Dilute and shoot (D&S) methods are promoted for use by analytical instrument vendors in clinical, forensic toxicology and bioanalytical laboratories. More sensitive LC-MS/MS systems make this strategy ever more appealing: the more sensitive the system, the more the sample can be diluted, thus minimizing matrix effects and consequently eliminating the need for proper sample preparation techniques. Problems arise, however, when internal standards (IS) do not coelute exactly with the analyte of interest, as it no longer compensates for matrix effects. Analyte suites are ever increasing and it is highly unlikely that all analytes will be matched to a sufficiently closely coeluting labelled IS.

Researchers from ARUP Institute for Clinical and Experimental Pathology and the University of Utah have tested to establish if D&S provides acceptable data when compared to an existing, validated, solid phase extraction (SPE) procedure for opioids in urine samples. In their findings, D&S underestimated oxycodone concentrations in a sample as much as 45% compared to the existing method utilising SPE sample preparation.

They emphasize the importance of investigating matrix effects early on in the method validation process on a large number of specimens, in order to determine whether D&S sample preparation is adequate or whether more comprehensive sample preparation such as SPE is necessary.

Or, as an end user puts it: "OK so you’ve just bought a $300k mass spec. Now you're literally going to urinate in the front end of it. Doesn’t sound so good now does it?"

Read More

Diluting and shooting yourself in the foot: Complications with sample-to-sample variations in signal suppression