ethyl 2-chloropropionate ( cas number 535 - 13 - 7 ) is a clear colorless liquid with a pungent odor .
its flash point is 100f and it is denser than water and insoluble in water . in recent years , as an important chemical intermediate and industrial reagent , it has been popularly applied in the synthesis of herbicides ( e.g. , phenoxypropionates , 2-(4-hydroxyphenoxy)propionate , and amino ( or aryloxy ) sulfonyl phenoxy propanates ) , plant auxiliaries ( e.g. , 2-chloroethyl trimethyl ammonium chloride , dimethylaminosuccinic acid ) , nonsteroidal antipyretic and anti - inflammatory drugs ( e.g. , naproxen , indomethacin , and ibuprofen ) , and so forth . though the synthetic process of ethyl 2-chloropropionate is relatively simple , the current offline quantitative method for its production monitoring and quality control can hardly meet the requirements of related researchers and producers .
previously , food and drug administration ( fda ) issued a guidance document to pharmaceutical industry regarding the implementation of process analytical technology ( pat ) in 2004 .
process analytical technology ( pat ) has been defined as  a system for designing , analyzing , and controlling manufacturing through timely measurements of critical quality and performance attributes of raw and in - process materials and processes , with the goal of ensuring final product quality  .
recently , the application of near - infrared ( nir ) spectroscopy has grown rapidly as an efficient online monitoring technique , which has been used as an ideal tool for pat .
the growing concentration on nir is probably a direct result for its advantages of outstanding sensitivity , high speed , low noise , nondestruction , and enabling the analysis of complex samples without the need for pure samples compared to others [ 35 ] .
near - infrared ( nir ) spectroscopy was used as a process analytical technology to monitor the amino acids concentration profile during hydrolysis process of cornu bubali by wu et al . .
and the use of near - infrared diffuse reflectance spectroscopy for qualification of ginkgo biloba extract was described as raw material for use in pharmaceutical products by rose and coworkers .
nir spectroscopy has also been used as an analyzer to determine the effect of several operating conditions on recovery , selectivity , and productivity for production of methyl isobutyl ketone ( mibk ) .
the use of this pat approach enabled the researchers to perform the necessary experiments in a time - efficient fashion and resulted in 30% improved productivity of mibk .
based on the above research status , the aim of this study was to use uv - nir spectroscopy for online and nondestructive analysis of synthesis process of 2-chloropropionate catalyzed by ion exchange resin for the first time .
the method of model updating was utilized to make the models more accurate and obtained better prediction results .
the developed method was supposed to provide foundation for further process chemical analysis and useful reference for similar online analytical research of synthetic reaction .
methanol , ethanol , 2-chloropropionic acid ( purity : 0.9956% ) , and ethyl 2-chloropropionate ( purity : 0.9913% ) were obtained from kelong chemical inc .
( chengdu , china ) . d001 strong acidic cation exchange resin was purchased from shengquan chemical inc .
nir spectrometer ( nirquest512 ) , dh-2000-bal deuterium light source , and optical fiber ( t300-uv - vis ) were obtained from ocean optics inc .
shanghai , china ) was used in the quantitative analysis , which was equipped with gh-300 hydrogen generator and flame ionization detector .
the scheme of experimental device is shown in figure 1 , which includes condensator , peristaltic pump , nir detector , workstation , optical source , detection cell , microsyringe connected with teflon tube , optical fiber , and signal line . in the three - necked flask ,
2-chloropropionic acid was esterified with ethanol under the catalysis of strong acidic cation exchange resin . because the synthesis of ethyl 2-chloropropionate was performed at 110c under refluxing , the condensator was added before the nir spectrometer in order to eliminate effects of temperature fluctuation on spectral signals .
meanwhile , the peristaltic pump was adjusted with the stable flow rate of 0.2  ml / s through repeated experiments to meet the requirement of ideal online detection . in the self - made detection cell ,
the fiber - optic probe acquired useful nir signals and the microsyringe could collect the samples in the sampling port .
the concentration ( c ) of the product ethyl 2-chloropropionate can be obtained through the following equation:(1)c = c0a2a2+fa1,where c 
 0 is the initial concentration of 2-chloropropionic acid ethanol solution , a 
 1 and a 
 2 are the peak areas of 2-chloropropionic acid and ethyl 2-chloropropionate in the collected samples , respectively , and f is the relative correction factor of 2-chloropropionic acid to ethyl 2-chloropropionate .
the sample of reaction product was drawn out from the pipeline of pat system at regular intervals .
it was diluted 5 times with dehydrated alcohol and then was filtrated by 0.22  m millipore filter .
5  l sample solution was injected into gc instrument and analyzed under the following conditions : nonpolar tm-1 capillary column ( 15  m  0.53  0.5  m ) , temperature programming ( started at 90c , holding for 0.5  min , and then increased to 165c at a rate of 25c / min ) , the temperature of injection port , and fid detector under 180c , n2 at 65  ml / min as carrier gas . as a result ,
the solvent of alcohol , the product of ethyl 2-chloropropionate ( t 
 r = 1.805  min ) , and the substrate of 2-chloropropionic acid ( t 
 r = 1.993  min ) could reach the baseline separation in 3 minutes .
the results of gc chromatogram of ethanol , ethyl 2-chloropropionate , and 2-chloropropionic acid are shown in figure 2 .
the nir spectroscopic data of training set for the reaction process were collected with 3 replications continuously every 30  s , and the average value was determined as the spectral absorption data of the sample at this time .
data of calibration set were the average of spectral values in three times which were collected once every 1  min in another batch synthetic process .
spectra acquisition conditions were presented as follows : in the transmission mode , the background of unreacted solution was used as a reference , and the detection wavelength ranged from 900  nm to 1800  nm with a resolution of 4  nm ; an integral time was 280  ms and the optical path was 2  cm .
spectral data were manipulated by identifying the optimal spectral regions and selecting appropriate pretreatment methods , and then they were correlated with the data measured by the reference assays using plsr to develop calibration models [ 10 , 11 ] .
the performance of the calibration models was assessed in terms of root mean square error ( rmse ) , correlation coefficient ( r 
 ) , root mean square error of cross - validation ( rmsecv ) , and relative standard error of prediction ( rsep ) . in plsr
algorithm , including more plsr factors in the model would better fit the modeling data , but the prediction accuracy of the other samples might become worse . this phenomenon was called  overfitting  of a model . in this case , the corresponding components should be eliminated effectively ( including noise , nonspectral measurement information ) [ 13 , 14 ] . the calibration models with the highest r and the lowest rmsec and rmsep with the least difference from each other were considered optimal .
based on the result of gc analytical conditions in section 2.3 , the standard compound of 2-chloropropionic acid was used to determine the relative correction factor ( f ) of ethyl 2-chloropropionate .
the related data of internal standard method are listed in table 1 , which are further linearly fitted as shown in figure 4 .
the linear equation is y = 0.3826x + 0.0164  ( r 
  = 0.9999 ) . as a result ,
f was determined as 0.3826 and a good linear relationship could be obtained in the range of molar ratio from 0.0362 to 7.3567 .
standard linearity was tested using linear regression and ethyl 2-chloropropionate together with 2-chloropropionic acid showed excellent linearity with correlation coefficient greater than 0.999 in the studied range .
within - run precision was measured using rsd for six replicate standards of 2-chloropropionic acid and ethyl 2-chloropropionate .
rsd values of retention time and the peak areas for 2-chloropropionic acid were within 0.16% and 1.56% . for ethyl 2-chloropropionate ,
the rsd values of retention time and the peak areas were within 0.37% and 1.98% .
the accuracy of the gc method was validated by adding two standards to the known concentration of their samples . as a result ,
a quantitative model was established by plsr , in which all the wavelengths could be analyzed and processed . but part of the signals in the spectrum ( produced by solvent , temperature , variance of flow rate , etc . )
was not related to the target compound , and the performance of the model would be affected at the same time .
a reasonable wavelength can improve the prosperity of the model with a small amount of computation .
the comparison of different regions is shown in table 2 . according to the results , the interval of 10001240  nm was finally selected to establish calibration model of ethyl 2-chloropropionate content .
the origin spectra collected between 899.07 and 1264.06  nm in the reaction process are shown in figure 5 .
it can be found that the absorbance in two regions of 1003.791134.40  nm and 1212.141240.18  nm will decrease with increasing reaction time .
because the content of 2-chloropropionic acid becomes less and less , the intensity of related absorbance band of cooh group will decrease . however , there is a turning point appearing in 1134.40  nm , and the absorbance in the region of 1134121.14  nm will rise continuously with the increasing content of ethyl 2-chloropropionate . the region is closely related with the characteristic absorption of cooet group .
nir spectra of the samples , which contain much chemical information , need to be pretreated to ensure accurate analysis .
there are many factors that would cause interference in the spectral measurement process , so most of the nir measurement methods require the use of chemometric treatment . taking measurement to eliminate outside interference to some extent
it can be seen from the table that the various pretreatment methods can improve the performance of models to different extent . in many cases
in addition , the standard normal variate ( snv ) was applied to reduce the changes in the path length and to reclaim the light scattering . considering that some pretreatments even exhibited negative values ,
obviously , the combination of snv and second - order derivative method was superior to other pretreatment methods , which presented the greatest value of r 
  and the smallest rmse .
therefore , snv + second - order derivative was selected to pretreat the original data .
the decomposition model of data compression with plsr method is different from other chemical decomposition models .
it explores the matrix of absorbance and the matrix of the reference value of the concentrations . respectively , scoring matrix and loading could be calculated ( the two scoring matrixes could exchange alternatively ) .
meanwhile , this method can make the reference of concentration related to the spectrum information better . in plsr process , cross - validation based on the samples from training set and corrected
set was conducted , and the predicted residual error sum of squares ( press ) was calculated at one time .
the plsr models with 012 factors were investigated , and the optimum number of factors employed in plsr was determined by press .
it can be concluded that the values of the previous seven main constituents are decreasing . and the result explains that the increasing main constituents are responsible for the content of ethyl 2-chloropropionate .
when the principal components were less than 7 , this established model was underfitting , and when the number of main components was greater than 8 , the press value would increase .
it indicated that the number of principal components was redundant and extra factors were considered in the establishment of models .
as comparison , the cross - validation method was also used to ascertain the number of significant factors ( latent variables ) in the pls algorithm , which left out one sample at a time .
the press was calculated in the same manner each time a new factor was added to the pls models , and the f - statistic was used to make the significance determination according to the suggestion of haaland and thomas .
the maximum number of factors used to calculate the optimum press was selected as 13 and the optimum number of factors obtained by the application of pls model was 9 as a result .
spectral data were manipulated by identifying the proper spectral region ( 10001240  nm ) and choosing snv + second - order derivative as appropriate pretreatment method , and then they were correlated with the data measured by the reference assays using plsr method to develop calibration models .
after the study of optimal number of principal components , the correlation diagram of reference and nir prediction of ethyl 2-chloropropionate in related reaction process is shown in figure 7 . the correlation coefficients ( r 
 ) of training set and calibration set are 0.9944 and 0.988 , respectively .
the performance of the calibration models is assessed in terms of root mean relative standard error of prediction ( rmsep ) . the calibration model with the high r 
  value and the low rmsep
it can be found that the predicted model and the references of ethyl 2-chloropropionate in the involved process have a similar trend . however , the uncertain factors , such as the pulse of peristaltic pump , temperature , tiny particles , and external vibration , all have impact on the nir spectra to different extent .
the results of online nir analysis and references can be also reflected by figure 7 . as a result ,
rmsec of training set = 0.018105  mol / l ( r 
  = 0.9944 ) , and rmsep = 0.036429  mol / l ( r 
  = 0.9788 ) .
the above performance can indicate that the prediction model is acceptable as a method of quantitative determination . on the basis of the above developed plsr model ,
21 batches of synthetic experiments have been used to verify the accuracy of the regression model .
the product contents of nir model and offline gc analysis are listed and compared in table 4 .
it can be found that the difference of the two contents is small except for sample number 9 , which could be the result of experimental error .
after this unqualified sample is deleted , the average relative deviation can reach 1.12% , and average predicted recovery is 101.12% . the results in table 4 can prove that the prediction ability of plsr model is good for pat analysis of ethyl 2-chloropropionate .
in the study , we investigated the synthetic process of ethyl 2-chloropropionate based on plsr analysis and the use of nir to achieve the online and noninvasive monitoring of the extraction process . the sampling , spectrum acquisition , and pat system were developed and the established quantitative models can detect the dynamic change of target product content during synthetic process accurately and in real time .
the origin spectrum was pretreated through first - order derivative , second - order derivative , msg , snv and sg smoothing , and so forth .
the values of r 
  and rmsec of plsr quantitative calibration model indicated that it had good performance .
offline gas chromatographic analysis was used to validate the predictive ability of the model , and the results of average relative deviation and recovery rate in the validation set were ideal . in brief , nir spectroscopy has potential to be extended to the whole reaction process , as a time - saving and continuous measuring method . and
online nir analysis technology was proved to be a fast and effective method to observe the extent of reaction successfully . as a fast , simple , and nondestructive analysis technology ,
online nir detection has become a helpful tool applied in various synthetic reactions and industrial production .
 online near - infrared spectroscopy was used as a process analysis technique in the synthesis of 2-chloropropionate for the first time . 
 then , the partial least squares regression ( plsr ) quantitative model of the product solution concentration was established and optimized . 
 correlation coefficient ( r 
 2 ) of partial least squares regression ( plsr ) calibration model was 0.9944 , and the root mean square error of correction ( rmsec ) was 0.018105  
 mol / l . 
 these values of plsr and rmsec could prove that the quantitative calibration model had good performance . 
 moreover , the root mean square error of prediction ( rmsep ) of validation set was 0.036429  
 mol / l . 
 the results were very similar to those of offline gas chromatographic analysis , which could prove the method was valid . 
1. Introduction
2. Experimental
3. Results and Discussion
4. Conclusions