TOC Analysis of Brine Samples: Sample Load Performance Evaluation Introduction

TOC Analysis of Brine
Samples: Sample Load
Performance Evaluation
Application Note
By: Brian Wallace
Introduction
The concentration of organic and inorganic carbon in samples containing sodium chloride is of considerable
interest. Coastal cities find the intrusion of seawater into their fresh water supplies. Many industries must
analyze process and outfall water that contains salt. A catalytic combustion Total Organic Carbon (TOC)
Analyzer provides one of the few ways of measuring TOC in brine solutions accurately. Other methods of
oxidizing carbon experience severe interference from the high level of chloride ion.
Objective
Determine the maximum sample load of brine solution that can be run on the Apollo 9000 and Apollo 9000 HS for
TOC analysis before analytical problems occur.
Experimental
The Apollo 9000 high sensitivity (HS) combustion TOC Analyzer was used for analysis of brine samples (28%
NaCl) until data failed an accuracy criteria of 80 to 120% recovery. Reconditioned catalyst, combustion tube and
a new halide scrubber were installed on the unit before analysis began. 3/4 inch of quartz wool was packed
below and above the recommended 20g of catalyst used in the combustion tube. The copper and tin of the halide
scrubber were replaced every 100 injections of 28% NaCl solution to protect the Non-Dispersive Infrared (NDIR)
detector cell liner from halide buildup which could cause corrosion and poor results.
The sample analysis was performed in the following order:
Sample Type
DI Water
DI Water w/ 28% NaCl
1 ppmC KHP w/ 28% NaCl
1 ppmC KHP Check Standard
5 ppmC KHP Check Standard
Number of Repetitions per vial
10
10
10
3
3
Accuracy Challenge Test
The average results were monitored for accuracy compliance of 20% for the 1ppmC KHP/28%NaCl. A 10%
accuracy window was used for the 1 ppmC and 5 ppmC KHP standard.
Method Parameters
To optimize the Apollo 9000 and Apollo 9000 HS, an update disk may be obtained from Tekmar-Dohrmann at our
website www.tekmar.com or by calling Technical Support for assistance (1-800-874-2004). This update disk will
change some of the procedures used in the sample injection process.
TOC_A-004.doc; 11-Jun-03
Sales/Support: 800-874-2004 · Main: 513-229-7000
4736 Socialville Foster Rd., Mason, OH 45040
www.teledynetekmar.com
Figure 1 High Salt Method Parameters
The following method criteria were changed from the original method “TOC 0-20 ppmC”. Most notably, the sample
injection volume was changed to 200 µl and the sparger volume was changed to 100 µl. The time the carrier gas
is off after the sample is injected into the combustion chamber is also minimized.
Calibration Linearity
The Apollo HS has the ability to be calibrated from 0 ppm to 5000 ppmC. The calibration curve shown below
demonstrates the low-level capabilities of the Apollo HS, calibrating from 0 ppmC to 5 ppmC.
Calibration Factor:
Y Intercept (Raw Data):
r-squared :
Standard ID
Raw Data
DI Water
172414
1 ppmC KHP
601771
5 ppmC KHP
2463347
2.301 E+06
158685
0.99983
Expected µgC
0.000
0.200
1.000
Measured µgC
0.006
0.193
1.001
Table 1 Calibration Curve used for Brine Samples
Discussion
The reproducibility remained good on average throughout the test as seen in the low standard deviations and %
RSDs. However, as the analysis approached failure, the accuracy of the 5ppmC check standard decreased to
less than 90% recovery as observed on the 20 repetitions preceding the 1ppmC brine failure. Reproducibility and
accuracy decreased dramatically after the first failure, as observed in the higher standard deviations or % RSDs.
In addition to the decreased reproducibility, the sample peak shapes were poor on occasional outliers. As the
precision decreased, the number of poor sample peaks increased, which eventually caused the accuracy to fail.
Sample ID
DI Water
1 ppmC KHP
5 ppmC KHP
28% NaCl in DI Water
1 ppmC KHP/ 28% NaCl
Average Results up to Failure
Standard Deviation (ppm)
Results (ppm)
0.04702
0.9999
4.878
0.4477
1.430
0.04590
0.02547
0.08809
0.04048
0.04730
% RSD
N/A
2.55
1.84
N/A
3.32
Table 2 Sample Average Results before Failure
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Calibration Verifications (During Brine Sample Run)
Results of 1 ppmC KHP in 28% NaCl Solution (NaCl bkgd subtracted)
2
1.8
6
5.5
1.6
5
4.5
1.4
ppm C
ppm C
4
3.5
3
2.5
2
1.5
1
1.2
1
0.8
0.6
0.5
0.4
0
1
6
11
16
21
26
KHP Standards (3 repetitions each marker)
1 ppmC KHP Standard
5 ppmC KHP Standard
31
0.2
0
1
6
11
16
21
26
31
Brine Samples
(10 repetitions per marker)
Figure 2: Calibration Verifications for 1 ppmC and 5
ppmC KHP Standards during Brine Sample Run
Figure 3: 1 ppmC KHP in Brine Solution Accuracy Compliance Chart
Brine Sample Load Evaluation
By subtracting the DI Water w/ 28% NaCl from the 1 ppmC KHP w/ 28% NaCl, the spiked brine 1ppmC KHP
sample remained within 20% accuracy for:
ƒ 384 injections of 0.2 ml of 28% NaCl samples
ƒ A total of 23.016g of NaCl by weight
(The KHP standards in DI water analyzed between salt samples do not need to be included in the salt injection count).
At this rate, typical duplicate analyses would last:
ƒ 164 samples, using 0.2 ml sample injections per repetition, before the 23.016g of NaCl limit were achieved.
ƒ Averaging (40) 28%NaCl samples a day, the analysis would last 4 days before the 23.016g of NaCl limit were
achieved.
Conclusion
The Apollo 9000 and 9000HS has the impressive ability to run brine samples (+ 20% NaCl) and accumulate up to
23.01 g of NaCl in the combustion tube before standards and samples fail accuracy criteria. The warning signs of
salt overload are poor precision and peak shape of samples. The same sample load criteria can be applied to
samples with lower amounts of salt, such as seawater (3-5% NaCl).
Also, the furnace temperature should not exceed 680°C when running salty samples. Higher temperatures will
increase the devitrification rate of the quartz combustion tube (causing it to crystallize). Increased devitrification
may cause the combustion tube to break.
One must be aware that with increased deposition of salts onto the catalyst eventually causes incorrect response
for organics at about 23g of NaCl. Therefore, to allow a safety factor for the analysis of salty samples, the Apollo
9000 and 9000HS can tolerate a maximum of 15 to 20 g of NaCl before the combustion tube and catalyst must be
cleaned. Both combustion tube and catalyst should be cleaned with 2 N HCl solution and DI water per the Apollo
9000 User’s Manual.2 The combustion tube, o-rings and injection port should be inspected for aging and salt build
up when cleaned. Fresh quartz wool should be used after each cleaning.
Reconditioned catalyst has been tested under the same experimental conditions. The data generated with
catalyst that has been reconditioned was found to be comparable to data generated with new catalyst.
References
1.
2.
3.
4.
5.
6.
Brown, T., Bursten, B. and LeMay, H. Chemistry the Central Science (5th Edition). (Prentice-Hall, Inc. Englewood Cliffs,
NJ. 1991), pp. 121, 733-736.
“Apollo 9000 User’s Manual”, Tekmar, Mason, OH. 1999-2000.
Booth, R., “Measuring Carbon in Salty Waters”, Application Note (Tekmar), summer 99.
Wallace, B., “TOC Analysis of Low-level Samples: A Catalytic Combustion Approach”, Application Note (Tekmar).
Booth, R., Furlong, J. and Wallace, B., “Selection of a TOC Analyzer: Analytical Considerations”, Application Note
(Tekmar).
Wallace, B., “Common Total Organic Carbon Terms”, Application Note (Tekmar).
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