Lessons learned from inter-laboratory studies of carbon isotope analysis of honey.

Forensic application of carbon isotope ratio measurements of honey and honey protein to investigate the degree of adulteration with high fructose corn syrup or other C4 plant sugars is well established. These measurements must use methods that exhibit suitable performance criteria, particularly with regard to measurement uncertainty and traceability - low levels of adulteration can only be detected by methods that result in suitably small measurement uncertainties such that differences of 1‰ or less can be reliably detected. Inter-laboratory exercises are invaluable to assess the state-of-the art of measurement capabilities of laboratories necessary to achieve such performance criteria. National and designated metrology institutes from a number of countries recently participated in an inter-laboratory assessment (CCQM-K140) of stable carbon isotope ratio determination of bulk honey. The same sample material was distributed to a number of forensic isotope analysis laboratories that could not participate directly in the metrological comparison. The results from these studies have demonstrated that the majority of participants provided isotope delta values with acceptable performance metrics; that all participants ensured traceability of their results; and that where measurement uncertainties were reported; these were fit-for-purpose. A number of the forensic laboratories only reported precision rather than full estimates of measurement uncertainty and this was the major cause of the few instances of questionable performance metrics. Reporting of standard deviations in place of measurement uncertainties is common practice outside metrology institutes and the implications for interpretations of small differences in isotopic compositions are discussed. The results have also highlighted a number of considerations that are useful for organisers of similar inter-laboratory studies in the future.

Single molecule detection in nanofluidic digital array enables accurate measurement of DNA copy number.

Digital polymerase chain reaction (PCR) is a promising technique for estimating target DNA copy number. PCR solution is distributed throughout numerous partitions, and following amplification, target DNA copy number is estimated based on the proportion of partitions containing amplified DNA. Here, we identify approaches for obtaining reliable digital PCR data. Single molecule amplification efficiency was significantly improved following fragmentation of total DNA and bias in copy number estimates reduced by analysis of short intact target DNA fragments. Random and independent distribution of target DNA molecules throughout partitions, which is critical to accurate digital PCR measurement, was demonstrated by spatial distribution analysis. The estimated relative uncertainty for target DNA concentration was under 6% when analyzing five digital panels comprising 765 partitions each, provided the panels contained an average of 212 to 3,365 template molecules. Partition volume was a major component of this uncertainty estimate. These findings can be applied to other digital PCR studies to improve confidence in such measurements.