Having trouble finding the slope of a line from the results of my lab work...x and y units are my prob?

Question

I'm working on a biology lab, trying to analyse the data, etc., and having trouble coming up with the slope of the results. The graph is the concentration of ortho-quinone as a measure of light absorbance (using a photospec) over time. The lab dealt with tyrosinase reacting with a catechol substrate, so ortho-Q was the product. Basically we were just measuring the rate of the reaction, which is the slope of the line (we tested the sample in the spec every 30s for 10 mins). MY PROBLEM IS...I'm unsure of the units to use to get the slope. If I use delta-X in seconds (600) and delta-y in percentage (about 40%), I get a different slope than if I use minutes and decimals between 0 and 1. What should I do? I'm just a bio student, not a scientist or anything, and I haven't been in school in years so my math certainly needs work, I guess...Any help is appreciated, and if you need to see the actual graph and data table (Excel file), let me know and I'll email. Thanks!

Answer 1

You shouldn't plot transmittance, but you'll need to plot Absorbance. On most spectrophotometers you can read either one, but Absorbance is directly related to concentration of a substance, but transmittance isn't.

Once you've calculated the absorbances, it really doesn't matter whether you plot that as a function of time in minutes or seconds. Yes, the value of your slope will be different, but so will the units of your slope.

Usually the lab information will specify whether to express the rate in terms of change in A/min or /sec. If it doesn't, then either one should be just fine.

Answer 2

It sounds like you have a data point for y at x = 30 seconds , 1 minute, 90 seconds etc. up to 600 seconds. This gives you a scatter plot to which you want to find a line that best fits the various data points The least squares method is one way of determining this line. You can use a TI 83+ caculator to enter the data and it will give you the line and its slope whichich best fits the data.