.)
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Figure 3. Comparison of 53Cr/52Cr results in 3 modes and 10 matrices (Enlarge image.)
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Figure 2 shows excellent agreement between the 65Cu/63Cu results in He mode (green bars) in all matrices. (All He mode results were <2% RPD.) This is in contrast to both no gas mode (blue bars) and reaction mode (H2 cell gas, red bars), where the incomplete removal of interferences from one or other isotope led to large differences between the results (up to 182% RPD between 65Cu and 63Cu results in no gas mode, and up to 915% RPD in reaction mode). Negative RPD values (e.g. 65Cu/63Cu in H3PO4 measured in H2 and no gas mode) indicate an interference on the primary isotope (31P16O2 on 63Cu in this case).
Figure 3 shows the same comparison for 53Cr/52Cr, again demonstrating the excellent agreement between the results from the two isotopes measured in He mode (<3% difference in all matrices). As with Cu, the residual (or newly-created) interferences in no gas and reaction mode gave poor agreement between the Cr isotopes (up to 453% RPD in H2 mode and 744% RPD in no gas mode).
Conclusions
Operating the 7700x ORS3 in He mode is a simple and effective method of removing matrix-based polyatomic interferences, allowing the user to measure additional isotopes for many analytes.
Measurement of secondary (qualifier) isotopes can be used to improve ICP-MS data quality in complex sample matrices, by validating the measured results from the primary isotope.
Agilent ICP-MS Journal
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