ECD Detector: The Most Sensitive Gas Chromatography Detector

1. Introduction

Electrochemical detection (ECD) is a detection technique used for ionization. It is characterized by its ability to provide accurate, reliable, controllable, and sustainable results with high precision and low cost, across a variety of environmental conditions. ECD aims to meet various environmental requirements, thereby enabling environmental protection and safety monitoring.

ECD is not only the most sensitive gas chromatography detector but also the earliest selective detector. It responds only to compounds capable of capturing electrons, such as halogenated hydrocarbons and compounds containing nitrogen, oxygen, sulfur, and other heteroatoms. Due to its high sensitivity and selectivity, the ECD has been widely used for the analysis of trace pesticides and polychlorinated biphenyls (PCBs) in environmental samples for many years.

2. Characteristics
   1. Powerful Ionization Capability:
   The ECD exhibits a strong ability to ionize compounds, providing an ample baseline current.
   
   2. Extremely Low Penetration Power:
   The ECD has very low penetration power, effectively ensuring the safety of individuals.
   
   3. Long Half-Life and Stable Performance:
   The ECD has a longer half-life, resulting in stable performance and an extended operational lifespan.
   
   4. High Temperature Resistance and Low Contamination:
   The ECD can withstand high temperatures and is less prone to contamination, allowing for a wider range of applications.

3. Working Principle

The ECD consists of a chamber covered with a layer of 63Ni radioactive source. This source emits β-rays, which cause the decomposition of the carrier gas inside the chamber, generating a series of secondary ions. These ions are influenced by the electric field due to the presence of the 63Ni radioactive source, resulting in a phenomenon known as the baseline current. Electronegative compounds within the ECD chamber are captured, leading to the generation of negative ions. As the “negative peaks” increase, they are amplified and eventually transmitted to the data processing system, providing a complete spectrum.

4. Classification

There are numerous classification methods for ECD, and a thorough understanding of these methods’ operational characteristics can assist us in selecting the most suitable classification method based on different analytical requirements.

4.1 Classification based on Ion Sources

ECD technology can be classified into two main categories: radioactive and non-radioactive. While non-radioactive ECD techniques may have certain advantages, such as the production of high-quality helium and the ability to provide power by adding rare gases, their structure and electronic devices are relatively complex. Additionally, there are still some operational drawbacks that need further improvement. Therefore, further advancements are required in non-radioactive ECD technology.

4.2 Classification based on the Type of Radioactive Source

ECD can be categorized into two major types based on the radioactive source: 63Ni and 3H.

Requirements for Radioactive Sources in ECD:

• Safety in use: Compared to alpha (α) and gamma (γ) radiation, beta (β) sources have more stable ionization and penetration capabilities, making them more suitable for ionization radioactive sources.
• Maximum radiation capability to meet the requirements.
• The range of radiation should be as short as possible to ensure structural design and safety. However, the demand for radiation energy should not be overlooked. Therefore, a balance needs to be struck between the two in practical applications.
• Sufficiently long half-life.
• High operating temperature.

5. Carrier Gas Selection

• N2 is commonly used as the carrier gas in ECD. It provides a large baseline current, and a purity of 99.999% or higher is required for the N2 gas.
• If Ar or He is used as the carrier gas, the presence of metastable molecules can lead to non-capture effects. Therefore, a tail-blow gas (also known as a protective gas) such as N2 should be used.
• To ensure the efficient operation of the system, the carrier gas supply line should incorporate dehydration and deoxygenation filtration devices to reduce the levels of O2 and H2O. This helps stabilize the magnitude of the baseline current.

6. Precautions

During the use of the ECD detector, it is important to maintain the cleanliness of the detection system and promptly address any system maintenance and contamination issues. To keep the ECD chamber clean, the following steps should be taken:

• Use high-purity gases for the carrier gas and tail-blow gas. Incorporate molecular sieve filters and hydrazine in the gas line, replacing them regularly.
• Prior to testing, age the sample inlet and chromatographic column adequately to prevent impurities from entering the ECD chamber during testing.
• Use low bleed chromatographic columns to prevent the leaching of stationary phase at high temperatures. The detector temperature should be set at least 20°C higher than the oven temperature.
• Perform regular high-temperature aging of the ECD (at 350°C).
• When the ECD is not in use for an extended period and the chromatographic column is removed, seal the detector interface (using solid graphite ferrules or similar inert seals).
• Due to the irreversible nature of ECD contamination, caution should be exercised in selecting appropriate solvents for ECD analysis. Avoid highly charged chemical reagents such as carbon disulfide, dichloromethane, chloroform, carbon tetrachloride, dichloroethylene, trichloroethylene, tetrachloroethylene, as they can severely damage the accuracy of the ECD and shorten its effective lifespan.
• The response of the ECD is related to the length of the chromatographic column extending into the chamber. Before installing each column, use the provided measuring tools to confirm the length of the column extending into the ECD chamber.