Total Hydrocarbon (THC) Analysis in Medical Devices – Technical Principles, Device Construction and Validation Criteria

1. Purpose of THC Analysis in Medical Devices

Total Hydrocarbon (THC) analysis is essential to identify volatile and semi-volatile organic compounds (VOCs) that may leach from medical devices into the patient environment. These compounds may originate from:

Lubricants, silicone oils, mold release agents
Residual solvents (IPA, hexane, toluene, xylene)
Ethylene oxide (EO) sterilization by-products
Packaging material emissions
PVC, PU, PE, PP, silicone, TPU polymer degradation products

THC analysis is directly related to ISO 10993-18 (Chemical Characterization) and ISO 10993-17 (Toxicological Risk Assessment).

2. Applicable Standards and Guidelines

Standard / Regulation	Scope
ISO 10993-18	Chemical characterization of medical device materials
ISO 10993-12	Sample extraction (surface area/volume ratio, solvent selection)
ISO 10993-5	Cytotoxicity testing of extracts
USP <467>	Residual solvent limits
TS EN 12619 / EPA 25A	THC/VOC measurement using Flame Ionization Detector (FID)
ISO 13485 / FDA 21 CFR 820	Process control and documentation

3. Analytical Technique – FID-Based THC Measurement

Flame Ionization Detector (FID) Principle

FID detects organic compounds containing C–H bonds by combusting them in a hydrogen-air flame, producing ions. The ion current is proportional to the hydrocarbon concentration.

Parameter	Description
Flame Gas	Hydrogen + Synthetic Air
Detection Limit (LOD)	~0.05 ppmC
Quantitation Limit (LOQ)	~0.1 ppmC
Output Units	ppmC (carbon equivalent), mg/m³

4. Sample Preparation – According to ISO 10993-12

Parameter	Specification
Extraction Ratio	6 cm²/mL (surface area-based extraction)
Extraction Media	0.9% NaCl, Ethanol 70%, PBS, IPA
Extraction Temperature	37°C or accelerated at 50°C
Duration	24–72 hours
Filtration	0.22 μm PTFE filter (low VOC adsorption)

5. Gas Chromatography – FID Method

GC-FID Operating Conditions

Setting	Value
Column	DB-624 (30 m x 0.32 mm x 1.8 μm)
Carrier Gas	Helium (1.5 mL/min)
Injection Volume	1 μL (Split 1:20)
Injector Temp.	200°C
Oven Program	40°C → 200°C at 10°C/min
Detector (FID) Temp.	250°C
Hydrogen Flow	40 mL/min
Air Flow	450 mL/min

6. Calibration Procedure

Calibration Standards

Gas Standard	Concentration	Certification
Propane (C₃H₈)	1% ±1%	ISO 6142 certified
Methane (CH₄)	1%	Alternative carbon reference
Zero Air	Hydrocarbon-free	Baseline correction

THC Calculation (ppmC to mg/m³)

$\\text{THC (mg/m³)} = \\frac{\\text{ppmC} \\times \\text{Molecular Weight}}{24.45 \\times \\text{Number of Carbon Atoms}}$

Example:
Measured Value: 20 ppmC (Propane equivalent)

$= \\frac{20 \\times 44}{24.45 \\times 3} = 12.0 \\text{ mg/m³}$

7. Acceptance Limits for Medical Devices

Parameter	Limit	Reference
Total Hydrocarbon Residue	< 10 mg/device	ISO 10993-12 & FDA
Residual Solvents (IPA, Hexane)	< 500 µg/device	USP <467>
THC in Sterilization Chamber	< 50 ppmC	TS EN 12619
EO Sterilization VOC Residue	< 250 µg/device	ISO 11135

8. Method Validation Criteria (FDA & ISO Requirements)

✔ System Suitability Test (SST) performed before analysis
✔ Blank measurement (zero air) < 0.1 ppmC
✔ Linearity: R² ≥ 0.995
✔ Repeatability: RSD < 5%
✔ Recovery: 80–120% (spiked QC samples)
✔ Positive Control: Heptane or Toluene standard solution

9. Reporting Requirements

A compliant test report must include:

Sample description (material, batch number, sterilization type)
Extraction conditions (time, temperature, solvent, SA/V ratio)
GC-FID chromatograms and calibration curves
Raw THC values (ppmC) and converted values (mg/m³, µg/device)
Comparison with ISO/FDA limit values
Risk assessment according to ISO 10993-17
Conclusion: Complies / Does Not Comply

Total Hydrocarbon (THC) Analysis in Medical Devices – Technical Principles, Device Construction and Validation Criteria