Many of us have encountered some ion chromatography (IC)/HPLC analyses in which the same sample is injected onto two different HPLC/IC systems, in order to ascertain the sample composition, with the only difference between the two HPLC/IC systems being the detectors used. This appeared to be a waste of time and other resources. However, two detectors were required, since together they could exploit the properties of each analyte found in the sample.
Years ago, one had very little choice, apart from having just one HPLC/IC system, where the detectors would be swapped, depending on the analytes to be detected. Since then, more options have arisen, and single chromatography software systems are often able to handle simultaneous signals from multiple detectors and columns.
Following are two common examples of the use of parallel flow with multiple detectors. Stream switching is available for fine-tuning the response from the chromatographic peak(s) of interest. Also, flow splitting will take the flow from one injection and use it for multiple paths, either into different columns or detectors. These options are valid and useful, but entail the use of additional expensive hardware and likely troubleshooting.
Figure 1 - PowerStream software (Cecil Instruments, Cambridge, U.K.) allows the user to monitor output from multiple detectors simultaneously.
One option that does not require the use of any additional hardware is to employ two or more detectors in simple series. Here, a sample is injected into one column and a single eluent flow is used to separate out all of the analytes. The eluent stream is passed through one detector; then the outflow from the first detector is passed into subsequent detector(s). The first detector is used to exploit the properties of some analytes contained in the sample. The second and/or subsequent detector(s) are then used to exploit the properties of other analytes. The virtually simultaneous chromatograms relating to each detector are then viewed on a tiled PC screen (see Figure 1).
Compared to stream switching and flow splitting, the advantages of using two or more detectors in simple series include:
- Cost—the only additional hardware required is a small length of tubing and a connection union to carry the output of one detector to the input of the next detector in the series
- Ease—only seconds are required to plumb the eluent flow into the detectors and to electrically plug the detector(s) to a software system
- Time—the analyst does not need to plan in advance to obtain specialized parts or apparatus; the analysis can be performed at a moment’s notice.
One slight drawback is potential band broadening. This can be overcome by using smaller injection volumes and by decreasing the overall dead volume as much as possible.
Only three conditions are required to achieve simple series detection: 1) The same column and eluent flow path should be used for all of the analytes in question; 2) if eluent is to be monitored after passing through a particular detector, then that detector should not alter the composition and nature of the analyte(s); and 3) the chromatographic software system should be capable of simultaneously collecting and displaying the signals from all the detectors used.
Figure 3 - The IonQuest ion chromatography system (Cecil Instruments), known for flat, stable baselines and high sensitivities, has a conductivity detector that can be used in series with the WaveQuest.
Figure 2 - The WaveQuest high-speed scanning UV detector (Cecil Instruments) can accommodate in series detection extremely well.
Examples of instances in which simple serial detection may be used in IC and HPLC include:
- Red wine: Sugars and alcohols are first detected using a refractive index detector (CE 4700, Cecil Instruments) and then the organic acids are detected using a conductivity detector (Cecil CE 4710, Cecil Instruments). An organic acid column and a weak sulfuric acid mobile phase are used
- Forensic blood plasma samples: Profile antibiotics are detected using a UV-VIS detector (CE 4201, Cecil Instruments), and then catecholamines are detected using an electrochemical detector (CE 4720, Cecil Instruments). A C18 column and a phosphate buffer/acetonitrile mobile phase are used
- Personal-care preparations: Ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and diethylenetriamine pentaacetic acid (DTPA) are first detected using a UV-VIS detector, and then arsenates are detected using a conductivity detector and an anion suppressor. An anion exchange column and a weak acid mobile phase are used
- Sugar cane molasses: Sugars are first detected using a refractive index detector, and then tracer pesticides are detected using a UV-VIS detector. An ODS 2 column (Restek, Bellefonte, PA) and a water/acetonitrile mobile phase are used
- Precious metal effluents: Alkali metals are first detected using a conductivity detector, and then the rare transition metals are detected by derivatization in a postcolumn reactor and detected by a UV-VIS detector (see Figure 2). A universal cation column and an oxalic acid/tartaric acid mobile phase are used.
The manufacturer supplies modular IC and modular HPLC systems plus a wide range of accessories (see Figure 3). These accessories include postcolumn derivatization reactors, fraction collectors, autosamplers, flow splitters, and column switching valves.
Dr. Kujore is Manager, Cecil Instruments Ltd., Cambridge, U.K. Dr. Sandor is Director, Tovatech LLC, 11 Harrison Ct., South Orange, NJ 07079, U.S.A.; tel.: 973-913-9734; fax: 973-327-4774; e-mail: [email protected].