Filter instruments

The most rugged instruments available, filter-based devices are capable of performing rather sophisticated analyses. With a filter wheel providing several dozen wavelengths of observation, multiple-analyte analyses are quite common. These multiple assays are more amenable to agricultural or food products than pharmaceuticals, if only for validation purposes. Fine chemicals, pharmaceuticals, and gases may be likely to have sharper peaks and are better analyzed via a continuous monochromator (i.e., grating, FT, AOTF). 

The strength, then, of filters is the ruggedness of design. The weakness lies in the broad band allowed through the filter and lack of full spectral signature. For simple analyses involving major components, the resolution of filters is sufficient. For fine work, a more sophisticated monochromator may be needed. 

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In the authors experience, the amount of carbon dioxide in 10 microliters of blood can readily be determined by adding the blood to an acid, through which bubbles an inert gas. The CO2 is then brought into the field of a long cuvette, of approximately 20" in length, and the carbon dioxide measured at the near infrared with a filter instrument. Instrumentation can be designed readily for measurement of the carbon dioxide content of as little as 1 l of plasma with this principle at the rate of approximately 40-60 per hour. 

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). 

The precision of absorption measurements depends upon the degree of sophistication of the instrumentation and on the chemical species involved, both of which can affect the apparent validity of the Beer-Lambert law. Where a single absorbing species exhibiting a broad flat maximum is to be determined, adequate results can often be achieved with a simple filter-photometer. In the visible region, this technique is known as colorimetry. The inherent disadvantage of colorimetric procedures using simple filter instruments with a broad bandpass lies in the invalidation of the Beer-Lambert law and the lack of compatibility between results from different... 

The samples had dry matter over 85% and were ground to pass through a 1 mm screen and then packed in a quartz-windowed cup. The unit was demonstrated at the 1971 Illinois State Fair. After the success of this instrument, Neotec (later Pacific Scientific, then NIR Systems, then Perstorp, then FOSS) built a rotating (tilting) filter instrument. Both instruments were dedicated, analog systems, neither of which was considered user-friendly. ... 

In the middle of the 1970s, Technicon Instruments had Dickey-John produce a filter instrument for them, named the InfraAlyzer. The first, numbered 2.5, featured dust proof optics and internal temperature control. This improved stability and ruggedness made it practical for consumers to operate. Technicon also introduced the gold plated integrating sphere. 

IR instruments are available in filter-based, grating-based, and FT-based models. The usual approach is to use a full-spectrum model to ascertain the working wavelengths for a particular reaction, then to apply simpler filter instruments to the process. This works where one, two, or three discrete wavelengths may be used for the analysis. If complex, chemometric models are used, and full-spectrum instruments are needed. 

Sampling of these substances has been carried out following three approaches liquid absorbents [47], solid-phase microextraction (SPME) fibres [43] and filter substrates (mostly quartz fibre filters but also PTFE membranes [1, 42, 48, 49]). When filter substrates are used, atmospheric particles are collected over 24-h periods using high-volume (dichotomous or single-filter instruments [1, 48]), medium-volume or low-volume samplers (operated to ensure collection of sufficient aerosol mass [37, 50]). Samples were always stored at low temperamres (refrigerated or frozen) to ensure sample preservation. 

One of the first reported couplings of GC-ICP-MS was by Van Loon et al. [115], who used a coupled system for the speciation of organotin compounds. A Perkin-Elmer Sciex Elan quadrupole mass filter instrument was used as the detector with 1250 or 1500 W forward power. The GC system comprised a Chromasorb column with 8 ml min 1 Ar/2 ml min-1 02 carrier gas flow with an oven temperature of 250°C. The interface comprised a stainless-steel transfer line (0.8 m long) which connected from the GC column to the base of the ICP torch. The transfer line was heated to 250°C. Oxygen gas was injected at the midpoint of the transfer line to prevent carbon deposits in the ICP torch and on the sampler cone. Carbon deposits were found to contain tin and thus proved detrimental to analytical recoveries. Detection limits were in the range 6-16 ng Sn compared to 0.1 ng obtained by ETAAS, but the authors identified areas for future improvements in detection limits and scope of the coupled system. 

The Aminco Fluoro-Microphotometer (Fig.3.56) is a filter instrument which is easily adaptable to liquid chromatography. The microflow cell has a capacity of 10 jul. A full range of excitation and emission filters are available. This detector has been adapted for use with the Technicon AutoAnalyser. The system uses a mercury lamp as the source and solid-state electronics. 

IR spectroscopy is one of the oldest spectroscopic measurements used to identify and quantify materials in on-line or near-line industrial and environmental applications. Traditionally, for analyses in the mid-IR, the technologies used for the measurement have been limited to fixed wavelength NDIR filter-based methods and scanning methods based on either grating or dispersive spectrophotometers or interferometer-based FTIR instruments. The last two methods have tended to be used more for instruments that are resident in the laboratory, whereas filter instruments have been used mainly for process, field-based and specialist applications, such as combustion gas monitoring. 

The calibration methods most frequently used to relate the property to be measured to the analytical signals acquired in NIR spectroscopy are MLR,59 60 principal component regression (PCR)61 and partial least-squares regression (PLSR).61 Most of the earliest quantitative applications of NIR spectroscopy were based on MLR because spectra were then recorded on filter instruments, which afforded measurements at a relatively small number of discrete wavelengths only. However, applications involving PCR and PLSR... 

Multiple linear regression is the usual choice with filter instruments and is also used with those that record whole spectra. It is an effective calibration approach when the... 

Air samples Sample collection on filters Instrumental NAA 2 ng/sample 95 Landsberger and Wu 1993... 

Grating-grating fluorometers are convenient for method development, because they permit selection of any excitation or emission wavelength. Filter-filter instruments, on the other hand, are simpler, easier in use, less expensive, more sensitive, and better suited for transferring an HPLC method between laboratories. 

Data obtained at Sunspot, N. M., during the summer of 1949 (45) with the simple photoelectric-filter instrument are given in Figures 6 and 7. Small changes in... 

It is no coincidence, then, that several companies specializing in NIR equipment should spring up in nearby communities of Maryland over the years [12]. In fairness, Dickey-John produced the first commercial NIR filter instrument, and Technicon (now Bran + Leubbe) the first commercial scanning (grating) instrument. Available instruments and the principles of operation of each type are covered in a later chapter. However, before looking at the hardware, it is necessary to understand the theory. 

Dickey-John introduced the first commercial filter instrument in 1971 at the Illinois State Fair. Technicon first licensed the patent for their own instruments, then proceeded to produce their own design as well as a rugged scanning grating type. Pacific-Scientific (now FOSS NIRSystems) added a twist ... 

Every type of instrument has some inherent advantage for any particular analysis. For example, water is the most analyzed material in NIR. If this is the only analysis to be run for a particular product, it may well be sufficient to use a simple filter instrument. Using a 60,000-100,000 instrument to measure only moisture is a waste of resources. 

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