Reduction of Solvent Effect in Reverse Phase Gradient Elution LC-ICP-MS

Field: Chemistry (Physical Sciences)

Quantification in liquid chromatography (LC) is becoming very important as more researchers are using LC, not as an analytical tool itself, but as a sample introduction system for other analytical instruments. The ability of LC instrumentation to quickly separate a wide variety of compounds makes it ideal for analysis of complex mixtures. For elemental speciation, LC is joined with inductively coupled plasma mass spectrometry (ICP-MS) to separate and detect metal-containing, organic compounds in complex mixtures, such as biological samples. Often, the solvent gradients required to perform complex separations will cause matrix effects within the plasma. This limits the sensitivity of the ICP-MS and the quantification methods available for use in such analyses. Traditionally, isotope dilution has been the method of choice for LC-ICP-MS quantification. The use of naturally abundant isotopes of a single element in quantification corrects for most of the effects that LC solvent gradients produce within the plasma. However, not all elements of interest in speciation studies have multiple naturally occurring isotopes; and polyatomic interferences for a given isotope can develop within the plasma, depending on the solvent matrix. This is the case for reverse phase LC separations, where increasing amounts of organic solvent are required. For such separations, an alternative to isotope dilution for quantification would be is needed. To this end, a new method was developed using the Apex-Q desolvation system (ESI, Omaha, NE) to couple LC instrumentation with an ICP-MS device. The desolvation power of the system allowed greater concentrations of methanol to be introduced to the plasma prior to destabilization than with direct methanol injection into the plasma. Studies were performed, using simulated and actual linear methanol gradients, to find analyte-internal standard (AIS) pairs whose ratio remains consistent (deviations {+-} 10%) over methanol concentration ranges of 5%-35% (simulated) and 8%-32% (actual). Quadrupole (low resolution) and sector field (high resolution) ICP-MS instrumentation were utilized in these studies. Once an AIS pair is determined, quantification studies can be performed. First, an analysis is performed by adding both elements of the AIS pair post-column while performing the gradient elution without sample injection. A comparison of the ratio of the measured intensities to the atomic ratio of the two standards is used to determine a correction factor that can be used to account for the matrix effects caused by the mobile phase. Then, organic and/or biological molecules containing one of the two elements in the AIS pair are injected into the LC column. A gradient method is used to vary the methanol-water mixture in the mobile phase and to separate out the compounds in a given sample. A standard solution of the second ion in the AIS pair is added continuously post-column. By comparing the ratio of the measured intensities to the atomic ratio of the eluting compound and internal standard, the concentration of the injected compound can be determined.