Causes of errors in the determination of macroelelements in soils and hard rocks using the X-ray fluorescence method

The effect of two main interfering factors (errors of the spectral line intensity measurements, and no agreement between the relationship of the line intensities with the element concentrations and the simplified methods of determining the fundamental parameters) on the X-ray fluorescence determination of macroelements in hard rocks and soils was considered. The use of reference elements in the presence of an interfering factor provides for the fourth category of accuracy as specified by the Scientific Council for Analytical Chemistry. In the simultaneous presence of two interfering factors, a two-mode method of measuring spectral line intensities was proposed for increasing the measuring accuracy. To decrease the effect of the determination error of an element on the error of determining another element, it was recommended to use the standard background method for the elements requiring correction instead of the fundamental parameter method. The regulating procedure of selective normalization was proposed for hard rock samples. The analysis of carbonate-free soil samples in the original powder form was analogous to that of hard rocks. Significantly better results were obtained in the analysis of calcined soil samples of simpler mineral composition.     

模拟降雨条件下不同土石比例弃土堆置体产流产沙研究

为研究不同土石比例条件下弃土堆置体水土流失特征之间的差异,以前期概化的4类弃土堆置体中坡顶平台有车辆碾压的弃土堆置类型为研究对象,采用人工模拟降雨试验,研究4种土石比例弃土堆置体在4种降雨强度条件下的产流产沙特征。研究表明:4种土石比例的弃土堆置体的开始产流时间随着砾石含量的增加而缩短;相同土石比例条件下,径流率和侵蚀速率随着降雨强度的增大而增大;相同降雨强度条件下,径流率和侵蚀速率随着土石比例的减小而减小;径流率随着降雨时间的推移在一定范围内呈波动式变化,侵蚀速率随时间推移呈稳定、波动两种变化趋势。不同土石比例弃土堆置体平均径流率、平均侵蚀速率随着砾石含量的增大而增大;总产沙量随着总径流量的增大而增大。     

Use of Lithium Metaborate to Determine Total Phosphorus and Other Element Concentrations in Soil,Plant, and Related Materials

Lithium metaborate was examined as an alternative to sodium carbonate for preparing samples for measuring total elements, especially P, by fusion in soil, geological, plant, and other natural organic compounds. The examination also included the effect of two concentrations (0.8 and 3M) of nitric acid used to dissolve the metaborate fusion products as preparation for multiple element analyses by inductively coupled argon plasma atomic emission spectrophotometry (ICP-AES). The three methods compared favorably and were able to reproduce a wide range of element concentration values documented for National Institution of Standards and Technology (NIST) mineral and organic samples. Neither lithium metaborate method measured sulfur (S) acceptably, especially in organic material. The sodium carbonate method traditionally uses sulfuric acid for dissolution, and therefore S measurements cannot be included. Also, sodium (Na) could not be included for the carbonate method. There were detection limit problems for measurement of some elements that are naturally present in small quantities in mineral and organic samples due to relatively low emission intensities for ICP-AES, and the greater sensitivity of ICP-mass spectrometry would be needed. There were only small differences between the two concentrations of nitric acid dissolution after metaborate fusion, but 3 M was adopted for routine use. The metaborate fusion with 3 M nitric acid method performed well on the routine analyses of soil samples. These results show that lithium metaborate is a good alternative to sodium carbonate fusion for the measurement of total P and many other elements (except S) in soil and related materials.     

Degradation products of polycondensed aromatic moieties (black carbon or pyrogenic carbon) in soil: Methodological improvements and comparison to contemporary black carbon concentrations

It was found that benzenepolycarboxylic acids (BPCA) do naturally occur in soils. They can be assumed as black carbon degradation products. We propose an improved method to analyse black carbon degradation products in soil. Method optimization comprised extraction duration, sample clean‐up, and chromatographic separation conditions. The method is also suitable for subsequent isotope ratio mass spectrometry of individual BPCA. Accuracy was checked with quartz sand and soil samples spiked with known amounts of BPCA. In soil samples with varying properties we compared contents of black carbon and black carbon degradation products, i.e., total and free BPCA contents, respectively. Results show that our proposed method can extract black carbon degradation products from soil after 48 h of extraction with 500 mM NaOH compared to 72 h of the original method. Polyvalent cations should be removed by Dowex 50 W X 8, 200–400 mesh, while original cation removal with Merck cation exchange resin was not successful. Free BPCA can be separated and quantified directly by ion exchange chromatography followed by ultraviolet spectroscopy or isotope ratio mass spectrometry, while the original method involved another two days for derivatization and isolation of derivatized BPCA. Recovery of spiked BPCA to a range of different soil samples was 81 ± 19%. Free BPCA concentrations of a Cambisol, Chernozem, Ferralsol, and Anthrosol varied between 0 and 0.24 g kg^?1 contributing 0–38% of total black carbon concentration. Free BPCA correlated with total black carbon, total organic carbon, and negatively with pH. However, further studies with a larger set of samples are necessary to systematically investigate conditions for black carbon degradation and, thus free BPCA formation in soils.     

Agronomic Efficiency of Indian Rock Phosphates in Acidic Soils Employing Radiotracer A‐Value Technique

Abstract

A greenhouse pot culture study was conducted to evaluate the agronomic efficiency of two rock phosphates from Mussoorie (MRP) and Purulia (PRP) in two acidic soils from Dapoli (Maharashtra) and Aruvanthklu (Karnataka), India, by growing maize (cv. Ganga) as the test crop and using ³²phosphorus (P) single superphosphate (³²P=SSP) as a tracer (A‐value technique). Dry‐matter yield and P uptake increased significantly with the application of P fertilizers compared to control treatment (without P) in both the soils. There was no significant difference with respect to dry‐matter yield among the P fertilizer treatments. However, P uptake by the shoots was found to be significantly higher in the PRP treatment in only Dapoli soil compared to other P fertilizer treatments. Phosphorus derived from fertilizer decreased in rock phosphate treatments compared to standard ³²P‐SSP treatment in both the soils, indicating an excess availability of P from the rock phosphates. A‐values of soil and rock phosphate indicate a relatively higher P availability from Aruvanthklu soil compared to Dapoli soil; A‐values for the rock phosphates were in the order PRP>MRP. The substitution ratio showed that the availability of P from both the rock phosphates were less than SSP in both the soils.     

海南胡椒种植基地土壤重金属评价及来源分析

在对海南省主要胡椒种植区-文昌、琼海两市种植地土壤重金属含量调查的基础上,采用单项污染指数法和综合污染指数法,结合绿色食品产地环境技术条件及土壤环境质量二级标准,对其重金属含量现状进行了评价;然后结合深层土壤、母质岩石样品及海南岛1:25万多目标区域地球化学调查成果,对其土壤重金属来源进行了综合分析。结果表明,相对所选标准,沉积砂岩区(F11~F13)、第四系松散沉积物区(F7、F14)和花岗岩区一种植地(F9)均符合绿色食品产地要求。玄武岩区(F1~F6)土壤Cu、Cr、Ni超标,花岗岩区另一种植地(F8)和变质岩区(F10)土壤Pb、As超标。综合分析结果显示,胡椒基地土壤环境质量整体良好,成土母质是土壤中重金属元素的主要控制因素,但部分种植地浅层土壤中Cd、Pb、As等重金属元素已受到人类活动的影响。     

Humification indices of water‐soluble fulvic acids derived from synchronous fluorescence spectra — effects of spectrometer type and concentration

The high variability of dissolved organic matter (DOM) in natural systems (concentration, composition) means rapid methods are required for its characterization so that a high number of samples can be analyzed. The objective of the present study was to quantify the effects of spectrometer type and dissolved organic carbon (DOC) concentration on the humification indices of water‐soluble fulvic acids (FAs) derived from synchronous fluorescence spectra, and thus enable the broader application of this method for DOM characterization. We used three standard FAs from the International Humic Substances Society, 24 water‐soluble FAs isolated from topsoil, groundwater and surface water in a fen area, and two different spectrometers. The wavelengths at which bands occurred were similar for all the FAs. Therefore, the differences between the spectra of the FAs studied could be described by humification indices (band ratios). The humification indices calculated correlated very well between spectrometers despite small differences in the wavelengths of bands and shoulders. The absolute values of these indices deduced from two spectrometers can only be directly compared if the spectra are corrected using a standard substance. Increasing DOC concentration resulted in a linear increase in humification indices with a sample specific slope. Therefore, we recommend using an uniformly low DOC concentration of about 10 mg C l^—1 for recording the spectra of samples with typically low DOC concentrations (aquatic samples, soil solutions). This value is a compromise between relatively low absorption to minimize inner filter effects and a sufficient signal‐to‐noise ratio.     

Geochemistry of Mercury Along a Soil Profile Compared to Other Elements and to the Parental Rock: Evidence of External Input

The vertical distribution of mercury along a weathering profile derived from a diabase was compared to the main geochemical and mineralogical characteristics of the soil and its parental rock. The sampling site was in a metropolitan area, nearby to an active quarry and relatively close to an industrial park. The samples of a 6-m-deep fresh exposure of the soil profile and also of fresh rock were collected during the dry season. Kaolinite, goethite, hematite, and residual primary minerals were identified in the soil samples. Typically, the concentrations of Hg in the soil are low. Whole samples contained between 1 (rock) and 37 ??g kg^?1 Hg, while the?<?63-??m soil fraction had up to 52 ??g kg^?1 Hg. The higher values of Hg corresponded to the upper layers of A (0?C10 cm) and B (200?C220 cm) soil horizons. Elemental gains and losses calculated against Zr resulted in the following order: Hg>>Pb?>?Zr?>?LREE?>?Nb?>?HREE?>?Al?>?Ti?>?Fe?>?Cr. Total organic carbon in soil samples varied between 0.2 and 5.1 g dm^?3, and correlation with Hg concentrations was moderate. The acid pH (4.2?C5.5) of the soil samples favors the sorption Hg species by predominant secondary phases like goethite and kaolinite. The Hg concentration of the rock is insufficient to explain the large enrichment of Hg along the soil profile, indicating that exogenic Hg, via atmospheric deposition, contributed to the measured Hg concentrations of the soil.     

Evaluation of soil analytical methods for the characterization of alkaline Technosols: II. Amorphous constituents and carbonates

Purpose

The aims of this study were to identify potential sources of error in common methods for determination of amorphous oxide concentrations and carbonate concentrations, as applied to a Technosolic material (bauxite residue), and where possible, suggest improvements to the methods.

Materials and methods

An acid ammonium oxalate (AAO) extraction was applied to fresh and weathered bauxite residues, at soil to solution ratios varying from 1:100 to 1:800. Two methods for carbonate concentration were compared: the ‘weight loss’ method, and the ‘difference in total C’ method. These were applied to six weathered bauxite residue samples, with CaCO₃ concentrations ranging from 0.1–2 % weight.

Results and discussion

Chemically extractable amorphous content was underreported in bauxite residue at the standard 1:100 extraction ratio, likely due to Al and Si saturation of the oxalate complex. A 1:400 soil: AAO ratio extracted the highest amount of amorphous material. Some crystalline minerals such as sodalite, inherited from the Technosolic parent material, were soluble in the acid ammonium oxalate extractant. The difference in total C method was more precise than the weight loss method for the determination of carbonate concentration in bauxite residues.

Conclusions

The high amorphous content of bauxite residues requires a wider soil to solution ratio (1:400) for acid ammonium oxalate extraction than is used for typical soil materials (1:100). The difference in total C method is recommended for the routine analysis of field samples where small variations in carbonate concentration need to be detected.     

Underestimation of chromium and zirconium in soils by hydrofluoric acid digestion and inductively coupled plasma-mass spectrometry

The concentrations of chromium (Cr) and zirconium (Zr) in 503 soil samples collected from 78 site in Japan were determined by using two utterly different methods. The first was the relatively widely used inductively coupled plasma-mass spectrometry (ICP-MS) combined with digestion with hydrofluoric acid (HF) and perchloric acid (HClO₄). The second was a non-destructive polarizing energy dispersive X-ray fluorescence spectrometric method (EDXRF) using pressed powder pellets. The Cr concentrations obtained by EDXRF were higher than those obtained by ICP-MS in ~75% of the samples. The EDXRF/ICP-MS ratios ranged from 0.48 to 9.65, and the geometric mean was 1.23. A typical example having big a difference in Cr content between the two methods was found in seven samples taken from a podzolic soil developed on sand dunes in northern Hokkaido. This sampling site is around 20 km from the northern edge of a serpentinite area that extends from north to south, forming the backbone mountain range of Hokkaido. The Cr concentrations obtained by ICP-MS for the seven samples were 27.9–52.2 mg kg^?1, similar to the average for all the samples, whereas those obtained by EDXRF were 101–460 mg kg^?1, falling within the range between the third quartile and the maximum for all the samples. It would be difficult to predict these results from the analytical data of other elements because the samples were high in silica (SiO₂) and low in magnesia (MgO), and did not show any characteristics typical of soils derived from serpentinite. X-ray adsorption near-edge structure (XANES) showed that Zr occurred as zircon (ZrSiO₄) in soils. The differences between the two methods were greater for Zr than for Cr. Zirconium concentrations obtained by EDXRF were higher than those by ICP-MS for all samples except three. The difference between the two methods was more pronounced in soils derived from granite that is widespread in western Japan. The EDXRF/ICP-MS ratios for seven soil samples from different horizons at a single sampling site ranged from 4.82 to 6.94 and were generally higher at the A horizon (Apg and Ag) and decreased with depth. Our results indicate that there is a high possibility of underestimation of Cr and Zr contents in soils for a considerable number of samples if determined with ICP-MS, and it could also overlook some important chemical characteristics of soils.     

Adenylate energy charge and ATP-to-microbial biomass C ratio in soils differing in the intensity of disturbance

We carried out an 8-days' incubation experiment with three different intensities of soil disturbance to analyse the effects on the ATP-to-microbial biomass C ratio and on the adenylate energy charge (AEC=(ATP+0.5×ADP)/(AMP+ADP+ATP). Single mixing of soil at 50% water holding capacity with a spatula during weighing of the samples into extraction jars at the end of the 8-days' incubation or 8-times repeated daily mixing for 2 min triggered the immediate formation of ATP, increasing both AEC and the ATP-to-microbial biomass C ratio. The energy for this extra ATP produced seems to be mainly derived from an accelerated turnover of C within the microbial biomass. In contrast, 8-days' continuous mixing led to a significant decrease in AEC and ATP-to-microbial biomass C ratio.     

广州土壤中的稀土元素

稀土元素是指镧系元素中原子序数由57至71的镧(La)、铈(Ce)、镨(Pr)、钕(Nd)、钷(Pm)、钐(Sm)、铕(Eu)、钆(Gd)、铽(Tb)、镝(Dy)、钬(Ho)、铒(Er)、铥(Tm)、镱(Yb)和镥(Lu).由于元素钇(Y)的化学性质和稀土相似、在自然界中常和稀土共存,因此地球化学家常把忆列入稀土元素.     

生产建设项目弃渣场安全选址方案研究

[目的]研究弃渣失稳后可能产生的滑移距离,为生产建设项目弃渣场安全选址提供参考依据。[方法]收集了1 032组国内外堆积体、滑坡体、火山流体数据,整理出有效数据882组,在此基础上,基于发生类似运动的概率提出了历史发生频率选址法,基于理论修正模型提出了模型预测选址法。[结果]根据历史发生频率法,对5,4,1级弃渣场宜分别取2.78,5.88,14.29倍落差的安全距离,对2级、3级弃渣场宜取7.14倍的落差的安全距离。利用模型预测法,根据不同的渣场大小等级(AL)和岩质类型(RT),可准确预测弃渣场选址的安全防护距离。[结论]历史发生频率法操作较简单,容易推广和实施。模型法理论性强,判断较为准确,但需进行试算迭代,使用要求较高。     

紫色土草地表层土壤137Cs含量与粒度和有机质的相关性研究

紫色土草地表层土壤的137Cs比活度与样品的筛分中值粒径(d50)相关较好,而与颗分中值粒径没有明显的相关性。137Cs主要被吸附于土壤颗粒的表面,土壤团聚体表面的矿物、岩石碎屑颗粒的137Cs吸附浓度高于团聚体内部的碎屑颗粒。筛分粒度中等的样品有机质含量高,粗和细粒度的样品有机质含量均低,137Cs比活度和有机质含量无明显相关。     

野外模拟降雨条件下矿区土质道路径流产沙及细沟发育研究

为研究矿区土质道路径流产沙及细沟形态发育特征,在野外调查的基础上,设计坡度(3°、6°、9°、12°)和雨强(0.5、1.0、2.5、2.0、2.5、3.0 mm/min)2个处理,在野外建立不同坡度的道路小区,采用人工模拟降雨的方法,测定了不同处理道路径流产沙参数和细沟形态指标。结果表明:1)各坡度道路径流率为1.12~8.24 L/min,与雨强线性关系极显著,随坡度变化不显著;除0.5 mm/min雨强3°~9°坡及1.0 mm/min雨强3°坡道路径流流态为层流外,其余为紊流,雨强-坡度交互作用(I×S)对流态影响显著;阻力系数只与坡度相关。2)各坡度道路剥蚀率为0.92~324.46 g/(m2·s),与雨强、坡度和径流率呈极显著幂函数关系(R2=0.968,P0.01),道路土壤发生剥蚀的临界剪切力和临界径流功率分别为2.15 N/m2和0.41 W/(m2·s)。3)3°道路以片状侵蚀为主,6°~12°道路细沟发育,细沟宽深比、复杂度、割裂度和细沟密度分别为1.80~3.75、1.07~1.55、0.20%~10.33%和0.067~2.01 m/m2,细沟发育程度是雨强和坡度交互作用(I×S)的结果。4)6°~12°道路细沟侵蚀量占总侵蚀量比例为18.0%~57.16%,总侵蚀量与细沟宽深比、细沟复杂度、细沟割裂度和细沟密度均呈显著的函数关系(R2=0.35~0.96,P≤0.01),割裂度是影响土质道路总侵蚀量的最佳细沟形态因子。结果可为矿区土质道路水土保持工程设计及生产安全提供参数支持。     

Application of the “standard cell” concept in research on soil formation

The concept of the “standard cell” of a rock, which compares numbers of cations associated with 160 oxygen ions (Barth, 1948) can be applied to the solid material of soils and saprolites as a means of tracing changes in element content at different depths in soil profiles weathered from igneous rocks. The “standard cells” can be derived readily from total silicate analyses of rock and soil materials and can be expressed as chemical formulae or in tables to facilitate comparisons. Standard cells for several layers from each of three profiles from southeast Queensland illustrate the type of information obtained. Standard cells of solum and weathered rock layers of a profile previously studied by mineralogical techniques permit a comparison of inferences about weathering which have been drawn by the two approaches.     

川中丘陵区侵蚀产沙的尺度单元及其研究方法

川中丘陵区地表以紫色沙页岩风化形成的紫色土壤为主,基岩风化剥蚀过程与流失过程交替进行.该区垦殖率高,植被盖度低,地形破碎,土地利用结构复杂,土壤侵蚀严重。根据川中丘陵区的景观特征和土地利用格局,认为地块应为该区土壤侵蚀研究的基本单元;探讨了在不同尺度单元上开展土壤侵蚀监测、评价和预测预报的方法。     

A Comparison of 18Oδ Composition of Water Extracted from Suction Lysimeters, Centrifugation, and Azeotropic Distillation

The representativeness of soil pore water extracted by suction lysimeters in ground-water monitoring studies is a problem that often confounds interpretation of measured data. Current soil water sampling techniques cannot delineate from which soil volume a pore water sample is extracted, neither macroscopic, microscopic, or preferential flowpath. This research was undertaken to compare δ¹⁸O and Br^? values of extracted suction lysimeters samples from intact soil cores with samples obtained by the direct extraction methods of centrifugation and azeotropic distillation. Also, the study was concerned with determining what portion of soil pore water is sampled by each method and explaining differences in concentrations of the extracted water from each method to allow a determination of the accuracy and viability of the three methods of extraction. Intact soil cores (30 cm diameter by 40 cm height) were extracted from two different sites. Site 1 was rapid infiltration basin number 50, near Altamonte Springs in Seminole County, Florida. Site 2 was the Missouri Management System Evaluation Area (MSEA) near Centralia in Boone County, Missouri. Isotopically (¹⁸Oδ) labeled water and bromide concentrations within water samples taken by suction lysimeters was compared with samples obtained by methods of centrifugation and azeotropic distillation. The ¹⁸Oδ water was analyzed by mass spectrometry while bromide concentration, applied in the form of KBr was measured using standard IC procedures. Water collected by centrifugation and azeotropic distillation data were about 0.25‰ more negative than that collected by suction lysimeter values from a sandy soil and about 2–7‰ more negative from a well structured soil. Results indicate that the majority of soil water in well-structured soil is strongly bound to soil grain surfaces and is not easily sampled by suction lysimeters. In cases where a sufficient volume of water has passed through the soil profile and displaced previous pore water, suction lysimeters will collect a representative sample of soil pore water from the sampled depth interval. It is suggested that for stable isotope studies monitoring precipitation and soil water, suction lysimeter be installed at shallow depths (10 cm). Samples should also be coordinated with precipitation events. The data also suggest that each extraction method samples a separate component of soil-pore water. Centrifugation can be used with success, particularly for efficient sampling of large areas. Azeotropic distillation is more appropriate when strict qualitative and quantitative data on sorption desorption, and various types of kinetic studies may be needed.     

THE MODIFICATION AND APPLICATION OF THE QUANTITATIVE X-RAY DIFFRACTION METHOD OF SCHULTZ (1964) TO THE MINERALOGICAL STUDY OF LONDON CLAY SAMPLES

Over 300 samples of London Clay from twenty-one widely scattered sample locations in the London and Hampshire Basins were to be examined in detail by X-ray diffraction techniques. The quantitative method of Schultz (1964) was chosen as being most suited for the analysis of the samples. Series of tests designed to investigate the reproducibility characteristics of the method were conducted. These showed that the sampling, sample preparation, and interpretation procedures used all introduced cumulative reproducibility errors of roughly equal magnitude which completely overshadowed errors introduced by machine-response variation. It was also established that minerals whose contents were below 10 percent of whole sample failed to be consistently quantitatively estimated. The accuracy of this method of analysis depended heavily on Schultz's quoted ‘intensity factors’ which were involved in the calculation of individual mineral contents. The authenticity of these factors with regard to the London Clay suite of minerals was checked by comparing accurately determined mineral percentages of six standard London Clay samples with the results obtained by use of the Schultz procedure and factors. The method was found to be basically sound although the intensity factors for quartz, total clay minerals, carbonates, and illite had all to be slightly adjusted. The accurate quartz percentages were established by three different X-ray diffraction techniques and the carbonate percentage checks were accomplished by specially designed wet chemical methods. The total clay mineral and illite contents checks were carried out by back-calculation of the Al₂O₃, and K₂O percentages as determined from full wet chemical analysis. Using the adjusted factors, quartz percentages are estimated accurate to ±6 percent, total clay minerals to ±7 percent, carbonate percentages to ±1.5 percent, and illite contents to ±4 percent of the whole rock.     

Comparison of standard and total Colwell procedures for measuring soil test phosphorus

Abstract

When soils are extracted with sodium bicarbonate to measure soil test phosphorus (P) by the standard Colwell procedure, the concentration of P in the extract solution is measured using the Murphy and Riley procedure. This measures P as orthophosphate (PO₄) and most of the organic P extracted is not measured. The total (inorganic plus organic) P extracted can be measured by digesting an aliquot of the extract solution with perchloric acid to convert all the extracted P to PO₄ before measuring the P concentration by the Murphy and Riley procedure. This is called total Colwell P. Data from one crop and two pasture field experiments in Western Australia are presented in which soil test P measured in 1991 and 1992 by the standard and total Colwell procedures were compared. Fertiliser P residues, including organic P, accumulate in the topsoil because, (i) the fertiliser is applied to the surface of pastures, and (ii) crops are being increasingly sown by minimum (conservation) tillage when P is concentrated in the top approximate 5 cm of the soil compared with about 10 cm when crops are sown conventionally. Consequently, to measure soil test P in the present study, soil samples were collected in February 1991 to either 5 or 10 cm depth. For the two Colwell procedures and both sampling depths, soil test P was related to: (i) the level of P applied in previous years and to (ii) plant yield measured later on that year (the soil test P calibration). Soil test P was larger for the total Colwell procedure and for soil samples collected to 5 cm depth. Total Colwell P as the dependent variable was closely correlated with standard Colwell P as the independent variable. The relationship between soil test P and the level of P applied, and the calibration relating yield to soil test P, were different for standard and total Colwell P. Neither method can be claimed to be significantly better than the other. It is concluded that use of the total Colwell procedure instead of the standard procedure is not warranted. Likewise, there was no evidence that collecting soil samples to 5 cm to measure soil test P provided better estimates of the current P status of soils than collecting soils to the standard 10 cm depth. There is therefore no need to change the sampling depth.