Tracking energy scale variations from scan to scan in nuclear resonant vibrational spectroscopy: In situ correction using zero-energy position drifts ΔEi rather than making in situ calibration measurements

Abstract

Nuclear resonant vibrational spectroscopy (NRVS) is an excellent modern vibrational spectroscopy, in particular, for revealing site-specific information inside complicated molecules, such as enzymes. There are two different concepts about the energy calibration for a beamline or a monochromator (including a high resolution monochromator): the absolute energy calibration and the practical energy calibration. While the former pursues an as-fine-as-possible and as-repeatable-as-possible result, the latter includes the environment influenced variation from scan to scan, which often needs an in situ calibration measurement to track. However, an in situ measurement often shares a weak beam intensity and therefore has a noisy NRVS spectrum at the calibration sample location, not leading to a better energy calibration/correction in most cases. NRVS users for a long time have noticed that there are energy drifts in the vibrational spectra’s zero-energy positions from scan to scan ([Formula: see text]Ei), but their trend has not been explored and utilized in the past. In this publication, after providing a brief introduction to the critical issue(s) in practical NRVS energy calibrations, we have evaluated the trend and the mechanism for these zero-energy drifts ([Formula: see text]Ei) and explored their link to the energy scales (αi) from scan to scan. Via detailed analyses, we have established a new stepwise procedure for carrying out practical energy calibrations, which includes the correction for the scan-dependent energy variations using [Formula: see text]Ei values rather than running additional in situ calibration measurements. We also proved that one additional instrument-fixed scaling constant (α0) exists to convert such “calibrated” energy axis (E′) to the real energy axis (Ereal). The “calibrated” real energy axis (Ereal) has a preliminary error bar of ±0.1% (the 2[Formula: see text]E divided by the vibrational energy position), which is 4–8 times better than that from the current practical energy calibration procedure.

Europium-151 and iron-57 nuclear resonant vibrational spectroscopy of naturally abundant KEu(iii)Fe(ii)(CN)6 and Eu(iii)Fe(iii)(CN)6 complexes

“…71 When a universal scaling factor is used to calibrate the energy axis for a sectional (skip) scan, which takes very different time at different data point, the energy axis can be over-scaled, resulting in a slightly higher energy positions for the characteristic peaks. 71…”

“…This can be understood with the concept proposed in a recent publication about NRVS energy re-calibration. 71 In a sectional scan or a skip scan, the energy should be calibrated point by point according to their accumulated scanning time, rather than scaling with a universal factor. 71 When a universal scaling factor is used to calibrate the energy axis for a sectional (skip) scan, which takes very different time at different data point, the energy axis can be over-scaled, resulting in a slightly higher energy positions for the characteristic peaks.…”

“…71 In a sectional scan or a skip scan, the energy should be calibrated point by point according to their accumulated scanning time, rather than scaling with a universal factor. 71 When a universal scaling factor is used to calibrate the energy axis for a sectional (skip) scan, which takes very different time at different data point, the energy axis can be over-scaled, resulting in a slightly higher energy positions for the characteristic peaks. 71 Although the exact peak position cannot be accurately determined due to the skip scan operation, the obtained peak features are sharp and clear as shown in Fig.…”

The True Nature of the Energy Calibration for Nuclear Resonant Vibrational Spectroscopy: A Time-Based Conversion

“…If each beamline can have a fixed α value as above, the corresponding α value is referred to as the energy calibration scale for that particular beamline or that particular HRM. Unfortunately, as mentioned in an earlier publication [40], and as will be detailed in the following, this is not the case: the measured α values are different from beamtime to beamtime and sometimes even from different calibration measurements within one beamtime. For example, the measured α values for SPring-8 BL09XU were once between 0.938 and 0.976 [5].…”

“…According to classical concepts [39], the instrumental scale (ruler) can be calibrated while additional dispersions (instabilities due to environmental differences) are attributed to random/un-calibratable source(s). However, not all the dispersions are imprecisions and at least some energy instabilities are also trackable and "calibratable" [40]. Therefore, there should be two categories of energy calibration for a monochromator, spectrometers, or other instruments.…”