Now most food testing laboratories are equipped with color-mass spectrometry, and the use of mass spectrometry alone has been very few. The only one used alone is to use an isotope mass spectrometer to detect the isotope ratio in foods such as honey to determine whether the product is adulterated. This article mainly introduces the main performance and functions that need to be considered when purchasing GC-MS.
Gas chromatography-mass spectrometry is a high-separation GC combined with an MS that can provide molecular information of the measured substance. The two instruments complement each other, making the instrument's analysis function more powerful. For example, mass spectrometry can provide specific molecular information of the analyte, and the qualitative of the compound is more accurate. However, mass spectrometry cannot distinguish isomers, and chromatographic separation of isomers is easy. Therefore, the function of the color-mass spectrometer is 1 + 1> 2.
At present, the gas chromatography part of the gas chromatography-mass spectrometry uses capillary chromatography with high separation performance, and different types of inlets can be selected, such as the most commonly used split / splitless inlet and (temperature / pressure) programmable Sample port. Multistage programmed temperature control of oven. When it comes to GC-MS performance, the performance and function of the mainstream model GC of the more common brands in the domestic market are not much different now. Therefore, no comparison is made in terms of GC.
There are many types of MS, usually divided according to the type of analyzer, including quadrupole mass spectrometry, ion trap mass spectrometry, time-of-flight mass spectrometry, quadrupole tandem mass spectrometry, high-resolution magnetic mass spectrometry, etc. Different models of different manufacturers have large differences in performance, function, price or cost performance. Therefore, this article will focus on MS. At present, the GC-MS combined instrument used in the food testing laboratory is mostly equipped with low-resolution MS. This type of instrument mainly focuses on the qualitative and quantitative of the target compound, and also has a certain qualitative function of unknown substances. There are two purposes for choosing this type of instrument:
First, and the main purpose is to analyze residues in food.
Since it is used for residue analysis, the sensitivity of the instrument is very important, and it should be considered first when selecting the instrument. But this is not the only indicator (especially you can not just look at the nominal indicator), but also comprehensively consider the resolution, mass stability, mass range, dynamic linear range, anti-pollution ability of the instrument (including instrument ion source, pre-quadrupole, etc.) Whether the cleaning and maintenance of components are convenient), and whether the software operation is convenient.
The application of gas chromatography-mass spectrometry in the analysis of residues is becoming more and more common because MS is a general-purpose detector that has a relatively good response to most organic compounds. On the other hand, there is a selective ion method (SIM method) for quadrupole mass spectrometry detection, which can improve the detection sensitivity by two or three orders of magnitude compared with the full scan method, and the detection sensitivity is higher than that of the hydrogen flame detector (FID) and flame photometric detection The detector (FPD) and nitrogen phosphorus detector (NPD) are high, which is slightly inferior to the detection of organic polyhalogen compounds by electron capture detector (ECD). Residue analysis is mostly target detection, so SIM detection is both broad-spectrum (for the response of compounds) and specific (for the characteristic ions of different compounds), so it is particularly suitable for use in The detection of various residues improves the analysis efficiency.
Now the technical indicators listed by the instrument company when buying the instrument are: sensitivity, resolution, quality stability, quality range, dynamic linear range, etc.
Why is the manufacturer's nominal sensitivity on the market so high?
Now the sensitivity is expressed using octafluoronaphthalene (OFN), such as: EI +, 1pgOFN signal / noise (S / N)> 100. The current signal-to-noise ratio is RMS (root mean square), and the value is much higher than the previous sensitivity value. In the past, the signal-to-noise ratio was the peak-to-peak ratio, that is, the peak height of the signal / the peak height of the baseline noise. It is more obvious at a glance, and all the energy is measured by the ruler. However, according to the manufacturer, there is a human error in choosing the baseline noise. Now the manufacturer compiles the signal / noise ratio into a fixed program, such as the ratio of the signal value to the noise in a fixed period of time (such as 1 ~ 2min, in fact the baseline of this period is relatively flat). However, the current measurement method manufacturers actually have many stealers. For example, when using the short test column (10m or 15m) that comes with the manufacturer, the scanning range of the quality is reduced, and the injection volume is increased (in the past, it was air-sample liquid-air 1μL, and now 1μL is the dead volume including the needle). No way, now manufacturers do this for competition, and users have to follow. Therefore, it is not enough to look at the manufacturer's nominal index now.
When doing sensitivity index, you should pay attention to several issues:
(1) Resolution should be done first, and sensitivity should be done when the unit mass resolution is guaranteed. As shown in the figure below, an approximation method can be used, that is, the peak width at half height is not less than 1/2 peak width. Sensitivity is inversely proportional to resolution. If the resolution is lost for sensitivity, the qualitative function of the mass spectrum will be reduced.
(2) The quality scan range should also be specified, for example: OFN, 200-300 amu, the reduction of the scan range can also improve the signal / noise ratio. These restrictive conditions should be determined when the contract is negotiated.
(3) The detection voltage should be the working voltage during normal detection. Different types of mass spectrometers have different meanings due to the parameters. Therefore, the recommended detection voltage value of each instrument is also different. However, the voltage during the sensitivity test should not be higher than the recommended operating voltage during normal use. Otherwise, there will be problems in actual work, because the actual sample is detected by matrix interference, high voltage can not improve the signal / ratio, and it will also reduce the life of the electron multiplier.
At present, there are some over-emphasis in China, or manufacturers over-promote their instruments with high sensitivity, which results in the nominal sensitivity becoming higher and higher. I heard that the RMS signal / noise ratio is given as 1000. In fact, the index of the standard product is just a reference. In the future, it is the key to obtain good and stable results from actual samples with complex matrices (such as animal internal organs). Now there are more and more units with instruments. You can make an actual sample to each instrument before purchasing the instrument to measure it and understand the reactions of various instrument users. This is better than just indicators.
The other indicators of the instrument are generally not too problematic.
For low-resolution mass spectrometry, there is generally no problem with resolution reaching unit resolution.
The mass range is now commonly referred to as 2 ~ 1025 (or 1500) u. This mass range is sufficient for GC-MS. Because GC-MS analytes are volatile or semi-volatile substances, the molecular weight is generally not too large. The only thing to note is that if the pollutants decabromobiphenyl (MW954) and decabromobiphenyl ether (MW 970) are tested, those with a mass of less than 1025u cannot be selected (the MS quality range of individual manufacturers is only 800u)
The stability of quality is generally 0.1 amu / 8hr, which is actually very important. A good instrument can be calibrated only once every few months, and a poor one should be calibrated every week. Although it does not affect the detection, it increases the workload of the operator.
The linear range is greater than 10e4, which is sufficient for residual analysis. These indicators need to be done carefully in accordance with the contract when accepting the instrument.
In addition, some functions of the instrument must also be done by hand when accepting the instrument, such as: chemical ionization source (CI) replacement, direct injection rod operation, composite ionization switching method (EI / CI), composite scanning method ( TIC / SIM) etc. Many pesticides contain halogens and electronegative groups, so they are electronegative. Negative chemical sources (NCI) can detect such substances with higher sensitivity. This is due to the lower background of NCI and higher signal / noise ratio when detecting electronegative substances. It can also play a supplementary confirmation role for qualitative. When doing NCI, the reaction gas needs to be vented, so the vacuum system of the instrument is better. At present, the GC-MS configuration provided by the manufacturer is optional. If it is equipped with NCI, it must be equipped with a vacuum pump with a large pumping rate, at least greater than 250L / min, and the highest configuration is 2 × 200L / min. In addition, the convenience of replacing the ion source should also be considered. Some models of instruments can replace the ion source without breaking the vacuum.
Residual analysis is usually the detection of targets, most of which are pesticides, veterinary drugs, additives, chemical contaminants, etc. Qualitative here is just to confirm the target. Two methods can be used for this qualitative analysis. One is to compare with the mass spectrum of the NIST spectral library (more than 140,000 pieces provided in the 2006 edition), and the other is to compare with the mass spectrum of the corresponding standard product. . The latter comparison method is better and more accurate in actual detection. Because, after the analyte undergoes pretreatment and capillary column, the interference of the matrix will cause the ion fragment and abundance ratio of the analyte mass spectrum and the mass spectrum of the NIST library (usually obtained by direct injection of pure product). deviation. In addition, a standard product is required for quantification.
The second analysis function is to analyze unknowns
The unknown here is not a substance that is completely unknown in the true sense. If it is the substance that is completely unknown, it is still difficult to achieve accurate confirmation by MS, especially low-resolution MS. The so-called unknown here is actually a substance that has been recognized by people. The mass spectrum information of this substance has been included in the NIST spectral library. It is just that the substance we detect does not know which of these substances is contained. For example, the ingredients of the same natural product in different regions are not the same. They are both rose essential oils. The composition of domestic and imported ingredients is different. Through MS analysis and comparison with the NIST library, you can find two What is the characteristic substance of essential oil, how much the amount is different, the difference is there. As another example, the fish in the fish pond suddenly died, and it is unclear what the reason was. Then we took the water in the fish pond and tested it. It is not clear what substance is contained in the water. Unknown. After getting the laboratory test, the NIST spectral library search and comparison of mass spectrometry showed that methamidophos was detected. For the sake of insurance, another shot of methamidophos standard was given. The retention time and ion abundance ratio were the same. It was finally determined that the methamidophos contained in the water was the cause of fish death. This kind of work encounters less in daily work. The requirement of the instrument is that the detected mass spectrum should be as close as possible to the NIST spectral library, so that the results obtained will be more accurate. Therefore, this is the best choice for quadrupole mass spectrometry, time-of-flight mass spectrometry or high-resolution magnetic mass spectrometry. The spectrum obtained by ion trap mass spectrometry, especially endogenous ion trap mass spectrometry, is much different from that of NIST library.
Some experience in configuring and purchasing mass spectrometry
1) The first set of mass spectrometers is recommended to choose quadrupole, and there are more funds recommended to configure some other types
According to the foregoing, it is generally recommended to purchase the first set of GC-MS to choose quadrupole mass spectrometry. For the second set of mass spectrometers, what type of mass spectrometer should be selected according to the laboratory's business situation. According to years of experience in laboratory work, it is recommended to configure another type of MS, because other types of MS also have their own characteristics and can play a complementary role.
2) Ion trap mass spectrometry is good at initial screening, structural analysis and confirmation
Ion trap mass spectrometry can be used for multi-stage mass spectrometry, especially suitable for the primary screening of pesticide residues in food. Due to the MSn (theoretically n = 10) function, matrix interference can be better eliminated by the second or even third-stage mass spectrometry. , Accurate confirmation. When the result is suspected to be positive, then further qualitative and quantitative. Some laboratories in the European Union do this. The quantitative reproducibility of ion trap mass spectrometry for the detection of complex samples is not as good as that of quadrupole mass spectrometry, but its ability to resolve compound structures is stronger than that of quadrupole mass spectrometry. The ion trap mass spectrometer is divided into an external ion source type and an internal ion source type. The price of the external ion trap mass spectrometer is equivalent to that of the quadrupole mass spectrometer, and the price of the internal source ion trap mass spectrometer is equivalent to the low spec. Of the quadrupole mass spectrometer.
3) The tandem quadrupole mass spectrometry is both quantitative and sensitive
If sufficient funds are available, consider configuring quadrupole tandem mass spectrometry. This type of mass spectrometry can be used for MS-MS. Although it can not be used as MSn for ion trap mass spectrometry, it is generally sufficient for secondary MS, because the residues detected are mostly small molecular compounds with molecular weights of 100-300. Quadrupole tandem mass spectrometry is better than single quadrupole mass spectrometry in anti-matrix interference and qualitative capability, and has better quantitative reproducibility and detection sensitivity than ion trap mass spectrometry. The prices of different models of quadrupole tandem mass spectrometers from different manufacturers are relatively large. The low ones are only higher than the high-match single quadrupole mass spectrometers, and the high ones are close to two sets of high-match single quadrupole mass spectrometers.
4) Time-of-flight mass spectrometry is good at qualitative and confirmation
There are also two types of time-of-flight mass spectrometry used in conjunction with the gas phase, one with high resolution and accurate mass, and the other with more emphasis on rapid detection capabilities, often used in conjunction with fast gas chromatography. For food testing laboratories, the former is more recommended. The high-resolution and precise mass functions are more accurate for characterizing compounds. When there are doubts about the test results, it is helpful to confirm the results by changing to another mode. The price is about 2 sets of single quadrupole mass spectrometer.
5) High-resolution magnetic mass spectrometry is only used in few fields
High-resolution magnetic mass spectrometry is generally not configured in laboratories, and is mainly used for the detection of residues such as dioxins and PCBs. And the sensitivity of such instruments is also high, so a sufficient set of funds, more confident in the results issued. One high-resolution mass spectrometer can be connected to two GCs, and the analysis efficiency, confirmation accuracy, and quantitative accuracy can be achieved at the same time, which is still very good. The price is of course expensive, at least equivalent to four sets of high-quality single quadrupole mass spectrometers.
6) GPC-GC-MS deserves attention
In addition, I also want to introduce a set of gel chromatography-gas chromatography-mass spectrometry (GPC-GC-MS) combined instrument. One of the current trends in residue analysis is multi-residue analysis. A major problem encountered when performing multi-residue analysis is that there are more matrix ions due to the more ions selected. In order to solve this problem, there are two approaches. One is to use instrument functions, such as secondary mass spectrometry, high-resolution functions, etc., to eliminate matrix interference; the second is to work hard in pretreatment and purification to remove interfering substances as much as possible. The former was mentioned earlier in the article. The GPC-GC-MS now introduced is to combine the purification instrument and the analysis instrument into a combined instrument. After online purification to remove matrix interference, there is the concept of multi-dimensional chromatography. This combination of analytical instruments and online purification devices is also one of the development directions of residual analytical instruments. The price of this set of instruments is comparable to that of the high-quality single quadrupole mass spectrometer, which deserves attention.
Application of water treatment process:
pure water, mineral water and other bottled water,water for food and beverage production.
Product's scope of water treatment process :
It is mainly composed of the following equipments:pre-treatment system (water tank,multi-medium filter, active carbon filter, ion exchanger, millipore filter), membrane separation system (ultra filter, nanometer filter, RO system), electrodialysis device, sterilization system (UV device, ozone device) ,product water tank and so on.
Water Treatment System,Reverse Osmosis Filtration System,Reverse Osmosis Filter,Ultraviolet Water Treatment Plant
Zhangjiagang EBA Machinery and Equipment Co., Ltd , https://www.eba-machine.com