Hans,
The fact that SIELC survived and expended with mixed-mode columns since 2003 probably shows that mixed-mode is a valuable approach. Since we revived mixed-mode six years ago other companies introduced new mixed-mode columns (Dionex, for example, introduced at least 3 new mixed-mode columns in the last 2 years). There are around 50 publications from SIELC customers published in the last 3 years in journals on Primesep and Obelisc mixed-mode columns
Here is on the subject of basic compounds:
Just want to add my 2 cents here.
1. XBridge is one of the most stable columns on the market. We tested it extensively in the lab and it behaves very well at low and high organic and in wide pH range. It si suitable for analysis of basic compounds at higher pH. One of the problems is that at higher pH your acidic molecules are going to be fully ionized and unretained, same is true for zwitter-ionic compounds. It is impossible to retain both acidic and basic analytes in one run. You can develop two methods one for basic analytes at higher pH and one for acidic analytes at lower pH. The draw backs ob high pH separations are described here:
http://www.sielc.com/pdf/SIELC_June_2005.pdf
2. Another approach is to use one of the RP columns designed for retention of polar compounds in highly aqueous mobile phase (0% organic). There are bunch of them on the market. These columns will allow you to retain polar compounds within normal pH range (2-7), although you will need to play with pH to make compounds less polar. Retention is better than on regular RP columns, but in some cases is not great. It is hard to go below 0% organic
to extend retention time.
3. Another approach is mixed-mode chromatography, where compounds are retained by combination of reversed-phase and ion-exchange mechanisms. Our company offers several mixed-mode columns:reversed-phase cation-exchange, reversed-phase anion-exchange and reversed-phase with both cation and anion-exchange groups. Presence of two or three mechanisms allows you to control retention time of different compounds independently and use low or moderate pH:
http://www.sielc.com/Technology_2D_Properties.html
You can retain wide range of compounds on the same column:
http://www.sielc.com/application_020.html
http://www.sielc.com/application_130.html
Including drugs and counter-ions:
http://www.sielc.com/pdf/SIELC_July_2009.pdf
In all our columns all functionalities are assembled on one ligand. It is not a physical mixture of two or more stationary phases (Scherzo SM-C18 from Imtakt, Hypersil Duet from Thermo). Physical mixtures of stationary phases were known for a long time, but they never gained too much popularity, due to few potential problems (change in selectivity due to different rate of hydrolysis/deactivation of two "mixed" phases during the use of the column, lower efficiency due to different nature of "mixed" phases, lower capacity, because only 50% of the phase particular phase is used, etc). When you have all functionalities on one molecule you are addressing these issues. Some of the mixed-mode columns (including our cation-exchange mixed-mode columns) have another draw back issue - you need to avoid alcohols. Alcohols can esterify stationary phase and change selectivity, but retention can be achieved at very low organic concentrations with the help of ion-exchange mechanism:
http://www.sielc.com/pdf/SIELC_April_2009.pdf
Here are few tips on method development for various analytes:
http://www.sielc.com/pdf/SIELC_August_2009.pdf
4. Reversed-phase column with ion-pairing reagent. This approach will allow you to retain polar ionizable compounds using corresponding ion-pairing reagents. The approach has some draw backs (not compatible with LC/MS, prolong equilibration, system dedication, etc.), but it widely used for decades. You can use any RP column.
5. HILIC columns. Good approach for polar analytes, will retain them very well. Have some draw backs, too - high consumption of ACN, low solubility of polar compounds at high organic, lower loadability in prep chromatography, poor peak shape in some cases. The cheapest and most economical way is to use bare silica column.
