Wavelength calibration accuracy

Wavelength Calibration And Spectral Resolution. The wavelength calibration accuracy of the SPD depends, in principle, only on the corresponding accuracy of the spectrometer s reciprocal linear dispersion. In this study, the wavelength-to-diode calibration was proven adequately linear to determine wavelength to within +0.02 nm over the entire 20 nrn spectral window (1024 diodes) of the array, from any pair of known spectral lines. 

Calibration. In general, standards used for instrument calibration are physical devices (standard lamps, flow meters, etc.) or pure chemical compounds in solution (solid or liquid), although some combined forms could be used (e.g., Tb + Eu in glass for wavelength calibration). Calibrated lnstr iment parameters include wavelength accuracy, detection-system spectral responsivity (to determine corrected excitation and emission spectra), and stability, among others. Fluorescence data such as corrected excitation and emission spectra, quantum yields, decay times, and polarization that are to be compared among laboratories are dependent on these calibrations. The Instrument and fluorescence parameters and various standards, reviewed recently (1,2,11), are discussed briefly below. 

Wavelength accuracy. In order to evaluate the ability of each system to locate spectral lines, a preliminary wavelength calibration was carred out with the emission spectrum of a mercury pen lamp and then the peak maxima of several atomic lines from an iron hollow cathode lamp were located. The root mean square (RMS) prediction error, which is the difference between the predicted and the observed location of a line, for the vidicon detector system was 1.4 DAC steps. Because it is known from system calibration data that one DAC increment corresponds to 0.0125 mm, the absolute error in position prediction is 0.018 mm. For the image dissector, the RMS prediction error was 7.6 DAC steps, and because one DAC step for this system corresponds to 0.0055 mm, the absolute error in the predicted coordinate is 0.042 mm. The data in Table II represent a comparison of the wavelength position prediction errors for the two detectors. 

Accuracy of wavelength calibration is maintained by thermostatic control of the monochromator. Water vapor may be harmful to optical components and in all instruments the internal atmosphere must be controlled by drying, gas filling, or evacuation. Carbon dioxide and water vapor absorb in the infrared and in single-beam instruments separate recordings of blank and sample spectra must be made. This is inconvenient, and double beam instruments, with automatic blank compensation and improved stability, are more commonly used. 

Solutions of potassium dichromate (K2Cr207) are also used for overall checks on photometric accuracy. These also provide an indication of wavelength calibration, linearity, cuvet light path, and freedom from stray fight in the UV region. 

Figure 20.1 Ishikawa diagram of the parameters which lead to the area of an HPLC peak. Mass means the weighed sample or analyte. For its dilution n measuring flasks (MF) and m pipettes (Pip) are needed. MPE is the maximum permissible error of a volumetric operation, i.e. the combined effects of calibration uncertainty and repeatability. Other abbreviations z = wavelength acc.= accuracy rep. = repeatability S/N = signal-to-noise ratio.

It is therefore essential that the user determines the effect of this "array discretness" on the accuracy of determination and accordingly set the reciprocal linear dispersion of the spectometer. Thermal insulation and/or control of the spectrometer SPD system should also be considered, because thermal expansion may reduce the accuracy of the diode-to-wavelength calibration. 

IR spectrometers must be calibrated for wavelength accuracy. FTIRs are usually calibrated by the manufacturer and checked on installation. Wavelength calibration can be checked by the analyst by taking a spectrum of a thin film of polystyrene, which has well-defined absorption bands across the entire mid-IR region, as seen in Fig. 4.1. Polystyrene calibration standard films are generally supplied with an IR instrument or can be purchased from any instmment manufacturer. Recalibration of the spectrometer should be left to the instmment service engineer if required. 

There are three major faaors that affect the accuracy and precision of quantitative absorption measuranents the instrument, the skill of the analyst, and the method variables. Instruments vary in the quality of their optical, mechanical, and electrical systems and also in their data processing. Each instrument has fixed limitations these mnst be understood by the analyst and optimized when possible. Wavelength calibration must be checked routinely using recognized wavelength standards. Holmium oxide standards are commonly used for this purpose. Stray light, transmittance, resolution, and other instrument parameters should be checked regularly. The analyst must optimize slit... 

However, the physical transfer of spectra between instruments is only one step in the complex chain of the standardization in spectra. The ideal is that a given sample provides a constant spectrum for a given physical state and a defined set of recording and sampling conditions. In the past, it was considered adequate to run a simple calibration standard, such as polystyrene. This is often sufficient as a simple validation of an instrument s performance relative to a prerecorded norm. However, it is not adequate for, and does not constitute, instrument standardization. Standardization implies a unified control of parameters, such as spectral resolution and band shape, actual spectral line position (wavelength calibration), and photometric recording accuracy, and all things that can impact these parameters in a practical measurement. 

More recently there have been reports [1,4, S] of the development of Fourier Transform (FT) Raman systems which have certain advantages over conventional dispersive systems. FT Raman systems have been described in Chapter 7, and are potentially highly efficient, with a high calibration accuracy. One particular advantage for polymers is that the use of IR excitation wavelengths... 

The precise formula will vary slightly with the instrument and the accuracy of wavelength calibration it is also dilferent for each solvent. 

For each grating position at least two lines of the Ne lamp could be focused on the detector simultaneously. In this way, after the initial grating drive running with an accuracy of about 2 arcmin, the wavelength calibration routine simply compares the expected to the measured neon line positions and, when required, initiates a final adjustment by successive grating rotation. At the end of the calibration routine the absorption line of interest is fixed with an absolute accuracy of a few percent of a pixel corresponding to about 0.5 arcsec and the spread of the continuum background spectrum is perfectly adapted to the detector dimension. 

Wavelength accuracy is a fundamental requirement in any spectrometer system and one that should be checked on a periodic basis. The normal method to check wavelength accuracy is to undertake measurements using a suitable line source lamp such as a low-pressure mercury lamp. This lamp provides a range of discrete lines that can be used to demonstrate the wavelength calibration of the system as well as wavelength linearity. At the same time it can be used to demonstrate the correct and repeatable operation of the grating turret. 

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