DE2045134B2 - Device for atomic fluorescence spectroscopy - Google Patents

Description

The invention relates to a device for atomic fluorescence spectroscopy with a device for

ίο Generating free atoms in the vapor of a sample Energy supply, an element-specific light source for irradiating the sample vapor and a detector for the radiation emitted by the sample vapor. Bd of the atomic absorption spectroscopy appara tively largely similar atomic fluorescence spectro copy becomes a sample to be examined by feeding thermal energy for example by means of a flame or an electric furnace in atomic steam convicted, in which many, on their fundamental

there are free atoms at energy levels. at If the sample vapor is irradiated with light, the free atoms are raised to higher energy levels and emit what is known as fluorescence radiation when they fall back to their basic energy level

»S with one or more wavelengths typical for the respective type of atom. The radiation emitted by the sample vapor is recorded and evaluated by a detector. Just like in absorption spectroscopy, in which the absorption is more specific, in each case

for a type of atom of typical wavelengths from the light with which a thermally excited sample is irradiated, is observed, there is the difficulty that for the type of atom sought in the sample typical wavelengths or spectral lines of the

to separate other radiation that is inevitable in the system. It is the latter mainly to components of the light used to irradiate the sample and to those through the thermal excitation of the sample generated unspecific so-called thermal radiation.

In order to avoid interference from the light irradiated on the sample, it is known to use the sample element-specific, d. H. to illuminate with light only those wavelengths that are required for what is sought in the sample Flement are typical. For generating element-specific light, monochromators are known which either a continuous broadband source are connected downstream, but then must have a very high resolution, or a line source in the form of the so-called hollow cathode lamp, in which case a lower resolution of the Monochromators are sufficient. However, it is also already known to use discharge lamps that work directly in an element-specific manner. These include one Mercury resonance lamp specified for absorption spectroscopy and specified for both absorption and fluorescence spectroscopy electrodeless discharge lamps excited by a microwave field, which are particularly sharp and inten generate sive spectral lines.

It is also known to use modulation methods to further or better differentiate the wavelength components of interest from the interfering modulation methods. These include the periodic interruption of the sample feed, all two-beam methods in which the light alternates between the Sample and a reference path is switched, and in particular for fluorescence spectroscopy

suitable modulation, in particular chopping of the light radiated into the sample. All modulation methods require a link, in particular a mutual subtraction of those obtained alternately according to the modulation Output signals from the detector or detectors are used to achieve the desired suppression of the "background", which in fluorescence spectroscopy is thermal radiation.

Light source can be determined. An analysis of avf can therefore be carried out even with a small amount of sample several items are run.

In the preferred embodiment of the invention the energy is supplied via a first switch, while the detector is only activated during the breaks What is effective is that its output is followed by a second switch which, together with the first Switch and in push-pull to this by means of a

Even with skilful use of all known io clocks, it is controlled with a certain frequency. Measures to improve the signal-to-noise ratio are other expedient embodiments of the invention

are characterized in subclaims. In the following, the invention is described with further

partial details on the basis of a schematic

to achieve it in fluorescence spectroscopy exact measurement results still necessary at the moment

the detector a monochromator or at least

a filter for separating the resonance lines of the exemplary embodiment presented is explained in more detail. In the

searched or selected element from the remaining, drawings shows

still present in the received radiation and FIG. 1 shows a combined schematic and block diagram

in particular a representation of a device according to the invention, which is based on thermal excitation,

to arrange outgoing components. In order to effectively refer to Fig. 2a bisegraphische representations of various

These devices must also have a high volume of signal shapes that are generated during operation of the device

have solvency. Since they only work on one Spectral line can be set to limit them when examining elements with a multitude of spectral lines, such as B. copper, nickel and iron, the intensity of the radiation typical of the element. A further disadvantage is that these devices for each element to be examined according to the different spectral lines of the elements have to be replaced, which is very costly and time-consuming

Fig. I occur.

FIGS. 3a to c show diagrams of spectral lines which are emitted by a sample to be examined In Fig. 1, a chamber 11 is shown, which can accommodate a sample of a substance as in relation examined for the concentration of one or more selected elements contained in this sample shall be. By means of a sample introduction device 13, the sample is introduced into the chamber 11 in such a form

is consuming. The latter is particularly introduced that it is present in the chamber as vapor if only take a small amount of the sample that can be dissociated and into free atoms. If the consumed in the conversion into atomic vapor, is present. An investigation for several elements would only be with a system, which is a fast switching between different mono 35 chromators permitted, possible; can be seen a such a system economically impracticable. The invention has for its object a very

precise and fast working device for atomic _.

to create fluorescence spectroscopy, which without mono- 40 and the sample in a suspension in the chamber

chromators or similar devices between feed. Solid samples can be applied directly to the interior

the sample and the detector. walls of the sample chamber itself applied and so

Starting from a device that initially is then carried out by z. B. evaporated an atomization process

According to the invention, this object will be achieved in the named type. The chamber 11 is closed by a vacuum

solved by that the energy supply to the sample steam 45 pump 15 at a low and constant pressure,

is pulsed and the detector is only used in the breaks of the typically in the range of 1 to 10 mmHg

Energy supply is effective. is held. The device according to the invention is based on a high-frequency energy source 17

Realization that the excitation of the sample high-frequency energy via a controlled container The thermal radiation generated in the pauses 5o 19 is fed to a winding 20 which surrounds the chamber 11 between the supply of energy and thus the excitation is wrapped around in the chamber an electric hero

to generate and thereby to excite the sample, so that a large number of molecules of the sample vapor in a plasma of free atoms dissociates, which are in their basic energy state. The high

Sample consists of a gas, the sample introduction device can consist of a simple economy valve, to introduce a continuous flow of gas into the chamber under a required pressure. at liquid samples, the sample introduction device 13 should contain a device to vaporize or nebulize the sample, whereby the sample can optionally be mixed with a carrier gas,

drops very quickly to a value that is low compared to the fluorescence radiation and the detector therefore, if you let it be effective in these pauses, it emits a signal, which is largely exclusive corresponds to the fluorescence radiation. A small amount of thermal radiation still present in the The output signal of the detector can then easily be completely eliminated using the known methods. at

55

frequency energy by which the sample is excited in order to dissociate it can be selected at a Frequency are emitted within a wide frequency range, the z. B. in the order of magnitude

the new device is not a monochromator, filter ^{6o is} from 10 to 2500 MHz. Although in the drawing or the like. To suppress the thermal radiation only a single coil 20 is provided to the

Plasma in the chamber

feed, too

necessary in front of the detector. This can directly receive the radiation emitted by the sample, which results in great cost savings, an increase in sensitivity, especially for elements with many spectral lines, and an increase in the measurement speed means so that z. B. several elements one after the other simply by switching the element-specific

11 Add high-frequency energy

_tu , _u . ,, w, other institutions untei

Using normal capacitive or inductive coupling techniques. E.g. can, if the high-frequency excitation source is in the microwave range, the sample chamber in one Resonance cavity are arranged.

It can, also other facilities related to random excitation of all atoms in the sample chamber to excite the sample to a plasma of free energy levels above the basic energy levels. For example, the energy source 17 can be effective. It turns out a random distribution of the Weeping through high voltage discharge circuit er atoms that are excited in this way and are set, 'and the coil 20 can drop to lower energy levels two consecutive times and Charge electrodes are replaced, which are arranged inside the thereby energy at a variety of wavelengths from sample chamber Ii. give. Which are given off in this way by all atoms ;; Several electrodesless. Metal vapor discharge level energy is called thermal radiation and Lamps 21, 23, 25 are aligned with one another, as is the fluorescent radiation in all of them that it emits its light all in the direction of a common ten directions. The thermal radiation is on release into the chamber 11 leading beam path. Each lamp consists of a small glass ampoule, disadvantageous in probability and the examination selectivity which affects a small amount of a certain element. It is therefore used in relation to the fluorescence or a composition that keeps this element de- centent radiation as small as possible, holds, enclosed along with an inert gas 15 All radiation received from the sample, i.e. H. is. The element contained in each lamp is both the fluorescent radiation and the thermiso chosen so that it is an element in the sample see radiation, is through a window on the side plasma that is to be examined. If the sample chamber 11 is radiated to a detector 43, that is, three different elements are fixed in the sample Suitable light-collecting lens elements (not shown) are to be set, the three lamps 21, 23, ao can each contain a different of these three elements between the sample chamber and the Detek-25. A geor can be arranged. The windows on the side and A suitable metal vapor discharge lamp is in the USA.- The ends of the chamber are made of a material described in patent specification 33 19 119. put that for the characteristic radiation of all The discharge lamps are energized in a manner similar to the elements to be examined in the sample as the sample chamber 11. Im as is casual. The three lamps 21, 23, 25 each through a window are sufficient for most elements. for certain items, however Coils 27, 29, 31 via switches 33, 35, 37 to the high windows made of quartz, sapphire, lithium fluoride or off frequency energy source 17 coupled. The switches require other materials. The window material 33, 35, 37 are operated in such a way that they must be able to withstand the plasma temperatures in the selected sequence radio frequency energy to the am- 30 sample chambers are generated and typically pull out. The switches 33, 35, 37 are also at 1000 ° C or higher, like the switch 19 indicated in dashed lines, the vacuum pump 15 is the plasma in to indicate that they can be designed in any suitable manner of the sample chamber 11 at a low pressure so that they hold for the particular, which is lower than atmospheric pressure in order to Frequency and the level of the voltage of the 35 to increase the yield of the fluorescent radiation and Energy source 17 supplied energy are suitable. the widening of the If z. B. to reduce the energy source 17 energy in the spectral lines. As a result, another microwave range is generated, the switches can control the entire examination sensitivity and Be waveguide circulators. the selectivity of the system is improved. Due to the The high-frequency energy from the energy low-pressure plasma also provides a radiation source 17, the gas in a selected ampoule is permeable in the far ultraviolet range of the specionized and the atoms contained therein, thereby reducing the examination area to a element excited, so that light in the waves- larger number of elements, including some lengths of the characteristic spectrum of the element non-metals, can be extended, is emitted. The radiation received from the sample is direct The light beam generated by the lamp passes through a window onto the detector 43. It doesn't need filters at the end of the chamber 11 in this one. The beam can or monochromators between the sample chamber , are amplified in the sample chamber in that 11 and the detector 43 are used so that the at both ends of the sample chamber window and detector all over a long distance ! two mirrors 39 and 41 are provided, radiation spectrum emr> extending between So length range j which the radiation traps through the sample chamber. The detector preferably comprises one or

■ is reflected through. That of the element-specific several photomultipliers, which are chosen so that they! tables discharge lamps emitted light will have a spectral responsivity, the '■ by the atoms of the specific element in the process from the infrared region to the far ultraviolet ·

■ bcnplasma absorbed, removing the atoms from their 55 area. For example, from wavelengths in the size of grand energy levels to higher energy levels, 8000 to 1000 A is sufficient. The detector 43 is to be excited. Then reverse many of the excited arranged that it only receives radiation emitted by the atoms to its ground energy level back and emit plasma is emitted, and no direct radiation from ^j: animals while in random directions again radiation the discharge lamps 21, 23, 25 receives .

j with one or more characteristic waves- * ° The output signal obtained from the detector 43

length. This phenomenon is called atomic fluorescence is amplified by an amplifier 45 and then

denotes a controlled switch 47 and a controlled

In addition to the beam switch 49, the sample plasma alternately emits two the mean value with the characteristic wavelengths of the circuits 51 and, respectively, which form the output signal. the light excited atoms also ^{supplied radiation at a 6} 5 53. As will be explained further below, 1 ^J another number of lengths of time. This is because the output signal from the circuit 51 corresponds

that due to the high frequency energy of the energy - the total radiation from the sample, the greatest -

auelle 17. by which the sample dissociates, also partly based on fluorescence and a small part

" 8th

thermal radiation, while the off distinguishes from one another in that the light radiated onto the sample path signal from the circuit 53 corresponds only to the thermal vapor with the aid of mechanical background radiation. The lower see chopper wheel 63, which is rotated about an axis 65, the large signal from the output of the circuit 53 runs parallel to the plane of the drawing, by a suitable output signal of the circuit 51 in a subtracting device (not shown), gliidd 55, subtracted generate a signal that is modulated or periodically interrupted. That is only proportional to the fluorescence radiation. Chopper wheel 63 has suitable segments in such a way that the output signal of the subtracter 55 is converted into a signal of the order of magnitude of z. B. 50 Hz interrupted dsis the concentration of a particular selected io. The controlled switch 49 is synchronized with element in the sample is proportional. The circuit operated by the chopper wheel. When the chopper wheel 57 is in principle a logarithmic function gene- 63 is in a position releasing the light, rotor; However, additional complex conversions can be provided around the circuit 51 for the output signal from the detector for the mean value detection functions. If the light beam cannot fall into the plasma in the range of detectable element concentrations up to 15, the detector will continue to expand. If a feedback line 58 is used, the signal is passed to the averaging circuit 53. At the Ausfogarithmic function generator 57 with the switching of the subtracter 55 is consequently, as already coupled ter 19, to the switching speed, with explained, a signal that is only proportional to the pure fluorescence radiation or the high-frequency energy to the sample chamber,
is applied to control and thus an overexcitation «o Instead of the mechanical chopper wheel, de« plasmas can be prevented. other facilities can also be used to achieve this

The supply of the excitation energy from the high light radiated onto the sample vapor to the modufrequency energy source 17 for the sample vapor takes place here. For example, the light sources themselves can be controlled by circuits which are pulsed with a predetermined repeat rate and which act on the switches 33,35, frequency of z. B. 500 Hz. This is achieved by acting, or an additional switch which is periodically opened and closed in that the switch 19 is under the control of a clock of the supply line for a lamp, periodically. and be turned off. In this case, the switch 49 controlled during a portion of each clock period would be applied to the sample in synchronism with the clock radio frequency energy to operate the circuits that operate the lamp switches. To excite atoms of the sample. During a part of the same clock period following the operation of the device shown in FIG. 1, this can best be understood from the diagrams in the high-frequency energy on the sample plasma. Fig. 2a shows the switches, whereby the thermal radiation drops rapidly from the average value of the radio frequency energy pulses, atoms in the sample. There is a period of time generated at the output of the controlled switch 19 after this drop, typically 1 to 35. The pulses begin at intervals of 110 msec, during which the atoms in the sample are still present in their dissociated form for 2 msec according to the clock speed. . During this corresponds to 500 Hz in the figure, the service life toe, the sample from one of the discharge lamps, each pulse of half a period of the Taktgeberdie the characteristic spectrum of the selected frequency; However, it can also have a shorter atomic type of use, be illuminated with light, and be selected permanently. Fig. 2b shows the mean Ausin .ir the free atoms of this type of atom can light output signals to the detector 43 absorb, which on the entire •. As already described above, radiation from the sample plasma is based. The beam stacks the atoms that absorb the light, this treatment is then off again during the high-frequency energy pulses, creating a maximum of fluorescence radiation. During the time intervals between the is generated. Consequently, during this period of time, a large amount of fluorescence radiation is obtained from the sample chamber, depending on whether light is emitted into the plasma with a relatively small thermal radiation or is blocked. In the absence of light radiation. Beam, the sample radiation quickly falls on a low

The detector output is only gauged during the third value from which the thermal radiation away from the thermal radiation speaks of the So, ^w * s through the part 67 of the curve of FIG. 2b energy source 17, whereby the largest part is shown. When light is irradiated, the result of the thermal background radiation is a sample radiation that is switched off by the undesired effects indicated by the dashed line. drawn part 69 of the curve indicated isi. This km the switch 47 is closed, so that the radiation largely corresponds to the fluorescence output signal obtained from the detector during radiation of the selected type of atoms and is greater in a predetermined period of time during each period and falls at a slower rate. when the high frequency switch 19 is open, as the radiation in the absence of light,
forward. The two switches 19, 47 are used for this purpose. From FIG. 2c it can be seen that the switch 47 is actuated jointly by the clock generator 59 in push-pull mode only during a predetermined time interval. In this case, the switch 47 is closed by means of a monostable toggle element 61 which is closed between the high-frequency energy pulses. The pulses shown in this figure are delayed only after the initial drop in the thermal speak to see the output pulses of the monostable radiation closed. Flip-flop 61. Every impulse is so delayed that it

Due to the above measures, the largest is based after a predetermined time interval I «after

Part of the output signal from the switch 47 already ⁶ S end of the HochfreqeenzEne ^ gieimpalses begins. the

on the fluorescent radiation from the plasma; it lies signals that are transmitted while the switch

however, a small proportion of the thermal jet is still closed. are shown in Figure 2d. They are

ment. These two components are further amplified by the detector output signals that occur.

9 10

after the thermal radiation has dropped somewhat. During the generation of the plasma correspond

is. the spectral lines are essentially thermal

Fig. 2e shows the output of the switch 47 in a radiation which is based on the high frequency excitation,

slightly larger timescale. While the light passes through The radiation of the light source is selected for the

the Ghopperrad 63 is locked, each several 5 element are characteristic and contributes to the generation

Signal pulses 67 emitted, since the chopper wheel at the "£" peaks. These peaks fluctuate in hers

Light beam interrupts with a frequency, the smaller intensity with the frequency with which the irradiated

than is the pulse rate for the radio frequency energy. Light is interrupted.

While the light beam through the chopper wheel in the Figs. 3b and 3c represent the spectrum of the The sample chamber 11 can be affected by the radiation from the sample when the generation of the Detector corresponding to several signal pulses 69 over plasma is switched off, d. H. during the time carrying. Since the switch 49 is synchronous with the chopper interval between the excitation pulses from the high wheel 63 is actuated, the signal pulses 67 reach the frequency energy source 17. The one in these two figures Circuit 53 and the signal pulses 69 to the circuit shown wavelength range is the same as that in FIG 51. By the subsequent subtraction, the 15 Fig. 3a; however, the light intensity / is through to a certain extent the thermal background radiation is clearly shown, that of a 100 times greater sensation Disruptive effect switched off. Liability corresponds to the scale in Fig. 3a. Figure 3b

As noted above, there are no filters - applies to switched off and Fig. 3c to switched on

or monochromator elements between the sample light. As can be seen from Fig. 3b, the

Kamrner 11 and the detector 43 are required and all radiation from the sample is very small if both

omitted the associated with such devices the plasma is not excited and the light is switched off.

Disadvantage. Notwithstanding these drawbacks, it is possible to use the tet. However, if, as shown in Fig. 3c,

If the light is switched on using a filter or a monochromator, the beam

The sensitivity of the examination increases the light only the selected atoms.

will. »5 excited, creating the well-defined spectral peaks

The advantages of the invention will be better produced by hand. As the excitation of the plasma increases 3a to c is switched off before this time, the thermal is obscure. In Fig. 3a the spectral distribution of the fundamental radiation from the sample is very low in radiation of the sample steam in the chamber 11 for equal to the characteristic radiation of the exhaust Case represented that by a pulse from the 3 ° selected element. In addition, since the entire in the Radiofrequency power source 17 that generates plasma graphs shown in Figures 3b and 3c is. Spectrum of the radiation directly on the detector 43

The intensity / of the spectral peaks is given arbitrarily, so that all characteristic spectral

shown in units up to 400. The special fixed lines contribute to the usable output signal is

the element to be delivered has certain characteristic 35 the examination sensitivity without the use

speciral fluorescence peaks denoted by E. The use of filters is very high. There can be a multitude

The other spectral lines represent the non-fluorescent elements in the sample that are thereby determined

ornamental transitions of the selected element such as that only the discharge lamps are optional

other types of atoms in the sample chamber are also turned on one after the other.

For this purpose 2 sheets of drawings

Claims (10)

Patent claims: 1. Device for atomic fluorescence spectroscopy with a device for generating free Atoms in the vapor of a sample by supplying energy, an element-specific light source for irradiating the sample vapor and a detector for the radiation emitted by the sample vapor, characterized in that the The energy supply is pulsed and the detector (43) only during the breaks in the energy supply is effective. 2. Apparatus according to claim 1, characterized in that the energy supply via a first switch (19) takes place and the output of the detector is followed by a second switch (47) is, and that the two switches are controlled in push-pull by a clock (59) with a certain frequency. 3. Apparatus according to claim 2, characterized in that the clock generator (59) is the second The switch (47) controls a delay element (61) with a delay time (f <t) corresponding to a fraction of the clock period. 4. Apparatus according to claim 1, 2 or 3 with a modulator for the sample vapor irradiated light, characterized in that a changeover switch (49) synchronized with the modulator (63) is provided which controls the output the detector (43) alternately connects to two circuits (51, 53) for averaging, at the output of which a subtracter (55) is connected. 5. Apparatus according to claim 4, characterized in that the frequency of the modulator (63) is less than the pulse frequency of the energy supply. 6. Apparatus according to claim 4 or 5, characterized in that the changeover switch (49) is connected downstream of the second switch (47). 7. Apparatus according to claim 4, 5 or 6, characterized in that the subtracter (55) a function generator (57) is connected, which has an output via a feedback (58) on the energy supply in order to avoid overexcitation of the sample vapor acts. 8. Device according to one of claims 1 to 7, characterized in that an evacuable and with a sample introduction device (13) in communication with chamber (11) for receiving rle «sample vapor is provided, which has an entry window for the irradiation of the sample vapor serving light and an exit window that cannot be reached by this light for the radiation emitted by the sample vapor, and one Means (20) for generating an electrical excitation field for the sample vapor is assigned. 9. Apparatus according to claim 8, characterized in that the device (20) for generating the electrical excitation field via the first switch (19) is connected to an RF source. 10. Device according to one of claims 1 to 9, characterized in that the elements Specific light source comprises several, optionally switchable, and each for * one element specific discharge lamps 0Ά, 23, 25)