Gas chromatographic analysis of 15 common peak shape variants - Database & Sql Blog Articles

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Analysis of 15 common peak shape variant sources in gas chromatography

In the routine chromatographic quantitative analysis, chromatographic peak shape or ghost peaks appear, which not only seriously affects the quantitative accuracy, but also makes the analysis work impossible. For this reason, we analyze the common types of peak shape distortion (15 species) and give Possible reasons, for reference by chromatographers with insufficient work experience. The peak shape variation we discuss here refers to deviations or unwanted peaks of certain chromatographic peak shapes when compared to known chromatograms that have been recorded after chromatographic methods are determined. In other words, for a chromatographic analysis method that has been set up, some peaks cannot be separated, tailing, or peak shape asymmetry, etc., which are not required or out of frustration, and do not affect the implementation of the method. Peak shape changes caused by inexperience or operational errors. Otherwise, the original analysis method needs to be re-examined or modified. In addition, it should be pointed out that the peak shape tailing that may occur due to the disorderly installation and use of the column that has not been evaluated, the separation is not good or the peak shape distortion is not a discussion. Obviously, it is called a general chromatographer. Under the normal working conditions, the quality of the column is judged to be higher. Before you suspect the peak shape change to find possible causes and troubleshooting methods, it is best to do the following work:

Carefully check the operating conditions and whether they are consistent with the analytical method requirements;

Compare with the standard chromatograms stored in the analysis to determine if there is a problem;

Observe the working state of the instrument or equipment item by item to see if there is any abnormality caused by the operation error.

Then analyze the possible causes and elimination methods based on the following 15 abnormal peak shapes.

1. Step peak:

(1) The TCD hot wire is corroded by elements such as halogen, oxygen and sulfur contained in the sample;

(2) The sudden change of gas flow is as follows: the injection pad suddenly leaks, the gas path is blocked, etc.;

(3) Record the chromatographic peak device failure such as: pull the wire;

2. Negative peaks:

(1) TCD uses nitrogen as a carrier gas. Due to the different concentrations of the components to be tested in N2, the heat conduction value is nonlinear and may have a negative peak, which can sometimes be overcome by changing the carrier gas flow rate or injection volume;

(2) When the ECD is operated, the injection volume is too large and the negative peak is generated. This is because the working principle is changed from electron capture to ionization detection, and the sensitivity is greatly reduced at this time;

(3) operating the FID, a low ionization efficiency solvent (such as CS2) or impurities appear, so that the higher baseline of the original base flow decreases and shows a negative peak;

(4) Operation FID, in the case of no polarization voltage, a large sample may have a negative peak;

(5) When the NPD and FPD are manipulated, the airflow ratio is not suitable, and the solvent or some components may have a negative peak;

3. "N" or "W" peak:

(1) TCD operation, using N2 as carrier gas due to nonlinearity of thermal conductivity;

(2) When the FID is operated, the sample solvent ionization efficiency is low (such as CS2), or when the gas flow ratio is poor;

(3) When the ECD is operated, the solvent peak or the content of the component to be tested is high due to the contamination of the detector, or the pulse power source is faulty;

4. Tongue peak (pre-extension):

(1) The vaporization temperature is low;

(2) Small carrier gas flow rate:

(3) The injection volume is large and the vaporization time is long;

(4) The vaporization chamber is contaminated and the sample has an adsorption effect;

(5) The sample is condensed at the column head or the column is contaminated;

(6) Poor injection technique (injection rate of volatile components is too slow);

(7) A "ghost" peak appeared before the peak.

5. Trailing peaks:

(1) If the column is not installed properly, the sample cannot enter the column in the form of a “plug”, and the dead volume of the column and the detector is too large;

(2) The sample failed to be injected into the column head (injection method of the column head);

(3) The vaporization tube is not installed or damaged, and the sample can only be tailed out into the column;

(4) The temperature of the chemical chamber is low or high;

(5) The carrier gas flow rate is low;

(6) Large injection volume;

(7) There is a leak in the carrier gas system (such as the injection pad);

(8) The injector (vaporization chamber) is contaminated by high boiling impurities or injection pad residues in the sample;

(9) The column is contaminated to cause the components to be analyzed and high-boiling pollutants to act;

(10) The supplemental gas is not open or low;

(11) The column temperature is low or invalid;

(12) Methanation of Ni catalyst fails;

(13) poor injection technique (if the speed is not suitable);

(14) There is just an interference peak (ghost peak) (such as misuse of contaminated injection needle);

(15) Non-polarization voltage (FID), at which time the sensitivity is low;

(16) There is something wrong with sample preparation;

6. Baseline shifts after peaking:

(1) The amount of sample is large, especially the solvent changes the working state;

(2) The FID is changed by the pollution status, or the air flow ratio changes;

(3) The system is leaking or clogging;

(4) The column is contaminated;

(5) improper sample processing, such as: some substances in the sample and the stationary phase;

7. The base flow increases (drift) when the temperature is programmed, and the noise increases:

(8) The column needs to be re-aged or failed;

(9) The purity of the new carrier gas is not good;

(10) The filter is invalid;

(11) Improper sample preparation, such as: too many impurities interfere with;

(12) The sensitivity is too high.

(13) The setting of the peaking parameter of the data processing device is unreasonable.

8. Dome wide peak

(17) The sample volume is large and the column capacity is increased;

(18) The vaporization temperature is low;

(19) The column is not installed as required;

(20) The detector is in the wrong working condition, such as the carrier gas is too small, and the gas is not supplied;

(21) The peaking parameter (half-peak width) of the data processing device is set to be large;

9. Flat peak (not full scale):

(1) The sample volume is large, the amplifier has a high range, the attenuation is large, and the signal output is saturated;

(2) the detector has been operating in the saturation zone;

(3) Data processing input signal polarity is wrong, or zero offset;

10. A wavy peak appears at the baseline:

(1) Before the high sensitivity operation instrument is not stabilized;

(2) When the TCD and ECD are operated, the temperature of the oven or the detector box changes periodically;

(3) The influence of ambient temperature on the temperature control of the instrument;

(4) The voltage is unstable, which affects the accuracy of the column temperature control;

(5) The over temperature protection setting is lower than the control temperature;

(6) The pressure (flow) regulating valve is out of adjustment and the cycle changes;

11. The peaks that could be separated can not be separated:

(1) Column installation is not satisfactory;

(2) The column is contaminated and needs to be reactivated;

(3) The column life has arrived and needs to be replaced;

(3) The newly replaced gas source has poor purity;

(4) The filter fails, re-ages or replaces;

(5) The column temperature and carrier gas flow rate need to be fine-tuned and optimized (chromatographic analysis is generally allowed);

(6) The working state of the detector changes (such as ECD leakage and FID airflow ratio is poor);

(7) The vaporization chamber is contaminated and the injection pad leaks;

(8) Improper sample handling, too many impurity interferences;

(9) The technique is too poor;

(10) The injection volume exceeds the column capacity;

(11) The peak value of the data processing, the half-peak width or the slope setting is unreasonable;

(12) Amplifier range or attenuation setting error;

12. Right angle peak

(1) The instrument outputs a negative signal beyond the scope of data processing;

(2) The data processing device zero point is not corrected, or the range setting is too large to determine the baseline position;

(3) The polarity of the input signal of the data processing device is reversed, and the zero point is set incorrectly;

13. With a burr peak

(1) The instrument is unstable and the noise is greater than the requirement;

(2) The peaking parameter of the data processing device, the half width and the slope setting are too small;

(3) The polarization voltage (FID) is unstable;

14. The operating conditions have not changed, and the peaks that can be discerned are gone:

(1) The column is contaminated or invalid;

(2) The gas path system is polluted (if the gas source is low in purity and the filter is ineffective);

(3) The injection pad leaks;

(4) The injection needle has poor sealing performance;

(5) The peaking parameters of data processing, such as: half-width and slope setting are too large;

(6) The injection method is incorrect;

15. "Ghost Peak" (Strange Peak, Super Peak, Memory Peak):

(1) The high-boiling impurity peak of the last injection naturally flows out;

(2) The carrier gas impure filter fails to condense the low-boiling contaminants in the column head, and normally flows out when the temperature is programmed;

(3) The pollution peak of the injection pad without aging or cleaning with a septum;

(4) The vaporization temperature is too high or heavily contaminated to decompose certain components of the sample;

(5) Some components of the sample have an effect on the contaminated stationary phase;

(6) The column temperature is too high and the stationary phase is decomposed;

(7) The use of contaminated injection needles (unqualified by themselves, hand touch or into contaminated samples);

(8) The sample is not processed properly or the wrong solvent is used;

(9) There is air in the sample;

(10) TCD, ECD, etc. have poor sealing performance (leakage);

(11) The power supply is unstable and has adverse effects on temperature control or amplifier.

(12) Improper use of column plugs, such as glass wool not treated as required;