ICP-MS Measurement

Resolution

Resolution is a property that should be understood. The peaks are considered to be resolved if the magnitude of the valley between two adjacent peaks in less than 10 % of the mean of the magnitude of the peaks:

(Intensity peak 1 + Intensity peak 2) / 2 > 0.1 (valley intensity)

NOTE:  This is for neighboring peaks of the same intensity

Most commercial quadrupole mass spectrometers are capable of 0.8 amu mass resolution (at 10% of the valley definition and having equal adjacent peak intensities).

Having adjacent peaks at the same intensity is not a realistic or typical situation. Therefore, the ability to measure a m/z peak at a low concentration adjacent to a high concentration m/z peak is a situation that must be considered. This very important consideration is referenced as Abundance Sensitivity. The concern is that tailing from the larger peak into the smaller peak will occur, giving false high results for the smaller peak. I personally experience this problem on a daily basis while attempting to measure ppb to ppt levels of impurities in our products that are typically diluted to 100 - 200 µg/g for ICP-MS trace impurity analysis.

For the quadruple mass filter, the abundance sensitivities for adjacent peaks on the low and high mass are not equal. This is because the peaks are asymmetric and tend to tail more on the low mass side. If we have a high concentration element with a peak intensity of I_h at mass M and want to measure a low concentration element with a peak intensity of I_L-1 on the low (M-1) mass side or I_L+1 on the high (M+1) mass side, the ratios I_L-1 / I_h and I_L+1 / I_h, are referred to as the low-mass abundance sensitivity and high-mass abundance sensitivity, respectively. For a peak width of 0.8 amu, expected values of 1 x 10^-5 and 1 x 10^-6 for the low-mass and high-mass abundance sensitivities are not unreasonable. This means the concentrations will be measured to be ~ 1 ng/g on the low-mass side and ~ 0.1 ng/g on the high-mass side, adjacent to a mass with a concentration of 100 µg/g. If you are working at dilution factors of 10 to 400, this translates into significant errors. In these situations, I prefer to use axial view ICP-OES for the low or high mass elements.

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