Stale flavour volatiles in Australian commercial UHT milk during storage

Methyl ketones, aldehydes and free saturated fatty acids were measured in the headspace of samples of two indirectly processed and two directly processed Australian commercial UHT milks during room temperature storage for 16 weeks. The analytes were isolated using headspace solid phase microextraction and analysed by gas chromatography coupled with flame ionisation detection. All methyl ketones and aldehydes increased during storage, With free saturated fatty acids exhibiting little change. On average, the total methyl ketone and aldehyde concentrations in the indirectly processed UHT milks were higher than those in the directly processed samples. A strong correlation was found between the concentration of methyl ketones and various heat indices (furosine, lactulose and undenatured whey proteins) in the milk samples.

Matrix effects: The Achilles heel of quantitative high-performance liquid chromatography-electrospray-tandem mass spectrometry

High-performance liquid chromatography coupled by an electrospray ion source to a tandem mass spectrometer (HPLC-EST-MS/ MS) is the current analytical method of choice for quantitation of analytes in biological matrices. With HPLC-ESI-MS/MS having the characteristics of high selectivity, sensitivity, and throughput, this technology is being increasingly used in the clinical laboratory. An important issue to be addressed in method development, validation, and routine use of HPLC-ESI-MS/MS is matrix effects. Matrix effects are the alteration of ionization efficiency by the presence of coeluting substances. These effects are unseen in the chromatograrn but have deleterious impact on methods accuracy and sensitivity. The two common ways to assess matrix effects are either by the postextraction addition method or the postcolumn infusion method. To remove or minimize matrix effects, modification to the sample extraction methodology and improved chromatographic separation must be performed. These two parameters are linked together and form the basis of developing a successful and robust quantitative HPLC-EST-MS/MS method. Due to the heterogenous nature of the population being studied, the variability of a method must be assessed in samples taken from a variety of subjects. In this paper, the major aspects of matrix effects are discussed with an approach to address matrix effects during method validation proposed. (c) 2004 The Canadian Society of Clinical Chemists. All rights reserved.