A Japanese group of researchers has designed an online internal standard correction technique for high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS) making use of an autosampler system.
Background:The hyphenation of high-performance liquid chromatography (HPLC) with inductively coupled plasma mass spectrometry (ICP-MS) is a very versatile and powerful tool for speciation analysis. However, while ICP-MS as a stand-alone instrument allows for high sample throughput for multi-element determinations, its' coupling with a chromatographic separation system is reducing the sample throughput significantly. Also, the mobile phase of HPLC might create some plasma instability, especially when changing its composition during gradient elution. As a result, the detection response and the baseline signals often fluctuate during analysis, limiting precision and accuracy of measurements. An approach to correct drift and measurement fluctuations is the use of an internal standard, which is usually used in ICP-MS. For effective correction, the selection of the internal standard element is critical. It is important that the internal standard is not contained in significant amounts in the samples and that the chosen element behave similarly to the analyte with respect to plasma fluctuations. When many samples have to be analysed, the addition of the internal standards to all the samples is a time-consuming step that also may cause some experimental errors. For the stand-alone ICP-MS the solution to this problem is the online addition of the internal standard via a Y-piece in front of the nebulizer.
The new study:A group of Japanese researchers have designed a technique for the online addition of an internal standard for HPLC-ICP-MS. The technique is using an autosampler system that was programmed to operate in the following sequence: it first takes up a portion of sample solution, rinses the needle, sucks air as a spacer, takes an internal standard solution and finally injects all of them into a sampling loop through an injection valve. By doing so, the repeatability of the sample injection (20 µL) was improved from 2.5 to 1.2 %.
The authors then applied the new technique for the speciation analysis of arsenic in food. Rhodium (Rh), Bismuth (BI), Ytterbium (YB) Thallium (Tl), Antimony (Sb), Rhenium (Re) and Tellurium (Te) were investigated as internal standards, since they are unlikely to be contained in food samples in significant amounts. Tellurium and Re both appeared as a single narrow peak in the chromatogram in the middle of As(V) and As(III), and no tailing was observed. Rhenium was finally selected because of better sensitivity and availability in nitric acid solution. The authors could demonstrate that the Re internal standard improved the accuracy of the As speciation analysis with HPLC-ICP-MS by compensating for the analytical bias and reducing the measurement uncertainty.
The original publication
Tomohiro Narukawa, Toshihiro Suzuki, Satoki Okabayashi, Koichi Chiba,
An online internal standard technique for high performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP-MS), Anal. Methods, 15/2 (2023) 240-246.
DOI: 10.1039/d2ay01696f
Used Instrumentation:
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