CN112525980B - Mass spectrometry detection method for melamine in dairy products - Google Patents
Mass spectrometry detection method for melamine in dairy products Download PDFInfo
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Abstract
The invention relates to a mass spectrometry detection method for melamine in dairy products. The method comprises the steps of performing protein denaturation and filtration on dairy products, then performing analysis test by using a laser-induced ultrasonic desorption source mass spectrometer, determining whether melamine exists or not by using a melamine characteristic mass spectrum peak m/z 126, and judging the melamine content range by semi-quantitatively judging the intensity of the mass spectrum peak m/z 126. Meanwhile, the method has the characteristic of high sensitivity, and the sample detection limit is as low as 10 pg.
Description
Technical Field
The invention belongs to the technical field of food component detection, and particularly relates to a method for detecting melamine in dairy products including milk.
Background
Melamine is a common chemical raw material, is commonly used for preparing melamine resin, is also used as a water reducing agent, a formaldehyde cleaner and the like, and is also used for preparing chemical fertilizers. In the food industry, it is often necessary to detect the protein content in milk and dairy products, and the protein content is determined by using the kjeldahl method, wherein the measurement principle is to estimate the protein content based on the nitrogen content in an analyte. The nitrogen content of the melamine is up to 66 percent and is far higher than the nitrogen content of 16 percent of the protein, so that the melamine is used as an additive by illegal traders to improve the protein content index in food, and serious food safety accidents are caused by national standard detection. Milk and dairy products using melamine as an additive can cause a series of health problems such as malnutrition, urinary diseases, cancers and the like to consumers, and can seriously and even endanger life, such as the three-deer milk powder event.
At present, the "method for detecting melamine in raw milk and dairy products" (GB/T22388-. In actual detection, although the standard method can provide high-sensitivity quantitative detection, in actual use, a sample needs to be subjected to purification pretreatment and then subjected to chromatographic separation detection, and the whole process is complicated.
Disclosure of Invention
The invention mainly aims to provide a rapid, simple and convenient mass spectrometric detection method for melamine in dairy products.
The technical scheme of the invention is as follows:
a mass spectrometry detection method for melamine comprises the following steps:
1) adding 0.1-1g solid sample or 0.1-1ml liquid sample into 40-60ml 1-10% dimethyl sulfoxide methanol solution, uniformly mixing for 5-15min, performing ultrasonic denaturation and precipitation for protein for 20-40min, centrifuging at 5000-10000rpm for 10-20min, taking supernatant, filtering with 0.1-1.0 μm filter membrane, completely evaporating filtrate to dryness, and collecting;
2) collecting a product, dissolving the product in 0.5-5ml of methanol solution again, dripping the product on the front surface of a titanium foil, completely drying, and then loading a titanium target on a target table of a mass spectrometer for testing, wherein the mass spectrometer comprises a laser-induced ultrasonic desorption source, an ionization system with the functions of electron bombardment ionization and photo ionization, an ion debugging and transmission system and a small-sized single-trans flight time mass analyzer; preferably, the mass spectrometer has an overall size within 500 x 400 mm.
3) And (3) determining whether melamine exists or not by using the melamine characteristic mass spectrum peak (m/z 126), and judging the melamine content range by semi-quantitatively judging the intensity of the (m/z 126) mass spectrum peak.
In a preferred embodiment, the laser energy is 14-16mJ with a frequency of 8-12Hz and a pulse width of 5-7 ns.
In a preferred embodiment, the laser-induced ultrasonic desorption source comprises a rotating target rod, a titanium foil target, a pulse laser and a beam focusing lens; wherein, the light beam focusing lens is positioned between the rotating target rod and the pulse laser; the titanium foil target is positioned at one end of the rotary target rod, which is far away from the laser, and the sample is positioned at one side of the titanium foil target, which is far away from the laser.
In a preferred embodiment, the rotating target shaft is off-axis from the ionization system, with the axis offset by 1-3 mm.
In a preferred embodiment, the target rods are rotated, each target point differing by 10-30 °, for a total of 12-36 test target points for the whole target.
In a preferred embodiment, the ionization system comprises an electron bombardment ion source and a photo ionization source which are respectively arranged at two ends of the ionization chamber.
In a preferred embodiment, the ion modulation transmission system comprises a three-lens ion transmission unit, and three groups of same single lenses are used for ion transmission.
In a preferred embodiment, each singlet lens is formed of a ring with an outer diameter of 18-22mm and an inner diameter of 4-6 mm.
In a preferred embodiment, the compact single-transflight time mass analyzer comprises an acceleration field, and the reflection field is arranged in the acceleration direction of the acceleration field; and a field-free region is arranged between the reflecting field and the secondary accelerating field.
In a preferred embodiment, the acceleration field comprises a primary acceleration field and a secondary acceleration field, wherein the reflection field of the secondary acceleration field is arranged in the acceleration direction of the primary acceleration field, and the acceleration directions of the secondary acceleration field and the primary acceleration field have an included angle of 50-150 degrees; the reflected field includes a primary reflected field and a secondary reflected field.
In a preferred embodiment, in a small single-transtype flight time mass analyzer, the total length of the acceleration zone is 20-30 mm; the primary acceleration field is 8-12 mm; the field-free region is 180-220mm, the total reflection field is 70-80mm, wherein the primary reflection field is 28-32mm, and the secondary reflection field is 40-50 mm.
The invention provides a melamine detection scheme based on a laser-induced ultrasonic desorption source mass spectrometer. The method comprises three processes of a testing instrument, a sample pretreatment method and sample detection.
Compared with the prior melamine detection method, the method has the advantages that:
1. the invention adopts a self-developed mass spectrometer which comprises a laser-induced ultrasonic desorption source, an ionization system with the functions of electron bombardment ionization and photo ionization, an ion debugging and transmission system and a small single-reverse flight time mass analyzer. Compared with the traditional method for separating melamine by using chromatography, the method only needs to use the dimethyl sulfoxide solution to remove protein components in the test sample, and is simpler and more convenient to operate and less in time consumption.
2. Through the detection of 1000ng of melamine with standard mass, m/z 126 is taken as a melamine qualitative peak, the detection limit uses the signal-to-noise ratio as 3 as a detection baseline, the noise value is the baseline standard deviation of the later section of a mass spectrum, the detection limit of the detection method is calculated to be 10pg, and the detection can be completed only by a small amount of samples.
Drawings
The invention is further illustrated by the following examples in conjunction with the drawings.
FIG. 1 is a schematic diagram of time-of-flight mass spectrometry of a laser-induced ultrasonic desorption source.
FIG. 2 shows the test pattern of 1000ng melamine.
FIG. 3 is a graph of the test of 1mg/Kg melamine content in milk powder.
Detailed Description
Referring to fig. 1, the present invention provides a miniaturized laser induced ultrasonic desorption source time-of-flight mass spectrometer, the overall size of which is limited to 500 x 400mm, comprising a laser induced ultrasonic desorption source 100, an ionization system 200 with both electron bombardment ionization and photo ionization, an ion modulation transmission system 300 and a miniaturized single transreflective time-of-flight mass analyzer 400.
The laser-induced ultrasonic desorption source 100 comprises a rotating target rod 130, a titanium foil target 140(15 μm), a 1064nm pulsed laser 110 (energy 10mJ, pulse width 5-7ns, frequency 10Hz) and a beam focusing lens 120(f ═ 500 mm). Wherein the beam focusing lens 120 is positioned between the rotating target 130 and the pulsed laser 110. The titanium foil target 140 is located on the end of the rotating target shaft 130 away from the laser 110 and the sample 150 is located on the side of the titanium foil target 140 away from the laser 110. The laser-induced ultrasonic desorption source 100 desorbs a sample 150 on the front side of a titanium foil target by focusing 1064nm laser into about 1mm light spots to irradiate the back side of the titanium foil target 140, the laser irradiation time of each target spot is 5s, the test target spots are changed by rotating a rotating target rod which is in a staggered design (the axis deviates by 2mm) with an ionization system, the difference of each target spot is 20 degrees, and the total number of the test target spots is 18.
The ionization system 200 includes an ionization chamber 210, an electron impact ionization source 230(EI), and a photo ionization source 220 (PI). The ionization chamber 210 after the optimization design is provided with an electron bombardment ionization source 230(70eV electrons) on the lower side and a photoionization source 220 (10.6 eV vacuum ultraviolet lamp) on the upper side, the ionization mode can be selected according to the requirement in the actual test, the photoionization source is selected for the complete mass number information, and the electron bombardment ionization source is selected for the fragment peak structure analysis.
The ion debugging and transmission system 300 adopts a design of three single lenses (310/320/330), corrects the spherical aberration of the ion beam through multiple times of focusing, and vertically introduces the ion beam into the time-of-flight mass spectrum, wherein each single lens is composed of three circular rings with the outer diameter of 20mm and the inner diameter of 5 mm.
The miniaturized single-reflection time-of-flight mass analyzer 400 employs dual-field acceleration and dual-field reflection technology, and the actual resolution can reach more than 1500. The specific structure of the instrument is as follows: the ion receiving and transmitting system 300 receives ions from the ion debugging transmission system and enters the electrostatic single lens 420 at the bottom left side of the analyzer, the accelerating field 430 is arranged at the right side of the electrostatic single lens 420, the upper end of the analyzer is provided with a secondary reflecting field, and a field-free region 410 is arranged between the accelerating field 430 and the reflecting field. The reflected field includes a primary reflected field 450 and a secondary reflected field 440. The reflected ions enter the MCP detector 460 on the right side of the bottom of the analyzer.
Wherein the total length of the accelerating area is 25 mm. The primary acceleration field is 10mm, and the secondary acceleration field is 15 mm. The field-free region is 200mm, the total reflection field is 75mm, wherein the primary reflection field is 30mm, and the secondary reflection field is 45 mm.
The reflected ions enter MCP detector 460 and the signal passes through amplifier 500, ADC converter 600, and to computer 700 for analysis.
The use principle of the invention is as follows:
the invention uses dimethyl sulfoxide solution to remove protein components in a sediment sample, carries out mild desorption through laser-induced ultrasonic desorption mass spectrometry, completes the conversion from a solid phase to a gas phase of a test sample, carries out ionization of the test sample through electron bombardment ionization and photoionization, and finally carries out high-sensitivity detection through flight time mass spectrometry.
Example 1
The self-grinding miniaturized laser-induced ultrasonic desorption source flight time mass spectrum is used as test equipment, commercially available milk powder is selected as a sample, 10g of milk powder is taken, 0.01mg of melamine is added, and the mixture is ground for 30min and uniformly mixed to serve as a test sample. Weighing 0.5g of test sample, adding 50ml of 5% dimethyl sulfoxide methanol solution, uniformly mixing for 10min, performing ultrasonic treatment for 30min to denature and precipitate protein, centrifuging at 6000rpm for 15min, taking supernatant, filtering with a 0.22 mu m filter membrane, completely evaporating filtrate to dryness, and collecting.
And dissolving the collected product in 1ml of methanol solution again, dripping the dissolved product to the front side of the titanium foil, drying the titanium foil in a 60 ℃ blast drying box, testing the titanium target device on a mass spectrometer target table after the titanium foil is completely dried, and collecting 20 targets in total for each target for 5s and 100s in total for each target.
250ng of melamine is loaded on the whole target surface, the test result shows that the peak strength S is 5326634 when the m/z is 126, the noise N is 53.27, when the S/N is 3, the sample amount is 5.97pg, and the detection limit is estimated to be 1.194 x 10 -4 mg/Kg。
Claims (1)
1. A method for detecting melamine in dairy products by mass spectrometry is characterized by comprising the following steps:
1) adding 0.1-1g solid sample or 0.1-1ml liquid sample into 40-60ml 1-10% dimethyl sulfoxide methanol solution, uniformly mixing for 5-15min, performing ultrasonic denaturation and precipitation for protein for 20-40min, centrifuging at 5000-10000rpm for 10-20min, taking supernatant, filtering with 0.1-1.0 μm filter membrane, completely evaporating filtrate to dryness, and collecting;
2) collecting a product, dissolving the product in 0.5-5ml of methanol solution again, dripping the product on the front surface of a titanium foil, completely drying, and then loading a titanium target on a target table of a mass spectrometer for testing, wherein the mass spectrometer comprises a laser-induced ultrasonic desorption source, an ionization system with the functions of electron bombardment ionization and photo ionization, an ion debugging and transmission system and a small-sized single-trans flight time mass analyzer;
3) determining whether melamine exists or not through a melamine characteristic mass spectrum peak, and judging the melamine content range through the mass spectrum peak strength semiquantitatively;
the mass spectrum is a miniaturized laser-induced ultrasonic desorption source flight time mass spectrum, the overall size of the mass spectrum is 500 x 400mm, and the mass spectrum comprises a laser-induced ultrasonic desorption source, an ionization system with the functions of electron bombardment ionization and photo ionization, an ion debugging and transmission system and a miniaturized single-transtype flight time mass analyzer;
the laser-induced ultrasonic desorption source comprises a rotary target rod, a titanium foil target, a 1064nm pulse laser 110, energy of 10mJ, pulse width of 5-7ns, frequency of 10Hz, and a beam focusing lens 120, wherein f =500 mm; wherein, the light beam focusing lens is positioned between the rotating target rod and the pulse laser; the titanium foil target is positioned at one end of the rotary target rod, which is far away from the laser, and the sample is positioned at one side of the titanium foil target, which is far away from the laser; the laser irradiation time of each target point is 5s, the test target points are changed by rotating the rotating target rods which are designed in a staggered mode with the ionization system, the difference of each target point is 20 degrees, and the total number of the test target points is 18;
the ionization system comprises an ionization chamber, an electron bombardment ionization source and a photo ionization source; an electron bombardment ionization source with 70eV electrons is arranged at the lower side of the ionization chamber, a photoionization source with 10.6eV vacuum ultraviolet lamp is arranged at the upper side;
the ion debugging and transmission system adopts a design of three single lenses, corrects the spherical aberration of the ion beam through multiple times of focusing, and vertically introduces the ion beam into a flight time mass spectrum, wherein each single lens consists of three circular rings with the outer diameter of 20mm and the inner diameter of 5 mm;
the miniaturized single-reflection type flight time mass analyzer adopts double-field acceleration and double-field reflection technology, and the specific structure of the analyzer is as follows: the system comprises an electrostatic single lens, an accelerating field, a secondary reflecting field and a non-field area, wherein ions from an ion debugging and transmission system are received and enter the left side of the bottom of an analyzer; the reflection field comprises a primary reflection field and a secondary reflection field; the reflected ions enter the MCP detector on the right side of the analyzer bottom;
wherein, the total length of the acceleration zone is 25 mm; the primary acceleration field is 10mm, and the secondary acceleration field is 15 mm; the non-field area is 200mm, the total reflection field is 75mm, wherein the primary reflection field is 30mm, and the secondary reflection field is 45 mm; the reflected ions enter the MCP detector, and the signal passes through an amplifier, an ADC converter, and to a computer for analysis.
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