Determination of Chloride and Sulfate in Bioethanol Using Dication and Trication Solutions

By: R. Köhling, N. Reichlin, AnalytiX Issue 3

Biodiesel and bioethanol are increasingly being used as renewable energy sources to replace the use of fossil fuels for common combustion engines. Diesel engines can be converted for use with plant oils or other fats, but signifi cant changes must be made to the vehicle. Modern biodiesel is an alternative to plant oils and used increasingly in diesel vehicles, e.g. trucks. There is also a renewable fuel source for gasoline engines which can run on ethanol-gasoline blends instead of pure gasoline. This sourcing of fuels from biomass is a common technique in countries with a large corn, sugar cane or general biomass emergence (USA, Brazil). The bioethanol can be obtained in large amounts through fermentation and distillation and is added to gasoline in varying amounts, e.g. 85% ethanol in E85 fuel1. Bioethanol and biodiesel contain several impurities despite passing through several cleaning steps. Especially concerning are dissolved salts which can damage modern engines. Thus, the determination of sulfate and chloride in ethanol-based fuels is an important quality criterion. A standard detection method for these analytes is ion chromatography confi gured with a conductivity detector2. It is a very sensitive analytical technique, but it lacks the capability to definitively identify the compounds in addition to retention time. An alternative to IC can be Di- (75128) and tricationic organic compounds (08675), which form positively charged adducts with chloride and sulfate anions, making them detectable for mass spectroscopy in the highly sensitive positive ESI mode3. The use of LC/MS also has the advantage of easy sample handling. For a large number of matrices, the samples can be injected without further treatments.

Independent of analytical techniques, the quality of quantitative results strongly depends on the precision and accuracy of the analytical reference standards. A robust calibration method still can lead to results with a high uncertainty, when the content of the reference material is not well defined. In this case, matrix samples are spiked with TraceCERT® anion standards for ion chromatography in order to ensure the highest accuracy for the calibration data.

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Method

There are two favored methods for the application of multiply charged cation solutions with LC/MS: (1) a direct injection of a mixture of cation solution, sample and solvent (e.g. methanol/water) into the MS, or (2) the injection of the sample into the constant fl ow of a mobile phase, which is mixed with a cation solution post-column via a T-connector (Figure 1).

 

Figure 1.Schematic drawing of the LC/MS set-up and the data processing of the MS data

Figure 1.Schematic drawing of the LC/MS set-up and the data processing of the MS data

The second method is preferred for the analysis of biofuels since it provides the highest sensitivity and opens the possibility to change all compositions during the method development. Additionally, an analytical column can remove parts of the matrix to prevent suppression effects. However, one should be aware that ionic analytes could interact with metal parts throughout the entire HPLC system, columns or tubing. If this happens and peak broadening occurs, then PEEK tubing, injectors and columns can prevent this negative infl uence on the peak height and shape.

Starting with the HPLC, the pump delivers a typical mobile phase water and methanol (90/10, v/v) at a low flow rate of 0.2 ml/min. Additives like formic acid, TFA or acetic acid should not be used, as they will form adducts with the multi-cationic reagent and will lower its efficiency. The solvents should have the best available quality to minimize bias and noise. Reducing the flow rate also reduces suppression effects caused by sample matrices4. The di- and tricationic fluoride solution is added to the mobile phase with a flow rate of 80 μl/h, but can be increased in case of low signal intensity of the adduct (ion pair of di-/trication and anion).

The calibration samples are prepared according to DIN 38402 part 51 and 32645 with equidistant concentrations. 5 calibration levels and 1 blank sample cover a concentration range from 2 to 10 μg/ml and result in a typical limit of detection (LOD) of 0.5 μg/ml and a lowest limit of quantifi cation (LLQ) of 1.2 μg/ml (Figure 2). Ethanol and water serve as matrix for the preparation of the calibration standards. Finally a sample of the lab water supply is analyzed without any sample pre-treatments.

 

Figure 2.Typical calibration curve for sulfate ions in water detected as trication-sulfate adduct. Similar results are obtained for chloride ions in ethanol detected as a dication-chloride adduct. Correlation coefficients of 0.996 to 0.999 are obtained for the linear fit of the calibration data. The calculation of the confidence and prediction bands is based on a confidence level of 0.95.

Figure 2.Typical calibration curve for sulfate ions in water detected as trication-sulfate adduct. Similar results are obtained for chloride ions in ethanol detected as a dication-chloride adduct. Correlation coefficients of 0.996 to 0.999 are obtained for the linear fit of the calibration data. The calculation of the confidence and prediction bands is based on a confidence level of 0.95.

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Discussion

Solutions of di- and tricationic compounds can easily be used for the determination of chloride and sulfate in ethanol or drinking water. The lab water supply was directly injected into the HPLC system and resulted in a typical chloride concentration of 4 ppm. The solutions of the di-/trications are supplied in a condition capable of being used directly in mixing with the mobile phase post-column. This set-up is installed on most LC/MS systems and does not need additional items except a syringe pump.

Mass spectroscopy with electrospray ionization (ESI) has the advantage of a very sensitive and selective detection, which is responsible for low LODs and LLQs as can be seen in Figure 2. Although 2 ppm is the lowest calibration level, it is possible to get lower LLQs even at a confi dence level of 0.99 because of the steep regression curve. One important advantage of MS is its capability to definitively identify analytes by their mass (exact mass), isotopic pattern and MS/MS spectra.

In addition to IC, this LC/MS method can also be a useful tool for the analysis of anions in complex matrices like biofuels, especially for those laboratories equipped with an LC/MS system.

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Materials

     

References

  1. www.bio-kraftstoffe.info
  2. Norm ASTM D4806 “Standard Specification for Denatured Fuel Ethanol for Blending with Gasolines for Use as Automotive Spark-Ignition Engine Fuel”.
  3. R. Köhling, N. Reichlin, “Highly Sensitive Detection of Organic and Inorganic Anions with Di- or Tricationic LC/MS Additives”, Analytix, 2, 2009.
  4. F. Gosetti, E. Mazzucco, D. Zampieri, M. Gennaro, “Comparison of Matrix Effects in HPLC-MS/MS and UPLC-MS/MS Analysis of Nine Basic Pharmaceuticals in Surface Waters”, J. Am. Soc. Mass. Spectrom., 2008, 19, 713–718.

 

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