Simultaneous background correction

Multichannel instruments are capable of measuring the intensities of the emission lines of up to 60 elements simultaneously. To overcome the effects of possible non-specific background radiation, one or more additional wavelengths may be measured and background correction (see Section 21.12) can be achieved. 

Figure 2.15 shows an instrument capable of doing this simultaneously and automatically. Lead is particularly prone to this problem, and Fig. 2.16 shows how background correction can be used to remove the interference of non-specific absorption when determining lead in chromium. Notice that the precision is also improved, mainly because the effects which give rise to the background are not very reproducible. 

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. 

In many modern atomic absorption spectrometers, this correction may be done automatically and simultaneously, time resolution of a few milliseconds being used to separate the two signals. Careful co-alignment of the two source beams is very important. To overcome this need, two other background correction systems have come into use over recent years, the Smith-Hieftje system and the Zeeman system. 

Two further limitations apply to the use of the method of standard additions. Simultaneous background correction must be employed because of possibly varying amounts of matrix material present in the tube during the several firings needed to make one determination. Furthermore, all readings must be within the linear portion of the calibration graph in order... 

Inductively coupled plasma-atomic emission spectrometry was investigated for simultaneous multielement determinations in human urine. Emission intensities of constant, added amounts of internal reference elements were used to compensate for variations in nebulization efficiency. Spectral background and stray-light contributions were measured, and their effects were eliminated with a minicomputer-con-trolled background correction scheme. Analyte concentrations were determined by the method of additions and by reference to analytical calibration curves. Internal reference and background correction techniques provided significant improvements in accuracy. However, with the simple sample preparation procedure that was used, lack of sufficient detecting power prevented quantitative determination of normal levels of many trace elements in urine. 

The advantages of coupled detectors (CCD, CID, etc.) are that they can detect and measure a wide range of wavelengths, hence elements. They do not require high voltages like PMTs, can detect unknown elements and carry out simultaneous analysis with background corrections. 

Figure 3.33 Simultaneous cyclic voltammetry charge determined from a coulometric analysis (v = 100 mVs-1, left ordinate) and reflectance of the voltammetric peak as a function of the (A, = 440nm, smoothed, right ordinate) background corrected AR/R based on the optical...

Fig. 91. Background correction with a computer-controlled simultaneous ICP-OES spectrometer.

For ICP-AES both sequential and simultaneous as well as combined instruments are used. In sequential spectrometers special attention is given to the speed of the wavelength access and in simultaneous spectrometers to the provision of background correction facilities. In combined instruments a number of frequently used channels are fixed and with a moving detector or an integrated monochromator... 

The possibilities of simultaneous background correction in the case of a two-channel spectrometer have been described in early ICP papers by Meyers and Tracy [405], In the case of brass alloys with internal standardization, RSDs below 0.1% can be obtained with simultaneous ICP-AES spectrometers for copper and zinc, where ICP-AES comes near to the precision achievable with x-ray fluorescence spectrometry for metal analysis [406]. 

Figure 19 is also an example of the background shift errors encountered with ICP-AES. The three emission (100 uL) pulses were measured with a PMT at 257.6 nm. Without the benefit of background correction the calcium concomitant caused an error of 52%, in the determination of the Mn in the Ca interfered solution. With the SIT, where an entire spectral window was simultaneously recorded for the Mn + Ca solutions, background subtraction was possible and resulted in a mere 3% error in the Mn measurement. This error is within the 1-3% sampling precision obtained with 100 uL samples. 

In principle, several samples could be analyzed simultaneously, using an xy-table with which the zeolitie materials could be positioned in the focus of the IR-beam. However, one problem is encountered using such procedures Typically, background correction is necessary,... 

Radziuk B, Rodel G, Stenz H, Becker-Ross H, Florek S (1995) Spectrometer system for simultaneous multielement electrothermal atomic absorption spectrometry using line sources and Zeeman-effect background correction. J Anal At Spectrom 10 127—136 Rao CRM, Reddi GS (2000) Platinum group metals (PGM) occurrence, use and recent trends in their determination. Trends Anal Chem 19 565-586 Rauch S, Lu M, Morrison GM (2001) Heterogeneity of platinum group metals in airborne particles. Env Sci Technol 35 595-599... 

These systems are very versatile. Absorption and emission measurements can be run by both channels. A dual-channel system permits the simultaneous determination of two elements. The sample amount and time needed are then only half of that required by a conventional AA system. This is of great use in analyses of small samples. The background correction can be performed by the other channel. Then the difference of the absorbance readings of the two channels are measured. It is also possible to obtain the absorbance ratio of the two channels, which can be used in working with the internal standards. 

Background correction by wavelength-modulation is not widely used, but seems to have considerable potential for simultaneous multi-element AAS with continuum sources. Alternatively, high-resolution echelle gratings may be used that can detect both the elemental line and the background at the same time. 

Only 1-3 pixels are used for registering the analyte absorption, so all the remaining pixels are available for correction purposes. Both fluctuations in lamp emission and transmission of the atomizer are accounted for, making the system a simultaneous double-beam system of high precision with simultaneous background correction. 

The CCD detector permits not only simultaneous background correction but complete correction of spectral interferences such as structured background from molecular species such as OH and NO. The resolution permits collection of the spectra from these species and removal of them from the absorbance of the sample. Direct-line overlap can also be corrected, if the matrix has an additional absorption line within the registered spectral range of the detector. Direct-line overlap is rare and usually is due to line-rich matrices such as Fe, so the system can reliably correct for the direct overlap of Fe on Zn, for example. 

---