Anal. Chem. 1900, 60,042-046
042
thermal lens formation, the measurements are susceptible to low-frequency fluctuations in the spatial profile of the probe beam (“spatial noise” (12)). I t has been suggested that the signals could be corrected for integrated intensity fluctuations in the beam by normalizing the spatial frequency domain results to the dc component. While the technique is certainly valid, it will not offer immunity to shape distortions which take place in the beam profile over time. The main source of spatial noise in the experiments reported here arises from low-frequency thermal fluctuations in the helium/neon laser cavity which distort the profile of the probe beam over time. I t may prove advantageous to measure transient changes in the spatial frequency spectrum of the probe beam although, a t present, there is no theoretical basis to indicate how these measurements should be carried out. In addition, the relatively slow readout rates of photodiode arrays suggest that a detection strategy involving the optical computation of the frequency spectrum of the probe beam may well yield results superior to those obtainable by using photodiode array detection. In summary, the present work demonstrates that analytical signals may be recovered from recordings of partially refracted probe beam profiles observed in mode-mismatched thermal lens experiments. Such techniques may provide up to an order of magnitude enhancement over conventional mode-matched thermal lensing using beam center measurement.
LITERATURE CITED Gordon, J. P.; Leite, R. C. C.; Moore, R. S.; Port0 S. P. S.; Whlnnery, J. R. J . ADD/. f h v s . 1065. 36. 3-8. HU, c.: wkinnery: J. R. A&. o p t . 1973, 72, 72-78, Leite, R. C. C.; Moore, R. S.; Whinnery, J. R. Appl. f h y s . Len. 1964, 5 , 141-143. Harris, J. M.; Dovichi, N. J. Anal. Cbem. 1080, 52,695A-700A. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1979, 57,728-730. Miyaishi, K.; Imasaka, T.: Ishibashi N. Anal. Cbim, Acta 1981, 724, 381-389. Igaishi, T.; Imasaka, T.; Ishibashi. N. Anal. Chem. 1983, 55, 1907- 19 10. Leslecki. M. L.; Drake, J. M. Appl. Opt. 1982, 21, 557-560. Power, J. F.; Langford, C. H. Anal. Chem., following paper in this issue. Buffle, J.; Deladoey, P.; Zumstein, J.; Haerdi, W.; Schweiz, 2 . Hydro/. 1082. 4 4 . 326-361. Miyaishi, K . ; Imasaka, T.; Ishibashi, N. Anal. Chem. 1082, 5 4 , 2039-2044. Jansen, K. L.; Harris, J. M. Anal. Chem. 1085, 57, 1698-1703. Siegman, A. E. An Introduction to Lasers & Masers: McGraw-Hill, New York, 1971; Chapter 8 . Berthoud, T.; Delorme. N.; Mauchlen, P. Anal. Chem. 1085, 5 7 , 1217- 1219. Nakanishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1085, 57, 1219-1223. Power, J. F., unpublished results. Bialkowski, S. E. Anal. Chem. 1086, 5 8 , 1706-1710. ~~
RECEIVED for review August 25, 1987. Accepted December 22,1987. The authors with to thank the Natural Sciences and Engineering Research Council for financial support of this project.
Optical Absorbance of Dissolved Organic Matter in Natural Water Studies Using the Thermal Lens Effect Joan F. Power*’ and Cooper H. Langford Chemistry Department, Concordia Uniuersity. 1455 West de Maisonneuue Boulevard, Montreal, Quebec, Canada H3G lM8
The thermal lens spectrometer described In the accompanying paper Is applled to cdorlmetrk determlnation of dlssdved organic matter (humlc substances) from a series of natural waters. The detection ilmit corresponds to dissolved organlc carbon (DOC) less than 300 pg L-‘ at a wavelength of 600 nm. As Is expected for thermal detection of absorbance, there Is no Interference in the measurement from light scatterlng; sample fHtratlon is not necessary. Analysis of the which characterlzes humlc absorbance parameter e ,/e materials, shows that it Is Independent of llght scattering but may be related to donor-acceptor complex formation in the polymers.
The common yellow to brown coloration of freshwaters is associated with dissolved organic matter (DOM) (1)of the type commonly referred to as “humic substances”. It is related to the organic matter of soil, but is only partly of soil origin. Typical freshwater concentrations may be in the range of 1-25 mg/L expressed as dissolved organic carbon (DOC). It has long been recognized that these materials play an important part in the speciation of trace metals in water (1,2) because Present address: Chemistry Department, McGill University, 801 Sherbrooke St. W., Montreal, Quebec, Canada H3A 2K6.
of their ligand sites for metal ion binding, and it is now recognized that most of these ions are associated with the colloidal particle size domain. Recently, it has become clear that light absorption by DOM can initiate a variety of photochemical processes (3), including solvated electron production, generation of oxidizing and reducing radicals, and production of triplet sensitizers. The interest in both metal ion binding and photochemistry raises important questions with respect to the measurement of optical absorbance of DOM. There are two difficult aspects to the analytical problem posed. First, the bands in the visible, especially the red, which distinguish the humic materials from other chromophores, which may be found in freshwater (e.g. small molecule metabolite and peptides), are weak and there is a sensitivity problem. Second, the interesting chromophores are frequently associated with particulates and absorbance must be distinguished from light scattering. Filtration to eliminate scattering particles is not acceptable, since filtering may entail the loss of an important part of the light-absorbing material sought. Thermooptic detection is the obvious approach to distinguishing light scattering from absorbance. The specific approach using the thermal lens effect promises the required sensitivity ( 4 ) . This paper discusses studies of several representative samples of organic matter extracted from freshwater and their comparison to a well-characterized soluble soil organic material, Armadale fulvic acid (5, 6).
0003-2700/88/0360-0642$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988 COLBECK
\#
LUTHER LAKE
843
Table I. Elemental Analysis Data for Freshwater Organic Materials sample
%C
%H
%N
%S
Wylde Lake Luther Lake Colbeck Big Creek Venison Creek Diedrich Creek Floradale Armadale fulvic acid
25.44 32.32 32.14 7.52 4.72 10.56 20.89 49.50
6.20 6.76 6.17 7.28 6.91 6.38 6.53 4.50
8.0 11.29
0.38
WYLDE LAKE-
FLORADALE
f
4 r‘
,/‘
‘.
\L
NIAGARA
DlEDRlCH CREEK VENISON CREEK
LAKE ERIE BIG CREEK
Figure 1. Map of sampling sites for extracted freshwater humic ma-
terials.
The studies focus on thermal lens detection of absorbance (as distinct from scattering) at the Ar-ion laser wavelength of 458 nm and the Rhodamine 6-G dye laser wavelength of 600 nm. These two wavelengths are chosen because of the usefulness of the E4/Es ratio (7)(which is the ratio of absorbance of humic substances at 465 and 665 nm) to the characterization of humic substances. Wavelengths convenient for laser spectroscopy allow calculation of a ratio, named e4/e6 here, which is similar to &/E6 and has parallel theoretical significance. That the spectrum of a humic is characterized by the ratio of absorbance a t only two wavelengths is a reflection of the fact that the absorbance of humic materials decreases smoothly from the near-UV to the red with no peaks or shoulders. The parameter E4/E6is a major tool used in the attempt to understand the origins of the broad featureless spectrum of humic materials. The parameter has been shown to be (i) correlated with particle size or molecular weight, (ii) sensitive to pH, (iii) correlated to free radical concentration, and (iv) concentration independent. The preferred pH for determination of the parameter is 7 to 8. We will show here that the ratio is not, as was once suggested, influenced by light scattering. Its behavior is determined by molecular features of absorption. The present results suggest a possible new model.
EXPERIMENTAL SECTION Samples and Materials. The sample set studied in this work incorporated freshwater materials originating from two general environments (see map, Figure 1): (a) Sandy low-carbon creeks. The surrounding land is agricultural with tobacco as the main crop. The organic carbon content of the soil is less than 2%. (Typical examples are the Venison, Diedrich, and Big Creek samples.) (b) Water bodies whose organic carbon content originated via runoff from soils derived from bedrock. These soils consist of fine clayey glacial tills or, in the southern sampling areas, coarse loamy tills. The organic carbon content of the soil is about 5% (e.g. Floradale). (c) Poorly drained areas giving rise to swamps and muskeg areas (Wylde Lake) with high carbon content. The surrounding land is mainly agricultural. Some examples of materials originating from this environment include the Colbeck, Wylde Lake, and Luther Lake samples. In addition to these materials, Armadale fulvic acid, a well-characterized standard humic substance (5,6), was studied. As a soil-derivedmaterial, its comparison with the
7.36 19.11 17.98 19.37 11.90 0.80
0.43 0.46 0.34 0.42 0.54 0.40
extracted freshwater materials with large run-off components is useful and it is a sample with a known functional group content. These freshwater samples were generously contributed by Dr. J. Carey of the National Water Research Laboratory, Canada Center for Inland Waters, Burlington, Ontario. The samples had been extracted as follows: The natural waters were filtered through 0.7 pm glass fiber microfdtersand pumped at 50 mL min-l through a column of XAD-2 macroreticular resin. No ultrafiltration was performed. The columns were eluted with 0.1 N ammonia in methanol. Filtrates were collected and evaporated to dryness. The freeze-dried samples were characterized by elemental analysis (Guelph Analytical Laboratories). The results are reported in Table I. Armadale fulvic acid was supplied by Mr. P. Aysola, Concordia University. The sample was extracted from the Bh horizon of a Prince Edward Island podzol. This sample has been described in detail elsewhere (5, 6, 8). Concentrated stock solutions of the humic materials were prepared on a weekly basis at concentrations of 100-400mg L-l. The stocks were stored in the dark at pH 2-4. Preliminary tests have shown the absorbance of humic materials to be stable over several weeks (8). Dilute solutions for analysis by thermal lensing were prepared daily, immediately prior to analysis. The “conventional spectrophotometer” absorbance of these samples was calculated from the measured absorbance of stocks assuming Beer’s law behavior still accounts properly for dilution at absorbance values too low for accurate measurement in conventional spectrophotometers. Measurements of sample pH were carried out with a meter constructed by Concordia University Science Technical Services. The meter was calibrated by use of Fisher and Canlab standard buffer solutions. Adjustment of the sample pH was made by addition of 0.100 M KOH or HCl (reagent grade) or, where indicated in early experiments on light scattering, by addition of standard buffer solutions. Water was either Nanopure H20 (conductivity 18 MQ cm) or glass double distilled. C0S04was used as an absorbance standard at 458 nm. CoS04.H20was recrystallized several times from hot water and 99% ethanol. Anhydrous CuS04 (Baker reagent) was recrystallized from hot water. The solvent was 1%aqueous H2S04. Stock solutions were prepared with absorbances of 0.100, as measured by visible spectrophotometry. Standards for thermal lensing were prepared by dilution of the stock. SiOzfor light-scatteringexperimentswas obtained as sea sand. The sand was prewashed in dilute chromic acid and thoroughly rinsed in distilled water. It was dried and ground to a fine powder in a mortar and pestle (particle size less than 10 wm). One- to two-milligram samples of Si02were weighed out and added to 10- or 25-mL samples of dissolved Armadale fulvic acid at a sample pH of 5-7. Apparatus. Visible absorption spectra of all concentrated stock solutions were recorded on a Perkin-Elmer 552 UV-visible spectrophotometer in a 1-cm glass cell. The thermal lens instrument used in these experiments is described in detail in the accompanying paper (4). The excitation source was a Coherent CR6 SupergraphiteIon laser equipped with a dye laser head (Coherent CR-590) and operated using Rhodamine 6G. The excitation wavelengths were 458,488, or 600 nm. The light scattering experiments also used “all line” argon ion excitation, consisting chiefly of 488-nm emission. The spectrometer was operated mainly in the mode-mismatched config-
844
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
Table 11. Absorptivity Data sample
Wylde Lake Luther Lake Colbeck Big Creek
Venison Creek Diedrich Creek
Floradale Armadale fulvic acid
e4, (g/L)-’ cm-l
e6, (g/L)-’ cm-’
0.987 f 0.076 0.341 f 0.011 0.434 f 0.020 0.046 f 0.005 0.043 f 0.002 0.081 f 0.010 0.221 f 0.0106 3.14 f 0.18
0.228 f 0.1 0.0614 f 0.0055 0.0639 f 0.0020 0.00793 f 0.0003 0.0064 f 0.0004 0.012 f 0.0033 0.0325 f 0.0028 0.8 f 0.0
uration ( 4 ) although early experiments on light scattering used mode-matched alignment. For e4/ee measurements,the spectrometer was calibrated with aqueous solutions of the absorbance standards C0S04and CuSO1. The range of absorbance of the standards was 5.00 x lo4 to 2.00 X Calibration curves of thermal lens response vs absorbance were linear (R1 0.999) at both wavelengths. The spectrometer was aligned and calibrated on a daily basis. The thermal lens response for the humic samples was directly converted to absorbance units from the calibration data.
RESULTS AND DISCUSSION Estimation of DOC and Humic Substances by Visible Spectrometry. The first test of thermal lensing as an analytical method for natural water applications is to determine the sensitivity of the method, ita tolerance to light scattering from particulates present in the sample, and the need for correction for fluorescence. With the detection limit defined as three times the standard deviation of the blank, the mode-mismatched thermal lens instrument is capable of detecting sample absorbances of 3 X in water in a cell path length of 1cm. This same detection limit was experimentally verified for dissolved humic materials at 458 nm and 600 nm using an excitation power of 50-60 mW. Calibration curves of thermal lens response vs absorbance for Armadale fulvic acid were run in parallel with measurements on the transmission standards CuSO,(aq) and CoSO,(aq). The thermal lens signal was reported as the relative change in the center intensity of a refracted probe beam before and after the formation of a steady-state thermal lens. This change is reported as I ( t = 0 ) - I(t = a)
-
_ . -
I,
I(t =
m)
where I is the center intensity of the beam; t = 0 and t = m correspond to the times of initial and steady-state conditions. This parameter is known to be linear in absorbance for weakly absorbing samples as is discussed in more detail in ref 9. The calibration curves showed the same value of slope, intercept, and correlation coefficient as the standards. This result demonstrates that the fluorescence quantum efficiency for the humic material excited at 450 nm is less than the error in the slopes of the calibration curves (
