Development of a portable laser-induced fluorescence system used for in situ measurements of dissolved organic matter

Optics & Laser Technology 64 (2014) 213–219

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Development of a portable laser-induced fluorescence system used for in situ measurements of dissolved organic matter Peng Chen a,b, Delu Pan a,b,n, Zhihua Mao b a

State Key Laboratory of Information Engineering in Surveying Mapping and Remote Sensing, Wuhan University, Wuhan 430072, China State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration, 36 Bochubeilu, Hangzhou 310012, China

b

art ic l e i nf o

a b s t r a c t

Article history: Received 18 March 2014 Received in revised form 5 May 2014 Accepted 23 May 2014

A portable laser-induced fluorescence system (total weight about 1.7 kg) has been developed for detecting the level of dissolved organic matter in water. The portable system consists of a high pulse repetition frequency (10-kHz) microchip laser at 405 nm, a reflective fluorescent probe and a broadband micro spectrometer. The stability and sensitivity of the new instrument were studied. The detection limit of this new instrument was 0.75 ug/L, and its baseline drift was only 0.45% per hour. Field results showed that the portable system can work as well as commercial laboratory spectrophotometers. A significant correlation (R2 ¼ 0.96) was found between measurements taken by the new instrument and thoses measured by laboratory spectrophotometer. In addtion, influence of temperature variation on measurements by the new instrument was investigated. The portable system is promising for monitoring dissolved organic matter in water, especially in the field. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Lasers Fluorescence spectroscopy Dissolved organic matter

1. Introduction Chromophoric dissolved organic matter (CDOM) represents the colored fraction of dissolved organic pool, which absorbs lights in the visible as well as the UV ranges. CDOM can strongly influences ocean optical properties, remotely sensed spectra and biogeochemical processes. The significance of CDOM to biogeochemical processes has been studied by many researchers [1–3]. CDOM is operationally defined as the colored fraction of material passing through a 0.2 μm filter and quantified by the absorption coefficient, ag(λ), as measured on filtered samples using a spectrophotometer or absorption meter [4]. Like other absorption measurements, fluorescence measurement is another wellknown tool for determining CDOM, which can be acquired more rapidly and with greater sensitivity in a variety of marine and freshwater applications. Various light sources, such as lasers, light emitting diodes, and lamps, can be used to stimulate fluorescence of aquatic constituents. The spectrally narrow laser emission provides improved selectivity and efficiency of excitation, and reduces the spectral overlap between the water Raman scattering and fluorescence bands of aquatic constituents [5].

n Corresponding author at: State Key Laboratory of Information Engineering in Surveying Mapping and Remote Sensing, Wuhan University, Wuhan 430072, China. E-mail address: [email protected] (D. Pan).

http://dx.doi.org/10.1016/j.optlastec.2014.05.021 0030-3992/& 2014 Elsevier Ltd. All rights reserved.

The laser-induced fluorescence (LIF) technique is a well-known analytical technique for rapid water environment monitoring, which is based on the measurements of laser-induced water emission spectrum, to obtain qualitative and quantitative information about the in-situ fluorescent constituents. In the past two decades, a variety of shipboard, shore-based, and airborne laser fluorescence systems were developed for analyzing oil pollution [6–8], phytoplankton [9–13], water transparency and turbidity [14–16], and characterization of CDOM [17–22]. However, many such instruments are too bulky for routine use in the field, and some detectors use only one or a few channels to measure one specific parameter (e.g., chlorophyll-a, CDOM, oil, or variable fluorescence) and do not provide full and detailed spectral information about other fluorescent constituents for more comprehensive characterization of aquatic environment [23,24]. To our knowledge, the utilization of LIF technique in the East China Sea (ECS) has rarely been studied. The ECS water belongs to Class II water, influenced by outflows from the Yangtze River and the Hangzhou Bay, which contain high concentration of suspended sediment. The optical properties of Class II water are significantly influenced by constituents such as phytoplankton, mineral particles, CDOM, or microbubbles, whose concentrations do not covary with phytoplankton concentration. They are often affected by terrigenous material output. Class II water are normally encountered present in coastal zones (estuaries, shelf areas, inlets, etc.) and possibly far from the coast in the case of extended shelves or shallow banks [25,26].

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Compared with traditional measuring methods for CDOM monitoring, LIF technique has the advantage of rapidly acquiring highresolution in situ profiles [27]. Studying the utility of LIF technique in the ECS is, therefore, of significance. The goal of this paper is to describe a novel portable LIF system and to utilize it for monitoring CDOM level in Chinese coastal and inland waters. A light-weight (about 1.7 kg) LIF system for fluorescence measurements was developed, which consists of a high pulse repetition frequency microchip laser at 405 nm, a reflective fluorescent probe and a broadband micro spectrometer. The signal to noise ratio of fluorescence spectroscopy instrument can be increased using high pulse repetition frequency laser [28]. A reflective fluorescent probe containing a dichroic beamsplitter could effectively eliminte the elastic scattering from fluorescence spectra. A broadband microspectrometer could provide full, detailed spectral information about fluorescent constituents in complex aquatic environments. In order to assess the performance of this new system, we compare the measurements taken by the instrument to those by laboratory absorption spectrophotometer or by spectrofluorometer. In addtion, the stability and sensitivity of the new instrument are studied in the lab. Influence of temperature variation on LIF measurement is investigated as well. Applications of the new system for water monitoring studies are illustrated by in situ measurements obtained in the Yangtze River Estuary and its adjacent sea and those obtained in the West Lake.

2. Instrument and methods 2.1. Experimental setup Fig. 1 shows the schematic of the setup of the portable LIF system. It consists of an excitation source module, a fluorescent probe module, a sample holder module, and a detection module. A micro violet laser with 405 nm wavelength (MM-405-100, Boson Tech) is selected as the excitation source, which is a mini size laser with the excitation wavelength in violet band. The laser has high pulse repetition frequency rate (greater than 10 k-Hz) and low power consumption (about 10 w); the size of the laser is 85 mm  32 mm  31 mm, and the weight is about 0.3 kg. A power supply module (PSU-H-FDA, Boson Tech) is used to provide stable power and to control the temperature of the laser source; it weighs 0.8 kg. A broadband microCCD spectrometer (USB4000, Ocean Optics) is integrated to record complete visible LIF emission PC for data acquisition Fluorescent probe

Spectrometer L4

L1

Fig. 2. Photograph of the optical system.

spectra with a range from 360 to 1000 nm, and with a full width at half maximum of 10 nm; its weight is about 0.2 kg. A fiber-optic fluorescent probe is used as the receiver in the fluorescence measuring system. A dichroic beamsplitter in the fluorescent probe is used to eliminte the elastic scattering from the laserstimulated emission. Three focusing lens are used for excited light collimation, and a collecting lens is used for emission collection. A sample pool is made from a quartz of the size 12.5 mm  12.5 mm  45 mm. The length of the internal optical path of sample space is 10 mm. Its four sides are polished and transparent. A reflecting mirror (74-msp, Ocean Optics) under the sample pool is used to enhance fluorescence signals. The SpectraSuite software (Ocean Optics software Inc.) is used to record fluorescence spectra. It provides a complete control of setting the parameters for all system functions, such as acquiring data, designing graph display and using spectra overlays. During measurement, the output of the laser is focused though a fluoresent probe into the quartz sample cell; then the emitted fluorescence light passes through a dichroic beamsplitter in the fluorescent probe and is collected by a collection quartz lenses onto the entrance slits of a spectrometer, which is interfaced to a computer through a USB port. The LIF spectral integration time is typically preset to 0.1–1 s, and the number of acquisitions is typically preset to 5–25, in order to average the fluorescence spectra. Each sample measurement typically takes 30–50 s. The corresponding water Raman signal is used for fluorescence normalization that accounts for highly variable optical properties of natural waters [29–32]. A photograph of the optical system is shown in Fig. 2. One can see that the whole system is light in weight, and is small in size. It is easy for transport and can be used aboard a small boat to sample organics and other fluorescing compounds in waters.

F1

405nm laser

2.2. Sensitivity and stability studies

L2

L3

MC

M1

Fig. 1. System chart of the LIF system. MC – measurement cell; F1 – Dichroic beamsplitter; L1, L2 and L4 – focusing lenses; L3 – collecting lens; M1 – reflecting mirror.

A fluorescence standard reference material, quinine sulfate [21,33], was used to measure the performance of the new fluorescence system. To measure the detection limit of the portable LIF system, the fluorescence of quinine sulfate (dissolved in 0.1-N sulfuric acid) was measured by varying the concentration of the sample from 0.15 to 15 ug/L. Replicate measurements were taken to create calibration curves of quinine sulfate at various concentrations. In addtion, in order to verify the stability of the LIF system, the baseline drift was measured, which is time variation of the peak baseline from a straight line. Measurements of Milli-Q water by the system were carried out once every second for one minute, and once every minute for one hour.

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Fig. 3. Distribution of sampling stations in the Yangtze River Estuary and its adjacent sea in March 2013.

2.3. Field test of the laser-induced fluorescence A total of 49 field samples were measured by the LIF system, including 25 samples in the Yangtze River Estuary and its adjacent sea, and the rest in the West Lake. The samples from the Yangtze River Estuary area were obtained in March 2013; the locations of the sampling sites are plotted in Fig. 3. The water samples from the West Lake were collected in November 2012 near 30.2461N, 120.1461E. Temperature, turbidity, PH, and salinity were obtained by a multi-parameter water quality monitor (Manta, Eureka Inc.) during the cruises. Water samples were collected from surface using a 1.5 L Plexiglas hydrophore during the cruises, and then stored into 1 L ambered glass bottles. These samples were then filtered through 0.2 um nucleopore membrane filters and kept refrigerated for absorption analysis later. In-situ fluorescence measurements were obtained using the LIF system. In order to assess the performance of the LIF system, comparisons with other laboratory systems (spectrofluorometer, absorption spectrophotometer) were carried out. Laboratory fluorescence measurements were obtained using a Shimadzu RF-5301 spectrofluorometer (Shimadzu Inc.). Prior to the measurements, the instrument was warmed up for 25 min. We then opened the software and checked the parameter settings (e.g., saving path, spectral number, scanning speed, excitation wavelength, and emission wavelength). The samples were measured in a 1 cm quartz cell, and the instrument was scanned over the range of 300–900 nm, with a spectral resolution of 1 nm. A Milli-Q water blank correction was applied to eliminate water Raman peaks. Absorption measurements were analyzed using a spectrophotometer after the cruises, approximately three days after the samples were taken. A PerkinElmer Lambada35 dual beam spectrophotometer (PerkinElmer Lambada35, Shimadzu Inc.) was used to measure the absorbance spectra. Prior to the measurements, the spectrophotometer was warmed up for more than 30 min. The samples were measured in 10 cm quartz cells and baseline corrected by Milli-Q water every five samples [34,35]. Absorbance was then converted into absorption coefficient (m  1) using the expression ag(λ)¼ 2.3  A(λ)/L [36], where A(λ) is the absorbance (optical density) and L¼ 0.1 m is the quartz cell's path length in meters.

Fig. 4. Examples of LIF measurements in various types of water. The overlapping spectra is integrated by the water Raman band at 470 nm and the CDOM constituent band at 508 nm.

2.4. Influence of temperature variation on LIF measurements The influence of temperature variation on LIF measurements in water was investigated in the laboratory. A beaker containing a water sample was first cooled to 4 1C in a dark refrigerator and then transferred to an incubator (HHS-1, Changfeng Tech), where they were gradually warmed to 30.5 1C over a period of 1.2 h withconstant stirring. Measurements of fluorescence and temperature were simultaneously logged at 1-min interval as the beaker was being warmed up.

3. Results 3.1. LIF measurements in various types of water Examples of LIF measurements in various types of water are presented in Fig. 4. The emission spectra by the laser fluorometer can be regarded as an overlapping spectra integrated by the water

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the detection limit of the instrument. Fig. 6 shows a linear response for concentrations ranging from approximately 0.75–15 ug/L. For concentrations lower than 0.75 ug/L, the signal falls off from

Raman band, the CDOM constituent band and different pigment bands. The peak nearby 470 nm corresponds to Raman scattering in water, and the peak nearby 508 nm corresponds to CDOM constituent. It reveals the capacity of the LIF technique for monitoring various types of water. The pure water without CDOM constituent has no CDOM peak, while the tap water has a small CDOM peak, which reveals that the tap water contains some CDOM constituent. Less apparent chlorophyll-a peak at 685 nm can be seen in the fluorescence spectra of the coastal water.

3.2. Sensitivity and stability studies In order to study the stability of the LIF system, we investigated the baseline drifts at the water Raman band in an hour and in a minute, respectively. The fluorescence spectrum of water Raman in an hour (Fig. 5a) shows that the water Raman peak is at 470 nm, and that there is little intensity variation in Raman peak over time. Fig. 5b shows the baseline drift over a minute, and Fig. 5c represents the baseline drift over an hour. We can see that the Raman peak baseline drifts 0.10% per minute and 0.45% per hour, respectively. The results show that the stability of the LIF system is good. In order to study the sensitivity of the LIF system, we investigated the calibration curve of quinine sulfate measured by the system. The linearity between fluorescence intensity and concentration indicates

Fig. 6. Calibration curve of quinine sulfate from fluorescence peak points at concentrations of 0.15–15 ug/L: the open circles indicate a linear response for concentrations ranging from approximately 0.75 to 15 ug/; the black triangles mean the signal falls off from the linearity when concentrations are lower than 0.75 ug/L.

Fluorescence spectrum of water Raman in an hour

fluorescence intensity(counts)

x 10

4

2 1.5 1

Water Raman peak

0.5 0 490 480

Wa

470

vele

ngth

(nm

460

)

450

0

10

20

30

40

Time(m

50

60

70

inute)

Fig. 5. Baseline drifts of the LIF system in an hour and in a minute: (a) the fluorescence spectrum of water Raman in an hour; (b) the baseline drift over a minute; and (c) the baseline drift over an hour.

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Fig. 7. Correlations between in situ LIF measurements and laboratory measurements by commercial instruments. (a) Correlation analysis between LIF measurements and absorption measured by an absorption spectrophotometer of the samples taken in the Yangtze River Estuary and its adjacent sea (N ¼ 25); and (b) correlation analysis between LIF measurements and fluorescence measured by a laboratory spectrofluorometer of the samples taken in the West Lake (N ¼ 24).

A temperature–fluorescence correction function was used to standardize CDOM measurements to a reference temperature [37,38]: IðTÞ ¼ IðT 0 Þ½1 þ ρðT  T 0 Þ

ð1Þ

where T is temperature, T0 is the reference temperature, I(T) and I (T0) respectively are the fluorescence intensity at T and T0, and ρ is the temperature coefficient. A range of 4–30 1C was used to find the temperature–fluorescence coefficient for the system. In this study, we found ρ¼  0.0119 with T0 ¼20 1C through our lab experiments. As Fig. 8 shows, the influence of temperature variation on LIF measurements can be corrected using Eq. (1), with T0 ¼ 20 1C and ρ¼  0.0119.

4. Discussion

Fig. 8. Influence of temperature variation on LIF measurements. The solid circle shows that fluorescence intensity decreases gradually as temperature increases; the open circle shows how the effect of temperature can be removed when the raw data are adjusted to a reference temperature of 20 1C using the equation I(T)¼ I(T0) [1 þρ(T  T0)], where ρ ¼  0.0119.

the linearity. The detection limit of the LIF system is estimated to be approximately 0.75 ug/L. 3.3. LIF measurements vs. commercial instruments' measurements Fig. 7 compares field LIF measurements and laboratory measurements by commercial instruments. Fig. 7a shows the correlation between LIF intensity (in Raman units) at CDOM band and absorption at 355 nm measured by an absorption spectrophotometer of the samples taken from the Yangtze River Estuary and its adjacent sea. The correlation (R2 ¼0.96) is significant. Fig. 7b shows the correlation between LIF measurements and fluorescence values measured by a laboratory spectrofluorometer of the samples taken from the West Lake. There is a high correlation (R2 ¼ 0.93) as well. These high correlations indicate the portable LIF system can work as well as the commercial instruments. 3.4. Influence of temperature variation on LIF measurements Fig. 8 shows that the fluorescence data measured by the LIF system are linearly correlated with temperature, and that the fluorescence intensity decreases gradually as temperature increases.

The LIF system was tested for in situ measurements of CDOM fluorescence. Using the laser fluorometer, in situ CDOM fluorescence profiles could be obtained rapidly. The high correlation between LIF measurements and commercial instruments' measurements (Fig. 7) confirms this new instrument is a good tool for CDOM monitoring. In particular, the significant correlation (Fig. 7a; R2 ¼0.96) between absorption coefficient at 355 nm and LIF intensity revealed that the fluorescence measured by the LIF system can be used as a good proxy for time-consuming absorption measurement. However, it should be taken into consideration that there are limitations when using any fluorescence method as a water quality analysis tool. For example, not all portions of CDOM have fluorescence properties; therefore, the nonfluorescing portions are not included in the data analysis, which can be problematic as these portions often contain revealing information about the sample [39]. Coupled with other kinds of monitoring instruments (e.g., Wetlabs ac9), the new system should work well in aquatic environments. In this study, we investigated LIF measurements in different types of water. As can be seen from Fig. 4, besides pure water, the spectra of other samples possessed two obvious peaks, centered at 470 nm and at 500–520 nm. Their peak heights, or peak shapes, differed, indicating variation in both species and amount of organic matters present in the samples. The spectra of the tap water sample in Fig. 4 has a small CDOM peak. It indicates the tap water may not be appropriate for drinking without being boiled first. The LIF sensor has the potential to be used as a warning tracer for providing an early warning of any potentially dangerous change in the CDOM levels or other spectral signatures [28,40]. Other laboratory tests

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using conventional water monitoring instruments could then be used to confirm the finding. The LIF fluorescence was found to decrease as temperature increases; on average, it decreases by 1.1%/1C relative to the intensity at the reference temperature of 20 1C (Fig. 8). This finding is similar to those reported in previous studies [37,38,41,42]. During the cruises in the Yangtze River Estuary and its adjacent sea, sea water temperature varied between 14.0 and 15.3 1C, which, assuming an invariant CDOM concentration, caused relative changes of up to 1.4% in fluorescence intensity. The influence of temperature variation on fluorescence measurements is small over a short time-period. In measurements at sea between summer and winter, the variation in water temperature can reach 10–15 1C, which can cause relatively large changes of up to 16% in fluorescence intensity. For long-term field deployments or in very dynamic environments, it is therefore necessary to correct the influence of large temperature change on CDOM fluorescence quantification. In our LIF field test in the Yangtze River Estuary and its adjacent sea, the CDOM absorption and fluorescence appeared conservative in waters of medium salinity (3–20 psu). This result is comparable to the values reported in [43]. CDOM was linearly correlated with salinity, which indicates that fresh water was the primary source of CDOM in the Yangtze River Estuary, and that the CDOM concentrations were controlled by conservative mixing of freshwater and seawater end members. Knowledge of CDOM distributions and spectral characteristics is important for identifying CDOM sources, composition, and fate [44]. Using existing laboratory or field instruments cannot fully meet the increasing need for acquisition of high-frequency, high-resolution, longer-term data series. The LIF system could provide rapid and high-resolution in situ measurements [23], which can help improve our understanding of biogeochemical processes in various waters at highresolution, high-frequency and over longer terms.

5. Conclusion We developed a novel small, light-weight LIF system. In this system, a high pulse repetition frequency laser was used to increase signal to noise ratio and a broadband spectrometer was applied to provide full, detailed spectral information. It is simple, inexpensive and portable for laboratory and field measurements in aquatic environments. High sensitivity and stability were observed, which indicates the utility of the LIF system. The LIF fluorescence intensity was found to decrease as temperature increased. At the stage of its development, we acquired a set of field observations representing diverse water types. The significant correlation (R2 ¼0.96) between fluorescence values measured by the LIF system in the ECS coastal waters and those measured by conventional instruments shows that the LIF system has a good ability in detecting CDOM levels in waters. It should be a good tool for research and observation in various waters at high-resolution, high-frequency and longer terms. Our on-going study is obtaining more measurements in various aquatic environments for other applications, such as phytoplankton and oil spills.

Acknowledgments This study is supported by the Public Science and Technology Research Funds Projects of Ocean (Grant no. 201005030), the National Science Foundation of China (Grant no. 41321004) and the National key Technology Research and Development Program of China (Grant no. 2012BAH32B01). We thank our colleagues from the Second Institute of Oceanography at the State Oceanic

Administration, who took part in the cruises, for their hard work on taking in-situ measurements.

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