Gas Chromatograph ECD Usage Precautions

Gas Chromatograph ECD Detector Precautions for Use

1. Keep the entire pneumatic system clean

ECD is very sensitive to impurities, so every step of the process should consider whether to bring in contaminated impurities. When foreign matter enters the ECD pool, two kinds of anomalies will occur: one is the surface pollution of the source, which reduces the ionization ability of the 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 specific cleaning measures are:

(1) Good air tightness of the system Through the airtightness test, it is ensured that the entire air circuit system from the air source to the detector outlet is airtight and no air leaks.

(2) High gas purity Both the carrier gas and the makeup gas are pure gas with a purity greater than 99.99%, or high purity gas greater than 99.999%.

(3) Small loss of septum The inlet septum is applied with a high temperature resistant mat. Before use, it can be aged in a column incubator at 250 ° C for 8-12 h, and even extracted with a solvent, and then used.

(4) Cleaning of the vaporization chamber The glass wool and glass cannula in the vaporization chamber should be replaced regularly.

(5) Small column loss The column should be fully aged at a temperature at least 25 ° C higher than the actual use temperature, and the column temperature should be used.

(6) Sample clean "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 Keep a small amount of purge gas through the ECD during temporary shutdown.

2. Detector pollution and its purification

According to one of the following signs, ECD may be contaminated:

(1) DC and constant frequency mode ECD base current drop or constant current mode base frequency increase. For example, the HP6890 gas chromatograph ECD, the design value is 100-600Hz, the fundamental 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.

For contaminated ECD, the operator can use one of the following labs to easily purify the ECD method.

(1) Thermal 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), and the column temperature is 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 washing

Replace the cleaned vaporization chamber intubation, remove the original column, replace it with a clean short empty column, and pass N ₂ gas to maintain the normal flow rate. The rise detector, vaporization chamber and column temperature were 300-350 ° C, 200 ° C and 150 ° C, respectively. A micro-injector was used to inject 50-100 uL of deionized water from the inlet for continuous injection for 10-20 injections. If the original capillary column is replaced, replace a short capillary column with no fixed solution, and inject 10-15uL each time for a total of 50-100 injections. 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 the cleaning takes 1-2 days, and has been used less frequently in recent years.

(3) Hydrogen baking

This is the 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 were room temperature, and the detector was 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.

If the above methods are invalid, you need to contact the manufacturer for processing.

3. Prevent ECD overload

In chromatographic analysis, column overload or detector overload may occur due to large injection volumes (or large sample concentrations). A typical 4 mm inner diameter packed column can tolerate up to 0.5-1 mg per peak and a WCOT column with an inner diameter of 0.25 mm is less than (2-5) 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 10-8-10-13g. 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.

4. Pay attention to safety

There are radioactive sources in the radioactive ECD. To prevent radioactive hazards, attention should be paid to the use:

(1) The detector outlet shall be connected to the outside by metal or polyethylene pipes, sheltered from wind, rain and off-roadwalks;

(2) The ECD may be dismantled after being trained and licensed to handle radioactive materials, otherwise it shall not be dismantled;

(3) According to the formal requirements, radioactive leak detection should be carried out at least every 6 months.