TLC- Related substances test

[olivia929]

[Impurity A and related substances]:

Examine by thin-layer chromatography (2.2.27), using a TLC silica gel GF254plate R.

Test solution (a) Dissolve 0.1 g of the substance to be examined in methanol R and dilute to 10 ml with the same solvent.

Test solution (b) Dilute 1 ml of test solution (a) to 10 ml with methanol R.

Reference solution (a) Dissolve 10 mg of propylthiouracil CRS in methanol R and dilute to 10 ml with the same solvent.

Reference solution (b) Dissolve 50 mg of thiourea R in methanol R and dilute to 100 ml with the same solvent. Dilute 1 ml of this solution to 100 ml with methanol R.

Reference solution (c) Dilute 1 ml of test solution (a) to 100 ml with methanol R.

Apply separately to the plate 10 µl of each solution. Develop over a path of 15 cm using a mixture of 0.1 volumes of glacial acetic acid R, 6 volumes of 2-propanol R and 50 volumes of chloroform R. Allow the plate to dry in air. Examine in ultraviolet light at 254 nm. Expose the plate to iodine vapour for 10 min.

In the chromatogram obtained with test solution (a), any spot corresponding to impurity A is not more intense than the spot in the chromatogram obtained with reference solution (b) (0.05 per cent) and any spot apart from the principal spot and any spot corresponding to impurity A is not more intense than the spot in the chromatogram obtained with reference solution (c) (1.0 per cent).
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I consider the sentence "any spot apart from the principal spot and any spot corresponding to impurity A is not more intense than the spot in the chromatogram obtained with reference solution (c) (1.0 per cent)" indicate that any other impurity excluded the principal substance(propylthiouracil) and impurity A is controlled less than 1.0 %. I also noticed that the the limit for single impurity (unspecified impurity) is somewhat less stringent.
Can anyone do me a favor to confirm this understanding right or not? and why so big impurity?

[Alex Buske]

Hi,

Without recalculating I understood:
Impurity A <= 0.05% (specified imp.)
any other impurity<= 1.0% (unspecified imp.)
"and why so big impurity?" because it is allowed. However, the assay spec says 98.0-100.5%, even though titration might not be specific.

Can anybody tell me what for is test solution (b)?

alex

[olivia929]

Hi,

Without recalculating I understood:
Impurity A <= 0.05% (specified imp.)
any other impurity<= 1.0% (unspecified imp.)
"and why so big impurity?" because it is allowed. However, the assay spec says 98.0-100.5%, even though titration might not be specific.

Can anybody tell me what for is test solution (b)?

alex

Thanks for your post.

I noticed and concerned the same question of test solution(b)?

[krickos]

In the chromatogram obtained with test solution (a), any spot corresponding to impurity A is not more intense than the spot in the chromatogram obtained with reference solution (b) (0.05 per cent) and any spot apart from the principal spot and any spot corresponding to impurity A is not more intense than the spot in the chromatogram obtained with reference solution (c) (1.0 per cent).

Ref solution B is the limit "spot" 0,05% for Impurity A.

[Alex Buske]

[quote]Ref solution B is the limit "spot" 0,05% for Impurity A. Wink[/quote]
test solution (b) is somewhat unclear

[krickos]

Ref solution B is the limit "spot" 0,05% for Impurity A. Wink

test solution (b) is somewhat unclear

My mistake

yes test solution (b)somewhat unclear, typo?

[olivia929]

Thanks for kindly discussion.

Tthis is the specific monograph Propylthiouracil of Ph. Eur.6.0.

I'd consider, it is for the purpose of resolution between the principal spot and impurity spot separatly obtained by test solution (b) and reference solution (a). it is just to determine if the test is valid.
This is j ust my thinking on this topic.

If it is true, then there is lost somelike words in the last paragraph : The test is valid if ....

any one following up please....