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need a math check

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#1 molbio1234

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Posted 14 February 2011 - 09:34 PM

I have 1ul of 10ng/ul solution, I add 19ul water to it so it is now 0.5ng/ul at 20ul total volume (where 1/20*10 = 0.5)

Next I take 2ul of the 0.5ng/ul and do a 1:5 dilution, so it is now 0.1ng/ul (where 1/5*0.5=0.1) at 10ul total volume.

But now I want more than 0.1ng/ul, so I add in 5ul more of the 0.5ng/ul. Now it's 15ul total volume, not 10ul anymore. What is the new concentration after this 5ul addition? I have that since I added 7ul total of 0.1ng/ul, it is (7/15)*0.5=0.2ng/ul. Is this correct and is there another way to do it?

#2 protolder

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Posted 14 February 2011 - 10:40 PM

Hola, Only at the end you have, to me, the confusion 5ul of 0.1ng/ul in 15 ul are 0.5ng/15ul<> 500pg/15ul<>33.3pg/ul. If they are 7ul/15 means 0.7ng/15ul<> 700pg/15ul or 46.6 pg/ul. Buen día

#3 fysio lab

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Posted 15 February 2011 - 01:35 AM

i think: if you have 0.1 ng/ul at 10 ul--> is 1 ng. You add 5 ul of 0.5 ng/ul, so you add 2.5 ng. Your total is 3.5 ng at 15 ul, so 3.5/15= 0.233333 ng/ul

#4 molbio1234

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Posted 15 February 2011 - 02:47 AM

fysio lab, on 15 February 2011 - 01:35 AM, said:

i think: if you have 0.1 ng/ul at 10 ul--> is 1 ng. You add 5 ul of 0.5 ng/ul, so you add 2.5 ng. Your total is 3.5 ng at 15 ul, so 3.5/15= 0.233333 ng/ul

hi, ok so if i understand you correct,  you added it in terms of how many ng is being put into the 15ul in the end.
so the first time, i added 2ul of 0.5ng/ul = 1ng into 10ul volume
then second time i added 5ul of 0.5ng/ul = 2.5ng
2.5ng+1ng = 3ng
3ng/15ul volume = 0.2333

but also my math gives the same (7/15)*0.5ng/ul = 0.2333ng/ul which i simplified to 0.2ng/ul

would like to know what others say. we get the same answer, so do both our approaches work or just an odd coincidence? thanks

#5 molbio1234

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Posted 15 February 2011 - 02:48 AM

double post deleted

Edited by molbio1234, 15 February 2011 - 02:48 AM.

#6 protolder

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Posted 15 February 2011 - 03:52 AM

[quote name='molbio1234' timestamp='1297766853' post='100648']
I´m terribly sorry, you two are correct, I´M wrong by a bad interpretation. Sometimes is better stay in silence. I hope you two can excuse me . Thanks

#7 fysio lab

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Posted 16 February 2011 - 01:26 AM

your way is also correct, different people--> different way of calculations