X-band rapid-scan EPR of nitroxyl radicals Deborah G. Mitchell b , Richard W. Quine a , Mark Tseitlin b , Sandra S. Eaton b , Gareth R. Eaton b,⇑ a School of Engineering and Computer Science, University of Denver, United States b Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80208, United States article info Article history: Received 8 October 2011 Revised 9 November 2011 Available online 20 November 2011 Keywords: Deconvolution Nitroxyl radicals Rapid scan EPR Resonator Q abstract X-band rapid-scan EPR spectra were obtained for dilute aqueous solutions of nitroxyl radicals 15 N-mHCTPO (4-hydro-3-carbamoyl-2,2,5,5-tetra-perdeuteromethyl-pyrrolin-1- 15 N-oxyl-d 12 ) and 15 N-PDT (4-oxo- 2,2,6,6-tetra-perdeuteromethyl-piperidinyl- 15 N-oxyl-d 16 ). Simulations of spectra for 15 N-mHCTPO and 15 N-PDT agreed well with the experimental spectra. As the scan rate is increased in the rapid scan regime, the region in which signal amplitude increases linearly with B 1 extends to higher power and the maximum signal amplitude increases. In the rapid scan regime, the signal-to-noise for rapid-scan spectra was about a factor of 2 higher than for unbroadened CW EPR, even when the rapid scan spectra were obtained in a mode that had only 4% duty cycle for data acquisition. Further improvement in signal-to-noise per unit time is expected for higher duty cycles. Rapid scan spectra have higher bandwidth than CW spectra and therefore require higher detection bandwidths at faster scan rates. However, when the scan rate is increased by increasing the scan frequency, the increase in noise from the detection bandwidth is compensated by the decrease in noise due to increased number of averages per unit time. Because of the higher signal band- width, lower resonator Q is needed for rapid scan than for CW, so the rapid scan method is advantageous for lossy samples that inherently lower resonator Q. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Rapid scan is an electron paramagnetic resonance (EPR) method in which the magnetic field is scanned through resonance in a time that is short relative to T 2 [1,2], and the absorption and dispersion signals are recorded by direct detection. Phase-sensitive detection at the magnetic field modulation frequency is not used. Oscilla- tions are observed on the trailing edge of the signal. Rapid scan sig- nals obtained with either triangular or sinusoidal scans can be deconvolved to give the conventional spectra [3,4]. Previously, rapid field scan EPR experiments were performed at 250 MHz (VHF) on a triarylmethyl (trityl) radical (T 2  11.5 ls) and on deoxygenated lithium phthalocyanine (LiPc) (T 2  2.5 ls) [4–7]. Because of the relatively long T 2 values for these samples, rapid scan conditions could be achieved with scan rates as low as 6 kG/s. The advantages of rapid scan EPR at X-band for the E 0 center in irradiated fused quartz (T 2  100 ls) [8] and for a,c-bisdiphenyl- ene-b-phenylallyl (BDPA, T 2  100 ns) [9] have been demonstrated. This paper reports the application of X-band rapid scan EPR to nitroxyl radicals. Nitroxyl radicals were selected for study because of their widespread use as spin probes in biomedical applications [10,11]. The two prior rapid field scan reports for nitroxyl radicals were at 250 MHz [12,13]. Rapid frequency scans for nitroxyls have been reported at W-band [14]. In dilute deoxygenated aqueous solutions at ambient temperature, nitroxyl radicals have T 2 values in the range of 500–700 ns [15]. These values of T 2 are much shorter than for trityl, LiPc, or irradiated fused quartz samples, so the scan rates required to achieve the rapid scan regime for nitroxyls are much faster. Sinusoidal scans with resonated coils allow the magnetic field to be swept at much higher rates than with triangular scans [9], which allows samples with shorter T 2 to be measured in the rapid scan regime. The nitroxides selected for study were 15 N-PDT (4-oxo-2,2,6,6-tetra-perdeuteromethyl- piperidinyl- 15 N-oxyl-d 16 ), because of its narrow lines [15], and 15 N-mHCTPO (4-hydro-3-carbamoyl-2,2,5,5-tetra-perdeutero- methyl-pyrrolin-1- 15 N-oxyl-d 12 ), because of its utility for oximetry imaging [16]. 2. Methods 2.1. Sample preparation 15 N-PDT with 98% isotope purity was purchased from CDN Isotopes (Quebec, Canada). 15 N-mHTCPO was prepared as previ- ously described [17] and provided by Prof. Halpern (University of Chicago). Solutions in 80/20 EtOH/H 2 O were 0.2 mM for 15 N-PDT and 0.1 mM for 15 N-mHTCPO. These concentrations are in a range where the contribution to relaxation from collisions is very small [15]. The samples, in 4 mm o.d.  3 mm i.d. quartz tubes, had heights of 3 mm, resulting in 3  3 mm cylindrical shapes. The 1090-7807/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2011.11.007 ⇑ Corresponding author. Fax: +1 303 871 2243. E-mail address: geaton@du.edu (G.R. Eaton). Journal of Magnetic Resonance 214 (2012) 221–226 Contents lists available at SciVerse ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr