product review
Spectrometers get small Miniature spectrometers rival benchtop instruments. James P. Smith
T
he marriage of fiber optics and semiconductor detector arrays has resulted in a new breed of spectrometer. Lenses, mirrors, and scanning monochromators are no longer needed, as bulky bench-top spectrophotometers are replaced with systems built around miniature spectrometers. These Liliputian spectrometers commonly consist of a light input slit, a diffraction grating, and a detector array—all sealed in a small box. Moreover, there are no moving parts and thus, these systems are quite robust. Yet their performance rivals many benchtop instruments. In these instruments, fiber optics connect a light source and a sample module to the spectrometer. Following analog-to-digital (A/D) conversion, the detector output is then stored or analyzed by one of several software applications. The wavelength range and optical performance of a miniature spectrometer are usually set at the time of manufacture, but various sample modules allow reflectance, transmission, absorption, Raman, or fluorescence analysis applications. For example, by changing a few fiber-optic connections, an absorption instrument can become a spectrofluorometer or a reflectance instrument. Even better, the relatively low price and versatility of these systems make them ideal for use in teaching labs, process monitoring, and field analyses.
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Table 1. Representative fiber-optic/semiconductor array miniature spectrometers. Model
Vis LIGA
PDA
SM-200
Millennium 3
MMS 1
Company
American Laubscher 80 Finn Ct. Farmingdale, NY 11735 631-694-5900 fax 631-293-0935
Control Development 3702 W. Sample St. South Bend, IN 46619 219-288-7338 fax 219-288-7339
CVI Spectral Products, East 111 Highland Dr. Putnam, CT 06260 860-928-1928 fax 860-928-2676
Equitech International 1359 Silver Bluff Rd. Aiken, SC 29803 800-768-8715 fax 803-642-0444
Hellma 118-21 Queens Blvd. Forest Hills, NY 11375 718-544-9534 fax 718-263-6910
URL
www.alcprecision.com www.microparts.de
www.controldevelopment. com
www.cvilaser.com
www.equitechintl.com
www.helllma.com www.Zeiss.de
Approximate price
$500
$2995
$1695
$7085
$1850
Dimensions (h 3 w 3 l)
0.04 3 0.55 3 0.91 in.
6.8 3 4.0 3 1.6 in.
5.6 3 2.75 3 0.87 in.
3.5 3 6.5 3 4 in.
2.8 3 2.4 3 1.6 in.
Dispersion
Self-focusing reflection grating, injection-molded as part of the package; 625 lines/mm; 560-nm blaze
Flat grating; 600 lines/mm; 500-nm blaze; 25-µm slit is standard
Several choices of ruled flat gratings and slits
Flat-field holographic grating; 250-nm blaze; 25-µm or 100-µm slit
Flat-field holographic grating; 366 lines/mm; 220-nm blaze; 70-µm slit
Detector(s)
PDA, 256 pixels, alternative CCD or PDA on request
PDA, 512 pixels (PDA), or high-sensitivity CCD
CCD array with 2040 pixels or PDA (choice of 128–1024 pixels)
CCD detector 64 3 1024 elements, first 400 columns UV-coated
PDA, 256 pixels
Wavelength ranges and resolution
380–780 nm; 7-nm resolution with 50/125 fiber optic, which acts as the slit
200–1100 nm; resolution is 2 nm in vis
200–1050 nm; resolution is 0.3 and up
190–790 nm; >0.75-nm resolution.
310–1100 nm; >10-nm resolution
Special features
Second diffraction order removed by blaze; operates from -20 to 40 °C; smallest spectrometer on the market
CCD available with UV upconverting coating; proprietary fast optics design, f/2.0 or f/3.0; free guaranteed wavelength calibration and amplitude calibration
Free acquisition software, LabView drivers, and DLL libraries; dual sample input mode; option of ISA, PCI, PCMICA, serial port, parallel port, or customer specification
Active-X Control; operates in either single-channel, or multichannel modes, which is configurable through software
Optical entrance options include a mechanical tubus, fiber cable, and slit
Table 1 lists features of representative miniature spectrometers. Such spectrometers are at the heart of spectrophotometer systems that are based on fiber-optic and semiconductor detector array technologies. The core spectrometer is often referred to as the “spectrometer engine”. To compare “apples with apples”, the spectrometers outlined in Table 1 are the basic, visible wavelength systems manufactured by companies that make their own spectrometer engines. By changing the dispersion system or detector, the wavelength range of each of these engines can be extended into the UV, or they can be optimized for near-IR applications. In addition, spectrophotometer systems can be made in various configurations. Interested readers should contact the companies for further details. Several companies, not listed in the
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table, also make spectrophotometer systems by using spectrometers made by other companies. Imaging spectrometers, which characterize the distribution of spectral properties on twodimensional heterogenous surfaces, are often based on miniature spectrometer systems, but these complex and expensive instruments are also not described in this product review. Thus, the table is best used to illustrate the diversity of spectrometer engines available, and individuals planning to purchase such equipment should use the table as a starting point. Most spectrophotometer systems contain these basic components: a light source, a slit, a light-dispersion unit, a sample module, fiber optics, a detector array, an A/D converter, a computer interface, and software. It is important to consider the choices available for these components, keeping in mind the
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specific application(s) planned for the system. Standard off-the-shelf, handheld field spectrophotometers are available, but quite often, spectrometers are customized to meet specific needs. Companies that build spectrophotometers with miniature spectrometers have learned to be adept at configuring instruments for specific applications.
Semiconductor detector arrays The choice of the best detector array for a particular application is critical. “There are three classes of devices,” says Bonner Denton of the University of Arizona. “These include true CCDs [charge-coupled devices], true photodiode arrays [PDAs], and several hybrid devices—often referred to as CCDs. But, in these hybrids, the actual light conversion is first done with photodiodes, then they are read out with a CCD shift register system.” Denton
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Table 1. Representative fiber-optic/semiconductor array miniature spectrometers (continued). Model
SpectraView 2000
PC 2000
MicroPac
Model 430
EPP 2000-C
Company
Korea Materials and Analysis 104-11 Moonji-Dong Yusong-Gu Taejeon 305-380 Korea +82-42-863-6868 fax +82-42-863-6867
OceanOptics 380 Main St. Dunedin, FL 34698 727-733-2447 fax 727-733-3962
Optical Coating Laboratory 2789 Northpoint Park Santa Rosa, CA 95407-7397 707-525-6957 fax 707-525-7841
Spectral Instruments 1802 West Grant Rd. Suite 120 Tucson, AZ 85745 520-884-8821 fax 520-884-8803
StellarNet 612 Shore Dr. E Oldsmar, FL 34677 813-855-8687 fax 813-855-2279
URL
www.kmac-12.etri.re.kr
www.oceanoptics.com
www.ocli.com
www.specinst.com
www.stellernet-inc.com
Approximate price
$2000
$1999
$799
$6000
$2495
Dimensions (h 3 w 3 l)
4.7 3 3.9 3 1.7 in.
1.25 3 6 3 2 in
0.63 3 1.4 3 1.4 in.
9 3 13 3 5.5 in.
6 3 4.25 3 2.75 in.
Dispersion
Linear grating 600 lines/mm; 4 slits available from 25 to 200 µm
Linear grating 600 lines/mm; 400-nm blaze; 25-µm slit
Linear variable (bandpass) filter
Linear grating; electronic slit by binning 5 pixels
Concave holographic grating; aberration corrected
Detector(s)
2048-element linear silicon CCD array
2048-element linear silicon CCD array
Choice of many
Linear CCD array with 3900 elements
Linear PDA, 2048 pixels
Wavelength ranges and resolution
185–1100 nm with 0.3–10nm resolution
350–900 nm with an average 1.5-nm resolution; UV range also available
400–700 nm with 5-nm resolution; 600–1100 nm with 7-nm resolution
350–980 nm with average of 1.2-nm resolution; UV range also available
200–850 nm with average of 0.75-nm resolution
Special features
Optics have no moving parts and a vibration tolerance property; compatible with any computer system by using a printer port for the signal transfer
Spectrometer and A/D interface board are inside the computer; ideal for educational systems
This is the only miniature spectrometer without a grating; very wide slits and mirrors allow more light to the filter; very stable with little thermal drift
Complete set of fiber-optic sampling accessories; acquisition time is very fast
As many as eight analyses can be performed simultaneously; daisy-chained units allow dual- and multiple-beam applications
says that, in general, the true CCD devices tend to have lower read noise and often higher quantum efficiencies. “PDAs tend to have read noises between 3000 electrons and 15,000 electrons, while the good scientific CCDs are around 5 electrons,” he said. The hybrid devices can have read noises between the CCD range and the photodiode range. When the incident light on a typical PDA falls within the range of 10212–1022 W, the achievable range of linearity is higher than 9 orders of magnitude, depending on the type of photodiode and its operating circuit. The lower limits of light detection for PDAs are determined by the noise characteristics of the device. PDA detectors are recommended for high light-level measurements or when a very high S/N is required, according to Denton.
A scientific CCD, on the other hand, is several orders of magnitude more sensitive than a PDA and is, therefore, the detector of choice for low-light measurements, such as Raman or luminescence. CCD arrays are either two-dimensional or one-dimensional (linear). The linear units are more commonly used in miniature spectrometers. There is a minimum noise level in CCDs that cannot be eliminated or reduced. This noise arises from read noise and shot noise; it is often the limiting factor in S/N or sensitivity. However, by lowering the temperature of the CCD, the dark signal and its associated shot noise are reduced to minimal levels. With thermoelectric cooling, the temperature dark signal can be less than one electron per pixel per hour. Such low noise level measurements are important for Raman spectroscopy, single-molecule, and evanes-
cent fluorescence studies. These are measurements of very small signals on a dark signal background. Denton warns that there are limitations to cooling. “For approximately each 8 ºC drop in temperature, the dark current is reduced by one-half,” he explains. For a CCD, the optimal strategy involves cooling the device until its charge transfer efficiency begins to suffer. For most scientific CCDs, this occurs near 2100 ºC. If cooled below that, Denton says the charge transfer efficiency suffers, leaving charge behind with each transfer. “A good CCD has 99.999% charge transfer efficiency.” He adds that cooling is usually not important for routine UV–vis absorption analysis, because the background light level is high compared with the dark current, and the analyst is measuring a decrease in this high level.
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David Landis of OceanOptics offers a detector strategy by comparing two detectors—a CCD with a 250:1 S/N for a single acquisition and a PDA with a 2500:1 S/N. He points out that equivalent S/N can be achieved by the two detectors in the same amount of time by using a single, long integration time with the photo-
from 400 to 700 nm with a resolution of 5 nm, or from 600 to 1100 nm with a 7-nm resolution. The distance between the two reflective surfaces varies from one end of the filter to the other; thus, a given wavelength passes through only a small area of the filter. The filters are placed adjacent to the detector array. A resolution of 5 nm is poorer than what is achievable The less-expensive ruled with grating systems, but many applications gratings work well, as long do not require higher resolution. as they fit the application. Although prisms have the potential for much better light throughput diode or multiple, short integration than a grating, they have not yet been times with the CCD. To understand incorporated into miniature spectromthis, one first needs to realize that eters. Light Form of New Jersey marthere is a trade-off between S/N and kets a spectral imaging system that sensitivity, because S/N is the square contains a small, unique prism, which root of the electron well depth of the might someday be adapted to miniadetector, and a larger well must conture spectrometers. tain more electrons before they are Diffraction gratings are manufacdetected. Signal averaging can increase tured by processes that include methe S/N. “With averaging, the CCD chanical ruling, replication, hologracan achieve a 2500 S/N, improving phy, and injection molding. They are as the square root of the number of characterized by their groove density, signals averaged,” says Landis. “So, blaze wavelength, and method of preto move from 250 to 2500, one aver- paration. The optimization of the groove profile to maximize grating ages 100 spectra. This is accomplishefficiency at a certain wavelength is ed in less than a second with a short called blazing. Denton says that the integration time.” Thus, says Landis, less-expensive ruled gratings work using a CCD and signal averaging well, as long as they fit the applicacan save approximately $800 over a tion. For example, most solution single-scan PDA with a 2500 S/N. absorption analyses do not require Wavelength dispersion high resolutions, nor extremely low Interference filters use selectively stray light. spaced reflecting surfaces to reinforce Holographic gratings are made the wavelength of interest and cancel by recording laser-generated interferothers. Harmonic frequencies are elim- ence patterns. “Since no mirrors are inated by glass cut-off filters. Several required, stray light [sometimes a low-tech instruments—colorimeters major problem in the UV range] is or photometers—also use filters for much less with holographic grating wavelength selection. Although these systems than with ruled gratings,” instruments are useful for many rousays Will Pierce, of StellarNet. “In tine analyses, they have not been inmany spectrometer designs, there are cluded in this review. aberrations built in due to the optical Only the MicroPac uses bandpass angles, but an aberration-corrected filtering for light selection. A variable holographic grating can be made with band-pass filter selects wavelengths just the opposite aberration, so they
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all cancel out.” StellarNet uses this type of grating in its EPP 2000-C spectrometer.
Fiber optics Quartz fiber optics are used in the miniature spectrophotometer systems: low water content quartz for the nearIR, and high water content in the UV. “Companies also use a solarization-resistant fiber optic, which is not degraded by UV,” says Landis. “There are color centers in fiber optics that absorb light below 250 nm, and these can decrease transmission over time.” There are several antisolarization fiber optics on the market, he notes. “The one we use has an aluminum jacket, and other than that, it is a quartzclad, quartz-core fiber—the same as the standard fiber.” However, Jack Zhou with CVI Spectral Products says the “jury is still out” on antisolarization strategies. Several are being tested now, and their success seems to depend on the applications and instrument parameters. Many investigators are interested in the spectral analyses in the 193-nm range, in which solarization is a concern. The diameters of fiber-optic cores vary from 10 to 400 µm in these spectrophotometer systems. Again, the choice should be based on the application. For example, the tiny LIGA spectrometer relies on the diameter of the fiber to act as the slit; a very small fiber provides improved resolution. Largerdiameter fibers require a slit, and the slit width determines resolution.
Fiber-optic sampling options The most compelling attribute of these miniature spectrometers is their versatility, and this is particularly illustrated by the collection of sampling modules that can be quickly attached to the instruments. Analysts who frequently convert benchtop instruments to different sample configurations will appreciate that these miniature spectrometers can often be converted by simply changing a few fiber-optic con-
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nectors. Some of the sample modules are listed below. Cuvette holders. Cuvette holders for sampling can be configured for absorption or for fluorescence measurements. The light should be well collimated for these applications. Flow injection analysis (FIA). FIA automates manual wet chemical procedures in routine laboratories, such as the quantitative mixing of reagents, cleaning, preparing blanks, and measuring standards. The miniature spectrophotometer system is ideal for this automation because very small volumes are needed. Immersion probe. Fiber-optic immersion probes eliminate traditional cuvettes. Probes are placed into the prepared sample, just like a pH probe. Typically, they have a 1-cm fixed pathlength and a transmission range of 190–980 nm. These rugged probes are generally constructed of stainless steel, two fibers made of fused silica, and a chemical-resistant epoxy coating. Standard probes can withstand temperatures of 120 °C and can be used in water and common organic solvents. One common immersion probe strategy is based on the transmission of light through one fiber from the light source to the tip. The light then travels 5 mm through the sample, reflects off a back-surface curved mirror or inside a 90º prism, and then passes back through another 5-mm length of sample to the input fiber. The light then travels back through the input fiber to the spectrograph. Reflectance probe. Fiber-optic reflectance probes are used for measurements of powders, solids, and liquid surfaces. The sample absorbs part of the light and reflects back a small portion. The reflected light carries the spectral characteristics of the sample. Attenuated total reflectance (ATR) probe. The ATR probe is used for the direct measurement of highly absorbing dyes and pigments and other highly absorbing materials in high concentrations. In the probe, light is transmitted by single-strand fiber to a sap-
phire crystal. The light is internally reflected in the sapphire to a receiving single-strand fiber cable. The spectrum of any strongly absorbing, lower-refractive index material in contact with the sapphire can be measured. Integrating sphere reflectance attachment. Fiber-optic coupled integrating sphere attachments provide diffuse, uniform illumination to measure larger spot sizes and samples of varying gloss and texture. The fiberoptic coupling allows measurements on large panels that are otherwise impossible to study with traditional benchtop spectrographs and color measuring instruments.
Light sources The most common light sources used in miniature spectrophotometer systems are deuterium–halogen lamps for deep-UV (185–400 nm) light; tungsten–halogen lamps, which provide a white light source (360–2000 nm); and pulsed Xe flash lamps, used for the UV–vis region (220–1000 nm). The deuterium–halogen lamps have a high photon output and are quite stable, but they require a large power supply and a high-voltage starting arc.
minimizing light loss from surfaces, dispersion, and focusing optics. Thus, the tiny instruments do not require as intense a light source as do the larger spectrophotometers. Often, two light sources are used simultaneously. Light-emitting diodes, which are found in some simple colorimeters, are sometimes placed in front of a tungsten–halogen lamp to enhance the blue region. A “shine-through” arrangement is used to combine the light from a tungsten–halogen lamp with that of a deuterium–halogen lamp. This can provide a wavelength range from 180 to 1100 nm.
Electronics and software The price of a spectrometer engine usually includes the A/D converter card and a computer interface, such as a cable or a card for mounting in the computer. Most spectrometer converters use 16-bit dynamic range electronics. The complete commercial miniature spectrophotometer systems can take many forms, from a hand-held instrument to a computer CPU with a fiber-optic connection for a sample module. As a result, there are several interface cable connections available for spectrophotometric systems, rang-
These miniature spectrometers can often be converted by simply changing a few fiber-optic connectors. This has limited their use in miniature spectrophotometer to UV applications. However, Huraeus recently introduced a light source containing a very small electrodeless deuterium lamp with a power consumption of only 3 W. Because fiber-optic solarization is a problem with UV radiation, the smaller UV lamp promises better fiber life than with a high-intensity UV lamp. Another consideration is that the optical components in the miniature spectrometer are also very close together, thereby
ing from parallel ports to credit-cardsized PCMCIAs to the newer universal serial bus connections. Spectrometer and spectrophotometer companies generally provide their own software with a system. The software should support each sample module and allow the flexibility that the hardware offers without being too comprehensive and complicated. Sample automation software is sometimes used. Other software are capable of sequentially recording the spectra of
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Miniature spectrophotometer
lation, which allows for real-time, on-line display, systems can take many forms, and predictions, as well as the storage of spectral from a hand-held instrument data and predicted analyte concentrations. The to a computer CPU. second type processes the data after its accumulation, and storage. several fibers stacked at the slit, single Selected wavelength ranges can be spectra in a sequentially multiplexed modeled via classical least squares, fashion, or dual spectrometer systems multiple linear regression, principal in sample/reference mode. The aucomponent regression, and partial tomation of FIA may also be necessary. least squares analysis. Most companies supply free software and DLL libraries to aid in further Quality assurance procedures software development. Active X conMany analysts face the task of ensuring trol libraries are also available for use that their measurements are legally dewith Visual Basic and other languages. fensible, such as in quality assurance. There are two types of software Analysts who work in environmental, data processing packages. The first quality control, industrial hygiene, functions at the time of data accumuand forensics understand that a defen-
sible analysis requires more than just a calibration curve. Jerry Messman, president of SpectroStandards Analytical and a leader in the field of spectroanalytical metrology, says, “As these new spectrophotometers enter the laboratory and process industries, standards and procedures need to be evaluated and updated as needed to insure the defensibility of their data.” Miniature spectrometer systems can be as small as a postage stamp; they can be bounced off the wall and still work. Their resolution and sensitivity can rival their big brothers on the bench top. Spectrophotometers can now be less expensive and much more versatile than ever before. But, as always, the validity of the analysis is in the hands of the analyst. James Smith is a freelance writer based in Lenox, MA.
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