Just to separate them or quantify them?

Quantify too.

Your question misses a lot of information needed:

Carboxylic acid is a group of organic acids, which one do you mean?

Chloride? Do you mean chloride as a dissolved salt?

In addition, what is the matrix of your sample (water / buffer / ?), what is the expected concentration of the components to measure and what other components are present.

In sample is phthalic acid and phthaloyl chloride in water. Not other components.

Please look at the following below links which may help you in your analysis:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3838891/pdf/nihms464392.pdf

http://www.academicjournals.org/app/webroot/article/article1379516398_Adegoke.pdf

https://www.sielc.com/Application-HPLC-Separation-of-Carboxylic-Acids.html

Thanks

Are these usefull ? The last one gives details about a HPLC method

https://academic.oup.com/chromsci/article/50/5/410/351633

Article Hydrolytic Stability of Terephthaloyl Chloride and Isophthal...

Chloride acids are very reactive and decompose in presence of water, any solvent that have some amount of saturation water will decompose the acid chloride. So reverse HPLC it is not an option. If you chose to "direct" HPLC they need to be inert to Acid chlorides and that are special equipment not very common. GC it not an option because acid chloride react with the solid phase or with the free OH groups of the silica if it is used. RMN or TLC are the ideal option.

Depending upon the relative concentrations of interest, and the accuracy and precision required in the analysis, there are a number of options. Derivatizing the phthaloyl chloride with an amine prior to HPLC, would give you an amide from the phthaloyl chloride, and depending upon the strength of the amine, no reaction, or an amine salt.

Alternatively, again dependent upon the sensitivity, accuracy and precision required, a spectroscopic technique such as infrared may give you a rapid method for the determination.

Either of these options will likely be less work and less problematic that direct HPLC.

If phthaloyl chloride is you only reasonably expected chloride, you could also use a silver nitrate titration of the chloride.

It is also worth noting that most TLC plates have -OH groups on them, and then there is the question of exposure to atmospheric water as well.

It is right that TLC has -OH groups but when you sensitized acid chloride and want to know in the vessel how the reaction is going, a quick TLC in hexane shows a spot in the origin of the acid and another spot with higher Rf with a tail. The tail is part of the acid chloride that was converted to acid during the TLC run.