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Topic: Concentration  (Read 18125 times)

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Offline Help

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Re: Concentration
« Reply #15 on: April 14, 2008, 12:10:17 PM »
Haha. I could send it to you if you'd like. They sent it to my e-mail, with a form to fill out answers for.

Thankyou for your advice. I bothered one of my old professers for some direction. I managed to figure it out by working backwards, which is good, because it was due at 11:00 am this morning.

Offline Arkcon

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Re: Concentration
« Reply #16 on: April 14, 2008, 12:23:23 PM »
Sure, drop me a PM on this forum, and I'll send you my email address.  I went and browsed their web site, and didn't see an obvious form for submitting yourself for a job, so I couldn't find it on my own.  I wouldn't worry too much about this question, they state, right at the top, that this question won't make or break your application.  IMHO, working something like this out this isn't a technician task, but more an associate chemist or level 1 chemist, if not a more senior level chemist.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline JGK

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Re: Concentration
« Reply #17 on: April 14, 2008, 01:54:43 PM »
I have arrived at some answers: 10,000 ppb and 13,333.3 ppb (adjusted for efficiency)

I can post the work-through if needed.
Experience is something you don't get until just after you need it.

Offline Help

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Re: Concentration
« Reply #18 on: April 14, 2008, 02:29:01 PM »
Oh no. I got 1.3335 x 10^-8 ppb Compound A. That's much smaller then yours.
« Last Edit: April 14, 2008, 02:44:56 PM by Help »

Offline Help

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Re: Concentration
« Reply #19 on: April 14, 2008, 02:29:38 PM »
I've already handed it in, I wouldn't mind seeing how you arrived at the answer.
« Last Edit: April 14, 2008, 02:45:16 PM by Help »

Offline JGK

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Re: Concentration
« Reply #20 on: April 14, 2008, 04:20:16 PM »
I have arrived at some answers: 10,000 ppb and 13,333.3 ppb (adjusted for efficiency)

I can post the work-through if needed.

Here it is:


No correction for extraction efficiency

2 µL injection of extract (ACN:0.5% HAc) yields 0.02 ng = Concentration 10 µg/mL
Extract Volume = 0.5mL which was obtained from 15 mL extract added to affinity column.  Extract Concentration = 0.333 µg/mL
The 15 mL was a sub-sample of a 50 mL volume (Amount in 50 mL volume 16.6667 µg) which in turn was made from a 10 mL portion of a 600 mL extract (total in extract 1000 µg, equivalent to 10 µg/g or 10,000 ppb)

If recovery is 75%
Then original concentration is 13,333.3 ppb
Experience is something you don't get until just after you need it.

Offline JGK

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Re: Concentration
« Reply #21 on: April 14, 2008, 04:23:51 PM »
Oh no. I got 1.3335 x 10^-8 ppb Compound A. That's much smaller then yours.

1 ppb = 10 ng/g

Must be one hell of a LC-fluorescence method if it's capable of measuring in the Femtogram range 
Experience is something you don't get until just after you need it.

Offline Arkcon

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Re: Concentration
« Reply #22 on: April 18, 2008, 10:12:38 AM »
That's the great thing about fluorescence, either with HPLC or a cuvette.  I've used it from time to time my whole career, and doing a serial dilution, trying to find the limit of detection, is always astounding to people on the sensitivity.  Of course, you have to find a excitation and emission maximum, but there usually is one, for large molecules, at least in the UV.  Yet, may people, when confronted by a new substance that they can't detect with an absorbence detector, just won't buy a fluorescence detector, preferring an RI or a ESD, despite their shortcomings.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline DrCMS

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Re: Concentration
« Reply #23 on: April 18, 2008, 10:52:42 AM »
I get 10ppb

2 µL injection of extract (ACN:0.5% HAc) yields 0.02 ng = Concentration 10 µg/mL

I think 0.02ng in 2µL is 10µg/L not per ml.

0.02ng = 2x10-11g
2µL = 2x10-6L

So concentration is 1x10-5g/L = 10µg/L or 10ng/ml

Besides an answer of 10000ppb is best written 10ppm.

Offline DrCMS

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Re: Concentration
« Reply #24 on: April 18, 2008, 04:36:56 PM »
No you've got it wrong again! 

Go back to maths class before answering these questions again.

1 ng = 1x10-9g    i.e. 1 billionth of a gram

so 1ng/g = 1ppb

10ng/g = 10ppb

Offline JGK

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Re: Concentration
« Reply #25 on: April 21, 2008, 08:09:04 AM »
No correction for extraction efficiency

2 µL injection of extract (ACN:0.5% HAc) yields 0.02 ng = Concentration 10 ng/mL
Extract Volume = 0.5mL which was obtained from 15 mL extract added to affinity column.  Extract Concentration = 0.333 ng/mL
The 15 mL was a sub-sample of a 50 mL volume (Amount in 50 mL volume 16.6667 ng) which in turn was made from a 10 mL portion of a 600 mL extract (total in extract 1000 ng, equivalent to 10 ng/g or 10 ppb)

If recovery is 75%
Then original concentration is 13.33 ppb

Sorry for the earlier errors, my confusion with ppb (US definition)
Experience is something you don't get until just after you need it.

Offline DrCMS

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Re: Concentration
« Reply #26 on: April 21, 2008, 09:04:43 AM »
JGK please do not remove your posts later as it makes the replies people then give look odd.
« Last Edit: April 21, 2008, 09:57:44 AM by DrCMS »

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