Determination of stearic acid and 12-hydroxyoctadecanoic acid in PEG-60 hydrogenated castor oil by HPLC-ELSD after alkaline hydrolysis
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Determination of stearic acid and
12-hydroxyoctadecanoic acid in PEG-60
hydrogenated castor oil by HPLC-ELSD
after alkaline hydrolysis
Acta Chromatographica
YU DUAN1, XIAN-ZHAO LIU2, EN-NIAN LI3,
DOI: ZHONG-JING GUO1 and XIN-JUN XU1p
10.1556/1326.2023.01116
© 2023 The Author(s)
1
College of Pharmacy, Sun Yat-sen University, Guangzhou 510006, China
2
Infinitus (China) Co., Ltd., Jiangmen 529100, China
3
Guangdong Yuansi South Pharmaceutical Biotechnology Co., Ltd., Guangdong 514699, China
Received: November 24, 2022 • Accepted: February 1, 2023
ORIGINAL RESEARCH
PAPER
ABSTRACT
A rapid and simple method for the determination of stearic acid and 12-hydroxystearic acid in PEG-60
hydrogenated castor oil by high performance liquid chromatography with evaporative light scattering
detection was established. The oil sample was first pretreated by alkaline hydrolysis. The analysis was
performed on a Zhongpu Develop XD-C18 column (250 mm 3 4.6 mm, 5 μm) with gradient elution of
methanol and 1% acetic acid aqueous solution at a flow rate of 1.2 mL$min1 and a column temper-
ature of 40 8C. The drift tube temperature of the evaporative light scattering detection system was set at
40 8C, and the pressure of carrier gas (N2) was 337 kPa. The regression equation revealed a good linear
relation (r 5 0.9993–0.9995) during the test ranges (119.1–1190.7 μg$mL1 for 12-hydroxystearic acid,
10.7–107.4 μg$mL1 for stearic acid). The detection limits of 12-hydroxystearic acid and stearic acid
were 1.1 and 2.5 μg$mL1, the limits of quantitation were 3.2 and 7.4 μg$mL1, respectively. And the
mean recoveries were 101.5 and 101.0%, the corresponding relative standard deviations (RSDs) were 2.1
and 2.8%, respectively. The RSDs corresponding to repeatability (n 5 6) were both less than 1.7% in
terms of precision. As to the stability, the test results remained stable after 8 h at room temperature
(RSDs were both less than 2.6%). The developed method showed high sensitivity, recovery, repeatability
and stability, which indicated that the method could be applied as a quality evaluation method for the
determination of stearic acid and 12-hydroxyoctadecanoic acid in PEG-60 hydrogenated castor oil.
KEYWORDS
HPLC-ELSD, hydrogenated castor oil, fatty acids, determination, validation
1. INTRODUCTION
Hydrogenated castor oil, produced by hydrogenation of natural vegetable oil extracted from
castor seeds, is a kind of important oil excipient. In the field of medicine, hydrogenated castor
oil is often used as an effective surfactant in the preparation of various emulsions to solubilize
hydrophobic drugs [1, 2]. In sustained-release formulations, hydrogenated castor oil is often
used to prepare sustained-release matrices [3, 4]. It is also used as a thixotropic agent in
p
various ointments and cream preparations to improve the rheological properties of the
Corresponding author.
E-mail: xxj2702@sina.com preparations [5]. In addition, hydrogenated castor oil has also shown excellent performance
and great application potential in cosmetics, pesticides, coatings, textiles, printing and dyeing,
chemical materials and other industries [6–8].
Hydrogenated castor oil is mainly composed of 12-hydroxystearic acid triglyceride, etc.
Fatty acid composition is the main item for quality control of hydrogenated castor oil and
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other oil excipients. According to the reported literature, the was determined by HPLC-ELSD for the first time, in order
detection methods of fatty acids include gas chromatography to establish a simple, reliable and applicable method for
(GC-FID) [9–13], high performance liquid chromatography determination of fatty acids. It also provides a reference for
(HPLC) [14–18], gas chromatography-mass spectrometry the quality evaluation of other oils.
(GC-MS) [19–21], liquid chromatography-mass spectrom-
etry (HPLC-MS) [22, 23], near infrared spectroscopy (NIR)
[24, 25], nuclear magnetic resonance spectroscopy (NMR) 2. MATERIALS AND METHODS
[26] and so on.
Due to the high sensitivity, low detection limit and good 2.1. Chemicals and reagents
repeatability, gas chromatography is the most widely uti-
lized, especially in the current effective quality evaluation 12-hydroxystearic acid (purity: 97%) reference substance
standards [27–31]. However, gas chromatography needs to was purchased from Shanghai Haohong Biopharmaceutical
go through the esterification pre-treatment step, which Technology Co., Ltd. (Shanghai, China). Stearic acid (purity:
prolongs the analysis time, complicated the analysis method, 99%) reference substance and glacial acetic acid (purity:
and may produce the by-products. It is also difficult to 99.9%) were produced by Shanghai Macklin Biochemical
ensure the complete esterification. Co., Ltd. (Shanghai, China). PEG-60 hydrogenated castor oil
In addition, although near infrared spectroscopy does test sample (batch number: 84050897V0, 12594475L0,
not need pre-treatment, the sensitivity of the method is low, abbreviated as 7V0, 5L0) was obtained from SEPPIC
the deviation is large, and it is time-consuming and labo- (Shanghai) Special Chemicals Co., Ltd. (Shanghai, China).
rious to establish a calibration model. The detectors Sodium hydroxide (purity: 96%) was purchased from
commonly used in HPLC, such as ultraviolet-visible ab- Tianjin Zhiyuan Chemical Reagent Factory (Tianjin, China).
sorption detector (UVD) and fluorescence detector (FLD), HPLC-grade methanol was produced by Tianjin Kemiou
also need to use some new extraction techniques or special Chemical Reagent Co., Ltd. (Tianjin, China).
pretreatment methods in the determination [14–16]. It is
mentioned in a small amount of literature that high per-
2.2. Solution preparation
formance liquid chromatography combined with refractive
index detector (RID) and electrochemical detector (ECD) 2.2.1. Preparation of 12-hydroxystearic acid and stearic
can directly detect fatty acids, but gradient elution can not be acid reference solution. 12-hydroxystearic acid and stearic
used, which affects the separation effect and sensitivity of the acid were weighed accurately and dissolved in methanol in
method [17, 18]. two different 100 mL volumetric flasks to make a standard
As a universal detector, the evaporative light scattering solution (1190.7, 107.4 μg$mL1) which was diluted with
detector (ELSD) has a unique detection principle. First, the methanol to produce a series of concentrations of the stan-
eluent of chromatographic column is atomized to form an dard solution (1190.7, 893.0, 595.3, 297.7, 119.1 μg$mL1
aerosol, and then the solvent is evaporated in a heated drift for 12-hydroxystearic acid and 107.4, 85.9, 53.7, 21.5, 10.7
tube, and finally the remaining non-volatile solute particles μg$mL1 for stearic acid).
are detected in a light scattering detection cell. Since the
2.2.2. Preparation of sample solution and blank solu-
solvent has been completely evaporated before detection, only
tion. The reaction container is the 20 mL headspace vial
the analytes with low volatility are detected by scattering the
with lid purchased by CNW Technologies GmbH. 30 mg
light emitted by the laser light source, the detection response
PEG-60 hydrogenated castor oil sample and 1 mL 1 mol L1
does not depend on the optical characteristics of the sample,
sodium hydroxide ethanol solution were added to the 20 mL
but only on the quality of the solute eluting peaks. And the
headspace vial, the lid was screwed and sealed. The vial was
detection is not affected by the solvent, so the high perfor-
placed in a water-bath at 90 8C for 30 min with continuous
mance liquid chromatography with evaporative light scat-
vibration. Then, 1 mL glacial acetic acid and 8 mL methanol
tering detector (HPLC-ELSD) also has a good resistance to
are precisely added when the above solution is cooled to
baseline shifting when using the gradient elution method. Due
room temperature to make the total volume of 10 mL, shake
to the above significant advantages, HPLC-ELSD has been
well, and the sample solution is obtained. According to the
widely used in the analysis of saccharide, lipids, amino acids,
same operation mentioned above, the blank solution was
antibiotics and natural active components [32]. However, it is
obtained by using 1 mL methanol instead of the test sample.
rarely used in the detection of oil excipients. For the analysis
of oil excipients, HPLC-ELSD can direct detect oil compo-
nents without esterification pretreatment, simple steps and
2.3. HPLC instrumentation and conditions
short analysis time. The method has the advantages of simple The HPLC analysis was carried out on a LC-20A HPLC
steps and short analysis time. The sensitivity and selectivity system (Shimadzu, Japan) equipped with a LT II evaporative
are high and the separation effect is good. Therefore, the light scattering detector (temperature from 25 to 100 8C,
HPLC-ELSD also has a good application prospect in the field gain from 1 to 12, Shimadzu, Japan), an autosampler (Shi-
of oil excipients analysis. madzu, Japan) and a column temperature controller (Shi-
In our study, the content of fatty acids in hydrogenated madzu, Japan). An XD-C18 column (250 mm 3 4.6 mm,
castor oil after a simple pretreatment of alkaline hydrolysis 5 μm) purchased from Zhongpu Technologies was utilized
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for separation. Methanol and 1% acetic acid aqueous solu- temperature was 40 8C. The drift tube temperature of the
tion were used as the mobile phase under a gradient elution ELSD system was optimized to 40 8C, the carrier gas was N2
condition at a flow of 1.2 mL min1 (Table 1). The column with a pressure of 337 kPa and the gain of the detector was 4.
The injection volume was 20 μL.
Table 1. Gradient elution condition
Time/min Methanol ratio
3. RESULTS AND DISCUSSION
0–10.0 85%
10.0–11.0 85→98%
3.1. System suitability and analysis
11.0–25.0 98% The HPLC chromatograms of blank solution, reference so-
lution and test solution are shown in Fig. 1. The retention
Fig. 1. HPLC chromatograms of blank solution (A), 12-hydroxystearic acid and stearic acid mixed reference solution (B), and sample
solution (C). (a) and (b) in figure B and C represent 12-hydroxystearic acid and stearic acid, respectively. AD2 in the upper right corner of
Figures A, B, and C indicates that the chromatogram was detected by the evaporative light scattering detector path
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times of 12-hydroxystearic acid and stearic acid in the where Y and X are the peak area and concentration,
standard solution were 8.46 and 19.75 min, respectively and respectively. The calibration curve obtained is in double
8.43 and 19.84 min in the sample solution, respectively. The log coordinate. The obtained regression equations of
determination of 12-hydroxystearic acid and stearic acid in 12-hydroxystearic acid and stearic acid were lg Y 5 1.3320lg
blank solution and sample solution was not disturbed. Thus, X þ 4.7539, lg Y 5 1.5290lg X þ 4.8329. The two calibration
hydrogenated castor oil samples produced 12-hydroxystearic curves showed good linearity and the correlation coefficients
acid and stearic acid by alkaline hydrolysis. In the sample were 0.9993 and 0.9995, respectively in the test ranges
solution, the separation degrees between 12-hydroxystearic (Table 2).
acid and its preceding and posterior adjacent peaks were
6.34 and 5.15, respectively, and the separation degrees be- 3.2.3. Repeatability. Six independent sample solutions
tween stearic acid and its preceding adjacent peaks were prepared from the same batch of PEG-60 hydrogenated
13.13, indicating that the separation effect was good under castor oil samples (batch No.: 5L0) were injected under the
the conditions. The adaptability of the system was evaluated set chromatographic conditions to confirm repeatability,
by five consecutive injections of standard solutions and the results were shown in Table 2. The RSD values of
(595.3 μg$mL1 for 12-hydroxystearic acid and 53.7 12-hydroxystearic acid and stearic acid were 1.1 and 1.7%,
μg$mL1 for stearic acid). The RSD values of the two respectively, indicating that the method has good
compounds were less than 2.6%. repeatability.
3.2. Method validation 3.2.4. Recovery. The recovery was determined by standard
addition method: low, medium and high levels of standard
solutions (80, 100 and 120% of the sample volume, respec-
3.2.1. LOD and LOQ. The reference solution of 12-hy-
tively) were added into triplicate sample solutions, and each
droxy stearic acid and stearic acid was diluted with
level was repeated thrice for analysis. The recovery can be
methanol. The corresponding concentrations of 12-
calculated as follows: recovery (%) 5 (CA)/B 3 100%,
hydroxystearic acid and stearic acid were determined as
where A is the amount of original sample, B is the amount of
LOD when the signal-to-noise ratio (S/N) was about 10:1
reference standards added, and C is the amount found. The
and LOQ when the S/N was about 3:1. The LOD values
results were shown in Table 2. Recoveries of 12-hydroxy-
of 12-hydroxy stearic acid and srearic acid were 1.1 and
stearic acid and stearic acid were in the range of
2.5 μg$mL1, respectively. And the LOQ values of 12-hydroxy
96.6–105.8% and the RSD values were both less than 2.8%,
stearic acid and srearic acid were 3.2 and 7.4 μg$mL1,
which indicated that the developed method has high
respectively (Table 2).
accuracy.
3.2.2. Linearity. Linearity was evaluated by five different
3.2.5. Stability. The stability of the sample was obtained by
concentrations of standard solutions (1190.7, 893.0, 595.3,
determining the composition content of the same PEG-60
297.7, 119.1 μg$mL1 for 12-hydroxystearic acid and 107.4,
hydrogenated castor oil sample solution stored at room
85.9, 53.7, 21.5, 10.7 μg$mL1 for stearic acid) with each
temperature for 0, 2, 4, 6 and 8 h. The determination results
concentration injected in duplicate. The regression equa-
are shown in Table 2. The RSD values of 12-hydroxystearic
tions were calculated in the form of lg Y 5 a 3 lg X þ b,
acid and stearic acid were both less than 2.6%, indicating
that the PEG-60 hydrogenated castor oil sample was stable
Table 2. Verification results of the HPLC-ELSD method for within 8 h after hydrolysis.
12-hydroxystearic acid and stearic acid
3.2.6. Determination of 12-hydroxystearic acid and stearic
Values acid in samples. The established HPLC-ELSD method was
12-hydroxystearic applied to the determination of 12-hydroxystearic acid and
Items acid Stearic acid stearic acid in two batches of hydrogenated castor oil. As
LOD (μg$mL ) 1
1.1 2.5 shown in Table 3, the RSD values were all less than 1.7%.
LOQ (μg$mL1) 3.2 7.4 The results show that the established method is simple,
Calibration curves lg Y 5 1.3320lg lg Y 5 1.5290lg effective and accurate.
X þ 4.7539 X þ 4.8329
Linear range 119.1–1190.7 10.7–107.4
Table 3. The percentage content of 12-hydroxystearic acid and
(μg$mL1)
stearic acid in two batches of hydrogenated castor oil (n 5 3)
Correlation 0.9995 0.9993
coefficient, r 12-hydroxystearic acid Stearic acid
Repeatability (n 5 6) 1.1 1.7 Batch Percentage RSD Percentage RSD
(RSD%) no. content (%) (%) content (%) (%)
Recovery (n 5 9) (%) 101.5 101.0
(RSD% 5 2.1) (RSD% 5 2.8) 7V0 20.3 0.6 2.3 1.2
Stability (RSD%) 1.5 2.6 5L0 20.2 1.3 2.3 1.7
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3.3. Discussion
3.3.1. Selection of mobile phase ratio. It is found that
increasing the proportion of methanol in the methanol-1%
acetic acid aqueous solution mobile phase can accelerate the
separation process, but reduce the degree of separation and
increasing the proportion of aqueous solution in the mobile
phase will make the separation better, but the retention time
will be prolonged and the peak shape will become worse.
Considering the above factors, setting the elution gradient as
shown in Table 1 and the flow rate of 1.2 mL min1 can Fig. 3. Optimization of alkaline hydrolysis time
achieve complete separation of compounds and save time,
which is satisfactory. Besides, adding 1% acetic acid to the
water can improve the peak shape and protect the chro-
temperature was set to 90 8C. The results of hydrolysis time
matographic column.
were shown in Fig. 3. The content of the two compounds
3.3.2. Optimization of ELSD parameters. We adopted a increased with the extension of hydrolysis time, but stopped
single factor optimization method. The carrier gas pressure when the hydrolysis time reached 30 min or above. Therefore,
was fixed at 337 kPa, and the injection of the reference so- the optimum hydrolysis conditions were determined to be 1
lution was used to investigate the effect of drift tube tem- mol L1 NaOH anhydrous ethanol solution, temperature of 90
perature on the response signal at 35, 40, 45 and 50 8C. The 8C and reaction time of 30 min.
results showed that with the increase of drift tube tempera-
ture, the number of theoretical plates increased, the column
efficiency became higher and the baseline became flat. But
4. CONCLUSIONS
excessively high drift tube temperature setting can increase
background noise and cause the loss of decomposition or The HPLC-ELSD method was successfully used to deter-
volatilization of the sample, which is not conducive to the mine the content of 12-hydroxystearic acid and stearic acid
detection of samples. The optimal temperature is the lowest in hydrogenated castor oil by gradient elution for the first
temperature that produces acceptable noise on the basis of the time. The method has been verified to have high selectivity
basic evaporation of the mobile phase. Considering compre- and good repeatability, recovery and stability. Compared
hensively, the drift tube temperature is selected as 40 8C. with the GC commonly used in the current quality standards
of oil excipients, it has the advantages of no esterification
3.3.3. Optimization of alkaline hydrolysis conditions. Consi- pretreatment, simple steps and short analysis time. This
dering the influence of hydrolysis time and temperature on the method opens up a new idea for the quality control of oil
hydrolysis process, the alkaline hydrolysis was carried out excipients and the quantitative analysis of fatty acids.
under different temperature conditions of 65, 75, 85, 90 and
95 8C (hydrolysis time was 30 min) and different time condi-
tions of 10, 20, 30, 40, 50 min (hydrolysis temperature was ACKNOWLEDGEMENTS
90 8C). The results of the single factor optimization experiment
showed that the content of 12-hydroxystearic acid and stearic The authors would like to thank Mr. Zhu Longping from
acid increased with the increase of hydrolysis temperature, and Sun Yat-sen University for his help in the instrument.
stopped growing when the temperature rose to 85 8C or above,
as shown in Fig. 2. In the actual determination, in order
to ensure the complete hydrolysis reaction, the hydrolysis
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