Stability lamp intensity

Variations in lamp intensity and electronic output between the measurements of the reference and the sample result in instrument drift. The lamp intensity is a function of the age of the lamp, temperature fluctuation, and wavelength of the measurement. These changes can lead to errors in the value of the measurements, especially over an extended period of time. The resulting error in the measurement may be positive or negative. The stability test checks the ability of the instrument to maintain a steady state over time so that the effect of the drift on the accuracy of the measurements is insignificant. 

Indeed, a possible problem may derive from the fact that even lamps of the same type set at the same operating point may not deliver an identical light intensity. However, by using a photodiode light sensor, LS, and an OPA amplifier, it is possible to convert the current into a stabilized output voltage proportional to the lamp intensity. 

Lamp intensity also is a function of temperature, where the optimum temperature will be different for different substances. Variations of lamp temperature will cause ffuctuations in the emission intensities of the lamps. Figure 10-7 illustrates (curve B) the stability of a zinc EDL. 

FIGURE 10-7. Stability and intensity of a zinc electrodeless discharge lamp. [From W. G. Schrenk, S. E. Valente, and H. R. Bohman, Microwave Discharge Tubes for Atomic Absorption Spectroscopy, Trans. Kan. Acad. Sci., 74, 249 (1971). Used by permission of the Kansas Academy of Science.]... 

Flame atomic absorption spectroscopy has the disadvantage of low lamp intensities and poor stability, but modifications of sample treatment and use of special gas combinations improve selenium detection [24, 39]. Flameless techniques have shown more promise [26, 29] and the detection limit of 72 picograms obtained by Baird et al. [5] shows that the necessary sensitivity is attainable. 

The stability of C60 and C70 solutions in vegetable oils has been examined also towards the action of UV light. A C60 solution in linseed oil has been irradiated in a quartz reactor with UV light from a 12 W low-pressure Hg lamp having its main emission at 254 nm under N2. In less than 1 hour irradiation, all the visible part of the electronic spectrum of C60 with bands at about 530 and 600 nm have been bleached. Simultaneously, a growth in absorption intensity as function of the irradiation time has been observed at about 410nm. 

A single-beam spectrophotometer, hke the one shown in Figure 1.5(a), presents a variety of problems, because the spectra are affected by spectral and temporal variations in the illumination intensity. The spectral variations are due to the combined effects of the lamp spectrum and the monochromator response, while the temporal variations occur because of lamp stability. 

A tenfold increase of the intensity may be obtained at the cost of a somewhat reduced thermal stability by omitting the draft shield and mounting the shutter and lamp assembly directly on top of the sample holder enclosure block. Since the heat production of the fluorescent lamp is very small the whole irradiation equipment could be mounted within the standard glove-box. 

Where vapour discharge lamp sources exist (for volatile elements such as Hg, Na, Cd, Ga, In, T1 and Zn) they can be used. Hollow-cathode lamps are insufficiently intense, unless operated in a pulsed mode. Microwave-excited electrodeless discharge lamps are very intense (typically 200-2000 times more intense than hollow-cathode lamps) and have been widely used. They are inexpensive and simple to make and operate. Stability has always been a problem with this type of source, although improvements can be made by operating the lamps in microwave cavities thermostated by warm air currents. A typical electrodeless discharge lamp is shown in Fig. 6.3. 

For some elements such as arsenic and selenium, which have their main atomic absorption wavelengths lying on the edge of the vacuum UV, the performance of hollow cathode lamps is often poor, the lamps displaying low intensity and poor stability. This, plus the search for more intense sources for AFS (see Chapter 1, section 10), resulted in the development of microwave-powered electrodeless discharge lamps (EDLs) as spectral line sources towards the end of the 1960s.3-5... 

The performance of hollow cathode lamps deteriorates slowly with use. After several months, or even a year or more, output tends to become progressively less stable and/or less intense. Because the loss in precision is gradual, it may well pass unnoticed. It is therefore useful to keep a recorder trace of signal stability after a new lamp has been in use for a few hours for comparison purposes. To be useful, however, it is important to have made a note of the slit width, photomultiplier gain setting, wavelength, and lamp current used, and the lamp position must of course have been carefully optimized. Some analysts prefer, on grounds of simplicity, to make a note of the absorbance attainable under optimized flame conditions from a specified determinant standard. However it must then be remembered that a decline in this parameter may be related to nebulizer deterioration rather than lamp deterioration. 

While conventional or high-intensity (boosted output)4 hollow cathode lamps are usually simply operated at room temperature, electrodeless discharge lamps are sometimes cooled with a regulated flow of air maintained at a constant temperature,5 and this flow too must be optimized with respect to signal-to-noise ratio. Sometimes these sources are operated in a vacuum jacket to enhance sensitivity and/or to improve stability.6... 

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