2.3.3.3. Micropipette

2.3.2. Chemicals, consumables and glassware

Figure 6. Diagram showing steps in UIMM assay.

Chemicals to be used for urinary iodine measurement should be more than or equivalent to analytical reagent-grade. Consumables to be used are non-sterile while the 96-well microtiter plates can be used either of flat- or round-bottom polystyrene ones. Microtiter plate lids should be covered with aluminum foil to prevent direct light onto reaction mixture. Sandell-Kolthoff reaction is sensitive to heat [14]. Thorough washing of glassware (test tubes, volumetric flasks,

Chemicals Purpose Ammonium persulfate Digestion Arsenious acid solution (As2O3, NaCl, H2SO4) Adding As3+ ions Ceric ammonium sulfate, H2SO4 Adding Ce4+ ions

Table 1. Purpose of each chemical addition in the urinary iodine micromethod (UIMM).

74 Quality Control in Laboratory

To check the micropipette performances, a maintenance-check-up every three-monthly is performed. Three points of volume should be tested, i.e. within the lowest, middle and highest ranges. For example, if the micropipette volume range is 100–1000 μl, then the pipette should be checked at 100 and next time around at 500 or at 1000 μl; it is up to the operator to decide. Water with the chosen volume should be pipetted into 10–20 clean disposable test tubes (LP3 or LP4 tubes). The weight of the tubes with and without water is recorded accordingly. Other information that should be recorded includes the brand and model of pipette, its code number, date of maintenance check-up and name of the person carrying it out. Mean, standard deviation (SD) and coefficient variation (CV) of the readings are determined. Inaccuracy is also determined as follows:


The maintenance check-up is repeated if the CV is >5% and inaccuracy is >10%. If the problem persists, request for instrument check-up and calibration should be lodged for further action.
