In the Laboratory

796 Journal of Chemical Education • Vol. 78 No. 6 June 2001 • JChemEd.chem.wisc.edu

At the Université du Québec à Montréal, we are com-mitted to offer a B.Sc. chemistry program that has a strongexperimental component. Our program unfolds over athree-year period. As part of the 90-credit chemistry degree,students are required to complete a number of 3-credit labo-ratory courses. Among the laboratory courses offered is a 3rd-year instrumental course on FTIR, NMR, AA, fluorescence,phosphorescence, and UV–vis spectroscopy. The lab class isdivided into groups of two students and each group is requiredto complete 11 experiments during the semester. The labsrun on a 6-hour period.

New technology has modified classroom academicssignificantly in recent years. Computers and a wide range ofsoftware facilitate data acquisition, allowing us to integratesignal-processing discussions into our course. We wanted toinclude such a discussion within an applied frame to showthe limits of these treatments when applied to real problems.To implement this concept, our lab needed instrumentationthat was both relatively inexpensive and capable of varioussignal-processing treatments. As these treatments are not offeredon simpler and less costly instruments such as the Spectronic20, such spectrophotometers were not a valid option. Mostspectrometers offer a variety of mathematical treatments butfew show their effect in a dynamic way.

In this paper, we discuss the effect of integrating opticmodules into our undergraduate-level laboratory classroom.Two miniature 2048-element CCD-array fiber-optic spec-trometers were purchased from Ocean Optics Inc., theCHEM2000 VIS and S2000 UV-VIS. They are briefly describedbelow. However, the object of this paper is not to comparedifferent spectrometers but to show that implementation ofthe Ocean Optics spectrometers has improved the quality ofour students’ learning.

Equipment Components

Like all such equipment, each instrument comprises anIBM-compatible computer and a spectrometer. The spec-trometer has five basic components:

1. Source

2. Sampling optics

3. Optic fibers

4. Micro-machined diffraction grating and detector(CCD-array)

5. A/D converter

The light sources vary according to the type of analysis.For the S2000, the light source, a deuterium tungsten-halogenlamp, combines a continuous spectrum in the UV and visibleregions. Its output ranges approximately from 200 to 1100 nm.The tungsten-halogen light source, on the other hand is ver-satile for the visible region, between 360 and 1700 nm.

The sampling chamber, capable of holding standard1-cm2 cuvettes, is directly connected to the source and thedetector via optical fiber cables. The cuvette holder has a5-mm-diameter f/2-collimating lens to collect the light andfunnel it to the exit solarization-resistant optical fiber. Ad-justment screws allow optimization of angles of incidence.The optical fiber cables used limit the range of the source.Attenuation is fairly flat in the visible but increases stronglyin the UV. In the NIR, water absorption bands at 750 and900 nm affect fiber attenuation (1).

The detector comprises a 2048-element linear siliconCCD-array and has an effective range of 200–850 nm,thereby limiting the spectral output range of both lightsources. The diffraction grating, the detector, and the A/Dconverter are housed on an ISA-bus card for PC. This as-sembly allows integration times of 5 ms to 60 s.

Finally, a Windows95-based program provides rapid ac-quisition and various processing functions, such as delay timeof acquisition and point-to-point averaging, both of whichare discussed below. This software is user friendly and requiresno prior experience. The basic concept of this software permitsimmediate time display of data, allowing the students to verifythe effectiveness of their experimental setup.

Compared to more conventional spectrometers, thesemodular instruments offer another major advantage: their lowcost. The S2000, the more versatile system, is 5 to 6 timesless expensive than its conventional competitors. However,Yee et al. have demonstrated that the dispersion and resolu-tion of modular spectrometers and a conventional low-endcommercial instrument are comparable (2).

The ExperimentsKnight and Farnsworth have successfully used the Ocean

Optics modules in their university classroom to provide livevisible spectra to a large audience (3). However, we know of nopublished accounts of direct implementation of miniaturespectrometers in lab courses.

Two experiments, previously performed with conven-tional spectrophotometers such as a Cary 1E (Varian) and a8800 (Pye-Unicam) model, were constructed with miniatureoptical fiber Ocean Optics spectrometers. We describe theexperiments that were successfully adapted using the modularspectrometers. Data obtained with all three instruments werecomparable.

Experiment 1In the first experiment, the CHEM2000 is used to deter-

mine (i) the ligand/metal ratio in a Fe(II)/phenanthroline com-plex using the standard method of Job and (ii) the dissociationcoefficient of bromocresol green. Job’s method establishes thatthe intensity at a fixed wavelength of a ligand/metal solutionof constant molarity is greatest when the molar fraction of the

Modular Spectrometers in the Undergraduate ChemistryLaboratoryPaul Bernazzani and Francine Paquin*Département de chimie, Université du Québec à Montréal, C.P. 8888, succ. Centre-Ville, Montréal, PQ H3C 3P8,Canada; *paquin.francine@uqam.ca

In the Laboratory

JChemEd.chem.wisc.edu • Vol. 78 No. 6 June 2001 • Journal of Chemical Education 797

ligand in solution is equal to that in the complex. Studentsshould conclude that three molecules of phenanthroline arefound in the coordination sphere of the Fe. Equation 1 showsthe formation of the complex.

Fe2+ + nPhenanthroline → (Fe(Phenanthroline)n)2+ (1)

Each of these components absorbs in a different region andthe students are asked to find the value of n.

In the second part of the experiment, students determinethe dissociation coefficient of bromocresol green. This caneasily be accomplished during the hour when the ligand/metalcomplex is forming. (Since the students have a limited amountof time to make a great number of solutions and measurements,they are forced to use their instrumentation time efficiently.)Students should find that the dissociation constant of bromo-cresol green is close to 4.72.

Experiment 2In the second experiment students use the S2000 spectrom-

eter to construct calibration curves for three aromatic acids basedon the UV intensities of benzoic, salicylic, and p-hydroxy-benzoic acid solutions. Using these curves they evaluate relativefractions of unknown solutions containing a mixture of theseacids. The correlation coefficients of the acids ranged from.784 to .999. The relative errors for the determination of theconcentration of the acids in the unknowns are of the orderof 0.2–4.5%. Both the correlation coefficients and the errorfactors of the mixtures studied were of the same magnitude asthose obtained using conventional UV–vis spectrophotometers.

Discussion: Signal ProcessingThese two experiments were chosen because the time

factor is critical in the first, whereas in the second, precisionis important.

Both experiments also provide an introduction to signalprocessing. The students extensively vary two parameters (curvesmoothing and scanning average) and observe the effects on theircalibration curves. They must evaluate the influence of eachparameter with respect to the objective they must achieve.They are shown that curve smoothing by the running averagemethod (dynamic averaging) is useful if the fluctuation ofthe noise is significantly greater than that of the signal. Themethod has two distinct drawbacks: (i) it eliminates somedata points at the extremities of the spectra, and (ii) over-smoothing can easily lead to loss of data. However, althoughthe Ocean Optics software will run the average over anynumber of points, we seldom encountered over-smoothingbecause the apparatus has a high resolution (0.6 nm with apixel resolution of 0.15 nm [4 ]).

The students must also verify that static averaging, whichrequires either taking multiple scans and averaging them orvarying the integration time, can improve the data as a functionof the square root of the number of scans (or more accurately,the time). Students can thus observe whether this is correct,depending on the limiting types of noise and their nature,and whether the number of scans or the time required islow enough to be of any value. If the square root relationshipbetween time and signal-to-noise ratio is valid, then whitenoise, defined as shot noise in the background or dark signal,is the limiting noise factor. Since we found that the signal-to-noise ratio varied proportionally with time, 1/f noise is thelimiting noise.

Because the Ocean Optics spectrometers are capable of rapidmeasurements over the complete region of interest, students havesufficient time to evaluate the effects of parameter settingsand optimize them. They can complete the experiment in lessthan the allotted 6 hours. However, two drawbacks of theseinstruments are noteworthy.

First, at the end of the course, students take home theirresults on floppy disk for subsequent evaluation. However,some students found exporting the data to be cumbersomebecause of difficulties with software incompatibility. Thispart of the software could be improved.

Second, the absolute values of intensity where found tofluctuate. Pokorny et al. used an S2000 to evaluate the colorof white wines (5). They found that using 30–36 responsesyielded an average standard deviation of the mean of the orderof 3%. In the best experimental conditions, we observe a 2%standard deviation. Compared to the 0.3% variation observedon the much slower Cary 1E from Varian, the Ocean Opticsspectrometer’s accuracy fares poorly. In the scope of our teach-ing goals however, this is not a problem because the precision(reproducibility) was comparable for the two apparatuses.Previous work also demonstrates the overall reliability of theinstruments (6 ). Since a major objective of the course is tointroduce signal processing, the Ocean Optics spectrometersare still advantageous.

EvaluationJudging by the students’ evaluation, these experiments are

a success. Students are eager to operate these miniaturespectrometers. They are fascinated by the speed of data acquisi-tion with the Ocean Optics spectrometer and its ease of use.They develop a curiosity about spectrometry and a basicunderstanding of signal processing methods.

Our evaluation of these modular spectrometers issummarized in the box. As this list shows, the advantagesoutweigh the disadvantages.

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In the Laboratory

798 Journal of Chemical Education • Vol. 78 No. 6 June 2001 • JChemEd.chem.wisc.edu

Conclusion

The acquisition of these modular spectrometer unitsenhanced our instrumental laboratory course. In general, weare very satisfied with the effect these modules have had onour curriculum. Their impact is twofold. First, they are lesscostly than the conventional equipment currently available.Second, their speed, ease of use, and capacity to vary signal-processing parameters facilitate the understanding of spec-troscopic principles. These instruments are a favorite amongboth students and teaching staff.

Literature Cited

1. Ocean Optics. Optical Fiber Performance Characteristics; Tech-nical document; http://www.oceanoptics.com/productsheets/

optical%5Ffibers%5Fperformance%5Fcharacteristics.asp (accessedFeb 2001).

2. Yee, G. M.; Maluf, N. I.; Hing, P. A.; Albin, M.; Kovacs, G. T. A.Sens. Actuators, A 1997, 58 (1), 61–66.

3. Knight, J. B.; Farnsworth, P. B.; Spectrochim. Acta, B 1998,53, 1889–1893.

4. Ocean Optics. Selected Components: Optical Resolution: http://www.oceanoptics.com/specifications/optical%5Fresolution.asp(accessed Feb 2001).

5. Pokorny, J.; Filipu, M.; Pudil, F. Nahrung 1998, 42, 412–415.

6. Waterbury, R. D.; Yao, W.; Byrne, R. H. Anal. Chim. Acta1997, 357, 99–102.

7. Ingle, J. D. Jr.; Crouch, S. R. Spectrochemical Analysis; Prentice-Hall Canada: Toronto, 1988.