## 5910 UV-ABSORBING ORGANIC CONSTITUENTS*

5910 A. Introduction

1. Applications



### Some organic compounds commonly found in water and wastewater, such as lignin, tannin, humic substances, and various aromatic compounds, strongly absorb ultraviolet (UV) radiation. UVabsorption is a useful surrogate measure of selected organic constituents in fresh waters, 1

–3 

### salt waters, 4

– 6 

### and wastewater. 7,8 

### Strong correlations may exist between UV absorption and organic carbon content, color, and precursors of trihalomethanes (THMs) and other disinfection by-products. 9,10 

### UV absorption also has been used to monitor industrial wastewater ef fluents 11 

### and to evaluate organic removal by coagulation, 10 

### carbon adsorption, 12,13,14 

### and other water treatment processes. 10 

### Specifi c absorption, the ratio of UV absorp

tion to organic carbon concentration, has been used to characterize natural organic matter. 10,15,16



### Although UV absorption can be used to detect certain individual organic contaminants after separation (e.g., by HPLC), as described in Part 6000, the method described here is not suitable for detection of trace concentrations of individual chemicals. It is intended to be used to provide an indication of the aggregate concentration of UV-absorbing organic constituents.

2. References

1. D OBBS , R.A., R.H. WISE & R.B. D EAN . 1972. The use of ultra-violet

absorbance for monitoring the total organic carbon of water and
wastewater. Water Res . 6:1173. 2. WILSON , A.L. 1959. Determination of fulvic acids in water. J. Appl. Chem . 9:501. 3. C OOPER , W.J. & J.C. Y OUNG . 1984. Chemical non-speci fi c organic analysis. In R.A. Minear & L. Keith, eds. Water Analysis, Vol. 3. Academic Press, New York, N.Y. 4. O GURA , N. & T. H ANYA . 1968. Ultra-violet absorbance of the seawater, in relation to organic and inorganic matters. Int. J. Oceanol. Limnol . 1:91. 5. O GURA , N. & T. H ANYA . 1968. Ultra-violet absorbance as an index of pollution of seawater. J. Water Pollut. Control Fed. 40:464. 6. F OSTER , P. & A.W. M ORRIS . 1971. The use of ultra-violet absorption measurements for the estimation of organic pollution in inshore sea
waters. Water Res. 5:19. 7. B UNCH , R.L., E.F. B ARTH & M.B. E TTINGER . 1961. Organic materials in secondary ef fl uents. J. Water Pollut. Control Fed. 33:122. 8. M RKVA , M. 1971. Use of ultra-violet spectrophotometry in the determination of organic impurities in sewage. Wasserwirtsch. Wassertech . 21:280. 9. S INGER , P.C., J.J. B ARRY , III, G.M. PALEN & A.E. S CRIVNER . 1981. Trihalomethane formation in North Carolina drinking waters. J. Amer. Water Works Assoc. 73:392. 10. EDZWALD , J.K., W.C. B ECKER & K.L. WATTIER . 1985. Surrogate parameters for monitoring organic matter and THM precursors. J. Amer. Water Works Assoc. 77(4):122. 11. B RAMER , H.C., M.J. WALSH & S.C. C ARUSO . 1966. Instrument for monitoring trace organic compounds in water. Water Sewage Works 113:275. 12. B ISHOP , D.F., L.S. MARSHALL , T.P. O ’ FARRELL , R.B. DEAN , B. O ’ C ONNOR , R.A. D OBBS , S.H. GRIGGS & R.V. VILLIERS . 1967. Studies on activated carbon treatment. J. Water Pollut. Control Fed. 39:188. 13. S ONTHEIMER , H.N., E. H EILKER , M.R. J EKEL , H. N OLTE & F.H. V OLIMER . 1978. The Mü lheim process. J. Amer. Water Works Assoc. 70(7):393. 14. S UMMERS , R.S., L. C UMMINGS , J. D EM ARCO , D.J. H ARTMAN , D. M ETZ , E.W. H OWE , B. M ACLEOD & M. S IMPSON . 1992. Standardized Protocol for the Evaluation of GAC. No. 90615, AWWA Research Foundation & American Water Works Assoc., Denver, Colo. 15. T HURMAN , E.M. 1985. Organic Geochemistry of Natural Waters. Martinus Nijhoff/Dr. W. Junk Publishers, Dordrecht, Netherlands. 16. O WEN , D.M., G.L. AMY & Z.K. C HOWDHURY . 1993. Characterization of Natural Organic Matter and Its Relationship to Treatability, No. 90631, AWWA Research Foundation & American Water Works Assoc., Denver, Colo. * Approved by Standard Methods Committee, 2000. Joint Task Group: 20th Edition

— James P. Malley, Jr. (chair), James K. Edzwald, Roger A. Yorton. UV-ABSORBING ORGANIC CONSTITUENTS (5910)/Introduction 5-71

## 5910 B. Ultraviolet Absorption Method

1. General Discussion



### a. Principle: UV-absorbing organic constituents in a sample absorb UV light in proportion to their concentration. Samples are fi ltered to control variations in UV absorption caused by particles. Adjustment of pH before fi ltration is optional.

UV absorption is measured at 253.7 nm (often rounded off to 254 nm). The choice of wavelength is arbitrary. Historically, 253.7 nm has been used as the standard wavelength; however, experienced analysts may choose a wavelength that minimizes interferences from compounds other than those of interest while maximizing absorption by the compound(s) of interest. If a wavelength other than 253.7 nm is used, state that wavelength when reporting results.

b. Interferences: The primary interferences in UV-absorption measurements are from colloidal particles, UV-absorbing organics other than those of interest, and UV-absorbing inorganics, notably ferrous iron, nitrate, nitrite, and bromide. Certain oxidants and reducing agents, such as ozone, chlorate, chlorite, chloramines, and thiosulfate, also will absorb ultraviolet light at 253.7 nm. Many natural waters and waters processed in drinking water treatment plants have been shown to be free of these interferences.

Evaluate and correct for UV absorption contributed by speci fi c interfering substances. If cumulative corrections exceed 10% of the total absorption, select an alternate wavelength and/or use another method. Because UV absorption by organic matter may vary at pH values below 4 or above 10, avoid these values. 1



### A UV absorption scan from 200 to 400 nm can be used to determine presence of interferences. Typical absorption scans ofnatural organic matter are featureless curves of increasing absorption with decreasing wavelength. Sharp peaks or irregularities in the absorption scan may be indicative of inorganic interferences orunexpected organic contaminants. Because many organic compounds in water and wastewater (e.g., carboxylic acids and carbohydrates) do not absorb signifi cantly in the UV wavelengths, correlate UV absorption to dissolved organic carbon (DOC) or soluble chemical oxygen demand (COD). However, use such correlations with care because they may vary from water to water, seasonally on the same water, and between raw and treated waters. In addition, chemical oxidation (e.g., ozonation, chlorination) of the organic material may reduce UV absorption without removing the organics and thus may change correlations. Because UV absorption and correlations with UV absorption are site-speci fi c, they may not be comparable from one water source to another.

c. Minimum detectable concentration: The minimum detectable concentration cannot be determined rigorously because this is a nonspeci fi c measurement. For precise measurement, select cell path length to provide an absorbance of approximately 0.005to 0.900. Alternatively, dilute high-strength samples. The minimum detectable concentration of a particular constituent dependson the relationship between UV absorption, the desired characteristic (e.g., trihalomethane formation potential or DOC), and any interfering substances.

Certain regulatory programs may limit holding time prior to analysis. Check applicable regulations before sample collection. 2. Apparatus



### a. Spectrophotometer, for use between 200 and 400 nm withmatched quartz cells providing a light path of 1 cm. For lowabsorbance samples use a path length of 5 or 10 cm. A scanning spectrophotometer is useful.

b. Filter: Use a glass- fi lter* without organic binder. Other fi lters that neither sorb UV-absorbing organics of interest nor leach interfering substances (e.g., nitrate or organics) into thewater may be used, especially if colloidal matter must be removed. Alternatively use fi lters of TFE, polycarbonate, or silver. Prerinse fi lter with sample of organic-free water to remove soluble impurities. If alternate separation techniques, fi lters or fi lter preparations are used, demonstrate that equivalent results are produced. Filter pore size will in fl uence test results, especially in raw waters. N OTE 

### : Certain regulatory usage of this method (e.g., EPA ’ s D/DBP Rule) may stipulate a speci fi c pore size. Check applicable regulation before selecting product.

c. Filter assembly , glass, TFE, or stainless steel, capable of holding the selected fi lters.

3. Reagents



### a. Organic-free water: Reagent water (see Section 1080) or equivalent water containing less than 0.05 mg DOC/L.

b. Hydrochloric acid (optional), HCl, 0.1 N

. c. Sodium hydroxide (optional), NaOH, 0.1 N

. d. Phosphate buffer (optional) : Dissolve 4.08 g dried anhydrous KH 2

### PO 4 

### and 2.84 g dried anhydrous Na 2

### HPO 4 

### in 800 mL organic-free water. Verify that pH is 7.0 and dilute to 1 L with organic-free water. Store in brown glass bottle at 4 ° C. Prepare fresh weekly or more frequently if microbial growth is observed.

4. Procedure



### a. Sample volume: Select sample volume on basis of the cell path length or dilution required to produce a UV absorbance between 0.005 and 0.900. For most applications a 50-mL sample is adequate. Use 100 mL sample if a 10-cm cell path length is required.

b. Sample preparation: Wash fi lter and fi lter assembly by passing at least 50 mL organic-free water through the fi lter. For speci fi c applications and correlations, sample pH may be adjusted with HCl or NaOH. In poorly buffered samples an appropriate non-UV-absorbing buffer system such as a phosphate buffer may be used. Take care to avoid precipitate formation during pH adjustment. UV absorbance of fulvic acid solutions apparently remains constant between pH 4 and 10. 1 

### Report sample pH value used with recorded absorbance. Once sample pH has been adjusted and/or measured, fi lter sample. Prepare an organic-free water blank and the sample in an identical manner.

c. Spectrophotometric measurement: Let spectrophotometer equilibrate according to manufacturer ’ s instructions. Set wave-

* Whatman grade 934AH; Gelman type A/E; Millipore type AP40; ED Scienti fi c Specialties grade 161; or other products that give demonstrably equivalent results. Practical fi lter diameters are 2.2 to 4.7 cm. 5-72 AGGREGATE ORGANIC CONSTITUENTS (5000)

### length to 253.7 nm and adjust spectrophotometer to read zero absorbance with the organic-free water blank. Measure UV absorbance at 253.7 nm of at least two fi ltered portions of sample at room temperature.

5. Calculation



### Report mean UV absorption in units of cm  1 

### using the fol

lowing notation. To report units in m -1 

### multiply the equation by one hundred.

UV  pH

 

#  A  b 

# D 

### where:

UV  pH  mean UV absorption, cm  1 (subscript denotes wavelength used, nm, and superscript denotes pH used if other than 7.0),
b  cell path length, cm, A   mean absorbance measured, and D  dilution factor resulting from pH adjustment and/or dilution with organic-free water.

D  final sample volume initial sample volume



### Correct results for absorption contributed by known interfering substances. If UV absorption contributed by interfering substances exceeds 10% of the total UV absorption do not use UV absorption at 253.7 nm as an indicator of organics. T ABLE 5910:I. PRECISION OF UV ANALYSES AND C ORRELATION TO KHP S AMPLES

Analysis UV 254 Result for Given KHP Sample Concentration*

cm  1

0.54 0.93 1.79 4.87 9.61 25.0 50.0 100.0
Laboratory 1 0.008 0.015 0.034 0.079 0.158 0.323 0.638 1.282 Laboratory 2 0.009 0.016 0.026 0.070 0.134 0.401 0.803 1.612 Laboratory 3 0.010 0.017 0.027 0.081 0.161 0.353 0.695 1.343 Laboratory 4 0.007 0.020 0.033 0.070 0.132 0.319 0.750 1.590 Laboratory 5 0.009 0.018 0.030 0.087 0.140 0.394 0.643 1.447 Mean 0.0086 0.0142 0.0300 0.0774 0.1450 0.3580 0.7058 1.4548 Standard deviation 0.0011 0.0019 0.0035 0.0074 0.0136 0.0384 0.0708 0.1461 % Relative standard

deviation † 12.8 11.1 11.7 9.56 9.38 10.7 10.0 10.0
* KHP sample concentration mg/L as C, measured as in Section 5310C. † The percent relative standard deviation is given by:

% RSD  

#  standard deviation ( S )
mean ( X  ) 

# 100

T ABLE 5910:II. S INGLE -O PERATOR PRECISION FOR UV A BSORPTION M EASUREMENTS OF FULVIC A CID S OLUTIONS

Replicate

No. Result

cm  1

DOC  2.5 mg/L DOC  4.9 mg/L DOC  10.0 mg/L

1 0.110 0.240 0.480 2 0.120 0.230 0.480 3 0.110 0.240 0.470 4 0.100 0.230 0.480 5 0.110 0.240 0.480 6 0.100 0.240 0.470 7 0.110 0.240 0.480 8 0.110 0.230 0.480 9 0.120 0.240 0.480 10 0.110 0.240 0.480 Mean 0.110 0.237 0.478 Standard deviation 0.00667 0.00483 0.00422 % Relative standard

deviation 6.06 2.05 0.882 UV-ABSORBING ORGANIC CONSTITUENTS (5910)/Ultraviolet Absorption Method 5-73

### 6. Quality Control



### a. Replicate measurements: Use at least two portions of fi ltered sample.

b. Duplicate analyses: Analyze every tenth sample in duplicate (i.e., duplicating the entire procedure) to assess method precision.

c. Baseline absorbance: Check system baseline UV absorbance at least after every 10 samples by measuring the absorbance of an organic-free water blank. A non-zero absorbancereading for the blank may indicate need for cell cleaning, problems with the reference cell if a dual-beam instrument is being used, or variation in the spectrophotometer response caused by heating or power fl uctuations over time.

d. Spectrophotometer check: Dif fi culties in comparing UV absorption data from different spectrophotometers have been reported. Potassium hydrogen phthalate (KHP), also known aspotassium biphthalate, standards were prepared in pH 7, phosphate-buffered (3 d) reagent water without acidi fi cation (see Section 5310B.3 c ) and analyzed in fi ve laboratories. The results are shown in Table 5910:I; these data suggest acceptable precision.These data also are useful for checking spectrophotometer results with KHP standards commonly used for TOC and/or COD analysis. A correlation equation for this 40-sample data set is:

UV 254  0.0144 KHP 0.0018



### with correlation coeffi cient ( r2 

### )  0.987, UV 254 

### expressed in

cm  1 

### , and KHP expressed as mg/L as C.

This equation can assist in verifying spectrophotometer performance. For example, if a set of UV 254 

### analyses is performed and the results are in the 0.010 range, prepare a KHP standard of 0.5 mg/L as C. The projected UV 254 

### of this KHP standard would be 0.009 cm  1 

### . If the measured UV 254 

### is outside 13% relative standard deviation (RSD) of 0.009 cm  1 

### , the spectrophotometermay be suspect and require maintenance. The correlation between UV 254 

### and KHP standards is presented solely as a useful means of verifying spectrophotometer performance. 7. Precision and Bias



### Table 5910:I shows interlaboratory precision data for 40 KHP samples. The percent relative standard deviations (% RSD) ranged from 9.38 to 12.8.

Single-operator precision data are presented in Table 5910:II for fulvic acid solutions. 2 

### The % RSD ranged from 0.9 to 6%. Because UV absorption is an aggregate measure of organic carbon, true standards are not available and bias cannot be determined.

The precision of analyses by this method was determined under the Information Collection Rule. 3 

### Precision was deter

mined as relative percent difference for duplicate analyses and was calculated only when both analyses in the duplicate pair showed concentrations at, or greater than, the ICR minimum reporting level (MRL  0.009 cm  1 

### ). Results were as follows: 4

Data Quality Percentile

Variable N 10 25 50 (median) 75 90
Precision (RPD) 33 306* 0.0 0.0 0.0 2.2 4.9

* 2744 excluded

— both samples less than MRL. N OTE : Median sample result was 0.040 cm

 1

. 

### 8. References

1. EDZWALD , J.K., W.C. B ECKER & K.L. WATTIER . 1985. Surrogate

parameters for monitoring organic matter and THM precursors. J. Amer. Water Works Assoc. 77(4):122. 2. MALLEY , J.P., JR . 1988. A Fundamental Study of Dissolved Air Flotation for Treatment of Low Turbidity Waters Containing Natural Organic Matter. Ph.D. thesis (unpublished), Univ. Massachusetts, Amherst. 3. U.S. ENVIRONMENTAL PROTECTION A GENCY . 1996. Monitoring requirements for Public Drinking Water Supplies: Cryptosporidium, Giardia, Viruses, Disinfection Byproducts, Water Treatment Plant Data and Other Information Requirements. Final Rule. Federal Register

. 61(94):24354. 4. FAIR , P.S., R. K. S ORRELL , M. KARAPONDO , B.J. B OHL , C.P. FREBIS & M.A. FEIGE . 2002. Quality of ICR monitoring data. In M. J. McGuire, J. McLain & A. Oblensky, eds. Information Collection Rule DataAnalysis. American Water Works Assoc. Research Foundation, Denver, Colo. 5-74 AGGREGATE ORGANIC CONSTITUENTS (5000)