Hi, I want to determine concentrations of heavy metals (e.g. Pb, Cr, As) in rainwater using ICP-MS, thus to calculate the wet deposition flux (concentration × rain volume × time). While, each heavy metal has more than one selectable isotope (e.g. Cr 50, Cr 52, Cr 53..). Our technician just simply advised me to choose as many as possible, and it turned out most of isotopes give reasonable but different sets of results. The problem is which set of result should I use to calculate the flux.
Isotopes will give different results because they have different natural abundances. If you must really choose one isotope from each element, you will of course choose the most abundant one: for Cr especially, because 52Cr is so much more abundant than the others; for As you have no choice, because it's a non-isotopic element. Also make sure to calibrate against the same isotopes as in your analytical runs.
Work with an internal standard for each of your target elements or a set of internal standards in a multi-element method.
We analyze 208Pb, 75As and 52Cr using the internal standards 103Rh, 72Ge and 72Ge, respectively.
The very low concentrations in rainwater requires the most sensitive analytical approach. Therefore, you should prefer and rely on the isotope which in nature has the highest percentage. This is for lead 208Pb (52.4 %). In order to verify the analytical results It can be a good idea to include the other isotopes of an element to the determination. However, in case of Pb the isotopes 206Pb and 207Pb occur with about 24 % and 22 %, respectively; by this yieldig about half the sensitivity, which could depress the signals below the detection limit. In case of Pb I used 193Ir as an internal standard (10 ppb). 53Cr occurs with about 84 %, which means that the low occurrence of the other Cr isotopes are too low to be of use fot rainwater analysis. Internal standard for Cr: 103Rh; 10 ppb. Arsenic is monoisotopic (75As) and an ICP-MS without collision cell device yields no reliable analysis of As. Rainwater certainly contains also 23Cl which combines in the plasma with 40Ar+ to yield 75(ClAr+). The mass coincidence hampers the As analysis. I used a collision cell rinsed with nascent Hydrogene by this deionizing Ar+; reliable and sensitive results can be achieved easily.
In case of As you should possibly have a look to oldfashioned atomic absorptio spectrometry (AAS) hydryde technique (flame method) where you can achieve a reasnonable sample throughput and detection limits in the range of 0.2 ppb (0.2 µg / L-1; depending on device) by this corresponding to the general detection capacities of ICP MS.
You need to be familiar with the isotopic abundances and possible interferences for the elements that you're interested in. 52Cr is the most abundant isotope, but may be interfered with by an ArC dimer. 75As is monoisotopic, but there is the well known ArCl interference. These interferences can be dealt with by interference equations (depending on the concentrations involved) or the use of a collision/reaction cell (or a high resolution magnetic sector instrument). Lead isotope abundance varies, since 206, 207, and 208, are all end products of radioactive decay chains. Lead is usually measured by ICP-MS as the sum of 206, 207, and 208. If you have any further questions, feel free to contact me; we have measured many elements in cloud water and other environmental matrices.
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