Air Based vs Liquid Filled Pipetting 2016-11-21T17:53:15+00:00

Air Based System Pipetting

Advantages

  • No system fluid reservoir, no liquid lines running throughout system, no hand tightened fittings and no remote syringes.
  • Uses same technology as hand held pipettes that are the standard in the lab.
  • New technologies allow the active monitoring of pipetting leading to more precise and validate liquid transfers.

Disadvantages

  • Air is more compressible than liquids so additional calibration might be necessary.

Liquid Filled System Pipetting

Advantages

  •  Liquid is less compressible than air which makes it possible to pull a smaller air gap between the sample and the piston to help with more accurate transfers.
  • Active flow through washing of the tips is possible with in-line pumps and a wash station.

Disadvantages

  • Dilution Effects – Liquid coats even the inside of teflon tubing so the more sample that is pulled up into the tubing for transfer, the more ‘dilution effect’ is noticed.  In a 1.2 ml aspiration for a 100 µL multidispense across a microplate, the leading edge of the liquid starts to get diluted form the residual system fluid.  This problem compounds itself when multipipetting starts since this mode uses a high speed dispense and high speed stop in order to shear the drops off the end of the tip.  This high speed stop starts to degrade the meniscus of the system fluid, the plunger, from the first dispense.  Therefore, after every dispense, the plunger loses its integrity, leaking system fluid into the sample by the 3rd or 4th multidispense leading to the final dispensing volumes diluted out of range.  Partition volumes surrounded by multiple air gaps may be tried to avoid some of the dilution effects where a slug of the sample or reagent can be pulled up surrounded by air gaps.  This partition fluid is meant to be super-diluted by the system fluid, as multidispensing is performed to reduce the actual dilution of the sample.  This has mixed results.
  • Stable System Fluid – Having the need for system fluid and liquid lines, the integrity of the fluid needs to be maintained by continuously washing and drying the reservoir to prevent the growth of microbials.  The system fluid lines need to be pumped cleaned in daily, weekly and monthly cycles.  In addition, it is usually recommended by manufacturers that de-gassed water is used as to not accumulate air bubbles during the pipetting process.  This de-gassed water should be replaced daily before each run and multiple purges should be run and each syringe and liquid line examined by the operator to assure no air bubbles are left in the system.
  • Hand Tightened Fittings – Having over 40+ hand tightened fittings leading from the system fluid reservoir, up into the system, through a wash pump, up to line splitters, in and out of the syringe drives and down finally to the tips, reproducibility issues across each tip and especially across each system arise depending on the manufacturing, installation and servicing.
  • Serviceability and Re-calibration – During a preventative maintenance (PM) visit for a liquid filled system, the entire liquid filled section of tubing, including the syringes, are replaced in the field.  During this process the entire pipetting calibration changes and unless fully revalidated via balance calibration or dye testing, the results of all post assays are usually in question.
  • Remote Syringes – Twenty years ago small syringe drive that could compact themselves to the 9 mm spacing needed to access microplates were not possible and larger syringe drives were designed up and away from the pipetting arm, thus requiring long pieces of tubing filled with liquid in order to perform even basic pipetting.