What's the difference between pipettes calibrated “TD”, “TC”, “Blow-out” and related terms?

Question

I've never blown-out the last drop of any glass pipette I've ever used during my undergraduate studies. Recently I re-read some notes I had lying around in order to prepare a class I had to give to a group of first year undergraduates and found out that there is a class of glass pipettes which need to be "blown-out" in order to deliver the nominal volume correctly.

I researched a little bit and found what seems to me to be completely inconsistent terminology and conventions to refer to this class of pipettes.

In https://en.wikipedia.org/wiki/Graduated_pipette it is clearly stated that there are "two kinds" of graduated pipettes. "serologic/blow-out" and "Mohr/drain-out". It is also stated that serologic pipettes have calibration marks all the way to the tip, whereas Mohr pipettes do not. The same page states that pipettes are either calibrated "to deliver" (TD) or "to contain" (TC). It states that "TD" is a far more common calibration than "TC" and explains that pipettes calibrated TC need to be blown-out in order to deliver the expected volume correctly, one would assume from this that TD is a synonym fro "blow-out". Judging from this there would seem to be two kinds of pipettes, "serologic/blow-out/TD" and "Mohr/drain-out/TC". This really contradicts my experience, I've been using pipettes with marks going all the way to the tip and never been told to blow out the last drop, however, there is always the chance I've been doing it wrong this whole time.

Digging a little bit deeper I found that this document http://www.brand.de/fileadmin/user/pdf/Information_Vol/Brochuere_Volumenmessung_EN.pdf (link downloads pdf) that wikipedia lists as a source doesn't mention "blow-out" pipettes at all, but does mention pipettes calibrated "TC". In page 13 there are some pictures and descriptions of pipettes resembling what one would think are "Mohr" and "Serologic" types, and both are said to be calibrated "to deliver" actually, reading this text one would think that ALL graduated and volumetric pipettes are calibrated to deliver (however, this document is from a lab equipment manufacturer, perhaps they only manufacture TD pipettes...?).

Digging yet deeper http://faculty.clinton.edu/faculty/Mike.Lawliss/My_webpage/Sci110/Other%20documents/Pipetting%20A%20Practical%20Guide_Guzman2001.pdf is another article cited as a source by wikipedia. Here the author pretty much identifies serologic pipettes with blow-out ones, but also states that blow-out pipettes "usually" have double rings (non-colored) near the top. There is, however, a drawing of a "serologic" pipette WITHOUT rings near the end of the article, close to an "Ostwald-Folin" pipette WITH rings which the author specifically identifies as blow-out. The definition of the term "serologic" pipette is, according to the author, "a type of measuring pipette used to measure variable volumes" which in my view would make ALL graduated pipettes serologic.

To add to all this confusion Sigma-Aldrich seems to manufacture "TD, blow-out, serologic pippetes" http://www.sigmaaldrich.com/catalog/product/sigma/z325422?lang=es&region=AR and also "TD/TC volumetric pippetes", which makes me lose all hope of making sense of any of the terms listed in the question at all.

In short, is there a STANDARD way in which these terms (TD, TC, IN, EX, Blow-out, Serologic, etc) are used? Is there a standard definition regulated by some sort of organization, or at least generally agreed upon? If you think that there is, could you please provide trustworthy sources? Thanks a lot.

This probably seems like a silly question (it is =D) but it kind of bugs me not to know something so basic as "what are the different kinds of graduated pipettes?"... It seemed like something that would be quite easy to settle when I started looking into it... guess I was wrong.

Answer 1

Since you are explicitly asking for standards, you might want to start with ISO 4787 Laboratory glassware – Volumetric instruments – Methods for testing of capacity and for use. This international standard provides methods for testing as well as for use of various volumetric instruments made from glass. Further requirements for individual volumetric instruments can be found in

• ISO 648 Laboratory glassware – Single-volume pipettes
• ISO 835 Laboratory glassware – Graduated pipettes
• ISO 385 Laboratory glassware – Burettes
• ISO 1042 Laboratory glassware – One-mark volumetric flasks
• ISO 4788:2005 Laboratory glassware – Graduated measuring cylinders

Most commercially available laboratory glassware is in accordance with these standards. However, some nonconforming volumetric instruments are also available, for example in non-standard sizes.

Due to retention of liquid on the inner surface of the volumetric instrument, the volume of liquid delivered is not identical with the volume of liquid contained by the volumetric instrument. Volumetric instruments are either adjusted ‘to contain’ (i.e. the capacity printed on the instrument corresponds to the contained quantity of liquid) or ‘to deliver’ (i.e. the capacity printed on the instrument corresponds to the delivered quantity of liquid; the wetting residue remaining in the instrument has already been taken into account in the calibration). According to international standards, the marking ‘Ex’ is used to indicate that the volumetric instrument has been adjusted to deliver; the marking ‘In’ is used to indicate that the volumetric instrument has been adjusted to contain. Most typical graduated pipettes, bulb pipettes, and burettes are usually adjusted to deliver, whereas typical graduated cylinders, volumetric flasks, and capillary pipettes (i.e. very small measuring pipettes with a capacity of up to 0.2 ml) are usually adjusted to contain.

ISO 648 and ISO 835 specify three classes of accuracy for pipettes:

• Class A (higher grade, without waiting time)
• Class AS (higher grade, with a specified waiting time)
• Class B (lower grade, without waiting time)

‘Waiting time’ is the period to be observed in order to ensure complete delivery after apparent completion of the liquid delivery of the pipette (i.e. after the meniscus appeared to come to rest) and before the final reading of the delivered volume is taken (i.e. before the tip of the jet is removed from the inner surface of the receiving vessel).

Furthermore, four types of graduated measuring pipettes are standardized in ISO 835:

• Type 1: Partial delivery (adjusted to deliver from zero line at the top to any graduation line; nominal capacity is represented by the lowest graduation line)
• Type 2: Total delivery (adjusted to deliver from any graduation line down to the jet; nominal capacity is represented by the highest graduation line)
• Type 3: Total and partial delivery (adjusted to deliver from zero line at the top to any graduation line; nominal capacity is obtained by delivery down to the jet)
• Type 4: Blow-out (adjusted to deliver from zero line at the top to any graduation line; nominal capacity is obtained by delivery down to the jet where the last drop of liquid in the jet is expelled by blowing. Blow-out pipettes are only adjusted to accuracy Class B.)

Typical graduated measuring pipettes with a capacity from 0.5 ml to 25 ml are commercially available in accordance with ISO 835 as Type 1, 2, or 3 and in Class A, AS, or B. Bulb pipettes with a capacity from 0.5 ml to 100 ml are available in accordance with ISO 648 in Class A, AS, or B. They are all adjusted to deliver (‘Ex’); i.e. the residual liquid still left in the tip has already been taken into account during calibration and must not be expelled into the vessel by blowing out.

Very small measuring pipettes with a capacity of up to 0.2 ml (capillary pipettes) are designed as Type 4 (blow-out) in Class B; however, most available capillary pipettes are actually adjusted to contain (‘In’), which is strictly not conforming with ISO 835. Though, the formal difference between blow-out pipettes adjusted to deliver and pipettes adjusted to contain may be small for total delivery since the last drop of retained liquid in the jet can be expelled by blowing in both cases. Nevertheless, when the greatest possible accuracy is desired, it is not sufficient to simply empty pipettes that are adjusted to contain by blowing out the liquid with a pipetting aid; such pipettes should also be rinsed two to three times with the diluting medium.

The general procedure for the use of pipettes that are adjusted to deliver (‘Ex’) is described in ISO 4787 as follows. However, when the greatest possible accuracy is desired, pipettes should be used as closely as possible to the manner in which they have been calibrated (which is also specified in ISO 4787, ISO 648, and ISO 648).

10.5.1 Pipettes adjusted to deliver (see ISO 648 and ISO 835)

After rinsing with the liquid or reagent to be used, fill the pipette by suction to a few millimetres above the selected graduation line. Remove any liquid remaining on the outside of the jet. The final setting of the meniscus shall then be made by dispensing the surplus liquid through the jet. Remove any drops of liquid adhering to the jet by bringing an inclined ground glass vessel into contact with the tip of the jet. Delivery shall then be made with the tip of the jet in contact with the inner surface of the inclined receiving vessel.

If the setting after delivery is done at a lower graduation line, the liquid flow has to be nearly stopped a few millimetres above the graduation line. After observing a waiting time, if specified, complete the final setting quickly.

A waiting time, if specified, shall be observed before making the final setting for delivery of a given volume.

The corresponding procedure for the use of pipettes that are adjusted to contain (‘In’) is described in ISO 4787 as follows. Note that, when the greatest possible accuracy is desired, it is not sufficient to simply empty the pipette by blowing out the liquid with a pipetting aid; the pipette should also be rinsed two to three times with the diluting medium.

10.5.2 Pipettes adjusted to contain

Rinse the pipette with the reagent to be used to a few millimetres below the desired graduation line. Fill the pipette by suction to as close as possible above the selected graduation line. Remove any liquid remaining on the outside of the jet. Make the final setting of the meniscus to the line by withdrawing the surplus liquid by means of filter paper. For the discharge, rinse the pipette several times with the diluting medium.

Answer 2

As you noted, "pipettes are either calibrated 'to deliver' (TD) or 'to contain' (TC)."

Because a bit of solution typically remains in pipettes, TC pipettes must be blown out.

Serological pipettes can be both TC and TD because they are graduated. So for a 10 ml pipette graduate to 0.1 ml, then the pipette will deliver TD from say between the 9.80 ml and 5.30 marks. That would be of course 4.50 ml. However to get all of the last 0.10 ml of liquid, then you'd have to blow out the pipette.

Answer 3

In olden days, (I was taught to both suck up and blow out a pipet by MOUTH), TD and TC about covered it. Now days, we're talking about micro-liters and automatic pipets and use for GC/MS calibration, and all sorts of more technical more SPECIALIZED uses. Wouldn't you expect that more specialized uses require more specific equipment? Temperature, gravitational field, density, surface tension, humidity, viscosity...those are the factors that can lead to variations in the amount of liquid left inside the pipet at the end of the delivery process. (I'm not saying all of them are important at sea level on Earth). FWIW, different needs will need different things. I think a good overview would be to discuss TD/TC and then explain one specific high accuracy micropipette. FWIW.