Spectrometer wavelength calibration

S. J. Choqnette, J.C. Travis, L.E. O Neal, C. Zhn, and D.L. Dnewer, SRM 2035 a rare earth oxide glass for the wavelength calibration of near infrared dispersive and Fonrier transform spectrometers. Spectroscopy, 16(4), 14—19... 

In physics, the use of calibration hierarchies is well established and is used in any laboratory, e.g. for balances, volumetric equipment, spectrometer wavelengths, cuvette light path lengths, thermometers, barometers and clocks. 

The major advantages of FTIR instruments over dispersive spectrometers include better speed and sensitivity, better light-gathering power, more accurate wavelength calibration, simpler mechanical design, and the virtual elimination of the problems of stray light and IR emission. Because of these advantages, nearly all the new IR instruments are FTIR systems. 

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. 

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. 

Figure 9.11 Emission spectra of the different types of sources in UV/Vis. A logarithmic scale accounts for the big differences of light intensity according to the wavelengths, notably for filamentless lamps. Below left and middle, general view of a lamp and that seen from above (reproduced courtesy of Oriel). Schematic presenting the circuit details for the lamp. The lamp is booted with a voltage of between 3 to 400 V. The anode is a molybdenum plate while the cathode is a filament of metallic oxide able of emitting electrons and connected to an electrical supply. The emission peaks of deuterium at 486 and 656.1 nm are often used to calibrate the spectrometer wavelength scale.

If the spectrometer is calibrated in wavelengths then the above equation can be converted into a more convenient form, as follows ... 

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. 

As in the case of dispersive Raman spectrometers (cf Section 4.4.1), it is necessary to calibrate the wavelength scale of dispersive UV/VIS spectrometers. The most accurate standards for checking the UV/VIS wavelengths are lasers of various types. The inexpensive helium-neon laser can be used to check at 632.8 nm. For spectrometers with a deuterium source, spectral lines at 486.6 and 656.1 nm can be used for calibration. A common method for wavelength calibration is the use of optical filters. A filter of didymium glass has many sharp absorption peaks, which can be used as a second wavelength standard (precision within 0.5 nm). 

The dispersive spectrometers suffer from greater wavenumber errors, of a less predictable form, owing to their general mechanical and thermal instability and can also be affected by non-uniform illumination across the monochromator entrance slit [26]. FT-spectrometers typically use a He-Ne laser as a reference beam to monitor the displacement of the moving optical element, so providing an active internal absolute wavelength calibration... 

C Tseng, J Ford, C Mann, T Vickers. Wavelength calibration of a multichannel spectrometer. Appl Spectrosc 47 1808-1813, 1993. 

D Carter, J Pemberton. Frequency/wavelength calibration of multipurpose multichannel Raman spectrometers. Part I Instrumental factors affecting precision. Appl Spectrosc 49 1550-1560,1995. D Carter, J Pemberton. Frequency/wavelength calibration of multipurpose multichannel Raman spectrometers. Part II Calibration fit considerations and calibration standards. Appl Spectrosc 49 1561-1576, 1995. 

Both spectrometers are equipped with a self-controlling wavelength calibration routine using the possibility to avoid the order pre-selection in fl-ont of the echelle monochromator. This takes up the well-known method from classical VUV spectrometry, where VUV lines diffracted from echellettes or concave gratings in higher orders were calibrated... 

This simple spectrum is often used as a wavelength calibration for infrared spectrometers. The principal feature of the spectrum is the mono-substituted aromatic bands, strong in the 13- to 15-/x region while weak in the 5- to 6-m region. Also of interest is the lack of other functionality. Polystyrene is often used as a cross-linking agent for polyesters and can be identified and quantitatively measured [ ]. 

Tables of Wavenumbers for the Calibration of Infrared Spectrometers, Butterworths Scientific Publications, London, England (1961). An excellent book for use in wavelength calibration. Calibration points for the 400-600 cm region are presented for both high-resolution and prism or small-grating spectrophotometers. Includes brief discussions of effects of pressure, temperature, and also presents material on preparation of samples, reliability of data, and experimental techniques.

These days, with modem instrumentation being so good, is not so essential to check the wavelength calibration of the spectrometer before running an infrared spectrum. This checking of the calibration may be done by examining a suitable reference substance (such as polystyrene film, ammonia gas, carbon dioxide gas, water vapour or indene) which has sharp bands, the positions of which are accurately known in the region of interest. 

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