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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: intelibp1600 on April 18, 2006, 09:58:14 PM

Title: atomic absorption sensitivity
Post by: intelibp1600 on April 18, 2006, 09:58:14 PM

A 12-ppm solution of lead gives an atomic absorption signal of 8.0% absorption. What is the atomic absorption sensitivity?


I don't know how to start this problem or which equation to use. Appreciate it if someone can help. Thanks!

Title: Re: atomic absorption sensitivity
Post by: mike on April 18, 2006, 10:35:16 PM

If 12 ppm corresponds to a 8 % absorption then 1ppm would correspond to 8/12 % = 0.7%.

So the atomic absorption sensitivity should correspond to a 0.7% absorbance increase with every 1ppm increase in lead concentration.

Title: Re: atomic absorption sensitivity
Post by: lemonoman on April 18, 2006, 11:17:43 PM

or 0.67%/ppm with significant figures ;)

Title: Re: atomic absorption sensitivity
Post by: Dude on April 19, 2006, 02:21:03 PM

Sensitivity has a precise definition in analytical chemistry.  See section 14 in this link (http://www.epa.gov/ttnamti1/files/ambient/inorganic/mthd-3-2.pdf). 

Sensitivity is the concentration of a given element that will absorb 1 % of incident radiation.  Thus, A = - log T where T = 0.99 or 1 % absorbed = 0.0044 absorbance units

Absorbance = 8 % presumably means that transmittance is 0.92
A = - l og 0.92 = 0.0362

My take would than be that the sensitivity for lead is  12 ppm* 0.0044 / 0.0362 = 1.46 ppm

Title: Re: atomic absorption sensitivity
Post by: intelibp1600 on April 19, 2006, 03:25:48 PM

Thanks so much for the *delete me*

Title: Re: atomic absorption sensitivity
Post by: mike on April 20, 2006, 08:25:58 PM

Quote

Sensitivity has a precise definition in analytical chemistry.

More information please.

Title: Re: atomic absorption sensitivity
Post by: Dude on April 21, 2006, 05:51:18 PM

Mike,

My mistake.  I believe you are correct.  Since absorbance is by definition unitless (a logarithm), I always filter a % answer as transmittance and then back calculate the absorbance.  If the absorbance were actually 0.08 (not 8 %), then your answer is correct 12* (.0044/.08) = 0.66.

Title: Re: atomic absorption sensitivity
Post by: intelibp1600 on April 24, 2006, 11:04:43 AM

I went to my TA today about this question and he told me it's calculated by (12/8)X100.

Title: Re: atomic absorption sensitivity
Post by: lemonoman on April 24, 2006, 11:25:10 PM

So he/she says that Atomic Absorption Sensitivity is: ( 12 ppm / 8.0 % ) × 100%

In other words, sensitivity is the ppm increase in concentration given by a 1% increase in absorbance.

This whole analytical sensitivity thing is crazy.  Different standards everywhere, as this post has demonstrated.  I did some digging, and it's definitely not uncommon for AA sensitivity to be described this way.  Question 7.6 at http://www.oup.com/uk/orc/bin/orc_demo/freelecturer/solutions/sols_ch07.pdf does it that way, and many other do to.

It's a convention.  One that few of us knew about.  I can't say whether or not Dude or mike knew...but I can honestly say that I've learned from this experience as well.  Your TA is right.

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P.S.  Ironically enough, if you google "ppm atomic absorption sensitivity"... the #5 hit is this forum!  http://www.google.ca/search?hl=en&q=ppm+%22atomic+absorption+sensitivity%22&meta=