Spectrometer Throughput Considerations

The signal-to-noise ratio (SNR) of FTIR spectrometers is direcdy proportional to the optical throughput (ff). For measurements at high resolution (approximately 2 cm ), throughput is determined by the allowable divergence of the infrared 

Scattering, reflection from window materials, and absorption by solvent contribute to strong radiation losses during infrared spectroelectrochemical measurements. Therefore, high-throughput instrument designs are generally recommended for these types of measurements [35]. 

The radiant power that reaches the detector depends in part upon the properties of the source. The spectral radiance of the source (Bp) can be approximated by Planck s radiation law  

The noise equivalent power (NEP) and specific detectivity (D ) are figures of merit that express the sensitivity of infrared detectors. The NEP is the root-mean-square (rms) power in a sinusoidally modulated radiation signal incident on the detector that gives a response equal to the rms dark noise in a 1-Hz 

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A considerable amount of time is necessary to reach the point at which sample analyses can commence, and it is essential that the stability and reliability of the mass spectrometer be high to ensure maximum sample throughput during the limited time available between calibration checta. 

If nontarget analysis should be realized, high-resolution mass spectrometer such as QTOF or Orbitrap would facilitate the analysis considerably. Even if a high sample throughput is still necessary, the QTOF would get precedence over the slow Orbitrap in high-resolution mode. However, regarding the resolution Orbitrap, in comparison with QTOF, is the more powerful system. The sensitivity of a QTOF is about a factor 10 lower than that of a triple quad, but detection limits in the lower parts per billion range are quite possible. 

For FT-IR spectrometers where the throughput is limited by the detector foreoptics (the constant throughput case discussed in Section 7.2), Jacquinot s advantage may be considerably less than the value calculated from Eq. 7.18. For example, if the resolution is changed from 2 cm to 8 cm, the throughput of the grating spectrometer can increase by a factor of 16 while / remains unchanged [9]. 

The systems reported above all rely on detection by ultraviolet absorbance, and require some separate means of identifying the desired product among the various components (and thus collected fractions) of the reaction mixture. Efficiency and throughput can be considerably enhanced by real-time detection and identification of products. Two groups have now reported preparative HPLC systems where fractionation decisions are based upon output from a mass spectrometer detector. The first preliminary report of such a system came from a collaboration between CombiChem and Sciex [21]. This system bases fractionation decisions on the output of a single ion mass chromatogram for the predicted molecular ion of the desired product. Reverse phase preparative HPLC is used in conjunction with electrospray ionization mass detection. A full report of this work has appeared in early 1998 [22]. The second report of such a system came from a collaboration between Pfizer and Micromass [23]. This system uses a flexible combination of UV and/or ion chromatograms to control fractionation. Unlike other systems, fractionation parameters are set by the mass spectrometer control software. Variants of both of the above systems will probably become commercially available in late 1997 [24]. 

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