Monitoring THMs and TOC in High-Purity Process Water Systems

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1 World Leader in Total Organic Carbon Analysis Model A-1000/A-1000XP/Access 643 Total Organic Carbon Analysis Systems Monitoring THMs and TOC in High-Purity Process Water Systems Introduction Trihalomethanes (THMs) are potential byproducts of the drinking water disinfection process and are formed when naturally occurring organic materials found in water react with chlorine, the most common drinking water disinfectant. These byproducts have been implicated to be carcinogenic 1 and are therefore regulated by the EPA through the Disinfection Byproducts Rule. Current EPA regulations allow no more than 80 ppb of total THMs in drinking water supplies. The amount of THMs allowed in drinking water supplies is important because of their impact on the source water or feedwater for process water purification systems in the semiconductor and pharmaceutical industries. By law the pharmaceutical industry is required to prepare water for injection and purified water from water complying with US EPA National Primary Drinking Water Regulations. 2 The semi-conductor industry also uses water that complies with these same drinking water regulations. Controlling THM Formation in Drinking Water The concentration of THMs in drinking water is controlled by monitoring the reduction in TOC between raw water and treated water. The Disinfection Byproducts Rule specifies a percentage removal of TOC by enhanced coagulation. Removal efficiencies from 20-50% are required based on the value of TOC and alkalinity concentration of the raw untreated water. Monitoring TOC in Drinking Water and High- Purity Water Systems TOC is monitored in drinking water facilities as outlined in the D/DBP Rule. 3 A common method of analysis is Standard Method 5310C. 4 Semiconductor manufacturing facilities continuously monitor and control TOC levels in their high-purity water systems due to the detrimental effects that TOC has on product yields. Pharmaceutical manufacturers must perform TOC testing according to USP Method <643> 2 and EP Method Semiconductor and pharmaceutical facilities most commonly use conductometric based TOC measurement technologies for low level TOC analysis. In all cases, TOC measurements are used as a process control attribute to monitor the performance of unit operations comprising the water s purification and distribution system. An increase in TOC levels can indicate the possible presence of THMs in high-purity water systems. THM Detection Methods and Sensitivity Gas Chromatography (GC) is the most commonly used method for analyzing total THMs and individual compound levels in water samples. The sensitivity of traditional GC methods is approximately 0.5 ppb. A new, high-sensitivity method combining solid phase microextraction (SPME) and GC-MS has been developed with detection as low as 0.04 ppb. 6 This new technique has been developed for determining the THM contamination in high-purity water systems. Key Words: Disinfection Byproducts Rule Trihalomethanes (THMs) Total Organic Carbon High-Purity Water Oxidation Efficiency

2 Examples of Removal Efficiency of THMs by High-Purity Process Water Systems Table 1 shows the results of two water system designs tested for THM removal as measured by SPME/GCMS. System 1 is a small laboratory system. The DI make-up loop includes a 5 µm filter, followed by an activated carbon unit, RO unit, EDI, storage tank, a mixed bed DI column and a 0.2 µm final filter. The polishing system contains activated carbon, mixed bed DI and UV. System 2 is a medium sized ultrapure water system. The DI make-up loop consists of multimedia, prefilter, RO, mixed bed DI and UV. The DI polishing loop on this system includes UV, mixed bed DI and a final filter. Sampling Point System 1 (total THM ppb) System 2 (total THM ppb) Incoming city water DI Make-up 16 not detected DI Polish System 1 (no UV) DI Polish System 2 (with UV) DI Polish Table 1: SPME-GCMS determination of THMs in high-purity water systems The data show that System 1 had more than the maximum allowed level of THMs (80 ppb) for its incoming city water. In the DI make-up loop the THM concentration was reduced to 16 ppb and consisted totally of chloroform. 6 Final polishing with UV reduced the chloroform levels to 0.06 ppb. This result demonstrates that THMs can be easily oxidized by UV and that the use of UV in the purification process dramatically reduces THM levels. System 2 had much lower THMs levels in the incoming city water and THMs were not detected after treatment. However, a small amount of THMs, namely chloroform, was found in the polished DI water. It was speculated that another source of chloroform existed in the polishing loop. These results show that THMs can be effectively removed by standard high-purity water purification processes, but can also reappear if the purification systems are not properly maintained. Ultrapure Water TOC and THM Contaminant Guidelines for Semiconductor Facilities Balazs Analytical Laboratory has published Ultrapure Water Monitoring Guidelines 7 for the year 2000 and beyond that have demonstrated acceptable product yields for semiconductor companies. Listed below are the maximum acceptable levels of TOC and total THMs compared to the line width of a device. Modern water purification operations such as those discussed in Table 1, can remove THMs down to or below these maximum acceptable levels. Test 16M 0.5 µm 64M 0.35 µm 256M 0.25 µm 1G 0.18 µm 4G 0.13 µm Resistivity TOC (ppb) On-line 2 2 < 2 < 2 < 1 THMs (ppb) < 2 < 2 < 2 < 2 < 1 Table 2: Balazs ultrapure water TOC and THM contaminant guidelines

3 Conductivity Based TOC Measurement Systems For low level TOC measurements, semiconductor and pharmaceutical water systems utilize conductometric based detection systems. Some organizations 7 have reported erroneous and falsely high TOC results in the presence of THMs from Anatel analyzers, without considering some very important experimental factors. The primary issues to be addressed are just how accurate are these TOC analyzers at such low concentration levels of both TOC and THMs, and the impact of THMs on their measurement accuracy. Two common conductometric based TOC analyzer designs are A) stop-flow, UV/TiO 2 oxidation, with differential conductivity measurement and B) continuous flow, UV oxidation, CO 2 membrane separation with differential conductivity measurement. Advantages and disadvantages of each system are as follows: Analyzer A (A-1000/A-1000XP/Access 643) Advantages Disadvantages On-line, no sample contamination, trending Limited resistivity range Limits of detection (parts per trillion levels) No Reagents or gases Analyzer B (Sievers Model 800, Model 400, Ultrapure PPT) Advantages On-line, no sample contamination, trending Limits of detection Disadvantages Limited resistivity range Cannot prove complete oxidation No oxidation catalyst in low range Incomplete CO 2 transfer through membrane A dosing experiment was conducted to evaluate the accuracy and oxidation efficiency of an Anatel A-1000XP and Sievers Model 800 analyzer. Background levels of TOC in an ultrapure water loop were determined with each instrument to generate a baseline and then a known TOC concentration was injected into the loop. A schematic of the experimental setup is shown in Figure 1. Sucrose and isopropyl alcohol were tested at 1 ppb, 5 ppb, 25 ppb, 50 ppb, 75 ppb and 100 ppb as carbon. Sucrose was selected to demonstrate oxidation efficiencies with a compound not typically found in water systems, and IPA was chosen because it can be a contaminant at points-of use (POU) where TOC measurements are performed. Figure 1: Dosing loop experimental setup

4 The A-1000XP routinely returned accurate (within 2% recovery) TOC values for all concentrations tested independent of the type of organic compound measured. The Sievers Model 800 consistently underreported (< 80% recovery at 25 ppb and below; <90% recovery at 50 ppb to 100 ppb) TOC values again independent of the type of organic compound. Representative results from a 25 ppb dose of sucrose and IPA are shown in Figures 2 and 3 respectively. Data collected from this dosing study indicate that the A-1000XP, due to its stop-flow method of analysis coupled with complete oxidation using dynamic endpoint detection, provides accurate and repeatable TOC values. The Sievers Model 800 because of its flow through method of analysis, reports lower than expected TOC values, presumably due to the instrument s reliance on an analytically inaccurate single-point calibration curve and/or partial oxidation of the TOC in the water. Figure 2: 25 ppb TOC Dose of Sucrose Figure 3: 25 ppb TOC Dose of IPA

5 Analyzing THMs with On-line Conductivity based TOC Analyzers As was stated previously, The Environmental Protection Agency (EPA) has set total THM concentration limits in all drinking waters at a maximum of 80 ppb. Chloroform is the most common THM. If it is assumed that all the THM present is chloroform, then 80 ppb of chloroform would be the limit that could be present in drinking water. Chloroform is 10% carbon: CHCl 3 = 121 g/mol 1 C = 12 g/mol (12gmol - /121 gmol - )100 = 10% 80 ppb of chloroform is 8 ppb as carbon. Table 1 shows the effectiveness of ultrapure water systems in removing THMs. Therefore, realistic levels of chloroform at the POU where on-line conductivity based TOC analyzers are operating should be close to 0 ppb or sub ppb levels. Using the same dosing setup as for the accuracy studies, a 9.24 ppb solution of chloroform was injected into the dosing loop and TOC measurements were taken with the A-1000XP and the Sievers Model 800. A 9.24 ppb chloroform solution contains ppb as carbon. Table 3 shows the average TOC values and percent error obtained from each analyzer. The A-1000XP read approximately 3 times higher than expected and the Sievers Model 800 read 1.5 times lower than expected. The following equation illustrates the oxidation of chloroform: CHCl 3 + HOH 1CO 2 + 3HCl H + + HCO (H + + Cl - ) Each H + ion contributes ~ 350 µs/mmol, the Cl - contributes ~ 60 µs/mmol, and the HCO 3 - adds approximately 45 µs/mmol. If only 1 mole each of H + and HCO 3 - were produced during oxidation the resulting conductivity would be: H + + HCO 3 - = 350 µs/mmol + 45 µs/mmol = 395 µs/mmol Since 3 moles each of H + + Cl - are also produced during oxidation, the resulting solution total conductivity is: H + + HCO H + + 3Cl - = 350 µs/mmol + 45 µs/mmol + 3(350) µs/mmol + 3(60 µs/mmol) = 1625 µs/mmol The presence of 3 moles of HCl elevates the solution conductivity by 1230 µs/mmol, which is approximately 3 times more than if only 1 mole each of H + + HCO 3 - were present. The A1000XP, and related products, determine TOC concentration by measuring the difference between an initial and final conductivity reading after oxidation. The oxidation of chloroform, using the A-1000XP, produces an elevated TOC reading as much as 3 times the actual value due to the additional 1230 µs/mmol of conductivity that is generated by the HCl. TOC levels in excess of 3 times the expected amount are not possible due to the chemistry previously demonstrated. Chloroform has limited solubility in water and does not dissolve easily. An emulsion is formed when preparing a solution of chloroform. This emulsified solution requires sonication for extended periods to dissolve the chloroform. If the chloroform is analyzed as an emulsion, extremely high TOC readings will result because it is not completely dissolved in the water. Abnormally high percent recoveries in excess of 3000%, as previously published by Ionics Sievers in MICRO magazine 8 and elsewhere are totally inaccurate and physically impossible. These high percent recoveries are due to the incomplete solubility of chloroform in the test solutions. A-1000XP Sievers Model ppb 0.62 ppb % error = 291% % error = 33% Table 3: Dosing of ppb chloroform as carbon The 1.5 times lower than expected % recovery by the Sievers Model 800 is assumed to be a result of partial oxidation due to its flow-though technology and its single point calibration. Both analyzers have trouble measuring TOC in the presence of THMs. According to the Balazs ultrapure water THM contaminant guidelines, 7 less than 2 ppb THM should be present in ultrapure water systems. At this level, the TOC concentration would be 0.2 ppb or less. Realistically, the A-1000XP, and associated products, could read approximately 0.6 ppb TOC. According to the results presented above, the Sievers Model 800 could read as low as 0.07 ppb ppb is the published detection limit for the Sievers Model 800. For standard analytical techniques, the limit of quantitation (LOQ), where reliable and accurate data can be obtained, is defined as 3 times the method detection limit. For the Sievers Model 800, the LOQ would be 0.15 ppb. Therefore, the Sievers Model 800 can not accurately measure or report THMs at the levels that are required to be in ultrapure water because the measured value is less than their LOQ.

6 Conclusions The accuracy of measuring TOC in ultrapure water systems using on-line conductivity based TOC analyzers is dependent upon the analysis techniques used to measure the organic carbon content. Reliable and accurate measurements of ultrapure water that contains THMs at recommended guideline levels (<0.2 ppb) cannot be achieved with the Sievers Model 800. At these levels, the Sievers Model 800 will report TOC concentrations that are less than the instrument s limit of quantitation. The flow-through technology of the Sievers Model 800 allows only partial oxidation of the organics in the water. This partial oxidation, in combination with a single-point calibration, results in under-reported TOC values. The Anatel A-1000XP, and related products, report TOC concentrations that are approximately 3 times higher than the actual water THM levels. The measurements from the A-1000XP, and related products, are reliable and predictable based on water chemistry and the instrument s stop-flow, dynamic endpoint analysis technique. The A-1000XP, and related products, completely oxidize the organics in the water, but in the presence of THMs, in particular chloroform, the instrument over-reports TOC values. At best, a false positive result obtained with an Anatel A- 1000XP, or other related products, will alert personnel if there is an upset or undesirable contaminants in the water system. Depending on the magnitude of the upset, the Sievers Model 800 will not indicate that an upset has occurred. If THMs are present, the most accurate way of determining actual concentration levels is to use GC-MS. References 1. Environmental Protection Agency s Information Collection Rule and disinfection byproducts monitoring; Federal Register 61, USP 23 Fifth Supplement; November 15, PHARMEUROPA; vol. 11, No. 1. March United States Environmental Protection Agency s Office of Ground water and Drinking Water; Microbial and Disinfection Byproduct Rules: Stage 1 D/DBP Rule, EPA 815-F , December 1998 ( th edition of Standard Methods for the Examination of Water and Wastewater, American Public Health Association, 1015 Fifteenth Street NW, Washington D.C Wang, K, and Samantha Tan, Solid Phase Micro-extraction and GC-MS Analysis of Trihalomethanes in Ultrapure Water Systems ; ChemTrace Corp; Presented at WATERTECH 99 Conference, October 5-6, Ultra Pure Water Monitoring Guidelines 2000; Balazs Analytical Laboratory, Sunnyvale, CA; Franklin, K. et al. Verifying the Accuracy of Low-Level TOC Measurements ; MICRO magazine, vol. 17, No. 8. September