背景校正方法的比较

Application Notice AA-15 Comparison of Background Correction Methods

The D2 Background Correction System of the AI 1100/2100:

There are three background correction methods currently being used by manufacturers of

atomic absorption spectrometers. They are: Smith-Hieftje (S-H), Deuterium (D2) and Zeeman. Regardless of the method of correction used the same factors contribute to the effectiveness of the measurement.

The keys to accurate background correction are:

a) The frequency at which the peaks are measured

GFAAS peaks are very narrow, typically with a width at half height of 0.2 to 0.5 seconds.

This is especially true for the AI 1100/2100 where temporally isothermal atomization

provides peak widths that are in the low end of this range. Normally, both analyte and background can exhibit similar rapid transient peaks. Often the change in the absorbance

of these peaks can be as high as 10 absorbance units per second.

Extensive studies have shown that for such fast peaks a 10 Hz sampling frequency can

result in significant errors in the measurement of peak height and/or peak area. For a 30

Hz sampling frequency this error, although largely reduced compared to 10 Hz, can still be significant. Therefore 10 and 30 Hz sampling frequencies are too slow to accurately

determine the peak shape. 60 Hz systems will give acceptable results. The AI 1100/2100 system, however, uses a sampling frequency as high as 120 Hz. This gives excellent peak definition while errors in the peak height and peak area measurements are negligible.

b) The interval between the total absorbance and the background absorbance

measurements

Sampling frequency is important, but it is not the only important time factor for achieving accurate background correction.

The net analyte signal is determined by subtracting the measured background absorbance

from the measured total absorbance. The AI 1100/2100 can quickly change between measuring the total absorbance and the background without introducing significant error

into the calculation of the net absorbance signal. These two measurements can not be

made simultaneously. Furthermore, because the background signal of a GFAAS

measurement can be changing very rapidly it is essential that the measurements be made as closely to each other as possible.

The AI 1100/2100 uses an industry leading D2/HCL modulation frequency of 1000 Hz which results in a time interval between the total absorbance and background absorbance measurements of less than 0.5 ms. The next best system is Varian’s which offers a 400 Hz modulation frequency.

c) The function used to calculate the net atomic absorption

Because the AI 1100/2100 uses such a fast switching speed between the background and total absorption measurement, a simple subtraction of the background signal from the total absorbance is used to determine the net absorbance.

Other manufacturers like Varian have to use interpolation to try to approximate the changes in the background that can occur during their 2 ms switching period. This leaves their correction technique open to large errors that the operator may never know exist.

d) The presence or absence of spectral or structured background

Less than 1% of the samples encountered in the real world exhibit spectral or structured background interferences that can not be overcome by optimizing the furnace heating program and/or using an appropriate chemical modifier.

e) The effect of the method on the linear working range

Due to complex splitting patterns the total absorbance measurement of an instrument equipped with Zeeman background correction can be non-linear. As a result when the background signal is subtracted from the total absorbance a roll over point can occur. i.e. two different concentration values correspond to the same absorbance value. Therefore, the linear dynamic range of the measurement is reduced. Conversely D2 background correction does not suffer from this problem.

Background Correction in General:

Advantages and disadvantages of the S-H method:

The S-H method is based on the source self-reversal, or self-absorption, behavior of the radiation emitted from the hollow cathode lamp. Its advantages include: no extra lamp to align, no need for a costly magnet and functionality for non-demanding background signals. However, it has the following disadvantages including: the requirement of special hollow cathode lamps (lamps are not available for all elements); the requirement that the high and low voltages of the lamp be stabilized before measurements can be made. This results in a typical sensitivity loss of 50%; a short linear dynamic range which can sometimes exhibit a roll over point; the incapability to correct for structured background;