14 replies to this topic

[jiajia1987]

Dear all,

I am always completely at a loss when it comes to calculations and I have always been bad at it. Do help me check if what I have done is correct, okay?

Basically, I want to dissolve a peptide in DMSO.

The peptide is 9.20mg and its molecular weight is 1890.7

The number of moles would be (9.20 x 10^-3) / 1890.7 = 4.8659 x 10^-6

I want to make 10mM. 10mM is a concentration that is equivalent to.... 0.01mole/L, right?

So that will mean... the amount of DMSO used to dissolve my peptide should be (4.8659 x 10^-6) / 0.01 = 4.8659 x 10^-4

Since 4.8659 x 10^-4 is in litres, that would mean I need 486.6uL of DMSO to dissolve my peptide.

AM I right?? Please help me!! Thanks in advance!!!

[altobarn]

My calculation is not good too but in that case, I would say - Yes, you are right! add 486.6 ul DMSO.

[pito]

edited: I was to fast with my answer. Mdfenko is indeed right.



But like josse said: can you really dissolve 9.2mg in 0.48ml?

Edited by pito, 16 July 2009 - 11:51 AM.

[mdfenko]

i would bring the final volume to 486.6 ul with dmso.

[josse]

mdfenko, on Jul 16 2009, 03:30 PM, said:

i would bring the final volume to 486.6 ul with dmso.

I do wonder if you can dissolve 9.2mg of a peptide in less then 0.5ml !

And what do you mean mdfenko, bring the final volume to 486.6µl ?

You mean see needs to add les then 486.6µl ? If she/he does that, then the endconcentration will not be  ok.

[mdfenko]

josse, on Jul 16 2009, 09:55 AM, said:

I do wonder if you can dissolve 9.2mg of a peptide in less then 0.5ml !

And what do you mean mdfenko, bring the final volume to 486.6µl ?

You mean see needs to add less then 486.6µl ? If she/he does that, then the end concentration will not be  ok.

the peptide will bring up the volume, as well.

bringing to the volume will give you the proper concentration (moles/volume=molar and final volume was calculated for the concentration). if you add the volume to the peptide then your concentration will be lower (volume will be higher). for large volumes (at lower concentrations) this may be insignificant but for the volume and concentration here there will be a significant difference in final concentration.

[jiajia1987]

wow... THANKS SO MUCH PEEPS!!! I am definitely glad I got the calculation right this time because amazingly enough (and sadly enough), I always get my calculations wrong.

But I am kind of confused about something. What I always do is to add the amount of DMSO required to the peptide (which is in powder form), is this considered bringing the volume to the required volume? I know that peptide will bring up the volume as well, but with such a small volume (486.6uL), it seems hard to 'see' for myself if the solution has reached that volume. I don't know if anyone gets what I am talking about. Does what I say make sense?

[pito]

jiajia1987, on Jul 17 2009, 03:08 AM, said:

wow... THANKS SO MUCH PEEPS!!! I am definitely glad I got the calculation right this time because amazingly enough (and sadly enough), I always get my calculations wrong.

But I am kind of confused about something. What I always do is to add the amount of DMSO required to the peptide (which is in powder form), is this considered bringing the volume to the required volume? I know that peptide will bring up the volume as well, but with such a small volume (486.6uL), it seems hard to 'see' for myself if the solution has reached that volume. I don't know if anyone gets what I am talking about. Does what I say make sense?

Not sure if I understand you but:


Like mdfenko said: you need to add DMSO untill you reach 486.6µl but also like you said: how can you in heavens sake see if you reached that volume.

I think this would be very difficult and maybe you should better recalculate the volume to a certain mass so you can see the mass on a very presice balans.
(I do think you have the weight, density of the DMSO and offcourse also of the peptide, so you can recalcute everything to know what mass you need)

Altough, I still wonder if it might be better for you to use a bit more peptide so you can have a larger volume to work with, because 9.2 mg dissolving in less then 0.5ml ? I really wonder if this will work?

[mdfenko]

to get to the proper volume:

suspend in less than the final volume

measure the volume after fully suspended with a pipette (if you have one, many of the electronic pipettes have a "measure" mode)

add solvent to the final volume required (final volume-measured volume=volume necessary)

[pito]

mdfenko, on Jul 17 2009, 05:39 PM, said:

to get to the proper volume:

suspend in less than the final volume

measure the volume after fully suspended with a pipette (if you have one, many of the electronic pipettes have a "measure" mode)

add solvent to the final volume required (final volume-measured volume=volume necessary)

Was thinking about that too, but I have one question for you then mdfenko:

If you use a pipette to measure the volume, in what would you put the peptide and dmso when dissolving it?
My experiences with very small volumes are that you can never pipette the complete volume.. you always leave a little bit behind in the cup or whatever you used to put the peptide in with the dmso or some of the volume will simple stay at the end of the pipette and not be calculated in the end volume.

normally there is real problem but you will lose some peptide, dmso and you are allready working with a very small amount.

Or is this no problem at all ?

[mdfenko]

it can be a problem but you can avoid it.

what i do:

with a manual pipette, set to a little less than the volume (easy to figure, you know how much solvent you put in) slowly (to accommodate wall (of the tube) effects) then slowly increase the volume of the pipette while immersed in the solution until it is all picked up. then read the volume.

with a electronic pipette, set the pipette to its slowest. pick up solution, occasionally stopping to accommodate wall effects.

in either case, expel the solution back into the tube slowly to ensure complete transfer.

[jiajia1987]

wow.. thanks a lot. i really learned some new stuffs from you people.

I am unable to increase the volume of the peptide because it comes in a powder form and it came in this bottle where the total mass is 9.2mg and I am supposed to dissolve all the powder before I can use it for my work..

[pito]

jiajia1987, on Jul 20 2009, 03:26 AM, said:

wow.. thanks a lot. i really learned some new stuffs from you people.

I am unable to increase the volume of the peptide because it comes in a powder form and it came in this bottle where the total mass is 9.2mg and I am supposed to dissolve all the powder before I can use it for my work..

I really wonder you can dissolve this amount in such low volume of DMSO.
Let us please know if it worked out or not.
I am really curious.

[jiajia1987]

Yes it works, I did it and it really dissolved.

Now I have this other peptide which is 6.2mg and has a molecular weight of 1834.98g/mol and I will be adding 337.9uL DMSO to make a 10mM stock.

[pito]

jiajia1987, on Jul 21 2009, 03:17 AM, said:

Yes it works, I did it and it really dissolved.

Now I have this other peptide which is 6.2mg and has a molecular weight of 1834.98g/mol and I will be adding 337.9uL DMSO to make a 10mM stock.

Yeah, you need to add DMSO untill you reach 337.9µl in total. (not adding 337.9µl DMSO till you reach 337.9µl + X l; total volume needs to be 337.9 as mdfenko allready explained.)