[rgncjn87]

Hello Forum,

For my analytical chemistry course, we were analyzing for lead content in the paint on old soda pop bottles. We have carried out all of the experiements and ran the samples through AAS. Using working standards of lead containing 3, 6, 9, 12, 15, 18 ug Pb / mL, I determined my calibration line equation to be:

y = 0.0191X + 0.0126

I now need to quantify the amount of lead in a sample, but I'm running into some difficulty. The original mass of the paint that was obtained from the bottle was 30.12 mg. This sample required a 200-fold dilution for measurement, and had an absorbance value of 0.0514.

For some reason, simply solving the equation for X (concentration) using the absorbance value of Y does not seem to make sense for me. Would someone be willing to walk me through the calculations needed to determine the quantity of lead in the sample, and in weight percent?

If you require any additional details, please let me know.

[marquis]

You need to calculate the total amount of lead and divide by the original sample weight.

You can determine the lead concentration from your slope calculation. There are other ways, as well.  Just be aware there is a concentration above which you curve will no longer be linear.

The calculation is something like:

Total ppm = [ppm (from curve) x volume (ml) x dilution factor]/ [ wt sample in g]

Usually, the sample prep has you take the sample and after the prep steps, put in a volumetric and dilute to volume.  The mls of  the volumetric is the number to use for volume in the calculation. 

Good luck and hope this works for you.

[rgncjn87]

Thank you.

We did dilute to a volume of 25.0 mL in a volumetric flask after the sample prep.

I'm not familiar with the terminology you used. Is total PPM the value I am looking for? Is the PPM value from the curve the answer I get from solving for X using the equation?

[marquis]

The total ppm mentioned is PPM by weight.  The difference in ppm values can be confusing.

The ppm you are getting from the curve is ppm in SOLUTION.  This has the unit of micrograms Pb per ml of solution or milligrams per liter.

The final answer you will report is ppm in the sample (or ppm by WEIGHT).  The units here are in micrograms Pb per gram of sample.  This is the final answer you are after.

If you solve your equation for X and then put in your absorbance value as Y and do the calculation, you will get the number mentioned as "PPM from curve"(or ppm in solution). Put this value in the equation in my previous email and add the correct dilution factors and voumes and you will get "total ppm"(or ppm by weight).

By the way, most instruments don't bother with the intercept, they just deal with the slope of the line.  You typically take care of the intercept by zeroing out the instrument before you run your samples.  A little extra work can't hurt, though.

Hope this isn't too confusing.  Good luck with your work.

[marquis]

You can report your answer in ppm (by weight).  Or..

You can convert ppm by weight to other units.  For example, ppm by weight divided by 1000 equals parts per thousand.  Ppm by weight divided by 10000 equals percent (%) by weight.

AA usually isn't used for percentage measurements.  The large dilution factor makes the final answer somewhat inaccurate (due to possible dilution errors).  But it can be done.