1. business and industrial
2. energy
3. coal

TOC Analysis of Humic Acid:
Sample Preparation is the Key
Application Note
By: Brian Wallace
Introduction
Measuring the amount of natural organic matter (NOM) in sources of surface waters has been of particular
interest for municipal drinking water purification and wastewater facilities. When raw water containing NOM is
chlorinated in the disinfection process, potential carcinogens known as disinfection by-products (DBPs), such as
trihalomethanes and haloacetic acids, are formed. In an effort to reduce NOM before disinfection, a direct
correlation between total organic carbon (TOC) and NOM has been established per Standard Methods 5310 and
the USEPA’s disinfectants/disinfection by-products rule. With the increased importance placed on TOC analysis
of source water, a historical question of how much NOM can be recovered using TOC has resurfaced. Humic
acid, a component of NOM, has been suggested as a standard for mimicking NOM in the laboratory. However
historically, persulfate oxidation TOC analyzers have shown an acceptable recovery rate, 80–85%, for this difficult
compound.1,2 Modern persulfate TOC analyzers have shown even better recovery, 90–95%, with the use of
optimized UV lamps and stronger concentrations of the persulfate reagent. However, no paper to date has fully
explored and documented the root causes for variations in recovery in these analyzers.
In this paper, the optimization of a modern UV persulfate TOC analyzer for humic acid recovery was evaluated
with a comparison to a modern, high temperature combustion (HTC) TOC analyzer. In the performance of TOC
analysis, UV persulfate instrumentation demonstrated 95% recovery of humic acid consistently across a linear
range of 1 to 100 ppmC, the range typically found in the NOM of source water. Experimental data showed the key
challenge for increasing humic acid recovery was representative sample transfer of this particulated matrix, after
the acidification employed in the inorganic carbon (IC) removal step, rather than oxidation of the humic acid.
Humic Acid
Humic Acid is a component of soil and NOM. Some of the questions of recovery in the past have existed because
of the highly complex polymeric properties of humic acids that can vary in form and structure depending on
environmental conditions. For instance, humic acids may fold in structure at high ionic strengths and open at low
ionic strengths.3 Structures of a neutral molecule may differ from those found in soils and sediments because of
the highly deprotonated properties of humic acids in aqueous environments.4 Also, complexation between humic
acids and metals or other cations can alter a predicted structure. 5
Humic acid is often used as a key compound for recovery studies. However, making and maintaining accurate
humic acid standards is very difficult due to the nature of the sample matrix. Humic acid is insoluble when pH is <
2.6 Therefore when a sample of humic acid is acidified, it is believed that humic acid becomes colloidal and either
precipitates and settles, or flocculates and floats.7, 8
IC Removal
The two typical TOC oxidation techniques traditionally have different ways of removing IC interferences from the
sample. Both techniques must acidify to a pH < 2 to convert all carbonate and bicarbonate forms of IC into
carbonic acid, as seen in figure 1. The carbonic acid, also known as dissolved CO2, is then sparged from the
sample before the TOC oxidation step. For the HTC instrumentation, the humic acid sample is acidified and
sparged to remove IC within the sample vial. This process also stirs the sample before it is injected into the
combustion tube, so that it includes insoluble forms of humic acid. With UV persulfate analysis, the humic acid
sample is transferred from the sample vial to the IC sparge chamber on the instrument. The sample is then
acidified and sparged to remove IC. Sequentially, the ‘sparged’ sample is then transferred to the UV reactor for
TOC oxidation. It is theorized that this transfer process may leave a portion of the humic acid sample on the walls
of the IC sparger chamber and transfer tubing resulting in lower humic acid recovery.
TOC_AP-009.doc; 29-Jul-03
Sales/Support: 800-874-2004 · Main: 513-229-7000
4736 Socialville Foster Rd., Mason, OH 45040
www.teledynetekmar.com
Figure 1. CO2 Distribution in Water versus pH 9
Method and Results: TOC Analysis Comparison
To prove the theory of sample retention in the IC sparger with the UV persulfate technique, a few experiments
were conducted with the Phoenix 8000 UV persulfate TOC analyzer. In the first experiment, IC removal from a
humic acid sample was performed in the UV reactor with the UV Lamp off. After IC removal was complete, the UV
lamp was turned on and persulfate reagent was added to oxidize the organic carbon. This process would include
the insoluble forms of humic acid. Wallace et al. found that this process was effective at recovering 100% of the
humic acid TOC portion at 10 ppmC in comparison to HTC oxidation.10 Similar benefits with the UV persulfate
technique were attained by Kaplan by extending the length of the oxidation process to increase the oxidation
efficiency for humic acids.11 These technical breakthroughs did display an improvement versus older UV
persulfate oxidation methodologies. However, having the UV lamp turn off and on during every analysis increases
the general time of analysis and may cause increased maintenance on the UV lamp. Therefore, another
alternative method was investigated with the UV persulfate technique that would allow the UV lamp to remain on
during the entire analysis.
A second experiment was conducted which combined removing IC in the sample vial and extending the allowable
oxidation time from 4 minutes to 6 minutes. All humic acid samples were run side-by-side for comparison with a
HTC oxidation TOC analyzer, the Apollo 9000, at various concentrations exceeding those which might occur in
NOM samples (typically 1–20 ppmC TOC concentration). Reagent grade purified water was used as a solvent for
a variety of concentrations of the sodium salt of humic acid, from Sigma-Aldrich Chemical Company, which had a
carbon assay value of 39.03% carbon.12 Each sample was analyzed with both HTC and UV persulfate oxidation
techniques with at least 3 replicates per sample. Precision results are displayed in table 1 and a linearity
comparison between HTC and UV persulfate is exhibited in figure 2.
TOC_AP-009.doc; 29-Jul-03
Page 2 of 4
HTC
Sample (n>3)
Result
(ppmC)
STD
(+/- ppmC)
UV Persulfate
RSD
Result
(ppmC)
STD
(+/- ppmC)
RSD
Recovery
1 ppmC Humic Acid
0.9864
0.0251
2.54%
0.9470
0.0295
3.11%
96.0%
10 ppmC Humic Acid
9.8076
0.1442
1.47%
9.3884
0.1408
1.50%
95.7%
50 ppmC Humic Acid
49.1629
1.4946
3.04%
46.7475
1.7343
3.71%
95.1%
100 ppmC Humic Acid
97.7801
1.4374
1.47%
93.9626
1.1088
1.18%
96.1%
200 ppmC Humic Acid
185.7058
1.0484
0.56%
170.907
0.1709
0.10%
92.0%
Table 1. Humic Acid TOC Recovery – HTC vs. UV Persulfate. All samples were acidified and the IC removed
from the sample vial on the autosampler. The maximum oxidation time was increased from 4 minutes (default) to
6 minutes on both oxidation techniques.
Figure 2. Linear regression of UV Persulfate versus HTC TOC results. Samples include TOC concentrations of
humic acid at 1, 10, 50, 100 and 200 ppmC.
The linear regression of humic acid sample recovery confirmed that the improvements of the second experiment
gave quality results across TOC concentrations 5 to 10 times that found in NOM. These results show that both
oxidation techniques for TOC analysis could be used for drinking water samples. However, persulfate oxidation
has much lower detection limits, more consistent blanks and a lower amount of instrument maintenance in
comparison to the HTC oxidation making it the superior choice for this application.
TOC_AP-009.doc; 29-Jul-03
Page 3 of 4
Relationship between Humic Acid and Trihalomethanes (THMs)
Because humic acid is partially insoluble at a pH < 2, the particulate TOC fraction has historically been measured
with greater accuracy using HTC oxidation technologies rather than UV persulfate. As the data from table 1 and
figure 2 shows, modern, optimized UV persulfate TOC analyzers include the particulate portion of NOM in the
analysis of TOC. Is, however, the particulate fraction of TOC from compounds in NOM actually important to the
prediction of disinfectant by-product (DBP) formation?
In a recent study of the disinfection of natural waters, dissolved fractions of TOC, known as dissolved organic
carbon (DOC), demonstrated a better prediction of DBP formation, compared to TOC measurements that
included the particulate fractions. Najm, et al., studied the relationship between TOC and DOC as predictors of
DBP formation.13 Water samples spiked with NOM were analyzed for TOC before disinfection and for
trihalomethanes (THMs), a DBP, after a disinfection period of 24 hours. All samples were measured with and
without filtration. Their findings demonstrated a significant relationship between filtered samples. On unfiltered
samples, the formation of THMs, compared to TOC measured, yielded a ratio of less than 5 µg THMs/mg TOC.
Whereas, samples, that were filtered through a 0.45 µm filter before analysis, yielded a significantly higher THMsto-DOC ratio of between 20 and 42 µg THMs/mg DOC. They concluded that DOC was a more appropriate
indicator of DBP formation potential in water treatment, than TOC. They theorized that the reaction between
disinfectants and leached-DOC content actually formed DBPs, rather than the particulate fraction of TOC.13 In
some sense, particulate fractions of TOC, such as in humic acid, may actually become an interference to
predicting THM’s generation.
In most drinking water utilities, particles are removed during the treatment process.14 Turbidity of settled water is
usually less than 1–2 NTU.13 Therefore, it is thought that DOC, not TOC, is actually what is being measured at the
point of disinfection. Consequently, the use of DOC samples created either by filtration in the laboratory or by
particulate settling in the treatment process, is commonly used for sample preparation and analysis. As described
in previously mentioned studies, a reduction in disinfectant by-product formation is most likely obtained by
decreasing DOC concentrations through appropriate water treatment.
Conclusions
The test results clearly show that improving sample transfer of particulated humic acid is the key to higher
recoveries. Using similar sample handling techniques, both persulfate and HTC oxidation technologies
demonstrated comparable results for humic acid TOC analysis. However, superior low-level TOC accuracy and
precision with less maintenance makes the persulfate technique the best technique for drinking water analysis.
Although not studied in this paper, future research should be directed towards the relationship between
disinfectant byproducts, such as trihalomethanes and haloacetic acids, and their precursors—most notably,
determining if there is a preference between TOC (with particulate fraction) or DOC for predicting disinfectant byproduct formation.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Koprivnjak, J.F., et al., Water Res., 1995, 29, 91
Wei, Y., Reavis, J., Meadows, C., Woung, L., and Greenlee, J., Proc. Water Qual Technol. Conf., AWWA, San Diego, CA, 1998, 2-E3 (on
CD-ROM).
Davis, J.A., Adsorption of natural dissolved organic matter at the oxide/water interface. Geochim. Cosomochim. Acta., 1982, 46, 2381.
Gaffney, J.S., Marley, N.A., and Clark, S.B., “Humic and fulvic acids and organic colloidal materials in the environment. In: Humic and
Fulvic Acids. Isolation, Structure and Environmental Role, ACS Symp. Ser., American Chemical Society, Washington, DC., 651, 1996, 2.
Kubicki, J.D., and Apitz, S.E., “Models of Natural Organic Matter and Interactions with Organic Contaminants”. Organic Geochemistry,
1999, 30, 911.
Schulten, H., and Schnitzer, M., “Chemical Model Structures for Soil Organic Matter and Soils”. Soil Science, 162, 115.
Langmuir, D., Aqueous Environmental Geochemistry, Prentice-Hall, Upper Saddle River, NJ, USA, 1992, 161.
Manahan, S.E., Environmental Chemistry, Lewis/CRC, Boca Raton, FL, USA, 6th edn., 1994, 80.
Joyce, R., Takahashi, Y. and Wirth, L., “Application of TOC Analyzer to the ppb-Level Measurement of CO2 Trapped in Ammoniated
Steam Condensate” Application Note (Tekmar-Dohrmann) Technical Reprint – 011, Rev. July 1991.
Wallace, B., Purcell, M., and Furlong, J., “Total Organic Carbon Analysis as a Precursor to Disinfection Byproducts in Potable Water:
Oxidation Technique Considerations”. J. Environ. Monit., 2002, 4,35.
Kaplan, L.A., Limnol. Oceanogr., 1992, 37, 1119.
Humic Acid Sodium Salt, CAS 68131044, Lot 04607BI. Retrieved July 24, 2003 from www.sigmaaldrich.com.
Najm, I., Marcinko, J., and Oppenheimer, J., J. Am. Water Works Assoc., 2000, 92,8,84.
Ruehl, K., Water Eng. Manage., 1999,146, 22.
TOC_AP-009.doc; 29-Jul-03
Page 4 of 4