The recommended upper limit of total lipid concentration for direct infusion-based approaches is approximately 100 pmol/μL in a 2:1 (v/v), 50 pmol/μL in a 1:1 (v/v), and 10 pmol/μL in a 1:2 (v/v) chloroform-methanol solvent system. However, when an extract contains a large amount of non-polar lipids such as TAG and cholesterol and its esters, this upper #click here keyword# lipid concentration limit should be substantially reduced, or alternatively, the upper limit remains for the polar lipid quantification after a pre-fractionation

with hexane or other non-polar solvent to remove most of the non-polar lipids from polar lipids. The estimate of the total lipid concentration of a lipid extract is based on pre-knowledge (e.g., approximately 300–500 nmol total lipids/mg of protein for organs such as heart, skeletal muscle, liver, kidney and for some cultured cell types; 1,000–2,000 nmol total lipids/mg of protein for brain samples) or trial experiments when working Inhibitors,research,lifescience,medical on an unknown sample with no pre-knowledge. The effects of lipid aggregation on quantification by direct infusion-based approaches have been appreciated by many investigators. In contrast, the effects of lipid aggregation on quantification by LC-MS-based approaches have been under-estimated. For example, a species eluted from a column is substantially concentrated at its peak time where formation

of aggregates Inhibitors,research,lifescience,medical (i.e., homo-aggregates from same species) potentially exists. Moreover, the mobile phase used in a reversed-phase HPLC column typically contains polar solvents (e.g., water, acetonitrile, high percentage of methanol, or salts)

that favor lipid aggregation in a relatively low concentration. These factors potentially Inhibitors,research,lifescience,medical affect the response factors of the lipid species eluted at different times and consequently their quantification especially if only one standard is used. Dynamic range is always one of the major concerns in quantitative analysis. The detectors used in mass spectrometers generally possess a very wide Inhibitors,research,lifescience,medical dynamic range and therefore do not limit the dynamic range also for quantitative analysis of lipids. The upper limit of dynamic range, indeed, is the concentration at which the lipids start to form aggregates while the lower limit of dynamic range is the lowest concentration that a method is capable of quantifying individual species (which is generally higher than the limit of detection). This concentration depends on the sensitivity of the instrument, the sensitivity of the method, the effects of matrices and others. For example, LC-MS/MS enhances the S/N through increases of duty cycle and selectivity and typically possesses an extended dynamic range in comparison to LC-MS. There are at least two different measures of dynamic range. One is the linear range of concentration of the analyte of interest.