I have to dilute my sample 1mL in 100mL distilled water.
If I use a plastic disposable syringe (1.0mL in 100mL) I get approx. 0.5% RSD on 5 dilutions.
If I use an 4 figure analytical balance (1.0g in 100mL) and convert to v/v with the sample density I get 1.0% RSD on 5 dilutions.
I would have thought the balance method would give a better RSD. Any thoughts?

My immediate thought is to ask what the precision of your system is i.e. the RSD when injecting 5 times from the same solution? The difference in RSD resulting from syringe vs. balance is small and could simply be due to chance.

Theoretically, I agree that the balance should be much more accurate than a disposible syringe. However, practically speaking, I prefer to handle liquids (using Gilson pippettes) when possible as I find it much more reproducible than weighing out powders in very small quantities (especially when static makes it fly off the spatula and cling to your gloved hand, grrrrr!!!)

Ann :o)

Hello Ann,
My RSD is 0.10 - 0.20% for 5 injections.
I agree with you about handling liquids - it makes life easier.
I suppose I am fortunate that my RSDs are only 0.5 - 1.0%. They could be worse!!
Just curious really.

(I have performed this balance vs syringe experiment before with the same result).

Thanks for your reply
HPLC Dave

Hi Dave

Another thought - how do you convert your 1% (w/v) to 1% (v/v), is there a dilution step? If so, I guess you would expect a 2-step process(balance plus dilution) to be more variable than a 1-step process (syringe dilution alone)?

Still thinking... :o)

Ann

How sure are you about the acurateness of your syringe? Just take 1 ml from this syringe and weigh it. I would not trust the disposable syringe too much.

Regards,

KHW

To KHW:

The RSD of 5 weighings of 1mL is approx. 0.5%.

HPLC Dave