Getting Started with Indirect Hard Modeling

This is a tutorial for PEAXACT software for quantitative spectroscopy from S-PACT. Its main objective is to get you familiar with one of the more complex though powerful spectra analysis methods: Indirect Hard Modeling (IHM). The tutorial addresses to PEAXACT users and persons interested in PEAXACT.

In this tutorial you learn how to:

  1. Create a Hard Model using IHM
  2. Calibrate the model
  3. Use the model to predict concentrations of unkown spectra

Data files used during the tutorial are available at the PEAXACT installation directory. If you have PEAXACT installed on your computer you may try this tutorial right away. If you don't have PEAXACT yet, get a free trial now.

Note that this tutorial was created for PEAXACT 3 and may look different in the newest version!

Preparations

  • Choose File > Preferences from the menu which opens the application settings window.
  • From the pulldown menu in the top-left corner select Raman, then click OK.

Pure Component Modeling

The first step is to create a new hard model for each pure component.

  • Start PEAXACT.
  • Choose Data > Load Table from the menu.
  • Browse to [PEAXACT Installation directory]\data\Raman - Mixing\calibration.
  • Select the Excel file dataTablePureComponents.xls and continue, which will load pure component spectra of Cyclohexene, Dioxane, and Toluene.
  • Select Cyclohexene.csv#1 in the Data Sets Panel.
  • Now, there a several ways to create a new Hard Model for the selected data set. For pure components, the easiest way is to right-click in the Data Sets Panel and choose New Hard Model for Active Sample from the context menu. This automatically creates a new model containing a Hard Model with a single component model named after the sample.
  • Choose Edit Model > Hard Model > Add & Autofit Peaks from the menu.
  • Enter 11 and continue. Peaks will always be added to the Hard Model's first component. You can always add more peaks if needed.
  • Choose File > Save Model from the menu to save the pure component model.
  • Select the next item in the Data Sets Panel and repeat the model building steps above. You may need a different number of peaks in order to fit the data well.
Component# Peaks
Cyclohexane11
Dioxane15
Toluene16
Recommended number of peaks to be fitted to each component spectrum
E.g., your pure component model of Toluene should look like this

Mixture Modeling

The second step is to create the hard model of the mixture by combining pure component models from before.

  • Choose Data > Load Table from the menu.
  • Browse to [PEAXACT Installation directory]\data\Raman - Mixing\calibration.
  • Select the Excel file dataTableCalibration.xls and continue, which will load 8 mixture spectra.
  • Select the first mixture (Mixture1-1.csv#1) in the Data Sets Panel.
  • Choose File > New Model from the menu.
  • Choose Edit Model > Hard Model > Import Components from the menu. You will be notified that a new Hard Model is added to the model.
  • Browse to the location where you saved the pure component models, select all three model files and continue.
  • The components are imported/copied to the new model.
  • Choose Edit Model > Hard Momdel > Adjust Components > W/S/B and Full Peaks from the menu. This fits the model to the selected sample and adjusts all model parameters, i.e. W/S/B parameters (component weight parameter, component shift parameter, baseline parameters) and peak parameters (position, amplitude, width, shape). Actually, this fitting step is optional. You can use it to change initial model parameters permanently, but since the hard model is later fitted individually to each mixture spectrum anyway, you don't have to do it.
  • Choose File > Save Model from the menu and save the mixture model.
Your mixture model should look like this if you did decide to adjust the components

Calibrate the Model

In order to calibrate the model, you need to provide reference concentrations for some mixture spectra. You can either enter reference values using the PEAXACT Data Table Editor or import reference values from an Excel file. Have a look at the Excel Data table you loaded before. The table did already contain the required reference concentrations.

  • Select all data sets in the Data Sets Panel.
  • Choose Edit Model > Calibration > Calibrate... from the menu which opens the calibration setup dialog.
    • Select a Indirect Hard Modeling (IHM) as calibration method (should be selected automatically)
    • Select all 3 components for calibration. For IHM calibration, you have to link each predictive feature (response variable) to a component of the hard model. If the model's component names match the feature names, components are linked automatically. Have a look at the Excel Data table you loaded before. The table contains both, feature names and concentration values in unit molar fraction [mol/mol].
    • Select ratios for the regression variables. You should use ratios if you want to calibrate relative concentrations like molar fractions or mass fractions. In other cases, choose normal.
Calibration Settings Dialog
  • Confirm the settings to start calibration. The hard model is fitted to all selected data sets and the calibration curve is calculated. When finished, calibration results are displayed in a result window. Use it to examine typical figures of merit and evaluate the calibration.
Calibration Report Dialog
  • Normally, in the bottom-left panel you would have to decide upon on a certain calibration function for each component. When choosing ratios for regression variables, though, the only option here is simple.
  • Click OK to accept the calibration.
  • You will be prompted to save a calibration report file. You can skip this for now and get the report later via File > Export > Model Summay.
  • Finally, choose File > Save Model to save the calibration.

Analysis

It is time to analyse some new spectra.
  • Choose Data > Unload All from the menu.
  • Choose Data > Load from the menu.
  • Browse to [PEAXACT Installation directory]\data\Raman - Mixing\analysis.
  • Select all data files and continue, which will load 8 mixture spectra. These spectra were taken from the same mixing process than the calibration spectra, i.e. they should have the same concentratios, but they were measured on a separate day.
  • Select all data sets in the Data Sets Panel and choose Analysis > Prediction from the menu.
  • When finished, predicted concentrations are displayed in a result window. You can now evaluate the results and export them by clicking Export.
 

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