TITLE: Particle size analysis by laser diffraction
AUTHORS: Jennifer Moore

[DESCRIPTION]
This method is a relatively rapid protocol for the determination of soil particle size analysis (texture) using laser diffraction. A critical step is to wet sieve the sand fraction prior to laser analysis.


[STEPS]
SECTION: Reagents
1. Sodium hexametaphosphate (NaHMP) 10% solution
Chemical formula: (NaPO3)6
Molar mass: 611.77 g/mol
Procedure: Mix 100 g  of NaHMP in 1000 mL  of deionized water.
SECTION: Sample preparation
3. Gather soil samples to be weighed. Samples should be dry and sieved to <2mm. Label the 125-mL wide-mouth Nalgene bottles ("small bottles") with the sample IDs.
Weigh 5 g  of soil into the small bottle with the corresponding sample ID. Record mass to three decimal places. 
 

Add 50 mL of 10% sodium hexametaphosphate solution.
Add 50 mL of deionized water.
Cap small bottles and shake vigorously.
Allow prepared samples to soak 5 min
Label and weigh smooth-walled metal tins. Record mass to at least three decimal places and set aside to be used in extraction.

SECTION: Laser analysis SOP settings
9.  

The appropriate SOP (currently file "20230825_soil.sop") has the following settings:
Stirring speed is 3500 rpm.
Sonication power is 100% and runs for 60 seconds pre-measurement. 
Mode is MIE scattering general purpose model for quartz material with non-spherical shape.
Particle refractive index is 1.55
Particle absorption index is 0.1
Water refractive index is 1.33
Density is 2.65g/cm3
Laser obscuration is default settings: 10-20%
 

Background measurement duration is 15 sec
Sample measurement duration is 30 sec
Level sensor threshold is 64%, which is the default value (for water)
Degas is checked
Align is checked
User defined sizes are selected as follows: 0.01 um, 6.325 um, 56.368 um, 2000 um. These are the particle size cut offs for clay (0.02-6 um), silt (6-56 um), and sand (56-2000 um).
SECTION: Calculations
11. Sand1 fraction (fsa1) = Total dried soil sample (MT) - Mass of sand particles following sieving and drying (Msa)/MT.
Clay fraction (f­cl) = [(MT - Msa) x f­cl-LD]/MT
Silt fraction (fsi) = [(MT - Msa) x f­si-LD]/ MT
NOTE: there is some fine sands that can be in the silt + clay fraction. Add the correct proportion from the dried sand fraction to the sand fraction from the laser. 
Sand2 (laser) fraction (fsa2) = [(MT - Msa) x f­sa-LD]/ MT
Total sand fraction (fTsa) = fsa1 + fsa2

f­cl-LD = Laser fraction cumulative volume of particles less than a clay-silt cutoff of 6.325μm.
f­si-LD = Laser fraction cumulative volume of particles greater than the clay-silt cutoff.
f­sa2 = Laser fraction cumulative volume of particles greater than a sand-silt cutoff of 56.368μm.
SECTION: Materials
2. Preparation
Dry, well-mixed soil sieved to <2mm
125-mL wide-mouth Nalgene bottles ("small bottles")
10% sodium hexametaphosphate solution
Deionized water
Extraction
500-mL Nalgene bottles ("large bottles")
Metal mixing cups and milkshake mixer (Hamilton Beach HMD 200 Drink Mixer)
Large funnel
53um sieves (diameter of sieve = 3")
Glass stir rod
DI water (in squeeze bottles and plastic beakers)
Smooth-walled metal tins
Stir plate and stir rod
Empty plastic beaker
Electric pipettor with a plastic 25-mL pipette
  Laser analysis
Stir plate and stir bars
DI water in plastic beaker
Empty plastic beaker
7-mL transfer pipette
"Mastersizer 2000" laser analysis machine
SECTION: Sample extraction
4. Prepared samples are ready for extraction after they have soaked overnight.
Gather prepared samples ready for extraction. Label the 500-mL Nalgene bottles ("large bottles") with the sample IDs.
Transfer the prepared sample from its small bottle into a metal mixing cup. 
Rinse any remaining soil from the small bottle and cap into the mixing cup. Using a plastic beaker containing DI water, fill the small bottle halfway, then cap it and shake vigorously. Pour the contents into the mixing cup. If needed, rinse again using a squeeze bottle. Try to use as little water as possible.
 

SECTION: Sample extraction
5. Place the metal mixing cup into the milkshake mixer on the upper position of the cup rest, which will activate the mixer. Mix the sample for 5 min on low speed. 
While sample is mixing, transfer other samples ready for extraction into remaining metal mixing cups. Keep the labeled small and large bottles close to the unlabeled metal mixing cups that hold the corresponding samples.
After 5 minutes, stop the mixing action by moving cup to the lower position of the cup rest. Once the mixer has stopped, remove the newly mixed sample from the milkshake machine. Rinse spindle as needed to remove any adhering soil particles. Place the next remaining unmixed metal mixing cup into the milkshake machine and begin agitating.
Gather a large funnel and 53um sieve. Place sieve into funnel and arrange above the large bottle labeled with the sample ID that corresponds with the newly mixed sample.  
Slowly pour mixed sample into sieve. If necessary, gently stir the liquid in the sieve with a glass stir rod to help the solution pass through.
Using a DI water squeeze bottle, rinse all remaining soil from the metal mixing cup into the sieve.

 

SECTION: Sample extraction
6. Sand particles larger than   will remain on top of the sieve. 
Using a squeeze bottle, rinse the sieve with DI water until all fine particles have passed through. Gently tilt the sieve back and forth, moving the particles across the top as you rinse them. Use the glass stir rod or small rubber spatula to ensure no fine particles remain. All fine particles have passed through the sieve when the water that exits the sieve is clear no matter which way you tilt it.  Be careful to not exceed the large bottle maximum volume.
Gather a labeled smooth-walled tin. Arrange the funnel and sieve over the tin. Flip the sieve over to transfer the particles larger than 53um into the tin through the funnel. Rinse the sieve and funnel until all remaining particles are in the tin. Try to use as little water as possible.
Place tin on an oven tray. When the tray is filled with tins, put it into the oven at 105 °C.
When all water has evaporated, cool the tins. Weigh and record mass to three decimal places. Tins may require 4-8 hours to fully dry. 
Cap the large bottle, gather the corresponding small bottle, and set both aside for the following steps.
 
 
SECTION: Sample extraction
7. Samples inside the large bottles must now be transferred back into the corresponding small bottles.
Assemble a stir plate, stir bar, empty beaker, and DI water beaker into a comfortable arrangement. Gather the labeled pairs of small and large bottles that had been set aside.
Drop the stir bar into a large bottle and place on stir plate. Stir on medium speed for at least 20 seconds, ensuring the formation of a visible vortex.
Using the 25-mL electric pipette, transfer approximately 100 mL  from the labeled large bottle into the correspondingly labeled small bottle. Rinse the pipette tip with DI water between different samples.
This volume does not need to be exact: fill the small bottle until it is almost full. The tip of the electric pipette should be located approximately halfway into bottle when extracting to ensure a representative sample.
Cap the small bottles and set aside on a plastic tray. Deliver the plastic tray with small bottles upstairs to be analyzed with the laser.
SECTION: Laser analysis measurement
10. Now that the samples have been fully separated and the  sand-sized particles have been extracted, the proportion of clay and silt-sized particles in the sample can be analyzed by the laser.

 

Make sure that a sample containing a stir bar is stirring on the stir plate at medium speed.
Once the SOP begins, it will ask for a sample ID. Enter the sample ID that corresponds with the sample that is on the stir plate. The SOP will then clean the tank, align the laser, and measure background light.
When the instrument is ready for sample to be added, the computer will indicate "Auto-Dilution is in progress... Please add sample". A soft beeping noise will also sound aloud.
Using a 7-mL transfer pipette, dispense the sample into the tank of the machine. Dispense enough sample to reach an obscuration level between 10-20% (approximately 14 mL).
The instrument will automatically begin to measure the sample. The process lasts for a duration of approximately 4 minutes.
 

While the instrument is measuring,
Remove the sample from the stir plate. Remove the stir bar, then cap and place on a plastic tray to be discarded later. 
Place the next sample containing a stir bar on the stir plate and begin spinning at a medium speed.
Clean the iron shavings from the used stir bar. When the stir bar is clean, uncap a new sample and drop the stir bar inside.

After the measurement is complete, the computer will ask if the SOP should be run again. Click "Yes". Enter the correct sample ID which corresponds with the next sample that is on the stir plate.

 

SECTION: Calculations
13.   
The attached spreadsheet can be used to generate a soil texture triangle plotting all samples in a given set.
SECTION: Calculations
12. Website for soil textural classification (calculator)
SECTION: Clean up
14. Always leave your workspace clean at the end of the day!
At the end of the day, wipe down benches, balances, and all instrumentation.
Turn off balance.
Log out and close all websites and any data sheets.
The mixer should be wiped down and completely cleaned each day.
The laser and hydro 2000S should be turned off at the end of each run.
SECTION: Sample extraction
8.