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High-performance size-exclusion chromatography (HPSEC) with ultraviolet absorbance detection (UVAD) has been widely utilized to estimate the molecular weight (MW) and MW distribution of humic acids (HAs). The MW estimated by UVAD was inherently inaccurate, however, because UVAD set at 254 nm only detects limited HA components, and the molar absorptivities of different HA constituents are not equal. The objective of this study was to evaluate the refractive index detection (RID)-based HPSEC method for quantifying the MW of HAs. Five HA samples were quantified with both UVAD and RID. The chromatograms obtained on the two detectors showed that the RID/UVAD response ratios were consistently >10, indicating that RID is more sensitive for the detection of HAs. The chromatograms obtained with RID had three peaks compared with two peaks shown on UVAD chromatograms because RID detected a late-eluting peak (F3) that was not shown on the UVAD chromatograms. Comparison of the RID/UVAD response ratio showed that the highest MW HA fraction (F1) and lowest MW HA fraction (F3) have sigher RID/UVAD response ratios, whereas the medium HA fraction (F2) had a lower RID/UVAD response ratio. These suggested that F1 and F3 may have relatively lower contents of UV-sensitive bonds such as C=C double bonds than F2. Compared with HPSEC-UVAD chromatograms, the HPSEC-RID chromatograms resulted in higher weight-averaged MWs, lower number-averaged MWs, and higher polydispersivity for the tested HA samples. This study indicated that RID is less selective than UVAD for detection of structurally highly heterogeneous HA molecules and is thus better for characterizing the MW distribution of HA molecules. [ABSTRACT FROM AUTHOR]

Standard laser-induced breakdown spectroscopy (LIBS) offers a potentially rapid, accurate, field-portable, and low-cost technique for the measurement of C content in soil samples. Neutral and singly ionized Fe lines form significant interferences, however, that can compromise the LIBS C measurement. In this study, the 247.8-nm line of atomic C (C I) was examined in detail to assess the effect of potential elemental interferences. These interferences and their spectral and temporal signatures were evaluated using control graphite and Fe oxide samples. A combination of high dispersion and appropriate time gating of the LIBS signal was found to generate very high signal/noise ratio spectra using low laser powers and therefore, allowed accurate determination of the C content down to the subpercent level in the presence of Fe interferences. Although the contribution of one singly ionized Fe line cannot be entirely eliminated, a method to quantitatively assess the contribution of this line was developed. The new technique was tested on soil samples with organic C content in the 0.5 to 3% (w/w) range, and a strong correlation of the LIBS C signal with measurements made by the thermal oxidation, dry combustion method was observed. The findings reaffirm the utility of standard LIBS for rapid and accurate laboratory measurement of soil C and the potential use of standard LIBS with further study and development for assessing soil C in situ. [ABSTRACT FROM AUTHOR]