[scar]

I am using some transfer pipets and some microcentrifuge tubes and i noticed when i measure out 1 mL on the pipet, it is only filling the micro. tube to about 0.9 mL.   I was wondering why there is such a huge discrepancy.  The pipet has to be filled about 2-3 mm above the 1 mL mark in order for it to register as 1.0 mL in the micro. tube.  Is one of them just a crap product or what?  I purchased them both from Santa Cruz Biotechnology, who seems to be a reputable company....

[Dan]

Well, how accurate do the gradations on these things claim to be? Check the specifications. I wouldn't be surprised if they are ±0.1 mL, it's not precision equipment.

[Arkcon]

I don't think it even matters how accurate the micropipette clams to be.  How accurately can you use it?  In my experience, if you draw up to the 1 ml mark, then lift the tip out of solution, you're going to not keep eactly the same finger force on the bulb.  Plastic, mass-produced transfer pipettes are literally just toys -- I use them to top up volumetric glassware, or when I don't need good accuracy.

[Corribus]

Right - if you need accuracy and precision when transfering volumes, do it gravimetrically. If this isn't possible or it's not convenient, do it volumetrically, using certified analytical glassware. (Or a certified pipetter... making sure you are using the correct type of pipetter for ths solvent you're using. I can't tell you how many times I've seen people transfering chloroform using a pipetter made for water and then wondering why their extinction coefficients were all over the place. /Facepalm)

[scar]

Thanks.   I actually am transferring chloroform in this case.
I have a plastic pipettor thing with detachable glass graduated pipet that works well and 'measures up' with the centrifuge tubes, but it's just like 2 feet long and not as convenient as the transfer pipets.
I was surprised though to find such a discrepancy with the transfer pipets; is it so hard to manufacture them with the marks 2-3 mm higher and get better accuracy?

[Arkcon]

You didn't answer what I asked.  Try this:  using an analytical balance, use the same pipette, and measure water to the same line.  How reproducible are you?  I said that these flimsy plastic bulbs aren't fully under your control.  We can certainly build very precise and accurate glassware, you said you have it, its just large and unwieldy.  Oh, and I use the cheap polyethylene pipette for aqueous solutions, generally.  You should check if they're attacked by solvents like chloroform, otherwise your sample, in addition to having analyte and chloroform, also has some plastic monomers and platicizers in solution.  Another vote for glass. http://en.wikipedia.org/wiki/Volumetric_pipette

[scar]

Sorry tweek didn't notice your question
i'm pretty good at controlling my fingers and watching the fluid level inside the pipet.  Given that 2-3 mm is about 0.1 mL, i think i can keep the volume within 0.01 mL of the mark.  When i lift out the pipet i can keep it pretty stable and watch the bottom to make sure im not letting in any air.  I took 10 measurements with the pipet: 0.93 g., 0.95, 0.92, 0.92, 0.93, 0.94, 0.94, 0.94, 0.95, and 0.95.  10 measurements with the glass pipet were slightly more reliable: 1.04 g., 1.02, 1.03, 1.03, 1.03, 1.03, 1.03, 1.03, 1.03, and 1.04

The plastic pipets are sterile and i assumed OK to use for our purposes since they were sold as part of a "kit"... but i suppose the designer of the kit could also not be too thorough....

But i will stick with the glass thank you