One of the most frequently asked questions about UV/Vis/NIR dispersive spectrophotometers is: “How high an absorbance value (or how small a %T value) can I accurately measure on my instrument?”
In the ‘good old days’, circa 1970s, there was a general ‘rule of thumb’ that stated, “all instrumental absorbance measurements should only be between 0 and 1 units”. The driving force behind this ‘rule’ was the fact that instruments of that era had ‘needle meters’ or strip chart recorders as output devices. Since absorbance is a logarithmic scale, the readable number of significant digits was best between 0 and 1, but declined rapidly for higher orders of magnitude. With the advent of digital displays one can read 3.1026 abs just as accurately as 0.1026 abs. The upper absorbance limit or instruments today is related to an instrumental specification called ‘stray light’. Today the stray light specification is the primary delineator of instrumental performance and cost.
Definition - Stray light is any light outside the spectral region isolated by the monochromator that reaches the detector. It is mainly produced by scatter from the optics and walls of the monochromator and is present in varying amounts in all spectrophotometers. Stray light is a constant for any given instrument design and is usually expressed in the specification values for that model as a %T value.
Example: Stray Light = 0.00007 %T for the UV/Vis region of the Lambda 950.
Stray light most frequently leads to deviations from the Beer-Lambert law and subsequent inaccuracy in photometric values. Double grating systems have lower stray light than a single grating systems, but are as one would expect more expensive.
The graph below is for a high performance instrument having very low stray light. The stray light level of an instrument fixes the maximum "actual" absorbance (blue line) measurable for the instrument (here 6A), Stray light also defines the maximum linear range (here 0A to about 5A). Remember: 0.0001 %T = 6 A
The % error (difference) between the absorbance with zero stray effect and the absorbance including stray light gives a sensitive measure on the non-liniarity of measurements with a given instrument. This % error can be used as a tool to help define the linear range of an instrument. Usually 5% error is the cut-off.
Conclusion: Knowing the stray light of a spectrophotometer enables one to calculate its maximum linear absorbance range and maximum upper absorbance limit. For more info go to http://www.perkinelmer.com
