Analytical instruments and Research in Guo Lab at UWM

A. Instruments for colloidal and nanoparticle characterization

• Asymmetrical Flow Field-Flow Fractionation (AFlFFF) system

The flow field-flow fractionation (FlFFF) is a chromatography-like technique capable of simultaneous separation and characterization of colloids, nanoparticles and macromolecules in aquatic environments.  Our new asymmetrical flow field-flow fractionation system (Postnova) purchased through a NSF Major Research Instrument grant (NSF-MRI award #1233192) can be coupled with a series of online detectors including a multiple angles light scattering (MALS) detector (21 angles), a UV absorbance detector, two fluorescence detectors with four different Ex/Em wavelength combinations, a refractive index, ICP-MS (e.g., Stolpe et al., 2010; Stolpe et al., 2013; Zhou et al. 2016), and other offline detectors/instruments, such as spectrophotometer, 3D fluorometer (e.g., Zhou and Guo, JCA 2015), EEM-PARAFAC (e.g., Lin and Guo, ES&T 2020), gamma- & alpha-spectroscopy, and SEM/TEM/AFM, supporting and augmenting graduate education and ongoing research and allowing broader applications in aquatic and environmental sciences and other emerging topics.

We are looking for motivated graduate and undergraduate students to join our research group and to work on this analytical technique and its applications.   Contact Dr. Guo at for more information.


Postnova asymmetric flow field-flow fractionation system, coupled with online detectors including MALS, UV-absorbance, fluorescence with 4 different Ex/Em combinations, refractive Index, etc (photo from Guo).


Separation of macromolecules and colloidal particles within FlFFF system


Separation of nanoparticles and/or colloids using our AFlFFF system (from Zhou and Guo, 2015, Journal of Chromatography A)


Examples showing the coupling between FlFFF separation techniques and fluorescence EEMs measurements. As shown in the EEM spectra, bulk DOM is highly heterogeneous with different chemical composition between DOM size fractions (from Zhou and Guo, 2015, Journal of Chromatography A).

Coupling the FlFFF size-fractionation with EEM-PARAFAC analysis to elucidate PARAFAC-derived DOM components in individual water samples and to decipher changes in DOM composition and optical properties with molecular weight within a specific sample (from Lin and Guo, 2020, ES&T).

Examples showing the dynamic changes in PARAFAC-derived fluorescent DOM components with molecular weight within an individual DOM sample from the Milwaukee River (from Lin and Guo, 2020, ES&T).

• SPLITT system: We also have a Postnova SPLITT system for size fractionation/separation of suspended particles and/or sediment/soil for further chemical and isotopic characterization.


Ultrafiltration systems

Our ultrafiltration systems have been widely used for size fractionation and isolating colloids or high molecular weight DOM for further chemical and isotopic characterization (Guo and Santschi, 2007, pdf; and Guo et al 2000).  These ultrafiltration units range from large engineering systems equipped with multiple spiral-wound or hollow fiber cartridges capable of processing hundreds and thousands of liters of water (Guo et al. 1996, 1997) to very small devices such as centrifugal units and stirred cell units for tracer studies and controlled laboratory experiments (e.g., Lin et al. 2015; Yang et al. 2015) and to quantify molecular size distribution of DOM and colloidal size spectra (e.g., Chen et al., 2004;  Xu and Guo, 2017, Water Research; Xu and Guo, 2018).


Different ultrafiltration systems, ranging from centrifugal to stirred cells, large volume spiral wound cartridges and engineering systems

A schematic diagram of a ultrafiltration system for isolating colloids/naoparticles from natural waters (Guo and Santschi 2007).

Large volume ultrafiltration (up to 100 liters) for collecting nanoparticles / macromolecules and/or sufficient amounts of freeze dried COM samples for isotopic and molecular characterization (Photo from Guo)

Atomic force microscopy image of aquatic colloids (from Santschi et al., 1998, L&O)

• Interactions of NOM with nanoparticles and metals

Natural organic matter (NOM) isolated using ultrafiltration and model macromolecular organic matter with different functional groups were used to understand the interactions between DOM and nanoparticles (see examples in Kteeba et al., 2017 ENPO;  Baalousha et al., 2018, Environmental Science: Nano) and effects of NOM on surface properties and toxicity of nanoparticles in aquatic organisms (see examples below).

slide1 slide2
Examples showing changes in zeta-potential of ZnO nanoparticles in the presence of different NOM and organic compound classes (from Kteeba et al. 2017, Environmental Pollution, ). Examples showing the role of dissolved organic matter in remedying the toxicity of nanoparticles to Zebrafish (from Kteeba et al., 2017, Environmental Pollution).

From Kteeba et al. (2017, Environ. Pollution)

Mitigative effects of natural and model DOM with different functionalities on the toxicity of methylmercury (Me-Hg) in embryonic zebrafish (Li et al., 2019, Environmental Pollution, 252, 616-626.)

  • Interactions of natural organic matter (NOM) with microplastics and nanoplastics in freshwater systems
Examples using FlFFF to elucidate the size distribution of nanoplastics in the presence and absence of natural organic matter

B. Instruments for Organic/Inorganic Characterization

  • High resolution sector field ICP-MS (ThermoFisher Element 2) (shared instrument, hourly charge).
  • Shimadzu TOC-TN analyzer (TOC-L, TNM-L and ASI-L) — capable of measuring total C (including dissolved organic carbon and dissolved inorganic carbon) and TDN (total dissolved nitrogen) at the same time. See examples in Guo et al. (1995) and Guo and Macdonald (2006).
  • Horiba 3-D fluorescence spectroscopy (Fluoromax-4) for the measurements of excitation-emmision matrices (EEMs) (see examples in Zhou and Guo, 2012 ; Zhou et al. 2013; and Zhou et al. 2016).
  • Agilent UV-vis spectrophotometer (Agilent 8453) – for the measurements of colored dissolved organic matter (CDOM) and other general chemical analyses (see examples in DeVilbiss et al. 2016)
  • Seal auto-analyzer (Model AA3) for measurements of nutrient species (NO3, NO2, NH3, DIP and dissolved silicate) in natural waters (e.g., Guo et al., 2004, Guo et al., 2012)
  • Ion chromatography (IC) for anion and cation analysis
  • GC-MS, FT-IR, LC-MS, Pyrolysis-GC-MS, HPLC (shared instrument, hourly charge).


High resolution sector field ICP-MS (Thermo Scientific Element 2) with laser ablation. See examples of coupling between FlFFF and ICP-MS (Stolpe et al., 2013, GCA).


Horiba 3-D fluorescence spectroscopy (Fluoromax-4) for the measurements of fluorescence excitation-emmision matrices (EEMs) of natural water samples (see examples in Zhou and Guo, 2012; Zhou et al., 2013; DeVilbiss et al., 2016; Zhou et al., 2016).

Examples of using EEMs techniques coupled with PARAFAC analysis for DOM characterization in aquatic environments (from Zhou et al., 2013, Marine Chemistry, EEMs of oil and seawater samples from the Deep-water Horizon oil spill in the Gulf of Mexico). Comparisons in the EEM spectra between pure caffeine (from Sigma), community coffee, black tea, green tea and natural river water samples (Guo et al., unpublished data).

C.  Instrumentation for radionuclides and stable isotopes

Canberra Gamma- and alpha- systems

  • Canberra Gamma spectroscopy with ultra-high purity Ge well detector for the measurements of naturally occurring radionuclides and radioactive tracers such as Th-234, Pb-210, Be-7, Cs-137, I-131, Ra-226, Pa-133, etc.
  • Canberra Alpha spectroscopy with 12 detectors for the measurements of Po-210, Th-228/Th-230/Th-232, Pu-239,240, and others
DSC08668 DSC02252

Slide2 Slide2
Transport trajectory of radionuclides from Japan to the U.S. during the 2010 Fukushima nuclear accident (upper) and major radionuclides identified in rainwater including Pb-210 (46 Kev), I-131 (364 Kev) and Be-7 (477 Kev). Atmospheric flux ratios of 210Po to 210Pb, with anomalously high 210Po/210Pb ratios observed during maximum 131I fallout from Fukushima (see Yang and Guo, 2012, JER).


 Slide1 Examples of using fallout radionuclides such as Cs-137 and Pb-210 for sediment chronology and for quantifying sedimentation rates and material fluxes in aquatic environments (Yang and Guo, 2018, Continental Shelf Research)


 Stable isotope analyzer

  • Picarro Cavity Ring Down Spectroscopy (L2130-i) for the analysis of oxygen and hydrogen stable isotopes (d18O and dD)
  • ThermoFisher Delta V Isotope Ratio-Mass Spectrometer (3 kV) with elemental analyzer (Costech Instrument) for the measurements of stable carbon and nitrogen isotopes in particulate matter, sediments and soil samples (EA-IR-MS, shared instrument).
 ES IR MS Delta V Isotope Ratio-Mass Spectrometer (3 kV )

D.  Additional shared Instruments/Facilities in the School

  • R/V Neeskay: provides year-round access to the Great Lakes and has a fully functional platform and floating laboratory
  • Instrument Shop: a full-service electronics, fabrication, and machine shop where parts and full assemblies are custom made.
  • McLane in-situ pumping system for collecting POM samples
  • Sediment coring
  • Other instrument
Slide2 Slide1