Polystyrene wavelength standard

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. 

Figure 8. HDC separation of a synthetic biomodal mixture of 380 A and 1760 A polystyrene standards at 220 nm and 254 nm wavelength (weight ratio is 1.00/120)...

In order to calculate particle size distributions in the adsorption regime and also to determine the relative effects of wavelength on the extinction cross section and imaginary refractive index of the particles, a series of turbidity meas irements were made on the polystyrene standards using a variable wavelength UV detector. More detailed discussions are presented elsewhere (23) > shown here is a brief summary of some of the major results and conclusions. 

Checking for wavelength repeatability by using one or more suitable standards (e.g. polystyrene or rare-earth oxides). The repeatability of measurements should be consistent with the spectrophotometer specification. 

The spectrophotometer should be calibrated so that the bands are observed at their proper frequencies or wavelengths. Proper calibration can be made with reliable standards, such as polystyrene film. 

Gel permeation chromatography (gpc) was performed on a Waters GPC-3 with a model 600 solvent delivery system, a 730 data module, a variable wavelength ultraviolet detector (uv), and a refractive index detector (Rl). Calculations were made on the uv detector response with the wavelength set at 325 nm. Three j/Styragel columns of porosities 105, 104, and 103 A were used and calibrated with polystyrene standards. Injection size was 50-125//I of 0.05% solutions with a flow rate of 1.4 ml/min. The solvent was HPLC grade N-methyl pyrrolidone (NMP) obtained from Aldrich Chemical Co. buffered with 0.03 M LiBr and 0.03 M H3P04.(2, 3)... 

Figure 10.9 HDC chromatograms at two different wavelengths for a bimodal mixture of 38-nm and 176-nm polystyrene standards.

The potential impact of detector wavelength on the resolution of separation is illustrated in Figure 10.9. Shown are two HDC chromatograms for a bimodal mixture of 38- and 176>nm polystyrene standards which were obtained on the same column, under the same operating conditions, but measured at two... 

The wavelength (wavenumber) of typical infrared spectrophotometers is often calibrated or verified with a standard spectrum of polystyrene, obtained from a polymer film. The wavelength match is also used as a diagnostic measure of spectrometer performance. The spectrum on the following page is that of polystyrene film (film thickness = 50 pm), and the table provides the wavenumber readings assigned to the peaks on the spectrum. - ... 

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. 

Another method used in determining the wavelength accuracy of NIR spectrophotometers is to use the overtone peaks of polystyrene in the transmittance mode, much as is done in mid-infrared spectroscopy. It has been suggested that polystyrene can also be used in the reflectance mode to provide a similar calibration standard. 

A sheet of polystyrene was compared to SRM 1921 (the NIST mid-IR standard) and shown that a thicker sample of polystyrene plastic that has been validated with the SRM 1921 standard would make an excellent reference material for verifying wavelength accuracy in a medium resolution FT-NIR spectrophotometer [6]. Traceable standards such as the SRM 2035 and SRM 1920x are also used and designed to fit in the sample position. 

Following the development of these methods of standardization, it was found that a simpler method of standardization could be employed for the NIR Systems line of instrumentation. Because wavelength alignment is accomplished with internal polystyrene and didymium filters within the instrument,... 

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