Utility of the Charge Detector in Ion Chromatography Applications

Transcription

1 Utility of the Charge Detector in Ion Chromatography Applications Mrinal K. Sengupta, Sheetal Bhardwaj, Kannan Srinivasan, Chris Pohl, and Purnendu K. Dasgupta Thermo Fisher Scientific, Sunnyvale, California, USA; University of Texas at Arlington, Arlington, Texas, USA

2 Overview Purpose: To demonstrate the utility of the Thermo Scientific Dionex QD Charge Detector for capillary ion chromatography (IC) applications. Methods: The performance of a Thermo Scientific Dionex QDC 3 Capillary Charge Detector Cell, which is connected in a series after a postsuppression Thermo Scientific Dionex CD Conductivity Detector, was evaluated. Several test standards, including strongly and weakly dissociated species, were prepared and evaluated. Additionally, real-life samples such as drinking water and various beverages were evaluated with this instrument. Results: The Dionex QD Charge Detector is sensitive and provides a very reliable technique for ion analysis. In conjunction with the Dionex CD Conductivity Detector, it can provide complimentary information about the analytes. Introduction The new Dionex QD Charge Detector detects ions for suppressed IC applications. The Dionex QDC 3 Capillary Charge Detector Cell is installed in a postsuppressor configuration and is plumbed in-line with a Dionex CD Conductivity Detector to provide another dimension to ion detection. The Dionex QD detector is useful for detection of anions and cations with Reagent- Free IC (RFIC ) systems. The detector is essentially a destructive detector, and therefore should be the last detector in the suppressed eluent stream. The charge detector contains both anion and cation exchange membranes in a configuration to deionize the influent suppressed stream. This approach results in a current signal that directly measures the transient analyte species as shown in the Figure. Strongly dissociated ionic species such as hydrochloric acid (analyte referred to as chloride) is fully dissociated, and therefore is removed completely by the deionization configuration of the Dionex QDC 3 Capillary Charge Detector Cell, transforming the analyte peak into deionized water. The obtained signal response is directly proportional to the concentration and provides a linear fit similar to conductivity detection, provided the influent concentration of analyte is completely removed. FIGURE. Dionex QD Charge Detector working principle. Methods Sample Preparation A test mixture comprising of 9 compounds of various organic acids and inorganic anions were prepared from the salt form with the concentrations listed in Figure. Ion Chromatography Conditions The Thermo Scientific Dionex ICS- capillary HPIC system was used with suppressed conductivity detection first, followed by detection with the Dionex QD detector powered at an applied voltage of 6V. The flow rate varied from µl/min with the Dionex IonPac AS-HC-µm Capillary (. mm) column, and µl/min with the Dionex IonPac AS-HC Capillary (. mm) and Dionex IonPac CS6 Capillary (. x mm) columns. The current setting was optimal and applied as recommended by the Thermo Scientific Dionex Chromeleon Chromatography Data System software (version 6.8 or higher) used for data analysis. Columns: Thermo Scientific Dionex IonPac AG-HC-µm Capillary Guard (. mm), P/N 83 Dionex IonPac AS-HC-µm Capillary Analytical (. mm), P/N 83 Dionex IonPac AG-HC Capillary Guard (. mm), P/N 83 Dionex IonPac AS-HC Capillary Analytical (. mm), P/N 89 Dionex IonPac CG6 Capillary Guard Column (. mm), P/N Dionex IonPac CS6 Capillary Analytical (. mm), P/N Utility of the Charge Detector in Ion Chromatography Applications

3 Eluent Source: Anions: Thermo Scientific Dionex EGC-KOH (Capillary) Cartridge with a Thermo Scientific Dionex CR-ATC Continuously Regenerated Anion Trap Column Cations: Dionex EGC-MSA (Capillary) Cartridge with a Dionex CR-CTC Continuously Regenerated Cation Trap Column Eluent: ) KOH, mm from to 8 min, 3 mm from 8 to 8 min, 3 6 mm from 8 to 38 min, 6mM from 38 to min (Dionex IonPac AS-HC-µm Capillary) ) KOH, mm (Dionex IonPac AS-HC Capillary) 3) MSA, 3 mm (Dionex IonPac CS6 Capillary) Flow Rate: ). ml/min, ). ml/min, 3). ml/min Inj. Volume:. µl Temperature: ) 3 C, ) 3 C, 3) C Suppressor: Thermo Scientific Dionex ACES 3 Anion Capillary Electrolytic Suppressor (AutoSuppression recycle mode) or Thermo Scientific Dionex CCES 3 Cation Capillary Electrolytic Suppressor Detection: A) Dionex CD Conductivity Detector B) Dionex QD Charge Detector Results vs Concentration Performance for Conductivity and Charge Detectors Charge detection provides similar response for equivalent concentration of fully dissociated species irrespective of charge, thus permitting reliable quantification of known and unknown compounds with a single standard calibration, as depicted in Figures 3 and. However conductivity detection provides unique response proportional to the equivalent conductance and concentration as shown in Figures and. Conductivity detection response for weakly dissociated species is dependent on the extent of ion dissociation, which is dependent on the pk a and pk b of the species involved and the concentration. This results in a nonlinear response with concentration under some conditions, as shown in Figures 6 and 8. With charge detection, a linear fit is obtained as long as the species is fully removed by the Dionex QDC 3 Capillary Charge Detector Cell, as shown in Figures and 9. A weakly dissociated species is more effectively dissociated by the Dionex QD Charge Detector deionization action, since the removal drives the ionization of the neutral fraction. At higher concentrations, nonlinear response can be expected due to poor removal. FIGURE. Dionex CD Conductivity for inorganic anions using the Dionex IonPac AS-HC Capillary column and a 3 mm KOH eluent. FIGURE 3. Dionex QD Charge for inorganic anions using the Dionex IonPac AS-HC Capillary column and a 3 mm KOH eluent. Thermo Scientific Poster Note PN96_PITTCON E_3/S 3

4 FIGURE. Dionex CD Conductivity for inorganic cations using the Dionex IonPac CS6 Capillary column and a mm MSA eluent. FIGURE. Dionex QD Charge for inorganic cations using the Dionex IonPac CS6 Capillary column and a mm MSA eluent. FIGURE 6. Dionex CD Conductivity for weak and strong acid anions. µs*min µs*min µs*min 6 R² = Formate (pk a 3.) R² = Chloride (pk a -6.) Fluoride (pk a 3.) R² = Concentration (mg/l) FIGURE 8. Dionex CD Conductivity for weak and strong basic cations. R² = Ammonium, pk b. R² = Morpholine, pk b. Methylamine, pk b 3.36 R² = 6 Dimethylamine, pk b 3.36 R² = Concentration (ppm) FIGURE. Dionex QD Charge for weak and strong acid anions. µa*min µa*min µa*min Fluoride (pk a 3.) R² = Formate (pk a 3.) R² = Chloride (pk a -6.) 6 R² =.999 Concentration (mg/l) FIGURE 9. Dionex QD Charge for weak and strong basic cations Ammonium, pk b. Dimethylamine, pk b 3.3 R² = Morpholine, pk b... R² = Methylamine, pk b 3.36 R² = Linear FIt R² = Concentration (ppm) Charge Detector as a Complimentary Detector When combined with suppressed conductivity detection, charge detection can be used as a confirmatory or complimentary detection method to provide additional analytical information. A peak confirmation scheme is shown in Figure. Based on the individual calibration of the Dionex CD Conductivity Detector and Dionex QD Charge Detector for Utility of the Charge Detector in Ion Chromatography Applications

5 a given analyte, two different concentrations will be obtained for an unknown peak, depicted here as Concentrations A and B. By definition, Concentrations A and B have to be identical for a pure analyte, hence peak confirmation is implicit. If coelution is encountered, then unequal concentrations will be predicted. A user-set criterion would determine the peak confirmation or coelution. Examples are shown in Table for two water samples and in Figure. FIGURE. Scheme predicting peak confirmation when the two detectors are in series. Conductivity Detector Calibration Curve Charge Detector Calibration Curve y =.8x +. y =.3x +. R² =.9999 R² = A. B Concentration Concentration Table. Predicting peak confirmation based on set criteria. Criteria = User Input Value % Condition Result Difference between A & B Criteria Peak Confirmed Difference between A & B > Criteria Possible Coelution µs FIGURE. s obtained from the Dionex CD Conductivity and QDC Charge Detectors Dionex CD Traces, F (. ppm) + Ac (. ppm) + Cl ( ppm).. Dionex CD Trace, F (. ppm) + Ac ( ppm) + Cl ( ppm) Dionex CD Traces, F ( ppm) + Ac (. ppm) + Cl ( ppm) µa..... Dionex QD Traces, F (. ppm) + Ac ( ppm) + Cl ( ppm) Coelution Predicted Dionex QD Traces, F ( ppm) + Ac (. ppm) + Cl ( ppm).8 Predicts. Fluoride Dionex QD Traces, F (. ppm) + Ac (. ppm) + Cl ( ppm).9 Coelution Predicted Table. Detection of anions present in drinking water using the Dionex CD Conductivity and Dionex QD Charge Detectors in series. Dionex Conductivity Detector Dionex QD Charge Detector Anion Concentration (mg/l) Concentration (mg/l) Tap Water Filtered Water Tap Water Filtered Water Fluoride.6 ±.6. ±..68 ±.. ±.9 Chloride.8 ±..6 ±.. ±.39.6 ±.6 Nitrate. ±.3 ±.3.99 ± ±. Sulfate. ±..9 ±.9.6 ±.8.9 ±.8 Application Examples A weakly dissociated species is more effectively dissociated by the Dionex QD Charge Detector deionization action, since the removal drives the ionization of the neutral fraction. Thus, many weakly dissociated species behave like strongly dissociated species and provide a stronger relative signal in the Dionex QDC 3 Capillary Charge Detector Cell versus conductivity detection. The trace in Figure shows enhanced signals for weakly dissociated species with charge detection compared to conductivity detection. The response for the Dionex QDC cell was normalized to the response for chloride peak height on the Dionex CD Conductivity Detector. The Dionex QD Charge Detector is useful for analysis of organic acids, inorganic anions, and inorganic cations in a variety of samples, such as drinking water, wastewater, fruit juice, wine, and a variety of beverage samples. An organic acid profile is important in characterizing the quality, purity, and flavor attributes. Analysis of beverage samples is demonstrated in Figures 3 and. In Figure 3, analysis of an old orange juice shows the presence of high concentrations of lactate and acetate, which is indicative of spoilage. Additionally, as depicted in Figure, some organic acids are more clearly detected with charge detection as opposed to conductivity trace detection. Thermo Scientific Poster Note PN96_PITTCON E_3/S

6 FIGURE. Normalized overlay of charge detection (red) and conductivity detection (black) for analysis of organic and inorganic anions using the Dionex Ion Pac AS-HC-µm Capillary column set and gradient mode. µa/µs Peaks:. Quinate mg/l 6. Bromide mg/l. Fluoride 3. Nitrate 3. Lactate 8. Carbonate. Acetate 9. Malonate. Propionate. Maleate 6. Formate. Sulfate. Butyrate. Oxalate 8. Methylsulfonate 3. Tungstate 9. Pyruvate. Phosphate. Valerate. Phthalate. Monochloro Acetate 6. Citrate. Bromate. Chromate 3. Chloride 8. cis-aconitate. Nitrite 9. trans-aconitate. Trifluoroacetate B A. 3 FIGURE 3. Orange juice analysis on both the Dionex CD Conductivity Detector (black) and the Dionex QD Charge Detector (red) using the Dionex Ion Pac AS-HC-µm Capillary column set. µa/µs Peaks:. Quinate. Fluoride 3. Lactate. Acetate. Formate 6. Pyruvate. Chloride 8 8. Unknown 9. Unknown. Unknown 9 B A. Carbonate. Maleate 3. Sulfate. Oxalate. Unknown 6. Phosphate. Citrate 8. cis-aconitate 9. trans-aconitate. Unknown 3 FIGURE. Comparison of the Dionex QD Charge Detector (red) vs the Dionex CD Conductivity Detector (in black) for analysis of the red wine sample µa µs... B) Charge Detector A) Conductivity Detection Conclusion Charge detection provides enhanced detection of weakly dissociated species when compared to conductivity detection and provides comparable detection for fully dissociated ions. For fully dissociated species, the charge detector offers universal calibration capability. Charge detection can be used in conjunction with conductivity detection to obtain confirmation or coelution information. References. Yang, B.; Chen, Y.; Mori, M.; Ohira, S.; Azad, A. K.; Dasgupta, P. K.; Srinivasan. K. Anal. Chem., 8 (3), Srinivasan, K.; Sengupta, M.; Bhardwaj, S.; Pohl, C.; Dasgupta, P. The Utility of the Charge Detector in Ion Chromatography and Selected Applications, presented at Pittcon Conference and Expo, Philadelphia, PA, Mar 8, Srinivasan, K. High-Pressure Ion Chromatography Charge Detection, webinar presentation, Oct 6,. 6 3 Peaks:. Quinate. Fluoride 3. Lactate. Acetate. Propionate 6. Formate. Butyrate 8. Pyruvate 9. Galacturonate. Chloride. Tartrate. Bromide 3. Nitrate. Glutarate. Succinate 6. Malate. Carbonate 8. Sulfate 9. Fumarate. Oxalate. Phosphate. Citrate 3. Isocitrate Utility of the Charge Detector in Ion Chromatography Applications

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