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. UV

absorption 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 effluents11 and to evaluate organic

removal by coagulation,10 carbon adsorption,12,13,14 and other water

treatment processes.10 Specific absorption, the ratio of UV absorption 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. DOBBS, R.A., R.H. WISE & R.B. DEAN. 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. COOPER, W.J. & J.C. YOUNG. 1984. Chemical non-specific organic

analysis. In R.A. Minear & L. Keith, eds. Water Analysis, Vol. 3.

Academic Press, New York, N.Y.

4. OGURA, N. & T. HANYA. 1968. Ultra-violet absorbance of the seawater, in relation to organic and inorganic matters. Int. J. Oceanol.

Limnol. 1:91.

5. OGURA, N. & T. HANYA. 1968. Ultra-violet absorbance as an index

of pollution of seawater. J. Water Pollut. Control Fed. 40:464.

6. FOSTER, P. & A.W. MORRIS. 1971. The use of ultra-violet absorption

measurements for the estimation of organic pollution in inshore sea

waters. Water Res. 5:19.

7. BUNCH, R.L., E.F. BARTH & M.B. ETTINGER. 1961. Organic materials

in secondary effluents. J. Water Pollut. Control Fed. 33:122.

8. MRKVA, M. 1971. Use of ultra-violet spectrophotometry in the

determination of organic impurities in sewage. Wasserwirtsch. Wassertech. 21:280.

9. SINGER, P.C., J.J. BARRY, III, G.M. PALEN & A.E. SCRIVNER. 1981.

Trihalomethane formation in North Carolina drinking waters.

J. Amer. Water Works Assoc. 73:392.

10. EDZWALD, J.K., W.C. BECKER & K.L. WATTIER. 1985. Surrogate

parameters for monitoring organic matter and THM precursors.

J. Amer. Water Works Assoc. 77(4):122.

11. BRAMER, H.C., M.J. WALSH & S.C. CARUSO. 1966. Instrument for

monitoring trace organic compounds in water. Water Sewage Works

113:275.

12. BISHOP, D.F., L.S. MARSHALL, T.P. O’FARRELL, R.B. DEAN, B.

O’CONNOR, R.A. DOBBS, S.H. GRIGGS & R.V. VILLIERS. 1967. Studies

on activated carbon treatment. J. Water Pollut. Control Fed. 39:188.

13. SONTHEIMER, H.N., E. HEILKER, M.R. JEKEL, H. NOLTE & F.H.

VOLIMER. 1978. The Mülheim process. J. Amer. Water Works Assoc.

70(7):393.

14. SUMMERS, R.S., L. CUMMINGS, J. DEMARCO, D.J. HARTMAN, D. METZ,

E.W. HOWE, B. MACLEOD & M. SIMPSON. 1992. Standardized Protocol for the Evaluation of GAC. No. 90615, AWWA Research

Foundation & American Water Works Assoc., Denver, Colo.

15. THURMAN, E.M. 1985. Organic Geochemistry of Natural Waters.

Martinus Nijhoff/Dr. W. Junk Publishers, Dordrecht, Netherlands.

16. OWEN, D.M., G.L. AMY & Z.K. CHOWDHURY. 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

filtered to control variations in UV absorption caused by particles. Adjustment of pH before filtration 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 specific

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 of

natural 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 or

unexpected organic contaminants. Because many organic compounds in water and wastewater (e.g., carboxylic acids and carbohydrates) do not absorb significantly 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-specific, 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 nonspecific measurement. For precise measurement, select

cell path length to provide an absorbance of approximately 0.005

to 0.900. Alternatively, dilute high-strength samples. The minimum detectable concentration of a particular constituent depends

on 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 with

matched 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-filter* without organic binder. Other

filters that neither sorb UV-absorbing organics of interest nor

leach interfering substances (e.g., nitrate or organics) into the

water may be used, especially if colloidal matter must be removed. Alternatively use filters of TFE, polycarbonate, or silver.

Prerinse filter with sample of organic-free water to remove

soluble impurities. If alternate separation techniques, filters or

filter preparations are used, demonstrate that equivalent results

are produced. Filter pore size will influence test results, especially in raw waters. NOTE: Certain regulatory usage of this

method (e.g., EPA’s D/DBP Rule) may stipulate a specific pore

size. Check applicable regulation before selecting product.

c. Filter assembly, glass, TFE, or stainless steel, capable of

holding the selected filters.

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, 0.1N.

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

d. Phosphate buffer (optional): Dissolve 4.08 g dried anhydrous KH2PO4 and 2.84 g dried anhydrous Na2HPO4 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 filter and filter assembly by

passing at least 50 mL organic-free water through the filter. For

specific 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, filter 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 Scientific

Specialties grade 161; or other products that give demonstrably equivalent results.

Practical filter 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 filtered portions of sample

at room temperature.

5. Calculation

Report mean UV absorption in units of cm1 using the following notation. To report units in m-1 multiply the equation by

one hundred.

UV

pH

  A



b D

where:

UV

pH  mean UV absorption, cm1 (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.

TABLE 5910:I. PRECISION OF UV ANALYSES AND CORRELATION TO KHP SAMPLES

Analysis

UV254 Result for Given KHP Sample Concentration*

cm1

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

TABLE 5910:II. SINGLE-OPERATOR PRECISION FOR UV ABSORPTION MEASUREMENTS OF FULVIC ACID SOLUTIONS

Replicate

No.

Result

cm1

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 filtered 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 absorbance

reading 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 fluctuations over time.

d. Spectrophotometer check: Difficulties in comparing UV

absorption data from different spectrophotometers have been

reported. Potassium hydrogen phthalate (KHP), also known as

potassium biphthalate, standards were prepared in pH 7, phosphate-buffered (3d) reagent water without acidification (see Section 5310B.3c) and analyzed in five 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:

UV254  0.0144 KHP 0.0018

with correlation coefficient (r

2

)  0.987, UV254 expressed in

cm1

, and KHP expressed as mg/L as C.

This equation can assist in verifying spectrophotometer performance. For example, if a set of UV254 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 UV254 of this KHP standard would

be 0.009 cm1

. If the measured UV254 is outside 13% relative

standard deviation (RSD) of 0.009 cm1

, the spectrophotometer

may be suspect and require maintenance. The correlation between UV254 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 determined 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 cm1

). 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.

NOTE: Median sample result was 0.040 cm1

.

8. References

1. EDZWALD, J.K., W.C. BECKER & 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 AGENCY. 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. SORRELL, M. KARAPONDO, B.J. BOHL, C.P. FREBIS &

M.A. FEIGE. 2002. Quality of ICR monitoring data. In M. J. McGuire,

J. McLain & A. Oblensky, eds. Information Collection Rule Data

Analysis. American Water Works Assoc. Research Foundation, Denver, Colo.

5-74 AGGREGATE ORGANIC CONSTITUENTS (5000)