Dear all. I have i food sample and i need to analyse lead in it . Should i choose lead 206 and lead 208 or only one is enough. If i choose the both how can i translate this in the calibration standards . Thank you
Dear Karim Mosa
To be on the safe side, measure the three main isotopes (206, 207 and 208) and take the sum of all intensities for calibration and evaluation. If your instrument software does not provide a method for summing ion signals, do it off-line. Also use certified reference materials and make spike additions to selected samples to verify that the calibration is valid.
Best
Bodo
@bodo hattendorf how can we spike a solid sample from 100 ppm standard . My procedure is to take 0.5 g sample ,make digestion and complete to 25 ml. So what is the calculations
Dear Karim
The calculation is straightforward. Basically you need to evaluate it the mass of analyte added via the spike is represents the difference in concentrations measured in the two samples.
You prepare an unspiked sample where you use a mass of food of A g (approx. 0.5 in your case) and after digestion "complete" that to a final mass of B g (approx. 25). The concentration you measure is C mg/g.
You prepare a spiked sample with U g of the food add V g of a spike standard with a concentration of W mg/g, "complete" to V g and measure a concentration of Z mg/g.
The recovery is then calculated as: Z * X / (U * C * B / A + V * W)
You may mulitiply with 100% if you prefer % notation.
MAKE Make sure to use the correct units for the values used in the equation.
If you prefer volumetric dilution (as I assume from your description) B, V and X will be in mL instead of g but the equation itself is unchanged.
Z * X represents the total mass of the analyte measured in the spiked sample.
V * W represents the total mass of analyte added to the solution by the spike
U * C * A / B represents the mass of analyte in the spiked solution originating from the sample.
Dear
In the ICP-MS analysis you have 4 different isotopes for Pb
Pb 204 (abundance = 1.4 %, interference = Hg, WO)
Pb 206 (abundance = 24.1 %, No interference)
Pb 207 (abundance = 22.1 %, No interference)
Pb 208 (abundance = 52.4 %, No interference)
From this data Pb 208 is found to be the best mass for lead determination.
Mahmoud Ghuniem
That depends. 208Pb is certainly the most sensitive choice. However, variations of the Pb-isotope abundances in nature can be easily detected and depend on origin, contamination etc.
For many applications these variation may not be critical but unless you have specific requirements with respect to sample size, transient signals etc, it should not make much of a difference for the analysis to compensate for the variability by summing all three main isotopes' intensities for the evaluation.
That still leaves you with an offset due to the mass discrimination of the ICP-MS but that is probably negligible compared to isotope ratio variations.
In most cases an average of Pb-206, Pb-207 and Pb-208 gives the concentration with not a very significant bias. Typically you sum the intensities of all three isotopes for both standards and the sample and base your calculations on this. ICP-MS software tipically recommends or this option. However, some natural sources of radiogenic lead (refractory minerals rich in Th, for example) are heavily influenced by unusual ratio between radiogenic isotopes of Pb. In this case, the easiest way is to verify the concentration by a non-mass-spectrometry method, such as ICP-OES. However, for food this is not very likely to be needed.
I regret I can't help you. My research focuses on late medieval food history
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