Researchers from Poland have compared microwave induced plasma optical emission spectrometry (MIP-OES) and inductively coupled plasma optical emission spectrometry (ICP-OES) for the detection of iron species separated by HPLC. The detection power of HPLC-ICP-AES was 10-20 time better than those obtained by HPLC-MIP-AES.
Background:Iron occurs in two thermodynamically stable and kinetically reactive species, namely Fe(II) and Fe(III). Interconversion between these species plays a fundamental role for the iron cycling in the environment, as well as its essential role for living organisms. Iron speciation analysis is therefore of great importance in order to follow these processes. Since interconversion during analysis has to be avoided, the development of an adequate extraction and separation procedure is the major challenge. While most often ICP-MS is used as the detection system for trace element speciation analysis, its excellent detection power in the ppb range and below is not required when iron is the target analyte since iron is the fourth most abundant element in the Earth’s crust. Also, developments in field of plasma-based atomic emission spectrometry (AES) systems have produced new possibilities of analytical applications.
The new study:Despite worse detection power in comparison to ICP-MS, plasma-based AES provides higher robustness and fewer apparatus costs. Recent developments for MIP-AES as an alternative source have also reduced the running costs, since this plasma source can be operated with nitrogen instead of the more costly argon. In order to exploit the possibilities of MIP-AES as detection system for on-line HPLC separation, researchers from Poland studied the iron speciation for sample types containing relatively high iron concentrations such as soils and sediments. For comparison, they coupled the used HPLC separation technique also to an ICP-AES detection system.
Solid samples (soil, sediments, ceramics) were extracted with 2 mol/L hydrochloric acid solution (HCl) by heating up to approx. 80°C with a reflux condenser for 30 min or by ultrasound assisted extraction with phosphoric acid.
The chromatographic separation of Fe(II)/Fe(III) was based on a cation-exchange column, isocratic eluted with a solution containing pyridine–2,6–dicarboxylic acid (PDCA) and formic acid (HCOOH) adjusted to pH 4.2 with potassium hydroxide (KOH) and potassium sulfate (K2SO4). The optimized elution flow rate of 2 ml/min allowed for the separation of the two species within 5 min.
The column eluent was directly fed to the nebulizer of either MIP-AES or ICP-AES detection system. Instrumental LODs (as 3–sigma) and LOQs (as 9-sigma) obtained by HPLC–ICP AES were 10–20 times lower than for HPLC–MIP AES. LODs obtained by HPLC-ICP-AES were 6.3 and 5.4 µg/L for Fe(II) and Fe(III) respectively. Precision obtained by HPLC-MIP-AES was below 3 % slightly better than those obtained by HPLC-ICP-AES (<5 %).
The authors concluded, that HPLC-MIP-AES allowed for a much cheaper analysis for cases of high total iron concentrations and Fe(II) / Fe(III) ratios below 25.
The original publication
Jędrzej Proch,
Przemysław Niedzielski,
Iron species determination by high performance liquid chromatography with plasma based optical emission detectors: HPLC–MIP OES and HPLC–ICP OES, Talanta, 231 (2021) 122403. DOI: 10.1016/j.talanta.2021.122403
Used Instrumentation: Shimadzu LC–10AT Agilent Technologies - MP–AES 4200 Agilent Technologies - 5110 ICP-OES
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