A comparative study of supervised machine learning algorithms for tracking copper using optical emission spectroscopy of solution cathode glow discharge

Abstract This paper compares the performance of five different supervised machine learning (SL) algorithms to identify the practical model for tracking copper using solution cathode glow discharge optical emission spectroscopy (SCGD-OES). A total of 4500 SCGD-OES spectra were collected, corresponding to 15 different copper concentrations ranging from 10.3 mg l −1 to 59.6 mg l −1 , for training and validating five models: K-nearest neighbors (KNN), random forest (RF), artificial neural network (ANN), convolutional neural network (CNN), and recurrent neural network (RNN). Then, performance of these models were evaluated based on accuracy, precision of predictions and training time. In terms of accuracy, the relative error (RE) was 3% for the CNN model and 4% for the ANN model. Conversely, the RE increased from 21% and 43% for the RF and RNN models to 103% for the KNN model due to the unpredictability of Cu concentration outside the training range. Regarding precision, the relative standard deviation (RSD) values were similar across all models, with the lowest RSD being 3% for the CNN model and the highest being 5% for the RF model. The training time indicated that the KNN and RF models provided the best learning speed, while the CNN model had the slowest learning speed, taking 5 times longer than the ANN model. The accuracy of the ANN and CNN models is in agreement with inductively coupled plasma-optical emission spectroscopy (ICP-OES), with a average difference not exceeding 6% when tracking six different Cu concentrations, both inside and outside the training range. Additionally, this research investigated the weight distribution in the hidden layers of the ANN, CNN, and RNN models and feature importances in the RF model to enhance understanding of the internal structure of models and assist analysts in selecting the practical model for tracking heavy metals based on SCGD-OES.