Process Monitoring: Moving from the Laboratory to the Line A variable filter array spectrometer can help defeat the environmental restraints of traditional infrared spectrometers. Paul Wilks In keeping with my stated goal of introducing new equipment without waiting for results to travel through normal channels, I am presenting this short piece by Paul Wilks (Wilks Enterprises; South Norwalk, CT). Paul has been designing, building, producing, and selling infrared equipment for more than half a century. Thus, when he has something new, I tend to listen sooner than with younger instrumentation parties. I have some experience with linear variable filters (LVFs), including a recently granted patent, and have been waiting to see more applications using the device. To those of you new to this technique, I am adding a little primer for your edification. An LVF device has, at its heart, an interference filter. A normal interference filter, simply, is two clear lenses separated by a clear material. Normally, the lenses are made of a higher refractive index (RI) material than the filler or spacer. In the normal filter, light is reflected between the materials of high and low indices of refraction (see Figure 1). The higher RI materials (windows) act as mirrors. With the middle layer thickness correct for the desired wavelength, the light reflecting between the two interfaces emerges with all the reflections in phase, intensifying the beam. At other wavelengths, the reflections are out of phase, canceling and reducing the intensity of the beam. Thus, the action of the light creates an interference effect; these filters are known as Fabry-Perot interferometers. They selectively allow only one wavelength to pass the filter. The wavelength passed is selected by the nature of the materials and the thickness of the layers. In an LVF, the interference filter is wedge-shaped (see Figure 2). This sets up a range of allowed wavelengths along the filter. The exact spacing and materials of fabrication determine the exiting wavelengths. When an LVF is bonded to a diode array (Figure 3), a spectrometer is born. Placing a light source and sample in line with the LVF and detector gives rise to a small, rugged spectrometer (Figure 4). Using this set-up in a commercial instrument produces an in-line process tool of great potential. The LVF is the heart of the tool that Paul and his people have built. Below, he gives some examples of what it can do. As I usually say, Enjoy! As the series of articles that have been appearing in this column attests, process analytical technology (PAT) is a subject of increasing importance to manufacturers as they strive to improve processing effectiveness and carry out on-site process monitoring. The overall objective is to move analytical procedures out of the laboratory to the process stream or to potential pollution sites. FDA also recently has begun encouraging pharmaceutical companies to move process evaluation out of the quality control laboratory and on to the production floor. Among the PAT methodologies being used or under consideration are gas and liquid chromatography, mass spectrometry, ultraviolet and visible spectroscopy, and vibrational spectroscopies (NIR, mid-ir, and Raman), as well as single-value measurements such as conductivity, mass, refractive index, and ph. An ideal PAT process monitor is rugged, has no moving Emil Ciurczak works as a consultant with Integrated Technical Solutions, 77 Park Road, Goldens Bridge, NY 10526. He can be reached via e- mail at: emil@ciurczak.com. Figure 1. Representation of an interference filter. Constructive and destructive interference occurring simultaneously. 24 Spectroscopy 19(9) September 2004 www.spectroscopyonline.com
Minimum parts, is small in physical size, and is easy to operate. The process stream interface should be simple, durable, and easy to clean. In a perfect world, the unit also would be interfaced with a standard PC or LIMS (or be wireless) so that the information could be easily networked to the scientists in the QA lab for more thorough evaluation of the data. Mid-IR is Well-Suited for PAT Applications The mid-range IR shows a large potential for both at-line and in-line monitoring. Consider these characteristics of mid-ir analysis: Nearly all molecules have a characteristic mid-ir spectrum. Obtain a pure spectrum of a molecule and it usually can be identified positively. This allows production supervisors to determine whether a reaction is following the correct path by identifying products, byproducts, and anomalies. Mid-IR spectra are additive. Thus, the concentration of a component in a mixture can be determined by measuring the depth of its characteristic absorption band in the presence of other components. Most importantly unlike gas chromatography and mass spectrometry mid-ir measurements are nondestructive to the sample and occur nearly instantaneously. These two factors allow for a secondary measurement on the same sample and nearly continuous measurements throughout a process. Wavelength (nm) Multiple measurements throughout a process leads to better control than when measuring fewer, widely spaced samples. Much effort, especially by the pharmaceutical industry, has been put into applying NIR measurements to process monitoring applications, resulting in many useful procedures. NIR has the advantage over mid-ir in that absorption bands are weaker, enabling the use of longer path absorption cells. Glass and quartz optics transmit in this region, leading to cheaper and more effective fiber optics. On the other hand, the very weakness and overlapping nature of NIR bands make the mathematics of sorting out concentration data from them complex and time consuming from a calibration standpoint. The Design of a Simple Mid-IR Source Maximum Figure 2. The actual wedge-like interference filter used to give an array of wavelengths from the thinnest end (shorter wavelengths) to the thickest (longer wavelengths). Spectrometer Recent developments in optical and electronic components are helping to overcome the two major barriers that have held back the widespread use of mid-ir outside the laboratory: the complexity and delicateness of the Fourier-transform IR (FT-IR) spectrometer and the requirement for extremely short-path absorption cells. Mark Druy has pointed out that using the attenuated total reflection (ATR) procedure, the required short effective pathlengths are obtained automatically (1). Unfortunately, many current FT-IR spectrometers might be too delicate for dedicated process-line applications. A new type of IR spectrometer, known as a variable filter array (VFA) spectrometer, recently has been constructed. Because it is a solid state instrument, it has a number of positive traits that might recommend it instead of the traditional scanning instrument. It consists of an ATR platform with an elongated pulsed source at one end and an LVF mounted on a pyroelectric detector array (2) at the other (see Figure 5). The sample plate is mounted on the top surface of a box approximately 5 in. (125 mm) x 3 in. (75 mm) x 1 1/2 in. (37 mm) in size. The associated electronics are installed in the box (see Figure 6) and output is fed to a PC. Two advantages of a solid state design are the absence of moving parts and an optical coupling to an ATR crystal, which eliminates the air path, thus removing the need for corrections for atmospheric absorptions. The power requirements are much lower than in a typical scanning spectrometer, requir- Diode array Filter Figure 3. A general diagram of an LVF: source, filter and array. 26 Spectroscopy 19(9) September 2004 www.spectroscopyonline.com
Source Aluminized surface Pulsable infrared source Reflector Interface connections Sample surface Sample Linear variable filter Pyroelectric array Figure 5. Schematic of the Wilks LVF-ATR device. Array detector Idealized multiple-wavelength photometer Linear variable filter Figure 4. A block diagram of an LVF spectrometer: the source, collimating lens, sample cell, LVF, and a diode array. Internal reflections Embedded microelectronics Aluminized surface Interface connections ing less than seven watts to operate using a nine-v dc supply. This VFA-based spectrometer can be operated in almost any plant or field environment. It is unaffected by ambient air conditions or vibration and thus is ideal for use as a PAT instrument: small, rugged, and low enough in cost for multiple units to be applied. Spectral Performance Most analysts are familiar with the performance capabilities of the conventional FT-IR spectrometers on the market. They normally have tunable resolution between < 1 cm -1 and 32 cm -1 or more, the scan speed for a single spectrum usually is less than 1 s (but higher resolution scans require 16 or 32 coadded scans, raising that number), and they are relatively simple to operate. The spectral range normally can cover the entire 2.5 25 µm region of the IR portion of the spectrum. The mercury cadmium telluride (MCT) detector, common to most FT-IR systems, requires liquid nitrogen to function at its peak. This alone makes it less attractive as a process tool. Most common attachments, including fiber optics, and ATR cells, are available for process work, but at a cost. The normal approach with a processhardened FT-IR is to take a laboratory model and place it in a NEMA enclosure for explosion protection. The final instrument is process-ready and will give excellent data, but the total cost of this grade instrument easily tops $100,000. Many simple process applications, however, merely require a monitor, making the standard FT-IR much more complex and precise than is needed. With a monitor, such as a small, self-contained LVF instrument, spectral performance varies with the ATR crystal material used and the wavelength range of the LVF. Typically, an LVF covers approximately an octave in terms of wavelength. Examples are 2.5 5 µm (the OH/CH region), 5.5 11 µm (the fingerprint region), and 7 14 µm (the gas fingerprint region). Sturdy materials, such as cubic zirconium crystals, are used with the 2.5 5 µm LVF, zinc sulfide (ZnS) with the 5.5 11 µm LVF and zinc selenide (ZnSe) with the 7 14 µm LVF. The ZnS and ZnSe plates are coated with a thin film of hard carbon material to make them acid and scratch resistant. A 64-element detector array 15 mm in length is used. Thus the theoretical resolution is 0.04 µm (31 cm -1 ) with the 2.5 5 µm LVF, 0.09 µm (14 cm -1 ) with the 5.5 11 µm LVF, and 0.11 µm (11 cm -1 ) with the 7 14 µm LVF. These Table I. Effective pathlengths for the LVF according to crystal type. ATR Material Wavelength (µm) Refractive Index Effective Pathlength for 10 Reflections (µm) Cubic zirconium 3.75 2.06 14 Zinc sulfide 8.25 2.2 19 Zinc selenide 10.5 2.4 17 28 Spectroscopy 19(9) September 2004 www.spectroscopyonline.com
Attenuated total reflectance 0.70 0.65 0.60 0.55 0.50 0.45 0.40 0.35 Plain yogurt Light yogurt Chocolate pudding Custard Pudding the bandwidth of key absorption bands in most of the materials to be measured (often, well over 100 nm). Furthermore, the lower resolution of this instrument results in a higher signal-to-noise ratio and hence greater overall sensitivity, which sometimes is cited as a reason for the good performance of NIR spectrometers. The effective pathlength of the VFA spectrometer with the ATR sample platform is determined by the formula: d p = 6 7 8 9 10 11 Wavelength (µm) Figure 6. Concentrations of so-called bad carbohydrates can be determined by absorption in the 9 10 µm region. diode arrays can be thermoelectrically cooled, if necessary. Liquid nitrogen is not required. The resolution, while not research-grade, usually is more than adequate to monitor a reaction or process. Although the resolution of this VFA instrument is low in comparison with laboratory FT-IR spectrometers, it is less than where d p = effective pathlength = wavelength at measurement n 1 = refractive index of the crystal n 2 = refractive index of the sample θ = angle of light Table I shows the effective pathlengths for the different crystals at the center of the LVF associated with them, assuming Circle 22 Circle 20
Attenuated total reflectance 0.70 0.65 0.60 0.55 0.50 0.45 0.40 Banana (low fat, high carb) Peaches (low fat, high carb) Sweet potato (no fat, high carb) Veal (high fat, no carb) 0.35 6 7 8 9 10 11 Wavelength (µm) Figure 7. Comparison of food values in different baby foods. 0.95 Attenuated total reflectance 0.90 0.85 0.80 0.75 Pain reliever, A Anti-inflammatory Pain reliever, B Antacid 6 7 8 9 10 11 Wavelength (µm) Figure 8. Most drug products can be readily identified from their IR spectra. the standard 10-reflection crystal and a sample refractive index of 1.4. Some Applications VFA-based spectrometers can be used much like FT-IR spectrometers, although some of their software programs are geared toward repetitive process analysis applications. An operator can run spectra against a source emission/crystal transmission background to generate the background or baseline (I o ). The analyst also can run spectra against a solvent background, for example, water. For routine QC applications, the analyst can run spectra, not as actual numeri- Circle 24
Attenuated total reflectance 0.8 0.7 0.6 0.5 0.4 0.3 cal data, but as differences from a standard mixture. Some examples of this are Toothpaste Shaving Cream Hand lotion Anti-perspirant 6 7 8 9 10 11 Wavelength (µm) Figure 9. Spectra of some personal care products from which concentrations of key components can be determined. Figure 6, where several types of yogurt and pudding are compared and followed over time; Figure 7, where various foods are examined for fat and carbohydrate comparisons; Figure 8, granulations of some common over-the-counter formulations; Figure 9, some personal hygiene products; and Figure 10, some carbonated and noncarbonated beverages. The ATR cell allows discrimination, even in aqueous solutions. Reactions easily can be followed in real-time with rapidly generated, repetitive spectra. The instrument can be configured to display numerical percentages or weight per volume of components in a mixture, continuously, in real-time. It should be pointed out that mid-ir spectroscopy, when used directly with liquid or solid samples as they occur in processes, is best suited to measure concentration ranges in the low- to midpercent range with a lowest detectable concentration (also, limit of detection) typically in the 0.1% range. Circle 25 Circle 26
Attenuated total reflectance 1.0 0.9 0.8 0.7 0.6 0.5 Diet Cola vs. water Regular Cola vs. water Grape juice Sports drink 6 7 8 9 10 11 Wavelength (µm) Figure 10. These spectra show the concentration of carbohydrates in sugared beverages versus diet versions. by relatively untrained personnel wherever analytical information is needed. It has been designed to make use of the information existing in the mid-ir wavelength region to provide data for process analysis, quality control, and environmental monitoring. And its point-of-measurement cost are less than NIR and FT-IR equipment, when these better instruments are used for multiple-point measurements. References 1. M.A. Druy, Spectroscopy 19(2), 60 63 (2004). 2. The Pyroelectric Array is manufactured by IR Microsystems, US Patent Number 6,420,708B2. Paul Wilks is president of Wilks Enterprise, Inc. (South Norwalk, CT). Conclusion Since the first commercial infrared spectrometers appeared in the late 1940 s, they have evolved gradually from prism instruments to gratings and finally to interferometers for providing spectral information. Throughout this period their use has been primarily restricted to the benign environment of the laboratory. A variable filter array spectrometer removes this environmental restraint and is usable Circle 28 Circle 29