Propylene Glycol Monocaprylate
Propylene glycol monooctanoate.
Octanoic acid, monoester with 1,2-propane diol.
Caprylic acid, monoester with propane-1,2-diol.
[31565-12-5].» Propylene Glycol Monocaprylate is a mixture of the propylene glycol monoesters and diesters of fatty acids composed predominately of caprylic acid. The requirements for monoester and diester content differ for the two types of Propylene Glycol Monocaprylate, as set forth in the accompanying table:
| Content of Monoesters (% ) | Content of Diesters (%) | |||
| Min. | Max. | Min. | Max. | |
| Type I | 55.0 | 80.0 | 20.0 | 45.0 |
| Type II | 90.0 | — | — | 10.0 |
Packaging and storage—
Preserve in well-closed containers and protect from moisture. No storage requirements specified.
Labeling—
Label it to indicate the type (Type I or Type II).
USP Reference standards 
11
—
USP Propylene Glycol RS.
USP Propylene Glycol Monocaprylate Type I RS.
USP Propylene Glycol Monocaprylate Type II RS.
11—
USP Propylene Glycol RS.
USP Propylene Glycol Monocaprylate Type I RS.
USP Propylene Glycol Monocaprylate Type II RS.
Identification—
B: Thin-Layer Chromatographic Identification Test 201—
201—
Test solution:
50 mg per mL, in methylene chloride.
Standard solution:
50 mg of USP Propylene Glycol Monocaprylate Type I RS (or USP Propylene Glycol Monocaprylate Type II RS) per mL, in methylene chloride.
Application volume:
10 µL.
Developing solvent solution:
a mixture of ether and hexane (70:30).
Spray reagent—
Prepare a 0.1 mg per mL solution of rhodamine 6G in alcohol.
Procedure—
Develop the chromatogram over a path of 15 cm, and dry the plate in a current of air. Spray the plate with Spray reagent, and locate the spots on the plate by examination under UV light at a wavelength of 365 nm: the RF values of the principal spots obtained from the Test solution correspond to those obtained from the Standard solution.
C:
It meets the requirements of the test for Fatty acid composition.
Acid value 401:
not more than 1.5.
401:
not more than 1.5.
Iodine value 401:
not more than 1.0.
401:
not more than 1.0.
Saponification value 401:
between 285 and 315 for Propylene Glycol Monocaprylate (Type I), and between 270 and 295 for Propylene Glycol Monocaprylate (Type II).
401:
between 285 and 315 for Propylene Glycol Monocaprylate (Type I), and between 270 and 295 for Propylene Glycol Monocaprylate (Type II).
Peroxide value 401:
not more than 6.0.
401:
not more than 6.0.
Fatty acid composition 401—
Propylene Glycol Monocaprylate exhibits the composition profile of fatty acids shown in the accompanying table, as determined in the section Fatty Acid Composition under Fats and Fixed Oils 401:
401—
Propylene Glycol Monocaprylate exhibits the composition profile of fatty acids shown in the accompanying table, as determined in the section Fatty Acid Composition under Fats and Fixed Oils 401:
| Carbon-Chain Length |
Number of Double Bonds |
Percentage (%) |
| 8 | 0 |  90.0 |
| 10 | 0 |  3.0 |
| 12 | 0 | 3.0 |
| 14 | 0 | 3.0 |
| 16 | 0 | 1.0 |
Water, Method Ia 921:
not more than 1.0%, using a mixture of methanol and methylene chloride (1:1) in place of methanol in the titration vessel.
921:
not more than 1.0%, using a mixture of methanol and methylene chloride (1:1) in place of methanol in the titration vessel.
Heavy metals, Method II 231:
not more than 10 µg per g.
231:
not more than 10 µg per g.
Total ash 561:
not more than 0.1%.
561:
not more than 0.1%.
Limit of propylene glycol—
Mobile phase—
Proceed as directed in the Assay.
Propylene glycol standard stock solution—
Prepare a solution containing a known concentration of about 4 mg per mL of USP Propylene Glycol RS in tetrahydrofuran.
Propylene glycol standard solutions—
Into four 5-mL volumetric flasks, introduce respectively 0.25 mL, 0.5 mL, 1.0 mL, and 2.5 mL of Propylene glycol standard stock solution, and dilute with tetrahydrofuran to volume. In a fifth 5-mL volumetric flask, introduce 5.0 mL of Propylene glycol standard stock solution.
Test solution—
Use the Assay preparation.
Chromatographic system—
Proceed as directed in the Assay.
Procedure—
Separately inject equal volumes (about 40 µL) of the Propylene glycol standard solutions and the Test solution into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Prepare a standard curve of peak area versus concentration, in mg per mL, of propylene glycol in the Propylene glycol standard solutions. Obtain the concentration, C, in mg per mL, of propylene glycol in the Test solution from the standard curve. Calculate the percentage of free propylene glycol in the portion of Propylene Glycol Monocaprylate taken by the formula:
100CV / W
in which V is the volume of the Test solution, and W is the weight, in mg, of Propylene Glycol Monocaprylate used to prepare the Test solution: not more than 1.5% is found for Propylene Glycol Monocaprylate (Type I and Type II).
Assay—
Mobile phase:
tetrahydrofuran.
Assay preparation—
Accurately weigh about 200 mg of Propylene Glycol Monocaprylate into a 5-mL volumetric flask, dissolve in and dilute with tetrahydrofuran to volume, and shake to mix thoroughly.
Chromatographic system (see Chromatography 621)—
The liquid chromatograph is equipped with a refractive index detector and a 7-mm × 60-cm column that contains 5-µm packing L21 (100
). The flow rate is about 1 mL per minute. The column and detector temperatures are maintained at 40
. [note—Two 7-mm × 30-cm L21 columns may be used in place of one 60-cm column, provided system suitability requirements are met.] Chromatograph the Assay preparation, and record the peak responses as directed for Procedure. The order of elution is diester, monoester, and propylene glycol. The relative standard deviation for replicate injections determined from the monoester peak is not more than 2.0%. [note—For information purposes only, relative retentions with reference to propylene glycol for diester are about 0.85, for monoester about 0.90.]
621)—
The liquid chromatograph is equipped with a refractive index detector and a 7-mm × 60-cm column that contains 5-µm packing L21 (100). The flow rate is about 1 mL per minute. The column and detector temperatures are maintained at 40. [note—Two 7-mm × 30-cm L21 columns may be used in place of one 60-cm column, provided system suitability requirements are met.] Chromatograph the Assay preparation, and record the peak responses as directed for Procedure. The order of elution is diester, monoester, and propylene glycol. The relative standard deviation for replicate injections determined from the monoester peak is not more than 2.0%. [note—For information purposes only, relative retentions with reference to propylene glycol for diester are about 0.85, for monoester about 0.90.]
Procedure—
Inject a volume (about 40 µL) of the Assay preparation into the chromatograph, record the chromatogram, and measure the responses for the major peaks. Calculate the percentage of monoester or diester in the portion of Propylene Glycol Monocaprylate taken by the formula:
(100 – D)(rU / rs)
in which D is the sum of the percentage content of propylene glycol and the percentage content of free fatty acids; rU is the peak response for monoester or diester; and rs is the sum of the responses of the monoester and diester peaks. Calculate the percentage content of free fatty acids using the formula:
144(A/561.1)
in which A is the acid value.
Auxiliary Information—
Please check for your question in the FAQs before contacting USP.
Chromatographic Column—
| Topic/Question | Contact | Expert Committee |
| Monograph | Hong Wang, Ph.D.
Scientist 1-301-816-8351 |
(EM205) Excipient Monographs 2 |
| Reference Standards | Lili Wang, Technical Services Scientist 1-301-816-8129 RSTech@usp.org |
USP32–NF27 Page 1327
Pharmacopeial Forum: Volume No. 33(2) Page 261
Chromatographic columns text is not derived from, and not part of, USP 32 or NF 27.