Background correction deuterium

In the previous section it has been shown that the measured sample absorbance may be higher than the true absorbance signal of the analyte to be determined. This elevated absorbance value can occur by molecular absorption or by light scattering. There are three techniques that can be used for background correction the deuterium arc the Zeeman effect and the Smith-Hieftje system. 

Deuterium arc background correction. This system uses two lamps, a high-intensity deuterium arc lamp producing an emission continuum over a wide wavelength range and the hollow cathode lamp of the element to be determined. 

All instruments should be equipped with a background correction facility. Virtually all instruments now have a deuterium arc background correction. The Zeeman system is also available in instruments marketed by the Perkin-Elmer Corporation and the Smith-Hieftje system by Thermo Electron Ltd. 

As an alternative approach, Koirtyohann et al (35) employed a mechanical chopper system for deuterium-background correction similar to the one that is marketed by Perkin-Elmer Co (49) One-half of the rotating chopper Is transparent and the second-... 

Manning, D. C. "Using the Perkin-Elmer Deuterium Background Correction System . At. Absorp. Newsl. (1972), 11, 112-113. 

A spectrometer with rapid response electronics should be used for electrothermal atomization, as it must follow the transient absorption event in the tube. Automatic simultaneous background correction (see Section 2.2.5.2) is virtually essential, as non-specific absorption problems are very severe. It is important that the continuum light follows exactly the same path through the furnace as the radiation from the line source (assuming a deuterium lamp is being used rather than Smith-Hieftje or Zeeman effect). The time interval between the two source pulses should be as short as possible (a chopping frequency of at least 50 Hz) because of the transient nature of the signal. 

Figure 14.12 —Schematic of an instrument showing deuterium lamp background correction. Perkin Elmer, model 3300 with a Littrow-type monochromator. This double beam assembly includes a deuterium lamp whose continuum spectrum is superimposed, with the aid of semitransparent mirrors, on the lines emitted by the hollow cathode lamp. One beam path goes through the flame while the other is a reference path. The instrument measures the ratio of transmitted intensities from both beams. The correction domain is limited to the spectral range of the deuterium lamp, which is from 200-350 nm. (Reproduced by permission of Perkin Elmer.)...

Beam chopping corrects for flame emission but not for scattering. Most spectrometers provide an additional means to correct for scattering and broad background absorption. Deuterium lamps and Zeeman correction systems are most common. 

Explain how the following background correction techniques work (a) beam chopping (b) deuterium lamp (c) Zeeman. 

Other background correction systems include the Zeeman effect and the Smith-Hieftje background correction. A detailed description of the operational principles of these methods is beyond the scope of this chapter and the required information can be found in the relevant literature.7,13 The advantages of these methods over deuterium lamps are that high background signals (up to 2.0 units) and structured backgrounds can easily be corrected for. 

Grabinski [12] has described an ion exchange method for the complete separation of the above four arsenic species, on a single column containing both cation and anion exchange resins. Flameless atomic absorption spectrometry with a deuterium arc background correction is used as a detection system for this procedure. This detection system was chosen because of its linear response and lack of specificity for these compounds combined with its resistance to matrix bias in this type of analysis. 

M. Hoenig, P. Van Hoeyweghen, Determination of selenium and arsenic in animal tissues with platform furnace atomic absorption spectrometry and deuterium background correction, Int. J. Environ. Anal. Chem., 24 (1986), 193-202. 

Procedure Determine the absorbances of the Test Preparation and the Standard Preparation at 284 nm in a suitable atomic absorption spectrophotometer equipped with a lead hollow-cathode lamp, deuterium arc background correction, and a single-slot burner, using an oxidizing air-acetylene flame. The absorbance of the Test Preparation is not greater than that of the Standard Preparation. 

Apparatus Use a suitable spectrophotometer (Perkin-El-mer Model 6000, or equivalent), a graphite furnace containing a L vov platform (Perkin-Elmer Model HGA-500, or equivalent), and an autosampler (Perkin-Elmer Model AS-40, or equivalent). Use a lead hollow-cathode lamp (lamp current of 10 mA), a slit width of 0.7 mm (set low), the wavelength set at 283.3 nm, and a deuterium arc lamp for background correction. 

Background correction is carried out with a continuum source, e.g. a hydrogen hollow-cathode lamp or a deuterium-arc lamp. 

Adjust the spectrometer variables of spectral band pass, wavelength and lamp current according to the manufacturer s recommended conditions for Ni and V. It will be necessary to employ background correction for the determination with most electrothermal atomisers, especially in the case of Ni. Where available on the spectrometer, set up the deuterium or hydrogen lamp background correction system as recommended by the manufacturer. Otherwise use a nearby non-absorbing line to estimate the background intensity. 

Check whether background correction is necessary, especially when determining Mg and Zn, by using a hydrogen or deuterium lamp or a nearby non-absorbing line. 

---