Filter-based instruments

The earliest commercial NIR instrumentation was based upon filters. (Karl Norris used a UV/Vis/NIR scanning Cary instrument, although that instrument was not designed for NIR per se.) A filter is a window that allows a particular slice of the spectrum to pass through (a bandpass filter) or blocks all wavelengths below or above a certain frequency ( ge and cutoff filters). 

In an interference filter, a transparent dielectric spacing material separates two partially reflective windows. This conformation forms a Fabry-Perot filter, allowing a specific set of wavelengths to pass. The outer windows are constructed of materials with higher refractive index than the center spacer, which determines the central wavelength via its thickness. 

The basic paradigm of a filter instrument is that many applications may be run using a few select wavelengths. Experience has shown that most organic materials have common moieties, i.e., C-H, O-H, and N-H 

Pharmaceutical and medical applications of near-infrared spectroscopy 

To produce a workable, high signal-to-noise spectrum, the analyst usually co-adds a number of single spectra (32 or 64 scans are typical) to reduce the noise apparent in an individual scan. This means that the typical laboratory grating-based instrument can scan about one sample per minute. This is fast compared with the first scanning spectrometers, but may be too slow for real-time quality control analyses. 

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Filter based instruments lack flexibility and are generally employed for the routine analysis of specific elements, whilst instruments utilizing a dispersing crystal have the greatest resolution. There is in principle no limit to this resolution as the detector may be placed at a large distance from the... 

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. 

Filter-based instruments are often limited to applications where there is simple chemistry, and where the analytes can be differentiated clearly from other species or components that are present. Today, we may consider snch analyzers more as sensors or even meters, and the analytical instrument community does typically not view them as trne instraments. Since the late 1980s a new focns on instrumentation has emerged based on the use of advanced measnrement technologies, and as such is considered to be more of the con-seqnence of an evolution from laboratory instruments. Some of the first work on full-spectrum analyzers started with an initial interest in NIR instruments. The natnre of the spectral information obtained in the NIR spectral region is snch that an analyzer capable of measnring multiple wavelengths or preferably a fnll spectrnm is normally reqnired. 

There are several measurement techniques that can be considered for use in IR-based instrumentation in the NIR and the mid-IR spectral regions, and these are summarized in Table 4.1. Some of these techniques are classical, as in the case of optical filter-based instruments and scanning monochromators. Others, such as acousto-optically tunable filter (AOTF), have been considered for 15+ years, but are mainly applied to NIR... 

Numerous styles and brands of instruments and sample cells have been used for the analysis of tablets. The authors currently use several brands of instrumentation for tablet analysis, including filter-, diffraction grating-, and acousto-optic tunable filter-based instrumentation. Detector configurations are evolving slowly toward an optimal design however, the designs of most manufacturers are suitable for many applica-... 

Sweat, J.A. and Wetzel, D.L. (2001) Near infrared acousto-optic tunable filter based instrumentation for the measurement of dynamic spectra of polymers. Rev. Sci. Instrum., 72 (4), 2153-8. 

The application of FTIR spectroscopy to the analysis of milk has been investigated, and its performance compared to that of conventional filter-based instrumentation [22]. Calibration of the FTIR spectrometer for the determination of fat, protein, lactose, and total solids was performed through the use of PLS. The FTIR method using modified Nicolet 510 research spectrometer was able to provide a four-component analysis of milk in 12 seconds per sample and met the AOAC specifications for milk analysis [22]. The results of this study demonstrated that the use of FTIR spectroscopy would allow payment laboratories to analyse for more components and with appropriate sample handling gain sample throughput speed. A commercial version of an FTIR milk analyser is presently on the market in Europe. An FTIR method for the direct determination of water in milk has also been reported [23]. 

Among the optical analysis methods, near-infrared (NIR) spectroscopy is the most popular method because of its non-destructive nature, the low operating cost and the fast response times (Armenia et al., 2007), and it also has been successfully applied to quality control in food (Pi et al., 2009 Leroy et al., 2003 Subbiah et al., 2008), petrochemical, pharmaceutical, clinical and biomedical and environmental sectors (Ripoll et al., 2008). Near-infrared (0.7-2.5pm 12900-4000cm-i) spectroscopy is further classified into NIR reflectance spectroscopy and NIR transmission spectroscopy. NIR can be non-dispersive (filter-based instrumentation), dispersive and use Fourier transform-based instrumentation. Table 1 lists some NIR spectroscopic applications suitable for pesticides determination. All these researches have shown the possibility and reasonability for determination of pesticide concentration using NIR spectroscopy. 

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-based 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. 

A standard error of calibration (SEC) of 0.18 MON and an SEP of 0.22 MON was obtained using a tunable acousto optic filter based instrument [7]. The spectral range measured was 900 to 1700 nm. Multiple linear regression (MLR) analysis was used to correlate the motor value with the NIR value. The paper did not specify how many samples were used in the analysis or verification set or if any preprocessing steps were used. 

Near-infrared spectra were collected on powdered samples, obtained by cmshing five tablets and underdosing or overdosing the samples as described. A Technicon InlraAlyzer 450 filter-based instrument was used for the analysis. Tablet drug content was determined by HPLC and water content by Karl Fischer titration. 

Noncontact NIR analyzers were known and widely used since the early 1970s. The first use of these analyzers was in measuring moisture content of samples moving on conveyers. Several two- or three-color interference filter-based instruments are still manufactured and sold for applications in the inorganic, tobacco, food, chemical, plastic, and other industries. 

In the case of the third requirement, instrument manufacturers have developed highly specialized Magnetic Sector mass filter-based instruments and time-of-flight mass filter-based instruments. Quadrupole mass filter-based SIMS instruments are ineffective in this area because of their inability to provide the high mass resolution required. Fourier Transform Ion Cyclotron Resonance (FT-ICR) as well as Orbitrap mass filter-based instruments, on the other hand, show significant promise. Mass filters are discussed in Section 4.3.2.I. 

Lastly, SIMS analysis has been carried out using a FT-ICR mass filter-based instrument. The example reported in the hterature (Smith et al. 2011) describes the capability of imaging high mass molecular secondary ions resulting from C o cluster ion impact to a mass resolution better than 100,000 from micron scale regions. Such mass filters also provide greater detection limits for high mass secondary... 

Figure 6 A filter-based instrument operating in reflectance mode. The sample is probed with the mirror in the sample position, and the beam is directed to the internal surface of the integrating sphere to measure the background. Courtesy of Bran+Luebbe Analyzing Technologies Ltd.

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