Unmatched Performance of High-End Cary UV-Vis-NIR
The Cary name has been around for a long time, and is synonymous with peerless spectroscoping prowess. We’ve seen Cary 5000/Cary 6000i and Cary 7000 UV-Vis-NIR spectrophotometers actually go above their rated photodynamic range (the smallest amount of light that can be reliably measured), hitting just a touch over 8 absorbance units and 10 absorbance units, respectively. Mind-boggling performance, when you consider the logarithmic scale of absorbance units (where 10 abs = 0.00000001% light transmitted through a sample). That’s almost like looking through a brick wall expecting to see photons coming through from the other side!
However, new High-End Cary UV-Vis-NIR customers attempting to make such measurements for the first time will notice the system can easily handle up to around 5 absorbance units, but not more. Why is that? What is needed to push the instrument further? Let’s dive right into it!
Currently Discharging Current
Learning how things work has always been a fun little hobby, and in this case a little knowledge on detectors can go a long way. Did you know that most light detectors need a bit of time to discharge any photons they’ve accrued before they can reliably measure more incoming photons? Under most measurement circumstances, this happens in a few dozen milliseconds. However, it also depends on the amount of photons which have been absorbed, or more plainly speaking, the amount of light to which the detector has been exposed. What happens when the detector doesn’t get sufficient time to discharge? To answer that, we need another piece of information—details on the High-End Cary UV-Vis-NIR measurement cycle.
Cycling Through Measurement Cycles
High-End Cary UV-Vis-NIR spectrophotometers continuously ensure optimal measurement conditions by measuring the reference beam, sample beam, then dark signal (i.e., the detector’s natural reading in absence of all light) 30 times a second using an optical chopper. Under typical circumstances, this process goes along swimmingly. However, something interesting happens when the amount of light between the reference beam and sample beam is very large. This usually occurs when a highly absorbing sample is placed in the sample beampath. See the below figure for a simplified data plot of what the detector experiences under both of these scenarios.
In the scenario where a highly absorbing sample is placed in the sample beampath, the detector doesn’t have enough time to discharge all the photos from the reference beam measurement before moving onto the sample beam. This severely limits measurement potential.
Let’s Attenuate That Rear Beam!
You guessed it—the solution here is to balance the reference and sample beams so that the detector must discharge less light from the reference beam before measuring the sample. Since we can’t do anything about the sample itself, the solution is straight-forward: attenuate (or lower) the amount of light going through the reference beampath. Sounds easy right? It actually is!
Simply place something that absorbs light evenly across all wavelengths in the rear beampath, and you’re ready to go! Mesh filters (such as Agilent’s Rear Beam Attenuation Kit, part number 9910047700, pictured above) work particularly well here since these are guaranteed to absorb evenly irrespective of what wavelengths are sent through.
There are only two constraints to keep in mind when attenuating the rear beam for high absorbance measurements. The first, the rear beam attenuator must remain in the rear beampath for the duration of the entire experiment (baselines, and all sample measurements). It is critical that the attenuator not be disturbed as this may change how much light is attenuated, and a new baseline will hence be required. Second, attenuating the rear beam by more than 3 absorbance units will usually not yield beneficial results (although there are some exceptions here).
It’s Time to Measure Photons Once More
With a rear beam attenuator in place, suddenly your High-End Cary UV-Vis-NIR spectrophotometer now becomes a spectroscopic beast, capable of measuring incredibly minute amounts of light. Naturally users may want to increase the signal averaging time (noted as “Ave Time” in the software) to a full second or more to reduce the noise level associated with low light level measurements, but it’s a small price to pay for such mind-blowing photodynamic range.
Further reading
- Application note: Photometric linearity and range of the new generation Cary 4000/5000/6000i Spectrophotometers
- Application note: Measuring optical densities over 10 Abs on the Agilent Cary 7000 Universal Measurement Spectrophotometer (UMS)
- Application note: Gaining Deeper Insights into Thin Film Response
- Application note: Optical Characteristics and Thickness of 2-layered Structures
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