Investigations beyond standard operating procedure on internal standard response.

Internal standard response is routinely monitored in regulated studies to accept or reject individual samples with outlier results due to potential sample processing or instrumentation errors, and processes are typically governed by standard operating procedures. However, acceptance or rejection of individual samples is not always sufficient. Internal standard response trends and substantial systemic differences between spiked and incurred samples using a method with an otherwise stable internal standard response should be investigated. Investigations may range from informal evaluations to detailed studies with formal investigation reports. Atypical internal standard response can be an indicator of systemic problems with a bioanalytical method and modification to allow for a relatively stable internal standard response across an analytical run may be essential.

Plasma phospholipids implicated in the matrix effect observed in liquid chromatography/tandem mass spectrometry bioanalysis: evaluation of the use of colloidal silica in combination with divalent or trivalent cations for the selective removal of phospholipids from plasma.

The feasibility of the use of colloidal silica in combination with a number of divalent or trivalent cations for the removal of plasma phospholipids was evaluated by sequentially adding the two reagents (i.e., colloidal silica and a cation) directly to blank plasma samples or plasma samples spiked with analytes. Three representative plasma phospholipids were monitored to determine the efficiency of the phospholipids removal under different reagent combinations. The recovery of each spiked analyte was also monitored under each condition in order to determine if any of the analyte was removed along with the phospholipids. By optimizing the amounts of the reagents used and the sequence of the addition of the reagents, quantitative and reproducible removal of the phospholipids was achieved. Using the finally selected lanthanum cation, the removal of phospholipids was achieved with minimal concomitant loss of the ten investigated analytes which were carefully selected to incorporate functional groups that could potentially interact with the added reagents and hence could be removed along with the phospholipids.