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# do I have enough sample?

calculations

5 replies to this topic

### #1 claritylight

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Posted 30 January 2013 - 03:56 PM

I have a few sample tubes. I need to add 50ng of each sample into 60ul water into new tubes. How much volume of each sample do I take to make the new tubes of 60ul each? Can I just divide the ng amounts into each other like one usually does with the concentration(ng/ul) ?

Sample 1 - 120ng in 42ul
Sample 2 - 65ng in 38ul
Sample 3 - 65ng in 95ul
Sample 4- 71ng in 76ul

Edited by claritylight, 30 January 2013 - 04:02 PM.

### #2 Curtis

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Posted 30 January 2013 - 07:07 PM

if your sample dissolves in water you need to dilute your sample first. Then calculate how many ng you have in every ul of your dilution. for example if you have 100 ng/ul then just take 0.5 ul of it and add 59.5 ul of water.

### #3 claritylight

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Posted 31 January 2013 - 04:48 AM

Is this correct then. I used 62ul instead of 60ul to have a little extra volume. Please correct me:

Given : 50ng/60ul = 0.833ng/ul
Sample 1 - 120ng in 42ul = 2.85ng/ul
Sample 2 - 65ng in 38ul = 1.71ng/ul
Sample 3 - 65ng in 95ul = 0.684ng/ul
Sample 4- 71ng in 76ul = 0.934ng/ul

Calculations:
sample 1 : (0.833/2.857)*62 = 18.0ul DNA + 44ul water
sample 2: (0.833/1.71)*62 = 30.2ul + 31.8ul water
sample 3: (0.833/0.684)*62ul = not enough
sample 4: (0.833/0.932)*62ul = 55.4ul + 6.6ul water

### #4 bob1

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Posted 31 January 2013 - 11:58 AM

Ok, the easiest way to do this is v=N/C (volume=number/concentration) so for sample 1 you want 50 ng (volume doesn't matter apart from needing less than 60 ul)

v=50ng/2.857
v=17.54ul make up to 60 with water (60-17.54=)... repeat for others.

### #5 claritylight

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Posted 01 February 2013 - 05:23 AM

Thanks bob1. That makes sense now. I can use that formula.

### #6 pito

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Posted 01 February 2013 - 05:36 AM

Thanks bob1. That makes sense now. I can use that formula.

One question: what are you trying to get? Fixed concentrations or fixed amounts?

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