calibration curves

In your next practical you will be constructing a calibration curve for glucose.... here is a little info about calibration curves...

So, a calibration curve is a graph that shows the relationship between the concentration of something and the result of some analytical process.... In your case, your calibration curve this week will show the relationship between glucose concentration and absorbance.

To create your calibration curve, you will make some solutions of glucose of known concentration using a serial dilution (post coming up soon on serial dilutions...). You will  then add a reagent to your glucose solutions (glucose oxidase) that reacts with the glucose to create a coloured compound - the more glucose there is, the greater the colour change will be. You will then be measuring the absorbance of light by each of your samples - more glucose will result in greater absorbance.

Once you have done this, you will draw a wonderful graph of glucose concentration against absorbance.

Example with Haemoglobin

Here is an example calibration curve relating haemoglobin concentration against absorbance (different example from what you will have on Tuesday, but same principle, just you will have glucose concentration on the x axis, instead of haemoglobin concentration):

So that is a calibration curve - NOTE it is a straight line not a curve!!

A calibration curve like this can come in really handy if you have a solution of unknown concentration - you can simply find the absorbance of this solution and then use your graph to find what concentration your absorbance reading corresponds to:

Watch out though, if your absorbance value is outside the range of your data, you will have to dilute your sample down. You cannot extrapolate your graph beyond your data. Never do this:

So in the example above, the absorbance value for the unknown solution of haemoglobin is outside of the data points you have on the graph. In this case you must dilute your sample until its absorbance is within the range of data on your graph... then when you have a concentration reading, you have to multiply this by your dilution factor to get back to the concentration in the original unknown sample.