How can I decrease tailing of a basic compound when running a reversed-phase method with a C18 column? - WKB92122

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Article number: 92122

ENVIRONMENT

C18 columns
Reversed-phase method

ANSWER

There are different options, based on the type of particle you are using. See the Additional Information section.

ADDITIONAL INFORMATION

Tailing of basic compounds on C18 columns occurs because surface silanol groups on the particles become ionized (deprotonated, negatively charged), and weak bases become ionized (protonated, positively charged). (Strong bases are virtually always ionized (protonated, positively charged)).

Here are some options, based on particle type, to help alleviate the tailing issue:

Hybrid packings:

Bridged ethylene hybrid (BEH) C18 (pH range 1-12):

Low-pH mobile phases (~pH 2.0 -3.0) can be used to neutralize silanol groups to lower the amount of tailing.
High-pH mobile phases (~pH 10.0) can be used to neutralize a weak base and alleviate tailing. High pH cannot be used to neutralize a strong base.
Ion pairing agents can be used to block the cation exchange mechanism between surface silanol groups and the positively charged basic analytes.
- 0.1% TFA (trifluoroacetic acid) in both the aqueous and organic mobile phases may be used.
  - TFA lowers pH to about 2.0, which neutralizes silanol groups.
  - TFA also ion pairs to the positive charges on charged bases to block the cation exchange mechanism.
- PIC (paired ion chromatography) reagents may be used to block the cation exchange mechanism and reduce tailing.

Charged surface hybrid (CSH) C18 (pH range 1-11):

On this packing, low-pH mobile phases (pH ~3.0) with 0.1% formic acid can reduce tailing of both weak bases and strong bases in many cases.
High-pH mobile phases (~pH 10.0) can be used to neutralize a weak base and alleviate tailing. High pH cannot be used to neutralize a strong base.
The positive surface on this packing charge helps repel the positive charges of the analytes, which minimizes the cation exchange mechanism, reducing tailing.
In some cases, use of ion pairing with 0.05% up to 0.1% TFA may help if 0.1% formic acid does not help reduce tailing enough.

Silica-based packings (pH range is generally 2.0-8.0):

High pH cannot be used to neutralize weak bases when using a silica-based C18. (Silica dissolves in pH higher than 8.0.)
Low-pH mobile phases (~pH 2.0 -3.0) can be used to neutralize silanol groups to lower the amount of tailing.
- Use of 0.1% formic acid in both the aqueous and organic mobile phases may help.
- Use other acidic modifiers.
Ion pairing agents can be used to block the cation exchange mechanism between surface silanol groups and the positively charged basic analytes.
- 0.1% TFA (trifluoroacetic acid) in both the aqueous and organic mobile phases may be used.
  - TFA lowers pH to about 2.0, which neutralizes silanol groups.
  - TFA also ion pairs to the positive charges on charged bases to block the cation exchange mechanism.
- PIC (paired ion chromatography) reagents may be used to block the cation exchange mechanism and reduce tailing.