Anion chromatography

Anion chromatography was used to quantify negatively charged products in the degraded amine solutions. The types of molecules quantified include carboxylate ions (formate, acetate, oxalate, glycolate, and pyruvate), nitrite and nitrate. Carbonate /bicarbonate were identified at the same time.

3.4.1 Apparatus description

The Dionex anion chromatography system includes a GP50 gradient pump module, EG40 eluent generation module (Serial No. 03120420), DC conductivity module, and LC25 chromatography oven. Attached to the system are a Gilson 231XL autosampler and a Gilson 402 syringe pump, which eliminates the need for manual user injection. The eluent contains varying concentrations of KOH in Milli-7 %&'( )*+' 3 $

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using an IonPac AG15 guard column (4×5mm) and an IonPac AS15 analytical column (4×250mm). The IonPac AS15 column substrate is composed of a macroporous resin bead consisting of ethylvinylbenzene crosslinked with 55% divinylbenzene. The anion-exchange layer is functionalized with alkanol quaternary ammonium groups. Dionex Chromeleon^® software analyzed the conductivity output and controlled the entire system.

The IC system contained a 4-mm Anionic Self-Regenerating Suppressor (ASRS 300) to remove cationic species before anionic species were detected with the conductivity cell. The suppressor comprises of two regenerant chambers and one eluent chamber separated by ion-exchange membranes on opposites of the chamber, as shown in Figure 3.11 (Dionex Corporation, 2009). Electrodes are placed along the length of the regenerant chambers. When an electric potential is applied across the electrodes, the water regenerate chambers undergoes electrolysis to form hydrogen gas and hydroxide ions (OH^-) in the cathode chamber while oxygen gas and hydronium ions (H3O⁺) are formed in the anode chamber. Cation exchange membrane allows H3O⁺ to move from the anode chamber into the eluent chamber to neutralize OH^-. Sodium ions (Na⁺) attracted by the electric potential applied, move across the membrane into the cathode chamber to combine with the OH^-generated at the cathode. Thus the analyte is converted to a more conductive acid form. It should be noted that the eluent used was KOH, not NaOH in this study.

Figure 3.11Schematic of ASRS 300 in autosuppression mode (Dionex Corporation, 2009).

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3.4.2 Analysis procedure

The samples were diluted 50:1 to 200:1 with Milli-7 # /'012 filter. The filtered sample was inserted into a 2 mL glass sample vial and placed into the Gilson autosampler that holds 60 samples. In Chromeleon^®, a sequence was made to direct the anion IC and analyze the samples of interested. A sequence contained all the information needed for the anion IC to determine which method to use, which sample to inject and an identifying name of each sample.

The experimental sample was introduced from the sample vial via an injection needle in

the autosampler. The 5 # #$ $ (124 5 #

ensure that there is no cross-contamination from a previous sample. The remaining sample was passed through the injection loop and carried by the mobile phase to the inlet of the columns.

A method was used to control the various components of the system as well as adjusting the eluent gradient and setting the overall time for analysis. The method used was titled

‘Anion Grad 60mM.pgm’ and is given in Appendix D2. In this method, there were eight minutes before each sample, indicated as negative time, which allowed the system to equilibrate to a lower KOH concentration after completion of the previous sample in preparation for the current sample. The eluent KOH concentration over the course of ‘Anion Grad 60mM.pgm’ is shown in Figure 3.12. At the end of each run of the samples, there was a final sample with the ‘Anion Stop.pgm’ method. In this method, the suppressor and detector were turned off at time zero. Also, the pump was shut down at time 1min.

-10 0 10 20 30

10 20 30 40 50 60

KOH Concentration (mM)

Time (min)

Figure 3.12KOH gradient in anion IC method.

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A typical chromatogram for anion IC analysis of a partially oxidized AMP sample is shown in Figure 3.13. A dominant formate peak can be seen at a retention time of 8.5 minutes.

Smaller acetate-, carbonate-, nitrite-, oxalate- and nitrate peaks can be seen at 8.1, 12.2, 12.6, 13.8 and 20.9 minutes, respectively. The peak of glycolate at 7.8 minutes is very small in this

“zoomed out” view. Calibration curves were created in order to quantify the concentration of species in the original sample. At least five or six calibration standards were analyzed for each anion of interest. Usually, standards were created in the range of 1 to 50 ppm for anions that are expected in low concentration such as formate, oxalate, nitrate, and so on. A quadratic regression was used to create the calibration curves. The output at the end of analysis was a ppm concentration of each analyte in the dilute sample. This ppm concentration was adjusted using the dilution factor for reporting.

5 10 15 20

Nitrate Nitrite

Oxalate Carbonate/bicarbonate Formate

Acetate

Glycolate

Conductivity

Retention time (min)

Figure 3.13A typical anion IC chromatogram for partially oxidized AMP sample.

3.4.3 Analysis error

The error associated with the anion IC method is mainly due to peak area deviation and unsteady baseline. When the baseline was high or had a large bulge in it, all the peak areas calculated were suspect because their areas were decreased from what they would normally be with good separation and a low baseline. When a 6 point standard curve for formate was run in quadruplicate, the average standard deviation across all concentrations was ±4.9%.

This gives the overall error in the anion IC measurements of ±4.9% which is much larger

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than the error associated with the cation IC measurements. The error in making the dilutions is much smaller than in cation IC since a much lower dilution ratio is used. In this case, the error associated with the dilutions will be less than 0.5%.

3.4.4 Sodium hydroxide treatment for amide analysis

Amides are molecules with a R1R2N-R3C=O functionality and are expected products in amine oxidation. Amides themselves are not readily detectable with the techniques employed for amine and carboxylic acid analysis. A base hydrolysis was used to hydrolyze the amide to its corresponding carboxylic acid and amine; the carboxylic acid can be detected by anion IC as described above. This method was employed to analyze the produced amides in degraded amine solvents by Sexton and Freeman (Sexton, 2008; Freeman, 2011).

The procedure for amide reversal was to treat a sample with an equal amount of 5M sodium hydroxide (NaOH). Generally, 0.5 g of each was used and the sample reacted for at least 24 hours before further dilution and analysis. The carboxylic acid released during the amide reversal was then quantified using anion IC. In the present work, some of the degraded AMP and PZ samples were treated by NaOH.