Wavelength calibration standards

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. 

Standard data reduction, i.e. bias and flat field correction, has been performed with Iraf. The Iraf task APEXTRACT/APALL was used to extract the spectra, with interactively selected background sampling, in order to avoid contamination for the star spectrum. The wavelength calibration has been done using daily He, Ne, HgCd arcs, and, in order to improve the calibration, wavelengths values for the transitions used were taken from http //physics.nist.gov/. 

If the use of the molar absorption coefficient is inappropriate then it may be sufficient to use a single reference solution of known concentration (known as a standard or calibrator solution) and to compare the test absorbance with that of this standard. The principle of quantitation is exactly the same as before except that the molar absorption coefficient is eliminated from the calculation by measuring the absorbance of both the test and standard solutions at the same wavelength and comparing their absorbance values. 

Standardizing the spectral response is mathematically more complex than standardizing the calibration models but provides better results as it allows slight spectral differences - the most common between very similar instruments - to be corrected via simple calculations. More marked differences can be accommodated with more complex and specific algorithms. This approach compares spectra recorded on different instruments, which are used to derive a mathematical equation, allowing their spectral response to be mutually correlated. The equation is then used to correct the new spectra recorded on the slave, which are thus made more similar to those obtained with the master. The simplest methods used in this context are of the univariate type, which correlate each wavelength in two spectra in a direct, simple manner. These methods, however, are only effective with very simple spectral differences. On the other hand, multivariate methods allow the construction of matrices correlating bodies of spectra recorded on different instruments for the above-described purpose. The most frequent choice in this context is piecewise direct standardization... 

There are many atomic emission lamps which give very precise line spectra. These are little used in photochemical applications, but are useful as wavelength calibration standards. A small selection of available wavelengths is listed in Table 7.1. 

Analyses are performed in accordance with standardized methods issued under the responsibility of a Technical Committee within the Health Ministry. Usually such measurements rely on a comparison of the measured quantity in the unknown sample with the same quantity in a standard , i.e. an RM, according to a specific measurement equation [6], after calibrating the instrument. Calibration of a photometric system for clinical analyses usually means the set of operations that establish, under specific conditions, the relationship, within a specified range, between values indicated by the instrument and the corresponding values assigned to the RMs at the stated uncertainty. Calibration of the photometer itself implies the calibration of wavelength and absorbance scale by means of proper wavelength and absorbance RMs [5], traceable to national standards. A calibration of the instrument is still needed in concentration units to check the indicated provided value. The measurement result is then verified by application of that method of measurement to a certified reference material (CRM). Both the comparator - a photometric device with narrow or wide bandwidth, and the RMs should thus be validated. 

Standards for calibration of absorbance and wavelength are necessary. Certified nichrome metal fihn filters can be used in the ultraviolet range but usually a solution of pure potassium dichromate is used, for which accepted specific absorbance values are known (Table 2). Other standards which have been proposed for the ultraviolet range are potassium nitrate in water and potassium chromate in dilute potassium hydroxide solution for the visible range, coppersulphate and ammonium sulphate havebeen used. 

For broader bandpass filters, neodymium glass filters are available for additional wavelength calibration in the visible region. Special NIST reference materials referred to as SRM (standard reference materials) numbers are available for calibration and verification from the National Institute of Standards and Technology. Information about these standards can be accessed directly from the Internet (http //www.i-nist. gov/itl/ div898 / index.html). 

Luminescent standards have been established for use in calibrating fluorescence spectrometers and have been suggested for Raman spectroscopy in the past (18). The standard is a luminescent material, usually a solid or liquid, that emits a broad reproducible luminescence spectrum when excited by a laser. Once the standard is calibrated for a particular laser wavelength, its emission spectrum is known, and it can provide the real standard output , d)i(AF) depicted in Figure 10.8. In practice, a spectrum of the standard is acquired with the same conditions as an unknown then the unknown spectrum is corrected for instrument response function using the known standard... 

Near-infrared spectroscopy was used to quantitatively determine the phase composition of a compound capable of existing in two polymorphic forms even though the NIR spectra were fairly similar.79 Using a five-wavelength calibration model, nearly 100% recovery was obtained in a series of spiked calibration samples, with relative standard deviation values ranging from 0.1% to 0.9%. The authors concluded that the use of NIR spectroscopy... 

Commercial spectrometers now come with powerful computers and software. Most of the newer ICP emission systems provide software that can assist in wavelength selection, calibration, background correction, interelement correction, spectral deconvolution, standard additions calibration, quality control charts, and report generation. 

A standard may be excellent for one purpose e.g. wavelength calibration) and less useful for another e.g. determination of instrumental resolution) so choose a standard appropriate for the task in-hand. The best samples to check the performance of the diffractometer at low angles are layer-like mica is one such material supplied by NIST, or silver behenate, which has a layer spacing of 58.38 A. ... 

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. 

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... 

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. 

Fluorescence chromatograms from the blank, test compound control, and sample incubations are superimposed. Additional peaks present only in the sample chromatogram are eonsidered adducts. Mass spectra (full scan and MS") of the peak provide additional confirmation. Quantitation is based on comparison of fluorescence peak area of adduct versus that of dGSH standard (external calibration). Quantitative analysis should not be performed if the test compound or its metabolites have fluorescence interference at the wavelength... 

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). 

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... 

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