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Vol. 3, No. 4. April, 2014

 In this issue...
Writing the Perfect Method
Part 4 - Columns
"That's the way we have always done it!"

Writing the Perfect Method!

To view previous sections, view our
Newsletter Archive.

This month we move on to the next section.

Part 4: Columns:
 
If you are writing the perfect method (or at least trying), this should be the easy part, right?  Just write down the description of the column and you are done.  Except that we see many methods where some critical information is not included.

Internal diameter, length, and type (e.g.., C18, 5% phenyl) are the obvious items.  But particle size (in LC) or film thickness (in GC) are also critical to producing the correct separation.  Also, it isn't just a C18 column.  Is it a Zorbax RX-C18, SB-C18, or XDB-C18?  They are all different. And Agilent C18 columns are different than C18 columns from Waters, Restek, Supelco, or Phenomenex, etc.

The GC column manufacturers have done a good job of producing equivalent columns.  Although they are not exactly equal, the results are often close to each other.  The concept of "equivalent" columns in LC is far more complicated.  There probably are no truly equivalent columns between different manufacturers.  The old USP system of "L" columns was a convenient way to describe column chemistry, but it was not useful for predicting actual chromatographic behavior.

There are some online tools now to help with selecting similar (or not similar) columns.  We cover this information in our advanced LC topics and in the Best Practices in HPLC Column Selection webinar. 

Do:

• You must include the following information for LC columns:
• Internal diameter (i.d.) in mm
• Length, in mm or cm (specify units)
• Particle size, in um
• Pore size, in Angstroms
• Stationary phase - manufacturer, brand/model, and any other unique descriptors
• You must include the following information for GC columns:
• Internal diameters, in mm or um
• Length, in m
• Film thickness, in um
• Stationary phase - manufacturer, brand/model, and any other unique descriptors.
• Use the term "or equivalent" for GC columns, but only if you have some evidence that a truly equivalent column exists, and you have tested it.
• Specify alternative columns, if you have tested them.  Show results.

Do Not:

• Use the term "or equivalent" for LC columns (see discussion above).
• Specify column types using USP designations (again, see above).  Sorry USP, but this labeling system is just not an accurate way to choose an equivalent column.
• Specify alternative columns that you think will work.  If it hasn't been tested, it should not be in the method.

Next Month: Operating Conditions


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Quote for the Day

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you can have it cheap,

you can have it good ...

– Anon.