Simultaneous Determination of Seven Anions of Interest in Raw

Article pubs.acs.org/EF

Simultaneous Determination of Seven Anions of Interest in Raw Jatropha curcas Oil by Ion Chromatography Yi Zhang,*,† Prapisala Thepsithar,‡ Xia Jiang,*,§ and Joo Hwa Tay∥ †

Department of Environmental Science and Engineering, Fudan University, 220 Handan Road, Shanghai, People’s Republic of China, 200433 ‡ Institute of Environmental Science and Engineering, Nanyang Technological University, Innovation Center, Block 2, Unit 237, 18 Nanyang Drive, Singapore 637723 § College of Architecture and Environment, Sichuan University, Chengdu, People’s Republic of China, 610065 ∥ Department of Civil Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada ABSTRACT: Various ions are of interest to the quality of biodiesel feedstock and products. In this study, a simple and laborsaving analytical method was developed to directly and simultaneously measure seven anions of interest in oil, utilizing an ion chromatography system with function of sample matrix elimination. Various eluent profiles were explored and calibration curves were made to analyze 13 raw Jatropha curcas oils for their contents of formate, acetate, nitrite, nitrate, sulfite, sulfate and phosphate. A 23 min program was found to sufficiently separate all the ions, and linear correlation higher than 99% was achieved for all the ions except formate (98.6%). High diversity was found in both the presence and concentration range of these ions. Formate, nitrate, and phosphate were more prevalent among the ions tested, such that 12, 10, and 11 samples showed their presence, respectively. Nitrite was found in only two samples with the concentrations lower than 10 mg kg−1. Formate concentration ranged from 0 to over 3000 mg kg−1, and nitrate and phosphate showed ranges of 0 to 100 and 0 to 300 mg kg−1, respectively. Acetate was less common than formate, and its concentration was universally lower (0 to 500 mg kg−1). In addition, the occurrence of acetate and nitrite seemed to be correlated to that of formate and nitrate, respectively, whereas sulfite and sulfate showed mutual exclusion. This method showed reasonably good detection limits and reproducibility, that concentrations of around 0.2 mg L−1 can be detected in the organic samples, and in most cases the ratio of standard deviation to average was below 25%. However, for phosphate, the accuracy and reproducibility need further improvement, possibly by decreasing sample dilution ratio and optimizing eluent profile.

I. INTRODUCTION As fossil fuel resources face depletion, efforts are being made to develop biofuel as a renewable and green alternative. Biodiesel is a major kind of biofuel that can be produced from various vegetable and animal oils and fats.1 The application of biodiesel in transportation fuel blends is increasing,2 and further commercialization may benefit from better evaluation of the quality of biodiesel feedstock and products. The quality of raw feedstock oils and biodiesel products can be evaluated by many parameters. Total acid number (TAN) indicates the acidity of the oil, which causes corrosion in oil pipes and tanks.3 It has been suggested that TAN alone is not adequate in describing oil’s corrosiveness, as only the short chain portion, especially formic and acetic acids, are the main causes.4 Sulfur and nitrogen exist in many raw vegetable oils,5,6 whose reduced forms can be oxidized during oil combustion, contributing to acid rain formation.7 Sulfur species in oil can also deactivate alkaline catalysts and reduce oil yield during fuel upgrading.8 Another important parameter is phosphorus, especially the organic phospholipids. Phospholipids in raw oil can cause quality deterioration by forming gum,9 and in oil processing can act as catalyst poison.10 Therefore, these parameters are of practical interest to biodiesel producers and users, and the development in their measurements is ongoing. Formic and acetic acids are short chain volatile fatty acids (VFAs) and are not specifically regulated by biodiesel quality © 2014 American Chemical Society

standards; therefore they have no corresponding standard methods. However, as simple organic acids, they can be measured by gas and liquid chromatography.11,12 On the other hand, phosphorus and sulfur in biodiesel are regulated, and are usually measured by standard methods which use inductively coupled plasma-optical emission spectrometry (ICP-OES) and ultraviolet fluorescence, respectively. These methods determine the total contents of P and S in a combusted form, sometimes after dilution with toxic solvent.13,14 Various alternative methods are being studied for short chain VFAs, phosphorus and sulfur components. For example, Strömberg and Sahlin15 developed an ion exclusion chromatography method for 6 short chain fatty acids, whereas phosphorus in oil has been measured by colorimetric method,16 graphite furnace atomic absorption spectrometry,17 molecular absorption spectrophotometry,18 FIA spectrometry,19 capillary electrophoresis,20 and so froth. However, these methods usually involve sophisticated instrumentation or complicated sample pretreatment procedures like solvent extraction or emulsion, which could restrict their use in practice. Another problem of measurement is the speciation of the elements. Sulfur, nitrogen and phosphorus in oil can exist in Received: December 12, 2013 Revised: March 25, 2014 Published: March 31, 2014 2581

dx.doi.org/10.1021/ef402451t | Energy Fuels 2014, 28, 2581−2588

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both inorganic and organic forms.20 For instance, total phosphorus is composed of organic P (phosphatides) and inorganic P (phosphate), and the relative amount can vary considerably with types and sources of oilseeds and the pressure and temperature used in the extraction process.21 The existing standards are for the total contents of these elements, without concern for their speciation. However, in practice, generally it is the organic part, e.g. phospholipids, that is of interest. To date, the information on the speciation of sulfur, nitrogen and phosphorus elements in oil is very limited, and it would be advisible to develop new test methods to determine the organic or inorganic parts of these elements separately. Ion chromatography (IC) can detect various organic and inorganic ions in liquid matrix, and has been previously explored in oil quality determination. Dugo et al.22 employed suppressed ion exchange chromatography to measure F−, Cl−, Br−, NO2−, NO3−, PO43−, SO42−, and I− in virgin olive oils. Recently an IC method with sample matrix elimination function has been successfully developed to directly determine the phosphate (PO43−) content in raw Jatropha curcas oil.23 Diluted oil samples are passed through a trap column, and the ions are retained within to be eluted and measured later. Therefore, this method requires minimal sample volume (30− 50 μL) and preparation procedure (organic solvent dilution only), which saves samples, labor, and time considerably. In addition, it has the potential to determine the inorganic species of S, N, and P elements. Combining this approach and the standard methods for total contents, both the organic and inorganic species of the elements could be obtained. In this study, it is intended to further apply this instrument and methodology to determine multiple anions of interest in raw Jatropha curcas oil. Jatropha curcas (J. curcas), a generally nonfood crop, produces seeds of significant raw oil content.24 Raw Jatropha curcas oil can be processed into high-quality biodiesel, and is currently being studied as a potential feedstock for stable and economic biodiesel production.25 The ions of interest included formate, acetate, NO2−, NO3−, SO32−, and SO42−, which were to be measured simultaneously in a single sample within a reasonable run time. Therefore, this study could provide further information on the quality of a promising biodiesel source, including its acid strength and speciation, as well as the inorganic ion contents. It might also be extended to other types of oils for easier quality control.

IC System and Conditions. A Dionex ICS-3000 system was used in this study for the aim of direct determination of ions in oil. The system was composed of a dual pump system, an EluGen II Potassium Hydroxide Cartridge (Dionex P/N 058900) to automatically generate KOH in DI water of desired concentration and gradient as the eluent, a CR-ATC (Dionex P/N 060477) anion trap column for eluent purification, a CRD 200−4 mm (Dionex P/N 062983) carbonate removal device, an ASRS 300−4 mm suppressor (Dionex P/N 064554), and a conductivity detector (Dionex P/N 061830) for anions. The system was also fitted with an AS-1 Autosampler (Dionex P/N 061773) and Chromeleon Chromatography Workstation with Chromeleon Version 6.8 SP3 as the operating software. The most significant advantage of this IC system is its ability to directly analyze organic samples, and the core component to that is the IonPac UTAC-LP1 concentrator (Dionex P/N 063079). It received the original organic samples and allowed the organic solvent to flow through but trapped the anions therein. The eluent, KOH solution of designated gradient, was then delivered into the concentrator to flush out the trapped ions at a flow rate of 1 mL min−1. The eluent containing various anions was separated by an IonPac AS18 analytical column (4 × 250 mm, Dionex P/N 060549) equipped with the corresponding guard column. Analysis was further done by the conductivity detector, the suppressor current of which was set at 112 mA. The sample volume was 100 μL and the temperatures of the cell, the column, and the compartment were set at 35, 30, and 30 °C, respectively. Eluent Profile. The gradient profile of the eluent was further explored to achieve both satisfactory separation and reasonably short runtime. Four profiles were first tested, which are depicted in Figure 1,

Figure 1. The five eluent profiles tested in this study. (■) No. 1, 23 mM for 10.5 min, to 45 mM in 0.1 min, hold for 5.4 min, to 23 mM in 0.1 min, hold for 2 min; total runtime 18.1 min; (◆) No. 2, 23 mM for 7 min, to 45 mM in 5 min, hold for 4 min, to 23 mM in 0.1 min, hold for 3.9 min; total runtime 20 min; (▲) No. 3, 10 mM for 8 min, to 45 mM in 6 min, hold for 5 min, to 10 mM in 0.1 min, hold for 10.9 min; total runtime 30 min; ( × ) No. 4, 10 mM for 10 min, to 45 mM in 13 min, hold for 4 min, to 10 mM in 0.1 mM, hold for 2.9 min; total runtime 30 min; (dotted line) No. 5, 10 mM for 5 min, to 45 mM in 13 min, hold for 4 min, to 10 mM in 0.1 min, hold for 0.9 min; total runtime 23 min.

II. EXPERIMENTAL SECTION Chemicals. Analytical grade n-hexane was obtained from Merck and used to extract raw oil from the J. curcas seeds. High performance liquid chromatography (HPLC) grade acetone (Thermal Fisher) was selected as the solvent to dissolve and dilute oil for IC analysis. Sodium sulfite, formate, and acetate of analytical grade were used to prepare the respective standard solutions. All were used without further purification. Multiple ions standard stock solution was obtained from Dionex (Dionex P/N 057590, P/N as Part Number). Deionized (DI) water with a specific resistance of 17.8 MΩ-cm or higher (Millipore) was used for eluent generation after filtration through 0.2 μm pore size membrane. J. curcas Seeds and Raw Oils. Thirteen kinds of J. curcas seeds were obtained from various farms in the Southeast Asia region, including India, Indonesia, and South China. The kernels were separated from the shells by mechanical pressing, and subsequently grinded into particles by a grinder (DeLonghi ABL-373) of lower than 2 mm diameter. Approximately 10 g of grinded seed particles were then used to extract the raw oil from via a standard Soxhlet chemical extraction procedure (BS EN ISO 65926), the detailed procedure of which was described in the previous work.23

and the resultant chromatograms of nine ions (chloride, fluoride, bromide, formate, acetate, nitrite, nitrate, sulfate, and phosphate, not in that order) are shown in Figure 2a−d. Comparing the chromatograms obtained, a further modification was made based on eluent profile No. 4, which further shortened the runtime to 23 min. The final gradient profile was: 10 mM for 5 min, to 45 mM in 13 min, hold for 4 min, to 10 mM in 0.1 min, and hold for 0.9 min. The resultant chromatogram is shown in Figure 2e, and this program was used for the subsequent calibration and measurement of all the raw oils. Standard Preparation. The formate and acetate stock solution was prepared by dissolving 0.3778 g of sodium formate and 0.3475 g of sodium acetate in 500 mL of DI water, resulting in a stock solution 2582


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Figure 2. Chromatograms of 9 or 10 ions with the 5 eluent profiles tested. (a) No. 1; (b) No. 2; (c) No. 3; (d) No. 4; (e) No. 5. of 500 mg L−1 of formate and acetate, respectively. As the Dionex multiple ion stock solution (in water) already contains nitrite, nitrate, sulfate, and phosphate, 1 mL of it was combined with 0.5 mL formate/ acetate stock, and the combination was then diluted to 50 mL with HPLC grade acetone. This resulted in a stock solution of 5, 5, 2, 2, 3, and 3 mg L−1 for formate, acetate, nitrite, nitrate, sulfate, and phosphate, respectively. The stock solution was further diluted with acetone to obtain the calibration standards of desired concentrations,

the ranges of which are presented in Table 1. Both the stock and the calibration standard solutions were prepared immediately before measurement, as acetone evaporation and SO42−/PO43− deterioration in acetone can bring error into the analysis. During preliminary studies, it was discovered that a small chromatography peak was present before the sulfate peak in some oil samples. Through retention time comparison, it was tentatively identified as sulfite ion. As a reduced form of sulfur, sulfite can be 2583


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oil samples were then diluted to 1.5 mL with acetone, and the vials were capped immediately and mixed thoroughly by hand. The final concentration was calculated as ion content in oil (mg kg−1) = ion concentration in acetone sample (mg L−1) × 1.5 mL/oil quantity in the sample (g).

Table 1. Calibration Series for the Seven Target Ions in Acetone ion

retention time (min)

standards series (mg L−1)

correlation (%)

acetate formate NO2‑ NO3‑ SO32‑ SO42‑ PO43‑

5.12 5.70 10.23 14.96 15.26 15.77 19.82

0.33, 0.83, 2.5, 5.0 0.33, 0.83, 2.5, 5.0 0.13, 0.33, 1.0, 2.0 0.13, 0.33, 1.0, 2.0 0.66, 1.98, 4.95 0.2, 0.5, 1.5, 3.0 0.2, 0.5, 1.5, 3.0

99.29 98.58 99.94 99.99 99.65 99.89 99.74

III. RESULTS AND DISCUSSION Ion Separation and Calibration. The ions present in acetone, the solvent used for oil solution, were first determined, and formate, acetate, nitrate, and sulfite ions were detected in it. However, after calibration, the concentrations of these ions in acetone were found to be too low to affect the calibration and test results. Therefore, the solvent matrix was considered to be noninterfering to the measurement. Various eluent gradient profiles were further tested and compared, and one program was selected for its separation effect and short runtime, which is shown in Figure 2e. It can be seen that complete separation and good peak shapes could be achieved for nine ions within 23 min. Calibration was then conducted with two standard series. A first calibration curve was established with a 6-ion standard series, that is, formate, acetate, nitrite, nitrate, sulfate, and phosphate, and a second series of sulfite was measured separately. The respective retention times, concentration ranges, and correlation coefficients of the individual ions are listed in Table 1, and the overlaid chromatograms of the standards are shown in Figure 3. Good correlation (R2 higher than 98.5%) was achieved with all the ions in the concentration range of 0.1−5 mg L−1, and most ions except formate showed R2 higher than 99.3%. Although various measures were taken to minimize sulfite oxidation during standard preparation, a small peak of sulfate was still observed in the sulfite series shown in Figure 3b. However, when the sulfate concentrations in the sulfite standards were measured using the standard curve obtained with the six-ion series, their level was found to be negligible, indicating a finite degree of auto-oxidation and the effectiveness of the preparation procedure. When comparing the intended and the measured concentrations, it was observed that inorganic ions had slightly better correlation than the two organic ions, especially formate. A higher concentration range was used for formate than the inorganic ions, but formate standards of lower concentrations showed more deviation from the standard curve. It is possible that 0.33 mg L−1 was around or below the lower detection limit of formate by this IC method, and a higher concentration range might provide better correlated standard curves. However, the practical standard series range would have to be determined with more tests using this method, as well as by the real concentration range of formate in the oil samples to be measured. Oil Sample Preparation. A total of 30 μL of raw J. curcas oils was aliquot into sample vials, and the oil quantity in each vial was measured by an analytical balance. The results of three parallel tests are presented in Table 2, which shows that the ratio of standard deviation to average was generally below 10%, except for No. 1 and No. 6 samples (23 and 11%, respectively). It was noted that the two samples were more difficult to dispense by pipettes, especially No. 1, due to their very high viscosity. A similar phenomenon was observed in a previous study where the IC system was applied to determine the inorganic phosphate content in raw J. curcas oils, that viscous samples showed bigger discrepancy in sample weight.23 Thus, improvement in quantification might be needed for oils with

oxidized during combustion, contributing to SOx formation; therefore, its quantification was also attempted in this study. However, the preparation of a stable standard solution proved to be a challenge, as sulfite can auto-oxidize rapidly when exposed to air. This was confirmed when first a high concentration aqueous sulfite stock solution was made then diluted into lower concentrations. Whether acetone or DI water was used for dilution, the resultant chromatograms all showed a significant degree of, sometimes, total oxidation into sulfate by the time of analysis. Second, a 10 mg L−1 sulfite stock solution in organic solvent was attempted by directly putting sodium sulfite crystals into acetone. However, the chemical did not dissolve and, therefore, sulfite stock could not be made without water. Subsequently, an acetone/water mixture was further tested as the matrix. It was found that a 50/50% (v/v) mixture of acetone and water can serve the purpose, whereas 40/60 and 30/70% (water to acetone, v/v) did not result in adequate solution even after prolonged mixing. Therefore, in the subsequent tests, the sulfite stock solution was prepared as follows: 50 mL of acetone was placed in a 100 mL glass volumetric flask, and 0.1575 g of Na2SO3 was precisely measured and poured into the flask. DI water was placed in an ultrasonic bath for 15 min, and the deoxygenated DI water was used to dilute the acetone in the flask. The mixture was hand-shaken until all the Na2SO3 crystal dissolved, which resulted in a 1000 mg L−1 SO32− stock solution. Various concentrations of SO32− standard solutions were made by further diluting the stock solution with acetone immediately before measurement, which helped to minimize auto-oxidation of sulfite. Sample Preparation. The contents of elements like sulfur and phosphorus in oils are expressed in the unit of milligrams per kilogram. In this study, raw J. curcas oils were aliquoted into sample vials, its quantity measured, and dilution was made with acetone. To determine the reproducibility of this method, triplicate samples of 30 μL of oil aliquots were dispensed into 2 mL glass HPLC sample vials fitted with preslit septa and matching caps, and the weight of each sample was determined by an analytical balance (results shown in Table 2). The

Table 2. Average Weights of 30 μL Raw J. curcas Oils of Three Measurements sample no. No. No. No. No. No. No. No. No. No. No. No. No. No.

1 2 3 4 5 6 7 8 9 10 11 12 13

oil ID

average weight (g)

standard deviation (g)

standard deviation/ average (%)

China China A China B Yunan Guangxi Java Indonesia Indian Indo Guangfeng 1 Guangfeng 2 Test VDH

0.0272 0.0246 0.0280 0.0267 0.0279 0.0305 0.0273 0.0279 0.0273 0.0297 0.0337 0.0275 0.0259

0.0064 0.0017 0.0016 0.0003 0.0025 0.0035 0.0013 0.0023 0.0013 0.0018 0.0020 0.0013 0.0012

23.4 6.72 5.57 0.94 8.83 11.5 4.68 8.13 4.76 6.03 5.91 4.68 4.45 2584


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Figure 3. Chromatograms of standard series used in the calibration and a real oil sample. (a) 6-ion series; (b) sulfite series; (c) a sample of raw J. curcas oil.

Table 3. Target Ions Concentrations in Various Oilsa

high viscosity, for example, to increase the volume. On the other hand, the pipet dispensing method can yield reasonably reproducible results of oil weights for oils with low viscosity. Measurement of Ion Contents in Real Oils. Thirteen real raw J. curcas oils were dissolved in acetone (1:49, v/v), and triplicate samples were measured using the IC method. The calibration curve was made immediately before the measurement, in order to correct for the change of instrument condition (if any). Figure 3c shows a sample run of a raw oil sample, and the measured concentrations of the seven target ions in the 13 raw oils are shown in Table 3. The results show that the presence of ions in various raw oils was diversified. Most of the oils contained formate, nitrate and phosphate, whereas the other ions were detected in only a few samples. Among the seven ions tested, nitrite was the rarest, which was found only in samples No. 1 and No. 2, and acetate, sulfite, and sulfate were present in four samples individually. On the other hand, the relative abundance of the individual ions varied considerably. Formate was relatively abundant in

a

2585

oil type

acetate

formate

NO2−

NO3−

SO32−

SO42−

PO43−

No. No. No. No. No. No. No. No. No. No. No. No. No.

334 148 0.0 0.0 0.0 64.9 0.0 0.0 0.0 0.0 469 0.0 0.0

1297 1583 86.1 60.3 152 329 33.5 81.7 18.3 79.7 3394 0.0 217

6.0 2.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

104 39.4 27.4 23.9 0.0 0.0 9.2 12.8 8.2 2.0 0.0 8.0 14.0

0.0 0.0 0.0 0.0 25.4 0.0 0.0 32.5 0.0 0.0 41.6 0.0 26.5

128 14.5 0.0 0.0 0.0 0.0 0.0 0.0 10.9 0.0 0.0 2.5 0.0

116 45.1 0.0 24.0 0.0 4.8 133 22.1 62.7 42.8 322 94.7 12.2

1 2 3 4 5 6 7 8 9 10 11 12 13

In milligrams per kilogram, average of three measurements.


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Reproducibility and Detection Limit. The ratios of standard deviation to average by three measurements of the oils are listed in Table 4. It can be seen that the ratio varied

these oils, with concentration up to approximately 3400 mg kg−1 in No. 11. However, the concentrations of nitrite were less than 10 mg kg−1 in the two oil samples where it was detected, which was lower than all the other ions. For a specific ion, the concentration range found among the oil samples also showed high diversity. Formate was not detected in No. 12, but was present with concentrations higher than 1000 mg kg−1 in No. 1, 2, and 11. In contrast, nitrate and phosphate, the other two most prevalent ions, showed concentration range of 0 to 100 and 0 to 300 mg kg−1, respectively. In most of the samples, nitrate concentrations varied between 10 to 40 mg kg−1, and phosphate between 20 and 100 mg kg−1, which was much less scattered than that of formate. This might be due to the different origins of these ions in oil. N and P species are natural constituents of the plant biomass, and therefore, their concentration range is restricted by the biomass composition. Formate, on the other hand, is originated from the degradation of long chain components of the oil, and its abundance is much affected by the oil processing and storage conditions,27 which could cause its high diversity among various samples. Interestingly, some ions seemed to show correlations in their abundance. Both formate and acetate, the two short-chain volatile acids, are the products of oil oxidation, and their presence could indicate the degree of oil quality deterioration. The results show that the occurrence of formate was more common than acetate, and its concentrations were uniformly higher than the latter. What’s more, only samples with formate concentrations higher than 300 mg kg−1 showed the presence of acetate, indicating that oil oxidation might be more likely to yield the single carbon compound of formate than acetate with 2 carbon atoms. In their study of short chain VFAs profiles in aged biodiesel samples, Strömberg et al.27 observed a similar phenomenon, that formic acid was generated much faster than longer chain acids and constituted a larger proportion of TAN after oil aging than in the original samples. Based on the results obtained, a rough linear correlation was found between the concentrations of formate and acetate (R2 = 0.89); however, more data are needed to elucidate the issue with more certainty. Similarly, an apparent positive correlation was observed between nitrite and nitrate, that only samples of high nitrate concentrations contained nitrite as well (threshold around 40 mg kg−1). However, sulfite and sulfate showed a completely different correlation, that the two ions seemed to be mutually exclusive. The samples containing sulfite all showed undetectable sulfate contents, and vice versa. In the preliminary screening, it was unexpected to find the sulfite peak between nitrate and sulfate, as sulfite proved to be extremely prone to auto-oxidation in aqueous medium. It seemed that a reducing environment might exist in some oil samples, preventing sulfite’s oxidation into sulfate, as during the time of sample storage and analysis, the sulfite peaks persisted with little variation in size. This agrees partially with the results of nitrite and nitrate, that samples with sulfate presence generally showed higher levels of nitrate, which could be due to the more oxidizing environment of the sample matrix. However, some samples, for example, No. 13, contained considerable levels of both sulfite and nitrate. Therefore, the relative abundance of the reduced and oxidized forms of nitrogen and sulfur inorganic ion species might be the result of complex interactions between the individual ion’s stability and the sample matrix’s redox environment. This phenomenon needs further investigation for better understanding.

Table 4. Ratio of Standard Deviation to Averagea oil type

acetate

formate

NO2−

NO3−

No. No. No. No. No. No. No. No. No. No. No. No. No.

25.0 8.90

13.9 5.04 20.0 19.7 14.6 9.02 22.9 27.3 15.3 38.3 5.00

37.4 29.6

1.24 5.66 25.2 11.2

a

1 2 3 4 5 6 7 8 9 10 11 12 13

4.78

24.3

7.41

SO32−

SO42−

PO43−

4.52 17.1

66.8 54.6 23.5

13.1 11.1 1.74 19.0 6.17

4.82 12.9 14.1

6.75 33.4

7.09 11.1

31.4 4.35 12.4 12.4 23.1 5.43 35.5 25.5

Percent based on three measurements.

significantly with different ion and oil types. For organic ions, the ratio in acetate measurement was higher for samples with higher acetate content (e.g., No. 1 and No. 11 in Table 3). For these oils, the acetate concentrations in the organic samples were around 10 mg L−1, which was higher than the concentrations used in the calibration series. This indicates that a higher concentration of acetate (e.g., above 10 mg L−1) might not be suitable for direct measurement. Further dilution of the samples might be needed for better reproducibility. However, for formate, higher ratios of standard deviation to average were observed for the oil samples in the lower concentration range (No. 3, 4, 7−10 in Table 3). These samples had the corresponding formate concentrations of around or below 1 mg L−1 in the acetone solution, which could be the lower detection limit of this ion by the present method. The results suggest that a higher calibration range might be needed for formate, and proper dilution of the original oil samples could bring both acetate and formate into the linear range of the method. If not, a separate dilution might be necessary to achieve satisfactory result for either ion. A higher deviation was also observed in the measurements of nitrite in No. 1 and No. 2, probably due to the low nitrite contents, 6.0 and 2.5 mg kg−1 in these two samples, respectively. The corresponding nitrite concentrations in the organic samples were around or below 0.1 mg L−1, which could be the lower detection limit of this IC method for nitrite. On the other hand, the ratio of standard deviation to average appeared to be generally lower for sulfite, sulfate, and nitrate, and was not related to the ions’ concentrations. For instance, samples No. 1 and 10 had the highest and lowest levels of nitrate, respectively, but exhibited similar high degrees of reproducibility, suggesting that the measurements could be more affected by the sample matrix. As the concentrations of these ions in the samples were generally within the linear range of calibration, their detection limits were not estimated. The measurement of phosphate, on the other hand, showed poorer reproducibility. The ratio was usually within the range of 10 to 30%, with samples No. 1 and 2 exhibiting values above 50%. In the previous study where the IC system was used to determine the phosphate contents in oil,23 a similar range of standard deviation to average ratio was observed, which was 2586


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below 20% for phosphate contents over 15 mg kg−1 and around 40% for those below 10 mg kg−1 (the lower detection limit). The phosphate contents in the samples used in this study were more widely scattered, and no significant correlation was observed with the reproducibility. It was discovered that during the sample runs various impurities can interfere with phosphate measurement by peak coelution. Moreover, the tailing became evident for the phosphate peak, and the residence time changed slightly after the system was used to analyze oil samples for a long time. These factors affected the accuracy and reproducibility of phosphate measurement and need to be studied in more detail in future work. Only a few studies to date have dealt with the analysis of anions in oil. Dugo et al.22 used a suppressed ion exchange chromatography to measure eight inorganic anions, that is, F−, Cl−, Br−, NO2−, NO3−, PO43−, SO42−, and I−, in commercial seed oils and virgin olive oils. They used a conventional sample preparation procedure of extraction with sonification, followed by centrifugation and filtration. As their objective of analysis was edible oils, the anions of interest were slightly different from this study. Nogueira and Do Lago20 applied capillary electrophoresis in the determination of cations Ca2+, K+, Mg2+, and Na+, as well as anions SO42−, PO43−, formate, acetate, and propionate in biodiesel. On the other hand, Strömberg and Sahlin15 specifically targeted six short-chain VFAs by an ion exclusion chromatography method. All these methods made use of organic-aqueous extraction to transfer the ions into a water or buffer phase, sometimes followed by sample purification. Compared with those studies, the IC method developed in this study showed relative low reproducibility, possibly due to the high dilution ratio (50 folds) used. Though the IC system could measure ions in oil samples, the ions are actually trapped in the concentrator and eluted with aqueous eluent. The system requires that the sample matrix is miscible with water; therefore, oil samples cannot be measured directly but must be diluted with a polar organic solvent like acetone, methanol, and ethanol. Therefore, the dilution ratio can be adjusted and the solvent be optimized to improve the accuracy and reproducibility. On the other hand, this method had the advantage of using commonly available and nontoxic solvent instead of specific and complex buffer systems. Conventional sample preparation like extraction might take hours for the organic and aqueous phases to separate and may introduce unknown interference into the samples. The IC method can measure ions in oil directly with minimal sample preparation procedures, which saves time and manual labor, and could be of great value to industrial application. What’s more, this study measured ions in raw oils instead of processed edible oil or biodiesel products. The more complex and viscous nature of the raw oil sample matrix could affect the analysis, and the IC system and method could be further optimized to deal with the measurement of raw biodiesel feedstock oil. Effective Life Span of the System Components. As mentioned in the last section, the measurement accuracy of some ions showed deterioration over time, suggesting the system components might be affected by the solvent matrix. In order to assess the duration of effective life span of the IC parts, spot checks with standards were periodically done, and the time when significant deviation occurred was noted. The number of samples from part installation to that point was recorded and presented in Table 5. It could be seen that the concentrator had the shortest life span, which was about 350−370 aqueous

Table 5. Estimated Life Span of the IC Components IC component

number of aqueous sample

number of oil sample

concentrator 1 concentrator 2 guard column analytical column AS 18 eluent generator

370 357 1123 916 1423

60 67 183 159 120

samples and 60−70 acetone samples. This is understandable, as the function of the concentrator was to trap the ions in oil to be eluted later, which can then be separated and analyzed by the analytical column. Therefore, the concentrator was in direct contact with acetone and oil, and was affected the most. The guard and the analytical columns can each measure around 900−1100 aqueous samples and 160−180 acetone samples, which were much more than the concentrator, indicating the effectiveness of the system’s function of sample matrix elimination. The life span of the eluent generator (1400 aqueous samples and 120 acetone samples) was determined by the eluent concentration and sample runtime more than by the sample matrix. Timely installation of new parts, especially the concentrator could help to ensure the quality of analysis, as well as protect downstream components like the analytical column and suppressor.

IV. CONCLUSION This study presents a practical, economical, and less laborintensive method using ion chromatography for simultaneous determination of seven organic and inorganic anions of interest in raw Jatropha curcas oil, a potential biodiesel feedstock. Both the presence and concentration range of the ions were found to be diversified in 13 various raw oils. Formate, nitrate, and phosphate were the most prevalent, whereas nitrite was only found in 2 samples. Formate concentration ranged from 0 to over 3000 mg kg−1, and nitrate and phosphate showed ranges of 0 to 100 and 0 to 300 mg kg−1, respectively. The presence of acetate and nitrate seemed to be correlated to that of formate and nitrite, respectively, but sulfite and sulfate showed mutual exclusion. Reasonably good accuracy and reproducibility of the result was achieved, except for phosphate, which was due to peak coelution and possible deterioration of IC components after exposure to organic sample matrix. Effective life span of the IC components was then identified, and the concentrator was found to be the part that needs to be replaced most frequently. Further improvements on the method could be made to enhance its applicability, which includes a preliminary screening of the ions’ contents in oils to set a proper dilution ratio, a bigger sample size (e.g., 50 μL) and injection volume (e.g., 200 μL), and further adjustment on the eluent profile and column conditions (column type, temperatures, etc.). In addition, solvents other than acetone to dissolve the oil samples could also be tested.

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AUTHOR INFORMATION

Corresponding Authors

*Y. Zhang. E-mail: [email protected]. Tel.: 86-21-5566 4354. Fax: 86-21-6564 2788. *X. Jiang. E-mail: [email protected]. Tel.: 86-28-8599 0936. Fax: 86-28-8599 0936. Notes

The authors declare no competing financial interest. 2587


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Article

ACKNOWLEDGMENTS The authors would like to express their sincere and deep gratitude to Dr. Yan Rong (deceased) of the Institute of Environmental Science and Engineering, Singapore, for her kind help and insightful guidance.

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Simultaneous Determination of Seven Anions of Interest in Raw

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