Sample Preparation Focus
Is your analyst contaminating your laboratory samples? Simon Osborne, Analytix Ltd
In recent years, many industries have been actively working towards reducing their detection limits often because of regulatory requirements. Another example is evident in the recent FDA proposal of a “close to zero” plan to eradicate trace metal contaminants from baby food [1].
Reducing detection limits presents some challenges and laboratories need to have specifi c protocols and the right set-up to control possible contamination and ensure a robust approach.
Any sample preparation for elemental analysis requires the addition of reagents and the handling of these reagents, even if ultrapure, is a key factor in achieving lower detection limits and ensuring reliable quantifi cation limits. Certainly, the quality of the acid plays a key role, but if not combined with proper handling, the reagent can easily be contaminated, affecting the blank values. Apart from reagent contamination, other sources of contamination include digestion vessels that have not been suffi ciently cleaned between runs and dirty glassware.
When working at trace level analysis, the choice falls on ultrapure reagents including acids and water. What is not often considered is the potential for the presence of the analyst to contaminate the reagents in routine operations that could lead to inconsistent results. In fact, human skin, hair and sweat contain contaminants such as Zn, Cd, Pb, Fe, Cu, Ni, Mn, and Na. In addition, the use of cosmetics and the presence of watches, rings, bracelets, and other jewellery further increases the risk of contamination during handling.
For example, Rodushkin et al.[2] reported increases in levels of 30 to 50 times for Bi and Sb probably originating from make-up worn by the analyst where these are used as additives for bacterial growth inhibition in some formulations or found in one of the components of the black pigment in mascara. The same publication also highlights the routine use of pipettes as a source of contamination when working at trace levels, either due to the presence of metal components in the pipettes or simply due to contamination on the tips. Even leaving a bottle of ultrapure reagent uncapped in the laboratory will get contaminated during regular operations.
This article will explore how the implementation of an automated dosing system can effectively overcome the challenges of reagent contamination and also investigate the critical task of adding concentrated acid and other reagents into digestion vessels, a process that has traditionally relied on manual methods like pipettes. This crucial step faces several limitations in addition to contamination:
- Safety concerns - Time consuming operation
Methods
The Milestone easyFILL from Analytix used in this study is an automated dosing station specifi cally design to be integrated into the sample preparation process (Figure 1). It controls direct addition of reagents in most digestion vessels and vials without exposing the operator to the reagents.
Figure 1: Auto dispensing easyFILL system.
To enable easy operation the system is controlled via a dedicated graphical interface (Figure 2). This allows new users to quickly understand the process of control and can simply select the type and volume of reagents and then the system begins the addition. For routine operations, the user has only to select a customised method and press ‘Start’ to begin the addition.
Figure 2: Graphical interface for simplifi ed procedures.
The system is equipped with six lines for different reagents that through a peristaltic pump are directly loaded into digestion vessels and vials. The six lines can be a combination of either solvents and water or acids and water however this study looks at the most common reagents used in food testing laboratories namely ultrapure water, trace metal grade nitric and hydrochloric acid, and hydrogen peroxide.
Two different procedures for adding reagents to vessels were compared: the conventional method using a bottle-top dispenser (manual addition) and an automated method using easyFILL. The analysis was performed with ICP-MS TQ on 67 elements representing both the major and trace elements typically analysed in food matrices. The easyFILL system had been in regular use for 12 months prior to the study.
Results
Table 1 shows the results for levels of major elements for manual dispensing with a bottle top system compared to auto dispensing using easyFILL. The levels for the auto dispensing method are signifi cantly lower compared to the manual method. As indicated by the colour codes there were a few sporadic values where the easyFILL bias was higher however these are not signifi cantly different and overall, the auto dispensing method shows favourable results.
Table 1: Major elements results for HNO3 , HCL, ultrapure water, and H2 O2
LABMATE UK & IRELAND - APRIL 2024
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