Researchers from LGC Ltd. (UK) developed a calibration method for speciation analysis using chromatographic separation that can correct for on-column analyte losses and signal drift and can be applied with any detector capable of recording time-resolved data, provided that enough species resolution can be achieved.
Background:Speciation analysis by highly sensitive inductively coupled plasma mass spectrometry (ICP-MS) requires the separation of species by a preferentially on-line coupled separation module such as HPLC. Unfortunately, such separation procedure is often time-consuming, prolonging the analysis time from a few seconds to minutes. Obtaining high accuracy from such analysis is hampered by matrix effects induced by the complex sample and the used mobile phase and signal drift during the analysis time. While signal drift in ICP-MS is often compensated by internal standardisation, its use for hyphenated HPLC-ICP-MS is hampered by interaction of the internal standard with the separation column or mobile phase. Matrix effects can be corrected by the method of standard addition, however in the presence of drift, results may be biased. When using ICP-MS as detection system under such conditions, the preferred method for quantification is isotope dilution (ID-MS). Unfortunately, in many cases IDMS calibration is either not applicable (e.g. for monoisotopic elements such as arsenic) or not feasible (e.g. limited by the cost and availability of isotopically enriched species).
The new study:In order to minimise the impact of drift and matrix effects on the accuracy and precision of the speciation data, the researchers from LGC Ltd. (UK) propose a novel on-column species-specific internal standardisation approach as an alternative to species-specific-IDMS. The method is based on the use of an internal standard that is the same species as the analyte, thus resulting in an ideal response. The method is using a manifold, allowing the precise positioning of the IS peak within the chromatogram for each particular speciation analysis.
Figure: manifold for on-column internal standardisation
- An automatic six-port valve is placed just before the analytical column
- While the sample is injected into the column, a loop is filled with a standard of the species of interest (A)
- After certain time, the valve is switched to inject position and the standard is injected onto the column (B)
- Standard and sample pass through the column simultaneously
The accuracy obtained by this approach was investigated by the determination of inorganic arsenic in rice samples. An expanded uncertainty (k = 2) of <10% was obtained for a mass fraction range of 60 to 300 mg/kg inorganic-As (i-As) in dry rice products. Moreover, the capability of the method to quantify more than one species under non-isocratic separation conditions was investigated through simultaneous determination of i-As and Methyl Arsonate (MA) in a standard mixture submitted to the same preparation procedure as the rice sample.
The authors concluded that the new method provides adequate accuracy as long as a good chromatographic selectivity is achieved that allows for the resolution between the analyte and the neighbouring peaks.
The original publication
Panayot Petrov, Simon Cowen,
Heidi Goenaga-Infante,
On-column internal standardisation as an alternative calibration strategy for speciation analysis: feasibility demonstration through analysis of inorganic As in rice, Anal. Methods, 13/33(2021) 3641-3648.
DOI: 10.1039/d1ay00699a
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Related EVISA Resources Link Database: Analytical Methods for Arsenic Speciation Analysis Brief summary: Standard methods for arsenic speciation analysis Brief Summary: LC-ICP-MS: The most often used hyphenated system for speciation analysis
Related EVISA News (newest first):
November 17, 2020: Transformation of arsenic species during ultrasonic sample pretreatment
last time modified: September 19, 2024
