The independent Forensic Scientific Service (FND) of the State Police in St Gallen, Switzerland, uses an XP205 semi-microbalance from Mettler Toledo to assist in delicate weighing tasks.
The FND conducts analytical and forensic tests with the aim of detecting proof of suspected crimes such as the illegal possession of drugs.
As samples are generally available in powder form and in rather small quantities, it selected an XP205 semi-microbalance for weighing tasks.
A balance in a forensic laboratory has to satisfy high demands as sample material is scarce and the quality of the investigation results depends considerably on the evidence found and the condition of the sample.
Drug materials such as heroin, cocaine or amphetamine are often found in very small quantities and the forensic scientific service is forced to examine the sample with great care.
Therefore, it was vital to find a balance solution that guarantees the weighing-in of small sample quantities down to 10mg, avoids influences caused by electrostatic charges on powdery substances, and facilitates the error-free positioning of up to five different flasks with volumes up to 200ml.
'On average, we weigh-in approximately 25mg of sample, and sometimes even smaller amounts down to 10mg,' said quality manager, Daniel Eugster.
'In this low weighing range, the measurement accuracy of the balance lies within one per cent, taking a safety factor of five into account.
'Additionally, we need a balance with a traceable measuring accuracy that meets the requirements of ISO/IEC17025,' he added.
The XP205 semi-microbalance fulfills these requirements with a weighing capacity of 220g and a readability of 0.01mg.
Performance of the balance was confirmed during installation by issuing a calibration certificate and minimum weight determination (according to DKD; German calibration service).
In addition, a service technician from Mettler Toledo maintains and calibrates the balance annually.
Electrostatic influences must be taken into account while weighing powdery substances, particularly if the air humidity is less than 45 per cent.
Even the sample container itself is usually electrostatically charged, transmitting this charge to the tare vessel with the sample loaded spatula.
An electrostatically charged sample influences the weighing cell and generates incorrect results.
The solution to this was simple: Mettler Toledo applied an external U-electrode combined with a point electrode.
During the weighing process, the technician first holds the sample container under the point electrode to discharge it.
Passing the spatula through the U-electrode into the weighing chamber further eliminates electrostatic charges that could adversely affect the measuring result.
Thanks to the special weighing pan Ergo-Clip Flask, the positioning of Erlenmeyer flasks on the weighing pan is a straightforward task for the laboratory staff.
'The sample is securely positioned during the weighing process, therefore we don't lose time and valuable material,' said crime technician, Ivan Schlatter.
'We don't even need to open a draft shield because of the special Minweigh door, which allows dosing through a small window in the draft shield door directly into the tare container.'
