Wavelength Anomalies in UV-Vis Spectrophometry Joel Tellinghuisen Department of Chemistry Vanderbilt University Nashville, TN 37235

Although the data in the I2 and Br2 absorption studies are low-resolution (1 nm), there is still a need for wavelength accuracy. In early attempts to estimate the three component bands for I2 in CCl4 as 3-parameter log-normal functions, the sum of weighted squared residuals dropped by half when an ad hoc wavelength correction, Dl = a sin[b(l-l0)], was added to the fit model. Furthermore, this gave remarkably precise parameters, a = 0.058(3) nm and b = 17.97(6) deg/nm (i.e., 20 nm period). (data error for the spectrum directly above)

Extensive measurements using atomic line sources (Ar, Ne, Hg) gave a more complex correction function, but still consistent with these indications.* * Appl. Spectrosc. 54, 1208 (2000).

l calibration employed narrowest slit width (0 l calibration employed narrowest slit width (0.1 nm) and smallest sampling interval (0.05 nm) available for this instrument (Shimadzu UV-2101 PC). Since the data for the I2 analyses were recorded at much larger slit width and sampling interval, I checked the wavelength accuracy for these: SURPRISE! Spectra were recorded w/ 1-nm slit, at 548-545 and 548-544 nm (0.05 nm interval), and 550-544 and 550-543 nm (0.1 nm interval). (Hg 546.07-nm line)

Still 1-nm slit, sampling at 0. 2 nm, 0. 5 nm, and 1. 0 nm Still 1-nm slit, sampling at 0.2 nm, 0.5 nm, and 1.0 nm. 552-540 nm and 553-540 nm; 562-530 nm and 562-529 nm; 574-510 nm and 574-509 nm.

5-nm slit, sampling at 0. 05 nm, 0. 1 nm, 1. 0 nm, and 0. 5 nm 0.5-nm slit, sampling at 0.05 nm, 0.1 nm, 1.0 nm, and 0.5 nm. Note the lack of smoothing for the last of these.

2-nm slit, sampling at 0. 05 nm, 0. 1 nm, and 0. 2 nm 0.2-nm slit, sampling at 0.05 nm, 0.1 nm, and 0.2 nm. No smoothing for any of these.

Dependence on starting wavelength for 1-nm slit width, 1-nm sampling (top) and 2-nm sampling (bottom).

Plea to Shimadzu and other instrument makers: Summary For slit widths > 0.3 nm, a smoothing algorithm takes effect for most sampling intervals, any time > 65 points are recorded. The instrument does go to the true wavelength when directed to do so, as at the start of a scan. For unsmoothed spectra there is effectively a 1-step miscount that causes the spectra to be shifted that much to longer wavelength. These problems are clearly in the software. Plea to Shimadzu and other instrument makers: Leave the data manipulation and “beautifying” of the spectra to us! At least make “step and record” the default.