A group of Chinese researchers propose a new system for coupling µHPLC to ICP-MS. The new system avoids any post-column dispersion, maintaining the molecular resolution obtained on μHPLC and the limit of detection (LOD) of ICP-MS.
Background:
Inductively coupled plasma mass spectrometry (ICP-MS) is a very powerful elemental detection system, that allows for sensitive determination of most elements. The hot environment of the inductively coupled plasma (ICP) enables the atomization of molecules and the ionization of atoms supporting compound independent calibration. Unfortunately, it also destroys all molecular information. In case that information about the element containing compounds is required, the ICP-MS has to be coupled with a separation system. The most versatile separation system for such speciation analysis is high-performance liquid chromatography (HPLC). Since HPLC works with liquid samples and the standard sample introduction system of the ICP is for liquid samples, coupling of the two systems seems to be relatively straight forward. Conventional interfacing is achieved by directing the effluent of the HPLC column into the nebulizer of the ICP-MS sample introduction system. In order to optimize the coupling, the nebulizer with a working flow-rate equal to the effluent flow rate should be selected. Even then one issue remaining is that the separated analyte-containing chromatographic peaks broaden and even remix prior to mass spectrometric quantification due to the inevitable molecular diffusion within the dead-volume introduced by the coupling components, especially the transfer capillary and the spraychamber.
The new study:
A group of Chinese researchers recently presented a new system for coupling µHPLC to ICP-MS, which they named "zero-interfacing". The system is based on a self-designed direct injection nebulizer with a tapered nozzle, in which a capillary chromatographic column can be harboured as the sample capillary while an Ar gas flow is blown through the nozzle mouth.
Figure: Schematic diagram of the COL-NEB for zero-interfacing µHPLC to ICP-MS
The column nebulizer assembly (COL-NEB) can be positioned just before the base of the Ar-plasma, with its gas flow creating the central sampling channel of the ICP torch for online nebulization and transportation of the analytes separated on μHPLC into the plasma. By avoiding any dead-volume for the hyphenation and the dispersion coming with the dead-volume, the full width at half-maximum of a SLUGT peptide chromatographic peak was reduced to 1.71 ± 0.07 s (n = 5) with a 0.72 fg LOD (3σ) of 80Se. The authors also reported the determination of 32 Se-containing peptides in the trypsin lysate of the water-soluble fraction (≥3000 MW) from Se-enriched yeast
CRM SELM-1 within a 10-min run, the highest record to date.
The authors conclude with their belief that their approach paves the way to determining accurate information on a heteroatom and its binding biomolecules that play key roles during life processes.
Comment:
For those who are not familiar with direct injection nebulizers, we would like to emphasize that the distance of the nebulizer nozzle from the plasma base is critical. A too small distance from the hot plasma will melt the nebulizer nozzle, thereby destroying the nebulizer. Special attention is necessary during the ignition phase of the plasma.
The original study
Yang Zhou, Xingrui Song, Xiaowen Yan, Limin Yang, Shi Chen, and Qiuquan Wang,
Zero-Interfacing μHPLC to ICPMS, Anal. Chem. 2022, 94, 49, 16975–16979.
DOI: 10.1021/acs.analchem.2c03951 Related studies (newest first)
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