Electronic capture detector use notes - Database & Sql Blog Articles

Electronic Capture Detector Usage Precautions The Electron Capture Detector (ECD) is a highly sensitive and selective detector. In gas chromatograph detection, ECD only has an output signal for substances with electronegativity, such as compounds containing S, P, halogen, metalorganic compounds and compounds containing carbonyl, nitro, conjugated double bonds; Very small compounds, such as hydrocarbons, have little or no output signal. The greater the electronegativity of the analyte, the smaller the detection limit of the ECD (up to 10-12 to 10-14 g), so the substance suitable for analysis by the ECD is a trace amount of electronegative compound. Although the linear range of ECD is narrow, ECD is still widely used in biology, medicine, pesticides, environmental protection, metal chelate and weather tracking.
The most important thing in the use of ECD is to always keep the system clean. If there is pollution, it should be discovered and eliminated in time. In order to make the quantification accurate, it is necessary to pay attention to prevent the ECD from being overloaded, and also pay attention to safety (no over-temperature use is strictly prohibited).

First, keep the entire pneumatic system clean
ECD is very sensitive to impurities, so every link in the use should consider whether to bring in contaminated impurities. When foreign matter enters the ECD pool, there will be two kinds of abnormalities: one is the surface pollution of the radioactive source, which reduces the ionization ability of the radio source, so that the DCD base current of the DC voltage and the constant frequency mode decreases or the fundamental frequency increases in the constant current mode; Direct capture of electrons in the ECD reduces the base current or increases the fundamental frequency. Both ultimately result in reduced sensitivity. The cleaning method of the ECD detector is as follows:
(1) The system is airtight. Through the air tightness test, it is ensured that the entire air circuit system from the air source to the detector outlet is airtight and there is no air leakage.
(2) Gas purity High carrier gas and makeup gas are both pure gas with a purity greater than 99.99%, or high purity gas greater than 99.999%.
(3) The septum is drained. The small inlet septum is applied with a high temperature resistant mat. Before use, it can be aged in a column incubator at 250 ° C for 8 ~ 12h, and even extracted with solvent, and then used.
(4) Glass wool and glass cannula in the vaporization chamber of the vaporization chamber should be replaced regularly.
(5) Column loss The small column should be fully aged at a temperature at least 25 ° C higher than the actual use temperature, and used at a low column temperature.
(6) The sample is clean and the “dirty sample” should be cleaned. The solvent is subjected to a secondary distillation of an alkane, an aromatic hydrocarbon or a monochlorohydrocarbon.
(7) Clean both ends of the capillary column When the temperature of the vaporization chamber or detector is high, the polyimide coating outside the capillary column may be decomposed into volatile components into the ECD pool. For this reason, a low temperature flame such as butane may be used. The lighter burns it to keep the ends of the column clean.
(8) The detector temperature is higher than the column temperature by 10 °C or more.
(9) Keep the purge gas. When the machine is temporarily shut down (no instrument is used within one week), keep a small amount of purge gas through the ECD.

Second, detector pollution and its purification
1. The phenomenon of judging the pollution of the ECD detector is as follows:
(1) DC and constant frequency mode ECD base current drop or constant current mode base frequency increase. Such as HP6890 gas chromatograph ECD, the design value is 100 ~ 600Hz base frequency is clean; in actual judgment, it is generally considered that more than 500Hz may be contaminated;
(2) The noise increases and the signal-to-noise ratio decreases;
(3) The baseline drift becomes larger;
(4) The linear range becomes smaller;
(5) A negative peak, usually with a negative peak after the large peak.
2. Common ECD detector cleaning methods:
(1) Hot cleaning method: This method is usually used for mild pollution. First, ensure that the pneumatic system is leak-free and non-polluting [(2), (3) the same]. Then, remove the column and block the fitting of the column detector with a bulkhead nut. Adjust the N2 tail gas to 50 ~ 60mL / min, raise the detector temperature to about 350 ° C (63Ni source), column temperature 250 ° C, for 4 ~ 8h. Finally, cool to the normal operating temperature and observe if the fundamental frequency drops to normal. If it is effective but not enough, it can be repeated.
(2) Hot water steam cleaning method: replace the clean vaporization chamber intubation, remove the original column, replace it with a clean short empty column, and pass N2 gas to maintain the normal flow rate. The rise detector, vaporization chamber and column temperature are 300-350 ° C, 200 ° C and 150 ° C, respectively. A small syringe was used to inject 50 to 100 μL of deionized water from the inlet, and continuous injection was performed for 10 to 20 injections. If the capillary column is connected to the original capillary tube, a short capillary column without a fixed solution is injected, and each injection is 10-15 μL, and a total of 50 to 100 injections are performed. In this way, the ECD pool is cleaned with a stream of hot water vapor. This method can remove most of the pollution. However, the operation is troublesome (if the autosampler can reduce some workload), it takes a long time, and it takes 1 to 2 days to clean once, and has been used less frequently in recent years.
(3) Hydrogen baking cleaning method: This is a more common method in recent years. Simply change the carrier gas or makeup gas to hydrogen and adjust the flow rate to 30-40mL/min. The vaporization chamber and column temperature are room temperature, and the detector is raised to 300-350 ° C for 18-24 hours to remove contaminants from hydrogen at high temperatures. After the hydrogen is baked, the system is returned to the original state and stabilized for several hours.

III. Preventing ECD overload In the chromatograph analysis, the injection volume may be large (or the sample concentration is large), and the overload or detector overload may occur. Generally, a 4 mm inner diameter packed column can allow a peak of 0.5 to 1 mg per peak and an inner diameter of 0.25 mm. The WCOT column is less than (2 to 5) x 10-8 g per peak. When using ECD for the analysis of trace contaminants in environmental samples, the amount of sample per peak is 10-8 to 10-13. This of course does not cause column overload, but for ECD with a narrow linear range, it is easy to achieve response saturation. It shows that the peak height no longer increases (or increases less), while the half-width increases. This is also known as ECD overload. Obviously, it will bring a lot of error to the quantitative. At this time, the sample must be diluted with the solvent to within the linear range of the ECD, and then measured.