Thanks Ethan Den Boer for the comment and document...actually my target compound available in ppt  level in environment(1-5ppt)..however my stock solution standard was in 200ppm.. Therefore i need to dilute the stock solution until ppt level...but my problem is hard to get the lowest level (ppt)...until now i only can prepare in 1ppb level...could you give me some tips to overcome this problem? im using a liquid standard which purchase from certified company.. to measure the concentration i using a lcmsms system.. 

If the concentration of compounds in your samples is in the range of instrumental  detection, you can dilute your standard into ppt level. Just dilution further like you dilute from ppm to ppb.

If the concentration of compounds in your samples is out of range of instrumental detection limit, you will need to concentrate your samples to concentrate the target compound concentration into higher detectable level.

So first test the instrument response by standard at ppb and ppt level.

Thank you Ethan Den Boer and Yuan-Yuan Zhao for the brief comment and explanation... it really helpful in my study..i will try to recheck before rerin back the standard calibration.. thank you once again for the information..:)

Thanks for your important question. I also need to calculate the same (from ppm level to ppt level..). Please suggest some possible ways. Thanks in advance 

Here I have shown the way to prepare solutions of 0.1ppt, 3 ppt and 5 ppt solution from 1000 ppm; that I prepared for my experiment. I believe this will help you. thanks