Like all food sectors, grain exporters must comply with relevant national and international regulations to ensure that all products are safe for consumption. For testing the inorganic content of grains, producers must meet the requirements of relevant international standards such as the Codex Alimentarius Commission1 or the importing country’s Maximum Levels (MLs) for contaminants such as heavy metals, including arsenic (As). Agilent ICP-MS instruments are widely used for the fast, accurate, multi-element analysis of a full range of foodstuffs.

Carbon enhancement of an argon inductively coupled plasma

If not identified and corrected, high concentrations of matrix elements can suppress or enhance the signal of some analytes in an argon plasma, leading to inaccurate results. However, matrix effects that enhance the signal can be leveraged to boost the sensitivity of the analysis of hard-to-ionize elements such as As and selenium (Se). It has been reported that an increased population of carbon ions or carbon-containing ions in the plasma facilitates a more complete ionization of certain analytes with slightly lower ionization energies than carbon2,3. The ionization potentials of C, As, and Se are 11.26, 9.82, and 9.75 eV, respectively.

The team at Australian Superintendence Company (ASC) routinely use a carbon-enhanced plasma during the multi-element analysis of grains, which are sampled, analyzed, and certified on behalf of clients before export. While the method is not widely used within food-testing laboratories, Sam Mallard and the team at ASC have evaluated its performance and have found it to be highly effective for detecting trace elements in grains.

Cost-effective sources of carbon

As a commercial laboratory, ASC must deliver competitively priced sample analysis to maintain client work, so reducing the cost per analysis can help secure repeat business. To reduce reagent costs per sample, the team investigated modifying the plasma of their Agilent 8900 triple quadrupole ICP-MS (ICP-QQQ) using low-cost carbon dioxide (CO2) rather than high-purity acetic acid.

As shown in Figure 1 (top), acetic acid is introduced to the plasma with the sample diluent or via the internal standard (ISTD) line and mixed with the sample using a tee-piece. In contrast, CO2 can be delivered directly into the total argon (Ar) flow of the plasma using a gas delivery system and an existing fifth plasma gas control line and mass flow controller of the 8900 ICP-QQQ (Figure 1, bottom). This approach means the carbon can be removed quickly and easily from the system, enhancing productivity.

A simple flow diagram showing how carbon enhancement is achieved using acetic acid or carbon dioxide

Figure 1. Top: A flow diagram demonstrating how carbon enhancement is achieved by adding the carbon source (acetic acid) with the internal standard. Carbon flows throughout the sample introduction system (represented in orange).

Bottom: Carbon enhancement with CO2 is achieved by adding the carbon source as an option gas in the ion source (represented in orange). This approach optimizes routine matrix loading with all the performance benefits of carbon enhancement in the plasma.

Economic analysis

To evaluate the economic case for using CO2 compared to acetic acid, the ongoing costs of both reagents were calculated. Upfront expenses of buying and installing the gas mixing equipment were also considered. The equipment includes a ballast tank for high-flow gas storage and a three-way valve for switching between CO2-enhanced and Ar-only plasma. As shown in Figure 2, the payback period for the CO2 system is around six years, based on current Australian dollar (AUD) prices. The figure also highlights significant long-term savings of the CO2 system over 20 years, which is the approximate lifetime of the CO2 bottle.

Graph showing a cost analysis of using carbon dioxide addition rather than acetic acid. The cost benefit is seen after 5 to 6 years use.

Figure 2. Cost of using acetic acid for carbon enhancement versus the cost of using a 10% ballast mix of CO2 over 20 years, inclusive of initial CO2 installation costs. All costs were in Australian dollars.

Analytical performance: Evaluation of signal enhancement

To evaluate the signal enhancement of adding CO2 to the plasma for various elements, 1 mL of each of the two Agilent PA tuning solutions (p/n 5188-6524) were diluted to 50 mL in 3% NHO3. The enhancement factors for selected elements using different concentrations of CO2 in Ar (5.4, 8.1, 8.4, and 13.4%) delivered to the plasma from the ballast tank are given in Table 1. Figure 3 illustrates a significant enhancement of the signal for As in the presence of CO2, supplied from the ballast tank, with optimal levels between 5 and 9%, peaking at approximately 8%. The introduction of the ballast gas into the plasma at a flow rate of 15% optional gas equates to a final carbon content within the plasma of 0.75 to 1.35%, with a peak of 1.2%.

Table 1. A selection of the most prominent elemental enhancement factors for carbon at various ballast tank percentages, when measuring the PA tuning solutions diluted to 50 mL in 3% NHO3 (n=7)

A table displaying the enhancement factors for several elements when using different concentrations of carbon dioxide in addition to the plasma.
Graph showing the measurement of arsenic with the 8900 ICP-QQQ using 75 to 91 mass transition in reaction mode. Enhancement of the arsenic signal is shown at different concentrations of carbon dioxide in the ballast tank with a maximum of 6 times enhancement at a concentration of 8% carbon dioxide in argon.

Figure 3. Enhancement factor for 75 -> 91 As at various percentage levels of CO2 in Ar in the ballast tank (n=7)

Cost and performance benefits of CO2

The study shows that CO2 is a viable and economical alternative to acetic acid for carbon enhancement in plasma ionization, benefiting commercial laboratories by reducing costs and improving analytical performance.

ASC recommend a carbon content of around 5% CO2 in Ar in the ballast tank to maximize benefits from carbon enhancement for multi-element analysis, while minimizing the adverse effects of additional oxide formation in the plasma.

Read our White Paper article to learn more.

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