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iS5 Instrument Will Not Pass EP Wavenumber Accuracy Test with ATR accessory without Advanced ATR Correction Applied


iS5 Instruments will not pass EP Wavenumber Accuracy Test with ATR accessory without Advanced ATR Correction applied


  • Nicolet iS5 (with iD7 accessory)
  • Nicolet iS10
  • Nicolet iS50


The iS10 and iS50 FTIR spectrometers have a more advanced optical beam path when compared to the iS5. The iS10 and iS50 utilize variable apertures to improve the collimation of the IR beam throughout the instrument. This improved collimation yields improved wavelength accuracy, especially when utilizing an ATR accessory.

Wavelength accuracy is impacted by the optical design of the ATR accessory. When an ATR accessory is inserted into the sample compartment, the peaks shift to a slightly lower wavelength. While the variable aperture inside the iS10 and iS50 often keeps that wavelength shift from falling out of the European Pharmacopeia guidelines, our iS10 and iS50 ValPro methods utilize an advanced ATR correction to account for the optical differences between an ATR and a transmission measurement.

We do not currently offer a ValPro certification for the ATR accessory on the iS5. Applying an advanced ATR correction to the iS5 data, however, can cause peak values to fall within the EP tolerance, as shown below.

The data in the attachment shows the impact of advanced ATR correction on the polystyrene data collected on your iS5 spectrometer with an iD7 accessory. We have included peak locations before and after the ATR correction.


When using the ATR Thick Polystyrene Standard, two of the four wavenumber accuracy tests did not meet EP wavenumber accuracy acceptance criteria. The Thick Polystyrene Standard (NIST traceable, Thermo Fisher part number 840-166700) was measured on two different iS5 instruments and it was found that the same bands failed the criteria. Specifically the thick polystyrene failed to meet the criteria for the bands at 3060.0 cm-1 and 2849.5 cm-1.

EP requirements are specifically designed for the transmission experiment. Acceptance criteria used by the European Pharmacopoeia (criteria shown below) was performed on the iS5 and passed in the transmission mode. The same test was applied to an ATR accessory in the sample compartment of the iS5 and it failed two of the four wavenumber acceptance criteria. This same criteria is then applied to an ATR accessory with the ATR Polystyrene Standard for the iS10, iS50 and Avatar FTIR models and they all passed the wavenumber criteria..


The fundamental issue with the iS5 instruments is that the transmission accessory meets regulatory requirements for a 1.5 mil polystyrene standard (transmission); however, with the thick ATR polystyrene standard used on the ATR accessory, it does not provide equivalent results.