Several Considerations of Gas Chromatography Trace Analysis

Trace analysis generally refers to qualitative and quantitative analysis of components in a pure substance or in a mixture containing 10-6 to 10-9 (volume or weight ratio). With the continuous development and progress of society, the solution of several major issues (resources, energy, and population environment) facing humanity is closely related to trace analysis techniques. Or, people's daily lives are increasingly inseparable from trace analysis. Recently received purchase orders for analytical instruments have almost always solved trace analysis projects.

Characteristics of Trace Analysis Samples

1 Wide range of sample sources;

2 kinds of kinds;

3 The composition is complicated;

4 low content;

5 The nature of the state varies. Therefore, relatively constant and micro-analysis is difficult, and it is a job that requires high standards in all aspects. Although many analytical methods such as chromatography, mass spectrometry, spectroscopy, and electrochemistry can be used for trace analysis, gas chromatography has many advantages (in most cases):

(1) The price of the instrument is low and the conditions for use are not harsh, which will facilitate popularization and promotion;

(2) The separation efficiency is high, the selectivity is large, which is conducive to the separation and detection of complex multi-components;

(3) High sensitivity, fast analysis speed, and small amount of direct injection;

(4) Combined with other instruments can solve more complex analysis problems. Chromatography is already the largest and most versatile method and instrument used in trace analysis. Of course, to complete a trace analysis task, in addition to first acquiring an excellent GC, it also involves sample collection, pretreatment, and analytical method establishment (selection of columns, optimization of analytical conditions, etc.), standard sample Preparation, data processing and other links. This article on gas chromatography trace analysis in the purchase of GC instruments should consider several aspects, to be summarized for your reference. If you have more specific technical problems, please contact us on the “Expert Advisory” section of this website.

I. Purpose and requirements of trace analysis

Because the purpose and requirement of trace analysis are different, there are great differences in the types, performance, and time-consuming of GC purchase. In the following, we divide the analysis purpose and requirements into three categories, and discuss the different considerations when purchasing different gas chromatographs:

(1): The analytical purpose is a quantitative analysis of known trace components:

In this case, the trace components and the chromatographic analysis method of the material being analyzed are known (there are standard analytical methods, or are known to have been analyzed by the same substance), and we can Performance, and even reference to the instrument model to buy directly. The remaining problem is the user's choice of column separation, control of operating conditions, etc. For example, 1 select suitable instrument conditions to meet the signal/noise ratio and separation requirements; 2 prepare standard samples; 3 set reasonable data parameters; 4 correct quantitative calibration; 5 prepare required analysis reports (forms and contents), etc. Achieve accurate analysis and accuracy requirements.

In addition, in order to meet different material trace analysis or various standard requirements, each GC manufacturer has specially developed and developed a special gas chromatograph, such as: 1 special gas chromatograph for natural gas, liquefied gas, gas, refinery gas, etc.; 2 transformer oil Special gas chromatograph for dissolved gas analysis; special gas chromatograph for trace C2H2 analysis in 3 liquid oxygen; 4 special gas chromatograph for impurity analysis in high purity permanent gas, etc. These special instruments generally include: 1 sample collection, pre-processing procedures and sampling methods; 2 instrument hardware, complete sets of systems and self-diagnosis and testing methods; 3 optimal separation of the chromatographic column and optimization of experimental conditions; 4 qualitative and quantitative method selection And standard analysis reports. Users can buy directly.

(2): The purpose and requirements of the analysis are qualitative and quantitative analysis of certain trace components in pure material:

Compared with the first case, the analysis of the target is definitive, but no written analysis method can be directly referred to. It is much more troublesome to select the GC. First of all, according to the analysis of the chemical composition of the components, boiling point, polarity, and analysis items in that field, to find relevant information and methods can be referenced, the initial development of the purchase of equipment and the required use of the column program. Then, look for relevant experts or manufacturing plants to see if there are more suitable instruments and methods. Otherwise, it may be necessary for the customer to first determine which column of GC to purchase is selected by pre-experimental screening of what columns, injection methods, detectors, and data processing are used. This is often said, the process of selecting the instrument (including auxiliary equipment) after the establishment of the chromatographic analysis method. If the general experience is not enough, customers can consult the GC manufacturer to see if they can assist in the development or modification of a dedicated GC for an analysis project. Of course, if the conditions permit, you can buy a multi-function, multi-test, multi-accessory instrument, after the establishment of chromatographic analysis methods through experiments.

(3): Qualitative and quantitative analysis of unknown components in totally unknown substances:

In fact, there is no information about the samples to be analyzed in advance. To complete such analysis tasks, it is not a question of how to purchase the instruments. Although most of the analysis topics can be solved through the combination of chromatography and other large-scale instruments, a large number of pre-experimental tasks need to be done to achieve the desired analytical purposes. For example: 1 What kind of sample collection and processing methods can be used to meet the analysis requirements (instrumentation) Accuracy and precision); 2 Through a variety of different types of chromatography separation experiments to determine the type of column and whether it is necessary to achieve a sample separation analysis through a multidimensional analysis system; 3 General-purpose detectors (such as: TCD) FID,) and selective detection (eg, ECD, NPD, FPD, HCD), etc., to see if it can meet qualitative and quantitative requirements. In addition, whether to consider the use of GC and other large-scale instrument combination method can solve the problem; 4 according to the type of column, the detector characteristics (large analytical instruments such as mass spectrometry, infrared spectroscopy can be seen as a GC detector) need to be matched What kind of performance data processing device, whether there is a dedicated data processing software available. Comprehensive analysis to see if it can establish a set of effective chromatographic analysis methods, or need to seek other analytical approaches. It should be pointed out that under current conditions, trace analysis of any analytical item cannot be resolved by instrumental analysis.

Through the analysis of the objectives of the above three cases and the rough analysis of the requirements of how to purchase the instrument, we can draw some conclusions about the purchase instruments discussed in this article, only for the first and second situations, that is, we used to The second and third categories of the user's classification for the optional equipment.

II. Trace Analysis GC Performance Requirements:

It can be said that trace analysis basically uses the maximum performance limit of the instrument (unless the sample is concentrated and converted) and various specialized functions. Therefore, to purchase instruments, we must start from those GCs with full-featured and excellent performance. Of course, 10-6 component analysis is sometimes not easier than 10-9. Such as: Analysis of liquid oxygen 0.05 × 10-6C2H2 and high-purity gas traces of CO, CO2, the former can be concentrated hundreds of times, the latter can be converted to CH4 under nickel catalysis with FID detection, can greatly reduce The instrument performance requirements. That is to say, when solving the trace analysis, we must also treat specific problems specifically. We must not blindly require that the higher the performance of the instrument, the better, and the stronger the function, the better.

1. The relationship between detector sensitivity, instrument detection limit, minimum detection amount, and minimum detection concentration with optional instruments:

The four technical indicators of sensitivity, detection limit, minimum detection concentration, and minimum detection amount are the basic conditions for measuring the quality of GC. However, due to the different physical meanings, it is necessary to clarify the relationship between the four, and it can be done when purchasing instruments. There are several.

(1) Detector sensitivity (S):

The physical meaning of the detector sensitivity is: the input unit changes to the detector unit concentration (or unit mass flow rate) how much electrical signal the detector changes to output, the greater the signal, the higher the sensitivity. According to different working principles, the concentration type (such as TCD) dimension: mv/mg/ml, mass type (eg: FID) c/g (coulomb/gram). It is better that the natural S value of the same type of detector used by different instruments is larger. However, it should be pointed out that a person's heart is not good enough to explain that this person's body is good. Therefore, the high sensitivity of the detector does not necessarily mean that this instrument can meet various requirements. It is only a prerequisite for selecting the instrument.

(2) Limit of detection of chromatograph (D):

The detection limit of chromatographs is commonly referred to as the sensitivity or sensitivity of the instrument. Its physical meaning is: It can generate the chromatographic peaks that can be judged instead of noise (double the noise for the minimum discriminable chromatographic peak) The concentration (or mass flow) of the component that needs to enter the detector, the dimension concentration Type detectors (eg TCD) mg/ml, mass detectors (eg FID) g/s. The detection limit of an instrument configured for a certain type of detector can be calculated by dividing the instrument's double noise by the detector's fitted detector's sensitivity. From the analysis of the physical meaning of the detection limit, it can be seen that if the D of a single instrument is reduced, not only the sensitivity of the configured detector is required to be high, but also the stability of the instrument is good, that is, the noise is small. In addition, because the chromatograph manufacturer does not know what specific analysis the customer bought back to the generic GC, the sensitivity and detection limit of an instrument's detector will become the main measure of the instrument's performance. For comparison, we have specified that the same sample should be used for the trial of S or D. However, sometimes the sample is far from the user's analyzed component properties. At this time, the large and small S and D do not indicate any problem. This point users should pay special attention when selecting instruments. From the chromatographic analysis process, we know that even the best instrument does not match well with a good column and qualified data processing device can not achieve the purpose of analysis, this point in the next part of the article to do a specific analysis.

(3) The minimum amount of real sample (m small):

The minimum amount of sample detected refers to the amount of the smallest sample (measured component) that needs to enter the chromatograph (through the gas injection valve or vaporization chamber), and it can be judged as a component peak instead of noise. From the chromatographic analysis, it is known that the same amount of sample is injected. The narrower the full width at half maximum of the peak (ie, the higher the peak) is, the smaller the minimum detection amount is. In the actual analysis, to obtain the minimum amount of detection, firstly the instrument is equipped with a detector with a high sensitivity, and the entire instrument must have a small noise, and the separated peaks of the measured components must be narrow and high. This also requires selection. A good column and appropriate analytical conditions. Therefore, when a customer asks a manufacturer of a model of general-purpose GC, the manufacturer can only answer in general if the minimum detection amount for each component is. Unless the manufacturer used this instrument for chromatographic analysis of the same sample. Otherwise their answer can only be used as a reference for purchasing instruments.

(4) The minimum concentration of the actual sample (C small):

The minimum concentration of the actual sample can be injected into the chromatograph for analysis and qualitative and quantitative results can be obtained. This is a key issue for the user to purchase the instrument. From the physical sense of minimum detection, we can see that when the two instruments have the same minimum detection amount, the maximum allowable sample volume is different due to the selected column (column Capacity) The minimum detection concentration of the sample is also different. For example, if the capacity of an instrument is 10 times that of another instrument, then the latter needs to concentrate the former sample by 10 times to obtain the same analysis result. That is, the latter has a smaller detection concentration than the former. Magnitude. In other words, in some cases, the former may not need to concentrate the sample, and it can directly analyze the sample, greatly simplifying the analysis process. Another benefit is also helpful to improve the accuracy of the analysis. This is why, although the separation peaks of the capillary column can be narrow and high, the capillary column capacity is 102 to 103 times smaller than that of the packed column, and the allowable injection volume is also corresponding to 102 to 103 times of the packed column, so sometimes it is To obtain the minimum detection concentration, the use of packed columns is not always worse than that of capillary columns.

Through the introduction and comparison of the above-mentioned detector sensitivity, instrument detection limit, the minimum detection amount of the tested sample components, and the physical significance of the minimum detection concentration of the measured component of the actual sample, the following conclusions can be drawn: It is appropriate.

The high sensitivity of the detector equipped with the instrument does not completely explain the quality of the entire instrument.

The low detection limit of the instrument does not necessarily meet the trace analysis requirements;

The premise of satisfying the trace analysis is that the sensitivity of a device equipped with a detector is high and the noise of the instrument is small. In addition, a good chromatographic analysis method is needed. It is not only the peak of the chromatogram that is high and narrow, but also allowed under the same conditions. Large sample volume;

Take capillary and packed column allowable sample volume as an example to illustrate the choice of column type or other such as: carrier gas, temperature, injection method and detector, etc. There is no absolute good or bad distinction.

Consult a general sales person, a certain type of instrument, to analyze the minimum detection concentration of a sample, and it is difficult to answer. The data provided at random may lead to disasters;

For the average customer, it is best to ask the manufacturer or an experienced unit to solve your analysis request. Not only to provide complete sets of equipment, but also to provide a complete chromatographic analysis method (the manufacturer calls it special chromatograph, and the customer calls it turnkey project) . If the user fails to meet the contract requirement of the minimum detection amount or the minimum detection concentration during the acceptance, the user may refuse the payment;

When the S and D detection standards provided by the manufacturer are different from those of the samples to be analyzed, it is necessary to perform conversion before knowing whether or not the sensitivity requirements can be met. This requires special attention when purchasing the instrument.

To purchase a GC for trace analysis, first select which detector to use for your particular sample analysis. We know that for the manufacture of a GC, the detector is the heart. Because the GC configures what kind of detector determines: 1 the overall structure of the instrument; 2 the configuration of the pneumatic system (such as: gas filtration system, sample introduction system, which columns to install, hermeticity and dead volume requirements, etc.); 3 Electrical Configuration (power, signal amplification, temperature control accuracy, anti-jamming measures, etc.) In addition, due to differences in performance, characteristics, and disadvantages of various detectors, we decided which detector to use, and we must also consider the sample processing procedures, sample injection methods, standard sample selection, quantitative methods, and data. Processing device types and performance options. This is why we have chosen the detector as the key reason for the detailed introduction. Due to the limited space, we will briefly introduce the adaptability, minimum detection amount, minimum detection concentration, operating characteristics and deficiencies of the commonly used detectors for customers' selection. Reference when purchasing.

(1) Hydrogen flame ionization detector (FID):

Applications: Especially suitable for organic compounds from constant to trace analysis. It is the best detection method for trace organic compounds in air and water in the environmental field. It is the only water detector in all conventional detectors. The FID is one of the current GC detectors.

Type: Quality general type (80% of analysis items);

Detection limit: D ≤ 2X10-12g/s; minimum detection amount: m small ≤ 1x10-10g (reference); minimum detection concentration: C small: 10-8 ∽ 10-9 (reference)

Linear range: 107

Response time: ~1ms (most suitable for direct injection for capillary chromatography);

Operating characteristics: a) is of quality type, the peak area does not change with the carrier gas flow rate; b) good stability: S and D are insensitive to airflow and detector temperature fluctuations; c) quantitative simplicity: the relative S value of homologues in organic substances Same; d) Approximate ideal detector, simple structure, long life, almost no maintenance; e) The only conventional detector that can enter water samples;

Insufficiency: a) Generally, three kinds of gas sources such as N2, H2, and air are used in the operation; b) Relative impurities in the gas path; the ignition ratio in operation or the best performance is required to repeatedly adjust the gas flow ratio, which is compared with other detectors. It seems troublesome.

(2) Electron Capture Detector (ECD):

Applications: For the analysis of trace amounts of electronegative organic compounds the most effective. Common negative compounds such as: halogenated hydrocarbons, phosphorus, sulfur, halogen compounds, metal organic compounds, hydroxyl, nitro and conjugated double bond compounds. In addition, trace oxygen analysis of 10-6 permanent gases is also sometimes used, or analysis of non-polar samples is converted. At present, ECD has been widely used in trace analysis in fields such as biochemistry, medicine, pharmacy, pesticide residue, environment, and meteorological tracer.

Type: Concentration type high selectivity Such as: carbon tetrachloride and n-hexane sensitivity ratio of more than 108;

Detection limit: D: 10-12~10-14g/ml; minimum detection amount: m small 10-13g (reference); minimum detection concentration: ≤ 1x10-11g/ml (reference)

Linear range: 102∽104 (depends on the working principle and working conditions);

Response time: several hundred milliseconds ∽ 1 second (mainly determine the pool capacity, with capillary analysis, should try to use a small response ECD);

Operation features:

a) The highest sensitivity in a conventional detector, one of the necessary detectors;

b) Concentration type: Peak area varies with carrier gas flow;

c) Stable: Stable is poorer than FID, S or D is particularly sensitive to carrier gas and temperature fluctuations;

d) With only one gas, daily operation is the simplest of all detectors (except for misoperation);

The main shortcomings:

a) A detector that is most likely to cause misunderstandings requires a lot of attention in daily operations. If you do not pay attention, it will immediately lead to deterioration of stability, S will decrease drastically, and linearity will become narrower (it takes longer to restore normal working conditions) Therefore, it is often mistaken to believe that it is the worst detector to operate.

b) The establishment of chromatographic methods is difficult and time-consuming. For example, sample pretreatment, column preparation aging, system cleanliness, and the use of solvent vessels must have special requirements.

(3) Flame photometric detector (FPD)

1 Application: It is a high sensitivity, high selectivity detector for trace analysis of sulfur compounds in phosphorus, sulfur organic compounds and gases, such as: mercaptans, COS, H2S, CS2, SO2 in petroleum distillation; Mercury in water pollution; H2S, SO2, and CS2 in air; Gas analysis of pesticide residues and sulfides in natural gas. The order of sensitivity of the FPDs to sulfides is: Sulfide> Carbon disulfide> Sulfur dioxide. FPD is not as good as NPD in measuring phosphorus, and when it is used to analyze phosphorus trace amount, it is better than FPD to choose NPD from many aspects.

Type: Phosphorus is mass type. When measuring sulfur, S is proportional to 1.5样品2 of the sample concentration (depending on the detector junction

Structural or operating conditions); high selectivity, selectivity to non-sensitive components greater than 104 ∽ 105;

Detection limit: Minimum detection amount for DP ≤ 1×10-12g/sDS ≤ 1×10-11g/s (reference): m Small (P) ≤ 10-10gm Small (S) ≤ 10-8g Minimum detection concentration (Reference) : C small (P) ≤ 10-6 ∽ 10-8C small (S) ≤ 10-5 ∽ 10-7;

Linear range: Phosphorus measurement: >103 sulfur measurement ≥ 102 (depending on the detector structure);

Response time: ≤ 0.1 seconds (normally available with capillary column analysis);

Operating features and deficiencies:

a) In gas chromatography, the use of FPD to measure trace sulfur is more sensitive and selective than other methods, and the instrument is easy to operate and stable. However, the minimum detection concentration (≤10-7) requires the purchase of a pulsed flame photometric detector (no production at home), the price is high, and promotion is still difficult;

b) According to the principle of FPD sulfur measurement, the dual flame has a lower sensitivity to sulfur detection than a single flame, but the linear width is advantageous for accurate quantification. In the purchase depends on the requirements;

c) Due to the chemical nature of sulfides, the inertness of materials used in the sample introduction system, column, and detection system is higher than that of other detectors. Attention should be paid to the adverse reactions of the above problems during operation.

d) Relative to the FID, the optimum gas flow ratio during FPD manipulation, the detector temperature is not only different in the measurement of phosphorus or sulfur, but also related to the chemical properties of a specific component, if not properly selected, sensitivity, selectivity, linear range All were affected, which caused great inconvenience to the operation;

e) The detection of phosphorus by NPD is one or two orders of magnitude lower than the minimum detection concentration of FPD. Although the cost of the two detectors is similar, the operation of NPD is relatively simple and convenient, and only NPD can be considered in the analysis of trace phosphorus content.

(4) Thermal ionization nitrogen and phosphorus detectors (TID, NPD, TSD, AFID):

Adaptation range: NPD is a high sensitivity, high selectivity detector, especially suitable for trace analysis of organic compounds containing nitrogen and phosphorus. Phosphorus as described above should be preferred when compared to FPD. At present, NPD is widely used in environmental protection, food (pesticides), drugs (narcotics, drugs, amino acid derivatives, etc.), biochemistry (nitrogen-containing metabolites), forensics, spices and other fields. However, it should be pointed out that due to lack of domestic hardware or lack of awareness, it is necessary to popularize it in the future.

Type: Concentration type or quality type response characteristics, depending on the operating conditions, should be chosen to work under quality type when working. The selectivity of high selectivity to non-sensitive components is greater than 105 or more.

Detection limit: DN ≤ 1 × 10-13g / s, DP ≤ 5 × 10-14g / s; minimum detection amount (reference): m small ≤ 1 × 10-12g;

The minimum detection concentration (reference): 1×10-11∽10-12;

Response time: <1 second (determine the detection structure and operating conditions);

Operating features and deficiencies:

a) It has a simple structure and low cost. It is basically the same as one of the FID and GC detectors.

b) With three sources, except that H2 flow needs to be fine-tuned, similar to FID, so it is possible to share an air circuit system with FID;

c) Major deficiency: i) The new alkali beads need to be stable for a period of time; ii) Finding the optimum gas flow ratio is slightly more difficult than the FID; iii) The alkali metal beads have a certain life span (determining the application of the operating conditions), so the operating cost is high

(5) Thermal Conductivity Detector (TCD):

TCD is a medium sensitivity, non-selective universal detector. In principle, it will not be suitable for trace analysis, but because of its simple structure and versatility, without destroying the sample, low cost and simple operation, people continuously improve it, such as: 1 using a new power supply principle; 2 use a new hot wire Materials; 3 reduce the pool capacity; 4 improve the gas path; 5 plus pre-amplification; 6 improve the temperature control accuracy of the pool, etc., greatly improve the signal / noise ratio, so in some permanent gas or low boiling point component analysis The minimum detectable concentration of the trace components in the can reach 10-6 ∽ 10-7. . Since the TCD is a concentration detector, it is sensitive to operating conditions such as temperature fluctuations and pressure flow fluctuations. Practice has proved that using it for trace analysis requires very strict operating conditions. Therefore, be careful when purchasing.

(6) Ionization detector for inert gas (æ°¦, æ°–):

Suitable for use in gas-solid chromatography, detectors for trace composition analysis in permanent gases, and ionization detectors with helium or argon as the carrier gas, and the basic working principle of both: helium and argon atoms in high energy electrons The bombardment is excited to a metastable state. The sample components enter the detector and collide with a metastable helium or argon atom to ionize the sample molecules, and the base current is greatly increased to detect the electrical signal. The metastable energies of helium and argon are 19.8 eV and 11.6 eV, respectively. At present, HID is mainly used for impurity detection in sorghum, and AID is used for trace impurity analysis in high-purity argon production and use. Since the above two detectors are mainly used for the analysis of permanent gas, the purity of the carrier gas itself is required to be particularly high, and the interference of the surrounding atmosphere with the work is also to be prevented, which brings many difficulties to practical operation. Be prepared for thinking when purchasing. In addition, due to the small production volume or the need for imports, high prices also hampered popularization and promotion.

Three: trace analysis of the basic configuration of the GC instrument requirements:

As mentioned before, if a special instrument is purchased from the manufacturer's agreement to meet the requirements of the trace analysis project, there is no need to consider how the instrument is configured, as long as the final acceptance is acceptable. However, if the customer himself chooses general general-purpose instruments and builds the column (outsourced) according to the analysis purpose, he or she can complete the analysis task. When selecting the instrument, it can be selected and compared from the following aspects (reference):

1) The concentration range of the component to be analyzed? Is it necessary to perform concentration and other pretreatments to determine the minimum or minimum detection concentration of the selected instrument?

2) According to the sample status and quantity requirements, the instrument uses a valve injection or a syringe injection. For example, although the precision of the gas injection is high, the injection of the injection needle is only necessary if the sample is only a few milliliters short of the six-way valve priming tube.

3) Since the sample is easily adsorbed and decomposed, does the instrument require a full glass system?

4) Does the instrument configure the type of column you use, such as packed or capillary columns?

5) Are you satisfied with the packing column specifications (column outside diameter, length and material) that you can use? Can the instrument be installed? Is it convenient?

6) Did you use the packed column injection vaporization system instrument?

7) Do you use the type of capillary column you are using?

8) Did you prepare the capillary column injection method?

9) Instrument oven operating temperature range, temperature control accuracy, can meet the requirements? Can you do a program to warm up?

10) What kinds of detectors can be configured and at the same time can operate several, can the sensitivity meet the requirements?

11) Can the detector do time programming (in the analysis process, signal attenuation, span adjustment, auto tuning, etc.)?

12) Is there an external event control function, such as: switching the two detector signal outputs and switching the column;

13) Can the capillary capillary quantitative shunt operation be achieved by modifying the instrument?

14) How large is the effective volume of the oven, which can meet the column length of several columns at the same time?

15) Can the instrument conveniently install other auxiliary devices such as: reformer, thermal desorber, cracker, etc.?

16) Does the equipment airway system structure and sealing method have advanced design principles? Is it leak-proof and pollution-proof? Is it easy to install and maintain?

17) Is it convenient and quick to replace common components (such as columns, filters, detectors, etc.)?

18) Are the requirements for the working environment harsh?

19) Is the pressure flow regulation of the instrument convenient and sealing? Can the minimum control pressure and flow rate meet the requirements?

20) Is the instrument parameter setting and adjustment convenient, simple, reliable, and reproducible?

Four: The relationship between the main technical performance of chromatographic data processor and the purchase of GC:

The basis of trace quantitative analysis is the peak area of ​​the measured component, so the correct peak value and accurate measured peak area (or peak height) are the most basic requirements for chromatographic data processing. As mentioned earlier, in gas chromatography trace analysis, the ratio of the main and trace components of the analysis target is generally 104 ∽ 107 or more, not only the electrical signal is very weak, the peak area is small, plus some trace analysis often Peaking or overlapping peaks can only occur on the trailing edge, which imposes higher requirements on chromatographic data processing. In the data processing practice, people find that due to poor data processing performance or functionality, or due to peak-argument parameters, the peak-measurement parameters should not be set reasonably, which will lead to a hard-to-use GC analysis system and do everything possible to optimize the chromatographic separation conditions. The trace component of the peak is distorted, either not detected or misjudged. In order to achieve the purpose of trace analysis and obtain accurate quantitative analysis results, trace analysis should be the top product in data processing equipment. Currently, data processing devices can be roughly classified into three categories. The commonly used long-pattern electronic potentiometer (ie, recorder) is not suitable for trace analysis due to its large dead zone and low sensitivity, difficulty in measuring and calculating peak areas, and poor accuracy. Chromatographic data processors (integrators) and chromatographic data processing workstations are electronic, fully automated data processing devices with similar basic processing functions. Can be used for trace analysis based on performance and function. Judging whether an integrator or chromatographic data processing workstation is good or bad, theoretically speaking, it is mainly to see whether the data processing and calculation methods used are more advanced, but designers and designers are never willing to indicate that they are used in comparison with other similar products. The advantages and disadvantages of the calculation method. Therefore, it is difficult for the user to determine the quality of the specification and the technical performance.

When purchasing chromatographic data processing devices, generally only:

1 understand the supplier's brand or reputation;

2 Comparison of actual use of peers or related units;

3 Please ask the insider to test and compare based on performance and function.

Practice has proved that the performance of a poor data processing device when doing data processing does not match the analysis requirements of the GC instrument, but it helps. If a data processing device with good performance and complete functions can not only accurately and accurately process data, but also has a certain soft noise reduction function, it can increase the S/N ratio by a factor of several to several tens of times. In this way, due to the use of an excellent data processing device, the performance of the chromatographic instrument is reduced and the analysis conditions are optimized. It is not difficult to see from the above brief description that the optional data processing device is not simpler than the GC. For more details, please refer to the relevant materials or consult the website.

Five: The relationship between the column and the optional GC:

In order to meet the requirements of trace analysis, gas chromatographs generally operate at the highest sensitivity (closest to the highest sensitivity). Selecting a high performance column at this time is more important than any chromatographic condition. Because an excellent chromatographic column can achieve: the tested components can not only achieve complete separation, short retention time (narrow peak), but also allow large injection volume, which is helpful to improve the minimum detection or minimum detection concentration of GC. However, because GC works with high sensitivity, the signal of the main component is particularly large in trace analysis. In most cases, a good column cannot be selected. To avoid masking or disturbing the trace components of the main component, consider the following points before purchasing the instrument:

1 Is it possible to use a column to achieve trace components in the peak of the main component;

2 Optional detector may be more suitable;

3 can use the multi-dimensional chromatography technology, the main component peak filter;

4 Concentration technology to increase the concentration of trace components (up to 2∽6 orders of magnitude);

5 increase the selectivity of the trace components to the main component by the conversion method;

6 Selecting capillary columns and special injection methods for trace analysis In most cases, it is more effective than using packed columns.

Sample collection, pre-processing, and purchasing GC relationships?

Through the above few lessons, we realized at one time that trace analysis with gas chromatography is indeed a comprehensive experimental technique. We can summarize the trace chromatographic analysis process into the following four stages:

1 sample collection;

2 sample preparation (pre-treatment);

3 chromatographic analysis;

4 data processing and expression of results.

If the sample collection and pretreatment are relatively successful, chromatographic analysis and data processing, even if the selected chromatographic instrument used with the detector sensitivity is not high, the analytical column separation efficiency is low, the data processing device performance, function in general, can also be obtained Ideal experimental results. Conversely, if a high-efficiency column is selected for the selected instrument and data processing device and configuration, the sample pretreatment process can be greatly reduced. In current trace analysis, time-consuming, laborious, and inefficient sample acquisition and processing remain the bottleneck in the overall chromatographic analysis. Sample collection and processing times sometimes account for two-thirds of the entire analysis time.

It should be noted that no matter what state-of-the-art chromatographs and equipments, true high-performance columns, or the most complete data processing devices, it is impossible to obtain satisfactory analysis results from an improperly collected sample. Therefore, when selecting the type and performance of an instrument (including a data processing device), it is necessary to consider how to make full use of the comprehensive analysis capability of the selected instrument in order to simplify the pretreatment process of the sample or to not require pretreatment of the sample at all.

In order to conduct a trace gas chromatography analysis, it is more effective to purchase a gas chromatograph and ancillary equipment. We have summarized the principles and methods of sample collection and preparation, and the relationship with chromatographic analysis methods as follows: (If you know more details Please refer to related professional books)

1. Sample collection:

In the current domestic analysis, general sample preparation (pretreatment) is performed by chromatographers, while sample collection is performed by other workers. In order to choose the right sample preparation method and the accuracy and reliability of the analysis results, we should mention that the analysts understand the source of the prepared sample, the collection method, and the collection process, and the person responsible for purchasing the instrument is no exception. Do you know about collecting samples?

What is the material composition and concentration of the sample?

What are the main components in the sample?

How to collect the sample site and site conditions: a) the best time to collect the sample; b) the location of the collected sample; c) the process of collecting the sample (effective time); d) the time interval for sample collection;

Should use destructive or non-destructive sampling methods?

Collecting samples for transportation and storage;

l. Which chromatography results are expected after sampling?

2. What are the principles that should be followed when selecting methods for sample collection and processing and their techniques?

a sample of the test component must be representative;

b The collection method and analysis purpose should be consistent to ensure that the desired sample can be collected;

c. How to prevent and avoid the change and loss of the tested components during sample processing;

d When the chemical reaction (derivatization, catalytic conversion) of the component to be measured must be known and quantitatively completed;

e Select the sample processing method should be as simple as possible, the processing device and the sample volume should be adapted;

3. Why choose a sample for processing?

The purpose of sample pretreatment can be summarized as: a) the components to be analyzed for separation; b) enrichment; c) conversion; d) derivatization (transformation into chromatographic state);

*Can't directly inject analysis:

Such as:

a) Variety of types (adverse effects of water, oxygen, etc. on instruments and columns);

b) The composition and concentration of the sample are complex and varied (macro interference to the trace components to be analyzed in the matrix);

c) Wide physical form of the sample (viscosity, solids, heterogeneity);

d) too many interference factors in direct analysis;

* Consider using sample pretreatment methods to make up for deficiencies in existing instruments or analysis conditions

a) Analyze the different quality requirements of the test;

The on-site environment is not allowed (eg time);

b) sample status, instability or chemical activity;

c) Existing analysis conditions are not allowed;

d) The optional equipment and equipment conditions are not available;

e) the technical level of the operator;

4. Common sample pretreatment technologies and equipment:

Although the sample pretreatment technology is still the bottleneck of trace chromatography analysis, with the development of science and technology, many traditional sample pretreatment technologies or equipment have been greatly improved and perfected, and new processing methods and technologies have also come out one after another. At present, the preparation methods of samples are in the process of coexistence of multiple processing technologies. Under the situation where new and old technologies are continuously combined, which sample processing technology is selected depends on the purpose of analysis, analysis methods, or existing conditions. In short, we must analyze specific issues.