By: Karnel R. Walker, Dan Davis, and Robert H. Clifford, Ph.D.

The goal of this article is to demonstrate that the Shimadzu TOC-VW can effectively oxidize the Humic Acid (HA) matrix at high accuracy and precision levels never before witnessed by the wet chemical TOC community. The Shimadzu TOC-VW Carbon analyzer is the only TOC on the market that uses three oxidation techniques of UV light, heat, and persulfate in a single analyzer.1 

Due to this advancement in Wet Chemical TOC technology, difficult-to-oxidize compounds such as HA can now be efficiently analyzed with high levels of accuracy and precision. This makes life significantly easier for members of the drinking water community who prefer a wet chemical TOC analyzer to a combustion TOC analyzer.

Humic Acid is a highly complex polymeric substance found in virtually every scoop of soil. It can be characterized at best as a mixture of polymers containing aromatic and heterocyclic structures, carboxyl groups and nitrogen. In environmental water chemistry, HA is also of importance, since it is a natural stream pollutant. It is of extreme current interest because it is a disinfectant byproduct (DBP) precursor. DBPs are toxic and potentially carcinogenic.

With respect to TOC analysis, HA is considered to be the most difficult compound to measure due to its solubility characteristics and highly polymeric matrix. At pH <2, HA is completely insoluble in water. Thus, when performing the acidify and sparge method of TOC, otherwise known as Non-Purgeable Organic Carbon (NPOC), it can easily precipitate out of solution and cause significantly low recoveries. Also, because of the bulkiness of the HA molecule, it is extremely difficult to oxidize.

Standard Methods 5310 has established that for difficult matrices such as HA, the most suitable TOC method is high-temperature combustion. High-temperature combustion TOC can efficiently recover HA matrices in the range of 100%. In the past, wet chemical TOCs have failed miserably at measuring relatively high levels of HA. Other TOC manufacturers have reported HA recoveries at only 80% using conventional wet chemical methods.

A Shimadzu TOC-VC (Combustion) and TOC-VW (Wet Chemical) were configured for side by side analysis of HA solutions prepared at 1ppm, 10ppm, and 50ppm concentration levels. The NPOC method was incorporated for both instruments. All samples were automatically pH adjusted by the instrument to achieve optimum NPOC results. Known control samples were analyzed with each sample group.

Section 1: TOC-V Wet Chemical versus Combustion for 1ppm Humic Acid Solution

A 1 ppm HA solution was analyzed using the TOC-V Wet Chemical and Combustion analyzers. For each oxidation method, results were compared against a 1 ppm KHP control standard using a 0, 1, and 10 ppm KHP calibration curve. The 1 ppm HA gave actual concentrations of 1.017 and 1.011 ppm for the wet chemical and combustion, respectively.

Table 1:   1 ppm Humic Acid Analysis

Instrument       Actual (ppm) % Recovery

TOC-Vw             1.017     101.7

TOC-VC             1.011     101.1

Section 2: TOC-V Wet Chemical versus Combustion for 10ppm Humic Acid Solution

A 10 ppm HA solution was analyzed using the TOC-V Wet Chemical and Combustion analyzers. For each oxidation method, results were compared against a 10 ppm KHP control standard using the 0, 1, and 10 ppm KHP calibration curve. The 10 ppm HA gave actual concentrations of 9.731 and 10.07 ppm for the wet chemical and combustion, respectively.

Table 2:  10 ppm Humic Acid Analysis

Instrument       Actual (ppm) % Recovery

TOC-Vw             9.731     97.31

TOC-VC             10.07     100.7

Brand X—        9.04        90.4

UV/persulfate type    

Section 3: TOC-V Wet Chemical versus Combustion for 50ppm Humic Acid Solution  

A 50 ppm HA solution was analyzed using the TOC-V Wet Chemical and Combustion analyzers. For each oxidation method, results were compared against a 50 ppm KHP control standard using the 0, 10, and 50 ppm KHP calibration curve. The 50 ppm HA gave actual concentrations of 45.58 and 50.53 ppm for the wet chemical and combustion, respectively.

Table 3:  50 ppm Humic Acid Analysis

Instrument       Actual (ppm) % Recovery

TOC-Vw             45.58     91.16

TOC-VC             50.53     101.1

Brand X-UV/  8.18        81.8

persulfate type               

It is important to reiterate that in years past, HA has been  a difficult compound to oxidize at 1ppm and 10ppm levels. Achieving recoveries in the range of 90% for 10 ppm HA is typical of other commercially available wet chemical TOC analyzers. Thus, achieving recoveries close to 100% at 1 ppm and 10 ppm is truly an accomplishment for the TOC-VW. At higher levels of 50ppm and greater, it becomes even more evident that this substance poses a dilemma in TOC analysis. Other commercially available wet chemical methods show recoveries around 80% at the 50 ppm range.

Thus, achieving recoveries above 90% at the 50 ppm range is also a major achievement for the TOC-VW. Consider that most natural streams contain on the order of 5 ppm TOC, and HA comprises 50 to 70% of the TOC present in these waters. The TOC-VW is able to efficiently analyze on the order of ten to twenty times higher the amounts of HA typically found naturally in the environment.

Furthermore, this results in approximately a 10% increase in the recovery of HA across three orders of magnitude using the TOC-VW analyzer.

From analyzing the results of the HA solutions, it is shown that the TOC-VW analyzer efficiently oxidizes this complex molecule with high precision and accuracy.

The efficiency of the TOC-VW is witnessed in the fact that the HA molecule has extremely high recovery rates compared to previous TOC wet chemical instruments on the market. The fact that the wet chemical instrument can efficiently oxidize a large amount of a difficult-to-oxidize substance, such as HA, demonstrates the power of having three forms of oxidation, UV, heat and persulfate, all housed into a single analyzer.1  

For further information, phone Shimadzu at 800-477-1227.