Ion Chromatography separates and measures ions and other charged molecules present in a liquid sample. The technique involves injecting a sample into a system that separates components based on their charge, size, and interaction with the system’s materials. The output is a graph called a chromatogram, which plots the detector’s electrical response against the time elapsed since injection. The distinct “spikes” or peaks visible represent the individual separated components, such as chloride or sulfate ions, reaching the detector.
Identifying Ions Through Retention Time
Each spike represents a specific ion that has traveled through the system and reached the detector. The ion’s identity is determined by its retention time, the precise time elapsed between sample injection and the apex of its corresponding peak. This retention time acts much like a chemical fingerprint, as it is unique to a particular ion when analytical conditions (column type, eluent composition, and flow rate) are kept the same.
A chloride ion, for example, consistently reaches the detector at a specific time, while a sulfate ion, which interacts differently, emerges at a later, characteristic time. To confirm a spike’s identity, analysts must first run known standard solutions containing the ions of interest. This creates a reference chromatogram, allowing the retention times of unknown sample peaks to be accurately matched to the known standards.
This comparison is the qualitative aspect of the analysis, confirming what is present in the sample. If a peak’s retention time matches that of a known standard, it is identified as that specific ion. The consistency of the retention time is crucial for reliable ion identification.
Determining Concentration From Peak Size
Beyond identifying the ion type, the spike size provides quantitative information, indicating how much of that ion is present. The concentration of an ion is directly proportional to the size of its corresponding peak. Specifically, the area under the peak, rather than just the height, is used for the most accurate quantification.
A larger peak area signifies a higher concentration of the ion. The detector, typically a conductivity cell, measures the change in electrical conductance as ions pass through it; more ions result in a stronger signal and a larger area. To convert this measured peak area into a meaningful concentration unit, such as parts per million (ppm), a calibration curve is required.
The calibration curve is generated by analyzing a series of standard solutions with known concentrations. Plotting the known concentrations against the measured peak areas creates a curve the system uses to calculate the concentration of the unknown sample by measuring the area of its spike. This method ensures the quantitative analysis is accurate and reliable across the detection range.
The Process of Ion Separation
Ion separation occurs due to the underlying chemical process of ion exchange. The core separation happens inside the column, which contains the stationary phase—a resin packed with fixed, oppositely charged functional groups. The liquid continuously flowing through the column, known as the mobile phase or eluent, carries the sample ions through the system.
As the sample travels down the column, charged sample ions engage in electrostatic interactions with the stationary phase. In anion exchange chromatography, negatively charged sample ions are attracted to the positively charged groups on the resin. The strength of this attraction, influenced by the ion’s charge, size, and hydration shell, determines how long the ion is temporarily held back.
Ions that interact weakly with the resin are quickly swept along by the mobile phase, resulting in a short retention time and an early spike. Conversely, ions that interact more strongly are retained longer, requiring more time to be displaced by the eluent ions, and appear as a spike later. This differential movement based on ionic interaction strength separates the mixture into individual components before they reach the detector.
Interpreting Spikes That Are Not Target Ions
While most spikes represent the ions being analyzed, a chromatogram can contain peaks that do not correspond to target sample components. One common feature is the “water dip,” a negative peak appearing early in the run, often before the first target anion. This dip occurs when the injected sample has a lower electrical conductivity than the mobile phase, causing a temporary drop in the detector’s baseline signal.
Other non-analyte features include “ghost peaks,” which are small spikes resulting from contamination within the instrument, such as impurities in the water supply or eluent, or residual material from a previous run. These are problematic if they co-elute, or overlap, with a target ion, leading to incorrect quantification. Small, random fluctuations in the baseline are considered noise and do not represent a chemical component.
Recognizing these non-target spikes is essential for accurate analysis, as they must be excluded from quantification to prevent misidentification or miscalculation. Analysts employ specialized software to differentiate these system-related peaks from true sample peaks. Troubleshooting often involves running blank samples of pure water or eluent to identify and eliminate the source of interfering spikes.