WSRC-TR-2001-00358
R. J. Ray
Westinghouse Savannah River Company
Aiken, SC 29808
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High concentrations of sodium ions (Na+) interfere with the analysis of ammonium ions (NH4+) by cation chromatography. Attempts to remove Na+ via ion-exchange without removing NH4+ have been unsuccessful. An alternative approach is to purge dissolved ammonia and trap it in acidic water for cation analysis. This study shows that dissolved ammonia can be reproducibly purged with heating from an alkaline aqueous matrix (via helium gas, needle purge) away from interfering ions, trapped and accurately analyzed using cation chromatography.
This method is developed in support of the High Level Waste (HLW) division waste acceptance criteria for ammonia. It is also developed for Central Laboratory Analytical Services (CLAS) in support of the H-Canyon HM process for determining the presence of ammonia in 1st and 2nd Uranium Cycles for both high activity waste (HAW) and low activity waste (LAW).
Keywords: Ammonia, Purge, Cation
This study is two fold as it addresses HLW division waste acceptance criteria for ammonia and CLAS compliance to provide analytical support to the H-Canyon process for transfers to HLW.
Studies have been done at SRS to develop Henry's Law constants for ammonia in SRS High Level Waste Pump Tanks, concentrated wastes. Results were obtained using a multiple headspace extraction gas chromatograph with flame-ionization detection. The data showed a high degree of variability, but was in close agreement with data obtained by the Pacific Northwest Laboratory (PNL) on one Hanford HLW salt solution. (Data results were documented in WSRC-TR-2000-00226.) In addition, the results did not provide the relief from PNLs restrictive Henrys Law constants that was initially expected. Development of a more precise analytical method would enable more accurate calculations of "ideal" gas law constants in HLW Pump Tanks for ammonia analysis. Moreover, analysis of ammonium by "purge-and-trap cation chromatography" is presented as a cost effective and viable method for accurately measuring dissolved ammonium in HLW Pump tanks.
In August 2000, for "As Low As Reasonably Achievable" (ALARA) reasons, CLAS revised its procedure for ammonium analysis on H-Canyons Tank 8.4 and Tank 8.6 due to increased sample radioactivity and matrix interference. Higher dilutions resulted which did not meet the HLW division waste acceptance criteria of 75-ppm ammonia for transfers from H-Canyon. CLAS was then tasked with having to design and set up a new system for ammonia determinations in 772-1F Hot Cells given the high levels of radioactivity in the 8.4/8.6 tanks. Consequently, H-Canyon is tasked by the HLW division to improve their methods sensitivity demonstrate over a period of about 6 months the absence of ammonia/ammonium at <30 ppm in both high activity waste (HAW) and low activity waste (LAW) streams. In addition, H-Canyon Technical Support requests a detection range from 10 ppm to 1000 ppm ammonia with a maximum +10% Uncertainty.
Ammonia has traditionally been analyzed as ammonium ion at the Savannah River Site (SRS) using cation chromatography. The presence of high concentrations of Na+ (along with other co-eluting, cationic species) impedes the analysis of ammonium by cation chromatography. As shown in Figures 1 and 2, the ammonium peak is masked by high concentrations of sodium ions. In addition, H-Canyon waste has interference present where ammonium ions elute due to high concentrations of nitric acid (Figure 4). Therefore, ammonia must be separated from the sample matrix for the purpose of instrumental analysis by cation chromatography.
3. Reagents
Methanesulfonic Acid 99% purity
Eluent - 30 mmolar methane sulfonic acid
High purity ammonium standard 1000 ug/mL
ASTM Type II Water
12.5 molar sodium hydroxide
4. Experimental
Given the boiling point of ammonia, it was feasible to purge it out of an alkaline solution and trap it in acidic water suitable for cation analysis. The equilibrium of NH4+/NH3(g) in solution was shifted to the right by increasing the OH- concentration in solution, thus causing soluble NH4+ to go to the less soluble NH3(g). NH3(g) was readily purged from the high salt, high alkaline matrix. Heating was only applied to speed up the volatilization of NH3(g).
A heated transfer line was connected to port #4 on the sample inlet housing to transfer purged ammonia from the sample into the pH adjusted water trap. The purge-and-trap cation chromatography system was configured as follows:
O-I Analytical Purge-and-Trap |
||
Infrared Heater |
Disposable Purge Tubes 18mm x 150mm |
|
Purge Gas Head Pressure 20 psi |
Purge Gas Helium |
|
Purge Flow Rate 40 mL/min |
Purge Temp Off |
|
Purge Time 15 minutes |
Desorb Temp Off |
|
Desorb Time 0 |
Bake Temp Off |
|
Bake Time 0 |
Water Management Temp On |
|
Pre-Purge Time 0 |
Pre-Heat Time 0 |
|
Dry Purge Time 2 minutes |
Sample Inlet Temp 80°C |
|
Valve Oven Off |
External Heater Off |
|
Transfer Line Temp 100°C |
Sample Temp - 80°C |
|
Autosampler not installed |
||
DX-500 Cation System |
||
Analytical Column CS-12A, 4x250mm |
||
Guard Column GC-12A, 4x50mm |
||
Eluent Flow Rate 1mL/min |
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CSRS Cation Self-Regenerating Suppressor |
||
CSRS Cation Self-Regenerating Suppressor |
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CSRS Cation Self-Regenerating Suppressor |
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Autosampler AS40 |
||
Eluent reservoir pressure 3-5 psi |
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Detector CD20 Conductivity Detector |
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Peaknet 5.1 Software |
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GP40 Gradient Pump |
The DX-500 system was calibrated using a 4-point calibration curve. The calibration points were 10, 25, 50, and 100 ug/mL. Linearity could not be established between calibration points; therefore, a Point-to-Point curve fit was used. The Point-to-Point curve fit gave the most accurate quantification and will be used in this method to quantify all samples and blanks for ammonia. All samples should be analyzed using bracket calibration verification standards. Calibration verification standards should be within +10% of the accepted value. In addition, a matrix spike sample should be analyzed once per sample matrix to verify analyte recovery and accuracy of the method.
Calibration Level |
Nominal Amount |
Response Factor (RF) |
Average. RF |
%RSD |
1 |
10 |
297351.6 |
210253.8 |
30.2 |
2 |
25 |
213356.0 |
|
|
3 |
50 |
179496.3 |
|
|
4 |
100 |
150811.5 |
|
|
Continuous Stirred Tank Reactor (CSTR) simulant was used as a high-salt matrix for method development. The simulant was measured out in 5-mL aliquots into disposable borosilicate glass purge vessels. The pH was >10.(Ammonia can easily be produced in the laboratory by heating an ammonium salt with a strong alkali such as NaOH in solution.) The hydroxide ion concentration was a minimum of 1 M in excess. Acidic samples from the H-Canyon process can be adjusted by adding 2 mL of 12.5 M NaOH to the purge vessel. Strong acids and bases should always be added to water when making dilutions. These samples were then spiked with 250 ug of ammonium and purged for 15 minutes at 80oC. A 2 minute dry purge was used to insure complete transfer of purged ammonia and to minimize contamination by carryover. Given the corrosivity and polarity of ammonia, some carryover was expected. It should be noted that the higher the ammonium concentration in the sample, the higher the carryover. Purging a water blank (purge blank) after each sample and extending the dry purge time further minimizes the dilemma of carryover. If ammonium is observed in the purge blank at >5% of the ammonium concentration found in the sample, a second purge blank should be run to clean up the purge system.(The purge blank is used as a method blank. According to SW-846 guidelines, method blanks are generally acceptable in volatile analyses by purge-and-trap if the analyte concentration in the blank does not exceed the MDL, 5% of the regulatory limit for that analyte, or 5% of the measured analyte concentration in the sample.) See carryover assessment in Table 2 below.
Test # |
Ammonium (ug/mL) |
% |
Prior Spk. Conc. |
P10-0 |
0.3 |
3.4 |
10 |
P10-2 |
1.1 |
2.2 |
50 |
P10-4 |
0.4 |
4.0 |
10 |
P10-6 |
0.4 |
4.0 |
10 |
P10-8 |
0.5 |
5.0 |
10 |
P10-10 |
0.4 |
4.0 |
10 |
P10-12 |
0.4 |
4.0 |
10 |
This method purges 5 mL of sample in a disposable glass purge vessel. Ammonium is quantitatively determined within a 4-point calibration range from 10 to 100 ug/mL. Calculated values that exceed the calibration curve must be diluted. Values that are less than the lowest calibration standard will be reported as less than values.
Detection limit studies were done using 2.4 molar reagent water as the matrix. The matrix was spiked with 50 ug of ammonium and purged immediately. The purged ammonia was trapped in 5.0 mL of acidic water and analyzed using cation chromatography. Data in Table 5 indicates that ammonia can be analyzed at a lower concentration. Lower method detection limits are anticipated by further method refinement.
Test # |
Ammonium (ug/mL) |
Nominal Amount |
Recovery |
Standard Deviation |
MDL (ug/mL) |
P10-3 |
8.5 |
10 |
85% |
0 |
10 |
P10-5 |
8.5 |
10 |
85% |
|
|
P10-7 |
8.5 |
10 |
85% |
|
|
P10-9 |
8.5 |
10 |
85% |
|
|
P10-11 |
8.5 |
10 |
85% |
|
|
Based upon the precision of ammonium recovery, a correction factor of 1.09 will be used to adjust analyte values which appear within the calibration range of detection. Values in Table 4, column 3 have been corrected using the correction factor. The overall Uncertainty of this method is expressed as the standard deviation about the arithmetic mean (%RSD) for ammonia recovery.
Test # |
Ammonium (ug/mL) |
Adjusted Value |
Spiked Amount |
Nominal Amount |
% Recovery |
%Uncertainty (n=5) |
P10-13 |
2.9 |
3.2 |
0 |
----- |
----- |
+2.2 |
P10-15 |
46.0 |
50.1 |
250uL |
50 |
100 |
|
P11-1 |
45.9 |
50.0 |
250uL |
50 |
100 |
|
P11-3 |
45.9 |
50.0 |
250uL |
50 |
100 |
|
P11-5 |
46.1 |
50.2 |
250uL |
50 |
100 |
|
P11-7 |
43.7 |
47.6 |
250uL |
50 |
95 |
|
Based upon the data presented in the method detection limit and accuracy/precision studies, analysis of ammonia by purge-and-trap cation chromatography is a viable method. Under the experimental conditions listed, this method has a low degree of variability and is robust enough for routine laboratory use.
Some noteworthy observations were made during this method development which will be investigated more fully. Ammonium is very strongly retained in a low-pH solution. However, it is easily converted to NH3(g) in an alkaline matrix at room temperature. Two solutions (A and B) were prepared at 1M NaOH and spiked with 250 ug of NH4+. The "A" solution was left open to the lab atmosphere, and the "B" solution was tightly capped. It was noted that the ammonium concentration decreased in container "A" but remained constant in container "B." Another solution was prepared at 1M NaOH, spiked with 5 mg of NH4+ and left open to the laboratory atmosphere. After one month, all ammonia had completely evolved from solution. It should be noted that dissolved NH3(g) readily evolves, over time, from an alkaline solution without agitation as a function of its vapor pressure at constant temperature. Moreover, when dissolved ammonia is left open to the atmosphere, NH3(g) evolution is sustained as the rate of vaporization exceeds the rate of condensation. Evolution of NH3(g) from solution is reduced or stopped when the rate of vaporization equals the rate of condensation. In addition, the speed of NH3(g) evolution from solution and changes in vapor pressure, under the ideal gas law would; therefore, be directly proportional to the temperature of the solution. These observations must be taken into consideration when sampling for ammonia. Exposure of alkaline samples to the atmosphere over time with respect to temperature and agitation will result in losses of ammonia during sampling. No significant losses have been observed when alkaline samples are tightly capped.
References
Appendix A