I would like to know what sort of difference in sensitivity and detection limits will be observed, if I use 2 mm columns instead of 3mm in Ion Chromatography separation of Anions and Cations.

Other things being equal, the internal volume of the column is proportional to the square of the diameter. In this case (3mm -> 2mm), it means that your analyte is dissolved in 44% of the original volume, so that your detection limits should go down by about a factor of 2.25.

*however*, be aware that you need to decrease the extra-column volume of your system by this same factor in order to avoid peak broadening, and you may need to limit the injection volume to avoid overloading the column.

Hope this helps.

-- Tom Jupille / LC Resources

Concur with Tom on a 2.25x improvement in detection limit assuming you are injecting the same volume and concentration of sample.

However, if you do not have a limited amount of sample you should find that the best detection limit is obtained by injecting the largest sample possible that does not degrade column performance. For RP-HPLC, this is typically about 2 to 10% of the column void volume, regardless of column ID. That is, if your column volume is 44% larger you can inject 44% more sample and you should see about the same detection limits and column performance. Due to extra-column dispersion sources, better performance may be seen with the larger column. You will use more sample and more solvent with larger columns, but the performance should be just as good or better.

Marc Foster

While I agree in general with the comments mentioned above, I would say that the answer to your question depends largely upon what injection strategy you are pursuing and the amount of sample you have available. If you are not doing trace analysis via a large injection and you're not sample volume limited then you should obtain sensitivity with a 3 mm column equivalent to that of a 2 mm column, provided you maintain loop volumes which are proportional to the relative cross-sectional area of the two columns (i.e. loop volume should be increased by the ratio of the square of the radius of each of the columns; 1.5 squared divided by 1 squared or 2.25 X when going from a 2 mm column to a 3 mm column). If you are making use of large loop injection, you are better off using the 2 mm column format inasmuch as this will permit a 225% increase in sensitivity upon injection of an identical volume of liquid. For example, if you compare the sensitivity achieved with a 1 ml injection on a 2 mm column to that of a 1 ml injection on a 3 mm column you will observe a 225% larger peak on the 2 mm column. If instead you used a 2.25 ml injection loop on the 3 mm column the peak areas should be identical. The main problem with using a larger loop in order to maintain proportionality of injection stems two issues. First, eventually preparing equivalent volume loops becomes prohibitive in terms of the amounted to be required and second, as loop volume increases it becomes increasingly difficult for the pumping system to deal with the pressure upset caused by the insertion of the sample loop into pressurized chromatographic flow stream. For both of the above practical reasons, best detection sensitivities when using large loop injection is achieved with 2 mm columns.