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The determination of trace metals in volatile organic solvents is of utmost importance to prevent catalyst poisoning and final product contamination. This article discusses the need for the analysis of trace metals in volatile organic solvents and reviews the challenges and problems that have been traditionally associated with this type of analysis when performed using inductively coupled plasma-mass spectrometry (ICP-MS). A new method is highlighted, combining ICP-MS with an advanced dual syringe pump sample introduction system. The unique capabilities of the technique to deliver direct, efficient, accurate, and contamination-free analysis of trace metals in volatile organic solvents are demonstrated in an application example.
Organic solvents are a chemical class of compounds used routinely in the production of a wide variety of products, including paints, varnishes, lacquers, adhesives, glues, degreasing and cleaning agents, polymers, plastics, textiles, printing inks, agricultural products, and pharmaceuticals. The usefulness of organic solvents lies in their capability to dissolve oils, fats, resins, rubber, and plastics. They exist in liquid form at room temperature and are also characterized by low molecular weight, lipophilicity, and volatility. The determination of trace metals in volatile organic solvents is important as trace metals can poison catalysts and contaminate the final products.
In the petrochemical industry, for example, the determination of trace metals in naphtha is essential as some elements can poison the catalyst during the cracking of hydrocarbons, leading to difficulties in subsequent processing as well as contamination of end products. Nonvolatile elements such as lead (Pb) are removed easily during refinement, but mercury (Hg) due to its volatility is more problematic and its analysis demands increased sensitivity and accuracy. As Hg acts as a poison to the expensive metal-based catalysts, production of low Hg-containing naphtha is of increased significance. Arsenic (As) also can poison catalysts at trace concentrations as low as 50 µg/kg and cause problems with high temperature naphtha cracking tubes due to the formation of coke build-up. This build-up can result in the eventual failure of the tubes and subsequently reduce production capability.
Direct and automated analysis of organic solvents using inductively coupled plasma–mass spectrometry (ICP-MS) often is seen as problematic and challenging, due to solvent loading and chemical incompatibility of the sample introduction system. A constant flow of solvent, regardless of sample viscosity or specific gravity, is necessary for accurate analysis. Pumping of solvents using a peristaltic pump, however, is problematic as any tubing found to be chemically resistant can also be a significant source of contamination. Self-aspiration is often the only alternative, but different uptake rates are likely due to changes in viscosity of the aspirated solution.
The high volatility of many organic solvents further complicates the analysis. When a volatile organic solvent is introduced into an ICP-MS analyzer, the sample transport efficiency is much greater than that of aqueous samples, which can lead to undesirable effects such as plasma instability. To introduce a solvent such as naphtha into plasma, the volatility must first be reduced. This can be done in two ways, either by dilution of the sample with another solvent such as kerosene, or by cooling the solvent before introduction into the plasma, which is typically done using a cooled spray chamber. The second of these two options is preferable as the first will degrade the sensitivity of the analysis. However, both methods result in reduced sample throughput while increasing the potential for sample contamination. Hg, for example, is a notoriously sticky element and requires extensive washing regimes when using traditional introduction systems to minimize contamination of subsequent samples.
Traditional analytical challenges can be overcome by coupling a quadrupole ICP-MS analyzer with an advanced dual syringe pump sample introduction system. This sophisticated method facilitates efficient, accurate and contamination-free analysis of an extended range of volatile organic solvents. The specially designed syringe pumps reliably deliver constant flow rates of organic solvents with different physical properties, while also eliminating the need for peristaltic pump sample delivery. In addition, the syringe pumps smoothly deliver highly volatile organic solvents at low flow rates from <5 µL/min to 50 µL/min, thus avoiding any overloading of the ICP-MS system.
All surfaces of the sample introduction system that the sample comes into contact with are made from PFA, facilitating an inert, noncontaminating sample introduction. Additionally, the valve system reduces uptake and washout times dramatically for improved sample throughput. An experiment was performed to explore how the new technology can optimize the analysis of trace metals in volatile organic solvents.
A quadrupole ICP-MS system (XSERIES 2, Thermo Fisher Scientific, Bremen, Germany) was coupled with a microFAST 2 advanced dual syringe pump sample introduction system (SC-FAST DX with Continuum, Elemental Scientific Inc, Omaha, Nebraska) to demonstrate the competence of the new method. The dual syringe pump sample introduction system uses syringe pumps to load samples onto and inject samples from a six-port vacuum loaded switching valve.
Naphtha: The multielement and single element Hg standards used in this experiment were sourced from CONOSTAN, USA. Naphtha from Fisher Scientific (Reference N/0050/PB17) was used in all analyses. The powerful collision cell technology (CCT) of the quadrupole ICP-MS system with a single H2/He cell gas mixture was used for the analysis of Ca, Cr, and Fe to remove spectral interferences resulting from the high carbon content in the sample. All other elements were analyzed in standard non-CCT mode.
Figure 1: Fully quantitative calibration for Hg in naphtha.
For quantification of trace elements in naphtha, fully quantitative external calibrations were performed. 95 Mo was used as an internal standard. Figure 1 shows the calibration line for Hg in naphtha. The background equivalent concentration (BEC) determined for 202 Hg in the naphtha sample was 53 ppt and a detection limit (calculated from 3 sigma of 10 blank measurements) of 10 ppt was determined.
Table I: Mercury memory test results
Mercury Memory Test: To determine the washout characteristics of the introduction system used, replicate analyses of pure naphtha were made directly after the analysis of a 50-ppb, Hg-spiked standard. Table I shows that 7 min after the analysis of the 50-ppb Hg naphtha standard, the Hg background recorded in a pure naphtha sample had dropped down to a background subtracted equivalent concentration of <50 ppt (less than 0.1% washout). To test the stability of the analysis, a sample of naphtha spiked with 10 ppb of a range of elements was repeatedly analyzed over a 4-h period. Normalized results illustrating the recovery of the 10 ppb spike are shown in Figure 2.
Figure 2: Recoveries for 10-ppb spiked naphtha sample over 250 minutes (200 runs).
Hexane: With the dual syringe pump sample introduction system, direct analysis of the challenging hexane solvent was possible. Fully quantitative calibration curves were generated for a number of trace elements in hexane (PN H303-1 or H/0355/15, Fisher Scientific) using the same ICP-MS acquisition parameters as naphtha. Figure 3 shows calibration lines of 24 Mg, 40 Ca, 52 Cr, and 56 Fe. BECs and limits of detection (LODs) were calculated for each analyte (Table II).
Figure 3: Calibration lines in hexane.
Typically a very challenging application, the direct trace metal analysis of volatile organic solvents can be performed efficiently and accurately using an advanced dual syringe pump sample introduction system coupled with a quadrupole ICP-MS analyzer. The dual syringe pumps provide a constant flow rate for different organic solvents in a contamination-free environment. The sample introduction system fully automates the sample handling so that batch analysis of problematic samples such as organic solvents and ammonia solution can become fully routine. The quadrupole ICP-MS analyzer operated in third-generation CCT mode provides a sensitive and interference-free solution for analytes suffering from polyatomic interferences, even for 52 Cr, which has a BEC of 24 ppt in undiluted hexane. Overall, the combination of advanced sample handling with the robust and high-performance ICP-MS technology provides a high-throughput, routine approach for the determination of trace elements in various undiluted organic solvents.
Table II: LOD and BEC data for trace elements in hexane.
Tomoko Oki, Julian D. Wills, Shona McSheehy, Meike Hamester, Torsten Lindemann, and Joachim Hinrich are with Thermo Fisher Scientific, Bremen, Germany.