Modulation Zeeman

Bishop [75] determined barium in seawater by direct injection Zeeman-modulated graphite furnace atomic absorption spectrometry. The V203/Si modifier added to undiluted seawater samples promotes injection, sample drying, graphite tube life, and the elimination of most seawater components in a slow char at 1150-1200 °C. Atomisation is at 2600 °C. Detection is at 553.6 nm and calibration is by peak area. Sensitivity is 0.8 absorbance s/ng (Mo = 5.6 pg 0.0044 absorbance s) at an internal argon flow of 60 ml/min. The detection limit is 2.5 pg barium in a 25 ml sample or 0.5 pg using a 135 ml sample. Precision is 1.2% and accuracy is 23% for natural seawater (5.6-28 xg/l). The method works well in organic-rich seawater matrices and sediment porewaters. 

To reduce spurious signals due to drifts of the EPR line setting arising from mechanical and thermal instabilities, double coding of the ENDOR information is often employed23). Normally a low-frequency Zeeman modulation (30-300 Hz) is applied while the rf field is frequency or amplitude modulated at frequencies of about 1-30 kHz. This modulation scheme, however, has two major disadvantages ... 

For maximum ENDOR enhancement, the Zeeman modulation amplitude has to be about one half of the width of the EPR line which is saturated at an extremum of its first derivative. However, in an EPR spectrum with line widths of typically 1 mT this Zeeman modulation contributes 20 kHz to the width of a proton ENDOR line. It turns out that in many cases a remarkably better resolution of the spectra may be obtained with a single coding in which only the rf field is modulated. 

In powder samples with broad EPR lines, large Zeeman modulation amplitudes have to be applied to improve the sensitivity. Such amplitudes often produce microphonic noise in the cavity and cause an uncertainty in the orientation selection in single crystal-like ENDOR spectra (Sect. 4.1). A modulation technique which avoids these problems in powder ENDOR studies has been proposed by Hyde et al.32). In this scheme the Zeeman modulation is replaced by a 180° modulation of the phase of the microwave signal. 

A simpler way is to use a frequency modulated rf field and to switch off the Zeeman modulation. With this single coding of the ENDOR line, the EI-EPR spectrum is directly displayed as an absorption signal20,35). 

The shift is proportional to 17-1 and, when 17 is large, a strong magnetic field is needed to shift the lines appreciably, especially v for which the Zee-man effect contains terms of opposite sign this is particularly inconvenient when Zeeman modulation is to be used. 

Both Zeeman and frequency modulation are used for line observation but each has its drawbacks frequency modulation is often accompanied by a steady drift of the recorder base line, while Zeeman modulation becomes inefficient when the electric field gradient is highly asymmetric, especially for the v lines. 

There are often reasons why the gas sample cell needs to be free of Stark plates, and in such cases the free-space cell has been used, sometimes with Zeeman modulation coils situated outside the cell. However, when Zeeman modulation is not appropriate... 

Microwave absorption spectroscopy has been used to investigate OH, OH and OD produced by electric discharges in the appropriate water vapour . The absorptions lie in the region 7.7 to 37 kMc.sec" and 2uise from transitions between the A-doublets, and (the pure rotation spectrum for OH, a light radical, is in the far infrared). Zeeman modulation was used with a conventional microwave spectrometer. 

This complex relationship means that if the cavity is held at resonance and the spectral line is swept, e.g. by Stark or Zeeman modulation, although other modulation schemes are possible, the cavity impedance changes and the reflected power incident on the Gunn device changes in sympathy with the spectral scan. This causes a current to flow in the Gunn oscillator circuit related to the spectral absorption profile, and therefore to its amplitude and area. That current can be readily transformer-coupled out of the Gunn bias circuit and detected S)mchro-nously with the modulation frequency. 

We have already discussed the use of electric field modulation as a means of providing the selective detection of those molecular absorption lines having the strongest Stark effects, using tunable lasers as sources ( 2.4). As in microwave spectroscopy, where field modulation is routine, magnetic fields may also be used for this purpose, as was demonstrated by Urban and Herrmann (1978). The spectrum of NO was recorded with extremely high sensitivity using Zeeman modulation with a spin-flip Raman tunable infrared laser. 

The spectrum near 600 cm of the V2 band of the CH3 radical generated by a glow discharge in di-tert-butylperoxide (Yamada et. al. 1981) provides a second example of this selectivity. With Zeeman modulation all the observed lines arise from CH3, but with source frequency modulation many additional lines from diamagnetic species are detected in the same spectral region. 

W. Urban, W. Herrmann, Zeeman modulation spectroscopy with spin-flip Raman laser. Appl. Phys. 17, 325 (1978)... 

The LOAS instrument consists of the following main units A tunable laser, a sample cell with an acoustic transducer (sometimes called a spec-trophone or SP), an amplifier, and a recording system. The acoustic response of the medium can be stimulated by amplitude or frequency modulation of the laser light or by Stark or Zeeman modulation of the absoiption line of the analyte. Both pulsed and cw lasers with mechanical or electro-optical amplitude modulation are often employed in LOAS analytical applications. 

Constants of the ground state and the v = 1 vibrational state were measured using a tunable IR diode laser in the range of the fundamental vibration band and the Zeeman modulation of the absorption lines. The true values depend on the unobserved state. From line positions, only effective B values, defined as B (2ri3 ) = Bv(1 + Bv/A) (A = fine structure constant, see p. 68) can be deduced [1,4], from which the following constants were derived [4] taking A = -177.3cm"i [1] ... 

The complete set of the three normal vibrations (0-0 stretch), vg (bend), and vg (0-F stretch), has only been observed by IR absorption of the radical isolated in various matrices. In the gas phase, the band center (at 1489 cm" ) was found with a Fourier transform IR spectrometer [2]. Only a small section of the band around 1480 cm" was observed by tuning the Zeeman splittings of vibration-rotation transitions to selected lines of a CO laser (laser magnetic resonance, LMR) [1]. Vg = 579.31839 0.00035 cm (band origin) was measured with a tunable IR diode laser and using Zeeman modulation of absorption lines [3]. 

In the literature one can find many descriptions of CW NQR spectrometers and oscillator-detectors for various purposes and frequency ranges. Particular interest is evoked by limited oscillators, which have a low noise level similar to that of marginal oscillators but do not need continual critical adjustment of circuit parameters. Figure 2 shows a simplified block diagram of a CW spectrometer, containing an oscillator-detector with a Zeeman modulation system and a computer for signal processing and frequency control. 

---