Researchers from the University of Graz propose to use 1,2-Hexanediol as eluent for RP-HPLC coupled to ICP-MS. This eluent avoids the necessity to operate the ICP-MS in the organic mode improving the detection power of the hyphenated system.
High-performance liquid chromatography (HPLC) is the most often applied separation technique for speciation analysis. While ICP-MS is the most sensitive element-selective detection system, its coupling with HPLC is hampered by the limited tolerance for organic solvents. In order to address this limitation, the ICP is operated in the "organic ICP-MS mode", using a well balanced addition of oxygen to the plasma gas to prevent the formation of carbon particles, reducing the flow rate to the sample introduction system, cooling the spraychamber to limit evaporation of the solvent and using Pt cones to avoid the fast corrosion of Cu/Ni cones under these conditions. Unfortunately the different components of the organic ICP-MS mode have a negative impact on sensitivity, reducing the detection power typically by two orders of magnitude. The tolerance of the ICP-MS for organic solvents is depending on some physicochemical properties of the solvent influencing the solvent load to the plasma including boiling point, vapor pressure, viscosity, surface tension, and density.
The new study:
Researchers from the University of Graz targeted their work to find alternative organic solvents that can be highly tolerated by the ICP and provide exceptionally strong chromatographic elution at lower eluent concentrations. The ideal solvent would enable the coupling of reversed-phase chromatography with ICPMS detection under default conditions and standard experimental setup without any of the components of the organic ICP-MS mode with their negative impact on sensitivity and robustness.
In order to compare different solvents, a standard separation system was configured, including a short C18 column operated under a standard set of parameters: mobile phase flow rate: 0.25 mL min−1; column temperature: 50 °C; injection volume: 1.0−3.0 μL; and mobile phase composition: formic acid 0.1% v/v with variable contents of the different organic solvents investigated. A group of hydrophobic compounds with a LogP within the range of 2.4−8.2 (computed using XLogP3 3.032) were selected as model compounds. The selected compounds are of medicinal/environmental interest and detectable via ICPMS through a heteroatom (see figure 1).
Chemical structures of the hydrophobic model compounds chosen by the researchers. ICPMS/MS detection was based on the highlighted heteroatoms.
Chromatographic experiments involving mobile phases containing >10−25% of methanol, acetonitrile, or isopropanol could not be performed with ICPMS detection under the standard conditions used due to plasma instability. 1,2-Hexanediol was selected as a candidate because of its extremely low vapor pressure of 2.7 Pa at 20 °C (0.02 mmHg) and a high boiling point of 224 °C which would be expected to result in low solvent load to the plasma. Even further, its high LogP of 0.7 would enable higher chromatographic elution strength at lower organic content. Using the separation of the model compounds with the different solvents, 1,2,-Haxandiol showed superior elution strength relative to commonly used solvents even when these were employed at much higher concentrations. The ICP operated under (aqueous) standard conditions tolerated mobile phases containg up to 30% v/v of 1,2-hexanediol which provides superior elution strength to that of >90% v/v methanol or acetonitrile. Also prolonged operation under such conditions produced no carbon buildup on the interface. 1,2-Hexandiol also has a positive influence on sensitivity, by enhancing the signal for all elements tested up to a concentration of 20%. The effect seems to be related to improved nebulization efficiency.
Finally the authors tested the applicability of 1,2-hexanediol by performing sulfur speciation in garlic and detection of arsenic-containing fatty acids in spiked human urine. They obtained high separation power and improved sensitivity compared to commonly used solvents. The authors concluded, that 1,1-Hexanediol can replace common organic solvents as a mobile phase for HPLC-ICP-MS, eliminating the negative impact of the organic ICP-MS mode, improving the detection power of the hyphenated system and reducing the health risk for the operator and the environment.
The original study
Bassam Lajin, Joerg Feldmann, Walter Goessler, Elution with 1,2-Hexanediol Enables Coupling of ICPMS with Reversed-Pase Liquid Chromatography under Standard Conditions,
Anal. Chem., 94 (2022) 8802−8810. DOI: 10.1021/acs.analchem.2c01769
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last time modified: July 6, 2022