Wavelength calibration measurements

Calibration. Wavelength calibration to measure the dispersion relation between wavelength and position on the detector requires illumination... 

The wavelength calibration can be checked by comparing the maximum absorbance wavelength for a known substance as measured by the instrument to what it is reported to be otherwise. 

Quality assurance measures such as pre-analytical checks on instrumental stability, wavelength calibration, balance calibration, tests on resolution of chromatography columns, and problem diagnostics are not included. For present purposes they are regarded as part of the analytical protocol, and IQC tests their effectiveness together with the other aspects of the methodology. 

During the course of laser resonance experiments it was noticed that the central wavelengths shift depending on the helium density. Thus, the resonance line shapes at various target gas conditions were measured precisely with a reduced laser bandwidth and an improved wavelength calibration [18]. Figure 5 shows resonance profiles taken for the 597.26 nm line at different pressures ranging from 530 mb to 8.0 bar at temperatures of 5.8-6.3 K. The results are summarized in Table 2. 

Two measurements [31,10] were conducted at a low-inductance vacuum spark plasma and a tokamak plasma respectively. In both cases only the w line was reported. The first study used a double Johann spectrograph and characteristic K lines were used for calibration [31]. The energy of the w was 5.20558(55) keV or a 105 ppm result The second study [10] used a tokamak plasma and claimed an uncertainty of 40 ppm. Close lying Lyman series lines were used for calibration so this was a relative measurement chain assuming one-electron QED. Shorter wavelength calibration lines and helium-like resonances were observed in second order diffraction suggesting the significant systematic shifts discussed above. The third study [11] was a relative measurement to the w line and, as such, can not be compared to absolute measurements. 

In most spectrophotometric analytical procedures, the absorbance of an unknown is compared directly with that of a calibrator or series of calibrators. Under these circumstances, minor errors in wavelength calibration, variation in spectral bandwidths, or presence of stray light are compensated for and do not usually contribute serious errors. Use of a series of calibrators covering a wide range of concentrations also provides a measure of Hnearity (i.e., agreement with Beer s law for a given procedure and instrument). When calculations are based on published or previously determined values for molar absorptivities or absorption coefficients, however, the spectrophotometer must be checked more rigorously. Performance verification of spectrophotometers on a periodic basis also improves reliability of routine comparative analyses. 

A calibration chain B sample treatment, C associated quantities. The numbers indicate the link between steps. TS transfer standard (masses, wavelength calibration devices, etc), RM laboratory or any non certified reference material, CRM certified RM, PCRM primary CRM e.g, pure substances or matrix CRMs certified with a primary method (balance, IDMS etc). RMs and CRMs can be used at various stages of the measurement process. For chain A PCRMs or CRMs do not exist for all substances in particular for organic and organo-metallic determinations. For step 10, 11 and 12, PCRMs or even CRMs rarely exist. For step 6 laboratories may prepare raw extracts to follow the purification stage. 

In general, when the absorbance is to be measured at a single wavelength, the absorption maximum is chosen. In order that the absorbance reflects the concentration, it is required that the spectral band A A reaching the detector and preselected by the instrument parameter called bandwidth, will be much narrower (10 times) than the width of the absorption band measured at mid-height (see wavelength calibration in Section 9.8.3). 

The determination of the pressure inside the diamond anvil cell requires a calibrated standard. The most commonly used standard is the R-line ( E —> Aj) emission from ruby. The wavelengths of the ruby Rj and Rj lines have been accurately calibrated as a function of pressure using fixed point standards [86] and the measured lattice constant of NaCl in conjunction with the Decker equation of state [87-89]. The shift rate of the lower energy Rj line is normally used to calibrate pressure. The room temperature Ri shift rate is linear up to 200 kbar (0.365 A/kbar (- 0.759 cm Vkbar)) [87]. Above 200 kbar, the shift becomes non-linear and has been quantified empirically [90-93]. The currently accepted Rj line wavelength calibration is valid up to 800 kbar and can be expressed [91] by... 

Figure 7.2a shows the intensity (as measured by a calibrated silicon photodiode) from a monochromator set to a 20 nm bandpass attached to a 1 kW Xe lamp. The wavelength calibration is set to the center of the bandpass. 

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