Identification and Quantification of Phosphate Ester-Based Hydraulic Fluid in Jet Fuel.
The hydraulic pumping system of the aircraft is immersed in the jet fuel tanks, which are operated by a hydraulic fluid. Any slight leaking failure in the system, which results in higher pressure of the hydraulic fluid than that of the surrounding fuel, can cause serious contamination of the fuel in the tanks. Hydraulic fluids used in aviation hydraulic systems contain a variety of compounds based on phosphate esters. These compounds play a significant role in fire resistance [1, 2, 3] but may also cause hot corrosion of turbine and metal bearings [4, 5, 6, 7]; damage composite parts [8, 9]; damage of corrosion inhibitor coatings, which are widely used in the aircraft industry [10, 11]; and swelling of polymer seals. Contamination of jet fuel with hydraulic fluid may endanger operating seals and engine parts, which are made of materials that are incompatible with phosphate compounds, in turn causing serious engine failure. Due to the corrosive effect of phosphate esters on cobalt, the level of phosphate ester contamination must not exceed 1 ppm of hydraulic fluid in engines manufactured using cobalt-containing alloys and must be less than 10 ppm in engines manufactured without cobalt alloys .
A multitude of chromatographic methods for the identification and quantification of hydraulic fluids [13, 14] and organophosphate [15, 16] contaminations in turbine engine oil, occupational air , human plasma , soil , water , and human urine  have been reported in recent years. Moreover, a chemical multisensory device as a detector for hydraulic oil contamination in air was developed . Methods for monitoring the quality of hydraulic fluids using acid-base indicators and chromatography have also been developed [23, 24]. However, despite the danger of hydraulic fluid contamination in jet fuel, only scarce methods for the identification and quantification of the contamination have been reported. One such method was described by Spila et al.  and another in a patent by Werner et al. . The two methods refer to a contamination caused by Skydrol hydraulic fluid. The presence of tri-butyl phosphate in jet fuel may be tested using other analytical techniques, for example, inductively coupled plasma atomic emission spectroscopy (ICP-AES), but the detection limit is too low . Another possible method uses a pulse flame photometric detector (PFPD) coupled with a gas chromatograph, which is very sensitive to many elements including phosphorous .
The Israeli Air Force (IAF) uses both brands of hydraulic fluid-ExxonMobil[TM] HyJet[TM] V and Skydrol[TM] LD-4 (Exxon and Skydrol). Quantification and identification of Skydrol can be performed according to its dibutyl phenyl phosphate component, as described by Spila et al. A similar analysis of the second fluid - Exxon - cannot be performed in the same way, since it does not contain dibutyl phenyl phosphate. Measuring the concentration of both Skydrol and Exxon can be carried out through analysis of their tri-butyl phosphate compound, which exists in both fluids. The mass spectrum of tri-butyl phosphate shows a main peak at 99 m/z that was considered nonsignificant for analysis because of the interference of peaks originating from the matrix of hydrocarbons. Failures of the hydraulic system in air refueling jets have led to contamination of the jet fuel. Together with the use of hydraulic fluid mixtures in the IAF, this has revealed a need for a universal method for identification and quantification of both hydraulic fluids in jet fuel. The lower detection limit of the methods developed had to be at least 1 ppm according to the policy of IAF and engine manufactures, though the specific limitation has not been proven yet .
The main goal of the present work was to develop a simple and reliable analysis method for the detection of contamination by either hydraulic fluid or a mixture of both in jet fuels at a concentration of at least 1 ppm using mass spectrometry (MS) or flame ionization detector (FID). Tri-butyl phosphate, a compound common in both Skydrol and Exxon, was separated from the matrix of jet fuel hydrocarbons using three methods presented herein. Two separation methods were developed by using solid phase extraction (SPE) columns. The use of SPE technique resulted in isolation of the 99 m/z signal originating from both hydraulic fluids, so that identification and quantification was carried out using a gas chromatograph equipped with a mass spectrometer (GC-MS) or by GC-FID. The third method is based on gas chromatographic separation and was carried out by a GC equipped with a polar column and MS analysis.
The SPE-GC-MS/FID methods were comprised of a separation stage that was carried out by an SPE technique followed by injection into a gas chromatograph where the analysis was performed by an MS or FID detector. The gas chromatographic separation method (GC-MS) was carried out in one stage by injecting the sample without any prior procedure. The gas chromatographic separation was performed by a polar column and analysis was carried out by an MS detector.
Commercial products were used in the preparation of the standard solutions: ExxonMobil[TM] HyJet[TM] V (ExxonMobil), Skydrol LD-4 (Solutia Inc.), hexane (anhydrous, 95%, Sigma-Aldrich), methanol (HPLC grade 99.9%, Sigma Aldrich), jet fuel JP8, and Strata[R] FL-PR Florisil SPE columns (170 [micro]m, 80 [Angstrom], 500 mg/6 mL).
2.2.1. GC-MS Method A Varian 450-GC gas chromatograph was equipped with a DB-WAX (30 m, 0.25 mm ID, 0.25 [micro]m df) column. The MS analysis was carried out by a Varian 300 MS system.
2.2.2. SPE-GC-MS/FID Methods The apparatus used for GC-MS analysis was comprised of a Varian 450-GC gas chromatograph equipped with a Rxi[R]-5 ms (Crossbond[R] 5% diphenyl 95% dimethylpolysiloxane, 30 m, 0.25 mm ID, 0.25 [micro]m df) column. The MS analysis was carried out in a Varian 300 MS system. The apparatus used for the GC-FID analysis was composed of a gas chromatograph and a Rxi[R]-5 ms column as described above. The analysis was carried out by an FID detector.
The stock solution was prepared by adding 0.08 mL of hydraulic fluid (Exxon/Skydrol) to 80 mL of jet fuel. Preparation of standard solutions was done by dilution of stock solution in jet fuel. Dilution was done according to the desired concentrations of standard solutions.
2.3.1. GC-MS Method The sample and the standard solutions were injected to the GC-MS without any preparation.
2.3.2. SPE-GC-MS/FID Methods Each standard solution was treated using the SPE technique: the SPE column was loaded with 20 mL of standard solution, 15 mL of hexane loaded in the column to extract jet fuel leftovers, and 2 mL of methanol used to extract the polar phosphate fraction from the SPE column. The methanol fraction containing the phosphate ester was analyzed by 1 [micro]L that was injected into the GS-MS or into the GC-FID system.
2.4. System Conditions
2.4.1. GC-MS Method GC-MS analysis was carried out under the following conditions: injection volume, 1 [micro]L; injector, split/splitless (type 1177); injector temperature, 240[degrees]C; splitless injection for 2 min; carrier gas, helium; column flow, 1.5 mL/min; oven, initial temperature 100[degrees]C for 1 min, heating to 230[degrees]C with rate 10[degrees]C/min; MS single-ion counting, 99 m/z; liner type, with quartz wool.
2.4.2. SPE-GC-MS Method GC-MS analysis was carried out under the following conditions: injection volume, 0.5 [micro]L; injector, split/splitless (type 1177); injector temperature, 240[degrees]C; split ratio, 1:20; carrier gas, helium; column flow, 1.5 mL/min; oven, initial temperature 100[degrees]C for 1 min, heating to 230[degrees]C with rate 10[degrees]C/min; MS single-ion counting, 99 m/z; liner type, with quartz wool.
2.4.3. SPE-GC-FID Method GC-FID analysis was carried out under the conditions: injection volume, 1 [micro]L; injector, split/splitless (type 1177); injector temperature, 240[degrees]C; makeup flow, 28 mL/min; hydrogen flow, 45 mL/min; air flow, 300 mL/min; column flow, 1.5 mL/min; detector temperature, 240[degrees]C. Oven, initial temperature 100[degrees]C for 1 min, heating to 230[degrees]C with rate 10[degrees]C/min; liner type, with quartz wool.
3. Results and Discussion
Both hydraulic fluids and jet fuel were separately injected to the GC-MS to examine their composition. Figure 1 shows that ion 99 m/z is common to both hydraulic fluids and originates from tri-butyl phosphate. Ion 99 m/z is inseparable from the saturated aliphatic ion fraction originating from the non-contaminated jet fuel (Figure 2).
Following the detection of ion peak 99 m/z in both hydraulic fluids and jet fuel chromatograms, three methods for the identification of hydraulic fluid contamination in jet fuel were developed: GC-MS with polar column method, SPE-GC-MS method, and SPE-GC-FID method.
3.1. GC-MS with Polar Column Method
The separation of ion 99 m/z originating from tri-butyl phosphate from the matrix of hydrocarbon peaks was achieved by using a DB-WAX column. It is evident that ion 99 m/z was not present in the chromatogram of uncontaminated jet fuel, but the former could be detected in the standard solutions beginning with 10 ppm of hydraulic fluid in jet fuel (Figure 3). The calibration curve in the range of 10-100 ppm of hydraulic fluid showed a linear relationship (y = 2562958x + 5531704, [R.sup.2] = 0.997). The limit of detection of hydraulic fluid contamination in jet fuel was 5 ppm.
3.2. SPE-GC-MS Method
In this separation method, the tri-butyl phosphate compound was separated from the jet fuel before the chromatographic analysis was performed. The separation was carried out by SPE that was performed according to the procedure described (Section 2.3.2). Solid extraction concentrates the sample and separates the jet fuel from the hydraulic fluid.
After separation, the methanol fraction containing the phosphate was analyzed by GC-MS. The chromatogram of the methanol showed a peak of ion 99 m/z. In the chromatogram of the jet fuel fraction, no peaks of ion 99 m/z were observed (Figure 4), indicating no traces of tri-butyl phosphate were left in the jet fuel after SPE. Quantification and identification of the contamination showed a lowest detection limit of 0.5 ppm of contamination in jet fuel. The calibration curve at a range of 0.5-100 ppm of hydraulic fluid showed a linear relationship (y = 2592269x + 3549189, [R.sup.2] = 0.997). The limit of detection of hydraulic fluid contamination in jet fuel was 0.1 ppm.
3.3. SPE-GC-FID Method
The other analysis method was based on GC-FID. Following the separation step described above, the methanol fraction containing the phosphate was analyzed by GC-FID. The chromatogram of the methanol showed a peak in retention time of 5.3 min, while the chromatogram of the jet fuel that passed the SPE column did not contain this peak, again indicating that no traces of tri-butyl phosphate were left in the jet fuel after SPE. The SPE-GC-FID method showed the same detection limit as the SPE-GC-MS method, at 0.1 ppm of contamination in jet fuel. The calibration curve at a range of 0.5-100 ppm of hydraulic fluid showed a linear relationship (y = 20903x + 551, [R.sup.2] = 0.998).
The quantification of contamination in all three methods mentioned above was based on the detection of the tri-butyl phosphate reagent that was separated from the jet fuel matrix. These quantification methods are not specific for only one type of hydraulic fluid contamination, unlike the methods described by Spila et al. and Werner et al., which were proven only for Skydrol. The GC-MS method is a single-step method, with no need for any sample preparation, unlike the method described in the patent of Werner et al. This method may be used when determination of massive concentrations of contamination is needed or for contamination detection in jet fuel used in engines not manufactured using cobalt metals. The SPE-GC-FID and SPE-GC-MS methods provided a better limit of detection of the contaminants, lower than in the method described by Spila et al. The SPE-GC-FID method allowed the use of more accessible equipment, since GC-FID is more common than GC-MS or GC-FPD, which were used in the article and in the patent mentioned above (Table 1).
Three straightforward methods for the identification and quantification of jet fuel contamination by hydraulic fluids were developed. Analysis was performed both by a gas chromatograph equipped with a mass spectrometer and a gas chromatograph equipped with an FID. Lowest level of detection reached was 0.1 ppm of tri-butyl phosphate in jet fuel, approximately ten times lower than the limit previously reported. Linearity was achieved for all analysis methods up to contamination of 100 ppm. The methods developed allow testing by two different detectors, thus minimizing dependency on a single instrument in the laboratory. In cases when one only needs to determine whether sample contamination is below/above a certain limit, a simple comparative test with a standard of a given contamination can be obtained within 1 h using the one of the methods reported herein.
Utilizing the methods developed during proactive maintenance of aircrafts can be successful in preventing failure, in which leakage detection is made possible before the problem becomes severe. These methods may prevent serious engine failure both in civil and military aircrafts.
This research opens a gate for further development of an online monitoring method. In addition, a method for purifying jet fuel from hydraulic fluid contamination may be developed according to our finding that the SPE method enables it.
The author and publisher would like to acknowledge that this article is based on a presentation at The International Association for Stability, Handling and Use of Liquid Fuels (IASH), Rome, Italy, September 10-14, 2017.
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Diana Stamker Gertopski, Konstantin Tartakovsky, and Moshe Rabaev, Israeli Air Force, Israel
Israeli Air Force
Depot 22, Materials Division, Fuel and Chemistry Department
Received: 26 Sep 2018
Revised: 19 Dec 2018
Accepted: 22 Jan 2019
e-Available: 13 Mar 2019
TABLE 1 Accuracy data and results for all analysis methods discussed in the article. GC-MS-method SPE-GC-MS method Linearity range (ppm) 10-100 0.5-100 Linearity regression 0.997 0.997 Sensitivity (ppm) 5 0.1 RSD (%, 10-15 ppm)/intra-day 3.8 (n = 5) 0.2 (n = 4) RSD (%, 10-15 ppm)/inter-day 6.1 (n = 2) 1.5 (n = 2) Sample preparation needed No SPE procedure Detection Exxon and Skydrol Exxon and Skydrol GC-MS method SPE-GC-FID method Spila et al.  Linearity range (ppm) 0.5-100 5-100 Linearity regression 0.998 0.997 Sensitivity (ppm) 0.1 2 RSD (%, 10-15 ppm)/intra-day 1.4 (n = 8) 3.0 RSD (%, 10-15 ppm)/inter-day 4.3 (n = 4) 5.1 Sample preparation needed SPE procedure No Detection Exxon and Skydrol Skydrol GC-FPD method Spila et al.  Linearity range (ppm) 5-100 Linearity regression 0.998 Sensitivity (ppm) 2 RSD (%, 10-15 ppm)/intra-day 4.0 RSD (%, 10-15 ppm)/inter-day 6.8 Sample preparation needed No Detection Skydrol [c] 2019 SAE International. All Rights Reserved
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|Author:||Gertopski, Diana Stamker; Tartakovsky, Konstantin; Rabaev, Moshe|
|Publication:||SAE International Journal of Fuels and Lubricants|
|Article Type:||Technical report|
|Date:||Apr 1, 2019|
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