You can use lower injection loops and may be reduce the flow rate to 0.7 ml/min.

What is the column and mobile phase concentration? The dilution of mobile phase can help and definitely you should reduce the injection volume.

Overlapping peak and consequent change in retention time is the Indication of column contamination ,Clean the column with appropriate solution . An analyte’s retention time is determined by the eluent flow rate and by the distribution of solute between the mobile and stationary phases. Repeat the analysis with different flow rate,make sure that your using the exact eluent concentration.

One thing to keep in mind in the case of weak acid cation exchange phases of the sort you are undoubtedly using is to make sure that the pH of your sample and/or standard isn't too low.  If your standard is made from salts dissolved in deionized water you should be okay although as suggested above it might be better to use a smaller injection volume (25 µL or less).  But if you are starting with individual elemental standards dissolved in acid to prepare your mix standard (as is common for standards used in elemental spectroscopy) then your standard pH may be compromising your separation performance.  Generally, it's best to keep the pH of your standard and samples below 4.

Hi Farooq

We use c4 column. cant reduce injection volume as we normally get concentration like .1ppm or less.

Hi Javad

we have tried your suggestion by reducing flow rate to .7, it worked well in first few sample but then it showing same result like .9.

Hi Ekaterina and Faisal

both of your suggestion showing good result till now.

Hi Chris we add HNO3 in every sample for cation analysis.pH of the samples are normally within a range of 3-4.

one thing i must mention that the number of theoretical plate is decreased almost 3 times for each of the ions. is it a indication of contamination?

If your efficiency is one third of what it used to be, it's probably not due to contamination unless the retention times are dramatically shorter as well.  Usually, contamination effects the efficiency of different ions to different extents.  So, considering the fact that you are seeing a similar loss in efficiency for all ions this suggests a more basic problem with your column.  Have you checked for headspace?  If you have headspace, it's time to get a new column.  Another possibility is that your inlet frit is partially clogged from samples containing particulate.  You might try changing the inlet frit since sometimes this can cause a problem of the sort you are reporting.

If you are very keen on good Na / NH4 separation, I highly suggest investing in the C6 column. The separation is fantastic, even up to several orders of magnitude of difference in concentration.

Thank you Alyson. do you find any tailing effect of calcium in c6 column?

Hello Arindam, the amount of tailing depends on the amount of PDCA you are using. Using only HNO3 as eluent, you will have some tailing in the divalent cations. Addition of PDCA (just as with the C4 column) helps the divalents elute more quickly and reduces the tailing. Could you tell me your current conditions (eluent strength + PDCA, temp, flowrate, column size, and guard column)?

hello alyson .. can we use only nitric acid as eluent for cation without PDCA? i previously used 1.7 mm/l HN03 and 0.7 mm/l PDCA. 

If you do not use PDCA, it should not affect your Na/NH4 so much. The PDCA is more to help the divalent cations (Mg 2+, Ca 2+, etc.) along. So if you use only HNO3 as eluent, or a HNO3/PDCA mixture, it will not greatly affect your Na/NH4 separation. You need a lot of theoretical plates, which like you said, seem to be decreasing a lot. Is there a possibility you can ask your Metrohm provider to test your sample with similar conditions (same eluent, same temperature, flow, etc.) on a C6 column? If this separation is very important to you, I am sure within a short time they can show you the differences between the C4 and C6 capabilities.