Atomic emission spectrometer with background compensation

A method of and a device for multi-element measurement of elements in a sample with correction for background emission. The method starts with atomizing a sample and then exciting the transformed atoms to emit light containing characteristic spectral lines for each element, followed by generating a spectrum of spectral lines characteristic of the elements, followed by measuring the intensity of selected spectral lines falling within a predetermined measuring range without changing their intensity. The next steps are sensing the background emission adjacent the selected spectral lines simultaneously with measuring the intensity of selected spectral lines and determining the concentration of each element from the measured intensity of the corresponding spectral line and sensed background emission.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention relates to atomic emission spectroscopy and more 
particularly to a background compensation apparatus and technique for an 
atomic emission spectrometer for multi-element measurement of elements in 
a sample. 
Atomic absorption spectroscopy (hereinafter "AAS") is a known procedure for 
measuring the concentration of a certain element in a sample. AAS utilizes 
the fact that atoms absorb light at certain wavelengths characteristic of 
the particular element. Atoms emit light in the form of a line spectrum 
when they are excited and the line spectrum is characteristic of the 
respective element. Correspondingly, the atoms absorb light only at the 
wavelengths of this line spectrum. 
In AAS, an atomizing device in the form of a flame burner or a graphite 
furnace for electrothermal atomization is utilized to generate an atomic 
vapor of the sample in which the atoms of the sample are present in their 
atomic state. A measuring light beam is normally generated by a hollow 
cathode lamp and consists of light with the line spectrum of the 
looked-for element. This measuring light beam is passed through the atomic 
vapor and is subjected to absorption indicative of the amount of the 
looked-for element in the sample. The other components of the sample, at 
least theoretically, do not influence the measuring light beam because 
their absorption lines do not coincide with the line spectrum of the 
measuring light beam. The measuring light beam impinges on a photoelectric 
detector and the concentration of the looked-for element is determined 
from the detector signal after suitable processing and calibration. 
In addition to the specific absorption caused by the atoms of the 
looked-for element (i.e., atomic absorption), there normally occurs a 
non-specific absorption referred to as background absorption which is 
caused by solid particles or molecules in the path of the measuring light 
beam. This background absorption can have the magnitude of the atomic 
absorption or can even be larger. Therefore, background absorption has to 
be determined and taken into consideration with highly sensitive 
measurements. In AAS, background absorption correction can be achieved by 
alternating between the line emitting light source and a light source 
emitting a continuum or by using the Zeeman effect in which either the 
emitted spectral lines of the light source or the absorption lines of the 
sample are shifted for the background measurement. These methods are quite 
expensive and require an additional light source or a strong electromagnet 
which is arranged to be energized or de-energized. 
A disadvantage of AAS is that the elements can only be determined one by 
one, i.e., one after the other. 
Therefore, another known analytical procedure is to measure the emission of 
a sample rather than the absorption, i.e., atomic emission spectroscopy, 
which allows multi-element measurement. 
In atomic emission spectroscopy, plasma burners are often used as the 
atomization and excitation device. In plasma burners, an emerging inert 
gas is inductively transformed to a plasma of high temperature and the 
sample is led into this plasma. In another prior art atomization and 
excitation device, a sample is electrothermally dried and ashed in a 
graphite furnace similar to the graphite tubes used in AAS. The graphite 
furnace is then evacuated and an inert gas is introduced. Subsequently, an 
electrothermal atomization of the sample is effected. A gas discharge is 
caused in the mixture of inert gas and sample vapor by an anode such that 
the graphite tube operates as a hollow cathode lamp. The graphite tube 
serves as a hollow cathode. 
A spectrum of the emitted light is generated by means of a polychromator. 
It is known to scan such a spectrum by means of a series detector or 
"detector array" consisting of a plurality of photodetectors. The entire 
spectrum is detected which results in a great amount of data and the 
signal processing is correspondingly complex. 
A polychromator is known in which a dispersion is effected in high order in 
a first direction by an echelle grating. The different orders overlap and 
a dispersion is effected in a second direction perpendicular to the first 
direction by a dispersion prism whereby the different orders are 
separated. This results in a two-dimensional spectrum with very high 
resolution in a focal plane. 
In the prior art polychromator, a mask with apertures at the location of 
the spectral lines of the spectrum which are characteristic of a certain 
element is arranged in the focal plane. These apertures are arranged to 
accommodate light pipes, each of which is guided to an associated 
photomultiplier. The number of available photomultipliers and thus the 
number of elements which can be analyzed simultaneously is therefore 
necessarily limited due to cost considerations. One mirror of the 
polychromator is movable through small angles to compensate for the 
background emission which occurs in a similar way as the background 
absorption described above. The generated spectrum is periodically shifted 
relative to the light pipes such that the light pipes detect the light 
outside the spectral lines. 
It is an object of the present invention to provide a new and improved 
atomic emission spectrometer with background emission compensation. 
Another object of the invention is to provide such an atomic emission 
spectrometer for multi-element measurement which attains simultaneous 
measurement of a relatively large number of elements with simultaneous 
measurement and correction of background emission. 
Another object of the invention is to provide a background emission 
correction device and technique for an atomic emission spectrometer which 
is economical. 
Other objects will be in part obvious and in part pointed out more in 
detail hereinafter. 
Accordingly, it has been found that the foregoing and related advantages 
are attained in an atomic emission spectrometer for multi-element 
measurement which includes an apparatus to atomize a sample and excite the 
atoms to emit characteristic spectral lines, a dispersion assembly to 
generate a spectrum of characteristic spectral lines in a focal plane, a 
first photodetector assembly for simultaneously sensing the intensity of 
spectral lines of a plurality of elements, a second photodetector assembly 
for sensing background emission outside the spectral lines sensed by the 
first photodetector assembly, and processing circuit means for determining 
the concentrations of the plurality of elements with correction for 
background emission. 
The second photodetector assembly has a plurality of semiconductor 
photodetectors disposed for sensing background emission adjacent the 
spectral lines for generating a correction signal which corresponds to the 
background emission at the wavelengths of the spectral lines measured. The 
first photodetector assembly may comprise a plurality of semiconductor 
photodetectors positioned for simultaneously sensing a plurality of 
spectral lines for each element of the sample to be tested and an 
evaluation circuit selects the semiconductor photodetectors sensing the 
spectral line for each element which has an intensity optimally within the 
sensing range of the respective semiconductor photodetector. The measured 
intensity is corrected with the background emission correction signal with 
the concentrations of the elements being determined from the corrected 
intensities.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Although specific forms of the present invention have been selected for 
illustration in the drawings, and the following description is drawn in 
specific terms for the purpose of describing these forms of the invention, 
the description is not intended to limit the scope of the invention which 
is defined in the appended claims. 
Referring to FIG. 1, the numeral 10 designates an atomization and 
excitation device which is illustrated here as a plasma burner. 
Alternately, a hollow cathode lamp such as disclosed in German patent 
C2-30,13,354 may be used which comprises a graphite tube for 
electrothermal drying, ashing and atomization of the sample and an anode. 
Such a hollow cathode lamp is evacuated after ashing and is filled with 
inert gas. After atomization, a gas discharge is generated during which 
the graphite tube assumes the function of a hollow cathode. 
A light beam 12 originates from the atomization and excitation device 10 
and is formed by light with line spectra of the different elements 
contained in the sample. These line spectra are emitted due to the 
excitation of the atoms of the elements contained in the sample. The line 
spectra are characteristic of the respective element and each comprises 
several spectral lines. The intensities of the spectral lines are 
proportional to the amount or the concentration of the element in the 
sample. The spectral lines of a line spectrum characteristic of a certain 
element have different intensities. Each line spectrum has spectral lines 
with a relatively high intensity and other weaker spectral lines with a 
relatively low intensity. 
The light beam 12 is limited by a main slit 14 extending horizontally in 
FIG. 1 and a transverse slit 16 extending perpendicular thereto. The light 
beam 12 is collimated by a collimator mirror 18 and is passed through a 
dispersion prism 20. The light beam 22, once spectrally dispersed by the 
dispersion prism 20, is incident at a large angle of incidence on the 
echelle grating 24, i.e., with a small angle between the beam and the 
grating. The echelle grating accomplishes a spectral dispersion of the 
light beam 22 by diffraction in a direction perpendicular to that in which 
the dispersion was accomplished by the dispersion prism 20, i.e., in a 
substantially vertical plane in FIG. 1. This diffraction is observed in 
high order. A very large spectral dispersion is effected but with a large 
overlap of the different orders. The diffracted light passes through the 
dispersion prism 20 again and is collected by a camera mirror 26 in a 
focal plane 28. 
A highly resolved spectrum with the lines of the single elements is 
generated in the focal plane 28. The different orders of the echelle 
grating 24 are separated by the dispersion prism 20 and are located in the 
spectrum side by side. The individual lines appear as light points. 
FIG. 2 is a schematical illustration of the arrangement of the spectral 
lines in the spectrum. For clarity, only spectral lines for two elements 
are indicated, which are designated by "A" and "B". 
A detector carrier 30 is arranged in the focal plane. Semiconductor 
photodetectors 32 are arranged on this detector carrier, each at the 
location of an associated spectral line. As can be seen in FIG. 3, one 
semiconductor photodetector 32 each are arranged at the location of 
several spectral lines of each element. In FIG. 3, semiconductor 
photodetectors 32A1, 32A2, and 32A3 are arranged at the locations of the 
spectral lines A1, A2 and A3, respectively, of the element A. 
Correspondingly, semiconductor photodetectors 32B1, 32B2, and 32B3 are 
arranged at the locations of the spectral lines B1, B2 and B3. The 
spectral line A1 is the main line of the element A and has the highest 
intensity compared to the intensities of the other spectral lines of the 
element A. The second spectral line A2 has an intensity which is lower by 
some orders of magnitude than the intensity of the main line A1. The third 
spectral line A3 is even substantially weaker than the second spectral 
line A2. The relations of the spectral lines of element B are similar. 
The semiconductor photodetectors 32 are connected to an evaluating circuit 
34 which is indicated by a block in FIG. 1. The evaluating circuit 34 
checks each semiconductor photodetector 32 for whether its signal lies 
within the measuring range of the semiconductor photodetector or whether 
the semiconductor photodetector is saturated by the intensity of the 
associated spectral line, A1, for example. The signal of such a 
semiconductor photodetector (32A1) is not processed further. Then the same 
check of the semiconductor photodetector 32A2 and, if required, also of 
the semiconductor photodetector 32A3 is carried out. If the signal of a 
semiconductor photodetector lies in its measuring range, the signal is 
processed further with a factor which corresponds to the ratio of the 
intensities of the respective spectral line and the reference line, 
spectral line A1, for example. 
Thereby, the ratios of the intensities of different spectral lines used are 
superposed to the dynamic range of the semiconductor photodetector such 
that a sufficiently large dynamic range is obtained. The selection of the 
signal to be processed can be made automatically by the evaluating circuit 
34. It is not necessary to adjust the different photodetectors prior to 
the actual measurement as with the prior art photomultipliers. 
Thus, the detector assembly comprises a plurality of semiconductor 
photodetectors 32, each of which is exposed to one of the characteristic 
lines. A plurality of semiconductors is provided for each element to be 
measured with the semiconductor photodetectors being exposed to different 
spectral lines of different intensities of the light spectrum emitted by 
the atoms of the element. The evaluating circuit is adapted to select, for 
measuring each element, one semiconductor photodetector for which the 
intensity of the associated spectral line lies in a part of the measuring 
range of the semiconductor photodetector which is as favorable as 
possible. 
Additional semiconductor photodetectors 36 are provided to correct for 
background, i.e., background emission. The semiconductor photodetectors 36 
are arranged outside the spectral lines, preferably close thereto, and 
provide the course of the background emission. A value of the background 
emission at the location of the spectral lines can be obtained from the 
signals of the semiconductor photodetectors 36. The measured intensity of 
the spectral lines can be corrected based upon this background value in 
order to obtain an exact measuring value of the concentration of the 
respective element in the sample. 
Accordingly, the plurality of semiconductor photodetectors 32 senses the 
characteristic spectral lines and the additional semiconductor 
photodetectors 36 are disposed closely adjacent the spectral lines so as 
to sense background emission. The signal evaluation circuit 34 selects a 
photodetector 32 (as described above) for each element and generates from 
the signals of photodetectors 36 a correction signal which corresponds to 
the background emission at the wavelengths of the spectral lines measured 
by the photodetectors 32. The signal evaluation circuit 34 makes a 
background emission correction of the measured intensities of the spectral 
lines and the concentrations of the elements are determined from the 
corrected intensities. 
The semiconductor photodetectors can be mounted on a carrier one by one in 
a two-dimensional arrangement. The semiconductor photodetectors can also 
be arranged in an integral unit. The semiconductor photodetectors are of 
such small dimensions and are relatively inexpensive such that a great 
number of sample elements and the background emission value can be 
simultaneously measured in the way described with a plurality of such 
semiconductor photodetectors being provided for each element. However, it 
is not necessary to detect each wavelength virtually continuously. 
Thereby, the expenditure for the signal processing is kept within 
acceptable limits. It is also possible to scan and process the signals of 
all semiconductor photodetectors virtually simultaneously with reasonable 
expenditure such that the concentrations of the different elements are 
associated with the same point in time. 
Accordingly, small and relatively inexpensive photodetectors can be used 
and the technical expenditure thereby becomes smaller. Using several of 
such semiconductor photodetectors for each individual element permits, on 
one hand, the required dynamic range to be achieved by such semiconductor 
photodetectors. On the other hand, the use of a plurality of 
photodetectors for each element becomes economically possible because of 
the use of the (inexpensive) semiconductor photodetectors. 
Furthermore, the use of semiconductor photodetectors offers the possibility 
to generate a background signal in the focal plane by additional 
semiconductor photodetectors at other suitable locations of the generated 
spectrum to compensate the measuring signal with respect to background. 
As can be seen, an atomic emission spectrometer for multi-element 
measurement has been described which attains simultaneous measurement of a 
relatively large number of elements with simultaneous measurement and 
correction for background emission. 
As will be apparent to persons skilled in the art, various modifications 
and adaptations of the structure above described will become readily 
apparent without departure from the spirit and scope of the invention, the 
scope of which is defined in the appended claims.