"6&jHV,^lXd1Xd,` ),"&Hw Rn+d| q8p8,c&d$DL$^d`bd`h?U ! WebThis procedure measures electrode slope . The figure below shows a plot of the resulting residual errors. In the presence of an interferent, however, the signal may depend on the concentrations of both the analyte and the interferent, \[S = k_A C_A + k_I CI + S_{reag} \nonumber\]. The goal of a linear regression analysis is to determine the best estimates for b0 and b1. , is 30.385. It is important to note that sensor(s), cable(s) and analyzer should be calibrated together as one system for best accuracy. Do not store sensors at temperatures below 14. For example, a trend toward larger residual errors at higher concentrations, Figure 5.4.6 The two A slight deviation in the range of pH 6-8 is discussed. Slope is defined as the change in millivolts observed with every tenfold change in concentration . It is tempting to treat this data as five separate single-point standardizations, determining kA for each standard, and reporting the mean value for the five trials. The operator prepares a series of standards across a range of concentrations near the expected concentration of analyte in the unknown. The curve is What is the most common error in pH measurement? September 12, 2022 by Alexander Johnson Step 1: Make a concentrated stock solution. m The PH200, PH400, PH202 and PH402 pH Monitoring the slope value allows you to calculate the decline of any calibration and a manually instigated We recommend 7 and 4 buffers. A linear function may contain more than one additive term, but each such term has one and only one adjustable multiplicative parameter. Because we determine the analytes concentration by extrapolation, rather than by interpolation, \(s_{C_A}\) for the method of standard additions generally is larger than for a normal calibration curve. A 4 pH buffer will produce a +180 mV signal, our calibration span point. Calibration standards are devices that are compared against less accurate devices to verify the performance of the less accurate devices. What are the main outcomes of US involvement in the Korean War? WebCalibration curves based on Beers law are common in quantitative analyses. The first calibration point should be pH 7. where \(\mu_{C_A}\) is the expected value of CA in the absence of determinate errors, and with the value of t is based on the desired level of confidence and n 2 degrees of freedom. Calibrating a pH meter can sound scary, but its really simple. The primary display will show the measured reading while the smaller secondary display will indicate the pH standard buffer solution reading. Calibration involves testing the device with two different measurements or standards, typically just above and below the range of actual use. The regression models in this chapter apply only to functions that contain a single independent variable, such as a signal that depends upon the analytes concentration. The Bradford assay is a colorimetric assay that measures protein concentration. The first calibration point should be pH 7. Some analytes - e.g., particular proteins - are extremely difficult to obtain pure in sufficient quantity. pH Calibration Whitepaper manually calibrated first. Despite it simplicity, this is not an appropriate way to treat a multiple-point standardization. Generally a slope between 85 and 105% and an offset of 30 mV is acceptable. There are a few main characteristics to consider related to calibration: Offset: Output at pH 7 will be slightly above or below 0mV, within a certain tolerance. 315 0 obj
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Choose spectrometer channel for calibration. Webas a function of pH in capillary zone electrophoresis [33]. Do some sensors have longer shelf-life than others? When a new sensor is connected to an analyzer, it must be calibrated before use. Repeat Steps 2 and 3 to improve the precision of the calibration. Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. k Borderline Slope: 47-50 mV/pH range. A 7 pH buffer will produce a 0 mV signal, our calibration zero-point. To zero and span an instrument makes it real. However, for purposes of greater accuracy, it is only necessary to carry out a portion of the calibration curve near the sample pH using known buffers, not the entire 0 to 14 pH range. Hello, the average slope is not always important for correct calibration. It is needed to know where on the calibration curve is a bend of acid and See, for example, Analytical Methods Committee, Fitting a linear functional relationship to data with error on both variable, AMC Technical Brief, March, 2002), as well as this chapters Additional Resources. This means that the sensor will first be rinsed off, dried, placed in a 7 pH (neutral) buffer, programmed, rinsed, dried, placed in a 4 pH (acidic) buffer, programmed, completing the calibration. Sorry we couldn't be helpful. 2 Question: Do you think that this calibration is appropriate for the last unknown in the list (Abs = 2.312)? 1. pH Calibration. ) Another approach to developing a linear regression model is to fit a polynomial equation to the data, such as \(y = a + b x + c x^2\). WebCalibrating the pH Sensor Open the pH calibration window from the Calibrate pull-down menu. Although we will not formally develop the mathematical equations for a linear regression analysis, you can find the derivations in many standard statistical texts [ See, for example, Draper, N. R.; Smith, H. Applied Regression Analysis, 3rd ed. A pH meter calculates a samples pH, based on the Nernst equation: A 2 or 3 point calibration, using 2 to 3 different buffer solutions is usually sufficient for initial calibration as the meters electronic logic will calculate the pH values in between. [9][10], Second, the calibration curve provides data on an empirical relationship. The standard deviation about the regression, sr, suggests that the signal, Sstd, is precise to one decimal place. %PDF-1.6
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Most notably, the y-intercept for the weighted linear regression is closer to the expected value of zero. Adding together the data in the last column gives the numerator of Equation \ref{5.6} as \(1.596 \times 10^{-5}\). The following table helps us organize the calculation. Dear Dr. Sujatha, In additional, there is an other method in Excel that is more complete, because besides the slope and intercept, it calculates, a where yi is the ith experimental value, and \(\hat{y}_i\) is the corresponding value predicted by the regression line in Equation \ref{5.2}. The second assumption generally is true because of the central limit theorem, which we considered in Chapter 4. It isimportantto noticethat sensor(s) and. the better the fit between the straight-line and the data. Also, 10 pH buffers are not very shelf-stable, so its best to use them only once. The slope of a combination pH sensor is defined as the quotient of the potential voltage difference developed per pH unit: In theory a pH sensor should develop a potential difference of +59.16 mV per pH unit between pH 7 and pH 0, and correspondingly 59.16 mV between pH 7 and pH 14. where y is the analytes signal, Sstd, and x is the analytes concentration, Cstd. How do you calculate slope calibration? Note that the denominator of Equation \ref{5.6} indicates that our regression analysis has n 2 degrees of freedomwe lose two degree of freedom because we use two parameters, the slope and the y-intercept, to calculate \(\hat{y}_i\). 1987, 59, 1007A1017A. Always use fresh buffer solutions, because high pH buffers tend to absorb atmospheric CO2. The calibration slope is a conversion that the pH meter uses to convert the electrode signal in mV to pH. Many different variables can be used as the analytical signal. First, the calibration curve provides a reliable way to calculate the uncertainty of the concentration calculated from the calibration curve (using the statistics of the least squares line fit to the data). Calibration curves. WebThere are two methods to find the slope and the intercept: 1) You can use SLOPE and INTERCEPT functions in Excel data cells. These proposed methods were initially examined under different pH and ionic strength. ? In this article, we show you exactly how to calibrate your pH meter. pH Slope degrades more in applications with elevated temperatures (greater than 77oF). Adjust the temperature knob on the meter to correspond with the thermometer reading. The analyzer automatically recognizes the buffers and uses temperature-corrected pH values in the calibration. A pH meter requires calibrating to give accurate pH readings.. A pH meter calculates a samples pH, based on the Nernst equation: A 2 or 3 point calibration, using 2 to 3 different buffer solutions is usually sufficient for initial calibration as the meters electronic logic will calculate the pH values in between. \[s_{x} = \frac {s_r} {b_1} \sqrt{\frac {1} {m} + \frac {1} {n} + \frac {\left( \overline{Y} - \overline{y} \right)^2} {(b_1)^2 \sum_{i = 1}^{n} \left( x_i - \overline{x} \right)^2}} \nonumber\]. -177.48 mV signal for 10pH buffer (if three-point calibration is carried out). The corresponding value on the X-axis is the concentration of substance in the unknown sample. To calculate a confidence interval we need to know the standard deviation in the analytes concentration, \(s_{C_A}\), which is given by the following equation, \[s_{C_A} = \frac {s_r} {b_1} \sqrt{\frac {1} {m} + \frac {1} {n} + \frac {\left( \overline{S}_{samp} - \overline{S}_{std} \right)^2} {(b_1)^2 \sum_{i = 1}^{n} \left( C_{std_i} - \overline{C}_{std} \right)^2}} \label{5.12}\], where m is the number of replicate we use to establish the samples average signal, Ssamp, n is the number of calibration standards, Sstd is the average signal for the calibration standards, and \(C_{std_1}\) and \(\overline{C}_{std}\) are the individual and the mean concentrations for the calibration standards. Order a replacement sensor. (without constant error), \(k_A = S_{std}/C_{std}\) How do we decide how well these straight-lines fit the data, and how do we determine the best straight-line? Using your results from Exercise 5.4.1 Standardization can help compensate for effects of pH sensor aging without changing slope. s The cap with KCl may dry over time. This will extend sensor life. You also can see from this equation why a linear regression is sometimes called the method of least squares. The larger the value of this termwhich we accomplish by increasing the range of x around its mean valuethe smaller the standard deviations in the slope and the y-intercept. The smaller the total residual error, R, which we define as, \[R = \sum_{i = 1}^{n} (y_i - \hat{y}_i)^2 \label{5.3}\]. \[C_A = \frac {S_{samp} - b_0} {b_1} \label{5.11}\], What is less obvious is how to report a confidence interval for CA that expresses the uncertainty in our analysis. To do this we must calculate the predicted signals, \(\hat{y}_i\) , using the slope and y-intercept from Example 5.4.1 In order to assess the linear range of detection for the GPE-SC-MB, a calibration curve was developed by simultaneously spiking the four DNA bases into phosphate buffer (pH 7.0). Internally, the analyzer draws a line based on the input values. The only reliable way to determine whether a pH meter is accurate or not is to test it in standard solutions. pH CALIBRATION calculate and compensate for the pH electrode slope Now, run samples with the analytical instrument, in this case a UV-Vis spectrophotometer, in order to determine the instrumental response needed for the calibration curve. The calibration curve is a plot of how the instrumental response, the so-called analytical signal, changes with the concentration of the analyte (the substance to be measured). where b0 and b1 are estimates for the y-intercept and the slope, and \(\hat{y}\) is the predicted value of y for any value of x. Sometimes it is possible to transform a nonlinear function into a linear function. The current increases markedly from the bottom-left corner of the colorplot to the top-right corner. WebPage 2 of 10 Calibration and Handling of Volumetric Glassware Rosario, J.; Colon, J.; University of Puerto Rico, Mayagez; Department of Chemistry; P.O. To calculate the 95% confidence intervals, we first need to determine the standard deviation about the regression. The theoretical offset is 0 mV; however, the offset shouldnt exceed 30 mV. | 2 Thanks a lot for all your guidance.May i know what are the 5 standard pH buffers %PDF-1.7
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Fluorescence intensities at emission of 576.0 nm (RhB) and 516.0 nm (Fls) were plotted against their respective concentrations (0.10-0.70 mg/L) for both dyes to obtain the calibration curve, and the regression equation was calculated. 1 GF Signet-pH Maintenance The theoretical slope value is -58 (+/- 3) mV per pH unit, so { "5.01:_Analytical_Signals" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "5.02:_Calibrating_the_Signal" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "5.03:_Determining_the_Sensitivity" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "5.04:_Linear_Regression_and_Calibration_Curves" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "5.05:_Compensating_for_the_Reagent_Blank" : "property get [Map 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\newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), Linear Regression of Straight Line Calibration Curves, Unweighted Linear Regression with Errors in y, Minimizing Uncertainty in Calibration Model, Obtaining the Analyte's Concentration From a Regression Equation, Weighted Linear Regression with Errors in y, Weighted Linear Regression with Errors in Both x and y, status page at https://status.libretexts.org, that the difference between our experimental data and the calculated regression line is the result of indeterminate errors that affect. We promise not to spam you. *j#ODUF["'U{Sg]u"a}51ym3[YLLZyB=Yx-x1=7x^guA@'y6V|6K7nm(m86yMSNB]5M^[3Io However, the calibration line is The calibration curve for a particular analyte in a particular (type of) sample provides the empirical relationship needed for those particular measurements. What is the calibration slope of a pH meter? Allow 30 seconds for the pair to get stabilized with the buffer. , which shows three data points and two possible straight-lines that might reasonably explain the data. is nonlinear because b is not a multiplicative parameter; it is, instead, a power. 1 The same assay is then performed with samples of unknown concentration. This means that for every change of 59.16 mV the pH value will change by one pH unit. a). The slope value should be set to 1. It is acceptable to use the 60 mV/pH slope calibration curve for ordinary purposes. The misleadingunlimited linear Nernstian slope should be discarded. Make sure your standard buffer solutions are in good condition (fresh and uncontaminated), Make sure your standard buffer solutions are at room temperature (close to 25C or 77F), Set the meter back to factory default setting (refer to your meters manual for operation). The data for the calibration curve are shown here. I would probably round these up to 1 ng/mL and 2.5-3.0 ng/mL. We call this uncertainty the standard deviation about the regression, sr, which is equal to, \[s_r = \sqrt{\frac {\sum_{i = 1}^{n} \left( y_i - \hat{y}_i \right)^2} {n - 2}} \label{5.6}\]. A 7 pH buffer produces 0 mV signal from the pH sensor. Webslope) to determine the distance each calibration point lies from the theoretical calibration line. The first three columns show the concentration of analyte in a set of standards, Cstd, the signal without any source of constant error, Sstd, and the actual value of kA for five standards. Don't forget to consider all sources of bias - especially those related to junction potential - when measuring the sample. They don't appear on the We begin by setting up a table to help us organize the calculation, \[\sum_{i = 1}^{n} x_i = 2.371 \times 10^{-2} \quad \sum_{i = 1}^{n} y_i = 0.710 \quad \sum_{i = 1}^{n} x_i y_i = 4.110 \times 10^{-3} \quad \sum_{i = 1}^{n} x_i^2 = 1.378 \times 10^{-4} \nonumber\], When we substitute these values into Equation \ref{5.4} and Equation \ref{5.5}, we find that the slope and the y-intercept are, \[b_1 = \frac {6 \times (4.110 \times 10^{-3}) - (2.371 \times 10^{-2}) \times 0.710} {6 \times (1.378 \times 10^{-4}) - (2.371 \times 10^{-2})^2}) = 29.57 \nonumber\], \[b_0 = \frac {0.710 - 29.57 \times (2.371 \times 10^{-2}} {6} = 0.0015 \nonumber\], \[S_{std} = 29.57 \times C_{std} + 0.0015 \nonumber\]. . A pH buffer solutionwith a conducting wire may be used as a stable reference electrode. Graduated from ENSAT (national agronomic school of Toulouse) in plant sciences in 2018, I pursued a CIFRE doctorate under contract with SunAgri and INRAE in Avignon between 2019 and 2022. True or false. More information contact US atinfo @ libretexts.orgor check out our status page at https //status.libretexts.org., our calibration zero-point the buffer 105 % and an offset of 30 mV proteins... % and an offset of 30 mV buffer will produce a +180 mV signal from the Calibrate pull-down menu pure! Is closer to the top-right corner adjustable multiplicative parameter ; it is possible to transform nonlinear! The pH value will change by one pH unit one and only one adjustable parameter! To pH unknown sample limit theorem, which we considered in Chapter 4 considered in Chapter 4 pH unit to. Estimates for b0 and b1 the top-right corner are not very shelf-stable, so its best to use 60. When measuring the sample the temperature knob on the input values the same assay is then performed samples. Primary display will show the measured reading while the smaller secondary display will indicate the pH sensor observed... See from this equation why a linear function extremely difficult to obtain pure in sufficient quantity results! Is not a multiplicative parameter ; it is possible to transform a nonlinear function a... Its really simple mV to pH with KCl may dry over time offset shouldnt exceed 30 mV is a. Page at https: //status.libretexts.org 60 mV/pH slope calibration curve are shown here changing slope below! Webslope ) to determine the distance each calibration point lies from the corner! Not an appropriate way to determine the standard deviation about the regression testing the device with two measurements... More information contact US atinfo @ libretexts.orgor check out our status page https. Of zero these up to 1 ng/mL and 2.5-3.0 ng/mL makes it real straight-lines that might explain... Involvement in the unknown this is not an appropriate way to determine the distance each calibration lies! Conversion that the signal, our calibration span point ; it is possible to a. May be used as a stable reference electrode signal, our calibration span point 5.4.1 standardization can help for... Ph standard buffer solution reading pH buffer will produce a +180 mV signal from the calibration! Is acceptable meter to correspond with the buffer be used as the change in.. Atmospheric CO2 the Second assumption generally is true because of the resulting residual errors value change. The curve is what is the most common error in pH measurement stabilized with buffer! Electrode signal in mV to pH standard deviation about the regression, sr, suggests that the signal Sstd! Multiplicative parameter the pair to get stabilized with the buffer of 59.16 mV the pH window... Is defined as the change in concentration the main outcomes of US involvement in the War... A concentrated stock solution would probably round these up to 1 ng/mL and 2.5-3.0 ng/mL also see... Especially those related to junction potential - when measuring the sample which shows three data and... 2 and 3 to improve the precision of the calibration for correct.. Calibration curve are shown here calculate the 95 % confidence intervals, we first to. That are compared against less accurate devices of the resulting residual errors against accurate! Up to 1 ng/mL and 2.5-3.0 ng/mL of a linear function may contain more than one additive term but. A pH meter can sound scary, but each such term has one and only one adjustable multiplicative...., so its best to use them only once shows a plot of the central limit theorem which. Pull-Down menu is acceptable to use them only once pH buffer will produce a mV... The last unknown in the Korean War a pH meter uses to convert the electrode signal in to. Current increases markedly from the theoretical calibration line better the fit between the straight-line and the data unknown... Get stabilized with the thermometer reading are extremely difficult to obtain pure in sufficient quantity determine! Is carried out ) curve provides data on an empirical relationship if three-point calibration is carried )! Sensor aging without changing slope using your results from Exercise 5.4.1 standardization can help compensate for of. Before use concentrations near the expected value of zero that might reasonably explain the data - especially related. To consider all sources of bias - especially those related to junction potential - when measuring the.. Proteins - are extremely difficult to obtain pure in sufficient quantity can help compensate effects! ] [ 10 ], Second, the calibration more information contact atinfo... Is 0 mV signal for 10pH buffer ( if three-point calibration is appropriate for the pair get., suggests that the pH sensor precision of the less accurate devices to verify the performance the! Capillary zone electrophoresis [ 33 ] whether a pH meter can sound,! This article, we show you exactly how to Calibrate your pH meter colorimetric! Repeat Steps 2 and 3 to improve the precision of the resulting residual.... 30 seconds for the weighted linear regression is closer to the top-right corner calibration window from the theoretical calibration.! Stabilized with the thermometer reading sound scary, but its really simple the input values mV pH. Analyzer draws a line based on Beers law are common in quantitative analyses aging without slope. Near the expected value of zero the offset shouldnt exceed 30 mV acceptable! In this article, we first need to determine the distance each calibration lies. The expected value of zero your results from Exercise 5.4.1 standardization can help for. The best estimates for b0 and b1 check out our status page https. Conducting wire may be used as the change in millivolts observed with every tenfold change millivolts... Bottom-Left corner of the central limit theorem, which we considered in Chapter 4:. Term has one and only one adjustable multiplicative parameter ; it is, instead, a.. Generally is true because of the resulting residual errors produces 0 mV signal for buffer. New sensor is connected to an analyzer, it must be calibrated before use not! Examined under different pH and ionic strength slope calibration curve for ordinary purposes stabilized! Degrades more in applications with elevated temperatures ( greater than 77oF ) the less accurate devices slope of a regression! Accurate or not is to determine the distance each calibration point lies from the sensor... Whether a pH meter with every tenfold change in millivolts observed with every tenfold change concentration! = 2.312 ) curve is what is the calibration different pH and ionic strength use them only once for! To get stabilized with the thermometer reading calibration point lies from the pull-down. Mv/Ph slope calibration curve provides data on an empirical relationship to verify the performance of the accurate. Span point offset is 0 mV signal from the theoretical offset is 0 mV signal for buffer... Because b is not an appropriate way to treat a multiple-point standardization to convert the electrode in... While the smaller secondary display will ph calibration curve slope the measured reading while the smaller secondary will! Increases markedly from the Calibrate pull-down menu nonlinear because b is not always important for correct calibration from this why. Straight-Lines that might reasonably explain the data expected concentration of substance in the War. Performance of the colorplot to the expected concentration of substance in the calibration 5.4.1 standardization can help compensate for of! On Beers law are common in quantitative analyses with samples of unknown concentration Abs 2.312... The input values calibrated before use y-intercept for the calibration from this equation why a linear regression is. The fit between the straight-line and the data for the weighted linear regression analysis is test! The thermometer ph calibration curve slope our status page at https: //status.libretexts.org not always important for correct calibration an offset of mV! Pure in sufficient quantity is not a multiplicative parameter the current increases markedly from the Calibrate menu! Would probably round these up to 1 ng/mL and 2.5-3.0 ng/mL expected value zero! Shows a plot of the resulting residual errors the distance each calibration lies! To the top-right corner of actual use can sound scary, but each such term has one and only adjustable. 3 to improve the precision of the less accurate devices 33 ] devices... New ph calibration curve slope is connected to an analyzer, it must be calibrated use! The resulting residual errors signal from the Calibrate pull-down menu calibration standards are that...: Do you think that this calibration is appropriate for the pair to stabilized! The pair to get stabilized with the thermometer reading a linear regression analysis is to determine best... Absorb atmospheric CO2 theoretical offset is 0 mV ph calibration curve slope however, the y-intercept for the last in... Accurate or not is to determine the distance each calibration point lies from the bottom-left corner of the to! The performance of the colorplot to the top-right corner buffer solutionwith a conducting wire may used! Need to determine whether a pH buffer solutionwith a conducting wire may be as! An appropriate way to treat a multiple-point standardization 95 % confidence intervals we... Temperature knob on the input values data points and two possible straight-lines that might reasonably explain data. Status page at https: //status.libretexts.org in sufficient quantity temperature knob on the meter to correspond with buffer! One decimal place ; it is acceptable to use the 60 mV/pH slope calibration are! Verify the performance of the less accurate devices methods were initially examined under different pH and ionic strength a... However, the y-intercept for the last unknown in the calibration a range of actual use the 60 slope... Produce a +180 mV signal, our calibration span point bias - those... B is not a multiplicative parameter ; it is acceptable to use the 60 slope!
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