Quantitative ECT: Comparison of Recovery Coefficient and Linearity of Detector Response for Single Photon and Coincidence Detection (F-18, B+ 511 keV)

The response characteristics of two tomographic systems were compared for imaging of positron emitters: a) a SPECT system with a 3/8 in crystal and 511 keV detector shielding, equipped with a specially designed 511 keV collimator, and b) a PET V system using coincidence detection. SPECT transverse plane resolution was 19 mm FWHM and 35 mm FWTM for a radius of rotation of 16 cm. Corresponding resolution for PET was 14 mm FWHM and 28 mm FWTM. Transverse images through a phantom containing cylindrical sources of various cross sections and uniform activity were obtained for each detector. The measured count density or recovery coefficient was found to decrease with source size, the dependence being similar for both systems. The theoretical values for recovery coefficients were calculated by convolution of a Gaussian fit to the SPECT resolution (FWHM, FWTM) values with the uniform cross section of each source. This simple mathematical model confirmed that the recovery coefficient dependence on source size was primarily related to the limited resolution of the detector. Experimental measurements demonstrated that the SPECT resolution for volume sources was sufficient for quantitation, although some limitations exist with respect to source sizes smaller than the detector resolution.     

Strong, low-energy two-photon absorption in extended amine-terminated cyano-substituted phenylenevinylene oligomers

Three quadrupolar oligophenylenevinylenes with five rings in the conjugated backbone, terminal donor groups, and various acceptors and/or donors along the backbone were synthesized and their two-photon spectroscopic properties investigated. These chromophores exhibit large two-photon absorption cross sections over a wide wavelength range and two distinct peaks, the strongest of which (deltamax > 3600 GM) is observed at 960-970 nm, a wavelength close to twice the value of the linear absorption maximum (2lambda(1)max). The findings on these chromophores are compared with those for analogous molecules with shorter conjugation length, for which the main two-photon band is at significantly shorter wavelength than 2lambda(1)max.     

Analysis of pesticide residues in mixed fruit and vegetable extracts by direct sample introduction/gas chromatography/tandem mass spectrometry

Direct sample introduction (DSI), or "dirty sample injection," was investigated in the determination of 22 diverse pesticide residues in mixed apple, green bean, and carrot extracts by benchtop gas chromatography/tandem mass spectrometry (DSI/GC/MS-MS). The targeted pesticides, some of which were incurred in the samples, included chlorpyrifos, azinphos-methyl, parathion-methyl, diazinon, terbufos, p,p'-DDE, endosulfan sulfate, carbofuran, carbaryl, propargite, bifenthrin, dacthal, trifluralin, metalaxyl, pendimethalin, atrazine, piperonyl butoxide, diphenylamine, vinclozolin, chlorothalonil, quintozene, and tetrahydrophthalimide (the breakdown product of captan). The analytical DSI method entailed the following steps: (1) blend 30 g sample with 60 mL acetonitrile for 1 min in a centrifuge bottle; (2) add 6 g NaCl and blend 30 s; (3) centrifuge for 1-2 min; (4) add 5 mL upper layer to 1 g anhydrous MgSO4 in a vial; and (5) analyze 11 microL extract, using DSI/GC/MS-MS. Sample cleanup is not needed because GCIMS-MS is exceptionally selective for the targeted analytes, and nonvolatile coextracted matrix components do not contaminate the injector or the GC/MS-MS system. Average recoveries of the pesticides were 103 +/- 7% with relative standard deviations of 14 +/- 5% on average, and limits of detection were <2 ng/g for nearly all pesticides studied. The DSI/GC/ MS-MS approach for targeted pesticides is quantitative, confirmatory, sensitive, selective, rugged, rapid, simple, and inexpensive.