Electronic-analytical balances (EABs) are known to provide accurate, precise, and reliable weight-measurement of a chosen sample. In this regard, a considerable number of weight-measurements in the laboratory are routine when using EABs, especially with the high-quality models currently available. However, there are a number of external factors that can confound the weight-measurement, including temperature, vibration, air turbulence, evaporation, hygroscopy, and electrostatic charge. Temperature, vibration, and air turbulence can easily be addressed by the location of the EAB. However, the evaporation (or volatility), hygroscopy, and electrostatic charge of the sample require special attention.
A high evaporation rate or hygroscopic rate of a sample requires a minimally-sized, tightly-capped weighing vessel to obtain a reliable weight measurement. But perhaps the most confounding and frustrating factor in EAB weighing is a high electrostatic charge of the sample.
A high electrostatic charge is the accumulation of an electrical charge on a non-conductive material, either the sample or weighing vessel. Telltale signs of an electrostatic charge problem include non-repeatable readings, drifting of the reading, and long EAB stabilization times. In addition, electrostatic charge causes reading errors of +/- 1-100mg and sample “jumping” from the spatula to the weighing vessel which disrupts the weighing process and results in a mess for the EAB user to clean-up.
Lyophilized dry powder is particularly vulnerable to hygroscopy and electrostatic charge and thereby troublesome to weigh. For example, pharmaceutical companies routinely synthesize hundreds of early drug derivatives that require precise weighing of 1-5mg of the lyophilized dry powder where hygroscopy and electrostatic charge play a role.1
Particulate matter (PM) collected on a PM-sampling filter is vulnerable to humidity and electrostatic charge. For example, regulatory agencies routinely monitor PM in the air collected on PM-sampling filters the require precise weighing under federal guidelines. 2
Electrostatic charge can be dealt with by either: a) preventing the charge before it builds or by b) dissipating the charge after it builds. Ways to prevent the charge before it builds include: using electrically-conductive or anti-static treated materials, use a Faraday cage, avoid contact between dissimilar materials (like air and latex gloves), avoid unnecessary rubbing of the weighing vessel, set relative humidity to 45–60%, and ground the balance. Ways to dissipate the charge after it builds include: use radioactive material (e.g., Polonium-210) to ionize the air, use an internal ionizer, and use an external ionizer.
The internal ionizer is installed inside the draft shield of the EAB and is the most convenient solution to dissipate the charge for the EAB user. The present-day internal ionizer automatically detects the electrostatic charge on the sample and weighing vessel, estimates the size of measurement error, and releases positive and negative ions that renders the electrostatic charge neutral. The detection occurs concurrently with the weighing process, requires only a few seconds, and speeds the weighing process while producing an accurate measurement.
The external ionizer is installed outside the EAB beside the EAB door. The present-day external ionizer automatically activates when the EAB door opens and de-activates when the EAB door closes. In this way, the electrostatic charge is neutralized as the sample and weighing vessel are placed into the weighing chamber. There are two types of external ionizers; point ionizers and U-electrode ionizers. Point ionizers are used for small samples and weighing vessels. U-electrodes are used for large samples and weighing vessels.
Conclusion
EAB users can take some simple steps to prevent an electrostatic charge from building in both the sample and weighing vessel. However, there are frequent cases where even preventive steps are unsuccessful and require dissipating the electrostatic charge after it builds. In these cases, the latest generation of EABs tackle the problem by automatically detecting the electrostatic charge, estimating its measurement error, and releasing ionization. The ionization is released by either an internal ionizer or an external ionizer both of which are integral components of the EAB. These ionizers make weighing these difficult samples accurate and easy for the EAB user.
References
1. Ioana Popa-Burke, Steven Novick, Melissa Mantilla, Keith McGrath: J Biomol Screen. 18 (3), 331-40, Mar 2013
2. R Allen, M Box, L J Liu, TV Larson: J Air Waste Manag Assoc. 51(12), 1650-3, Dec 2001