The Modern Student laboratory: Spectroscopy Determining Optimum Spectral Bandwidth An Instrumental Methods Laboratory Exercise Don D. Gilbert Northern Arizona University, Flagstaff, AZ 8601 1 Appropriate instrumental data acquisition for a particular chemical question is a learning objective of instrumental methods courses. The availability of instruments with a wide range of spectral bandwidths in research and production laboratories requires students to have a firmunderstanding of how this parameter affects spectrophotometric data. This experiment is easily adapted to a laboratory course with one instrument having one variable or with two or more instruments having significantly different spectral bandwidths. The effect of different spectral bandwidths on observed absorbance data is studied with two absorbing species: the chromate ion with a relatively large natural bandwidth, a n d p-cresol with multiple peaks and small natural bandwidths. Spectmphotometers Sargent-Welch ChemAnal spectrophotometersystem; 350-650 nm: soectral bandwidth: 20 f 4 nm. Hewlett-packard (HP)model 8 4 5 i diode array sprctmphotometer; 180-820 nm; diudc array resolution (spectral handwidth,: 2 nm. Rcckmsn Modrl 3270 reenrdrng rprrtrophotum~trr.190 -3000 nm; ~pertrnlhandwidth: 0.1-8 nm. Experimental Ultraviolet spectra of p-cresol in a 2,2,4-trimethylpentane solunatural bandwidth: 3 nm at 260 nm) tion (10 pW250 mL; 0.05 were obtained with Hewlett-Packard and Beckman spectrophotometers. The Beckman spectrophotometerwas operated with the automatic slit-control program, which gave a spectral bandwidth of 0.45 nm at 286 nm. Visible spectra of potassium chromate solution (0.0400 g K2Cr04/L in 0.0500 MKOH; natural handwidth: 49nm at 370 nm) were obtained with the Hewlett-Paekard and Beckman spectrophotometera. The spectral bandwidth of the Beckman instrument was 0.40 nm at 370 nm. The absorbance of the potassium chromate solution was measured at 370 nm with a Sargent-Welch ChemAnal System spectrophotometer.
a;
rlgure I . new~en-racnarowxn (aavv = L nmj aosorpuon spectrum of pcresol in 2,2,4-trimehtylpentane(0.05 g1L).
A278
Journal of Chemical Education
Figure 2. Beckman 5270 (SEW = 0.45 nm) absorption spectrum of pcresol in 2,2,4-trimethylpentane(0.05g1L). Results and Discussion Figure 1 shows the dramatic result of a mismatch between the HP diode array spectral bandwidth (SBW) and the natural bandwidths (NBW) of the absorption bands of p-cresol. Figure 2, on the other hand, showsa satisfactory match of the spectral bandwidth of the Reckman 5270 and the absorption characteristics ofp-,mesol. Spectral bandwidth is an instrumental characteristic, defined a s the ranreofwavelengths leaving the monochrornator with one-haif the intensity of the most intense wavelength, that is, the nominal wavelength to which the monochromator is set. Natural bandwidth is a characteristic of absorbing species, defined as the wavelength spread a t one-half the maximum absorption of the band. I t is well-established that, if the instrument spectral bandwidth is one-tenth the natural bandwidth of the absorbing species, that is, SBW/NBW = 0.1, the observed spectrum is an accurate representation of the actual absorption band (1-6).However, as the spectral bandwidth increases, that is, SBW/NBW > 0.1, the observed absorption peak heights decrease, and the widths increase. Errors in absorbance measurements are introduced, and resolution of multiple peaks is reduced. The 25%lower absorbances and decreased resolution in the p-cresol spectrum obtained with the Hewlett-Packard diode array instrument are the result of obtaining data with a n instrument with a large spectral bandwidth relative to the natural bandwidth of the absorbingspecies. With the diode array instrument, SBWMBW is 0.67, whereas, with the Beckman 5270, the ratio is 0.13. The latter ratio (ContinuedonpageA281)
The Modern Student laboratory Potassium Chromate Data, 370 nm
Instrument ChemAnal HewlettPackard 8452A Beckman 5270
SEW, nrn 24 2 0.40
SBWINBW Absorbance 0.49 0.688 0.041 0.992 0.008
1.002
'NEW = 49 nm.
is a good value for getting accurate data on absorption bands. The table summarizes absorbance data for the chromate solution, measured at 370 nm. Chromate ion has a rather wide natural bandwidth, 49 nm. SBW/NBW's of both the HP diode array and Beckman spectrophotometers are less than 0.1 and yield similar absorbance measurements. The absorbance of the same chromate solution, measured with the same cuvette, decreases by more than 30% as the spectral bandwidth increases from 0.40 nm with the Beckman instrument to 20 nm with the ChemAnal instrument. The data clearly demonstrate the potential problem with making absorption measurements using instruments with large spectral bandwidths. Students are instructed to write a short report, summarize their data, and account for the differencesin observed resolution of the p-cresol spectra obtained with the Hewlett-Packard diode array and Beckmau instruments. They are also asked to explain the differences in the absorbance values of the potassium chromate solution measured with the three instruments. They propose desiderata for spectrophotometers to be used for different purposes: a Beer's law determination of an absorbing species, calcula-
tion of a cuvette path length from potassium chromate absorbance data, determination of an absorbance spectrum of an unreported absorbing species, and calculation of its absorptivity at maximum absorbance. Both the spectral bandwidth of a n instrument and the natural bandwidth of a n absorbine s~eciesmust be considered when choosing an instrument for absorbance measurements. For routine Beer's law determination of an absorbing species, spectral bandwidth is not an overriding issue. As long as the same instrument with a constant spectral bandwidth is used to measure blanks, standards, and sam~les.a n accurate determination is Dossible. Some sensitiviiy, however, will be lost if an instiument with a s ~ e c t r a lbandwidth greater than 0.1 of the natural dandwidth of the absorbing species is used. On the other hand, when spectra of new comwunds are run, when absorpti&ies or &vette path lengtG are calculated, or when spectral data are used for identification or fundamental sp~ctroscopiccalculations, accurate absorbance measurements must be made. That is, SBW/NBW should be about 0.1. If a recording, automatic scanning spectmphotometer is used, pen period and scanning speed must also be optimized to obtain accurate absorption data
-.
(3,4).
Literature Cited 1.Stmng,F. C., IIIAnol Cham. 1976,48,2155-2161.
2.Owen.AJ.ThsDiodo-ArmyAdoontqeiiWiV~ibleSp~etmsmpy;Hiilet%P~~Lard PublieationNo. 12-6954-8912.1988. 3. O p t h u m Parameters for S ~ c t m p h o i o m f r yVarian ; Instrument Diuision: Palo Alto. California, 1977. 4. Gilbert, D. D.; Mounts, T. T.;fioat,A. A J Chem Educ
198s.59,661-663.
5.Chenn.K L.:Youne.VY.InInstrumntolAnolvsis, 2nded.;Christian,GD.:OReilly. J.E., Eda.;AIIG & Bacon: Boston, 1986: p~17&179. 6. Willard, H.H.:Menitt, Jr., L. L.: Settle, F. A.Insfmmntal MethadsofAnolraia, 7th ed.; Wadsworth: Belmont, 1988; p 128.
Volume 68 Number 11 November 1991
A281
