Causes and solutions for common peak abnormalities in gas chromatography

The following chromatographic technicians from Wuhan Tentworth Technology Co., Ltd. analyzed the common causes of peak anomalies and discussed possible solutions. As a mature and stable separation and analysis technology, gas chromatography is widely used in the petrochemical industry. Wuhan Tetworth Technology Co., Ltd. is the leading gas chromatograph supplier in China. When qualitative and quantitative detection is carried out by gas chromatography analysis technology, the occurrence of peak abnormality often causes serious interference to the result judgment. Analysis of the causes of such problems in gas chromatography analysis, to find out the corresponding solutions, can reduce the interference in the processing results, and thus improve the accuracy of the analysis results.

The analysis process of chromatography is complicated. Many factors can affect the results. Unexpected phenomena often occur. Peak abnormalities are one of the frequently encountered problems. Generally, the chromatographic peaks are collected. If it is not open, the chromatographic peak will be tailed, the peak of the chromatogram will appear, and there will be no peak after injection. The appearance of these problems seems to be irregular, and most of the traces can be traced.

1 The cause and solution of the ghost peak

1.1 Influence of the inlet silicone pad

(1) Analysis of the cause. It is related to the quality of the silicone pad and the quality of the injection needle. There are several aspects: 1 The quality of the injection needle is not good, the edge is not flat enough, and the silicone pad is easily damaged. 2The silica gel pad is used too much or the aging time is long. The quality of the silica gel pad is not good, the material elasticity is poor, and the silica gel pad debris enters the vaporization chamber, and a ghost peak is formed. 3 The silica gel pad is contaminated. If the sample droplet attached to the needle is injected by the silica gel septum during the injection, it will be desorbed and then enter the column after the analysis.

(2) Solution. Check the syringe and silicone pad used for targeted replacement of poor quality syringes, silicone pads that have been used for too long or have a quality problem.

1.2 Inlet, detector or liner and split plate contamination

(1) Analysis of the causes There are many reasons for the pollution, which can be attributed to the following aspects: 1 The periodic maintenance of the chromatograph is not performed for a long time. 2 The composition of the sample to be tested is complicated, the inlet temperature setting is not high enough, the sample vaporization is incomplete, and the heavier components are retained in the inlet. 3In the vaporization chamber and the liner, some deposits and high-boiling substances are often collected. If a high-boiling substance is detected, the aggregated material will escape from the excess peak when the temperature of the inlet rises. The non-volatile matter in the groove of the device and the splitter plate will accumulate slowly, causing the ghost peak to appear irregularly.

(2) Solution Clean the parts of the instrument according to the actual pollution situation. 1 Clean the inlet. First remove the column, then inject the absolute ethanol or acetone from the inlet under the premise of heating and ventilation, repeat 3 to 5 times, and finally heat the aeration to dry the inlet. 2 Replace the liner or clean the liner. When the liner is contaminated, it can be clearly seen that there is particulate deposit or the color of the quartz wool is darkened, and a new liner needs to be replaced when the pollution is serious. Method of cleaning the liner: Remove the quartz wool in the liner, and immerse the liner in the chromic acid washing solution for 24 hours, then take it out, wash it with distilled water, methanol, acetone, and then dry. 3 split plate cleaning. The split liner is placed in an organic solvent such as chromatographically pure methanol for sonication and drying. Note that it is not possible to directly touch it during the process of disassembling and installing the splitter plate to prevent contamination. 4 detector cleaning. Thermal conductivity detector: Select the appropriate cleaning solvent according to the degree of contamination of the detector, fill the measuring tank of acetone, decalin and other solvents into the measuring cell of the calibrator, soak it for about 20 minutes, then pour it out, and repeat it several times until it is poured out. The solution is relatively clean. If a solvent cannot be used for washing, the high boiling point solvent may be used for immersion cleaning according to the nature of the contaminant, and then repeatedly washed with a low boiling point solvent. After washing, heat the solvent out, install it back on the instrument, heat the detector to about 150 °C, and turn it on for about 4 hours.

Hydrogen flame detector: When the pollution is not serious, the column can be removed, the inlet is connected with the detector by a pipe, then the carrier gas is passed and the temperature of the detector is raised to above 120 °C, and 20 μl is injected from the inlet. Distill the water to the left and right, and then inject several tens of microliters of acetone solvent for cleaning. When the pollution is serious, the detector must be removed for cleaning. The collector, the positive electrode and the nozzle should be removed in sequence. If the nozzle is made of stainless steel, the dirty part can be carefully polished with fine sandpaper. Then use ultrasonic cleaning, finally clean with methanol, and put it in an oven to dry.

1.3 Effect of the column

(1) Analysis of the cause. 1 The column is not fully aging, the column temperature is too low and the aging is insufficient, and the temperature is too high to cause the liquid phase to be lost. 2 The column is used for a long time. The column itself has adsorption characteristics, and a high concentration sample remains in the column or a high-boiling substance remains in the column, and the column head is contaminated. In addition, the capillary column has a low load and requires a highly sensitive detector, which is particularly prone to ghost peaks.

(2) Solution. Cut off the contaminated stigma section, raise the inlet temperature to a suitable value, and perform column head aging. After performing a complex sample analysis, the column is purged by programmed temperature to remove residual material from the column. During the temperature programming process, the set column temperature must be lower than the column use temperature by about 20 °C to avoid column loss.

1.4 Instrument analysis conditions are not suitable

(1) Analysis of the cause. When analyzing an unknown new sample, some interference peaks may be generated due to improper setting of the instrument conditions. The general reasons are: 1 The sample may degrade under analytical conditions to produce some interfering components. If these components respond well under these conditions, ghost peaks may occur. 2 The selected column is not suitable, which is not conducive to the analysis of high boiling compounds.

( 2) Solution. By consulting the literature, conducting experiments, searching methods, and understanding as much as possible the various properties of the test sample, such as polarity, molecular structure, molecular weight, etc., select appropriate analytical conditions, including the oven, inlet, and detector. Use temperature, column polarity, film thickness, length, inner diameter, etc.

1.5 Sample contamination

The sample is contaminated during the processing before injection, and the ghost peak appears regularly during the detection, with good repeatability, and the solvent blank does not contain this peak, which is easier to judge.

Solution: Since chromatographic analysis is a trace analysis method, all items used in each process of sample processing must be cleaned to prevent interfering substances from entering the sample.

2 Reasons for the separation of peaks and solutions

2.1 Carrier gas flow rate is too high

(1) Analysis of the cause. The carrier gas flow rate has an effect on the column efficiency. The carrier gas flow rate is fast to speed up the analysis and reduce the molecular diffusion. However, if the flow rate is too fast, the separation effect will be poor, and the peaks will be tailed or overlapped, so that the peaks cannot be effectively separated. (2) Solution. Select the carrier gas flow rate suitable for the sample. Generally, the column carrier gas flow rate is 20-100 ml/min.

2.2 The injection volume is too large

(1) Analysis of the cause. Excessive injection volume in chromatographic analysis results in less separation and affects chromatographic separation. (2) Solution. Select the appropriate sample volume for injection. For liquid samples, the depth and position of the needle should be fixed, and the injection speed should be as fast as possible.

2.3 Chronic column loss and pollution

(1) Cause analysis All the columns will have a column loss phenomenon, which is derived from the eluted substance produced by the stationary phase degradation due to various reasons. When the column is heavily drained, the efficiency of the column is reduced, which is manifested by a decrease in resolution and a trailing or overlapping peak in the spectrum. 1 The loss of the column will increase as the temperature increases. 2 If there is oxygen in the carrier gas, it will oxidize the fixative and cause serious loss.

(2) Solution 1 Select the appropriate column temperature. 2 Ensure the purity of the carrier gas.

For the column that has been lost, if the loss is not serious, the column can be aged, and the targeted solvent is purchased at the time of high temperature aging, which is generally easy to age. If the loss is serious, replace the column with a new one. You can also use the column as a pretreatment column or for individual analysis. Refer to this article for column contamination issues.

2.4 gasification chamber is too large

In general commercial instruments, the size of the gasification chamber is fixed, so the so-called dead volume is also fixed. The large dead volume is likely to cause the sample to diffuse and backflush, resulting in a broadening of the chromatographic peak, peak asymmetry and peak separation. Since the gasification chamber is generally not replaceable, it is important to note that the position of the column head should be appropriate when installing the column. There are specific requirements for on-column injection, gasification chamber injection, capillary, packed column, etc. In addition, care should be taken not to change the length of the liner when changing the liner.

2.5 Column is too short

(1) Analysis of the cause. The column is too short and the column efficiency is too low, which causes the chromatographic peaks to not be completely separated, affecting the analysis of the results.

(2) Solution. The column of the appropriate length is replaced according to the nature of the sample being analyzed, but the analysis time of the column is too long and the column pressure is increased, so the longer the column is, the better the column length should be.

2.6 Column temperature is too high

(1) Cause analysis According to the column temperature of Fan's equation, the column efficiency, resolution, selectivity and column stability are affected. The high column temperature is conducive to mass transfer, but the column temperature is too high and the partition coefficient becomes small, which is not suitable for separation.

(2) Solution Select the appropriate column temperature according to the nature and composition of the sample. If the analytical component has a wide boiling range, the temperature can be programmed.

3 Reasons and solutions for peak tailing

(1) The carrier gas flow rate is too high, refer to 2.1 in this article; (2) The injection volume is too large, refer to 2.2 in detail; (3) The column is seriously lost or contaminated, refer to 2.3 in detail; (4) The gasification chamber has too much dead volume. Refer to 2.4 in detail; (5) Analysis of the initial column temperature of the column is too high. When the initial column temperature is set too high, the target analyte has not yet condensed in the column head and enters the column to start separation with a wider band, so the peak shape is wide, tailing or even bifurcation occurs. 2 solutions. By lowering the initial column temperature, the target analyte vaporized at the inlet first condenses on the column head (while the solvent remains gaseous) and gradually aggregates, the so-called "hot concentration" or "thermal focus" process. Thus, the analyte begins the chromatographic separation process with a narrow initial band, resulting in a sharp, symmetrical peak shape.

(6) The temperature of the gasification chamber is too low 1 analysis. If the temperature of the gasification chamber is too low, the sample with high boiling point may not be vaporized instantaneously, and it may not be fully brought into the column by the carrier gas, causing the expansion of the chromatographic peak and causing tailing. 2 solutions. The correct gasification temperature of the liquid sample is an important condition for obtaining the peak shape of the symmetrical chromatogram. Especially for the sample with high boiling point and easy decomposition, it is required that the sample is vaporized instantaneously without decomposition at the gasification temperature. The choice of gasification temperature requires consideration of factors such as the boiling point of the sample, the amount of injection, and the sensitivity of the detector. The gasification temperature does not have to be higher than the boiling point of the test substance, but generally needs to be 50 to 100 ° C higher than the column temperature.

(7) If the gas carrier system of the carrier gas system leaks, the carrier gas pressure will be low, the flow rate will be slow, and tailing will easily occur. 1 Reason analysis. a. The pipeline leaks. b. If it is a capillary, check if the capillary is broken (the probability of cracking is often greater than the probability of breaking). c. The sample pad leaks. 2 solutions. a. Check the pipeline, apply soapy water to the pipeline with a brush, determine the location, and repair. b. Replace the new capillary. c. Replace the new sample pad.

4 Reasons for no peak after injection and solutions

No gas chromatographic instrument can be injected without peaks after peak injection. Only one straight line is drawn at the baseline.

(1) Check if the flame of the detector is extinguished. If it is extinguished, it needs to be re-ignited. If it can't be lit or it is easy to extinguish after clicking, the following checks can be made: 1 Check if the ignition coil is working normally. If it is not red, it should be The ignition pole is partially faulty. 2 Check whether the flow rate of gas in each gas path is in the working range. When it is not easy to ignite, the hydrogen flow rate can be appropriately increased. 3 When using the thermal cell detector, check if the detector thermal element and bridge current settings are normal.

(2) Connection check Check the connection of the ion signal line to the detector and amplifier board, and whether the output signal line is properly connected to the instrument or integrator/workstation.

(3) When the system and other systems check that the zero adjustment is not normal, consider whether the circuit system is faulty. Please check if it is a fault of the signal line, the amplifier circuit board, the output signal line or the integrator.

(4) After entering the conventional solvent such as methane, if there is no peak in the chromatogram or the retention time becomes large, the heating of the column oven can be turned off. After the temperature has dropped to room temperature, the following items are checked: 1 Check the column Has it been broken somewhere? 2 Check if the carrier gas enters the column and FID detector normally, and the flow is normal.

(5) There are still some reasons for not peaking. You can check 1 syringe for clogging or damage according to the following items. 2 Check whether the analysis conditions are set properly, such as column temperature, injector temperature, detector temperature, range setting, etc. 3 Check the sample concentration and the sample injection amount within the detection range. 4 Check the sample selection and column selection.

The complexity of chromatographic analysis determines the diversity of causes of peak anomalies. In addition to the above, there are still some problems that need to be tried and used in practice and practice to find a solution, but we must think twice before avoiding blindness. Operation, damage to the chromatograph. I hope this article can help you in the operation of chromatography, and use gas chromatography faster and better.

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