Chromatography Forum

Dear All,

Can any one suggest me the acceptance criteria for the Acuracy (%difference) and precision (%RSD ) for the calibration of micropipette of variable ranges.

Thanks/ Regards

Which range does the pipette cover?

Regards Bert

i asume you are talking about a pipet in the microliter range. more importand then the ranges is how you are testing. we in our lab heve a special device in our balance to prevent the water used for testing to evaporat.
if you are pipeting 5 µl of water for testing, evaporation of 1 µl wil give a eror of 20%.
contact metler or something like that.

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Can any one suggest me the acceptance criteria for the Acuracy (%difference) and precision (%RSD ) for the calibration of micropipette of variable ranges.

Accuracy = how close to the correct amount delivered your pipet is
Precision = how reproducible your result is

You'll have to check the specs on the particular pipettor as they vary as a function of both manufacturer and volume to be delivered. Variable ones tend to be very reproducible and fairly accurate at the high end of the range, not as good at the low end.

In my experience, none of them are class "A" but I find them to be reproducible enough that I use them if I have to do 1:1 dilutions of (for example) dissolution samples (after they're filtered, and at room temp.)with mobile phase.

Pipettor calibration can be tricky depending on your demanded level of accuracy. Some just use water & a density correction factor, others delve deeply into the Kell equation (not fun). Many hire outside vendors for this service (they tend to be quite reasonable).

Another of these weird phenomena. Spending enormous effort for satisfying rules, feeling that ones duty has been done now, then slopping up when the real work comes. We noticed there was a strong dependence on individual pipetting method, it took some effort to get equal precision in the lab (during actual sample handling), even though everybody had no trouble getting the calibration straight. For instance, after filling, slowly, one should drag the pipets along the inside of the container while taking it out, then pipet onto the wall of the receiving container.
(Sloppy pipetting easily gives 20% errors at volumes around 100µL)

Also, one should not pipet volatiles with pipets that work with an air piston, the air heats up from the hand, liquid drops out of the pipet. There are "positive displacement pipets" for this.........
Radioactive and biologically active samples should also be handled only with the latter.

Dear All

We are using the variable pipettes from 1 ul to 100 to 1000 ul range.

Another of these weird phenomena. Spending enormous effort for satisfying rules, feeling that ones duty has been done now, then slopping up when the real work comes. ...
(Sloppy pipetting easily gives 20% errors at volumes around 100µL)

Also, one should not pipet volatiles with pipets that work with an air piston, the air heats up from the hand, liquid drops out of the pipet. There are "positive displacement pipets" for this.........
Radioactive and biologically active samples should also be handled only with the latter.

Agreed. When using pipettors (a rare thing for me, by design), I tend to walk around with them in my hand for several minutes before starting. I too have noticed a lot of variation attributable to technique (or lack thereof) and that it varies as a function of volume. That's why I generally regard them as tools of the devil, to be used only if adding small, equal aliquots of samples and diluent and then, only if a fairly high RSD is to be expected anyway (dissolution profiles).

I have also found that lots of people fail to appreciate that pipettor specs are not generally a match for the specs of the class "A" glassware that they are trying to replace.

we use pipettes from 0,5 to 10000 ul. We dont/cant calibrate below 10 ul (i think its not a good practise to pipette such a small volumes either). We test each pipette at lower and upper volume (except 0,5-10 ul only at 10ul) twice a year. We took the limits recommended by the manufacturer and added some "safety marginal" as we dont have specially controlled room (temp, humidity). Our limits are 0.6-3.6% for accuracy and 0.2-2.8% for precision depending on the pipette size. We are using the air pressure/temperature conversion table for the water density but in my experience it makes no difference in our conditions and with 5 decimal balance. Air humidity is an important factor with the lower volumes and you may want to use somekind of trap or if you can just place a small water containers on each corner of the windscreen of the balance.

Since one sees only what DR calls pipettors in the med area, I automatically thought this is what was talkd about, also since class pipets used to be calibrated at manufacture. But even with traditional glass pipets one needs to be careful, for instance, when is it emptied?
I use positive displacement "pipettors" even for some fairly serious work. Unfortunatly, most "pipettors" seen almost everywhere are the air cylinder type (even drawing up the liquid into the tip is not that easy to repeat).

Since one sees only what DR calls pipettors in the med area...

:: :: I work in generic pharma, we have pipettors that are not positive displacement. I discourage the use of them regularly for anything but 1:1 dilutions of similar liquids. The only other instance where I would use them for prep is to deliver an approximate volume of a well characterized solution to a tared container on a balance (then I would use mass & density instead of volume in calculations).