Why Black walled Cells?
Why it’s a good idea to use self masking cuvettes…
This article explains why it is important to use black walled cells for low volume work. We performed this experiment on a Varian Cary 500 a number of years ago to demonstrate this. The experiment:
- Take a suitable visible and stable chemcal, and make up a solution to give you a peak Absorbance measurement of 3+ Absorbance. In this case I used diluted Starna Green, measured at 428 nm, with a 1.0 nm SBW.
- Call this ‘100’ as a top standard – and serially dilute to produce ’75’, ’50’, and ’25’ respectively.
- Using a VAOBS1 clear wall cell – fill with water, and zero the Cary.
- Re-measure the blank, and then each standard in turn.
- Repeat 3, and 4 using a VAOBM1 black walled cell.
- Plot, and compare the ‘Beer-Lambert’ linear response!
How the Optical characteristics of the cells affect the Chemical measurement
1. Using the VAOBS1 clear wall cell- On this evidence, I don’t think I’d want to use this type of cell (without any masking) to measure more than 0.5 A?
Clear Walled | Blank Corrected | ||
0 | 0.0004 | 0 | 0 |
25 | 0.8177 | 25 | 0.8173 |
50 | 1.473 | 50 | 1.4726 |
75 | 1.8064 | 75 | 1.806 |
100 | 1.8999 | 100 | 1.8995 |
2. Using the VAOBM1 black wall cell – what a difference! I’ve restored my faith in my analytical expertise – the series 0 – 75 (2.5 A) is a perfect straight line (linearity = 1). Linearity to 3.3 (100) isn’t bad either at 0.9993, but you are beginning to see the stray light characteristics of the Cary 500 instrument at this point.
Black Walled | Blank Corrected | ||
0 | 0.0004 | 0 | 0 |
25 | 0.8508 | 25 | 0.8505 |
50 | 1.7149 | 50 | 1.7145 |
75 | 2.5611 | 75 | 2.5607 |
100 | 3.3106 | 100 | 3.3103 |
Black Walled | Clear Walled | Difference | % Difference. | |
0 | 0 | 0 | 0 | 0 |
0.8505 | 0.8173 | 25 | 0.0331 | 3.9 |
1.7145 | 1.4726 | 50 | 0.242 | 14.1 |
2.5607 | 1.806 | 75 | 0.7547 | 29.5 |
3.3103 | 1.8995 | 100 | 1.4108 | 42.6 |
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