CN117433865A - Heavy metal-containing puer raw tea matrix standard substance and preparation method thereof - Google Patents
Heavy metal-containing puer raw tea matrix standard substance and preparation method thereof Download PDFInfo
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Classifications
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- G—PHYSICS
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N1/00—Sampling; Preparing specimens for investigation
- G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
- G01N1/286—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q involving mechanical work, e.g. chopping, disintegrating, compacting, homogenising
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N1/00—Sampling; Preparing specimens for investigation
- G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
- G01N1/38—Diluting, dispersing or mixing samples
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N1/00—Sampling; Preparing specimens for investigation
- G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
- G01N1/44—Sample treatment involving radiation, e.g. heat
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
- G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
- G01N21/64—Fluorescence; Phosphorescence
- G01N21/6402—Atomic fluorescence; Laser induced fluorescence
- G01N21/6404—Atomic fluorescence
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/62—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode
- G01N27/626—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating the ionisation of gases, e.g. aerosols; by investigating electric discharges, e.g. emission of cathode using heat to ionise a gas
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N1/00—Sampling; Preparing specimens for investigation
- G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
- G01N1/286—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q involving mechanical work, e.g. chopping, disintegrating, compacting, homogenising
- G01N2001/2866—Grinding or homogeneising
Abstract
The invention belongs to the technical field of preparation of measurement professional standard substances, and particularly relates to a puer raw tea matrix standard substance containing heavy metals and a preparation method thereof. The standard substance provided by the invention is prepared by taking puer tea raw materials in a main puer tea production area of Yunnan puer tea as a matrix through the steps of selecting, drying, crushing, grinding, uniformly mixing, irradiating and the like, and the puer raw tea powder with good uniformity is prepared and obtained by adopting microwave digestion, inductively coupled plasma mass spectrometry and atomic fluorescence photometry to cooperatively perform fixed value, uniformity inspection and stability monitoring of the standard substance in a plurality of laboratories. The Pu' er raw tea powder containing heavy metals, prepared by the method, has good uniformity and stability, and can be used in the fields of detection analysis and method evaluation of heavy metal content in tea in a food detection laboratory, quality control of analysis process, laboratory capability assessment and the like.
Description
Technical Field
The invention belongs to the technical field of preparation of measurement professional standard substances, and particularly relates to a puer raw tea matrix standard substance containing heavy metals and a preparation method thereof.
Background
The tea production area is widely distributed, and the tea has wide market supply and demand in China. Pu' er tea is one of the famous geographical mark products representative in China, is also a special industry in Yunnan province, is rich in catechol, catechin, vitamin E, flavonoid and other substances, and has various health care functions such as antioxidation, anti-aging, sterilization, anti-inflammation and the like. Tea is also a very common agricultural product, and is extremely easy to be polluted by toxic chemical substances, such as pesticide residues, heavy metal residues, mycotoxins and the like, in the processes of planting, processing, storing, transporting, selling and the like, the factors seriously affect the quality and safety of the tea, the health of a human body is greatly threatened, and along with the improvement of the living standard of people, the heavy metal residues become a second focus of attention affecting the quality and safety of the tea. The accurate determination of the content of heavy metal elements in puer tea has important significance for guaranteeing food safety and maintaining consumer rights and interests.
Because the puer tea geographic mark product protection area relates to 11 states (markets) in Yunnan province, the puer tea of different producing areas in Yunnan province has different quality characteristics, and according to the production process, the puer tea has different human intervention degrees in the processing process, and is classified into two types of raw tea and cooked tea, and the different production processes lead to different heavy metal contents in the puer tea.
Currently, in the daily detection process of puer tea products, a laboratory generally adopts heavy metal (cadmium, chromium, arsenic, lead, mercury, copper and the like) element solution standard substances to control the quality, or adopts other matrix heavy metal standard substances (pork liver powder, total chromium in water) as measurement standards to trace or calibrate the source. Because the matrix effect of the sample is ubiquitous, the metallic element solution standard substance or other matrix standard substances are adopted as quality control substances, and the detection result of the puer tea heavy metal is influenced. At present, the accuracy and the reliability of the detection result of the puer tea heavy metal are still doubtful. When the instrument analysis method is used for detecting the heavy metal content in the puer raw tea sample, the matrix effect interference generated by inconsistent matrix of the standard solution and the sample to be detected must be considered to be eliminated, and the matrix effect interference can cause the measured element content of the sample to be detected to have errors. Therefore, when the element to be detected of the sample is detected, the consistency of the standard curve matrix and the matrix of the sample to be detected must be ensured so as to reduce errors caused by interference of matrix effects.
Therefore, in order to ensure the accuracy, traceability and comparability of measurement data in the quality detection and research of Yunnan Pu' er tea products, it is necessary to deeply research the standard substances of heavy metal component analysis of different matrix types.
Disclosure of Invention
In view of this, considering that the matrix standard substance is a standard substance having the characteristics of an actual sample, the matrix and the target compound are combined in a natural manner, and are more consistent with the actual sample, the influence of the matrix (mass) effect on the analysis of the substance components during the detection process can be effectively avoided. The first aim of the invention is to disclose a preparation method of heavy metal standard substances by taking Yunnan Pu 'er raw tea as a matrix, so as to ensure consistency of standard curves and sample matrixes in the process of detecting Pu' er raw tea samples and minimize errors caused by matrix effect interference.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
a preparation method of a puer raw tea matrix standard substance containing heavy metals comprises the following steps:
(1) Numbering: numbering the collected representative puer raw tea samples, and registering in a book;
(2) Pretreatment: dust is raised on Pu' er raw tea and loose tea leaves by a dustpan, light drift matters such as dust, fragments and the like in the tea leaves are removed, and then the tea leaves are spread and dried on a bamboo sieve, so that non-tea substances are removed manually;
(3) Baking: placing the pretreated puer raw tea powder tea into a low-temperature oven for baking for 24 hours;
(4) Crushing: crushing dried puer raw tea powder tea leaves by a crusher;
(5) And (3) screening: pouring crushed loose tea leaves into a 40-mesh sieve, and sieving a sample by ultrasonic pulse;
(6) Grinding: putting all the tea screening samples passing through the 40-mesh sieve into a corundum ball milling tank of a planetary ball mill for repeated grinding, and enabling the tea screening samples to pass through a 80-100-mesh nylon sieve to form tea powder samples;
(7) Mixing evenly: uniformly mixing a tea powder sample passing through a nylon sieve with 80-100 meshes on a powder mixing machine;
(8) And (3) primary uniformity inspection: carrying out primary uniformity detection on the tea powder sample after uniform mixing;
(9) And (3) irradiation: performing irradiation sterilization on the sample passing the uniformity primary detection;
(10) And (5) subpackaging: and packaging the irradiated and sterilized sample by using a medical packaging bottle made of HDPE material with an inner cover, and packaging by using a food-grade PE double-sided net-shaped vacuum bag, wherein the packaging bottle is sequentially cleaned by washing agent ultrasonic cleaning-tap water-distilled water-ultrapure water cleaning, and then is dried for use.
(11) And (3) preserving: vacuum packaging the sample, and storing in a drier at normal temperature;
(12) Uniformity and stability monitoring: carrying out uniformity inspection and stability monitoring on the prepared puer raw tea sample containing heavy metals by adopting a microwave digestion method, an inductively coupled plasma mass spectrometry (ICP-MS) method and an Atomic Fluorescence Spectrometry (AFS) method;
(13) And (5) constant value and uncertainty assessment: and carrying out microwave digestion, carrying out cooperative constant value on ICP-MS and AFS by an atomic fluorescence spectrometry in a plurality of laboratories, and carrying out constant value on the content of heavy metal elements in the prepared puer raw tea sample in the plurality of laboratories to determine a nominal value and an uncertainty range.
It is worth to say that the matrix standard substance is a standard substance with the characteristics of an actual sample, and is combined with the target compound in a natural way by the matrix, and is more consistent with the actual sample, so that the influence of the matrix (mass) effect on the analysis of the substance components in the detection process can be effectively avoided. The method for preparing the heavy metal component analysis standard substance by taking the Yunnan Pu 'er raw tea as the matrix ensures the consistency of the standard curve and the sample matrix in the detection process of the Pu' er raw tea sample so as to reduce the error caused by the interference of the matrix effect to the greatest extent.
Preferably, the heavy metals include one or more of cadmium, chromium, arsenic, lead, and copper.
Preferably, the low-temperature baking temperature in the step (3) is 35-45 ℃.
Preferably, the mixing time in the step (7) is 6-8 hours.
Preferably, the irradiation in the step (9) adopts gamma rays generated by using radioactive isotope cobalt-60, and the irradiation dose is 6kGy.
Preferably, the split charging of the step (10) is 20g per bottle, and 200 bottles are split charging.
Preferably, the microwave digestion, the inductively coupled plasma mass spectrometry and the atomic fluorescence spectrometry are cooperated to perform fixed value, uniformity test and stability monitoring on standard substances in a plurality of laboratories, and the detection method comprises the following steps:
1) Reagents and samples: nitric acid: the quality is higher than that of the high-grade pure; hydrogen peroxide: high-grade purity; water: ultrapure water; elemental standard solutions; tuning solution: li, co, Y, ce, tl; blank liquid: 5% nitric acid; internal standard solution: in, ru, rh, bi;
2) The method for setting the value comprises the following steps: preparing a standard curve by adopting a single-element primary standard substance, calculating and tracing the content of an element to be detected in a sample, simultaneously measuring heavy metal elements in a quality control sample and a puer raw tea sample, and directly comparing the measured mode fixed values;
3) The method comprises the steps of pre-measuring the moisture content of a puer raw tea sample, performing pretreatment by adopting microwave digestion, and measuring the heavy metal element content in puer raw tea by adopting an inductively coupled plasma mass spectrometry (AFS) method; wherein,
the inductively coupled plasma mass spectrometer has the working conditions that: RF power/kW: 1.55; RF matching voltage/V: 1.8; plasma gas flow rate Ar:15.0 L.min -1 The method comprises the steps of carrying out a first treatment on the surface of the Auxiliary air flow rate Ar:1.0 L.min -1 The method comprises the steps of carrying out a first treatment on the surface of the Atomizer flow rate Ar:1.0 L.min -1 The method comprises the steps of carrying out a first treatment on the surface of the Scanning mode: peak jumping; sampling taper hole diameter 1.0 mm; intercepting a taper hole with a diameter of 0.45 and mm; sampling depth 10.0 mm; sample injection pump speed 0.2 rps; pre-flush time 25 s, single element integration time 0.1 s;
the atomic fluorescence photometer working conditions: negative high pressure 300V, as hollow cathode lamp current 50 mA, atomizer height 8 mm, ar carrier gas flow rate 300 mL/min, shielding gas 900 mL/min, measuring mode: standard curve method, reading mode: peak area, reading time 7s, delay time 1.5s, and number of repetitions of measurement 3.
The invention discloses a preparation method of a puer raw tea matrix standard substance containing heavy metals, which takes puer raw tea as a matrix, and the standard substance obtained by the preparation method can ensure the accuracy, comparability and traceability of the measurement results of related heavy metals, and has great social benefit and economic benefit for food safety detection.
In order to overcome the defect of interference of matrix effect in the prior art, the second aim of the invention is to provide the puer raw tea matrix standard substance containing heavy metal, which is obtained by the preparation method.
The puer raw tea matrix standard substance containing heavy metals, which is prepared by the invention, has good uniformity and stability, greatly reserves the background characteristics of matrix samples, reduces errors caused by matrix effect interference, can be used for analysis method evaluation, quality control and laboratory capability assessment of the analysis process of heavy metal content detection in puer raw tea, can be used for calibrating metering devices and evaluation and measurement methods, is simple to operate and is easy to popularize.
Compared with the prior art, the standard substance provided by the invention is prepared by taking the puer tea raw material of a puer tea main production area of Yunnan puer tea as a matrix through the steps of carefully selecting, drying, crushing, grinding, uniformly mixing, irradiating and the like, and the puer raw tea powder with good uniformity is prepared and obtained by adopting microwave digestion, an inductively coupled plasma mass spectrometry and an atomic fluorescence photometry to cooperate with constant values in a plurality of laboratories, so that the constant value, uniformity inspection and stability monitoring of the standard substance are carried out. The Pu' er raw tea powder containing heavy metals, prepared by the method, has good uniformity and stability, and can be used in the fields of detection analysis and method evaluation of heavy metal content in tea in a food detection laboratory, quality control of analysis process, laboratory capability assessment and the like. The biomass material which plays a role of a scale in the process of detecting the heavy metal content in the puer tea is converted from the puer tea biological resource, has good uniformity and stability, and realizes the matching degree of standard substances and a sample matrix in the puer raw tea heavy metal content detection, so that the accuracy and the reliability of a detection result are improved, and the standard substances can be sold as laboratory quality control samples, and can create considerable economic benefits.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required to be used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only embodiments of the present invention, and that other drawings can be obtained according to the provided drawings without inventive effort for a person skilled in the art.
FIG. 1 is a flow chart of the preparation of the standard substance of the puer raw tea matrix containing heavy metals.
Detailed Description
The following will clearly and completely describe the technical solutions in the embodiments of the present invention, and it is obvious that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The word "embodiment" as used herein does not necessarily mean that any embodiment described as "exemplary" is preferred or advantageous over other embodiments. Performance index testing in the examples herein, unless otherwise indicated, was performed using conventional testing methods in the art. It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure.
Unless otherwise defined, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; other test methods and techniques not specifically identified herein are those commonly employed by those of ordinary skill in the art.
Numerous specific details are set forth in the following examples in order to provide a better understanding of the present application. It will be understood by those skilled in the art that the present application may be practiced without some of these specific details. In the examples, some methods, means, instruments, devices, etc. well known to those skilled in the art are not described in detail in order to highlight the gist of the present application.
On the premise of no conflict, the technical features disclosed in the embodiments of the present application may be combined arbitrarily, and the obtained technical solution belongs to the disclosure of the embodiments of the present application.
The invention provides a preparation method of a puer raw tea matrix standard substance containing heavy metals, wherein the matrix is puer tea raw materials in a puer tea main production area of Yunnan puer tea, and a proper preparation method is adopted to complete standard substance research and development work.
The invention provides a puer raw tea matrix standard substance containing heavy metals and a preparation method thereof, and aims to overcome the defect of matrix effect interference in the prior art, and provides the puer raw tea matrix standard substance containing heavy metals, which has good uniformity and stability, and can realize matrix matching degree of standard substances and samples in puer raw tea heavy metal content detection, thereby improving accuracy and reliability of detection results.
The present invention will be further specifically illustrated by the following examples, which are not to be construed as limiting the invention, but rather as falling within the scope of the present invention, for some non-essential modifications and adaptations of the invention that are apparent to those skilled in the art based on the foregoing disclosure.
Example 1A Pu' er raw tea base stock containing heavy metals and its preparation method
1. Preparation
In this embodiment, raw materials, that is, raw puer tea samples containing heavy metals, need to be obtained first.
The source is as follows: pu 'er raw tea planted in Pu' er village of Yunnan province is Gu Xianniu, all the raw tea samples are required to be loose tea, sun-dried green raw tea, strong and tight strip rope, obvious milli, even and orderly surface and grey green.
1. Pretreatment of puer raw tea samples:
(1) The collected puer raw tea samples are numbered and registered in a book.
(2) Dust is raised on Pu' er raw tea and loose tea leaves by a dustpan, light drift matters such as dust, fragments and the like in the tea leaves are removed, and then the tea leaves are spread and dried on a bamboo sieve, so that non-tea substances are removed manually.
2. Preparation of Pu' er raw tea sample
(1) Baking: and (3) placing the puer raw tea sample into a low-temperature oven at 35-45 ℃ for baking for 24 hours.
(2) Grinding: crushing the dried Pu' er raw tea and loose tea leaves by a crusher.
(3) And (3) screening: the crushed loose tea leaves are poured into a 40-mesh nylon sieve to screen samples.
(4) Grinding: and (3) putting all the tea samples passing through the 40-mesh sieve into a corundum ball milling tank of a planetary ball mill for repeated grinding, and enabling all the tea samples to pass through a 80-100-mesh nylon sieve to obtain tea powder samples.
(5) Mixing evenly: and uniformly mixing all tea powder samples passing through a 80-100-mesh sieve on a laboratory powder mixer for 6-8 hours.
(6) And (3) primary uniformity inspection: and (5) carrying out primary uniformity inspection on the tea powder sample after uniform mixing.
(7) And (3) irradiation: and (3) carrying out irradiation sterilization on the sample by gamma rays generated by radioactive isotope cobalt-60 after sub-packaging, wherein the irradiation dose is 6kGy.
(8) And (5) subpackaging: the sample passing through the uniformity primary inspection is firstly packaged by a medical packaging bottle (30 mL) with an inner cover HDPE material, then is packaged by a food-grade PE double-sided net-shaped vacuum bag, and is sequentially cleaned by a detergent ultrasonic cleaning-tap water-distilled water-ultrapure water cleaning, and is used after being dried, wherein a packaging unit is as follows: each bottle contains 20g and 200 bottles are split.
(9) And (3) preserving: vacuum packaging the sample, and storing in a drier at normal temperature.
And (3) carrying out uniformity inspection and stability monitoring on the puer raw tea sample prepared by the method.
2. Uniformity inspection and stability monitoring
Uniformity inspection
Uniformity is a fundamental property of a standard substance that characterizes the spatial distribution of the characteristics of the standard substance. Uniformity tests must be performed during development of the standard substance to demonstrate good uniformity.
1. Detection method
(1) Pretreatment conditions
About 0.25g of the sample is weighed (accurately weighed to 0.0001 g), placed in a microwave digestion tank, and digested by adding 5mL of nitric acid and 2mL of hydrogen peroxide. Digestion conditions are shown in table 1. After complete digestion, taking out the digestion inner tube from the digestion tank, cooling the digestion liquid to room temperature, transferring the digestion liquid into a 50mL volumetric flask for constant volume, flushing the digestion tube wall and the sealing cover with pure water for 2-3 times, collecting the solution together into the digestion tube, and transferring the solution into the volumetric flask for constant volume to obtain a clear and transparent solution.
Table 1 microwave digestion condition program and parameter setting
(2) Detection test
The above-mentioned microwave digested sample was subjected to measurement of the Cr element content in the sample by inductively coupled plasma mass spectrometry (ICP-MS) (standard curve method). Referring to GB5009.268 for multi-element determination in food, an inductively coupled plasma mass spectrometer is adopted to determine Cr element, an ICP-MS (inductively coupled plasma mass spectrometer) adopts tuning liquid to reach an optimal working condition, bi, rh and Ge are adopted as internal standard substances to respectively measure blank samples, standard curves and instrument response values of puer raw tea samples to be determined, and after the test is finished, the instrument automatically deducts the blank values of the samples and automatically calculates the results.
ICP-MS standard curve: the standard solution of Cr element having a mass concentration of 1000. Mu.g/mL was diluted with 5% nitric acid by stepwise dilution, and the mass concentration of the standard solution is shown in Table 2.
TABLE 2 ICP-MS measurement standard curve
The mass concentration of Cr element has good linear relation with the corresponding signal value, and the linear regression correlation coefficient is 0.9995. The average and standard deviations were calculated by repeating the measurement using 5% nitric acid as a reagent blank, and the ratio of the standard deviation to the slope of the calibration curve was 3 times as the detection limit, and the linear relationship and detection limit of the Cr element are shown in table 3.
Table 3 shows the linear relationship and detection limit of Cr elements
(3) Method validation
To further verify the accuracy of the method, the method was performed using GBW10052a green tea, a primary standard substance of the existing homogeneous matrix, as a quality control sample, the quality control sample was processed using an optimized sample processing method and instrument conditions, and the Cr element in the quality control sample was measured using ICP-MS, and the results of the determination were verified to fall within the uncertainty range of the standard substance nominal value as shown in table 4 below.
TABLE 4 verification results of ICP-MS measurement of quality control samples
2. Step (a)
According to annex A in JJF1343-2022 standard substance constant value, uniformity and stability evaluation, randomly extracting 13 bottles of puer raw tea heavy metal component standard substance samples from 200 packaging units which are already packaged and numbered respectively, adopting single-sequence measurement, weighing about 0.25g of each bottle of the samples respectively, adopting microwave digestion pretreatment, measuring Cr element content by an inductively coupled plasma mass spectrometer (ICP-MS), measuring 3 times in parallel, taking an average value of 3 times of measurement, using dry basis for the test data (the average water content of the samples is 8.05%) as a measurement result of uniformity test, and then carrying out variance analysis on 13 groups of parallel measurement data.
3. Data analysis
Sample uniformity was examined using analysis of variance at 95% confidence, and test data and statistical analysis results are shown in table 5.
TABLE 5 test data and results of Cr element uniformity test of Pu' er raw tea heavy metal component analysis standard substance
From the experimental data and statistical analysis results, it can be seen that: the statistic F value of the Cr element uniformity test measurement result of the puer raw tea heavy metal component analysis standard substance is smaller than uniformityThreshold value of sexual test F α(12,26) :2.148, the standard substance proved to be homogeneous.
Calculating the total uncertainty of the standard should include the uncertainty introduced by the non-uniformity. Uncertainty introduced by the inhomogeneity is estimated by using standard deviation of inhomogeneity among samples, and a calculation formula is as follows:
wherein:-inter-group variance; />-intra-group variance; n-number of intra-group measurements.
If it is>/>The uncertainty component introduced by uniformity is: u (u) Uniformity of =/>。
If it is</>The measurement method is limited by sensitivity, so the standard deviation of uniformity is calculated as follows:
wherein, in the formula-freedom of variance within the group.
The standard deviation from the non-uniformity was introduced into the final uncertainty of the fixed value, and the non-uniformity introduced uncertainty of the samples is shown in table 6.
TABLE 6 uncertainty in uniformity of Cr element in Pu' er raw tea heavy metal component analysis Standard substance
(II) stability monitoring
The stability of a standard substance is defined as the time-dependent change of a characteristic amount of a given value, and is affected by factors such as physical, chemical, storage conditions, and the like. Stability is a fundamental property of a standard substance that describes the time-varying nature of the characteristic magnitude of the standard substance, and stability assessment not only enables assessment of measurement uncertainty associated with the stability of the standard substance, but also enables definition of appropriate storage and transport conditions for the standard substance.
1. Method of
The stability monitoring adopts a characteristic value time-varying curve to judge whether the characteristic value has a unidirectional variation trend, and adopts a straight line as an empirical model. In the test data, the time is taken as a horizontal axis, the content of heavy metal elements in the sample is taken as a vertical axis, a straight line is fitted, and the stability of the sample is judged by evaluating the final regression variance.
The stability assessment base model can be expressed as:
beta in 0 ，β 1 -regression coefficients; x-time; y-the characteristic value of the standard substance candidate;
β 1 standard deviation S (. Beta.) of (2) 1 ) The calculation formula is as follows:
beta-based 1 Standard deviation of (1)The method adopts F-test to judge the significance. If sig.F in regression analysis<0.05, the linear relationship of the regression equation is significant, whereas it is not.
2. Data analysis
According to the technical specifications of standard substances and the principle of front tightening and rear thinning, a classical stability evaluation scheme is adopted to evaluate the long-term stability of the puer raw tea heavy metal component analysis standard substances. The samples were tested using microwave digestion, inductively coupled plasma mass spectrometry, standard curve analysis rules (internal standard method) at equal intervals of 0 month, 0.5 month, 1 month, 2 months, 3 months, 6 months up to 36 months (between 7 months in 2019 and 6 months in 2022), the test data were expressed on a dry basis (the water content of the samples was averaged according to 8.07%), and the puer raw tea heavy metal component analysis standard substance Cr element was monitored for long-term stability, and the results are shown in table 7.
TABLE 7 Long-term stability test data of Cr element as a standard substance for heavy metal component analysis of Pu' er raw tea
The results of the analysis of variance are shown in Table 8.
TABLE 8 regression analysis from Cr element long-term stability data of the standard substance of heavy metal component analysis of Pu' er raw tea of TABLE 7
The analysis result shows that the regression of the heavy metal component analysis standard substance Cr element in the raw puerh tea is not obvious (sig.F > 0.05) within 36 months, and the quantity value of the heavy metal component analysis standard substance Cr element does not have obvious trend change, so that the heavy metal component analysis standard substance Cr element is stable.
Adopting an analysis stability method, and adopting a univariate linear model to regress the standard deviation S corresponding to the analysis of variance b To estimate the standard uncertainty introduced by long-term stability, the uncertainty contribution of long-term stability for the period t=36 months is:
the uncertainty introduced by the long term stability of the samples is shown in table 9.
TABLE 9 uncertainty of long-term production of Cr element as a standard substance for analyzing heavy metal components of Pu' er raw tea
3. Constant value
1. Method of
The method adopts a plurality of methods of a plurality of laboratories to cooperatively determine the value, the fixed value adopts a star form to compare the route, and the fixed value method is ICP-MS and AFS. In the process of value determination, each laboratory randomly extracts 3 packages from all samples, randomly takes out 1-3 subsamples from each bottle, and adopts an internal standard method to determine the content of heavy metal elements in the samples.
2. Statistical analysis of data
After the data of each collaborative fixed-value laboratory are recovered, firstly, removing suspicious data among groups, and removing laboratory data with standard deviation exceeding 10% according to the prior convention; secondly, checking whether the data accords with normal distribution; performing intra-group suspicious value inspection by using a Dixon (Dixon) method again to determine reserved data; again, the glaubes (Grubbs) method is adopted to reject suspicious data among groups; and finally, performing equal-precision test by adopting a Cochran method to determine fixed value data, and finally adopting an arithmetic average value of the determined fixed value data as a fixed value result. Chromium (Cr) is used as an example, as shown in table 10.
Table 10 quantitative results of analysis of heavy metal content of Pu' er raw tea
3. Constant value
Referring to GB 5009.3 "determination of moisture in food", the moisture content in a standard substance is determined by a direct drying method (absolute measurement method), about 5g (accurate to 0.0001 g) of a puer raw tea heavy metal component analysis standard substance sample is weighed, placed in a glass weighing bottle with constant weight in advance, dried in an electrothermal constant temperature drying oven at 105 ℃ for 3.5 hours, cooled for 0.5 hour and then weighed, and the measurement result of the moisture content in the puer raw tea heavy metal component analysis standard substance is calculated as shown in Table 11.
Table 11 measurement results of heavy metal component analysis standard substance moisture of Pu' er raw tea (n=6)
The final content X (mg/kg) =constant concentration/(1-moisture) of Pu' er raw tea gave a Cr element constant of 1.07 mg/kg (dry basis).
4. Uncertainty assessment
The total uncertainty of the standard value of the standard substance consists of three parts. Uncertainty introduced during uniformity testing, stability monitoring, and thresholding, respectively, where existing thresholding data introduces class a standard uncertainty (u a ) There is also a balance weighing, a class B uncertainty (u b ) The total uncertainty of the post-synthesis needs to be evaluated separately.
1. Uncertainty resulting from ten laboratory collaboration values: the estimation is performed using an analysis of variance, 0.0482mg/kg。
2. uncertainty component introduced by water content of sample: 0.0206mg/kg
3. uncertainty passing method for incomplete introduction of sample digestionStatistical analysis and estimation of recovery rate or standard-added recovery rate data, namely, the uncertainty of the relative standard introduced by the recovery rate is as follows:
4. uncertainty u introduced in standard solution formulation process Standard solution =0.0019mg/kg
5. Uncertainty component u introduced by calibration of standard curve Calibration of =0.0050mg/kg
6. Uncertainty of sample inhomogeneity introduction, as described previously, u Uniformity of =0.0228mg/kg
7. Uncertainty introduced by sample stability, as previously described, u Stability of = 0.0504mg/kg, all of which need to be accounted for in the total uncertainty.
8. Total uncertainty of synthesis
The total uncertainty of the synthesis of the Cr element in the heavy metal component analysis standard substance in the puer raw tea is calculated as shown in the following formula, and the calculated data are shown in Table 12.
TABLE 12 total uncertainty of Cr element as a standard substance for analysis of heavy metal component in Pu' er raw tea
The quantitative result x=1.07±0.16 (mg/kg) (p=95%) of the Cr element of the standard substance for analyzing heavy metal component in the raw puerh tea.
In conclusion, the puer raw tea heavy metal component analysis standard substance developed by the method is subjected to uniformity inspection and stability monitoring, and a plurality of laboratories carry out value determination by adopting more than two different detection principles, so that the puer raw tea heavy metal component analysis standard substance can be applied to measurement such as instrument calibration and quality control, and the use result is satisfactory. The successful development of the standard substance ensures the accuracy, comparability and traceability of the measurement result of the related heavy metal residues, and has extremely important social and economic benefits for food safety detection.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (8)
1. The preparation method of the puer raw tea matrix standard substance containing heavy metals is characterized by comprising the following steps:
(1) Numbering: numbering the collected representative puer raw tea loose tea samples, and registering in a book;
(2) Pretreatment: dust is raised on Pu' er raw tea and loose tea leaves by a dustpan, light drift matters in the tea leaves are removed, then the tea leaves are spread and dried on a bamboo sieve, and non-tea substances are removed manually;
(3) Baking: the pretreated Pu' er raw tea loose tea leaves are placed in a drying oven to be baked for 24 hours at low temperature;
(4) Crushing: crushing dried puer raw tea powder tea leaves by a crusher;
(5) And (3) screening: pouring crushed loose tea leaves into a 40-mesh nylon sieve, and sieving a sample by ultrasonic pulse;
(6) Grinding: putting all the tea screening samples passing through the 40-mesh sieve into a corundum ball milling tank of a planetary ball mill for repeated grinding, and enabling the tea screening samples to pass through a 80-100-mesh nylon sieve to form tea powder samples;
(7) Mixing evenly: uniformly mixing a tea powder sample passing through a nylon sieve with 80-100 meshes on a powder mixing machine;
(8) And (3) primary uniformity inspection: carrying out primary uniformity detection on the tea powder sample after uniform mixing;
(9) And (3) irradiation: performing irradiation sterilization on the sample through the sample subjected to the uniformity primary detection;
(10) And (5) subpackaging: packaging the irradiated sample with medical packaging bottles made of HDPE materials with inner covers, and packaging with food-grade PE double-sided net-shaped vacuum bags, wherein the packaging bottles are sequentially subjected to ultrasonic cleaning with a detergent, namely tap water, distilled water and ultrapure water, and air-dried for use;
(11) And (3) preserving: vacuum packaging the sample, and storing in a drier at normal temperature;
(12) Uniformity and stability monitoring: carrying out uniformity inspection and stability monitoring on the puer raw tea sample containing heavy metal obtained in the preparation process by adopting a microwave digestion method, an inductively coupled plasma mass spectrometry (ICP-MS) method and an Atomic Fluorescence Spectrometry (AFS) method;
(13) And (5) constant value and uncertainty assessment: and carrying out microwave digestion, carrying out ICP-MS (inductively coupled plasma mass spectrometry) and AFS (atomic fluorescence spectrometry) cooperative constant value in a plurality of laboratories, and carrying out constant value on the content of heavy metal elements in the prepared puer raw tea sample in the plurality of laboratories to determine a nominal value and an uncertainty range.
2. The method for preparing the puer raw tea matrix standard substance containing heavy metals according to claim 1, wherein the heavy metals comprise one or more of cadmium, chromium, arsenic, lead and copper.
3. The method for preparing the raw puerh tea matrix standard substance containing heavy metals according to claim 1, wherein the low-temperature baking temperature in the step (3) is 35-45 ℃.
4. The method for preparing the raw puerh tea matrix standard substance containing heavy metals according to claim 1, wherein the mixing time in the step (7) is 6-8 hours.
5. The method for preparing the standard substance of the puer raw tea matrix containing heavy metals according to claim 1, wherein the irradiation in the step (9) adopts gamma rays generated by using radioactive isotope cobalt-60, and the irradiation dose is 6kGy.
6. The method for preparing the raw puer tea matrix standard substance containing heavy metals according to claim 1, wherein the split charging of the step (10) is 20g per bottle, and 200 bottles are split charging.
7. The method for preparing the standard substance of the puer raw tea matrix containing heavy metals according to claim 1, wherein the method for preparing the standard substance comprises the steps of carrying out fixed value, uniformity inspection and stability monitoring on the standard substance by adopting microwave digestion, inductively coupled plasma mass spectrometry and atomic fluorescence spectrometry in a plurality of laboratories in a cooperative fixed value mode, and the detection method comprises the following steps:
1) Reagents and samples: nitric acid: the quality is higher than that of the high-grade pure; hydrogen peroxide: high-grade purity; water: ultrapure water; elemental standard solutions; tuning solution: li, co, Y, ce, tl; blank liquid: 5% nitric acid; internal standard solution: in, ru, rh, bi;
2) The method for setting the value comprises the following steps: preparing a standard curve by adopting a single-element primary standard substance, calculating and tracing the content of an element to be detected in a sample, simultaneously measuring heavy metal elements in a quality control sample and a puer raw tea sample, and directly comparing the measured mode fixed values;
3) The method comprises the steps of pre-measuring the moisture content of a puer raw tea sample, performing pretreatment by adopting microwave digestion, and measuring the heavy metal element content in puer raw tea by adopting an inductively coupled plasma mass spectrometry (AFS) method; wherein,
the inductively coupled plasma mass spectrometer has the working conditions that: RF power/kW: 1.55; RF matching voltage/V: 1.8; plasma gas flow rate Ar:15.0 L.min -1 The method comprises the steps of carrying out a first treatment on the surface of the Auxiliary air flow rate Ar:1.0 L.min -1 The method comprises the steps of carrying out a first treatment on the surface of the Atomizer flow rate Ar:1.0 L.min -1 The method comprises the steps of carrying out a first treatment on the surface of the Scanning mode: peak jumping; sampling taper hole diameter 1.0 mm; intercepting a taper hole with a diameter of 0.45 and mm; sampling depth 10.0 mm; sample injection pump speed 0.2 rps; pre-rinse time 25 s, single unitPixel integration time 0.1 s;
the atomic fluorescence photometer working conditions: negative high pressure 300V, as hollow cathode lamp current 50 mA, atomizer height 8 mm, ar carrier gas flow rate 300 mL/min, shielding gas 900 mL/min, measuring mode: standard curve method, reading mode: peak area, reading time 7s, delay time 1.5s, and number of repetitions of measurement 3.
8. A raw puerh tea base standard substance containing heavy metals prepared by the preparation method of any one of claims 1-7.
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