Preparation

Darius Goergen

Last updated: 2019-08-19

Checks: 7 0

Knit directory: polymeRID/

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Reference Data

For this project we used a data base published by Primpke et al. (2018) online. The data base can be downloaded here. The authors state the samples were collected based on the FTIR-spectrometer Bruker Tensor 27 System for the spectral range 4000 to 40 1/cm. Additionally, some data of polymer-based fibers and spectra of biological origins were received from the Bremer Faserinstitut. During preprocessing, they applied a concave rubberband correction based on 10 iterations and 64 baseline points. They also excluded the C02 band between 2420 to 2200 1/cm by setting the data points to 0. This should be kept in mind, since the inclusion of additional reference samples requires the same procedure for the data base to stay in a consistent state. The data provided by Primpke et al. (2018) shows a spectral resolution of 2.1 1/cm. Additional reference samples need to be resampled to the same spectral resolution and the same baseline correction should be applied.

To ensure consistency, the data base was read into R and the wavenumbers were saved in a separate file for the future use of resampling additional reference spectra.

library(openxlsx)
url = "https://static-content.springer.com/esm/art%3A10.1007%2Fs00216-018-1156-x/MediaObjects/216_2018_1156_MOESM2_ESM.xlsx"

data = openxlsx::read.xlsx(url)
# extract wavenumbers from first row
wavenumbers = as.numeric(names(data)[2:1864])
# saving wavenumbers to reference sample directory
saveRDS(wavenumbers, paste0(ref, "wavenumbers.rds"))

An important feature of any data base is the distribution of the different classes. Here, we only print the 20 most common classes, since there are a lot of reference samples only found once or twice within the data base.

data$Abbreviation = as.factor(data$Abbreviation)
summary(data$Abbreviation)[1:10]
  PES    PP  LDPE  HDPE   PET    PE Nylon    PA    PS   PUR 
   15    12    11    10     9     8     7     7     7     7 

Construction of the Data Base

We are interested in assigning the correct class to potential plastic particles. The most important classes found in the data base to us are thus the ones of artificial polymer origin. However, sometimes also particles of biological origin will be subject to a spectral analysis, because they resemble the appearance of microplastics in environmental samples. Any machine learning algorithm trained only with reference samples from plastics would eventually assign a plastic-class also to the particles of biological origin. It will just assign the class with the greatest similarity to the classes it learned. This can lead to so-called false positive errors. To reduce the error of false positives, we include some of the samples of biological origin as well. We summaries these samples to broader classes.

# furs and wools
indexFur = grep("fur", data$Abbreviation)
indexWool = grep("wool", data$Abbreviation)
furs = data[c(indexFur, indexWool), ]
furs = furs[ , c(2:1864)] # leave out index column
names(furs) = paste("wvn", wavenumbers, sep="")
furs$class = "FUR"

# fibres
indexFibre = grep("fibre", data$Abbreviation)
fibre = data[indexFibre, ]
fibre = fibre[ , c(2:1864)] # leave out index column
names(fibre) = paste("wvn", wavenumbers, sep="")
fibre$class = "FIBRE"

# wood
indexWood = grep("wood", data$Abbreviation)
wood = data[indexWood, ]
wood = wood[ , c(2:1864)] # leave out index colums
names(wood) = paste("wvn", wavenumbers, sep="")
wood$class = "WOOD"

# synthetic polymers
polyIndex = which(data$`Natural./Synthetic` =="synthetic polymer")
syntPolymer = data[polyIndex,]
counts = summary(syntPolymer$Abbreviation)
polyNames = names(counts)[1:10] # only major polymers
syntPolymer = syntPolymer[which(syntPolymer$Abbreviation %in%  polyNames) , ]
classes = droplevels(syntPolymer$Abbreviation)
syntPolymer = syntPolymer[ , c(2:1864)] # leave out index column
names(syntPolymer) = paste("wvn",wavenumbers,sep="")
syntPolymer$class = as.character(classes)

# lets group together some synthetic polymer classes
syntPolymer$class[grep("Nylon",syntPolymer$class)] = "PA"

Class Distribution

We now bind the reference samples together and take a look at the distribution of classes in the resulting data frame, which is the concrete data base for the following calculations.

data = rbind(furs,wood,fibre,syntPolymer) 
data$class = as.factor(data$class)
summary(data$class)
FIBRE   FUR  HDPE  LDPE    PA    PE   PES   PET    PP    PS   PUR  WOOD 
   27    23    10    11    14     8    15     9    12     7     7     4 

In total, 93 (53%) reference samples of plastic polymers are present in the data base and 44 (47%) of biological origin. Within the plastic samples, we find that the data is very balanced with no single class showing less than 7 samples. For the samples of biological origin, however, the class FIBRE dominates the distribution. This could prove as an disadvantage if a machine learning algorithm picks up this unbalance by minimizing its error rate simply by more frequently predicting the FIBRE class. At this point, we will leave the resulting data base as it is and save it to disk. We save the data in individual files as well as in a comprehensive data base in csv format. This way we ensure that later extensions to the data base are easy to manage.

write.csv(data, file = paste0(ref, "reference_database.csv"), row.names=FALSE)

# writing class control file
classIndex = as.character(unique(data$class))

for (class in classIndex){
  tmp = data[data$class==class , ]
  write.csv(tmp, file = paste0(ref, "reference_", class, ".csv"), row.names=FALSE)
}

write(classIndex, paste0(ref, "classes.txt"))

Spectra of Reference Samples

Below one can find the spectra found within the data base. By selecting a class in the drop-down menu the plot of the respective class will be shown. The solid grey line in the center of the plot indicates the mean value for all samples for the respective wavenumbers. The grey ribbon indicates the standard deviation from that mean value while the dashed lines show the minimum and the maximum found in the data base.

Citations on this page

Primpke, Sebastian, Marisa Wirth, Claudia Lorenz, and Gunnar Gerdts. 2018. “Reference database design for the automated analysis of microplastic samples based on Fourier transform infrared (FTIR) spectroscopy.” Analytical and Bioanalytical Chemistry 410 (21). Analytical; Bioanalytical Chemistry: 5131–41. https://doi.org/10.1007/s00216-018-1156-x.


sessionInfo()
R version 3.6.1 (2019-07-05)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Linux Mint 19.1

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.7.1
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.7.1

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
 [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
 [5] LC_MONETARY=de_DE.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=de_DE.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=de_DE.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] tensorflow_1.14.0         abind_1.4-5              
 [3] e1071_1.7-2               keras_2.2.4.1            
 [5] workflowr_1.4.0.9001      baseline_1.2-1           
 [7] gridExtra_2.3             stringr_1.4.0            
 [9] prospectr_0.1.3           RcppArmadillo_0.9.600.4.0
[11] openxlsx_4.1.0.1          magrittr_1.5             
[13] ggplot2_3.2.0             reshape2_1.4.3           
[15] dplyr_0.8.3              

loaded via a namespace (and not attached):
 [1] reticulate_1.13  tidyselect_0.2.5 xfun_0.8         purrr_0.3.2     
 [5] lattice_0.20-38  colorspace_1.4-1 generics_0.0.2   htmltools_0.3.6 
 [9] base64enc_0.1-3  yaml_2.2.0       rlang_0.4.0      later_0.8.0     
[13] pillar_1.4.2     glue_1.3.1       withr_2.1.2      foreach_1.4.7   
[17] plyr_1.8.4       munsell_0.5.0    gtable_0.3.0     zip_2.0.3       
[21] codetools_0.2-16 evaluate_0.14    knitr_1.24       SparseM_1.77    
[25] tfruns_1.4       httpuv_1.5.1     class_7.3-15     highr_0.8       
[29] Rcpp_1.0.2       xtable_1.8-4     promises_1.0.1   scales_1.0.0    
[33] backports_1.1.4  jsonlite_1.6     mime_0.7         fs_1.3.1        
[37] digest_0.6.20    stringi_1.4.3    shiny_1.3.2      grid_3.6.1      
[41] rprojroot_1.3-2  tools_3.6.1      lazyeval_0.2.2   tibble_2.1.3    
[45] crayon_1.3.4     whisker_0.3-2    pkgconfig_2.0.2  zeallot_0.1.0   
[49] Matrix_1.2-17    assertthat_0.2.1 rmarkdown_1.14   iterators_1.0.12
[53] R6_2.4.0         git2r_0.26.1     compiler_3.6.1