CN117990462A - Preparation method of coffee matrix standard substance containing heavy metals - Google Patents
Preparation method of coffee matrix standard substance containing heavy metals Download PDFInfo
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Abstract
The invention discloses a preparation method of a coffee matrix standard substance containing heavy metals, and belongs to the technical field of standard substance research and development in the field of metering science. The method comprises the following specific steps: the method comprises the steps of (1) baking, (2) crushing and grinding, (3) uniformly mixing, (4) mixing a substrate, (5) adding, (6) freezing, (7) vacuum drying, (8) secondarily crushing, (9) secondarily uniformly mixing, (10) primary uniformity inspection, (11) irradiation, (12) sub-packaging and (13) preserving. Through uniformity inspection and stability monitoring, and collaborative value setting in a plurality of laboratories, the standard substance is finally determined to meet the design of the expected plan, the background characteristics of the matrix sample are reserved to a great extent, the consistency of the standard solution matrix and the matrix of the sample to be detected can be ensured, the method can be applied to the works of quality control, analysis instrument calibration, detection method confirmation, laboratory capability assessment and the like in the coffee heavy metal content detection process, and the method has important significance for the coffee product quality detection work.
Description
Technical Field
The invention relates to the technical field of standard substance research and development in the field of metering science, in particular to a preparation method of a coffee matrix standard substance containing heavy metals.
Background
Coffee is a beverage crop with high economic value, and is called as three major beverages in the world together with cocoa and tea, and the yield, the production value and the consumption amount of the three major beverages are the first. Because coffee contains rich substances such as protein, fat, carbohydrate, caffeine and the like, the coffee has strong aroma and delicious taste, and is favored by consumers. The provinces of Chinese coffee planting are Yunnan, hainan, guangdong, guangxi and Sichuan, wherein Yunnan is the largest coffee planting and export base in China, and the planting area and the yield account for 99% of the whole country. Coffee is used as an economic crop, the possibility of enriching heavy metals in soil and the environment exists in the planting growth and production and processing processes, and the harmful elements can enter the coffee beverage in a soaking and leaching mode and be drunk by people, and the heavy metals are accumulated in the human body to cause serious harm to the human body.
For the above reasons, in recent years, researchers have conducted researches on the quality of coffee, for example, gas chromatography-mass spectrometry (GC-MS) for detecting 49 kinds of pesticide residues in coffee (Jiangsu agricultural science, 2013,41 (7): 287-289, qian Zongyao, etc.) for measuring 49 kinds of pesticide residues in coffee; heavy metal content of coffee beans was studied in analysis of heavy metal elements in small-grain coffee beans of Yunnan province (food industry, 2022,43 (10): 260-264, li Xiulin, etc.). In the coffee heavy metal content detection process, a metal single-element solution standard substance is generally adopted for confirmation, but the detection cannot control the interference introduced by the matrix effect in the detection process, namely, quality control and capability evaluation are carried out by using the traditional pure single-element solution standard substance, so that the detection requirement is difficult to meet.
Therefore, how to prepare a coffee base standard substance containing heavy metals is a problem to be solved by the person skilled in the art.
Disclosure of Invention
Aiming at the defect of interference of matrix effect in the prior art, the invention provides a preparation method of a coffee matrix standard substance containing heavy metals.
In order to achieve the above purpose, the present invention provides a method for preparing a coffee base standard substance containing heavy metals, which comprises the following specific preparation steps:
(1) Baking: continuously drying coffee beans without heavy metal elements in a drying oven at 45-50 ℃ for more than 120 hours;
(2) Crushing and grinding: repeatedly pulverizing, grinding, and sieving with 80 mesh sieve to obtain coffee powder;
(3) Mixing evenly: mixing coffee powder for 6-8 hr, and mixing completely;
(4) Mixing the substrates: pouring the heavy metal element solution standard substance into pure water according to the target concentration of the standard substance, uniformly stirring, slowly adding the coffee powder into the mixed solution, and fully stirring to obtain a coffee powder mixed suspension containing heavy metal elements;
(5) And (3) adding: repeatedly stirring and refrigerating the mixed suspension until the mixed suspension is not layered after standing;
(6) Freezing: the mixed suspension is placed in a material tray and quickly frozen;
(7) Vacuum drying: vacuum drying the frozen sample until the temperature of the sample gradually returns to room temperature;
(8) Secondary crushing: pulverizing the sample again, and sieving with 80 mesh sieve;
(9) And (3) secondary mixing: uniformly mixing for more than 6 hours at the rotating speed of 20-30 rpm;
(10) And (3) primary uniformity inspection: performing uniformity primary detection on the uniformly mixed coffee powder sample;
(11) And (3) irradiation: performing irradiation sterilization on the coffee powder sample passing the uniformity primary detection;
(12) And (5) subpackaging: packaging with medicine packaging bottles which are cleaned in advance and dried in the air and have no dissolved substances, wherein each bottle is 8g;
(13) And (3) preserving: and (3) packaging the sample by adopting a film, and then placing the packaged sample in a dryer at normal temperature for preservation to obtain the coffee matrix standard substance containing heavy metals.
Further, crushing in the step (2), wherein the rotating speed is 25000rpm, the power is 1.2kW, the crushing bin cover is opened for 20-30 seconds, cooling is carried out for 8-10 minutes, and the temperature of the crusher bin is lower than 40 ℃ and the crushing is continued; repeating for 8-10 times.
Further, in the step (2), the grinding is carried out at the rotating speed of 200-300rpm and the power of 0.75kW, and every 30 minutes, the tank body is opened to observe the grinding condition of the raw coffee beans, so that the hardening of powder materials is avoided; repeating for 4 times.
Further, the target concentration in the step (4) is 0.8-1.0mg/kg of heavy metal element content; the weight ratio of the coffee powder to the pure water is 1:1.5-3.0.
Further, stirring in the step (5), wherein the rotating speed is 180-200rpm, the power is 0.75kW, and the time is 4-6 hours; the cold storage is carried out for 13 to 15 hours at the temperature of 3 ℃; and repeatedly stirring and refrigerating for at least 48 hours.
Further, the materials contained in the separating discs in the step (6) are 150-160g; the temperature of the frozen cold trap is-54 ℃ and the time is 18-24 hours, and when the temperature of the material is stabilized between- (50-53) ℃ for 3 hours continuously, the freezing is completed.
Preferably, the beneficial effects of adopting above-mentioned technical scheme are: the coffee matrix sample prepared by adopting the freeze drying technology can effectively maintain the characteristics of raw coffee beans because the content of volatile aromatic substances and grease in the coffee is high, and the caramelization reaction and the pyrolysis reaction of the coffee beans caused by the temperature change are avoided, so that the raw coffee beans are changed into cooked beans; the characteristics of the matrix can be reserved to a great extent after the freeze-dried raw beans are digested, the shelf life of the beans can be well controlled, the content of the contained water is easy to control, the uniformity and the stability are good, and the method is a basis for ensuring the accuracy and reliability of the heavy metal analysis test data in the coffee.
Further, the vacuum drying in the step (7) is carried out, the vacuum degree is 10.0-14.9Pa, the temperature is 50-53 ℃, and the continuous drying is carried out for more than 72 hours.
Further, the secondary crushing in the step (8) is carried out, the rotating speed is 2000rpm, the crushing is carried out for 10-30 seconds, the bin cover is opened, the cooling is carried out for 5-8 minutes, and the 80-mesh sieve is used for sieving the sample after the secondary crushing; grinding coffee bean powder which cannot pass through a 80-mesh sieve, wherein the grinding rotating speed is 200-300rpm, the power is 0.75kW, and the tank body is opened every 30 minutes to observe the grinding condition of the raw coffee beans, so that the hardening of the powder is avoided; repeating for several times to obtain coffee powder with 80 mesh sieve.
Compared with the prior art, the invention discloses a preparation method of a coffee matrix standard substance containing heavy metals, which has the following beneficial effects:
(1) The process is easy to produce and control, the prepared coffee matrix standard substance containing heavy metals accords with the expected planning design, the background characteristics of matrix samples are reserved to a great extent, the consistency of the matrix of the standard substance and the matrix of the sample to be tested can be ensured, the error caused by interference of matrix effects is reduced, the process can be applied to the works of quality control, calibration of analysis instruments, confirmation of detection methods, laboratory capability assessment and the like in the coffee heavy metal content detection process, and the process has important significance for the quality detection work of coffee products.
(2) The laboratory detection shows that the measured heavy metal content of the heavy metal-containing coffee matrix standard substance prepared by the invention is basically consistent with the pre-added heavy metal content, the sample uniformity and stability are good, the requirements of the laboratory standard substance are met, and no alternative scheme is available temporarily to achieve the effect.
(3) The invention establishes a preparation method of a heavy metal-containing coffee matrix standard substance through a freeze-drying technology and a sample exogenous addition combined technology.
According to the standard NY/T604-2006 of the original Ministry of agriculture (raw coffee) and the sanitary index requirement of the raw coffee, the content of lead (calculated by Pb) is definitely required to be less than or equal to 0.5mg/kg, after the standard is revised, the limit requirement of pollutants in food is in accordance with the relevant regulations of the pollutant limit in food safety national standard food of GB2762, the pollutants which are important to pay attention to are definitely required by different food types, wherein the maximum allowable content level of lead (calculated by Pb) in the raw coffee beans is definitely 0.5mg/kg.
According to the geographic marking product standard DB 53/T371-2012 of Yunnan province, namely the geographic marking product Baoshan small particle coffee, the special safety index of the Baoshan small particle coffee is made clear to pay attention to heavy metal elements of lead and arsenic, the content of lead (calculated by Pb) is required to be less than or equal to 0.5mg/kg, and the lead content in coffee beans is regulated in the national and local standards related to heavy metal content detection in coffee, so that the application provides a preparation method of a lead-containing coffee matrix standard substance, which is used for quality control of inspection and detection work of coffee products, and can be said to have very strong pertinence; meanwhile, the application provides a freeze-drying preparation method of the lead-containing coffee matrix standard substance, which is mainly used for preparing other heavy metal elements by carrying out related tests on lead elements and obtaining satisfactory test results, and can also be used for preparing other heavy metal elements and provides good technical reference for preparing standard substances of other heavy metal elements in a coffee matrix.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely, and it is apparent 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.
Research on a method for detecting heavy metal content in food has become a focus of attention of the whole society, and currently in the detection process, a detection laboratory generally adopts a standard substance of a single element solution of detected heavy metal (cadmium, chromium, arsenic, lead, mercury, copper and the like) to control quality, or adopts a standard substance of other matrix heavy metals (pork liver powder, total chromium in water) as a metering standard to trace or calibrate.
The base body effect of the sample is ubiquitous, so that the accuracy and the reliability of the detection data result of heavy metals contained in many agricultural and sideline products are doubtful at present. When the heavy metal content in the coffee sample is detected by using an instrumental analysis method, the elimination of matrix effect interference generated by inconsistent matrixes of the standard substance and the sample to be detected is considered, and the matrix effect interference can cause errors in the content of the element to be detected in the sample to be detected. Particularly, the grease and protein-rich sample like coffee beans has the phenomenon of false positive and inaccurate quantification due to the interference of a matrix effect on an analysis instrument, and the accuracy of a detection result cannot be ensured. Therefore, when the element to be detected of the sample is detected, the standard curve matrix or the matrix of the quality control sample is required to be ensured to be basically consistent with the matrix of the sample to be detected as far as possible, so that errors caused by interference of matrix effects are reduced.
The invention aims to overcome the defect of interference of matrix effect in the prior art, prepare the coffee bean matrix standard substance (or reference substance) which takes Yunnan Arabica coffee beans as a matrix and has known concentration content of the heavy metal elements, meets the requirements of quality inspection and detection work of products in the Yunnan coffee industry, has an important role in evaluating the quality of coffee, and can be used for analyzing and detecting heavy metal components in coffee in the fields of food, scientific research and the like, controlling the quality of production departments and detection laboratories, calibrating analytical instruments, evaluating analytical methods and the like.
The matrix standard substance is a standard substance with the characteristics of an actual sample, and is combined with a target compound by a target matrix to be closer to the actual sample, so that the influence of matrix effect on the analysis of the substance components in the detection process can be effectively avoided, and the magnitude detection is closer to a true value. At present, a plurality of metering mechanisms develop and produce different types of matrix standard substances, and the development of the matrix standard substances of heavy metal components in coffee is blank in China. Based on the method, a coffee heavy metal component matrix standard substance is developed, and the method aims to provide technical support for heavy metal content measurement, measurement result magnitude tracing and the like in coffee.
Examples
The technical method is described by taking a method for preparing a lead-containing coffee matrix standard substance by freeze-drying as a specific example.
1. Pretreatment of
1) Planning: preparing a coffee matrix standard substance with Yunnan Ababi coffee beans as a matrix and lead element concentration content of (0.8-1.0) mg/kg, and examining the feasibility of preparing the coffee matrix standard substance containing lead heavy metals by a freeze-drying technology.
2) Preparing materials: 3.0-5.0 kg of Yunnan Abiraka coffee beans without heavy metal lead elements are prepared.
3) Material ratio test: mixing coffee powder with pure water in different mass ratios (1:1.5, 1:2.0 and 1:30), standing, observing whether a uniform mixed suspension is formed, and confirming that 1:2.0 is the optimal material ratio.
2. Preparation
(1) Baking: the prepared coffee beans are placed in a drying oven at 45 ℃ for at least continuous drying for more than 120 hours.
(2) Crushing and grinding: crushing the dried raw coffee beans by using a crusher, wherein the rotation speed of the crusher is 25000rpm, the power is 1.2kW, the crushing is performed for 20-30 seconds each time, then opening a crushing bin cover, placing and cooling for 8-10 minutes, and continuously crushing until the temperature of the crushing bin is lower than 40 ℃, and repeatedly crushing for 9 times; then placing the coffee powder into a corundum ball milling tank of a planetary ball mill, rotating at 250rpm and 0.75kW power, opening the tank body every 30min to observe grinding conditions of raw coffee beans, avoiding powder hardening, repeatedly grinding for 4 times, and taking the coffee powder which is sieved by a 80-mesh nylon sieve for later use.
(3) Mixing evenly: 2.5kg of powder passing through an 80-mesh nylon sieve is mixed in a mixer for 7 hours, and fully mixed for standby.
(4) Mixing the substrates: according to the mass concentration requirement of a planning target, preparing 500mg of lead (Pb) solution standard substance with the mass concentration of 1000 mug/mL, pouring the standard substance into 1000g of pure water, fully and uniformly stirring, slowly adding 500g of fully and uniformly mixed coffee powder dry powder into the mixed solution, and fully stirring and mixing to obtain the coffee powder mixed suspension containing metal elements.
(5) And (3) adding: and (3) placing the coffee powder mixed suspension into a planetary ball mill in a split manner, stirring for 5 hours in a rotary manner at the rotating speed of 200rpm and the power of 0.75kW, putting into a refrigerator at the temperature of 3 ℃ for refrigeration, taking out the coffee powder mixed suspension from the refrigerator after 14 hours, observing whether the solution has layering phenomenon, continuously putting into the planetary ball mill for continuous stirring for 5 hours, and repeating the steps for 4 times until the coffee powder metal mixed solution is not layered after standing.
(6) Freezing: and (3) placing the coffee powder metal mixed suspension in a material tray, controlling the weight of each tray of material to be 150-160g, rapidly placing the material tray in a freeze dryer with a cold trap temperature of-54 ℃ for cooling for 20 hours, and ensuring that the temperature of the material is stabilized between- (50-53) ℃ for 3 hours continuously, so that the freezing process can be considered to be completed.
(7) Vacuum drying: placing the above materials under vacuum degree of 10.0-14.9Pa and temperature lower than that of-
And (5) continuously drying in vacuum for more than 72 hours in a hollow bin of the freeze dryer at the temperature of (50-53) ℃ until the temperature of the sample gradually returns to the room temperature, determining that the drying process is finished, and taking the sample out of the freeze dryer.
Repeating the steps (3) to (7), and preparing 2 kg of coffee base standard substances containing lead heavy metals.
(8) Secondary crushing: placing the frozen and vacuum dried sample into a pulverizer again at 2000rpm for 10-30 seconds each time, then opening the pulverizer, cooling for 5-8 minutes, and screening the secondarily pulverized sample by using an 80-mesh nylon screen; when necessary, the coffee bean powder which cannot pass through the 80-mesh sieve is put into a corundum ball milling tank of a planetary ball mill, the rotating speed is 300rpm, the power is 0.75kW, every 30 minutes, the tank body is opened to observe the grinding condition of the raw coffee beans, and the hardening of the powder is avoided. Repeatedly crushing for 4 times to obtain coffee powder which is sieved by a 80-mesh nylon sieve for standby.
(9) And (3) secondary mixing: the twice crushed coffee powder samples are mixed for 7 hours in a three-dimensional mixer, fully and uniformly mixed, and the spindle rotating speed is 30rpm.
(10) And (3) primary uniformity inspection: carrying out primary uniformity detection on the coffee powder sample after secondary uniform mixing;
(11) And (3) irradiation: performing irradiation sterilization on the coffee powder sample passing the uniformity primary detection;
(12) And (5) subpackaging: packaging with medicine packaging bottles which are cleaned in advance and dried in the air and have no dissolved substances, wherein each bottle is 8g;
(13) And (3) preserving: and (3) packaging the sample by using a film, and then placing the packaged sample in a dryer at normal temperature for preservation to obtain the lead-containing coffee matrix standard substance.
And carrying out uniformity inspection and stability monitoring on the coffee powder sample prepared by the method.
3. 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 (accurately weighing to 0.0001 g) of a lead-containing coffee matrix standard substance is weighed, 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 25mL 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 digestion standard substance was measured for the content of lead (Pb) in the sample by inductively coupled plasma mass spectrometry (ICP-MS) (standard curve method). Referring to GB5009.268 "determination of multiple elements in food", an inductively coupled plasma mass spectrometer is used for determining lead (Pb), an ICP-MS (inductively coupled plasma mass spectrometer) adopts tuning liquid to an optimal working condition, bi is used as an internal standard substance, instrument response values of a blank sample, a standard curve and a coffee standard substance to be determined are respectively measured, and an instrument automatically calculates the result.
ICP-MS standard curve: the standard solution of lead (Pb) 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 2ICP-MS measurement standard curve
The mass concentration of the lead element (Pb) has good linear relation with the corresponding signal value, and the linear regression correlation coefficient is 0.9999.
(3) Method validation
To further verify the accuracy of the method, the method was verified using the existing first-order standard substance GBW10052a green tea as a quality control sample, the same pretreatment method was used for treatment, and ICP-MS was used to measure lead element (Pb) in the green tea sample, and the results of the measurement were verified to fall within the uncertainty range of the standard substance nominal value as shown in table 3 below.
TABLE 3 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 coffee matrix standard substances from 200 packaging units which are already packaged and numbered respectively, measuring by adopting a single sequence, weighing about 0.25g of each bottle respectively, carrying out microwave digestion pretreatment, measuring by using an inductively coupled plasma mass spectrometer (ICP-MS) to measure the content of lead element (Pb), measuring 3 times in parallel per bottle, taking the average value of the 3 times of measurement, expressing the test data by using a dry basis (the average water content of a sample is 4.15%) as a measurement result of uniformity test, and 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 4.
TABLE 4 lead (Pb) element uniformity test data and results in coffee matrix heavy metal content standards
Analysis of variance results:
from the above test data unit element analysis of variance:
the inter-group variance is:
the intra-group variance is:
Fα(ν1,ν2)＝2.148,α＝0.05
From the experimental data and statistical analysis results, it can be seen that: the statistic F value of the lead (Pb) uniformity test measurement results in the coffee matrix is smaller than the uniformity test critical value 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; /(I) -Intra-group variance; n-number of intra-group measurements.
If it isThe uncertainty component introduced by the uniformity is: u uniformity of ＝sbb.
If it isThe 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 5.
TABLE 5 uncertainty in the uniformity of lead (Pb) element in coffee matrix heavy metal content standards
(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:
Y＝β0+β1X
wherein β0, β1-regression coefficients; x-time; y-the characteristic value of the standard substance candidate;
the standard deviation S (β1) of β1 is calculated as follows:
Based on the standard deviation of beta 1, the method adopts F-test to judge the significance. If sig.F <0.05 in the regression analysis, the linear relationship of the regression equation is significant, whereas it is not.
2. Data analysis
According to the technical specifications of standard substances, a classical stability evaluation scheme is adopted to evaluate the long-term stability of the heavy metal component analysis standard substances in the coffee matrix according to the principle of front tightening and rear thinning. The samples were tested using microwave digestion, inductively coupled plasma mass spectrometry, standard curve analysis rules (internal standard method) at 0 month, 0.5 month, 1 month, 2 months, 3 months, 6 months time intervals (between 4 months 2023 and 10 months 2023), the test data were expressed on a dry basis (the water content of the samples was averaged according to 4.15%), and the long-term stability monitoring was performed on the lead element (Pb) of the heavy metal component analysis standard substance in the coffee matrix, and the results are shown in table 6.
TABLE 6 Long-term stability test data for the analysis of the heavy metal content of the Standard substance lead (Pb) in coffee matrices
The results of the analysis of variance are shown in Table 7.
TABLE 7 regression analysis results from long term stability data
The analysis result shows that the regression of the heavy metal component analysis standard substance lead element (Pb) in the coffee matrix is insignificant (sig.F > 0.05) within 6 months, and the magnitude thereof does not undergo obvious trend change, which indicates that the coffee matrix is stable.
The method for analyzing stability is adopted, standard uncertainty introduced by long-term stability is estimated by using standard deviation S b corresponding to single-variable linear model regression variance analysis, and the uncertainty contribution of long-term stability in the effective period t=6 months is as follows:
μ Stability of ＝Sb×t＝Sb×6
the uncertainty introduced by the long term stability of the samples is shown in table 8.
TABLE 8 Standard deviation s b and introduced uncertainty component u from regression analysis of long term stability data Stability of
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 standard substance 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 standard substance 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, eliminating suspicious data among groups by adopting a Grubbs method; the method comprises the steps of performing intra-group suspicious value detection by using a Dixon (Dixon) method, detecting whether data accords with normal distribution or not by using a Dagostenor method, performing equal-precision detection by using a Cochran method, and determining whether to reject a data group with significant differences after performing technical examination. After the data statistical analysis processes of suspicious value elimination, normal distribution inspection, equal precision inspection and the like are carried out, each data and the average value have no significant difference, and the total average value can be calculated to be the standard value. The calculation is as follows:
TABLE 9 determination of the lead (Pb) element values in coffee matrix heavy metal content standards
According to the above fixed value result, obtaining a preliminary fixed value result of the content of lead (Pb) element in the coffee matrix heavy metal content standard substance:
3. Constant value
Referring to GB5009.3 "determination of moisture in food", the moisture content in the standard substance is determined by direct drying method (absolute measurement method), about 5g (accurate to 0.0001 g) of sample of heavy metal component analysis standard substance in coffee matrix 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, weighed, and the measurement result of the moisture content in heavy metal component analysis standard substance in coffee matrix is calculated as shown in Table 10.
Table 10 results of determination of lead content analysis standard substance moisture in coffee (n=6)
The final content X (mg/kg) =constant concentration/(1-moisture) of lead element in the heavy metal component analysis standard substance in the coffee base, and the result of the constant value of lead element (Pb) was 1.082mg/kg (dry basis).
4. Uncertainty assessment
The total uncertainty of the standard value of the standard substance consists of three parts. The uncertainty introduced in the processes of uniformity inspection, stability monitoring and constant value respectively, wherein the uncertainty of class A standard introduced by constant value data (u a) and the uncertainty of class B introduced by balance weighing and pipette preparation (u b) are required to be respectively evaluated and then are combined into the total uncertainty.
1. Uncertainty resulting from ten laboratory collaboration values: the estimation is performed using an analysis of variance,
2. Uncertainty component introduced by water content of sample: u Moisture content =0.00508 mg/kg
3. Uncertainty introduced by incomplete digestion of the sample can be estimated by statistical analysis of the method recovery or the scaled recovery data, i.e., the relative standard uncertainty introduced by recovery is:
4. Uncertainty u standard solution preparation = 0.0033mg/kg introduced during standard solution formulation
5. Uncertainty component u Calibration of =0.0249 mg/kg introduced by standard curve calibration
6. Uncertainty of sample inhomogeneity introduction, as described previously, u uniformity of =0.0113 mg/kg
7. Uncertainty introduced by sample stability, as previously described, u Stability of = 0.02994mg/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 heavy metal component analysis standard substance lead element (Pb) in the coffee matrix is calculated as shown in the following formula, and the calculated data are shown in Table 11.
UX＝k×u Element(s)
TABLE 11 Total uncertainty of heavy metal component analysis Standard lead element (Pb) in coffee matrix
The quantitative result of the analysis of the heavy metal component in the coffee base with respect to the standard substance lead element (Pb) is x=1.082±0.104 (mg/kg) (p=95%).
In conclusion, the heavy metal component analysis standard substance in the coffee matrix developed by the method is subjected to uniformity inspection and stability monitoring, a plurality of laboratories carry out value determination by adopting more than two different detection principles, and the measured heavy metal content is proved to be basically consistent with the content of the heavy metal added by planning through testing. The heavy metal component analysis standard substance in the coffee matrix can be applied to measurement such as instrument calibration, quality control and the like, and the using 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 (10)
1. The preparation method of the coffee matrix standard substance containing heavy metals is characterized by comprising the following preparation steps:
(1) Baking: continuously drying coffee beans without heavy metal elements in a drying oven at 45-50 ℃ for more than 120 hours;
(2) Crushing and grinding: repeatedly pulverizing, grinding, and sieving with 80 mesh sieve to obtain coffee powder;
(3) Mixing evenly: mixing coffee powder for 6-8 hr, and mixing completely;
(4) Mixing the substrates: according to the target concentration of the standard substance, placing the heavy metal element solution standard substance into pure water, uniformly stirring, slowly adding the coffee powder into the mixed solution, and fully stirring to obtain a coffee powder mixed suspension containing heavy metal elements;
(5) And (3) adding: repeatedly stirring and refrigerating the mixed suspension until the mixed suspension is not layered after standing;
(6) Freezing: the mixed suspension is placed in a material tray and quickly frozen;
(7) Vacuum drying: vacuum drying the frozen sample until the temperature of the sample gradually returns to room temperature;
(8) Secondary crushing: pulverizing the sample again, and sieving with 80 mesh sieve;
(9) And (3) secondary mixing: uniformly mixing for more than 6 hours at the rotating speed of 20-30 rpm;
(10) And (3) primary uniformity inspection: carrying out primary uniformity detection on the coffee powder sample after secondary uniform mixing;
(11) And (3) irradiation: performing irradiation sterilization on the coffee powder sample passing the uniformity primary detection;
(12) And (5) subpackaging: packaging with medicine packaging bottles which are cleaned in advance and dried in the air and have no dissolved substances, wherein each bottle is 8g;
(13) And (3) preserving: and (3) packaging the sample by adopting a film, and then placing the packaged sample in a dryer at normal temperature for preservation to obtain the coffee matrix standard substance containing heavy metals.
2. The method for preparing a coffee matrix standard substance containing heavy metals according to claim 1, wherein in the step (2), the crushing is performed at a rotation speed of 25000rpm and a power of 1.2kW for 20-30 seconds, a crushing bin cover is opened, the cooling is performed for 8-10 minutes, and the temperature of a bin of a crusher is lower than 40 ℃ and the crushing is continued; repeating for 8-10 times.
3. The method for preparing a heavy metal-containing coffee matrix standard substance according to claim 1, wherein the grinding in the step (2) is carried out at a rotating speed of 200-300rpm and a power of 0.75kW, and the pot body is opened every 30min to observe the grinding condition of the raw coffee beans, so that the hardening of powder is avoided; repeating for 4 times.
4. The method for preparing a heavy metal-containing coffee base standard substance according to claim 1, wherein the target concentration in the step (4) is 0.8-1.0mg/kg of heavy metal element content; the weight ratio of the coffee powder to the pure water is 1:1.5-3.0.
5. The method for preparing a heavy metal-containing coffee base standard substance according to claim 1, wherein the stirring in the step (5) is performed at a rotation speed of 180-200rpm and a power of 0.75kW for 4-6 hours; the cold storage is carried out for 13 to 15 hours at the temperature of 3 ℃; the total time of repeated stirring and cold storage is not less than 48 hours.
6. The method for preparing a heavy metal-containing coffee base standard substance according to claim 1, wherein the material contained in the separation tray in the step (6) is 150-160g; the temperature of the frozen cold trap is-54 ℃ and the time is 18-24 hours, when the temperature of the material is stabilized between- (50-53) ℃ for 3 hours, the freezing process is completed.
7. The method for preparing a heavy metal-containing coffee base standard substance according to claim 1, wherein the vacuum drying in step (7) is performed at a vacuum degree of 10.0 to 14.9Pa and a temperature of- (50 to 53) ℃ for more than 72 hours.
8. The method for preparing a heavy metal-containing coffee matrix standard substance according to claim 1, wherein the secondary crushing in the step (8) is carried out at a rotation speed of 2000rpm for 10-30 seconds, the bin cover is opened for cooling for 5-8 minutes, and the sample after the secondary crushing is sieved by a sieve of 80 meshes; grinding coffee bean powder which cannot pass through a 80-mesh sieve, wherein the grinding rotating speed is 200-300rpm, the power is 0.75kW, and the tank body is opened every 30 minutes to observe the grinding condition of the raw coffee beans, so that the hardening of the powder is avoided; repeating for several times to obtain coffee powder with 80 mesh sieve.
9. The standard substance prepared by the method for preparing a heavy metal-containing coffee base standard substance according to any one of claims 1 to 8.
10. The use of a standard substance according to claim 9, characterized by the use of the standard substance for analysis, evaluation and quality control, analytical instrument calibration and detection method validation, laboratory ability assessment in coffee heavy metal content detection.
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