Funding will be sought from the School’s NERC studentship quota

Contact Dr Clare Wilson

Climate change and biodiversity in Scotland’s montane habitats: cascade effects of changes in plant community composition on soil invertebrate assemblages

Funding for this project may be available from NERC or SNH

Contact Prof. Phil Wookey

Developing spatially explicit predictive capabilities in erosion, sediment flux and storage modelling within subcatchments of the Tay

This project will be funded by SAGES; SEPA is an additional CASE partner

Contact Dr. Andrew Tyler

Historical ecology of fuel resource utilization in Sahelian Africa

Funding will be sought from the School’s NERC studentship quota

Contact Dr Paul Adderley

Micro-chemical analysis of grave soils: unlocking the hidden archive of archaeological human burials

This project will start in spring 2010 and is ERC funded in collaboration with University of York

Contact Dr. Clare Wilson

Modelling landscape sensitivities: Are future human-induced landscape changes quantifiable?

Funding is available through SAGES in collaboration with University of Aberdeen and University of Glasgow

Contact Dr Paul Adderley

Scottish soils and the C cycle: How will climate change and ‘re-wilding’ affect soil organic matter dynamics and C fluxes?

Funding will be sought from the School’s NERC studentship quota

Contact Prof. Phil Wookey

Soil disturbance as a result of tree stump extraction practices

In collaboration with Forest Research, funding is being sought from the Scottish Forestry Trust

Contact Dr Clare Wilson

Temperature responses of soil N transformations

Joint SCRI / University of Stirling PhD Studentship

Contact Prof. David Hopkins or Prof. Phil Wookey

The role of mineral interactions in soil organic carbon stabilisation

Funding will be sought from the School’s NERC studentship quota

Contact Dr Clare Wilson

Use of trace elements as tracers of soil – sediment relationships and identification of source areas in eroding catchments: A tool for validating catchment scale modelling of soil and sediment flux

Funding will be sought from the School’s NERC studentship quota and / or a NERC CASE studentship

Contact Dr. Andrew Tyler

Buehler: Tel: 02476 692242 Email

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Meyer & Burger (Switzerland) Tel: ++41 33 439 05 05 Email

Brunner Machine Tools agents for above Tel: 0181 992 6011 Email

MK Tile saws (USA) Tel 1 800 938 7925 Email

Malvern Lapidary Tel: 01684 561537 Email

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South Bay Technology (USA) Tel: +1-949-492-2600

Lapping Machinery

Buehler Tel: 02476 692242 Email

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Logitech Tel: 01389 875444 Email

Struers Tel: 01389 877222 Email

Jones & Shipman Tel: 0116 289 6222 Email

Lapmaster Tel: 01752 893 191 Email

South Bay Technology (USA) Tel: +1-949-492-2600

Polishing Machinery & Materials

Logitech Tel: 01389 875444 Email

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Engis (US) Tel: +1-800-993-6447 Email

Lapmaster Tel: 01752 893 191 Email

Kemet International Tel: 01622 755287 Email

South Bay Technology (USA) Tel: +1-949-492-2600

Resins

ABL (Stevens) Tel:01270 766685 Email

Aeropia (formerly B&K) Tel: 01293 459500 Email

Buehler Tel: 02476 692242 Email

MetPrep: Tel: 024 7642 1222 Email

Logitech Tel: 01389 875444 Email

Promatech Tel: 01285 644211 Email

Scott Bader Tel: 01933 663100 Email

Struers Tel: 01389 877222 Email

Testbourne Ltd Tel: 01256 467005 Email

Devcon Tel: 01933 675299 Email

Production Techniques Tel: 01252 616575 Email

Robnor Resins Tel: 01793 823741 Email

Intertronics Tel: 01865 842 842 Email

Chemicals

Fisher Scientific Tel: 01509-231166 Email

Sigma Aldrich Tel: 0800-717181 Email

Merck

Glass Slides & Coverslips

Logitech Tel: 01389 875444 Email

Chance Propper Tel: 0121 553 5551

Abrasives

Buehler Tel: 02476 692242 Email

MetPrep: Tel: 024 7642 1222 Email

Logitech Tel: 01389 875444 Email

Peter Wolters Tel: 208 544 1800 Email

Washington Mills Electro Minerals Ltd Tel: 0161 848 0276

Shakespeare Abrasive Tech Tel: 01484 716977

1. Bonding soil block to slide : The impregnated soil block is cut with an abrasive cutter (Buehler Petrocut) into approximately 1cm thick slices. Each slice is ground on the lapping plate (LP40/50) using 15 µm calcined aluminium oxide in water (or in the case of clay or peat in ethylene glycol). This is to ensure a flat uniform surface for bonding to the slide. The sample slice is then bonded to a glass slide with an etched surface using 301 epoxy resin (Epotek 301). This is clamped overnight in spring mounted jigs to ensure a good sample/slide bond and to minimise the thickness of the epoxy resin layer.

2. Cutting off the excess: The next day excess material is cut off using an abrasive cutter. Depending on the machinery used care should be taken to cut off as much material as possible without damaging the slide.

3. Grinding: The slide is now ready to be lapped on the LP40/50 machines. The thickness of the glass is measured using a micrometer. The target thickness for a sample is 30-40 µm therefore this is added to the slide thickness plus an undercut value of 30 µm which gives the total setting for the lapping jigs (PLJ2). Slides are held in the PLJ2 jigs by means of a vacuum so it is important to keep the surface of both the jig and the slide clean and free from debris.

The LP40/50 machines have a cast iron radial grooved plate and the grinding medium is drip fed onto the plate. The majority of soil samples are lapped with 15 µm calcined aluminium oxide in water except in the case of clays or highly organic samples where ethylene glycol is used in place of water. The time taken to lap a sample to it’s target thickness is dependent upon the starting thickness, the hardness of the material and whether water or ethylene glycol is used. In general most samples can be lapped to completion in anything between one to 5 hours.

Samples are checked for thickness towards the end of the lapping process using a microscope to determine the colour of quartz grains under crossed polarised light. For a thickness of 30 – 40 µm they will appear pale to light yellow.

4. Polishing: When the desired sample thickness has been achieved the slides are next polished on the CL-40 polishing machine with 3 µm diamond in oil suspension. Slides are then cleaned with a non solvent cleaner to remove any residual oil.

5. Coverlipping :The final stage is to coverslip the completed slide to protect the surface from damage. The coverslip is bonded to the slide using Epotek 301 epoxy resin and placed in the bonding jig overnight.

The crystic resin/catalyst mixture is determined by the type of sample, however for most soil and sediments the mix ratios in the table below are used. The catalyst for crystic resin is methyl ethyl ketone peroxide (MEKP) and the accelerator is Cobalt Octoate (Cobalt 2-Ethylhexanoic acid).

There are many other resins which could be used to impregnate soils but the only others used here at Stirling with any regularity are epoxy resin (Araldite MY750 + hardener HY956) and Poly Ethylene Glycol (PEG 8000). Epoxy resin is often used in cases where soil thin section slides are to be subjected to micro-probe analysis for example since it is more stable in these cases than the polyester resin. There are unfortunately significant problems in using epoxy resin, firstly it is more expensive so may be prohibitive for large numbers of samples, the curing time is also significantly less than that for polyester resin and there is the danger of incomplete impregnation of the sample and lastly it is much more viscous that polyester resin and needs to be thinned but is less miscible with acetone than polyester resin. See the mixing guide below.

Once the choice of resin (polyester or epoxy) and mix has been decided the samples are normally placed under vacuum overnight. Care should be taken not create to great a vacuum so as to avoid disturbing the sample, experience has shown that a vacuum of ~20mm of Hg is optimal.The samples are then removed from the chamber and left in the fume cupboard to continue curing. During this stage the  samples need to be monitored regularly as the solvents evaporate the level of resin will drop so this needs to be topped up two or three times to maintain the level above the surface of the sample.

Crystic impregnated samples will cure generally at a slower rate that those with epoxy resin and as stated before this is preffered. Depending on the resin mix used crystic samples will be kept in the fume cupboard for around 2-3 weeks, after which they are placed in an oven at 40^oC (which is also vented to the fume cupboard) to ensure that any residual styrene and acetone solvents are removed which usually takes a further week.

Samples impregnated with epoxy resin will cure at a faster rate and do not need to be placed in the oven at all. The main problem with this resin is ensuring the curing rate is slow enough to fully impregnate the soil sample.

The third ‘resin’ used at Stirling is Poly Ethylene Glycol (PEG 8000). This was selected for a very particular set of samples where water removal could not be performed due to the risk of losing volatile organic compounds. PEG 8000 is water soluble and has a low melting point of around 59^oC. Samples are taken directly from the field site and molten PEG poured onto them ensuring that the sample is completely covered in PEG at all times. The samples are then placed in an oven at around 60^oC to keep the PEG liquid and monitored regularly. After about 1-3 weeks (depending on sample size) they are removed and allowed to reach room temperature and harden.

The acetone vapour drying process involves removing both of the lids from the tin, replacing the bottom one with a perforated lid to allow the free flow of acetone vapour, then placing the soil sample on a raised platform above a bath of acetone. A container of anhydrous calcium chloride can also be added to facilitate drying. The acetone is changed every three or so days. Over a period of time most of the water will be removed from the soil sample and be replaced by acetone. It is important to be able to monitor water removal to ascertain when sufficient water has been removed from the sample. Methods for monitoring of water removal employed by other workers have included; NMR spectroscopy, (Murphy 1), solubility in petroleum spirit, (Fitzpatrick 2 ); and enthalpimetry, (Moran & McBratney 3). The densimetric method used at Stirling was developed by Muriel MacLeod and is cheap, accurate and easily carried out. Solvent guide tables do not give detailed values between 0 – 10% water in acetone mixtures, therefore standards were produced from which calibration graphs could be drawn (see table image below). Standards were made, using calibrated pipettes, as follows: 0 cm³ to 10 cm³ (in 1 cm³ increments) of water made up to 100 cm3 in volumetric flasks with acetone. However, these standards are v/v, all comparable data are expressed in w/w. Since the specific gravity of acetone at 20°C is 0.7911 then 1 cm³ of acetone will weigh 0.7911 g at this temperature. The specific gravity of water at 20°C can be taken as 1.0 Therefore all v/v data can be converted to w/w data, and this can be repeated at various temperatures (see table) In all cases the specific gravity of water was assumed to be 1.00 as the variability over the 4 degree range at such low concentrations would have little effect.

Keeping the temperature constant at 20°C in a thermostatically controlled water bath, the specific gravity of solutions over the range 1% to 10% water in acetone was measured. This produced a calibration graph for 20°C ambient temperature. The process was repeated at 18°C and 22°C, these being the most probable ambient temperatures of the acetone baths.

Experience has shown that one need only start to monitor the water content of the acetone in the baths after about 6 changes of acetone. This can be done very simply by removing about 100 cm³ of the acetone into a measuring cylinder, recording the temperature, and the specific gravity by use of a 0.790 – 0.800 range hydrometer. The water content can then be calculated by reference to the calibration graph for the recorded temperature. At the point when the water content reaches 0.5% or less the soil samples are then ready for impregnation with resin. The method has been in use now for several years and has proven to be quick, easy, very inexpensive and, above all, reliable.

References:

(1) Murphy, C.P. (1986) Thin Section Preparation of Soils and Sediments. AB Academic Publishers, Berkhamsted.
(2) Fitzpatrick, E.A. (1984) Micromorphology of Soils. Chapman and Hall Ltd.
(3) Moran, C.J. and McBratney, A.B. CSIRO Divisions of Soils, Technical Memorandum 13/88 “A method for dehydration and impregnation of clay soil”.

]]> http://www.thin.stir.ac.uk/2008/06/03/methods-drying/feed/ 2 http://www.thin.stir.ac.uk/2008/06/03/soil-sampling-for-thin-sections/ http://www.thin.stir.ac.uk/2008/06/03/soil-sampling-for-thin-sections/#comments Tue, 03 Jun 2008 12:36:33 +0000 admin http://www.thin.stir.ac.uk/?p=10 The main aim of sampling is to obtain soil samples in as undisturbed a state as possible. The process involves the careful use of Kubiena tins inserted into soil profiles, removed, sealed and bagged for transport to the lab.

back in the lab samples are normally dried because for most purposes the resins used for impregnation are hydrophobic. This can be achieved by oven drying, air drying or by solvent exchange (acetone). The first two methods can be extremely detrimental to the soil structure causing shrinking and cracking, therefore the preferred method at Stirling is solvent exchange in the vapour phase, this ensure minimal disturbance to the sample.

The choice of resin for impregnation is determined by the intended use of the soil thin section slide. For ‘normal’ slides produced for standard microscopy and image analysis techniques the resin of choice is a polyester called crystic 7449. This resin is readily thinned by acetone and the curing time can be controlled easily by increasing or decreasing the amount of catalyst (methyl ethyl ketone peroxide, MEKP). An ‘accelerator’ can also be added to shorten the curing time. If the soil thin section slide is to be used in SEM or Microprobe analysis for example then the resin of choice is epoxy (Araldite MY750 with hardener HY951). This resin shows greater integrity under electron beams than crystic resin. If, for some reason it is not possible to dry the sample then polyethylene glycol (PEG) is used, this is a waxy material which is soluble in water, has a low melting point and is reasonably easy to handle.

Processing of soil thin section slides involves the skilled manipulation of material bonded to glass slides to produce thin sections of around 30µm in thickness. These are then polished and coverslipped for protection.