Toc

 Below is an updated **Standard Operating Procedure (SOP)** for **Total Organic Carbon (TOC)** analysis in **solid samples** using a TOC analyzer with a solid sample module (e.g., Shimadzu TOC-L with SSM-5000A). This SOP incorporates a detailed **calibration procedure** with specific criteria, establishes **criteria for quality control (QC) samples**, and includes the **computation for obtaining TOC** (TOC = TC - IC). The procedure is tailored for solid samples (e.g., soil, sediment, sludge) and includes an example for clarity.

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### Standard Operating Procedure (SOP) for TOC Analysis of Solid Samples

#### 1. Purpose

To provide a standardized procedure for determining Total Organic Carbon (TOC) in solid samples using a TOC analyzer with a solid sample module, ensuring accurate, reproducible, and compliant results.

#### 2. Scope

This SOP applies to the analysis of TOC in solid samples (e.g., soil, sediment, sludge, or biomass). It covers sample preparation, instrument calibration, quality control, TOC computation, and data reporting.

#### 3. References

- ASTM D7573: Standard Test Method for TOC in Solid Samples

- EPA Method 9060A: TOC in Soil and Sediment

- USP <643> (for general TOC principles, if applicable)

- Instrument manufacturer’s manual (e.g., Shimadzu TOC-L with SSM-5000A)

#### 4. Definitions

- **TOC**: Total Organic Carbon, carbon bound in organic compounds.

- **TC**: Total Carbon, sum of organic and inorganic carbon.

- **IC**: Inorganic Carbon, carbon in inorganic compounds (e.g., carbonates).

- **TOC = TC - IC**: TOC is calculated by subtracting IC from TC.

- **Control Chart**: A graphical tool to monitor instrument performance over time.

#### 5. Equipment and Materials

- TOC analyzer with solid sample module (e.g., Shimadzu SSM-5000A)

- Analytical balance (0.1 mg precision)

- Ceramic or quartz boats (pre-combusted at 900°C)

- Drying oven (105°C)

- Desiccator

- Mortar and pestle or ball mill (for homogenization)

- Reagents:

  - Sucrose (for TC calibration and QC standards, 42.1% carbon by weight)

  - Calcium carbonate (CaCO₃, for IC standards, 12% carbon by weight)

  - Phosphoric acid (5–10% v/v, for IC removal if needed)

- Quality control standards (e.g., certified soil reference material or sucrose)

- Ultra-pure water (TOC < 50 ppb)

- Spatula and forceps (cleaned)

- Computer with data acquisition software

#### 6. Safety Precautions

- Wear PPE (gloves, lab coat, safety glasses).

- Handle acids (e.g., phosphoric acid) in a fume hood.

- Use caution with high-temperature furnaces (up to 900°C).

- Dispose of chemical waste per local regulations.

#### 7. Procedure

##### 7.1 Sample Preparation

1. **Collect and store samples**:

   - Collect representative samples in clean, airtight containers.

   - Store at 4°C and analyze within 28 days (or per regulatory guidelines).

2. **Dry samples**:

   - Air-dry or oven-dry at 105°C to constant weight to remove moisture.

   - Cool in a desiccator.

3. **Homogenize**:

   - Grind samples using a mortar and pestle or ball mill to a fine powder (<0.2 mm particle size).

   - Sieve to ensure uniformity.

4. **Remove IC (optional)**:

   - If high carbonate content is suspected, treat a subsample with 5–10% phosphoric acid to remove IC (add dropwise until fizzing stops, then dry).

   - Use this subsample for direct TOC measurement (TOC = TC).

##### 7.2 Instrument Preparation

1. **Turn on the TOC analyzer** and solid sample module.

2. **Set furnace temperatures**:

   - TC combustion: ~900°C.

   - IC reaction: ~200°C (for IC measurement).

3. **Check gases**:

   - Ensure carrier gas (e.g., high-purity oxygen or nitrogen) flows at ~500 mL/min.

4. **Clean boats**:

   - Pre-combust ceramic/quartz boats at 900°C for 10 minutes to remove residual carbon.

   - Store in a desiccator.

5. **Perform system blank**:

   - Run an empty boat to ensure TC < 0.1 µg C (or manufacturer’s limit).

##### 7.3 Calibration Procedure

1. **Prepare calibration standards**:

   - **TC standards**: Use sucrose (42.1% carbon by weight).

     - Weigh 1.19, 2.38, 4.75, 11.88 mg sucrose to yield 0.5, 1.0, 2.0, 5.0 mg C, respectively.

   - **IC standards**: Use calcium carbonate (12% carbon by weight).

     - Weigh 4.17, 8.33, 16.67, 41.67 mg CaCO₃ to yield 0.5, 1.0, 2.0, 5.0 mg C, respectively.

   - Place standards in pre-combusted boats.

2. **Run calibration**:

   - Analyze each standard in triplicate.

   - Record peak areas for TC (at 900°C) and IC (at 200°C with acid reaction).

   - Generate calibration curves:

     - Plot peak area vs. carbon mass (µg C) for TC and IC separately.

     - Use linear regression: y = mx + b (y = peak area, x = µg C, m = slope, b = intercept).

3. **Calibration criteria**:

   - **Correlation coefficient (R²)**: ≥ 0.995 for both TC and IC curves.

   - **Slope**: Must be consistent with instrument sensitivity (e.g., within ±10% of historical values).

   - **Intercept**: Near zero (e.g., < 5% of lowest standard’s peak area).

   - **Relative standard deviation (RSD)**: ≤ 5% for triplicate injections of each standard.

   - **Calibration check standard**: Run a mid-range standard (e.g., 2 mg C sucrose) after calibration. Recovery must be 95–105% of expected value.

4. **Re-calibration**:

   - Perform if any criterion fails or after significant instrument maintenance.

##### 7.4 Sample Analysis

1. **Weigh samples**:

   - Weigh 10–100 mg of dried, homogenized sample into a clean boat (adjust based on expected carbon content).

   - Record exact weight to 0.1 mg.

2. **Run sequence**:

   - **Blank**: Empty boat to confirm TC < 0.1 µg C.

   - **Calibration check standard**: Sucrose (e.g., 2 mg C) to verify calibration (95–105% recovery).

   - **Quality control sample**: Certified reference material or sucrose (e.g., 2 mg C).

   - **Samples**: Analyze in duplicate or triplicate.

   - **TC measurement**: Combust at 900°C.

   - **IC measurement**: React with acid at 200°C (if needed).

3. **Optional direct TOC**:

   - If IC was removed by acid treatment, measure TC of treated sample as TOC.

##### 7.5 Quality Control

1. **System Blank**:

   - Criteria: TC < 0.1 µg C (or manufacturer’s limit).

2. **Quality Control Sample**:

   - Use a certified reference material (e.g., soil with known TOC) or sucrose (e.g., 2 mg C).

   - Run every 10 samples or at least once per batch.

   - **Criteria**:

     - Recovery: 90–110% of certified/expected value.

     - Precision: RSD ≤ 5% for triplicate measurements.

     - If QC fails, investigate (e.g., check sample preparation, re-calibrate) and re-run.

3. **Control Chart**:

   - Plot QC results (%C or mg C) over time.

   - Calculate mean and ±2 standard deviations (control limits) from at least 20 historical QC results.

   - **Criteria**:

     - Results must fall within control limits.

     - No trends (e.g., 7 consecutive points above/below mean or increasing/decreasing).

     - Investigate and take corrective action if criteria are not met.

4. **Duplicates**:

   - Relative percent difference (RPD) between duplicates ≤ 10%.

   - RPD = |(C1 - C2)| / [(C1 + C2)/2] × 100.

##### 7.6 Computation for Obtaining TOC

1. **Determine TC and IC**:

   - Use calibration curves to convert peak areas to carbon mass (µg C).

   - TC (µg C) = (TC peak area - TC intercept) / TC slope.

   - IC (µg C) = (IC peak area - IC intercept) / IC slope.

2. **Calculate TOC**:

   - TOC (µg C) = TC (µg C) - IC (µg C).

3. **Convert to %C or mg C/g**:

   - %TOC = (TOC in µg C / sample weight in µg) × 100.

   - mg C/g = (TOC in µg C / sample weight in mg).

4. **Example Calculation**:

   - Sample weight: 50 mg (50,000 µg).

   - TC calibration: y = 1000x + 50.

   - IC calibration: y = 950x + 30.

   - TC peak area: 4500 → TC = (4500 - 50) / 1000 = 4.45 µg C.

   - IC peak area: 1000 → IC = (1000 - 30) / 950 = 1.02 µg C.

   - TOC = 4.45 - 1.02 = 3.43 µg C.

   - %TOC = (3.43 / 50,000) × 100 = 0.00686% (68.6 ppm).

   - mg C/g = 3.43 / 50 = 0.0686 mg C/g.

##### 7.7 Data Recording

1. Record sample weights, TC, IC, and TOC (µg C, %C, or mg C/g).

2. Save calibration curves, QC data, and control charts.

3. Document deviations, corrective actions, and instrument maintenance.

#### 8. Maintenance

- Clean combustion tube and boats after 100–200 runs or if contamination is suspected.

- Check gas regulators and replace if pressure drops.

- Perform annual maintenance by a certified technician.

#### 9. Troubleshooting

- **High blank values**: Re-combust boats or check carrier gas purity.

- **Calibration failure**: Verify standard weights, re-prepare standards, or clean furnace.

- **QC failure**: Check sample homogeneity, re-calibrate, or investigate contamination.

- **Inconsistent duplicates**: Ensure uniform grinding or check boat placement.

#### 10. Example

**Scenario**: Analyzing TOC in agricultural soil.

- **Sample Preparation**:

  - 100 g soil dried at 105°C, ground to <0.2 mm.

  - Sample weight: 50 mg.

- **Calibration**:

  - TC standards: 0.5, 1.0, 2.0, 5.0 mg C (sucrose).

  - IC standards: 0.5, 1.0, 2.0, 5.0 mg C (CaCO₃).

  - TC curve: y = 1000x + 50, R² = 0.998, RSD ≤ 3%.

  - IC curve: y = 950x + 30, R² = 0.997, RSD ≤ 4%.

  - Calibration check (2 mg C sucrose): 2.02 mg C (101% recovery, acceptable).

- **Analysis**:

  - Blank: 0.05 µg C (acceptable).

  - QC (2 mg C sucrose): 2.1 mg C (105% recovery, RSD = 2%, acceptable).

  - Sample TC: 4.5 µg C, IC: 1.0 µg C.

  - TOC = 4.5 - 1.0 = 3.5 µg C.

  - %TOC = (3.5 / 50,000) × 100 = 0.007% (70 ppm).

  - mg C/g = 3.5 / 50 = 0.07 mg C/g.

- **Control Chart**:

  - QC results (last 20 runs): Mean = 2.05 mg C, SD = 0.06 mg C.

  - Control limits: 1.93–2.17 mg C.

  - Current QC (2.1 mg C) within limits, no trends.

#### 11. Records

- Maintain a logbook for sample weights, calibration, QC, and results.

- Archive data for at least 5 years (or per regulatory requirements).

#### 12. Approval

- Prepared by: [Analyst Name]

- Approved by: [Lab Manager Name]

- Effective Date: [Date]

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This SOP provides a comprehensive framework for TOC analysis in solid samples, with explicit calibration criteria, QC sample criteria, and TOC computation. Let me know if you need further customization, additional details, or clarification!