Calibration curve

About: Calibration curve is a research topic. Over the lifetime, 6552 publications have been published within this topic receiving 95128 citations.

An iterative EPID calibration procedure for dosimetric verification that considers the EPID scattering factor

Abstract: There has been an increasing interest in the application of electronic portal imaging devices (EPIDs) to dosimetric verification, particularly for intensity modulated radiotherapy Although not water equivalent, the phantom scatter factor of an EPID, Spe, is generally assumed to be that of a full phantom, Sp, a slab phantom, Sps, or a mini phantom This assumption may introduce errors in absolute dosimetry using EPIDs A calibration procedure that iteratively updates Spe and the calibration curve (pixel value to dose rate) is presented The EPID (Varian Portal Vision) is irradiated using a 20 x 20 cm2 field with different beam intensities The initial guess of dose rates in the EPID is calculated from ionization chamber measurements in air, multiplied by Sp or Sps The calibration curve is obtained by fitting EPID readings from pixels near the beam central axis and dose rates in EPID to a quadratic equation The Spe is obtained from EPID measurements in 10 X 10 cm2 and 20 x 20 cm2 field and from the calibration curve, and is in turn used to adjust the dose rate measurements and hence the calibration curve The above procedure is repeated until it converges The final calibration curve is used to convert portal dose to dose in the slab phantom, using the calibrated Spe, or assuming Spe = Sp or Spe=Sps The converted doses are then compared with the dose measured using an ionization chamber We also apply this procedure to off-axis points and study its dependence on the energy spectrum The hypothesis testing results (on the 95% significance level) indicate that systematic errors are introduced when assuming Spe = Sp or Spe=Sps and the dose calculated using Spe is more consistent with ionization chamber measurements Differences between Spe and Sps are as large as 2% for large field sizes The measured relative dose profile at dmax using the EPID agrees well with the measured profile at dmax of the isocentric plane using film in a polystyrene phantom with full buildup and full backup, for open and wedged fields, and for a broad range of field sizes of interest The dependence of the EPID response on the energy spectrum is removed once the calibration is performed under the same conditions as the actual measurements

Determination of Trace Impurities in High-purity Quartz by Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry Using the Slurry Sampling Technique

Abstract: A method has been developed for the determination of 14 relevant trace impurities in high-purity quartz based on ETV-ICP-MS using slurry sampling. The ETV device consisted of a double layer tungsten coil. Experimental conditions were optimized with respect to the temperature program, the carrier gas flow, the i.d. of the aerosol tubing, ICP-MS measurement parameters and internal standardization. Excluding U, calibration had to be carried out by the standard additions method because of non-spectral matrix interferences. For U, simple quantification via calibration curves, recorded with aqueous standards, was possible. The observed interferences also aggravated the background evaluation, which seriously limited the determination of Al and Fe. The method was applied to the determination of Al, Ba, Co, Cr, Cu, Fe, Li, Mg, Mn, Na, Pb, Sr, U and Zn in two quartz samples of different grades of purity. The accuracy of the results was checked by their comparison with those obtained by independent methods including instrumental neutron activation analysis. The achievable detection limits are between 2 ng g -1 (Li, U) and 70 µg g -1 (Al).

Calibration transfer for solving the signal instability in quantitative headspace-mass spectrometry.

Abstract: It is reported that calibration transfer is able to compensate the variations in sensitivity in direct coupling of a headspace sampler to a mass spectrometer when used for quantification purposes using multivariate calibration techniques. This strategy of signal stability compensation allows the use of models constructed from large calibration standard sets without having to repeat their measurement even though variations occur in sensitivity, which may or may not be constant along the mass range. This technique offers advantages over the use of internal standards in this methodology and only requires the measurement of a small number of transfer samples with each set of unknown samples. The results obtained in the determination of six volatile organic compounds-benzene, toluene, ethylbenzene, and m-xylene (BTEX), methyl tert-butyl ether (MTBE), and mesitylene-are reported. To obtain an appropriate calibration set, a Plackett-Burman design with five levels of concentration for each component was employed. A PLS multivariate calibration model was constructed with a group of 25 samples. For selection of the optimum number of principal components, an external validation set (5 samples) was used and the prediction capacity of this set was checked with an additional group of samples that had not been used either in the construction or in the validation of the model. The results obtained can be considered highly satisfactory, and the methodology was successfully tested with natural matrixes (river and tap water).