SEED/2012/Day 1

Spectrophotometer data

Thanks to Bao and others for being our scribes :)

Sample	Absorbance
Blank	0
0	3.000
1	1.330
2	0.356
3	0.073
4	0.014
5	0.001
6	-0.003

Notes

(Sample number = the number of times it was diluted from the stock food coloring, with each dilution being a factor of 4. So sample 0 is stock, sample 1 is 1/4 as concentrated, sample 2 is 1/16 as concentrated, etc. Blank, of course, is just water.)

Note that the spectrophotometer, like all measuring instruments, has a finite range, and indicates an error or gives wonky readings when asked to make measurements outside of that range. One of the questions on the post-lab assignment deals with this issue. This is a situation where serial dilutions can be very useful. If you want to measure the concentration of a very concentrated sample, it helps to dilute it by a known factor until the absorbance gets down into the measurable range. Then, since you know the dilution factor, you can back-solve the concentration of the original sample. If you're not quite sure how much to dilute it, you can prepare a set of serial dilutions and measure them all.

(Of course, just knowing the absorbance doesn't directly tell you the concentration. You would need to do a "standard curve": take a sample of known concentration ahead of time, make serial dilutions, and measure them, so you know what concentration gives what absorbance reading on your particular spec. Many specs can do an automatic conversion for common samples like DNA in water solution, but for anything else it's good practice to do a standard curve. And every spec will be slightly different.)

You may wish to read the Wikipedia page on absorbance for a discussion of the equations defining it.