The independent Forensic Scientific Service (FND) of the State Police in St. Gallen, Switzerland conducts analytical and forensic tests with the aim of detecting proof of suspected crimes such as the illegal possession of drugs. Since samples are generally available in powder form and in rather small quantities, an XP205 analytical balance (Mettler Toledo AG, Greifensee, Switzerland) was the customer’s solution to assist in its delicate weighing tasks.
Handle with care
A balance in a forensic laboratory must satisfy high demands, since 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, and amphetamine are often found in very small quantities, and the forensic scientific service needs to examine the sample with great care. Therefore, it was vital to find a balance solution that ensured the weighing-in of small sample quantities down to 10 mg, avoided influences caused by electrostatic charges on powdery substances, and facilitated the error-free positioning of up to five different flasks with volumes up to 200 mL.
Small quantity, big impact
Daniel Eugster, Quality Manager at FND, explained that on average, the laboratory weighs-in approximately 25 mg of sample, or even smaller amounts down to 10 mg. In this low weighing range, the measurement accuracy of the balance lies within 1%, taking a safety factor of 5 into account. Additionally, the laboratory needed a balance with a measuring accuracy that is traceable and meets the requirements of ISO/IEC17025.
Figure 1 - XP205 analytical balance.The XP205 analytical balance (Figure 1) fulfills this easily with a weighing capacity of 220 g and a readability of 0.01 mg. 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 GmbH maintains and calibrates the balance annually.
Free of electrostatic influence
One has to take electrostatic influences into account while weighing powdery substances, particularly if the air humidity is less than 45%. 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 was simple; the manufacturer 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.
Precise and secure positioning
Due to the special Ergo-Clip Flask weighing pan, the positioning of volumetric flasks on the weighing pan is a straightforward task for the laboratory staff. Because the tare vessel is securely positioned during the weighing process, there is no loss in time or valuable material. In addition, there is no need to open a draft shield because the MinWeigh Door allows dosing through a small window in the draft shield door directly into the tare container. The laboratory requires reliable and accurate laboratory instruments to obtain precise results with the absolute minimal measurement uncertainty. The XP205 analytical balance satisfies this requirement.
Ms. Hupert is ePR Program Manager, Mettler Toledo AG, Im Langacher, 8606 Greifensee, Switzerland; tel.: +41 44 944 3385; e-mail: [email protected].