Quantitation of Genetox Impurities Using a Surrogate Standard Approach 171
Table 5. Comparison of known spiked amounts of PGI’s to those calculated utilizing the surrogate standard method (APCI
positive ion mode).
Surrogate Analyte Amount Spiked into API Amount Calculated in the API
N,N-dimethylaniline N,N-dimethyltoluidine 21 ppm 26 ppm
3-methoxy-2-nitro-pyridine 3-ethoxy-2-nitro-pyridine 19 ppm 26 ppm
(a) (b)
Figure 2. Structure of the PGI, substituted 3-methyl indole
(a) and the surrogate, indole-6-carboxylic acid (b).
4. Conclusions
An analytical method for quantitation of PGI’s in API’s
utilizing surrogate standards was developed and demon-
strated to ppm levels. This method can be utilized for
assessing the levels of PGI’s when authentic standards
are not readily available.
For small volatile PGI’s of similar structures, the
GC-MS-EI source gave good RRF agreement. Similarly,
when utilizing the LC-MS-APCI source in both the posi-
tive and negative modes surrogate standards can be also
successfully used provided that the species chosen for the
surrogate and analyte have similar functionalities, with
isomers of the actual PGI being the preferred compounds
of choice.
It should be also emphasized that the results obtained
using this methodology are based on calculation of rela-
tive response factors determined in a given system. The
RRF data obtained in the study indicate that there can be
significant variation in the amounts predicted depending
on the analytical method and surrogate standard chosen.
Therefore, the quantitive results obtained using the sur-
rogate methodology should only serve as an approxima-
tion of the PGI impurity present. Additionally, data for
this surrogate approach are useful in determining which
PGI’s need to be prepared as standards based on the rela-
tive amounts determined and the levels allowable using
the staged TTC guidelines.
5. Acknowledgements
The authors thank the Merck & Co. Inc., MRL Summer
Intern Program for their support.
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