Product
Review
Open-path FT-IR takes
the long view
The market is small, but a new EPA reference method could spark more interest
Open-path FT-IR moves one of analytical chemistry's best-known techniques off the lab bench and into the gritty world of refineries and Superfund cleanup sites. At these types of sites, open-path FT-IR has been used to locate fugitive leaks of hazardous air pollutants and to provide "fence-line" monitoring for emissions of environmentally regulated volatile compounds. "The advantage [of open-path FT-IR] is that you get chemical compositions over wide spatial areas," says Donald Gurka of EPA's National Environmental Research Laboratory (Las Vegas, NV). Basically, the open-path approach replaces the classic 10-cm gas IR cell with a column of atmosphere that, if the path is unobstructed, can stretch as long as 1 km. Although ubiquitous atmospheric water vapor and carbon dioxide blank out large sections of the IR spectrum, usable windows are available at roughly 700-1300 cm-1, 2000-2250 cm"1, and 2400-3000 cm"1 (Environ. Set. Technol. 1994,25,224 A231 A). Within these spectral windows, many environmentally sensitive volatile
compounds have IR fingerprints that allow qualitative and quantitative measurements, even when the analytes are found in a mixture. Detection limits depend on the individual molecule's strength of absorption, the pathlength used, the number of scans collected, the atmospheric conditions, interferences caused by overlapping absorption bands of atmospheric species and, to a lesser degree, the spectral resolution; but for many common volatile organics, those values fall into the range of 50 ppb or lower. (Because the technique actually measures the path-integrated concentration, the units are often reported as concentration times pathlength.) Open-path FT-IR has been used by the military, applied to indoor air measurements, and helped analyze volcanic plumes. However, the application most instrument manufacturers are targeting is outdoor environmental air monitoring. Acceptance of open-path FT-IR in this arena has been slow, in part, because after years of testing, EPA has not approved a suggested method for using the technique. This
Analytical Chemistry News & Features, January 1, 1997 4 3 A
Product
Review
Table 1. Summary of representative products
Product Company
I Open-Path FT-IR Bomem Instruments 450 St-Jean-Baptiste Quebec, Canada G2E-5S5 418-877-2944
URL or e-mail Approximate price Weight (lb) Interferometer design
[email protected] $75,000 < 125 Dual input, dual output Michelson and ZnSe optics and beamsplitter
Light source
SiC, replaceable with a quartz halogen lamp tor alignment purposes
Telescope
10-in. Newtonian with 45-in. focal length
Measurement Up to 1 km pathlengths (one way) Detectors
Range (cm-1) Nominal resolution Retro-reflector (monostatic)
Interchangeable LN-cooled MCT (1 mm x 1 mm); LNcooled InSb (1 mm x 1 mm); Sterling-cooled MCT; or Sterling-cooled InSb 5000-510 (MCT), 7000-1800 (InSb) Selectable from 32 to 1 CITT1 apodized Gold-coated 15-in. diameter corner cube, 3 arc-s accuracy
Instrument power Hardware/Software
12 VDC/120 VAC NEMA 4 Industrial Grade notebook, GRAMS software in ARRAY BASIC, Continuous Automation Analysis Program, and Hanst Quantitative Reference Library
Options
Sandwich MCT and InSb detectors Sterling cooled
Special features
Permanently aligned; operates in bistatic, monostatic, and single modes
Reader service number 401
I AirSentry-FTIR Environmental Technologies Group 1400 Taylor Ave. Baltimore, MD 21284-9840 800-635-4598 INA 150 Continuous-scan Michelson; ZnSe optics and beamsplitter 360E; corner cube wishbone; scanning system; and HeNe calibration and alignment laser SiC glowbar
12-in. Cassegrainian
INA 20-100 Plane-mirror Michelson, and ZnSe or KBr optics and beamsplitter
SiC glower; remote source for bistatic mode with 20-in aperture requiring 120 VAC or 2 amp at 12 V 10-or 14-in. Newtonian
IRAM-2000 AIL Systems Commack Rd. Deer Park, NY 11729 800-264-7477
[email protected] JNA 110 Ruggedized, temperaturecontrolled Michelson interferometer
SiC glower
10-in. (14-in. option)
10 to 1000 m (monostatic), multiple 1-km paths when using stealth scanner LN-cooled MCT Sterlingcooled MCT
Up to 1000 m (bistatic), up to Up to 1000 m (bistatic), 500 m (monostatic) up to 500 m (monostatic) LN-cooled MCT, Sterlingcooled MCT
LN-cooled MCT (other detector option)
5000-700
5000-700 (ZnSe), 5000-600 (KBr) 0.5 cm- 1
600-5000
0.6 cm- 1
From 30 2.5-in. cube array up INA to 120 2.5-in. cube array, gold coated 120 VAC/240 VAC 120 VAC/12 VDC Windows-based continuous Choice of 486 or 586 PC, or monitoring software, local area 486 notebook PC, and network, remote networking Windows-based AutoQuant and diagnostics, alarm and software warning system outputs, and automated plume modeling with meteorological data to predict downwind impact and emission rates Closed-cycle cryocooler for Closed-cycle cryocooler 24-h continuous scan, LN autofill for continuous 24-h monitoring, StealthScan for 360° scanning for multiple 1-km paths, and heated optics and retroreflector Explosion-proof enclosure, Operates in bistatic and internal QA/QC gas cell, auto- monostatic modes mated QA/QC checks on each measurement
402
LN, liquid nitrogen; INA, information not available
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I AM Systems MIDAC Corp. 17911 Fitch Ave. Irvine, CA 92714 714-660-8558
Analytical Chemistry News & Features, January 1, 1997
403
0.5 to 16 cm- 1 Corner cube array with highreflective, hardened coating foi durability and easy cleaning 120 VAC/240 VAC Windows-based continuous monitoring software with realtime display, custom data reports with two-alarm triggers, automated control of auto positioner (optional), and automated control of meteorological station emission rate/receptor impact module Automatic LN refill system and remote modem software
Consolidated file structure allows complete meteorological data to be recorded with each spectral file and combined with position, instrument status, and results to provide traceability 404
Custom Integrated System Radian Corp. 15508 Bratton Lane Austin, TX 78728 512-244-0100 www.radian.com Starting at $75,000 30 to 200 High-stability dash-pot FT-IR, dynamic alignment, and KBr or ZnSe optics
Computer-switchable SiC or quartz halogen lamp
6-in.//15 Dall-Kirkham (90-in. effective focal length) or 12-in.//15 Dall-Kirkham (180-in. effective focal length) Up to 700 m (monostotic) with displacing retro-reflector LN-cooled MCT with autofill system, uncooled deuterated triglycerine sulfite detector, or InSb for special applications with LN autofill 4000-400, 8000 optional 0.125 cm- 1 capability, 0.5 cm- 1 standard 1-m or 12-in. aperture-displacing retro mirror, Al with SiO overcoat, heated 120 VAC/240 VAC Custom software (Windows-based) for unattended operation, real-time display, custom data reporting, integrated alarm triggering, integral meteorological data and optional CHARM code integration, and Pentium computer
LN autofill system, closed-cycle cryocooler, local area network link software, and remote modem software
Dual telescope design for superior S/N and low-ppb-level detections, real-time software designed for industrial use and interfacing
slowness is about to change with the issuance of Compendium Method TO 16 for open-path FT-IR, according to William McClenny of EPA's Office of Research and Development. Moreover, Robert Spellicy, Radian's manager of Optical Remote Sensing Systems, reports that ASTM is releasing a Standard Guide for using the method. Will these developments lead to increased sales? Table 1 listsfivevendors of open-path FT-IR who are betting that the market will grow for this technique. Bomem says that so far its main market has been research-grade instruments and that their product is not a turnkey system. The other companies do supply turnkey units for more applied uses. Radian International is an environmental engineering and services company that has built a system around a Nicolet interferometer and its own optical systems and software. AIL introduced its first system this past June, according to Robert Kagann, senior technical adviser. (A general discussion of FT-IR spectrometers and a product table on benchtop systems are found in Anal. Chem. 1995, 67, 381A-385 A.) The promise of open path
"The one area that people can really use something like open-path FT-IR is in the perimeter monitoring of industrial plants, something that is permanently set up," says University of Idaho spectroscopist Peter Griffiths. "There are some neat ways of flipping mirrors around so that an [IR] beam passes around the perimeter and 'looks' all around the plant." Company representatives echo Griffiths' analysis. "Any growth in sales [of open-path systems] would need to be as a result of being incorporated into plant-wide emergency-warning and early-detection systems," says Dennis Glennon, regional sales manager with Midac. Other monitoring locations and situations listed by the experts Analytical Chemistry talked to included hazardous waste handling, production monitoring, and emergency response. Site monitoring can be as straightforward as taking an upwind spectrum for background and a downwind spectrum for analysis, although this is not possible with permanently installed FT-IR systems, and more sophisticated methods are required in this case. Unlike other air-sampling techniques such as canisters or adsorbents that require further workup for analysis by GC or GC/MS, open-path FT-IR avoids samAnalytical
Chemistry
ple handling and offers near real-time analysis. Moreover, canisters and absorbents are point samplers. A non-detect with a point sampler doesn't mean that an analyte isn't present at high concentrations, points out William Vaughan, president and principle scientist of the consulting firm Remote Sensing Air (St. Louis, MO). However, a non-detect with an openpath system does provide an upper limit on what may be present. "You can use open-path to give you a conservative worse case," Vaughan points out. Vaughan adds that he will use both open-path FT-IR and canisters at some sites. "The comparisons come out pretty good," he says. In fact, a 1991 formal intercomparison sponsored by the University of Kansas at Lawrence and EPA's Region VII found that concentrations determined by open-path FT-IR agreed to ± 20% with values from canister samples collected along the same path and analyzed by GC {Environ. Sci. Technol. 1992,26,2175-81). Open path also works well for finding fugitive gas leaks. "You may have to set up a series of canisters and move them around [to find a leak]," says Gurka. "But with open path you set up the equipment and look over distances as long as a kilometer." Point samplers, on the other hand, generally offer better detection limits. The prime example is benzene, says Spellicy. Because of interferences with carbon dioxide in ambient air, the best IR band available for measuring benzene offers detection limits of 30-40 ppb. When extractive IR is used, where the air sample is scrubbed of carbon dioxide before being analyzed in a gas cell, detection limits for benzene can be lowered to 2 ppb. Canisters and solid adsorbents provide similar improvements in detection limits. However, when open-path FT-IR is used for regulatory or safety compliance, as well as accidental releases, the lower detection limits may not be required. FT-IR also gives "a boatload of data," according to Tom Wisniewski, an FT-IR specialist with ETG. "If something is coming across your fence line and it is an IR absorber, it's likely to be in the spectrum." Indeed, a number of the experts related stories of inadvertently stumbling across fugitive gas leaks from facilities adjacent to a customer's or a user's site. However, the notion of lots of data is not always appealing to potential customers. "If chemical company X is using our system and the neighborhood decides to News & Features, January
1, 1997
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Product
Review
eters tend to be fairly temperaturesensitive. "One would like an instrument that compensates for [temperature changes] and maintains its internal optical alignment—ideally, for weeks," says Griffiths. All the commercial instruments compensate for these temperature changes; many systems use some sort of dynamic alignment to keep the instrument aligned through temperature drifts and even vibration. Our reviewers find that some designs work better than others and that purchasers should carefully check claims of thermal stability. Software poses several problems. "The steps needed to get good quantitative and even qualitative data are pretty important. You need good software," warns Griffiths. "The analytical software used to extract concentrations must take into account a Figure 1 . Setups for open-path FT-IR. variety of subtle effects to produce reliable (Adapted from Environ. Sci. Technol. 1994, 28, 224 A-231 A) results," says Spellicy. The proper analytical routines must also be coupled to good reference spectra to obtain precise and sue, these data can help, but they can also and IR source at one site and uses a retroaccurate data under a range of atmoreflecting mirror to bounce back the IR hurt," says Wisniewski. spheric conditions. light to the detector. Although the bistatic The other limitation of open path is "A big concern is the accuracy of the that it is at the mercy of the weather. "FT- mode is cheaper to set up, the monostatic mode is preferred because it requires reference data," says Griffiths. Open-path IR gives in one scan many compounds," power at only one rather than two locations instruments yield an integrated spectrum says Luc Rochette of Bomem Instruof typically a mixture of analytes. "You can ments. "The disadvantage is that there are and, as a result of modulating the IR light compare the mixture spectrum with indimany parameters than can't be controlled, from the source, it is less susceptible to vidual spectra and start subtracting out such as the wind." (Ironically, by correlat- bias caused by recording stray light. spectra and see what's left over," says ing observed gas concentrations with Open-path FT-IR equipment weighs Gurka. "A database of under 100 chemiwind direction, it is frequently possible to around 100 lb, and therefore these syscals is typically adequate." find the direction of a gas release and, tems are transportable but not lightwith more than one FT-IR even the apweight. However, like any optical system, In practice, however, software matches proximate location of the source.) resolution and sensitivity improve if the the unknown mixture by fitting it with a instrument is permanently mounted on a classical least-squares sum of library speccement platform to produce a stable align- tra. This forms a series of library spectra, Transportable and rugged ment without optical drift or vibrations. As each of which is scaled to a specific conThe typical open-path FT-IR system concentration to generate a best match with sists of a Michelson-type interferometer, a a result, when running short-term studies that require a temporary setup, Vaughan the unknown. This approach has the admercury-cadmium-telluride (MCT) phosays he is "uncomfortable" taking meavantage of picking up analytes at very low todetector (operating at liquid nitrogen levels; rapid quantification; and simultatemperatures, which adds another consid- surements at distances greater than 500 m. One kilometer is probably the top neous fitting, which generally produces eration to field work), and associated dedistance for the current generation of per- better results. tection optics piggy-backed onto a collimanently mounted instruments, primarily mating telescope. Although the system In many situations, analysts know which because atmospheric interferences limit can be operated passively by relying on volatiles are present Nevertheless, our exmeasurement beyond that point. ambient IR light, the detection limits sufperts also report running into unexpected fer, and the measurement techniques are In choosing which FT-IR system to buy, compounds that must be analyzed manually. not as easy. Instead, field-monitoring stud- our experts cite ruggedness, thermal stabilQuantitative reference spectra can, of ies usually use an IR source. Windows and ity, and software as key elements to evalucourse, be generated by the analyst in the beamsplitters are generally composed of ate. In general, commercial systems earned laboratory by preparing calibration samples rugged and nonhygroscopic zinc selenide. high marks for ruggedness from our rein a gas cell. The library spectra should viewers. "I've had my system for four years, match as much as possible the conditions How these key components are put toand nothing much has gone wrong with it," under which the open-path spectra are gether depends on the choice of experisays Vaughan. What fails typically, says taken. Gurka, however, says that these mental setup (Figure 1). In the bistatic analyst-produced spectra can suffer from mode, the IR source and optics typically sit Gurka, is the IR source, which lasts about one year but is inexpensive. problems such as leaking cells and adsorpat one end of the pathlength, and the intertion of material onto the cell surface. "It is ferometer and detector sit on the opposite Open-path systems are expected to go not easy to quantitatively generate these end. The monostatic mode, on the other anywhere and to work under diverse hand, places the interferometer, detector, weather conditions. However, interferom- calibration curves in the lab." 46 A
Analytical Chemistry News & Features, January 1, 1997
New! Commercial libraries of data are an attractive alternative to doing it yourself. "Most of the data are very good, but some are not," says Griffiths. "And the average user wouldn't be able to tell what is good and what is bad." Not surprisingly, Vaughan says that high on his wish list of technique improvements are standardized spectra that would be NIST-traceable, like the reference materials. 'The libraries really need government funding so that they are standardized and intercomparable," he says. NIST, in fact, is now producing a series of reference standards for some common volatile organic compounds. (According to Kagann, NIST has already prepared spectra standards for 13 of the 189 hazardous air pollutants listed in the Clean Air Amendment legislation approved by Congress in 1990.) Another solution to the library problem would be to collect spectra using instruments with lower resolution. These would allow access to larger commercial libraries of gas-phase IR spectra. "Historically, open-path spectroscopists have thought that you need resolution all the way down to 0.1 cm-1," says Gurka. "To the extent you can operate at lower resolution such 8 cm-1 there is cess to a larger database and sensitivity gets better" Griffiths, in particular, has become an advocate for lower resolution. "We feel that it is not necessary to go to very high resolutions, providing you've got a good S/N," he says. Quantitation, in fact, improves as the resolution drops. Other experts in thisfieldquestion the use of lower resolution, arguing that for some compounds, sensitivity worsens. "There is a heated debate in the scientific community about what resolution is optimum and under what conditions you can use lower resolution without loss of specificity and detection limit," "ays Sppllicy. Concern over the loss of specificity arises from the ubiquitous interference from narrow absorption lines of water, says Kagann. "At low resolutions, water lines will be smeared out to the point where they will be difficult to detect in the multicomponent analysis." This can lead to a bias in the calculated concentration values.
On the other hand, lower resolution may mean less expensive instruments. EPA says Gurka, would like to see the cost of open-path systems drop, which would make the technique more attractive to cash-strapped environmental laboratories. For example, Griffiths' group is working with EPA to reduce the cost of the retro-reflectors used in the monostatic mode. Griffiths reports success in spincasting mirrors, lowering the cost of each one in his laboratory to less than $500, excluding the cost of labor. Fortunately, according to company representatives, the costs of FT-IR systems are already dropping. A larger market for these instruments, point out the commercial suppliers, would also aid in cutting costs. A technique for everyone?
If open-path FT-IR spectroscopy is now an EPA compendium method, does that mean anyone can run the technique? Our experts answer "yes" and "no". "It was never intended to be a technique done without sophistication. You need to have someone who knows what the client is looking for and what the purpose of the beam is in order to set it up properly " says Vaughan This means that if the instrumentation is installed and set UD correctly in the first place it can be run successfully as a more routine automated techni e Griffiths seconds that notion. "It is one of the drawbacks [of open-path FT-IR]— setting it up requires something of an expert." However, he believes that "there are several approaches to handling openpath data, including the application of neural networks and genetic algorithms, that should allow these drawbacks to be overcome in the future." All of this suggests that the commercial market for open-path FT-IR instrumentation could finally begin to grow. "The key thing is to be gun-shy of exaggerated sales claims, but don't dismiss the technique because of the enthusiasm of the salesman," argues Vaughan. "Technically, there are plenty of good reasons to use these systems." Alan R. Newman
There is a heated debate in the scientific community about what resolution is optimal.
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Analytical Chemistry News & Features, January 1, 1997
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