Introduction
The NIR spectra of many gases and solids exhibit narrow bands containing vibrational fine structure. Resolving these peaks, which are on the order of <>
Theory
The resolving power of an absorption spectrophotometer depends on a number of different mechanical instrument parameters. These include the grating resolution (lines per mm), focal length of the monochromator, the minimum slit width and the minimum stepping interval. These parameters, along with the detector sensitivity across the wavelength range of interest, dictate the minimum resolution. A common, but incorrect, assumption often made is that the minimum resolution is only dependent on the spectral band width. If there are not enough data points collected, or the signal-to-noise (S/N) is extremely low, then the fine spectral characteristics of the sample will not be resolved.
To resolve a peak, it is a general requirement to set the SBW to 1/3rd the natural peak width, and the data interval to 1/10th the natural peak width. However, the narrower the SBW setting, the lower the amount of light hitting the detector, hence, the sensitivity of the detector becomes a very important factor if good S/N spectra are to be collected. Varian’s high performance spectrophotometers can measure the most demanding samples and the release of the new generation Cary 5000 and Cary 6000i UV-Vis-NIR absorption spectrophotometers has raised the bar to a new level. In theory, the Cary 6000i can resolve a peak of between 0.06 – 0.1 nm natural bandwidth in a relatively short time. Without a doubt, the new generation Cary range are the best UV-Vis-NIR instruments available on the market today.
Discussion
The difference between the Cary 5000 and 6000i is the minimum resolution and sensitivity. As mentioned earlier, the narrower the SBW setting, the less light hits the detector, which results in poorer S/N or requires a much longer averaging time. The Cary 6000i has a minimum SBW of 0.02 nm and uses an InGaAs detector, which has approximately 100 times greater sensitivity than the traditional PbS detectors used in most NIR measurements. This is evident in Figure 2, where signal averaging of only two seconds was required to achieve excellent S/N compared to the Cary 5000 instrument using ten seconds signal averaging time, as shown in Figure 1. Also, the SBW setting on the Cary 5000 was 0.05 nm compared to 0.02 nm on the Cary 6000i. This difference in resolution results in the peak at 1380 nm being better resolved into a doublet on the 6000i compared to the 5000, as shown in Figure 3. For comparison, this 1380 nm peak cannot be resolved into a doublet on other UV-Vis-NIR absorption spectrophotometers.
Conclusion
The Cary 5000 and 6000i instruments are by far the best spectrophotometers on the market today if high sensitivity and fine spectral resolution are required. Both instruments can resolve a peak of <0.5 nm bandwidth, with the additional sensitivity of the Cary 6000i InGaAs detector providing an order of magnitude increase in speed and throughput. Both instruments have provided new technology that will benefit customers in all areas of research.