Achieving consistency in sample preparation to ensure low blank levels in trace metal determination

In recent years, various industries have been inclined towards reducing their detection limits, as 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 specific protocols and the right set-up to control possible contamination and ensure a robust approach. Among other parameters, the handling of the reagents, even if ultrapure, is a key factor in achieving lower detection limits and ensuring reliable quantification 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. Any sample preparation for elemental analysis requires the addition of reagents. When working at trace levels, the choice falls on ultrapure acids. However, the presence of the analysts in routine operations could contaminate the acid and the sample, as well as 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, etc. further increases the risk of contamination during handling. Even the routine use of pipettes can be 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. On top of this, there is human error. As reported by Rodushkin Et. Al,2 the level of contaminants could raises from 5 to 35 times, moving from 74 ng/L to 368 – 2,627 ng/L. Those data demonstrate how easily a bottle of reagent lose its purity and gets contaminated in regular operations. In this study we will focus on understanding how easyFILL, an automated dosing station, helps to control blanks in elemental analysis in term of reduced risk of contamination and consistency.

INSTRUMENT AND REAGENTS

• Milestone’s easyFILL
• Bottle-top dispenser
• ICP-MS/TQ
• UltraPure water
• HNO₃ (Trace Metal Grade)
• HCl (Trace Metal Grade)
• H₂O₂ (Analytical reagent grade)

easyFILL is an automated dosing station specifically design to be integrated into the sample preparation process. In fact, it enables direct addition of concentrated acid in most digestion vessels and vials, without exposing the operator to the reagents. Through a dedicated user interface, the technician simply selects the type and volume of acids and then the system begins the addition. easyFILL is equipped with 6 lines for different reagents that through a peristaltic pump are directly loaded into digestion vessels and vials. For routine operations, the user has only to select a method and press “Start” to begin with the addition. Operator can create customized methods by choosing the volume of reagents, the type of reagents and even selecting a different chemistry for each position.

PROCEDURE
In both bottle-top dispenser and easyFILL procedures, reagentes were directly added into a 50mL falcon tube then directly analyzed in ICP-MS TQ.

In this study, we compared two different procedures for adding reagents: the conventional method using a bottle-top dispenser (manual addition) and the automated method using Milestone’s easyFILL. For this purpose, the most common reagents used in food testing laboratory such as UltraPure water, Trace Metal Grade nitric and chloridic acids, and Analytical grade hydrogen peroxide were analyzed in the blank tests. The analysis was performed with ICP-MS TQ on 67 elements representing both the major and the trace elements analysed in food matrices. Tables 3 to 6 report the major elements while Tables 7 to 10 report trace elements. For each element analysed, a comparison is made between the bottle-top dispenser and easyFILL, with the bias reported for evaluating differences between the two procedures.
In order to assess the consistency of the blank values over time, tests were carried out on an easyFILL that has been in regular use for 12 months.
The same measurements were performed also on a brand new easyFILL and no significant differences was reported (for easier reading of the data, only the data obtained on the used system were reported in this document).