Instrument Calibration
Summary
This general procedure is based on determining the weighing result of water samples delivered by the syringe. True volume is calculated based on the density of water at specific temperatures.
Limitations
The method is not recommended for volumes below 2uL. There is no upper volume limit.
Equipment, Materials, Environment
- Laboratory balances required for the test method should meet or exceed the following performance specifications, be calibrated regularly with the appropriate traceable weights, and be regularly maintained.
| Test Volume, uL |
 |
Balance sensitivity, mg |
 |
| 1-10 uL |
|
0.001 mg |
|
| 10-100 uL |
|
0.01 mg |
|
| 100-1000 uL |
|
0.1 mg |
- Use a balance table, or suitable equivalent to minimize vibration. Cover the working surface directly in front of the balance with a dark, smooth, non-glare material. Keep the balance area reasonably free of draft currents and the ambient area free of excessive dust.
- Use a calibrated thermometer to measure the temperature of the water.
- Use a weighing vessel that has a total volume of about 10 times the test volume, or 500 µL, whichever is larger. This is for evaporation control. If possible, also use a cover that fits over the outside of the vessel top (do NOT allow the cover to come into contact with the test liquid). The vessel should be plastic, glass, metal, or some other non-porous material. The cross-sectional area of the opening should be as small as possible to further control evaporation.
- Handle the vessel with forceps or tweezers.
- Use distilled water.
Procedure
- Introduction: Deliver a total of n samples into a weighing vessel, and weigh each sample after delivery. Replicate all motions and time intervals in each sampling cycle as precisely as possible. Keep the distance between the balance and the diluter/dispenser to a minimum.
- Preparation: Select the analytical equipment and materials. Select the syringe to be evaluated, including/removable and disposable parts. Make sure the syringe is clean. Ensure the room, equipment and materials, including the prepared water, are thermally equilibrated. Ensure the electronic balances have had sufficient warm-up time to stabilize.
- Place a small amount of water in the weighing vessel (between 2 and 30 sample amounts or a minimum of 500 µL).
- Fill a water reservoir. Aspirate water into the syringe. Remove any bubbles by slowly aspirating and quickly dispensing water several times. (When testing the sample side of a diluter, use the probe to aspirate and dispense the water.
- Open door of balance chamber, place the weighing vessel on the balance pan and close the door of the balance chamber.
- Tare the balance. Aspirate one sample. Retrieve the weighing vessel from the balance chamber, deliver complete sample, and return the vessel to the balance pan, closing the door to the chamber. Observe and record balance readout.
- Repeat the previous step until 10 samples have been weighed. Note: Perform the weighing cycles as quickly as possible, but without compromising the integrity of the liquid delivery or the precision of the technique of the operator.
- Measure and record the water temperature.
Calculations
- Calculate the volume of each dispense (Vi) by dividing each mass value by the density of water at the measured temperature. Refer to the table below for density values.
Density of Water at Various Temperatures
Taken from CRC Handbook of Chemistry and Physics, 50th edition, 1969, page F-4
| C° |
g/cc |
C° |
g/cc |
C° |
g/cc |
|
| 17 |
0.998774 |
22 |
0.997770 |
27 |
0.996512 |
|
| 18 |
0.998595 |
23 |
0.997538 |
28 |
0.995646 |
|
| 19 |
0.998405 |
24 |
0.997296 |
29 |
0.995944 |
|
| 20 |
0.998203 |
25 |
0.997044 |
30 |
0.995646 |
|
| 21 |
0.997992 |
26 |
0.996783 |
|
|
- Single dispense (in)accuracies can be calculated from the volume dispensed (Vi) and the expected volume (V o): Accuracy (%) = 100 x (Vi - Vo) / Vo
- Calculate the average dispensed volume from the individual dispensed volumes, Vi (where I is 1 to10): V avg = V1 + V2 + V3 + . + V10) / 10
- Calculate the syringe accuracy: Accuracy (%) = 100 x (Vavg - Vo) / Vo
- Calculate the standard deviation (SDEV) of the calculated volumes, then determine the coefficient of variation: CV (%) = 100 x SDEV / V avg
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