Standard Operating Procedure: Determination of Total Suspended Solids (TSS)

This Standard Operating Procedure (SOP) outlines the gravimetric method for determining Total Suspended Solids (TSS) in water and wastewater samples. TSS represents the portion of total solids retained by a glass-fiber filter after filtration of a well-mixed sample, followed by drying to a constant weight at 103–105°C. Adherence to this protocol is critical for ensuring regulatory compliance, maintaining data integrity, and minimizing analytical variance in environmental monitoring.

1. Preparation and Equipment Calibration

• Filter Preparation: Place glass-fiber filters (e.g., Whatman 934-AH or equivalent) into the filtration apparatus with the wrinkled side up.
• Pre-washing: Apply three successive 20 mL portions of reagent-grade water to the filter under vacuum suction. Continue suction for one minute after filtration is complete.
• Drying: Remove the filter from the apparatus and transfer to an aluminum weighing dish. Dry in an oven at 103–105°C for at least one hour.
• Desiccation: Transfer the dish/filter to a desiccator to cool to room temperature.
• Initial Weighing: Weigh the filter/dish assembly using an analytical balance (precision to 0.1 mg) and record as the "Tare Weight." Repeat until a constant weight (within 0.5 mg) is achieved.

2. Sample Collection and Processing

• Mixing: Ensure the sample is representative by shaking the container vigorously before drawing the aliquot.
• Volume Selection: Select a sample volume that will yield a residue between 2.5 mg and 200 mg. If the filtration time exceeds 10 minutes, reduce the sample volume.
• Filtration:
  • Place the pre-weighed filter onto the filtration apparatus and apply vacuum.
  • Wet the filter with a small amount of reagent-grade water to seat it properly.
  • Measure the sample volume precisely using a graduated cylinder.
  • Filter the sample through the glass-fiber filter and rinse the measuring vessel/filter funnel with three 10 mL portions of reagent-grade water, allowing drainage between rinses.
  • Continue suction for at least 3 minutes after filtration is complete.

3. Drying, Weighing, and Calculation

• Drying: Carefully remove the filter using forceps and place it back into the weighing dish. Dry in the oven at 103–105°C for at least one hour.
• Cooling: Transfer the dish to a desiccator and cool to room temperature to prevent moisture absorption from the atmosphere.
• Final Weighing: Weigh the filter/dish immediately after cooling.
• Calculation: Use the following formula:
  • mg TSS/L = [(A - B) * 1,000,000] / Sample Volume (mL)
  • Where A = Weight of filter + dish + residue (g) and B = Tare weight of filter + dish (g).

Pro Tips & Pitfalls

• The "Clumping" Pitfall: If the sample contains large floating particles or clumps, do not attempt to filter them as they will clog the filter and bias the result. Use a wide-bore pipette for sampling if necessary.
• Desiccator Maintenance: Always ensure the silica gel in the desiccator is active (check for color changes). A saturated desiccator will lead to artificially high weights due to moisture re-absorption.
• Handling: Always handle filters with clean, non-serrated forceps. Fingerprints contain oils that add significant weight at the microgram scale.
• Filter Type: Ensure you are using glass-fiber filters without organic binders. Filters with binders may leach material during the drying process, skewing results.

Frequently Asked Questions (FAQ)

Q: What should I do if the sample takes too long to filter? A: If filtration takes longer than 10 minutes, the filter is likely blinding. Reduce the sample volume for the next run or use a larger diameter filter if the equipment permits.

Q: Why is it necessary to dry to a "constant weight"? A: Constant weight ensures that all physically occluded water has been evaporated and that no further chemical decomposition of the sample is occurring at the drying temperature.

Q: Can I store the filtered samples before weighing? A: Yes, if immediate weighing is not possible, the filters can be stored in the desiccator, but they should be weighed as soon as they reach room temperature to minimize the risk of environmental contamination.