Ion Chromatography (IC) – or more specifically, suppressed anion chromatography – is an instrumental separation technique that allows us to identify and quantify levels of many different negatively charged inorganic and organic constituents (anions) from a single water sample. Cation chromatography allows us to identify and quantify levels of many different positively charged inorganic and organic constituents (cations) from a single water sample.
Basics of Ion Chromatography
An ion chromatograph is an instrument with several key components to allow for the separation of dissolved charged particles (anions & cations) based on their chemical and physical properties. For the Beaver Run Project, we employ a Metrohm 850-IC Professional chromatograph.
Pumps and Filters
Because we're analyzing aqueous samples (dissolved in water), it's important to be able to move a lot of liquid through the machine. It's also important to have the precise types of liquids present to make sure we get the best results possible. All solutions are filtered through a 0.45-μm before placing on the instrument. During analysis, the instrument uses an additional filtration system to prevent clogs and contaminations (0.20-μm filter). High-precision pumps are used to maintain the smoothest flow of liquid possible.
Currently, we are analyzing anions (negatively-charged ions) in solution. As such, our mobile phase is comprised of an aqueous mixture of sodium carbonate () and sodium bicarbonate (). We are running a suppressed method, meaning that we suppress the background signal (see Detector below) by neutralizing the mobile phase with sulfuric acid.
The most important component of a chromatograph is the column, where actual separation of chemicals occurs. As the sample, which is moved along by the mobile phase, enters the column, it encounters the solid phase. In this case, the solid phase is comprised of microscopic silica-based beads that have been coated with very specific polymers that attract the sample anions.
In short, there is a "tug-of-war" between the mobile phase and the stationary phase for each particular anion passing through the column. For an anion like fluoride (), the mobile phase is more attractive and so it passes through the column very quickly. For an anion like sulfate (), the stationary phase is more attractive and so it stays on the column for a longer time. We can use this phenomenon to separate rather complex mixtures in a short time. Not only can we identify many anions, but we can calculate at what concentration they exist in the sample by using a set of standards in a calibration curve.
In order for us to be able to identify and quantify each anion in a complex mixture, we have to be able to see it. On the ion chromatograph, we see anions by a conductivity reading. A conductivity detector measures the electrical conductance of the solution as it exits the separation column. When no anions are passing through the detector, we are only reading the background conductivity – which is kept very close to zero by neutralizing the mobile phase basic anions ( and ) with acid.
However, when an anion such as chloride () or nitrate () exits the column, the electrical conductivity increases until the anion passes through the detector. In order to identify and quantify each anion in our mixture, we rely on a calibration curve.