This post is designed to offer assistance to users of cadmium coils in the analysis of Nitrate. Since the use of the cadmium coil is fairly complicated and because there are many possible applications, we want to discuss the different sizes that are available as well as the advantages of the various sizes. Cadmium coils were designed to avoid the “messiness” of using granulated cadmium in packed columns. Originally, these coils were developed by Alpkem to replace packed columns and were configured in two basic sizes a 12 and 24 inch but we have added a 6 inch and a 36 inch to improve various applications.
6 inch Cadmium Coil:
This is a very small section of coiled cadmium designed for very high level Nitrate analysis. Many agriculture applications have samples that can reach into the 100’s or even 1000’s PPM. By using a very small section of cadmium the overall reaction is significantly reduced thus allowing for a high very high calibration range helping to reduce the amount of dilutions required by the analyst thus increasing throughput and productivity. Accuracy is not compromised since all samples pass through the same section so the reaction is very stable and equalized across the range. Of course, low range samples cannot be analyzed with this coil.
12 inch Cadmium Coil:
This coil was originally designed to be used on RFA instruments that used a slower pump speed and a reduced volume of solutions. We continue to provide this coil for those platforms that still exist. They tend to provide good function across the normal EPA ranges of 0-2 ppm on these platforms because the flow volume and rate is so low/ slow. This coil can also be used for mid-range concentrations above the normal EPA ranges to similarly reduce the signal similar to the 6 inch in ranges of approx. 5-100 ppm. These ranges vary from instrument and matrix but is effective at spreading out a range while keeping in mind that the low end will be much higher.
24 inch Cadmium Coil:
The new style instruments such as the OI Analytical FS3000, FSIV, FS3100 and the FS3700 are designed to be used with a 24-inch coil. This size is the optimal length for most applications even down to the ppb levels as well as 5-10 ppm, but can vary among various configurations. As the coil gets longer the upper range begins to diminish since the reaction will be so strong that the color response will occlude light in the flowcell (Beer’s Law).
36 inch Cadmium Coil:
This length of coil is at times useful for very low analysis in the sub ppb range or a combination of low level with difficult matrixes such as seawater or extractions (plant or soil). The longer time in the cadmium coil allows for a greater exposure to the reduction process to insure a good reduction occurs.
In closing we want to mention that all of these coils can be used on any type of Autoanalyzer or Discrete Analyzer. There is no difference if the system is a Segmented Flow Analysis (SFA) or Flow Injection Analysis (FIA) system. Additionally, we can make any kind of fitting connection required for no extra charge. Ranges that are required are based on a combination of pump tubing sizes, pump speed, FIA Loop Size, SFA injection times, mixing coils, etc. – the variety of cadmium coils listed here is one more tool for the analyst to get to the optimal analysis range required. Thank you for your time please feel free to contact us for any chemistry issues you may have we are very confident we can solve nearly any issue you have with colorimetric analysis or even just increase your range to help make your analysis more efficient and productive.