S Frisk | PubMed Health

PubMed2006

Quantification of mtDNA mixtures in forensic evidence material using pyrosequencing.

Analysis of mtDNA variation using Sanger sequencing does not allow accurate quantification of the components of mtDNA mixtures. An alternative method to determine the specific mixture ratios in samples displaying hetero-plasma, consisting of DNA contributions from several individuals, or containing contamination would therefore be valuable. A novel quantification system for mtDNA mixture analysis has been developed based on pyrosequencing technology, in which the linear relationship between incorporated nucleotides and released light allows quantification of the components of a sample. Within five polymerase chain reaction fragments, seven variable positions in the mtDNA control and coding region were evaluated using this quantification analysis. For all single nucleotide polymorphisms quantified in this study, a linear relationship was observed between the measured and expected mixture ratios. This mtDNA quantification assay is an easy to use, fast and accurate quantification system, with the ability to resolve and interpret major and minor mtDNA components in forensic mixture samples.

DNA Fingerprinting↗

PubMed2004

Determination of the molecular orientation of poly(propylene terephthalate) fibers using polarized Raman spectroscopy: a comparison of methods.

For the first time, four different methods to determine the degree of molecular orientation from polarized Raman spectroscopy measurements are compared. The great influence of molecular orientation on the properties of polymers has driven the development of multiple experimental techniques and procedures. This study is based on the C(1)-C(4) ring stretching vibration of poly(propylene terephthalate) (PPT) at 1614 cm(-1). It is shown that simply ratioing the band intensity obtained with the polarization parallel and perpendicular to the unique axis of the sample provides a good qualitative method to observe the evolution of orientation in a series of similar samples. To quantitatively compare the degree of orientation one needs to utilize a more complex method yielding the second- and fourth-order parameters of the orientation distribution function (P(2) and P(4), respectively). To date, most studies have been based on the assumption of a cylindrically symmetric polarizibility tensor. It is shown that this assumption is highly questionable although this method has been used fairly successfully in the past. This method results in orientation parameters that are clearly different from those obtained with the two more complex procedures. The most complex method, both theoretically and experimentally, requires the most measurements per sample. Major problems have occurred when trying to calculate the desired parameters, in particular for samples with high birefringence. These problems are related to experimental complexities occurring for measurements when the samples are tilted with respect to the polarization direction of the incident light. These measurements are replaced by a simple determination of depolarization ratio in the third method. This method assumes that the depolarization ratio is independent of changes in molecular orientation and structure. It was found that this assumption is not correct. Thus, the most complex method is the method of choice to quantitatively determine the second- and fourth-order parameters of the orientation distribution function, unless one has knowledge of the depolarization ratio of each sample being studied. That knowledge permits the use of an experimentally simpler method to obtain the desired parameters.

Algorithms↗

PubMed2003

Rotational invariants for polarized Raman spectroscopy.

A new method has been developed to determine an orientation-independent Raman scattered intensity based on various polarized Raman measurements. The equivalent term in infrared spectroscopy is the structural absorbance, which has existed for many years. As with the structural absorbance, the calculated Raman intensity allows one to observe spectral changes that are due uniquely to morphological changes in a set of different samples in the presence of orientation differences. The full theoretical development is presented, followed by an example based on a set of polymer fibers processed under different conditions leading to different morphologies and degrees of molecular orientation.

Algorithms↗