Product
Review
Near-IR
Gets the Job Done
Infrared brings to mind spectra with narrow, well-separated peaks; anybody expecting something similar from near-IR needs to think again. Just as in the mid-IR, information in the near-IR arises from molecular vibrations. The mid-IR spectrum contains the intense, narrow peaks of the fundamental vibrations, whereas the near-IR spectrum isfilledwith combinations and overtones (harmonics) of those fundamental vibrations. The peaks are broader, weaker, and overlapping. Finding an isolated peak whose intensity can be correlated with the analyte concentration is impossible. Instead, the near-IR, by its very nature, requires multivariate calibration techniques. Because the measurements are made with overtone and combination bands, which are broad and tend to overlap, near-IR is not a particularly sensitive technique. However, says Gary Small of Ohio University, a lack of sensitivity can also Celia M. Henry
be an advantage. "You can have a complicated matrix, and lots of things don't interfere simply because they're not present in large enough quantities or because their molar absorptivities aren't high enough to really contribute to the spectrum. Sensitivity is a two-edged sword. If something is sensitive, then everything interferes." Near-IR instruments have been marketed as application-specific analyzers, particularly for the agricultural, food and beverage, petrochemical, chemical, and pharmaceutical industries. These instruments are being used for on-line, at-line, and offline quality control and process monitoring. Near-IR is particularly popular for industrial applications because it can rapidly and nondestructively analyze samples with minimal preparation. Analytical Chemistry last reviewed near-IR analyzers in 1995 (Anal. Chem. 1995, 67, 735 A-740 A). Now, as then,
these instruments are available from many companies. The 22 companies listed in Table 1 are a significant portion of the manufacturers but may not be all of them. Rather than listing specific products, the table identifies which types of near-IR analyzers each company manufactures and which industries each company targets. Readers are encouraged to contact the manufacturers individually to learn about specifications and each company's product rancre Multiple choice Four types of near-IR instruments are available—grating, discretefilter,acoustooptic tunable filter (AOTF), and Fourier transform (FT)—which are differentiated by their means of wavelength discrimination. The first three types can be classified as dispersive instruments. The design of the grating instruments closely resembles that used for UV-vis spectros-
Analytical Chemistry News & Features, September 1, 1999 6 2 5 A
Product
Review
Table 1 . Representative manufacturers of near-IR instruments.
Company
Types of spectrometers
Application areas
Reader service number
Analytical Spectral Devices 5335 Sterling Dr. Boulder, CO 80301 303-444-6522 www.asdi.com
Fast-scanning grating
Chemical, pharmaceutical, petro- 401 chemical, food and beverage, pulp and paper, textile, paint, tobacco
Bio-Rad 237 Putnam Ave. Cambridge, MA 02139 617-868-4330 www.bio-rad.com
FT
Chemical, pharmaceutical petro- 402 chemical, food and beverage, pulp and paper, textile, forensic, paint, tobacco, polymers
Bomem 450 ave St-Jean-Baptiste Quebec, Quebec Canada G2E 5S5 418-877-2944 www.bomem.com
FT
Petrochemical, chemical, polymer, pharmaceutical, dairy product, oil and fat, meat processing
403
Bran+Luebbe 1025 Busch Pkwy. Buffalo Grove, IL 60089 847-520-0700 www.bran - luebbe.com
Grating, discrete filter, AOTF, FT
Lab, at-line, and on-line analysis; food and dairy; feed and grain; pharmaceutical; cosmetics; chemical; petrochemical; textile; tobacco; pulp and paper
404
Brimrose 5020 Campbell Blvd. Baltimore, MD 21236 410-931-7200 www.brimrose.com
AOTF
Chemical, petrochemical, pharmaceutical, dairy, food, grain and seed, pulp and paper, thin film, tobacco, meat, ceramics and ore, polymer, solvent recovery
405
Bruker Optics 19 Fortune Dr. Manning Park Billerica, MA 01821 978-667-9580 www.bruker.com/optics
FT
Chemical, polymer, pharmaceu- 406 tical, petrochemical, agricultural, food and beverage, textile, recycling
FOSS NIRSystems 12101 Tech Rd. Silver Spring, MD 20904
Grating
Pharmaceutical, petrochemical, polychemical
Grating
Diode/light source characteriza- 408 tion, materials analysis, process control
Jasco 8649 Commerce Dr. Easton, MD 21601 410-822-1220 www.jasco.co.jp
Grating, FT
Pharmaceutical, chemical, agricultural, petrochemical
409
LECO 3000 Lakeview Ave. St. Joseph, Ml 49085
Discrete filter
Petrochemical, food and grain, pharmaceutical
410
Scanning grating
Pharmaceutical, food, chemical, petroleum
411
407
301-680-9600 www.foss - nirsystems.com Instruments SA 3880 Park Ave. Edison, NJ 08820
732-494-8660 www.lnstrumentsSA.com
616-985-5496 www.leco.com LT. Industries 6110 Executive Blvd. Rockville, MD 20852
301-468-6777 www.ltindustries.com
626 A
Analytical Chemistry News & Features, September 1, 1999
copy, requiring only that the holographic gratings, the filters, and the detectors be changed. The choice of analyzer type is a question of selectivity and flexibility versus simplicity. "In the conventional thinking, selectivity is based on how many different wavelengths you can measure," says Small. He says that for several applications with well-characterized samples— such as water or protein in wheat—it may be possible to build a filter system that is adequate using only several wavelength bands. Filter-based analyzers are the least flexible of the near-IR instruments. They generally contain wheels with filters selected for specific applications. They are used for the analysis of samples whose composition is well known (e.g., quality control analysis of foods, textiles, and tobacco). "For something that's more complicated or where you're going to use the instrument for a number of different applications, more flexibility would be good," says Small. "Flexibility means having greater control of which wavelength bands you're able to measure." Three sets of bands are found in the near-IR—the combination, thefirstovertone, and the second overtone. The combination bands, which are located between 2000 and 2500 nm, are the strongest of the near-IR bands. "If your instrument doesn't have the ability to measure [the combination] bands, in my opinion, it's quite limited," says Small. "Ideally, you'd like to be able to go out to 2500 nm. Past that, you're into the mid-IR and run into a big water band." The AOTF, which diffracts light at wavelengths determined by a radio frequency applied to the crystal, is a solidstate device with no moving parts. Although the wavelengths can be scanned, they are also randomly accessible. The primary advantages of an AOTF instrument are its speed—wavelengths can be changed in —250 us—and ruggedness. However, its resolution is low (which is really not an issue in the near-IR), and it is affected by changing temperatures, which can cause wavelengths to drift. Just what spectral resolution is required in the near-IR? "That's a whole big debate in the field, simply by virtue of the fact that the spectral bands are fairly broad," says Small. "The conventional thinking is that you don't need the same spectral resolution that you would need if
you were looking at the gas phase in the mid-IR." Indeed, the question of the necessary resolution is one that will likely never reach consensus. Small says that empirical studies have been conducted to establish the resolution required for particular problems. However, as he points out, the issue is "so problem-dependent that you can't make a general statement." The resolution depends on the sample matrix and the degree of spectral overlap between sample constituents. "The conventional wisdom is that you don't need high resolution," says Small. "That's borne out by the fact that a lot of industrial analyses are carried out with filter systems that don't pretend to have good resolution." Mark Arnold of the University of Iowa believes that FT instruments have advantages over dispersive instruments. "I think the FT instruments are inherently better in terms of signal to noise," he says. 'The ability to get absolute reproducibility with wavelength gives you the ability to signal average quite effectively." He admits that data acquisition is slower with FT instruments, but for typical applications the time difference does not pose a problem. The advantages that are usually associated with the FT in the mid-IR—increased energy throughput and high resolution— are less applicable in the near-IR. FT near-IR instruments, however, offer wavelength reproducibility. "From one spectrum to the next, the spectral points line up very reproducibly—very amenable to doing lots of mathematical manipulations. You don't really ever worry about spectral point 23 varying in terms of its wavelength because of the laser referencing system," says Small. "One of the key advantages that gives you is very precise, reproducible wavelength determinations. That makes it very amenable to doing a lot with spectral subtraction. Almost all of the multivariate data handling down to being able to add and subtract spectra and not be worrying about the wavelengths moving around" Advances in instruments
Small and Arnold agree that detector technology has been an area of change for near-IR instruments. "It's not that there are different detectors," says Arnold. "It's just that the detectors that are available are higher quality than they were five years ago." He attributes these advances to im-
T a b l e 1 (cont'd.). Representative manufacturers of near-IR instruments. Types of spectrometers
Application areas
Reader service number
LUMEX 19 Moskovskii Pr St. Petersburg 198005 Russia 7-812-315-15-17 www.lumex.ru
FT
Food, feed and grain
412
Mattson 5225 Verona Rd. Madison, Wl 53711 608-276-6300 www.mattsonir.com
FT
Chemical, pharmaceutical, petrochemical, plastic and polymer, food and beverage, pulp and paper, paint and coating, academic
413
Midac 17911 Fitch Ave. Irvine, CA 92614 949-660-8558 www.midac.com
FT
Chemical, petrochemical, semiconductor gases, pharmaceutical, paint, tobacco
414
Nicolet 5225 Verona Rd. Madison, Wl 53711 608-276-6100 www.nicolet.com
FT
Pharmaceutical, chemical
415
Ocean Optics 380 Main St. Dunedin, FL 34698 727-733-2447 www.oceanoptics.com
Grating
Laser and laser diode characterization, QC testing of fiberoptic components
416
Optical Solutions 9477 Greenback Ln., Ste. 521 Folsom, CA 95630 919-989-3900 www.optical-solutions.com
Diode array, discrete filter
Process, lab chemical, and petrochemical analysis systems
417
Orbital Sciences 2771 N. Garey Ave. Pomona, CA 91767 909-593-3581 www.orbital-int.com
FT, fixed-diode grating
Petrochemical, chemical, pharmaceutical, food, pulp and paper, steel, process development, research
418
Perkin Elmer 761 Main Ave. Norwalk, CT 06859 203-762-4000 www.perkin-elmer.com
FT
Pharmaceutical, food and beverage, agricultural
419
Shimadzu 7102 Riverwood Dr. Columbia, MD 21046 410-381-1227 www.shimadzu.com
Grating, FT
Optics, filters, coatings, mirrors, lenses, chemical, pharmaceuticals
420
Varian 2700 Mitchell Dr. Walnut Creek, CA 94598 800-926-3000 www.varianinc.com
Grating
Material science, food and grain, petrochemical, pharmaceutical
421
Zeltex 130 Western Maryland Pkwy. Hagerstown, MD 21740 301-791-7080 www.zeltex.com
Discrete filter
Petrochemical, food and grain, pharmaceutical
422
Company
provements in the growth of the detector materials themselves. "There s probably been an increase in multichannel detectors, [such as] InGaAs
array detectors," says Small. 'You are probably seeing more use of multichannel detectors than you did a few years ago." Other detectors, depending on the wave-
Analytical Chemistry News & Features, September 1, 1999 6 2 7 A
Product
Review
length region, include InSb, HgCdTe (also called MCT), silicon, PbSe, and PbS. In recent years, an interesting development in near-IR instrumentation has been "sample multiplexers". Using optical fibers, these devices allow a single spectrometer to monitor multiple process streams. The multiplexing can be handled in one of two ways. In the first method, light is divided between the optical fibers and detected by separate detectors. In the second method, the device switches sequentially from one sample to the next under computer control. Such sample multiplexers are used in food, pharmaceutical, and refining applications. Making sense of the spectrum
'There's obviously a big emphasis on data handling in near-IR spectroscopy because the spectra are so awful," says Small. 'The bands are broad, and they tend to overlap, so you never find a clean spectral band sitting by itself, which you can measure the height of to correlate with your analyte's concentration. It's an inherently multivariate technique in terms of calibration." Calibration involves running a "training set" that reflects all the conditions that could conceivably be encountered in samples. Users have little choice of whether to use multivariate techniques, but they do have a choice about which technique to use. The most common chemometric techniques are partial least squares (PLS) and principal components regression (PCR). All the manufacturers include software with their products that handles the calibration models. In addition, users can select software packages from other companies. "Clearly, there is a reasonable market in software packages that users can work with to work out their own calibration," says Small. 'That depends on the particular customer, whether they've got the personnel resources available to be heavily involved in developing calibrations." But are the third party packages better? 'The analogy I would use would be a car stereo. If you buy a Chevy, the car stereo that's in the Chevy will typically not be as good as if you go with an Alpine. It's a question of what your needs are," says Small. "I'm not prepared to say that the software distributed by the instrument vendors is not good. It's certainly going to be less flexible than if you have a software package from a third party ven628 A
dor. If you have a fairly routine analysis that doesn't take anything special, it may not be worth your effort to purchase third party software." Work published by Arnold and Small indicates that the training sets must be carefully chosen for the calibration to be valid. In 1998, they demonstrated that a calibration model could appear to predict glucose when, in fact, none was present (Anal. Chem. 1998, 70,1773-81). In that example, there was a correlation between time and the glucose concentrations in the training set. "It's extremely important in getting accurate calibration models that you're really getting information about the species of interest," says Arnold. "For bioreactor monitoring, it's very important because when cells are metabolizing glucose, they're generally producing lactate. If you just run a bioreactor, collect samples periodically, and measure the glucose levels independently, you can build a calibration model for glucose." However, he says, that calibration model will incorporate lactate spectral information, even though the concentrations of glucose and lactate are inversely related. Thus glucose predictions could be wrong. Arnold circumvents this problem with a technique known as adaptive calibration, which destroys the correlation between analytes (Biotechnol. Prog. g.199 14, 527-33). "We change the composition of samples to adapt them for calibration purposes. If we're measuring glucose, we will take the samples, measure the concentration of glucose, then spike those samples with a known level of glucose, but do it randomly so that it will destroy the glucose-lactate correlation. If you don't do that, the lactate spectral information will be incorporated into glucalibration model." The calibrations need to be checked periodically, but a complete recalibration is required infrequently. "You have to characterize the system that you're trying to measure. The frequency with which you have to recalibrate really depends on how much variance there is and over what period of time," says Arnold. "I believe that, in the [agricultural] industry, they have to recalibrate every season. For reactor monitoring or process monitoring, you may have to recalibrate more often than that. You certainly have to check the calibration's accuracy in some periodic fashion. It depends on how sta-
Analytical Chemistry News & Features, September 1, 1999
ble your system is." Work is ongoing to determine the validity of transferring calibrations—which can be instrumentdependent—from one instrument to another. Near-IR is also finding a place in process monitoring. Hoeil Chung, a senior research scientist at SK Corp., a refinery and petrochemical company in Korea, is using near-IR spectroscopy in petroleum refining. "Spectroscopy is a good candidate for optimizing the processes. Previously, [the process engineers] relied on lab data, which they can collect about once per day. We can do it on-line." The insensitivity of near-IR is something of a disadvantage for process monitoring, according to Chung. However, he diminishes the problem by using the near-IR to measure physical properties, which tend to arise from all components rather than a few specific components. "In that case [physical properties], near-IR is generally working better than the other spectroscopies," he says. Chung believes that the most important characteristics of a process monitoring technique are reproducibility and simplicity. "Near-IR is not a sensitive technique, but it is ducible compared to other techniques because it is not sensitive. It's a tradeoff actually " he states. A wish list
What would the researchers like to see in the future? 'We're involved with trying to measure glucose in biological samples. There you're looking at a very small component of a strongly absorbing sample matrix," says Small. "We're continually up against the problem that we don't have detectors that are sensitive enough or light sources that are bright enough." The intrinsic detector noise is the limiting factor, which can be circumvented by using brighter light sources or making detectors with less noise. "It would be nice to have more intense sources," Small continues. "One of the hopes is that you will have more options, such as diode lasers in the near-IR. You have more variety, cheaper prices, and greater degrees of tunability there." One of the potential roadblocks is that the development of diode lasers has been driven by the needs of the telecommunications industry rather than those of analytical chemists. Celia M. Henry is an associateeditorof Analytical Chemistry.
