Calibration Sources

In addition to fulfilling the in-house requirements for quality control, state and local air monitoring networks which are collecting data for compliance purposes are required to have an external performance audit on an annual basis. Under this program, an independent organization supplies externally calibrated sources of air pollutant gases to be measured by the instrumentation undergoing audit. An audit report summarizes the performance of the instruments. If necessary, further action must be taken to eliminate any major discrepancies between the internal and external calibration results. 

Visibility Amplitude (III.7) This is the most readily measured observable, the maximum contrast of the interferogram. It contains essential photometric information for images of the source and all information for circularly (or el-liptically) symmetric sources. A high precision measurment requires calibrator sources with known visibilities and / or monitoring of system parameters for calibration. 

Procedures for calibrating both monochromators in a fluorescence spectrometer using narrow line sources have been discussed (IS,18) care must be taken with placement of the calibration source. 

Procedures for determining the spectral responslvlty or correction factors In equation 2 are based on radiance or Irradlance standards, calibrated source-monochromator combinations, and an accepted standard. The easiest measurement procedure for determining corrected emission spectra Is to use a well-characterized standard and obtain an Instrumental response function, as described by equation 3 (17). In this case, quinine sulfate dlhydrate has been extensively studied and Issued as a National Bureau of Standards (NBS) Standard Reference Material (SRM). 

Programming a CAM for fluorometry is far more complex than for spectrophotometry. Spectrophotometry is simple because it is based on the ratio of light in to light out. But fluorometry creates many of the problems associated with true radiometry—measuring the emission spectrum of an unknown source. The logic may become circular. Radiometry to determine an emission spectrum requires the relative spectral sensitivity of the photometer to be known, but how can this be determined without a source with a known emission spectrum Fortunately, physicists in our national standardization organizations provide us with calibrated sources and photometers. 

Radiation thermometers, 24 453 calibration of, 24 454 calibration source for, 24 458 Radiation thermometry, uncertainty of,... 

It is necessary, however, to correct for volume if the calibration tool is not a true primary calibration source. Our wet test meter is supposed to be accurate within 4 0.5%, and the volume has been calibrated by personnel at the University of N.C. at Chapel Hill against their primary standard. There are also small differences in flow rates if there are large temperature changes between the calibration site and the sampling site (3). 

As is the case for LIF, calibration to obtain absolute concentrations is a challenge. In the instrument shown in Fig. 11.45, a calibration source based on the photolysis of water at 185 nm is installed in the inlet. From the absorption cross section of HzO gas at 185 nm, its concentration, the light intensity, and the sample flow rate, the concentration of OH generated by the photolysis can be calculated. However, not only is there significant uncertainty in the absorption cross section for HzO at 185 nm (e.g., see Lazendorf et al., 1997 Hofzumahaus et al., 1997, 1998 and Tanner et al., 1997), but the measured calibration factor was highly variable from day to day, by as much as a factor of two (Tanner et al., 1997). 

Schultz, M., M. Heitlinger, D. Mihelcic, and A. Volz-Thomas, Calibration Source for Peroxy Radicals with Built-In Actinome-try Using H20 and 02 Photolysis at 185 nm, . /. Geophys. Res., 100, 18811-18816 (1995). 

Vecefa, Z and P. K. Dasgupta, Measurement of Ambient Nitrous Acid and a Reliable Calibration Source for Gaseous Nitrous Acid, Environ. Sci, Technol., 25, 255-260 (1991). 

Many specific and highly sensitive fluorometric and electrochemical detection methods for various analytes are available. The combination of such detection schemes with a DS-based collection system provides a combination of sensitive and affordable instrumentation for atmospheric measurements. A step-by-step construction and operation manual for a DS-based fluorometric H202 analyzer is available (94). With a change in the reagents, the calibration source, and the conditions of the fluorometric measurement, such an instrument is readily reconfigured for a different analyte. The 1992 fabrication cost of a complete DS instrument that utilizes fluorometric detection and includes a thermostated calibration source from commercially available components is approximately 12,000. 

Emission measuring sensors also need an internal calibration source. In the case of absorption sensors, the external source most often served as the standard. In the case of emission, a reference source is usually incorporated in the instrument to certify instrument performance and provide an absolute reference. 

Beta particle calibration sources span energies from about 100 to 3,000 keV for proportional counters, and down to a few keV for liquid scintillation counters. In this experiment, a low-background, gas-flow, end-window proportional counter with automatic sample changer for alpha- and beta-particle counting is calibrated. Beta-particles sources are counted with pulse-height discrimination to eliminate interference from alpha particles the discriminator may be turned off when no alpha particles are present. 

Step 8. When the procedure is approved, collect equipment and reagents. Place instruments and equipment into operational form, test with calibration sources, and determine null or background values. Prepare needed solutions and dilutions. Calibrate tracer and carrier, if needed. 

Most precision spectroscopy of medium Z ions has been conducted at accelerators or tokamak plasmas, but the recent development of the electron beam ion trap (EBIT) has offered a new spectroscopic source to experimenters. Our experimental method takes advantage of the Doppler free and relatively clean spectra produced by an EBIT and is coupled with an external calibration source to allow absolute measurement of highly charged ions. These are the first precision X-ray measurements conducted at the NIST EBIT [12],... 

We locate the EBIT source and calibration source inside the Rowland circle by design. Bragg diffraction angles of calibration lines are in the range 29-45° while the helium-like resonances are observed around 39°. The plane of crystal dispersion is parallel to the electron beam axis. The crystal acts as a polarizer at Bragg angles near 45° and radiation polarised perpendicular to the electron beam axis is the dominant diffracted component. 

The calibration source consists of a 20 keV electron gun and a series of metal targets (Mn, Cr, V, Ti) located on the opposite port to the spectrometer as indicated in Fig. 1. Calibration spectra are collected for a range of Ka and K/3 characteristic wavelengths (1.9A-2.8A) about the helium-like resonances of vanadium. Ka and Ka2 are well resolved in our system and so the Ka doublet provides two reference wavelengths at one detector location. This gives an absolute calibration of detector scale [17,18],... 

One example of a systematic shift is that caused by the calibration source not being in the same location as the EBIT source. Our theoretical modelling determines the shifts of < 1 arcsecond associated with this mis-location. The dispersion function is not a simple relationship between angle and wavelength but a complex (but smooth) function of reference wavelengths, clinometer values, detector scale and systematic shifts. 

The uncertainty of the dispersion function determination reported in Table 1 is the estimated total uncertainty of the factors that contribute to the determination. The overall contribution of calibration source size and alignment uncertainty is 5 ppm. The statistical error associated with calibration lines is 2-3 ppm and the error associated with calibration profile fitting is < 5 ppm. 

Nitric Oxide The ro-vibrational line selected for NO measurements is in the 1900 cm 1 region. Bottled gas of NO in Ne at a concentration of a few ppmv traceable to a National Bureau of Standards determination has been used as the calibration source for NO. The response time of the system for this gas is the residence time in the cell of 3 sec. The minimum detection limit MDC defined as a signal to noise of unity is better than 40 pptv (parts per 10ls by volume) for an integration time of 5 min fhe... 

Nitrogen Dioxide The line selected for NOa measurement lies in the 1600 cm-1 region. A commercial permeation tube is used for the calibration source for NOB. The permeation rate is determined by weight loss and also checked by N0/03 titration with NO then serving as the calibration standard. The response time is 3 sec and the MDC is 25 pptv. 

Surface-barrier detectors are very useful for detection and measurement of a particles. The internal counting efficiency for a particles is = 1.0. If the geometry G of the arrangement of a source and detector is well-defined and self-absorption S in the sample can be neglected, absolute activities of a emitters can be determined. The performance of surface-barrier detectors is conventionally tested by recording the spectrum of a calibration source, e.g. a Am source. 

Finally, the influence of the dead time D in eq. (7.3)) has to be taken into account, particularly if the dead time of the detector is high (as in the case of Geiger Muller counters) and if the counting rates of the sample and the calibration source are markedly different. 

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