Interference filter instrument

Extremely quiet and rapid monochromators are now available from numerous vendors. As covered in detail in Chapter 2, top-notch monochromators (grating and interferometer), diode arrays, accousto-optic tunable filters, as well as modern interference-filter instruments now exist. Fiber optic probes, multiple detector modules, and transmission attachments all lend themselves to superior sample handling of powders and solid dosage forms. 

Another type of filter system consisted of the interference filter instrument (Figure 4.5). This instrument type uses prespecified research grade discrete interference filters. The filters are mounted... 

A simple instrument for measuring absorbance that uses absorption or interference filters to select the wavelength. 

Unlike the carbon monoxide measuring instrument discussed above, the Lion Intoximeter 3000 uses an interference filter to produce monochromatic radiation... 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

Flame emission spectrometry is used extensively for the determination of trace metals in solution and in particular the alkali and alkaline earth metals. The most notable applications are the determinations of Na, K, Ca and Mg in body fluids and other biological samples for clinical diagnosis. Simple filter instruments generally provide adequate resolution for this type of analysis. The same elements, together with B, Fe, Cu and Mn, are important constituents of soils and fertilizers and the technique is therefore also useful for the analysis of agricultural materials. Although many other trace metals can be determined in a variety of matrices, there has been a preference for the use of atomic absorption spectrometry because variations in flame temperature are much less critical and spectral interference is negligible. Detection limits for flame emission techniques are comparable to those for atomic absorption, i.e. from < 0.01 to 10 ppm (Table 8.6). Flame emission spectrometry complements atomic absorption spectrometry because it operates most effectively for elements which are easily ionized, whilst atomic absorption methods demand a minimum of ionization (Table 8.7). 

As with prisms, there are other devices that have been historically used for dispersing or filtering electromagnetic radiation. These include interference filters and absorption filters. Both of these are used for monochromatic instruments or experiments and find little use compared to more versatile instruments. The interested reader is referred to earlier versions of instrumental analysis texts. 

A continuously monitoring detector of high sensitivity is required and those that measure absorption in the ultraviolet are probably the most popular. These may operate at fixed wavelengths selected by interference filters but the variable wavelength instruments with monochromators are more useful. Wavelengths in the range of 190-350 nm are frequently used and this obviously means that a mobile phase must not absorb at those wavelengths. 

In the mid-IR, unlike NIR, it is customary to use microns or micrometers (um) for the wavelength scale, not nanometers (nm). Also, for filter-based instruments the wavelength scale is traditionally used instead of the wavenumber (cm ) frequency based scale this tends to be historical based on thin film interference filter technology. 

Experience at Barringer Research Laboratory demonstrated that the most effective method to reduce stray light is to combine several reduction procedures. The lead/calcium selectivity of the instrumentation as received was 160, but this has been increased to greater than 1,000,000 with the manufacturer s modifications. The actual steps included replacement of the calcium 393.3 nm line with the 315.9 nm line, replacement of the lead 405.8 nm line with the 220.3 nm line, installation of an interference filter mask over the lead photomultiplier, and computer correction of the residual calcium interference. The stray light reduction obtained by installation of interference filters is presented for three common concomitants in Table II. In many cases the stray light levels were less than or equivalent to the detection limit. The interference filters and the photomultiplier masks (which reduce the entrance angle to the photomultiplier to include only the receiver mirror) improved the detection limits for many elements in the array by decreasing the system band pass. 

A slightly less accurate determination is possible using a colorimeter with a wide bandpass filter, e.g. a simple (non-interference type) purple-red filter but an interference filter in a quality instrument gives results comparable to a spectrophotometer. 

Luminescence Lidar (light Detection and Ranging) is an active instrument, which sends out coherent waves to the object concerned. A fraction of the transmitted energy is transformed by the objects and sends back to the sensor. lidar instriunents measure both the traveUng time interval between sensor/object/sensor as well as the difference between emitted and returning energy, providing information on the exact position of the objects and on the material the objects are made of. Spectral selectivity was achieved usually with the aid of narrow band interference filters. 

Calibrations which are generated on the top-of-the-line instrument can be transfered to a less expensive instrument for routine use by less skilled personnel. Computer programs handle all the details, such as a filter transform (to make data collected via monochromator look like it was collected via interference filter), a data reduction (to limit a large data base of as many as 700 wavelengths to only those 19 wavelengths represented by the filters in a smaller instrument), and a calibration (the creation of an equation by examining known standards and maximizing the correlation as described above). 

Many diffuse-reflectance instruments are available. Some employ several interference filters to provide narrow bands of radiation. Others are equipped with grating monochromators. Ordinarily, calibration is often a stringent requirement as samples must be acquired of the material for analysis that contain the range of analyte concentrations likely to be encountered. It may be useful to grind solid samples to a reproducible particle size. Equations are developed and used for the analysis. Once method development has been completed and validated, solid samples can be analyzed in a few minutes. Accuracy and precision are reported to be of 1 to 2% relative. 

Phosphorescence spectra (uncorrected, front face) were recorded on a Perkin-Elmer LS-5 fluorescence spectrometer using a pulsed excitation source ( 10 ps) and gated detection. The instrument was controlled by a P-E 3600 data station. The samples were typically excited at 313 nm using the instrument s monochromator and an additional interference filter. Excitation and emission bandpasses were 2 nm. Typically the emission spectra were recorded using a 50 ps delay following excitation and a 20 ps gate. The samples were contained in cells made of 3x7 mm2 Suprasil tubing, under a continuous stream of N2, 02 or 02/N2 mixtures of known composition. 

Tunable filters in the form of AOTF devices, liquid crystal tunable filter (LCTF) and also tunable cavity Fabry-Perot etalon (FPE) devices have been considered in non-moving part instrument designs for many years. Today, the AOTF and the LCTF devices are used in the NIR spectral region.9,10 Originally, designs were also proposed for mid-IR AOTF devices, but these have not become available, mainly because of fabrication issues (cost and material purity). Tunable FPE devices, which are really just variable cavity interference filters, have been developed for the telecommunications industry. While these have been primarily used in the NIR, in most cases they can be fabricated to work also in the mid-IR, the latter being only an issue of material/substrate selection. 

Note that instruments 1-4 were photometric devices with less then 0.01 absorbance accuracy evaluated against reference neutral filters at 546 nm and A = 1.000, traceable to INM. The bandwidth provided by the interference filters equipping the absorption photometers was within the range of 4-10 nm. Instrument 5 was a 10 nm bandwidth photometric device with less than 1.0% absorbance linearity, evaluated at 405 nm and 500 nm, against liquid absorbance RMs type 16.02 and 16.03 [5], Enzymatic colorimetric methods for determination of glucose and urea were used. The o-cresoftalein colorimetric method was used for calcium determination. 

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