Translator for HPLC HINTS and TIPS for Chromatographers

Saturday, December 26, 2015

Common pKa Values for ACIDS & BASES used in HPLC and LC/MS Method Development





pKa (25°C)                              ACID
0.3                                           Trifluoroacetic acid
2.15                                          Phosphoric acid (pK#1)
3.13                                          Citric acid (pK#1)
3.75                                          Formic acid
4.76                                          Acetic acid
4.76                                          Citric acid (pK#2)
4.86                                          Propionic acid
6.35                                          Carbonic acid (pK#1)
6.40                                          Citric acid (pK#3)
7.20                                          Phosphoric acid (pK#2)
8.06                                          Tris
9.23                                          Boric acid
9.25                                          Ammonia
9.78                                          Glycine (pK#2)
10.33                                        Carbonic acid (pK#2)
10.72                                        Triethylamine
11.27                                        Pyrrolidine
12.33                                        Phosphoric acid (pK#3)



Notes: (1) This is a general list of commonly used acids & bases for chromatography applications and not meant to be a comprehensive list of all values. (2) TFA is an overused and very strong acid for many chromatography applications. It also has strong ion pairing properties and can result in high UV noise, vacuum degasser and/or MS contamination. If you must use it, try and use the lowest concentration which results in the desired pH. Example: 0.1 % TFA ~ pH 2.0, 0.02% TFA ~ pH 2.7. (3) Formic acid is a popular alternative to TFA for many applications, esp LC/MS. (4) Not all acids/bases provide "buffering" on their own.

Reference: CRC Handbook of Chemistry & Physics.

3 comments:

  1. I just liked the article. It was Very refreshing post with attractive ideas.It was great to read your blog.

    ReplyDelete
  2. Please I need to know if using TFA 0.1% could harm c18 column as after using it I reinject samples with another method and the second peak appeared after its time by 2 minutes but the first one is in its place

    ReplyDelete
    Replies
    1. "harm" is not the word I would use. TFA is a strong ion pairing agent and may alter the surface chemistry of the HPLC support. In other words, you may not be able to reproduce a different method after using TFA (or any other strong ion pairing agent) on the column. *This is why we dedicate specific columns to specific methods.

      Good Laboratory Practice: We recommend that you always apply a special label to any column which has been used with an ion paring agent so it does not get used with methods which do not use one. For dedicated sample analysis methods, we always use a dedicated HPLC column. In fact, we maintain different sets of columns which are used with neutral (no ion pairing agents), basic and acidic agents (or modifiers).

      Columns are low cost consumable items compared to the TIME spent to run the methods on them.

      Delete

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