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Leading-edge ion analysis with the Agilent 7100 Capillary Electrophoresis system

By Martin Greiner
Agilent Product Manager, Capillary Electrophoresis

Labs have traditionally performed the separation of small ions by ion chromatography using conductivity detection, but the cost-per-analysis can be high and matrix interferences are common. You can overcome these problems by taking a different approach – one that uses the Agilent 7100 Capillary Electrophoresis system with specialized detectors.

Capillary electrophoresis (CE) offers fast separations with exceptional efficiency and resolution for charged substances such as biomolecules, low-molecular-weight basic or acidic drugs, and especially ions. These separations are often difficult to achieve with HPLC.

CE also excels where sample amounts are very limited, and requires less buffer than liquid or ion chromatography. In addition to the speed of analysis (typically less than 6 minutes for cations and less than 10 minutes for anions), sample preparation often is reduced to a dilution or filtration step. CE enables many types of separations, the most popular being capillary zone electrophoresis (CZE).

Less costly ion analysis with CZE

For decades, labs have successfully employed ion chromatography (IC) to separate small ions. However, the technology has some limitations. You need expensive and sometimes short-lived columns, and the mobile phase is usually a strong acid or base. Pumping systems may require frequent maintenance for peak performance. Matrix effects are commonplace, you may need to perform extensive sample preparation, and the cost-per-analysis can be high. Capillary zone electrophoresis is an alternative that is well-suited to separate these species.

The basis for a CZE separation is the differences in mobilities among the various ions. You can readily exploit these differences to perform rapid separations of small ions with superior speed and resolution compared with IC. You can replace the often-expensive column required for ion-exchange chromatography with a fused-silica capillary that costs very little. With CZE methods, the cost-per-analysis is considerably less than comparable chromatographic methods.

Figure 1. Indirect photometric detection allows you to detect ions that do not absorb in the UV range. (Enlarge image.)

Figure 2. This CZE analysis of wastewater from a municipal waste treatment plant shows excellent sensitivity. (Enlarge image.)

IPD detects compounds that lack chromophores

Most small ions do not absorb in the ultraviolet (UV) or visible regions of the spectrum. This detection problem is solved with the indirect photometric detection (IPD) technique that can be applied with Agilent’s 7100 Capillary Electrophoresis system. In this technique (Figure 1), a UV-absorbing solute of the same charge as the separands (a co-ion) serves as an additive to the background electrolyte (BGE). This additive, known as a visualizing reagent (VR), elevates the baseline. When sample ions are present, they displace the visualizing reagent, as required by the principle of electroneutrality. As the separated ions migrate past the detector window, the detector measures them as negative peaks relative to the high baseline. By reversing the signal and reference wavelengths of the diode-array detector (DAD), a positive signal is obtained. CZE-IPD is suitable for virtually all ionic analytes that lack chromophores.

Sensitivities for CZE-IPD are usually in the ppm range, but you can reduce detection limits down to the ppb range, depending on buffer systems and application. Figure 2 gives an IPD example for the detection of anions in wastewater.

If you want to get started quickly and easily with CE analysis, the complete Agilent CE Solution Kits, and the included methods, enable reproducible results for a wide variety of samples in food, pharmaceutical, forensic, or environmental analysis. You can find more information in the new Agilent compendium, “Ion Analysis with Agilent Capillary Electrophoresis Systems” (publication number 5990-5244EN).

Alternative detector interfaces seamlessly

In case background electrolytes are undesirable for separation reasons, another option for small ion detection is a commercial contactless conductivity detector (CCD). You can seamlessly add a CCD to the Agilent 7100 CE system, because the required analog/digital converter is now an integral part of the CE system. No external circuit boards are necessary. Because the 7100 CE system uses air cooling rather than liquid cooling to control the temperature of the capillary cassette, you can easily access and open the cassette. And the cassette arrives ready to attach the CCD, making the switch from UV detector to CCD an easy task.

The Agilent 7100 Capillary Electrophoresis system, with detection by either IPD or CCD, overcomes several problems with IC analysis of small ions and delivers lower cost-per-analysis in less time. If you want a better alternative to IC, learn more about the use of CE for ion analysis, and request your copy of the new Agilent ion analysis compendium.

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