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| Author(s): |
Fabiana Diuk de Andrade
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Carlos. |
| Institution: | Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT) |
| Defense date: | 2011-05-20 |
| Examining board members: |
Luiz Alberto Colnago;
Eduardo Ribeiro de Azevêdo;
Hidetake Imasato
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| Advisor: | Luiz Alberto Colnago |
| Abstract | |
Low Resolution Nuclear Magnetic Resonance (LR-NMR) has been applied in quality control and certification in industry. To speed up the analysis the online NMR has been proposed. Despite LR-NMR online potential, some problems related to equipment overload occurs. It is due to methodologies used that can cause an increase in sample temperature and consequently errors in measurements as well reduction on equipment durability. Thus, Carr-Purcell, CP and Meiboom-Gill, CPMG using low refocusing flip angles (LRFA) were developed. It was observed that LRFA as low as 45° (CPMG45) can provide transverse relaxation time values (T2) with errors below 5% for homogeneous field (Δν ≤ 15Hz). For a less homogeneous magnetic field (Δν ≥ 100 Hz) the choice of the LRFA has to take into account the reduction in the intensity of the CPMG signal and the increase in the time constant of the CPMG decay that also becomes dependent on longitudinal relaxation time (T1). The Carr-Purcell (CP) pulse sequence, with LRFA, produces echoes midway between refocusing pulses that decay to a minimum value dependent on T2* (Free Induction Decay time constant - FID). When τ > T2*, the signal increased to reach a Steady-State Free Precession regime (SSFP) after the minimum value and was composed of FID signal after each pulse and an echo, before the next pulse. CP90 signal increased from the minimum value to the steady-state regime with a time constant (T*) = 2T1T2/(T1+T2), identical to the time constant observed Continuous Wave Free Precession (CWFP). The Steady-State amplitude obtained with CP90 (MCP90) = M0T2/(T1+T2) was identical to CWFP. Therefore, this sequence was named CP-CWFP because it is a CP sequence that produces results similar to the CWFP. However, CP-CWFP is a better sequence than CWFP for measuring the longitudinal and transverse relaxation times in single scan, when the sample exhibits T1 ~ T2. When phase alternation is applied in CPMG90AF (y\'/-y\') e CP90AF (x\'-x\') the signal reaches a Steady-State. CPMG90AF and CP90AF showed similar behavior to CP-CWFP, so one more alternative method to measure T1 and T2. Therefore, the T2 measurements can be performed with 90° refocusing pulses (CPMG90), which use only 25% of the RF power used in conventional CPMG. This reduces the heating problem in the probe and reduces the power deposition in the samples. The CP-CWFP sequence can be a useful method in low-resolution NMR and can be widely used in the agriculture, food and petrochemical industries because those samples tend to have similar relaxation times in low magnetic fields. (AU) | |