Effective Cleanup and Analysis of Metronidazole from Foaming Facial Cleanser

When working with complex matrices such as personal care products, sample preparation is perhaps the most difficult step of the analysis process. By utilizing a targeted sample preparation technique, such as ion-exchange SPE, analysis can be significantly improved as compared to less targeted techniques such as liquidliquid extraction. Our work successfully extracted metronidazole from foaming facial cleanser using a polymeric cation-exchange SPE sorbent, Strata™ -X-C, followed by a rapid LC–MS-MS analysis on a Kinetex® 2.6 µm XB-C18 HPLC/UHPLC core-shell column.

Materials and Methods

Sample Pretreatment:

1. Dissolve 0.250 g of foaming facial cleanser in 10 mL of 0.1 N HCl

2. Vortex until homogeneous

3. Centrifuge sample at 5000 g for 5 min

Solid-Phase Extraction (SPE)

The pretreated sample is further cleaned up and concentrated using SPE.

Cartridge: Strata-X-C, 30 mg/3 mL

Part No.: 8B-S029-TBJ

Condition: 1 mL methanol

Equilibrate: 1 mL 0.1 N HCl

Load: 3 mL of pretreated sample

Wash 1: 3 mL 0.1 N HCl

Wash 2: 3 mL methanol

Wash 3: 6 mL ethyl acetate

Dry: 5 min under full vacuum

Elute: 1 mL 5% NH₄OH in methanol (v/v)

Dry down: Evaporate under a stream of nitrogen gas at 50 °C until dry

Reconstitute: Reconstitute samples with 200 µL of methanol/0.1% formic acid (10:90)

Results and Discussion

We were able to effectively extract metronidazole from foaming facial cleanser using a targeted SPE procedure on Strata-X-C polymeric SPE sorbent. The cation-exchange properties of the Strata-X-C SPE sorbent targeted the sp2 hybridized nitrogen at the 3 position of the imidazole ring in the metronidazole structure, forming a tight bond between the sorbent and the metronidazole compound. The strong interaction allowed a strong solvent wash of ethyl acetate to be performed which removed a significant amount of matrix interferences (Figure 1).

Figure 1: The vial on the left was not subjected to a strong organic wash and, therefore, contains matrix interferences such as foaming agents. After a strong ethyl acetate wash, matrix interferences are no longer present in the vial on the right.

Conclusion

By implementing a targeted SPE method, matrix interferences were significantly removed from a foaming facial cleanser matrix. This clean-up process allowed for a sensitive LC–MS-MS method that could detect metronidazole at low levels, down to 100 pg/mL. (Visit www.phenomenex.com/Application and search for Application No. 20631 for LC–MS-MS conditions).

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