Transcend your calibration transfer woes in 10 steps (with Transpec)

A New Year means new beginnings! Heading into 2022 we’ve added some new members to our group: Isaac Rukundo and Kristen Frano are two product specialists who have recently joined the BUCHI NIR team! They contributed to the content of this post.

We’re now amid that post-holiday trudge! Were you good enough last year for Santa to bring you a new NIR system for Christmas to replace your old one? If you were, you’re probably excited about upgrading your instrument for a newer model. But for close readers of our blog who have learned about the importance of robust calibration models, perhaps you’re feeling overwhelmed by the intimidating task of starting NIR calibration development all over again! (We wouldn’t blame you).

Once robust calibrations are developed for NIR instruments, measurements are fast and simple to operate. But initial calibration development can indeed be a daunting challenge; for some products, it can take months or even years to acquire a data set with a sufficient calibration range and sample variation. That’s why – instead of starting calibration development from scratch – it is preferred to transfer your hard-earned existing calibrations to your shiny new NIR system so you can start collecting data ASAP. Let’s discuss what BUCHI NIR calibration transfer looks like.

NIR calibrations are usually developed on a specific instrument. Essentially this means that each calibration model will have its own spectral range, spectral resolution, and noise built into it. The ideal scenario for calibration transfer is to transfer calibrations between units of similar hardware configurations and specifications.

Before the addition of the ProxiMate to the BUCHI NIR product line, BUCHI NIR systems have traditionally been Fourier transform-NIR benchtop instruments (NIRFlex N-500 and NIRMaster). The ProxiMate instead uses a photodiode array with a fixed diffraction grating instead of an interferometer to disperse light into its characteristic wavelengths (you can read more about these hardware differences in our previous blog post Finding the right fit: FT vs Dispersive NIR).

While the diode array technology of the ProxiMate means that it is more rugged for its intended non-laboratory environments than FT-NIR systems, the tradeoff is (there’s always a tradeoff!) that it can’t match the spectral range and resolution of the high-end FT-NIR systems. Nonetheless,  calibrations developed on FT-NIR systems will typically perform equally well on diode array systems. We can use basic statistical figures of merit, such as standard error of prediction (SEP) to judge these how well these calibrations transfer. However, it is important to note that although calibrations developed on a high-end FT-NIR instrument can usually be transferred to either another instrument of equivalent quality or to a diode array system, the transfer of a calibration developed on a dispersive NIR system to an FT-NIR system is less likely to be successful. Long story made short: you can transfer from high-end to low-end, but not from low to high.

The tool that BUCHI NIR uses for calibration transfer is Transpec. Transpec enables calibrations to be transferred from one NIR system to another. In this example, let’s consider transferring a calibration from a BUCHI FT-NIR (Instrument A) to a ProxiMate system (Instrument B). The workflow of Transpec to transfer calibrations from one system to another is:

  1. Select at least 20 representative samples with a wide range of property reference values.
  2. Assign IDs to each of these 20 samples (i.e. “Sample1”, “Sample2”…)
  3. Measure the 20 samples on Instrument A (e.g. a NIRFlex N-500). Make sure to assign the IDs from Step 2 to the correct spectral measurements.
  4. Export the spectral data from Instrument A and save the files to a known location.
  5. Measure the 20 samples again, this time on Instrument B (here, the ProxiMate). Make sure to assign the IDs from Step 2 to the correct spectral measurements.
  6. Export the spectral data from Instrument B and save the files to a know location.
  7. Provide a spreadsheet that identifies the respective sample IDs from each instrument. They should be easily identified to allow the linking of data from Instrument A to that from Instrument B (see Table 1, below).
  8. Send a folder containing the two sets of spectral files, the spreadsheet, and all calibrations for the properties on Instrument A that are to be transferred to Instrument B (e.g. the calibration models for protein, ash, and moisture in corn flour).
  9. BUCHI will apply Transpec to the data.
  10. You will receive transferred applications that are ready to be used on your new Instrument B!

Table 1. Example sample table.

Figure 1 (below) shows an example of Transpec, specifically the transfer of a moisture calibration from an FT-NIR instrument to a ProxiMate. The graph shows the prediction results of the samples from a NIRFlex N-500 versus the prediction results of the samples measured on a ProxiMate system. After Transpec is used to process the data, the results from the ProxiMate are in good agreement with the results from the NIRFlex N-500.

Figure 1. FT-NIR prediction data compared with ProxiMate NIR data. The blue series shows the data unadjusted, while the green series shows the data after adjustment with Transpec.

Transpec is also not limited to transferring calibrations between BUCHI NIR systems. Existing calibrations from a non-BUCHI NIR unit can be transferred to a BUCHI NIR unit as long as the calibrations are available in a  compatible file format (preferably JCAMP). The Transpec workflow would be similar to the aforementioned steps.   

Just like you can’t wait to open a new gift on Christmas morning, you probably can’t wait to start using a new instrument to its fullest extent! Transpec is a valuable tool, enabling years of calibration data collected on an existing instrument to be transferred to a new instrument so that you can start using it right away. Do you still have questions about transfer of your existing calibrations? Contact us!

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