Emission of trace gases from the marine diatom Thalassiosira pseudonana (CCMP1335) was continuously monitored with a proton transfer reaction-mass spectrometry (PTR-MS) in an axenic batch culture system under a 13:11-h light:dark cycle. Substantial increases in the signals at m/z 49, 63, and 69, attributable to methanethiol, dimethyl sulfide (DMS), and isoprene, respectively, were observed in response to increases in cell density. Signals at m/z 69 showed diurnal variations throughout the experiment whereas those at m/z 49 were more pronounced at the beginning of the incubation. Interestingly, the signals at m/z 49 and 69 changed immediately following the light-dark and dark-light transitions, suggesting that light plays a crucial role in the production of methanethiol and isoprene. However, in the latter half of the experiment, methanethiol showed negligible diurnal variations regardless of light conditions, suggesting the production of methanethiol from enzymatic cleavage of DMS. The trend in signals at m/z 63 was similar to that of the abundance of senescent cells plus cell debris rather than vegetative cells. The results suggest that aging or death of phytoplankton cells could also substantially control DMS production in natural waters along with the other microbial processes related to bacteria and zooplankton.
The main aim of this study was to assess the free lipid composition of plant community in a Chinese peatland. Twelve plant species from the Dajiuhu Peatland were analyzed for the compositions of n-alkanes, n-fatty alcohols and sterols in leaves and roots. The lipid compositions are significantly different between root and leaf for most plants. In some cases, the roots yield more long chain n-alkanes and n-alkanols than the leaves of the corresponding plant. The long chain n-alkanes of the roots in half of the plant species are characterized by a higher Cmax (homologue with the maximum concentration) relative to the leaves. The root-derived sterols and steroidal ketones may be important sources for corresponding compounds within the peat. Because of the different lipid compositions of roots and leaves, more attention should be paid to root-derived lipids for investigations of the lipid composition and their source assessment in soils and terrestrial sediments, where root-derived organic matter can be an important source. The contribution of root-derived lipids may be especially important in peatlands, where acidic and/or anoxic conditions in the subsoil limit the degradation of organic matter and the contribution of leaf litter.
Radioisotope concentrations, such as 238U and 232Th, of bulk sediments are good environmental indicators and useful for radiometric age determinations. Pressed pellets of sediment can be analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) with little preparation time. However, the matrix effects caused by the different properties between the external standard and measured sample may yield unreliable results. This study examines the preparation method of pressed powder pellets using geological rock reference materials, and tests them as external standards. Powders of the rock reference materials (JA-1, JA-2, JB-1 and JB-2 from the Geological Survey of Japan), a lacustrine sediment (4A-18 from Lake Baikal, Russia), and solid glass reference standards (SRM 610 and 612 from the National Institute of Standard and Technology) were analyzed. In calculating 238U or 232Th concentrations, one sample was selected as an external standard and other six samples were treated as unknown samples. 29Si was chosen as an internal standard. When the grain size is small enough (about <10 μm) compared to the laser spot size, and the powders are sufficiently homogenized, the relative standard deviation (RSD) of analytical data of 238U was small enough (average: 14%) to produce reliable data. In the case of 232Th, accuracy was affected by the matrix effect regardless of grain size: the concentrations of pressed pellets measured by SRM glasses as external standards are always lower than the reference values, while those measured by other pressed pellets as external standards are almost concordant to the reference value within a range of ±10.1% RSD. This is most likely due to incomplete digestion of large particles from the pressed pellet in the ICP-MS. Therefore, the pellet-pellet combination for an unknown sample and an external standard is recommended although behavior of different matrix element composition or different mineralogy should be investigated further. In contrast to 232Th, 238U appears to be free from matrix effect so that both pellet-pellet and pellet-glass analyses are feasible.
Using laser-ablation inductively coupled plasma mass spectrometer (LA-ICPMS), we have improved the reliability of the abundance data for trace-elements in geochemical samples using a glass bead ablation method. The glass beads were made of mixture of 0.1 g sample and 1.0 g of lithium-tetraborate preliminary prepared for an analysis of major components using a X-ray fluorescence (XRF) technique. The present method has several advantages: 1) higher sensitivity than that achieved by the XRF method, 2) obviation of erroneous measurements due to incomplete dissolution of heavy minerals, and 3) simple, rapid and user friendly sample preparation procedures for the analysis of both the major and trace elements. Development of this method constitute: 1) femtosecond laser-ablation for minimal elemental fractionation during the laser ablation, 2) new software to control all the laser, sample stage movement as well as triggering the data acquisition using the ICP-MS, and 3) a newly designed sample cell to enhance the transport efficiency of the sample aerosol into the ICP. Moreover, to improve the data quality for both the major and trace elements, calibration lines were defined based on the Li-normalized signal intensities and the reported abundance values for the analytes in well distributed GSJ geochemical reference samples. These improvements enabled us to analyze whole-rock compositions at ≤100 sec/sample. Using this method, the precisions of analyses were better than 10% for Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Mn, Co, Ga, Rb, Sr, Y, Zr, Nb, La, Pr and Nd; 20% for P, Zn, Sn, Ce, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Th, and U, and 30% for Fe, Cs, Ba, and Eu. For Ni and As, precisions of the measurements was not better than 30%. Reliabilities of analyses were better than 10% for Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Mn, Co, Rb, Sr, Y, Zr, Cs, Ba, La, Ce, Nd, Sm, Gd, Tb, Dy, Yb, Lu, Hf, Th and U; <20% for Zn, Ga, Nb, Sn, Pr, Sm, Eu, Ho, Er, Tm and Ta, and 30% for P. For Fe, Ni and As, reliability of the measurement was not better than 30%.
In order to determine the distribution of sediments, bottom-withdrawal settling velocity tubes (SVTs) are used. This technique is, for example, often applied in estuaries. Yet, these tubes have serious disadvantages. Several papers call for the use of video applications in preference. However, not only that this technique has many disadvantages as well, it generally does not suit the need of hydro-/marine geochemical research. Particularly if a project needs to analyse different fractions separately, for example in order to determine and model the transport of suspended materials, video analyses are not a feasible alternative. Therefore, this note aims to describe a number of improvements to compensate for previously observed weaknesses. Particularly the addition of a simple funnel, mimicking a natural environment without walls, could increase the general reliability of bottom-withdrawal techniques with sediment settling velocity tubes.