Measuring Chlorophyll a with a Spectrofluorophotometer

Significance of the topic

Chlorophyll a is the primary pigment used to estimate phytoplankton biomass and is therefore a fundamental indicator for assessing marine primary productivity, coastal ecosystem status, and water quality. Accurate, sensitive measurement of chlorophyll a at low environmental concentrations is essential for monitoring biological responses to natural events (e.g., storms) and anthropogenic impacts. This study compares spectrophotometric (absorbance) and fluorometric (spectrofluorophotometer) approaches for chlorophyll a quantification in seawater extracts and evaluates the effect of extraction solvent and sample condition on analytical performance.

Objectives and overview of the study

The study aimed to:

• Compare absorbance-based and fluorescence-based measurements of chlorophyll a following solvent extraction of field seawater samples.
• Evaluate three common extraction solvents (N,N-dimethylformamide (DMF), 90% acetone, and methanol) in terms of extraction efficiency and spectral behavior.
• Illustrate how sample condition (good weather vs. post-typhoon turbidity) and location (Tama River mouth vs. upstream Daishi Bridge) affect measured chlorophyll signals and measurement reliability.

Methodology

Sample collection and pretreatment:

• Sampling sites: two locations on the Tama River—near the river mouth and at Daishi Bridge 2.4 km upstream.
• Sampling conditions: two weather states—good weather and after a typhoon—to capture differences in turbidity and suspended material.
• Filtration: approximately 200 mL seawater (consistent with guideline recommendations) was vacuum-filtered through glass-fiber filters (47 mm diameter, 1.0 µm pore size).
• Extraction: filter residues were transferred into 10 mL of solvent (DMF, methanol, or 90% acetone) and held in a dark, cold environment at −20 °C, typically for ~24 h to promote pigment extraction.

Absorbance measurement (spectrophotometry):

• Instrument: UV-2600i Plus UV-Vis spectrophotometer.
• Scan range and settings: 350–850 nm, 1 nm interval, slit width 1 nm, medium scanning speed.
• Quantification approach: chlorophyll a absorbance evaluated near 663–665 nm with baseline correction by subtracting absorbance at 750 nm to remove scattering contributions.

Fluorescence measurement (spectrofluorophotometry):

• Instrument: RF-6000 spectrofluorophotometer.
• Settings: excitation at 436 nm, emission scanned 600–800 nm (1 nm interval), scanning speed 200 nm/min, slit widths Ex 5 nm/Em 5 nm, low sensitivity setting used for the study.
• Chlorophyll a fluorescence recorded near 670 nm; fluorescence peak intensity and wavelength used for comparative evaluation.

Used Instrumentation

• Shimadzu UV-2600i Plus UV-Vis Spectrophotometer — absorbance scans (350–850 nm).
• Shimadzu RF-6000 Spectrofluorophotometer — fluorescence excitation/emission scans for chlorophyll a.
• Standard field sampling materials: glass-fiber filters (47 mm, 1.0 µm), vacuum filtration apparatus, cold storage at −20 °C for extraction.

Main results and discussion

• Sensitivity: Fluorescence measurements provided substantially higher signal intensity and sensitivity compared with absorbance. Samples with low absorbance (~0.05 Abs) still produced strong, reproducible fluorescence signals, whereas equivalent absorbance readings after dilution would approach instrument noise and have poor reproducibility.
• Effect of light scatter: Absorbance quantification required subtraction of a 750 nm baseline to compensate for scattering by particulates. High scatter introduced large uncertainty in absorbance-based chlorophyll estimates; fluorescence measurements were less affected by scattering and therefore more stable for turbid extracts.
• Solvent performance: DMF generally extracted the most chlorophyll a in three out of four field samples when assessed by fluorescence, followed by 90% acetone and then methanol. Absorbance results were more variable across solvents, likely because light scatter and baseline issues amplified errors in low-absorbance samples.
• Environmental influence: Chlorophyll a signals at the river mouth decreased after the typhoon relative to good-weather samples, attributed to increased turbidity from upstream sediment runoff. Upstream samples did not show the same decrease, possibly due to localized inputs (e.g., debris) or different particulate composition affecting extraction and scattering.
• Spectral observations: Fluorescence peaks clustered near 671–674 nm depending on solvent and sample; small peak shifts between solvents were observed, indicating matrix- and solvent-dependent spectral behaviour that should be considered during method calibration.

Benefits and practical applications of the method

• Fluorometry offers higher sensitivity and lower detection limits for chlorophyll a than UV-Vis absorbance, making it preferable for monitoring oligotrophic waters or low-biomass coastal zones.
• Reduced susceptibility to light scatter makes fluorescence more robust for field-collected extracts, especially when turbidity is variable (post-storm conditions).
• Choice of extraction solvent (DMF shown as most effective here) materially affects measured pigment yield; method selection should match monitoring objectives and matrix characteristics.
• Practical applications include routine coastal monitoring, storm impact assessments, baseline phytoplankton surveys, and laboratory-based validation of in-situ fluorometer readings.

Future trends and potential uses

• Advances in in-situ fluorescence sensors and autonomous platforms (buoys, gliders) will enable higher-frequency chlorophyll monitoring; laboratory fluorometry will remain important for calibration and ground truthing.
• Integration with hyperspectral fluorescence and multivariate correction algorithms could further reduce scattering artifacts and disentangle accessory pigments.
• Standardization of extraction protocols (solvent choice, standing time, centrifugation) and improved sample cleanup (centrifugation/filtration) will reduce inter-laboratory variability.
• Coupling fluorometry with chromatographic pigment analysis (HPLC) improves taxonomic resolution and provides reference-grade calibration of fluorescence-based biomass estimates.
• Miniaturized, automated extraction and measurement systems could accelerate sample throughput for large-scale monitoring programs.

Conclusion

Fluorometric analysis using a spectrofluorophotometer gives higher sensitivity, better signal-to-noise, and reduced influence of light scatter compared with UV-Vis absorbance for chlorophyll a in solvent extracts of seawater. In this study DMF was the most effective extraction solvent in most samples by fluorescence. Careful sample preparation (appropriate extraction time, centrifugation/clarification) remains critical to minimize scattering artifacts and achieve reliable absorbance-based results. Overall, fluorescence-based measurement is recommended for low-concentration environmental samples and for conditions with variable turbidity, while maintaining solvent- and matrix-appropriate calibrations.

References

• The Oceanographic Society of Japan, Guideline of Ocean Observations 5th Edition, Vol. 4, Chap. 4, 2023.
• Nitrogen compounds in PM2.5 atmospheric pollution are growing ocean phytoplankton, Ocean and Earth Information Site JAMTEC BASE (accessed on June 1, 2026).
• Water Standard Test Methods 2020 Edition III Organic Materials, Japan Water Works Association.