Interferences background radiation

High power pulsed lasers are used to produce plasmas and thus to sample and excite the surfaces of soHds. Improvements in minimum detectable limits and decreases in background radiation and in interelement interference effects result from the use of two lasers (99) (see Surface and interface analysis). 

An inductively coupled argon plasma eliminates many common interferences. The plasma is twice as hot as a conventional flame, and the residence time of analyte in the flame is about twice as long. Therefore, atomization is more complete and signal is enhanced. Formation of analyte oxides and hydroxides is negligible. The plasma is remarkably free of background radiation 15-35 mm above the load coil where sample emission is observed. 

Once the detector has been chosen, the problem remains of demonstrating that any radiation observed is that of the oxygen systems, and of separating the bands from each other and from any background radiation. For the 7620 A band it is possible to use a spectrograph with photographic detection (as, for example, in the study of Clyne et al.27), while a monochromator may be used with photoelectric devices as exemplified by the experiments of Whitlow and Findlay.15 In many cases, however, the intensity of emission is too weak to be detected after dispersion, and narrow-band interference filters can then be used to advantage. 

Molecular band emission can also cause a blank interference. This is particularly troublesome in flame spectrometry, where the lower temperature and reactive atmosphere are more likely to produce molecular species. As an example, a high concentration of Ca in a sample can produce band emission from CaOH, which can cause a blank interference if it occurs at the analyte wavelength. Usually, improving the resolution of the spectrometer will not reduce band emission, since tbe narrow analyte lines are superimposed on a broad molecular emission band. Flame or plasma background radiation is generally well compensated by measurements on a blank solution. 

Upon transiting the boiler, the beams entered the detector optics enclosure, also mounted to a flange on the boiler. Here the beams were separated using a beam splitter and passed through interference filters to reduce background radiation and crossfalk befween fhe two channels. The 1.56 and 0.813 pm beams were then directed to a thermoelectrically cooled InGaAs detector and uncooled silicon photodiode, respectively. 

There are two types of spectral interferences in flame OES (1) background radiation and... 

The sample enters the plasma tube through the aerosol inlet and the usual vaporization, atomization, and excitation steps occur at an effective excitation temperature of approximately 7000° K. Excitation occurs in a chemically inert environment thus some possible interference effects are reduced in magnitude. No electrodes are used, so contamination possibilities are reduced. Spectra produced do, however, include lines, some OH band emission at 2600-3250 A, and weak band emission of NO, NH, CN, and C2. The spectra are relatively free of general background radiation. 

Spectral interferences occur whenever any radiation overlaps that of the analyte element. The interfering radiation may be an emission line of another element, radical, or molecule, unresolved band spectra, or general background radiation from the flame, solvent, or analytical sample. If the spectral interference does not coincide or overlap the analyte element, spectral interference may still occur if the resolving power and spectral band pass of the monochromator permit the undesired radiation to reach the photoreceptor. 

In a few cases, a measurement of contamination may be made by directly reading contamination monitors. Such a measurement will include both fixed and non-fixed contamination. This will only be practicable where the level of background radiation from the installation in which the measurement is made or the radiation level from the contents does not interfere. In most cases the level of non-fixed contamination will have to be measured indirectly by wiping a known area for a smear and measuring the resultant activity of the smear in an area not affected by radiation background from other sources. 

Fortunately, Np is the most commonly available isotope of Np and its rather long half-life (Ti/2 = 2 x 10 y) makes its specific activity low. The radioactivity of the Np sample is a problem only with respect to the health hazard, but does not interfere seriously with the Mossbauer measurement. Standard transmission geometry can be used. One should not place the absorber too close to the detector to keep down nonresonant background radiation. It is recommended to triply seal the absorber container, especially if powder samples are used. Details are given in Potzel et al. (1983). 

The advantages of this technique over emission flame photometry lie in the fact that the interferences due to physical inter-element effects, background radiation and scattered light are absent. Unfortunately chemical interferences still exist so that the effect of phosphate on the absorption of the calcium line is the same as its effect on the emission. 

The high potentials required for electrospray show that air at atmospheric pressure is not only a convenient, but also a very suitable, ambient gas for ES, particularly when solvents with high surface tension, such as water, are to be electrosprayed. The oxygen molecules in air have a positive electron affinity and readily capture free electrons. Initiation of gas discharges occurs when free electrons present in the gas (due to cosmic ray or background radiation) are accelerated by the high electric field near the capillary to velocities where they can ionize the gas molecules. At near-atmospheric pressures, the collision frequency of the electrons with the gas molecules is very high and interferes with the electron acceleration process. 

Bunsen designed a special gas burner for his spectroscopic studies. This burner, the common laboratory Bimsen burner, produces very little background radiation to interfere with spectral observations. 

The procedure is strictly analogous to that used for absorbance measurements in UV and visible molecular spectrometry (p. 355). To avoid interference from emission by excited atoms in the flame and from random background emission by the flame, the output of the lamp is modulated, usually at 50 Hz, and the detection system tuned to the same frequency. Alternatively, a mechanical chopper which physically interrupts the radiation beam, can be used to simulate modulation of the lamp output. 

Instrumental correction for background absorption using a double beam instrument or a continuum source has already been discussed (p. 325). An alternative is to assess the background absorption on a non-resonance line two or three band-passes away from the analytical line and to correct the sample absorption accordingly. This method assumes the molecular absorption to be constant over several band passes. The elimination of spectral interference from the emission of radiation by the heated sample and matrix has been discussed on page 324 et seq. 

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