Assay of Ibuprofen Esters & Impurities

Paul Lee, Jenna Milliken

Merck Laramie, WY

Read more about

• Introduction
• Experimental Conditions
• Results
• Conclusion

INTRODUCTION

Ibuprofen, the active pharmaceutical ingredient in many brand-named products, is one of the most prolific APIs on the market and in our households. This application note describes an analysis of Ibuprofen-related compounds and impurities using an HPLC method similar to the compendial method described by the United States Pharmacopeia and European Pharmacopeia. The described ibuprofen assay method separates ibuprofen from various esters and intermediate impurities on the solid-core Ascentis^® Express 90Å C18 column (USP L1) with 0.1% phosphoric acid in both water and acetonitrile as mobile phase. Due to the low absorbance maximum for ibuprofen at 220 nm, a slower gradient maximizes the separation and minimizes baseline drift.

Figure 1. Chemical structures of ibuprofen and ibuprofen-related impurities.

EXPERIMENTAL CONDITIONS

Column:	Ascentis® Express C18, 150 x 4.6 mm I.D., 2.7 µm (53829-U)
Mobile Phase:	[A] 0.1% phosphoric acid in water; [B] 0.1% phosphoric acid in acetonitrile
Gradient:	Time (min)     %A     %B     0                           48       52       3                           48       52       13                        15       85       16                        15       85
Flow Rate:	1 mL/min
Pressure:	194 bar (2814 psi)
Temperature:	Column: 30 °C Autosampler: 12 °C
Detection:	DAD @ 220 nm, 254 nm
Injection:	7 µL
Samples
Diluent:	Acetonitrile and water (50:50)
Sample concentration:	Ibuprofen Standard - 0.4 mg/mL  Resolution Solution - 0.07 mg/mL (each compound)

Results

Figure 2. Chromatogram of ibuprofen standard 0.4 mg/mL.

Figure 3. Chromatogram for ibuprofen and its related impurities.

TABLE 1. CHROMATOGRAPHIC AND DETECTION DATA FOR IBUPROFEN ANDITS RELATED IMPURITIES

Peaks	Compound	Retention Time (min)	Relative Retention Time	Resolution to Preceding Peak	USP Tailing	λmax
1	1-(4-Isobutylphenyl)ethanol	5.224	0.92	-	1.07	220 nm
2	Ibuprofen	5.679	1.00	3.66	1.10	220 nm
3 | 4	2-Hydroxypropyl 2-(4-Isobutylphenyl)-propanoate)	7.212 | 7.517	1.27 | 1.32	11.70 (2.38)	1.39 | 1.03	220 nm
5	2-(4-Isobutyrylphenyl)propane	9.763	1.72	18.40	0.99	254 nm
6	Ethyl 2-(4-Isobutylphenyl)propanoate	12.696	2.24	24.19	0.99	220 nm
7	4-Isobutylstyrene	13.817	2.43	9.30	1.00	254 nm
8	Isopropyl 2-(4-Isobutylphenyl)propanoate	14.253	2.51	3.66	1.06	220 nm
9	1-Ethyl-4-isobutylbenzene	15.217	2.68	8.11	1.01	220 nm

Calibration and Linearity Data for Ibuprofen and its Related Compounds

Table 2. Calibration data for ibuprofen and its related impurities

	Ibuprofen	2-(4-Isobutyrylphenyl)propane	2-Hydroxypropyl 2-(4-Isobutylphenyl)propanoate	Isopropyl 2-(4-Isobutylphenyl)propanoate
Concentration (μg/mL)	Mean Area	Mean Area	Mean Area	Mean Area
0.4	5.8	10.5	4.7	5.0
1.2	17.8	33.1	13.5	16.3
2.0	30.5	55.6	23.0	27.2
2.8	43.0	78.4	32.2	39.6
4.0	60.5	107.8	46.2	55.3
Std Error	0.31	1.28	0.15	0.47
STDEV	0.69	2.87	0.34	1.05
Slope	15.26	27.18	11.58	14.03
LOD (µg/mL)	0.14	0.32	0.09	0.22
LOQ (µg/mL)	0.46	1.05	0.30	0.75

	1-Ethyl-4-isobutylbenzene	4-Isobutylstyrene	1-(4-Isobutylphenyl)ethanol	Ethyl 2-(4-Isobutylphenyl)propanoate
Concentration (μg/mL)	Mean Area	Mean Area	Mean Area	Mean Area
0.4	3.7	9.1	6.5	4.7
1.2	11.7	27.7	20.3	16.3
2.0	20.3	47.2	34.4	28.3
2.8	29.4	66.8	47.7	39.7
4.0	41.8	98.5	67.8	56.8
Std Error	0.28	0.91	0.24	0.23
STDEV	0.63	2.03	0.55	0.51
Slope	10.67	24.82	17.03	14.50
LOD (µg/mL)	0.18	0.25	0.10	0.11
LOQ (µg/mL)	0.59	0.82	0.32	0.35

Table 3. Linearity - ibuprofen and its impurities

	Ibuprofen	2-(4-Isobutyrylphenyl)propane	2-Hydroxypropyl 2-(4-Isobutylphenyl)-propanoate)	Isopropyl 2-(4-Isobutylphenyl)propanoate
Concentration (μg/mL)	Mean Area	Mean Area	Mean Area	Mean Area
0.4	5.8	10.5	4.7	5.0
1.2	17.8	33.1	13.5	16.3
2.0	30.5	55.6	23.0	27.2
2.8	43.0	78.4	32.2	39.6
4.0	60.5	107.8	46.2	55.3
RSQ	0.9998	0.9987	0.9999 | 1.0000	0.9994
Concentration (μg/mL)	Mean Area	Mean Area	Mean Area	Mean Area
100	1516.8	3561.9 (125 µg/mL)	1138.5	1387.9
200	3193.9	7346.3 (250 µg/mL)	2377.4	2841.5
300	4612.4	10740.2 (375 µg/mL)	3567.0	4349.1
400	6202.9	13648.6 (500 µg/mL)	4767.7	5863.3
500	7493.0	-	5828.0	7123.3
RSQ	0.9983	0.9963	0.9993 | 0.9992	0.9990
Combined RSQ	0.9994	0.9988	0.9998 | 0.9998	0.9997

	1-Ethyl-4-isobutylbenzene	4-Isobutylstyrene	1-(4-Isobutylphenyl)ethanol	Ethyl 2-(4-Isobutylphenyl)propanoate
Concentration (μg/mL)	Mean Area	Mean Area	Mean Area	Mean Area
0.4	3.9	9.1	6.5	4.7
1.2	11.7	27.7	20.3	16.3
2.0	20.3	47.2	34.4	28.3
2.8	29.4	66.8	47.7	39.7
4.0	41.7	98.5	67.8	56.8
RSQ	0.9994	0.9993	0.9999	0.9999
Concentration (μg/mL)	Mean Area	Mean Area	Mean Area	Mean Area
100	926.6	2011.4	1714.5	1537.4
200	1896.6	4321.2	3526.5	3212.7
300	2940.9	6568.6	5349.3	4748.2
400	3722.4	8838.6	7085.1	6012.3
500	4654.7	11273.2	8683.4	7672.4
RSQ	0.9973	0.9998	0.9993	0.9981
Combined RSQ	0.9992	0.9995	0.9998	0.9993

CONCLUSION

This method demonstrates excellent resolution for Ibuprofen and ibuprofen-impurities. But the stereochemical identification was not included in the scope of the analysis conducted (2-Hydroxypropyl 2-(4-Isobutylphenyl)propanoate comprises two diastereoisomers as synthesized).

The maximal range of linearity is approximately 500 µg/mL, particularly for 2-(4-Isobutyrylphenyl)propane and 4-Isobutylstyrene, which have strong absorption at 254 nm rather than 220 nm. The method enables ibuprofen to be separated from its impurities with excellent peak shape, with limits of quantification of 1 µg/mL or less.

See also Small Molecules Analysis & Quality Control (QC) for more information on other applications or use our chromatogram search.

See more chromatograms on ibuprofen here.