Principles for the development of new methods for pesticide residue detection (1)

By contacting many people who are just engaged in testing, they can quickly master the necessary GC basic knowledge, and even quickly operate the instrument
.

But when they received a batch of new tasks, they didn't know where to start in the face of samples

1.
The source of the sample and its pretreatment method.
The sample that can be directly analyzed by GC must be gas or liquid.
The solid sample should be dissolved in an appropriate solvent before analysis, and it must be ensured that the sample does not contain components that cannot be analyzed by GC (such as Inorganic salt) or may damage the components of the chromatographic column
.

In this way, when we receive an unknown sample, we must understand its source, so as to estimate the possible components in the sample and the boiling point range of the sample

If there are components in the sample that cannot be directly analyzed by GC, or the sample concentration is too low, necessary processing must be performed, including the use of some pre-separation methods, such as various extraction techniques, concentration methods, and purification methods
.

One point that needs to be emphasized is that whether it is the sample processing method or the determination of the GC analysis conditions to be discussed below, the literature and standard method investigation are very important method development steps
.

Therefore, you should check the literature before starting the experiment

2.
Determine the instrument configuration The so-called instrument configuration refers to what sampling device, what carrier gas, what chromatographic column and what detector is used to analyze the sample
.

For example, to use GC to analyze chlorothalonil or determine the residual amount of trace chlorinated pesticides in water, an electron capture detector is needed

3.
Determine the initial operating conditions When the sample is ready and the instrument configuration is determined, the tentative separation can be started
.

At this time, the initial separation conditions should be determined, which mainly include the injection volume, the temperature of the injection port, the temperature of the detector, the temperature of the chromatographic column, and the flow rate of the carrier gas

The injection volume should be determined according to the sample concentration, column capacity and detector sensitivity
.

The injection volume is usually 1~5uL, and for capillary columns, if the split ratio is 50:1, the injection volume generally does not exceed 2uL; when splitless, the injection volume is generally 1μL

The inlet temperature is mainly determined by the boiling point range of the sample, and the use temperature of the chromatographic column must be considered.
That is, firstly, ensure that all the samples to be tested are vaporized, and secondly, ensure that all the vaporized sample components can flow out of the chromatographic column without being in the column.
Medium condensation
.

In principle, it is advantageous for the inlet temperature to be higher.

The determination of the column temperature is mainly determined by the complexity of the sample and the vaporization temperature
.

The principle is not only to ensure the complete separation of the analyte, but also to ensure that all components can flow out of the chromatographic column, and the shorter the analysis time, the better

After the above-mentioned initial conditions are set, tentative analysis of samples can be carried out
.
Generally, the standard sample is separated first, and then the actual sample is analyzed
.
In this process, the conditions should be continuously optimized according to the separation situation
.

4.
Separation conditions optimization Separation optimization is a big topic, there is a special optimization theory, here only a brief introduction to the optimization of operating conditions from a practical point of view
.

In fact, after the sample and instrument configuration are determined, the most common work of the analyst, in addition to replacing the chromatographic column, is to change the chromatographic column temperature and the carrier gas flow rate (column flow rate) in order to achieve the optimal separation
.
The influence of column temperature on the separation result is greater than that of carrier gas
.

Simply put, the optimization purpose of separation conditions is to achieve satisfactory separation results in the shortest analysis time
.
Therefore, when the resolution R of the difficult-to-separate substance pair in the sample is greater than 1.
5 under the initial conditions, measures can be adopted to increase the carrier gas flow rate, increase the column temperature or the heating rate to shorten the analysis time, and vice versa
.
The more difficult problem is to determine whether the peak on the chromatogram is a single component peak
.
This can be compared with a standard sample, or a mass spectrometer can be used to determine the peak purity
.
If a peak of interest is a co-eluting peak of two or more components, the task of optimizing the separation is more difficult
.
When changing the column temperature and carrier gas flow rate can not achieve the purpose of baseline separation, you should replace a longer chromatographic column or replace a chromatographic column with a different stationary phase, because in GC, the chromatographic column is the key to the success or failure of separation
.