Spin concentration calibration standards

Cage B, Weekley A, Brunei LC, Dalai N. K3CrOg in KjNbOj as a proposed standard for g-factor, spin concentration, and field calibration in high-field EPR spectroscopy. Anal Chem 1999 71 1951-7. 

The use of NMR spectroscopy as an analytical technique is well established ( 1 8). In order to quantitate our spin-echo height to the number of protons present, we performed an independent calibration using standard solutions of naphthalene in carbon tetrachloride. Concentrations for the standards were chosen to correspond to the anticipated supercritical C02 solubilities, and all calibration measurements were performed using a sample cell of the same dimensions as the solubility sample cell previously described. The response of our spectrometer to the standard solutions was linear over the concentration range. The reproducibility for independent measurements of the calibration curve was 3 . Throughout the experiment, all spectrometer conditions (pulse lengths, phases, receiver amplifier gain, etc.) were closely monitored, and frequent checks on the calibration of the spectrometer were performed. In this way we were able to obtain the molar solubility of solid naphthalene in supercritical carbon dioxide to an estimated experimental accuracy of 6%. 

A glance at the sensitivity eq. (3-1) should convince us to shy away from absolute spin measurements and compromise with the "calibration" technique using suitable primary or secondary standards. This becomes relatively straightforward if suitable primary standards with accurately known concentration can be found. The comparison is based on the fact that the area under the ESR absorption is directly proportional to the number of spins, provided the absorption is not saturated and all variable spectrometric parameters remain constant. In practice, two major groups of problems may develop. 

Measurements of the concentrations are most accurately performed by comparison with a standard sample. A double cavity can be used, with the sample in one half, the reference in the other. Other designs have a built in reference that is recorded simultaneously with the sample. Absolute calibration is complicated and large deviations often occur in comparisons between different laboratories. The highest reported sensitivity of commercial instruments is ca. 10 AB spins, where AB is the width of the signal in mT. This applies to instruments operating at the normal X-band wavelength, k = 3 cm. As mentioned the sensitivity increases at higher frequencies, but the sample volume decreases, and the instrument also... 

The colloidal state inevitably brings about difficulties for the experimentalist when separation of the disperse phase from the dispersion medium is needed. This is the case when the speciation and concentration of only the free soluble species have to be determined. Separation of the ionic solution from the small colloidal particles for conventional chemical analysis is nontrivial, although separation techniques such as ultracentrifugation, dialysis, and field-flow fractionation have been successfully used. If the soluble species of interest have an active nuclear spin, the liquid NMR technique wiU constitute an alternative and simpler way to characterize and quantify those species without being affected by the disperse phase. An exception is the case where the colloidal species gives a signal that fully overlaps the sharp resonance of the solution entity. As NMR is quantitative, the absolute concentration of the species can be estimated based on an internal reference of known concentration but different chemical shift relative to the sample signals. Alternatively, a calibration curve can be established from a set of external standard solutions (preferably the same substance found in the sample) measured under the same experimental NMR conditions as those applied to the sample. 

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