Advantages of FT – NMR over Continuous Wave NMR

Nuclear Magnetic Resonance helps provide structural details on molecules. It is based on transitions between nuclear spin states of NMR active nuclei by positioning the sample in an external magnetic field. Observations have been made on Continuous wave (CW) and FT – NMR techniques. The benefits of FT – NMR over CW – NMR are discussed in the present article. However, before discussing the benefits it is important to have a clear understanding of the two techniques.

Continuous wave NMR

Continuous wave NMR uses a fixed magnetic field and sweeping of frequency by varying the current in a frequency coil to achieve resonance absorption signals. CW -NMR spectrometer finds routine use for 1H NMR studies at 60 MHz. They are commonly seen in university laboratories as they require low maintenance and operational costs. Such instruments require only water cooled electromagnets unlike the liquid helium cooled high-speed superconducting magnets required for FT – NMR spectrometers.

Fourier transform NMR (FT – NMR)

The Fourier transform technique offers similar advantages as in case ofFT - IR.。磁场中的核暴露于持续时间短（约10μs）的射频辐射脉冲。脉冲持续时间足以激发所有NMR活性核的存在。连续脉冲之间的延迟允许所有兴奋的细胞核放松回到他们的地面状态。放松被称为自由诱导衰减。利用垂直于静态磁场的RF线圈检测自由感应衰减。在一段时间内从多次连续脉冲从多次的连续脉冲中添加这些信号以提高信噪比。使用傅里叶变换功能，从时域转换为频域的自由感应延迟响应导致类似于CW光谱的频谱记录。

FT - NMR的好处

Time-saving

NMR active nucleipresent in the molecule are scanned simultaneously and this results in greater useful structural information from samples containing nuclei other than 1H.

更高的灵敏度

Co-addition of spectral response over a given time period improves signal-to-noise ratios and this leads to improved sensitivity. Unlike CW – NMR which is used primarily for 1H studies NMR information on other nuclei like 13 C, 31 P and 19 F give poor response due to their lower isotopic abundance. FT- NMR affords greater sensitivity for studies on such nuclei due to signal averaging. Remarkable increase in sensitivity often results in useful information from the limited available sample amounts.

FT-NMR仪器虽然运行昂贵，但维持有丰富的信息，而CW仪器是不可能的。如今，大多数制造商已停止CW仪器的制造。