# log plot calculation - (Sep/30/2009 )

Hi,

I wonder if a maths whizz can help me out.. I'm stumped on an equation.

Basically I'm trying to calculate unknown concentrations of samples from a standard curve, in log format.

I performed a standard curve with a tenfold serial dilution of my 'Standard' compound, and also have my sample measurements (luminescence).

The compound was diluted in concentrations ranging from1^-10 tp 1^-16 moles (as per manufacturers instructions).

When I plot this on Excel, luminescence is on the Y axis (Log scale), and compound concentration is on X scale (Log).

I add a trendline to get the equation, and get y=4E + 3x^{ 0.8406}

In order to get my x concentration I'm not sure what to do. (I normally work it through using y=mx +c)

Can anyone help me out? thanks

OKSO on Oct 1 2009, 12:05 AM said:

I add a trendline to get the equation, and get y=4E + 3x

^{ 0.8406}

I think you should be reading this as: y = (4E3) x

^{0.8406}

which makes x = (y/(4E3))^(1/0.8)

Thanks for that help

Yes, I've worked out some y values from this and they correlate with my standard curve.

So briefly if my y value (luminescence) was 1500 then x turns out to be

x=<(1500)/4E3>^(1/0.8)

x=<(0.375)^(1.25)>

x=0.29

Does this tally up with your thoughts?

thanks again

Looks good. Just don’t get caught with my crude rounding (^(1/0.8) is different to (^(1/0.8406)