product review
Portable FTIR spectrometers get moving Experts predict more applications will develop in the future. Rajendrani Mukhopadhyay
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TIR spectrometers are no longer confined to laboratory benches. First responders, geophysicists, and even pig farmers are using portable FTIR spectrometers for various applications. A wide range of users find the spectrometer attractive partly because, as Robert Yokelson of the University of Montana explains, “It’s the closest thing we have to an ‘everything’ detector.” Portable FTIR spectrometers are designed for use in the field, where real-time measurements are necessary. However, because many benchtop designs are inherently lightweight, Gerald Auth of MIDAC says, “‘Portable’ has a very flexible definition.” Some experts say that “portable” should strictly refer to handheld or backpack devices, while others contend that a spectrometer is portable if it’s mobile in some form, be it on a cart or in a helicopter. Peter Griffiths of the University of Idaho says, “I have heard that field instruments should be divided into three categories: ‘portable’ (handheld), ‘luggable’ (backpack devices), and ‘transportable’ (mounted in a vehicle of some kind).” In this article, “portable” will be used to encompass all three categories. Portable FTIR systems have several features that make them attractive. The spectrometer doesn’t take long to warm up, and analysis is quick. A tiny amount of sample is usually sufficient for identification purposes—in some cases a single crystal of sugar can give a signal. Several spectrometers are designed to run off battery power. Table 1 lists some commercially available portable FTIR spectrometers. The table is not meant to be comprehensive; some vendors may offer products not listed here.
Making the FTIR spectrometer portable In a typical benchtop FTIR spectrometer, IR radiation is split into two beams that undergo constructive and destructive interference to produce a waveform. The radiation interacts with a sample, and molecules in the sample selectively absorb radiation at particular wavelengths. The resultant radiation is then directed to a detector and subjected to a fast FT to generate a characteristic spectrum of the sample. To make a traditional benchtop FTIR spectrometer portable, the interferometer and electronic circuits are miniaturized. Many developments in portable FTIR have resulted from decreasing © 2004 AMERICAN CHEMICAL SOCIETY
the size of the electronics. However, there have also been efforts to reduce the dimensions of the interferometer. The interferometer consists of a beam splitter and two mirrors. The most common type is the Michelson interferometer, in which one mirror remains stationary while the other mirror moves back and forth. Auth explains the advantage of sizing down the interferometer: “The mirror suspension system behaves as springs and forms resonant structures. Any resonance is a killer. So we make [the springs] as stiff as possible to avoid those resonances. Stiff generally means small.” But he adds, “Small has its benefits; however, the plane-mirror Michelson [type] interferometer is most sensitive to small tilts in the mirror. To take advantage of the small size of the plane-mirror Michelson interferometer, manufacturers have to take great care in ensuring the interferometer is a stable structure.” The optical alignment of the laser in portable spectrometers is designed to withstand mechanical vibrations. “The [spectrometer] can be moved and shaken, and the light will still be coming out pointed in the same direction. You don’t have to realign as much, if the system can withstand mechanical shock,” says Yokelson. Spectrometers currently on the market operate in the midIR region. Solids, liquids, and gases can all be studied, but the data are collected in different ways. Portable spectrometers are O C T O B E R 1 , 2 0 0 4 / A N A LY T I C A L C H E M I S T R Y
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Table 1. Selected portable FTIR spectrometers.1 Product
OPAG22
Model 102F Portable FTIR RovIR Spectrometer
RAM-601 Roaming Air Monitor
Open-Path FT Spectrometer
Company
Bruker Daltonics, Inc. 40 Manning Rd. Billerica, MA 01821 978-663-3660 www.bdal.com
Designs and Prototypes 1280 Hopmeadow St., Unit E Simsbury, CT 06070 860-658-0458 www.designsandproto types.com
Hamilton Sundstrand Applied Instrument Technologies 2771 N. Garey Ave. Pomona, CA 91767 909-593-3581 www.hs-ait.com
MIDAC Corp. 130 McCormick Ave. #111 Costa Mesa, CA 92626 714-546-4322 www.midac.com
OPTRA, Inc. 461 Boston St. Topsfield, MA 01983-1290 978-887-6600 www.optra.com
Cost (U.S.D.)
$76,000
$70,000
$50,000 without sampling options
$49,500
$50,000
Dimensions (cm)
40 37 25
36 20 23
94 84 74
40 40 30
27 22 18
Weight (kg)
18
7
145
16
30
Display
IBM-compatible PC
Backlit, full-daylight-visible transflective liquid crystal display
Remote PC
External laptop
Graphical user interface
Compact battery, 12 V supply, or worldwide universal power supply and battery charger
115 and 220 V ac
12 V dc (4 A); 110–220 V ac
30 W (drawn from PC cards)
2200–1300
4500–600
1400–700
Backpack transport; can operate in open-cell or conventional closed-cell extractive mode
Optimized corner-cube retroreflector for active measurements
Power requirements 12–36 V
Wavenumber range (cm–1)
1300–700
1600–200
Other features
Open-path system for passive measurements
Thermally stabilized inter- Portable on cart; process ferometer; calibrated out- run-time and developput with optional thermal- ment software ly stabilized blackbody
1
Some companies offer multiple instruments. Contact the vendors for full product lines.
generally equipped with crystals of high refractive index that allow the spectra of solids and liquids to be measured by attenuated total reflection spectrometry. For the determination of trace components in the atmosphere, spectra are measured by either active or passive techniques. In active measurements, the radiation from a standard IR source passes through the interferometer, is expanded by a telescope, and then passes through the air along a measured path (often ~100 m) to a retroreflector. The reflected radiation returns along the same path and is collected by the telescope and focused onto a detector. In passive measurements, radiation emitted by surfaces such as rocks or soil is detected. Passive measurement is sometimes called “standoff” or “remote” detection and is used by the military for detecting chemical warfare agents.
Design specifications The design of portable FTIR spectrometers depends on the applications they are used for. Manufacturers tend to build their spectrometers to closely mesh with their customers’ needs, rather than following a one-size-fits-all approach. For this reason, they largely focus on niche markets and make spectrometers for dedicated purposes. However, many spectrometers have various sampling accessories available to change the interface; this flexibility allows for the analysis of different types of samples. 370 A
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Because the spectrometers have to be mobile, the design specifications of portable FTIR spectrometers differ tremendously from those of benchtop instruments. “Somebody in the field is not going to treat their spectrometer the same way I like my graduate students to treat spectrometers in my lab,” says Griffiths. This means a portable spectrometer, besides being relatively compact and lightweight, has to be extremely rugged. Kevin Bartoe, the assistant chief of the fire department at the Robins Air Force Base in Georgia, describes an experience with a portable FTIR system. “We had our [FTIR spectrometer] on the back of a truck. We received a call and off we went. I forgot [it] was there. After the call, a guy came up to the fire station and pulled out a debris-covered box from his trunk. [The spectrometer] had flown off the back of our truck at approximately 45 miles per hour and went tumbling into the grass. . . . I removed the spectrometer from its box and fired it up. It aligned itself and has been working fine since.” Auth says he has customers in the military whose spectrometers have to survive occasional rocket hits. For some applications, protective clothing can get in the way of analysis. First responders often have to wear bulky gear, including three layers of gloves, while working in a contaminated “hot” zone. Spectrometer manufacturers often keep this in mind when designing portable FTIR systems. Jim Fitzpatrick of Smiths Detection–Danbury (formerly SensIR) says, “We had
product review
Table 1. Selected portable FTIR spectrometers (continued).1 Product
IROX 2000
HazMatID
SOC-400
GASMET FTIR Multicomponent Gas Analyzer
Nicolet Compound ID portable IR Analyzer
Company
Petrolab 874 Albany-Shaker Rd. Latham, NY 12110-1416 518-783-5133 www.petrolab.com
Smiths Detection–Danbury 14 Commerce Dr. Danbury, CT 06810 203-207-9727 www.smithsdetection.com
Surface Optics Corp. 11555 Rancho Bernardo Rd. San Diego, CA 92127 858-675-7404 www.surfaceoptics.com
Temet Instruments Air Quality Analytical, Inc. P.O. Box 204084 Austin, TX 78720 512-331-0073 www.airqa.com
Thermo Electron Corp. 81 Wyman St. Waltham, MA 02454 781-622-1000 www.thermo.com
Cost (U.S.D.)
$27,000
$50,500
$39,855 with Diamond ATR Not available accessory
Dimensions (cm)
20 32 22
45 28 18
25 20 25
43 19 43
62 48 30
Weight (kg)
11
10
10
16
31
Display
Backlit graphic display
Backlit, touch-screen, embedded display, finger or stylus controlled
Notebook computer
External laptop
High-resolution, touchscreen liquid crystal display
12 V
100–240 V ac or 12 V dc
Standard ac/batteries/ vehicle power charger
Power requirements 100/120/230 V ac, 65 W, or Internal battery with 2-h 12 V/4 A dc operation
$49,500 or less, depending on configuration
Wavenumber range (cm–1)
1400–650
4000–650
4000–400
4200–900
6500–650
Other features
Automatic sample introduction; built-in density meter; specialized software for detailed fuel analysis
Waterproof; wirelessenabled; resistant to extreme environments
Accessories include diffuse reflectance, specular reflectance, and grazingangle specular reflectance; works in any orientation; no limit to sample size
Automatic quantitative multicomponent analysis with proprietary CALCMET software
2-lb proprietary Seal IR removable sampling plate; removable sampling module for transmission analysis
1
Some companies offer multiple instruments. Contact the vendors for full product lines.
to make a system to allow [first responders] to run all their samples, operate the computer software, have it give them an answer, and be visible to [someone] inside a Level A suit with a visor and respirator on.” In addition, portable FTIR systems have to be resistant to extreme environmental conditions, especially water. “Any spectrometer that’s used in the field has to really withstand water. Water is the enemy of IR because it absorbs IR radiation very strongly,” says Griffiths. For first responders, the spectrometer also has to tolerate being completely submerged in bleach for decontamination purposes. When it comes to data display, portable FTIR spectrometers either have a built-in computer or can be hooked up to an external computer. Some offer both options, but which one is preferable really depends on the user. Users like the first responders want an onboard computer so they can carry out an analysis quickly and take immediate action. Those who do basic research prefer to use a laptop loaded with various software. Yokelson says, “With an external laptop, you have a lot more capability to visualize and analyze data.” Auth also points out that a built-in computer may affect the spectrometer’s performance. “I struggle very hard to maintain a quiet analog world in [the spectrometer]. You stick in a computer, and you have a tremendous noisemaker.”
Searching databases Spectra generated by both benchtop and portable FTIR spectrometers are usually searched against a library of compounds. Manufacturers have their own proprietary databases that are packaged with the spectrometer software. But a match to a compound in a library cannot be blindly accepted as the correct answer. “Very often when you’re doing search comparisons, you come back with the best match. It’s not necessarily the correct match, and spectral similarity doesn’t necessarily mean structural similarity,” says James de Haseth of the University of Georgia. “The bigger problem, I think, is finding things not in the database and then having the spectra incorrectly assigned to something in the database. That’s where the problems lie because no manufacturer can be expected to put millions of compounds that are available into a single database. The cost would be prohibitive. They have to be limited to likely suspects, which is a very reasonable approach,” says de Haseth. For this reason, users tend to build their own libraries of compounds that they regularly work with, using them in addition to the manufacturer’s database to ensure proper matches. de Haseth says that although portable FTIR systems are designed to be used by nonspecialists, ultimately someone with training in spectroscopy has to decide whether the analyO C T O B E R 1 , 2 0 0 4 / A N A LY T I C A L C H E M I S T R Y
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sis is correct or not. Manufacturers keep this in mind and sometimes design software with two tiers. The first tier is aimed at the nonspecialist and has a user-friendly interface with no access to the spectrometer settings. The user is only expected to place the sample on the spectrometer, push a button, and wait for an answer. The second tier of software is meant for the spectroscopist who has access to the spectrometer specifications and can change the spectrometer setup to meet the demands of the application at hand.
Market set to explode? Experts are brimming with ideas about how the market for portable FTIR spectrometers could expand. A prominent niche market for portable FTIR spectrometers has emerged following the September 11, 2001, terrorist attacks and the anthrax mailings in the United States. First responders faced a desperate need for instrumentation that allowed them to rapidly identify unknown substances on site, rather than wait the typical two days for a lab analysis. Other applications for portable FTIR spectrometers cover a wide range of areas, including analysis of paint coatings on aircraft, monitoring of vapors above pig-waste lagoons, and studies of photochemistry in smoke plumes of forest fires.
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Agriculture and the petrochemical industry are also fields in which a portable FTIR spectrometer would be useful. In addition, U.S. Customs and Border Protection officers could use them at borders for quick sample analyses. Federico Izzia of Thermo Electron gives a different example: “I was talking to a customer in restoration archeology. He was telling me how new tombs were discovered in the Giza pyramid area, but the Egyptian government won’t allow samples to be taken away. So [archeologists] are very interested in a portable system which they can take into the tombs and do analysis without taking samples away from the site.” But enthusiasm for portable FTIR spectrometers is tempered by some experts. Jim Engel of OPTRA feels that the price tag on portable FTIR spectrometers may prohibit widespread adoption. “I’m just concerned that without regulatory pressure and statutory requirements to install this type of equipment, it will really never go very far.” The overall feeling, however, is that IR spectrometry will see a rapid growth in its portable reincarnation. “This is just the beginning,” says de Haseth. “It’s going to depend on people’s imagination, and there’s plenty of that around.” Rajendrani Mukhopadhyay is an associate editor of Analytical Chemistry.
