Proper storage of soil samples is critical for ensuring the integrity and accuracy of analytical results, particularly when testing for volatile organic compounds (VOCs). The choice of vial type used for soil sample storage can significantly impact the preservation of analytes and the reliability of the data generated.In this blog post, we will explore the various vial types recommended for soil sample storage and their advantages and limitations. We will also discuss best practices for sample collection, preservation, and handling to maintain sample integrity during storage and transportation.
Importance of Proper Soil Sample Storage
Soil samples collected for VOC analysis are highly susceptible to analyte loss due to volatilization, biodegradation, and other chemical and physical processes. Improper storage conditions, such as exposure to air, high temperatures, or incorrect preservatives, can lead to significant underestimation of VOC concentrations in the soil. To ensure the accuracy and reliability of VOC analysis in soil samples, it is essential to use appropriate vial types and follow established protocols for sample collection, preservation, and storage. The U.S. Environmental Protection Agency (EPA) and other regulatory agencies have developed guidelines and methods for soil sample storage, such as EPA Method 5035 and ASTM D4547-91.
Types of Vials Recommended for Soil Sample Storage
Several types of vials are recommended for soil sample storage, depending on the specific requirements of the analysis method and the expected VOC concentrations in the soil. Some common vial types include:
- 40 mL VOA vials: These vials are commonly used for collecting and storing soil samples for VOC analysis. They typically have a volume of 40 mL and are equipped with a septum-sealed screw cap to minimize headspace and prevent the loss of analytes.
- En Core samplers: En Core samplers are specialized devices designed for collecting and storing soil samples for VOC analysis. They consist of a cylindrical chamber with a plunger that allows the sample to be extruded directly into the sample container, minimizing disturbance and volatilization losses.
- Syringes: Modified syringes, such as the 10 mL syringe with the tip and rubber plunger removed, can be used for collecting and storing soil samples for VOC analysis. The sample is collected directly into the syringe and extruded into the sample container upon arrival at the laboratory.
- Encore-type samplers: These samplers are similar to En Core samplers but are designed to be used with pre-prepared vials containing a preservative, such as methanol or sodium bisulfate. The sample is collected directly into the sampler and extruded into the vial, which is then sealed to minimize headspace.
When selecting a vial type for soil sample storage, it is essential to consider factors such as the expected VOC concentrations, the specific requirements of the analysis method, and the availability of laboratory equipment and expertise.
Sample Collection and Preservation Protocols
To ensure the integrity of soil samples during storage and transportation, it is crucial to follow established protocols for sample collection and preservation. These protocols typically include:
- Sample collection: Soil samples should be collected using appropriate sampling techniques, such as coring devices or syringes, to minimize disturbance and volatilization losses.
- Headspace minimization: The sample container should be filled to minimize headspace, which can lead to the loss of volatile analytes.
- Preservation: Depending on the analysis method and the expected VOC concentrations, soil samples may require preservation with a chemical agent, such as methanol or sodium bisulfate, to inhibit biodegradation and maintain analyte stability.
- Cooling: Soil samples should be cooled to 4°C (±2°C) immediately after collection and maintained at this temperature during storage and transportation to minimize volatilization and biodegradation.
- Holding time: Soil samples should be analyzed within the specified holding time, which varies depending on the preservation method and the analysis method. For example, samples frozen in a coring device have a holding time of 48 hours, while samples extracted from a coring device within 48 hours and frozen in vials or preserved with methanol have a holding time of 14 days.
By following these protocols and using appropriate vial types, environmental professionals can ensure the integrity of soil samples and obtain reliable analytical results for VOC analysis.
Advantages and Limitations of Different Vial Types
Each type of vial used for soil sample storage has its own advantages and limitations:
- 40 mL VOA vials:
- Advantages: Widely available, cost-effective, and suitable for a wide range of VOC concentrations
- Limitations: Require manual sample collection and transfer, which can lead to volatilization losses and potential contamination
- En Core samplers:
- Advantages: Minimize disturbance and volatilization losses, allow for direct extrusion into the sample container
- Limitations: More expensive than other vial types, require specialized equipment for sample collection and extrusion
- Syringes:
- Advantages: Cost-effective, allow for direct sample collection and transfer into the sample container
- Limitations: Require manual sample collection and transfer, which can lead to volatilization losses and potential contamination
- Encore-type samplers:
- Advantages: Minimize disturbance and volatilization losses, allow for direct extrusion into a pre-prepared vial containing a preservative
- Limitations: More expensive than other vial types, require specialized equipment for sample collection and extrusion
When selecting a vial type for soil sample storage, it is essential to balance the advantages and limitations of each type and choose the option that best suits the specific requirements of the project and the available resources.
Best Practices for Soil Sample Storage
To ensure the integrity of soil samples during storage and transportation, it is essential to follow best practices, such as:
- Use only laboratory-provided, pre-cleaned and pre-weighed vials: These vials are certified to be free from contamination and have a known tare weight, which is necessary for calculating the sample mass.
- Avoid exposing samples to air for extended periods: Soil samples should be collected and transferred to the sample container as quickly as possible to minimize volatilization losses.
- Maintain a consistent chain of custody: Ensure that the samples are handled by authorized personnel and that any transfers or changes in custody are documented.
- Conduct quality control checks: Regularly analyze blank samples, duplicates, and spiked samples to verify the accuracy and precision of the analytical results.
- Collaborate with accredited laboratories: Work with laboratories that are accredited by the National Environmental Laboratory Accreditation Program (NELAP) or other recognized accreditation bodies to ensure that the analysis of the samples is conducted in accordance with established protocols and quality standards.
By following these best practices and using appropriate vial types for soil sample storage, environmental professionals can ensure the integrity and reliability of VOC analysis in soil samples.
Case Studies and Examples
Several case studies and examples demonstrate the importance of using appropriate vial types and following established protocols for soil sample storage:
- Groundwater monitoring at a hazardous waste site: En Core samplers were used to collect and store soil samples from a hazardous waste site to monitor the presence and concentration of VOCs. The use of these samplers ensured that the samples accurately represented the site conditions and provided reliable data for assessing the effectiveness of remediation efforts.
- Soil sampling at a former industrial site: 40 mL VOA vials were used to collect and store soil samples from a former industrial site to assess the presence and concentration of VOCs. The use of these vials, along with proper sample collection and preservation protocols, ensured that the data generated was reliable and reproducible.
- Comparison of collocated samples collected with a syringe vs. En Core sampler: A study compared the recovery of trichloroethylene (TCE) in soil samples collected using a syringe and an En Core sampler. The results showed that samples collected with the En Core sampler had a recovery of 85-97% relative to the syringe samples, demonstrating the effectiveness of the En Core sampler in preserving VOCs in soil samples.
These case studies and examples highlight the importance of using appropriate vial types and following established protocols for soil sample storage to ensure the accuracy and reliability of VOC analysis in soil samples.
Proper soil sample storage is essential for ensuring the integrity and accuracy of analytical results, particularly when testing for volatile organic compounds (VOCs). The choice of vial type used for soil sample storage can significantly impact the preservation of analytes and the reliability of the data generated.
