Investigation on sources of contamination, for an accurate analytical blank estimation, during measurements of the Fe content in seawater by isotope dilution inductively coupled plasma mass spectrometry

Abstract

It is well known that direct measurement of the Fe content in seawater by means of inductively coupled plasma mass spectrometry (ICPMS) is hardly possible. For open ocean water samples, typically, it requires a separation from the matrix associated to significant concentration factors. Furthermore, in the low ng kg⁻¹ range, a correct assessment of the analytical blank is of crucial importance. Despite years of experiments and publications, blank estimation remains a fundamental analytical challenge for the realisation of reliable profiles of dissolved Fe data. Not only must this blank be low and reproducible, which is particularly difficult considering the ubiquity of Fe and the complexity of the seawater matrix, but also realistic. Thus, the complete analytical sequence must be applied to real seawater with zero concentration of Fe to produce a “field blank” (FB), according to the International Union of Pure and Applied Chemistry terminology. This paper investigates the sources of contamination and describes two ways of establishing reliable field blank values for the isotope dilution (ID) ICPMS procedure described by Wu and Boyle (J. Wu and E. A. Boyle, Anal. Chim. Acta, 1998, 367, 183), based on a multiple steps protocol, including a co-precipitation with magnesium hydroxide after ammonia loading, and consecutive dissolution with hydrochloric acid. The analytical protocol is optimised to achieve reproducible separation–pre-concentration of ∼100% Fe under stable pH conditions. Typically, at ∼30 ng Fe kg⁻¹ level a concentration factor of up to 15 can be achieved, leading to samples containing residual salinity ∼0.06%. The first approach proposed for field blank determination applies mostly to Fe mass fractions >500 ng kg⁻¹ and results from the comparison of two IDMS-based Fe content values of an identical sample, where the first is produced by Mg(OH)₂ co-precipitation and the second after a simple dilution. For the second approach, adapted to lower Fe content samples (down to 3 ng Fe kg⁻¹), the absolute field blank is the intercept of a linear regression between sample masses and corresponding absolute Fe contents for a given set of sample replicates. The estimated field blanks for both approaches were 16 ± 12 ng kg⁻¹ and 6 ± 2 ng kg⁻¹, respectively. We found that manipulations (i.e. sample handling and the environment) are by far the largest source of contamination as they contribute ∼75% of the total. The 2% nitric acid used to dissolve the precipitate and the instrumental background come next, with, respectively, 3–10% and 5–9% contributions. The measurement procedure was validated through the use of reference materials, a systematic assessment of factors influencing the result, by calculating the combined measurement uncertainty, and from the results obtained on a test material of a recent inter-laboratory comparison.