DE102005058160A1 - Atom fluorescence spectrometer e.g. spark emission spectrometer, for analyzing metallic samples, has spark generator producing plasmas in area between electrode and sample surface, and radiation source producing fluorescence radiation - Google Patents

Abstract

The spectrometer has an auxiliary spark generator (55) producing plasmas in an area between a counter electrode (3) and a sample surface, where a spectrometer optics has an inflow gap, a dispersive unit and a number of sensors. An auxiliary radiation source is arranged for optical radiation of the areas between the electrode and the sample surface, and provided for producing fluorescence radiation. An additional sensor is provided for detecting the fluorescence radiation, where the radiation passes through the optics or the inflow gap with a variable width. An independent claim is also included for a method for analyzing metallic probes by an atom fluorescence spectrometer.

Description

was standing of the technique

The Spark emission spectrometry (spark AES) is the most common Method for the routine analysis of metallic samples.

1 shows the general state of the art with reference to a schematic representation of the structure of such systems. A spark stand ( 1 ) allows the application of a sample ( 2 ) at a distance of 0.5 to 5 mm to a counter electrode ( 3 ). The excitation generator ( 4 ) first generates a high-voltage pulse, which ionizes the atmosphere between the sample surface and counter-electrode (air or inert gas) and thus makes it low-impedance. In 2 the ionized plasma channel is shown ( 8th ). Through the ionized spark gap, a current pulse is passed, which has a duration of 50 microseconds to 2 ms. It forms an expanding thermal plasma with temperatures between 4000 K and 20,000 K, in which free atoms and ions are excited to emit a line spectrum. The emitted light is transformed into an optical system ( 5 ), on whose focal curve ( 6 ) the spectral lines are sharply imaged. A new spark may only be started (with the aid of an ionizing high-voltage pulse) if there is no longer conductive bridge of ions between the counter electrode and the sample. This limits the maximum possible spark repetition frequency to 200 Hz to 800 Hz. It depends on the sample material, the purge rate, the purge gas (usually argon) and the flow conditions in the radio chamber.

• 1. Bestückung mit Austrittspalten und Photomultipliern (PMT) (3) Auf der Fokalkurve (6) werden einzelne Linien durch so genannte Austrittspalte (15) ausgeblendet. Die durch die Austrittspalte fallende Strahlungsmengen wird von Photomultipliern (16) in elektrische Ladungen umgesetzt. Die Ladungsmenge wird analogelektronisch über die gewünschte Messzeit integriert.
• 2. Bestückung mit elektrooptischen Multikanal-Sensoren (4) Es sind elektrooptische Multikanal-Sensoren verfügbar, die aus nebeneinanderliegenden lichtempfindlichen Elementen, so genannten Pixeln bestehen. Die Breite der Pixel ist in der Größenordnung ähnlich der Breite eines Austrittspaltes wie unter <1.> beschrieben. Jedes Pixel ist in der Lage, die auf sie fallende Strahlung in Ladung umzusetzen und diese Ladung gleichzeitig zu sammeln. elektrooptische Multikanal-Sensoren mit 128 bis über 10000 Pixel sind in verschiedenen Technologien kommerziell verfügbar (CCD, CCD, CMOS oder Photodiodenarrays). Werden auf der Fokalkurve (6) einer Konkavgitteroptik elektrooptischen Multikanal-Sensoren montiert, können ganze Spektralbereiche simultan erfaßt werden.

The radiation emitted by the plasma passes either on the direct light path or via a light guide ( 18 ) in a spectrometer optics. Spectrometer optics with concave gratings ( 14 ) are in use. In this type of optics spectral lines are used as wavelength-dependent diffraction patterns of the entrance slit ( 13 ) on a focal curve ( 6 ). To capture this spectrum, two design principles are common.

• 1. Assembly with exit slits and photomultipliers (PMT) ( 3 ) On the focal curve ( 6 ), individual lines are separated by so-called exit slits ( 15 ) hidden. The amount of radiation falling through the exit slit is measured by photomultipliers ( 16 ) converted into electrical charges. The amount of charge is integrated analogelectronic over the desired measurement time.
• 2. Assembly with electro-optical multi-channel sensors ( 4 ) There are electro-optical multi-channel sensors available, which consist of adjacent photosensitive elements, so-called pixels. The width of the pixels is on the order of magnitude similar to the width of an exit slit as described under <1.>. Each pixel is able to convert the radiation incident on it into charge and to collect this charge at the same time. Electro-optical multi-channel sensors with 128 to over 10000 pixels are commercially available in various technologies (CCD, CCD, CMOS or photodiode arrays). Be on the focal curve ( 6 ) mounted concave grating optics electro-optical multi-channel sensors, whole spectral ranges can be detected simultaneously.

Out the measured quantities of charge belonging to certain spectral lines an unknown sample can be used via appropriate Calibration functions whose element concentrations are calculated.

• 1. Es soll möglichst ein linearer Zusammenhang zwischen Elementkonzentration und abgestrahlter Linienintensität bestehen
• 2. Ist ein Analyt nicht in der Probe vorhanden, sollte ein möglichst geringes Signal auf der zugehörigen Spektrallinie gemessen werden
• 3. Die gewählte Spektrallinie sollte nicht von Linien anderer Elemente überlagert sein

The following criteria apply to the selection of spectral lines:

• 1. It should as possible be a linear relationship between element concentration and emitted line intensity
• 2. If an analyte is not present in the sample, the lowest possible signal on the associated spectral line should be measured
• 3. The selected spectral line should not be superimposed by lines of other elements

In the spark AES points two and three, however, are never perfect to fulfill. High-resolution lines overlap often with less sensitive lines of other elements. By condensed Metal particles and the hot sample surface the line spectrum is superimposed by a continuum caused by temperature radiation. Temperature fluctuations lead fluctuations in the Kontiuumstrahlung and thus limit the detection sensitivity spark emission spectrometry. Note that this continuum inevitably is present, as it is only in a several 1000 K hot plasma to excite the emission lines to be measured. Are interference lines superimposed, limit accuracy of determination of the interfering element and variations of the interfering signal the detection limit of the analyte. Also in the choice of the spark duration, and the current during A spark has to compromise between optimal material evaporation and optimal stimulation.

Radio emission spectrometer and the limits of detection that could be achieved with it were at Kipsch [4], Mika [5], Kudermann [6], Slickers [7], Thomsen [8] and Lührs [9] in detail Detection sensitivity, structure and requirements described.

A variety of applications are known which require detection limits that are difficult or impossible with emission spectrometry are rich. Such applications include the determination of lead marks below 2 ppm in cast iron, the measurement of Te, Sb and As traces in electrolytic copper, the quantitative determination of impurities in precious metals and the measurement of Au, Ir, Pd and Pt in the content range 10 ppb up to 100 ppm in lead.

to Improving the detection limits are spectrometer optics spectral resolutions used up to 10 pm. This increases the relationship between line and line Background signal. Disturbance lines beyond 10 pm are hidden. The high resolution is characterized by narrow entrance and exit slots, long focal lengths and reached high grating line numbers. Long focal lengths and narrow However, gaps cause a low in real spectrometer designs optical conductance and thus low signal intensities, what the detection capability again limited. In addition, lead Grid focal lengths over 1000 mm to unwieldy large optics systems.

One further way to improve the detection sensitivity exists in that, during the radiation emitted in the first microsecond of the spark does not enter the Include measurement. While This first spark phase has the current leading channel between sample surface and Counter electrode only a few micrometers in diameter. This comes There it too high current densities and peak temperatures of over 10,000 K, which cause strong continuum radiation.

Also this method only reduces the disturbing continuum radiation without her entirely too remove.

• • Es gibt keine überlagerten Störlinien, weil nur Linien des Analyten zur Fluoreszenz angeregt werden
• • Der Einfluß des thermische Untergrund kann durch Pulsen der Fluoreszenz-Anregungsquelle wirksam unterdrückt werden

Atomic fluorescence spectrometers are known, which are mainly used for single element determination of gases and liquids. 5 outlines an exemplary setup used to analyze Pb in water and Fe in etching gases. A detailed description can be found in Grazhulene / Khvostikov [2] [3]. A tantalum spiral is wetted with a drop of the liquid to be analyzed. The tantalum spiral is heated up so much by a current pulse that the sample material is evaporated and atomized. The resulting atomic cloud is illuminated by a directed radiation of a pulsed Pb hollow cathode lamp. It comes to fluorescence: the lead atoms are excited by the radiation of the Pb hollow cathode lamp and emit when the electrons fall back into a lower energy state light of a Pb spectral line. The fluorescent light, unlike the excitation light, propagates in all directions. If a photomultiplier is arranged at right angles to the excitation light source and the incidence of excitation radiation is prevented by a suitable diaphragm, only fluorescence radiation is measured. A lock-in amplifier ensures that only light of the modulation frequency of the hollow cathode lamp is integrated. This eliminates the thermal background of the cloud and the tantalum spiral. The atomic fluorescence spectrometry (AFS) has the following advantages over the spark AES:

• • There are no superimposed interfering lines because only lines of the analyte are excited to fluoresce
• • The influence of the thermal background can be effectively suppressed by pulsing the fluorescence excitation source

For Pb in Water became with the device described in [2] and [3] a Detection limit of 0.35 ppb achieved.

One The use of AFS for the direct analysis of metals is currently not known.

description the invention

object the invention is a device with a matched measuring method, which allows an atomic fluorescence measurement of metals and because the elimination of influences lower detection limits from thermal background and interference lines allowed. This will mainly affect the detection sensitivity the radio spectroscopy limiting interfering radiation sources off.

The degradation of the sample ( 2 ) is carried out by a spark with the usual parameters in the spark AES. Approximately 300 μs after the current-carrying phase of a single spark discharge, the emission has largely subsided. The metal vapor cloud has relaxed and cooled almost to normal pressure / room temperature. However, there remains an atomic cloud ( 10 ) between sample ( 2 ) and counterelectrode ( 3 ), which only completely coagulate to condensate particles a few milliseconds later. If the analyte is now to be determined, the cloud of atoms can be exposed to a monochromatic radiation of such a wavelength that can be absorbed by the analyte atoms. The excited analyte atoms leave the excited state after typically 10 ^-12 s. Upon return to the ground state, radiation of the same wavelength as the exciting radiation is emitted. This is called resonance fluorescence. However, not all atoms return to the ground state, some take energy levels higher than the ground state. When switching to a level above the ground level, the emitted radiation has a lower frequency than the stimulating one. The emission of longer-wave radiation, triggered by excitation with short-wave radiation from the ground state is referred to as shifted or Stokes fluorescence net. Favorable irradiation times for fluorescence generation are between 50 and 500 μs. With increasing time interval from the spark event, the atomic cloud is removed (blown) by the purge gas from its original position.

The measuring device of the invention ( 5 ) is constructed as follows: To generate the fluorescence, an auxiliary radiation source ( 21 ) used.

• 1. Durch ein Funkenplasma nach Patentanspruch 3 In diesem Fall besteht die Hilfsstrahlungsquelle aus einem zweiten Funkenstand, einem zweiten Anregungsgenerator und einer Metallprobe, die fest auf dem zweiten Funkenstand montiert ist. Die Metallprobe enthält nur Elemente, die zur Fluoreszenz vorgesehen sind. Der Funken des Hilfsgenerators wird zeitlich verzögert gegenüber dem Hauptfunken ausgelöst, nämlich dann, wenn dort nach einer Entladung eine erkaltete Atomwolke vorliegt.
• 2. Durch ein Bogenplasma nach Patentanspruch 4 Alternativ kann statt eines Funkengenerators ein Bogengenerator verwendet werden. Es empfiehlt sich, einen Generator mit variablen Bogenstrom zu benutzen. Der Bogen brennt dann kontinuierlich mit niedrigem Strom, wenn keine Fluoreszenz benötigt wird. Sobald die Atomwolke im Hauptfunkenstand zur Fluoreszenz angeregt werden soll sorgt eine Stromüberhöhung für ausreichende Signalintensität. Es ist zu bemerken, dass der Strom nicht beliebig überhöht werden kann. Bei starken Strömen kommt es zu Verbreiterungen der Analytlinien und zu Selbstabsorbtionseffekten. Damit ist eine Verringerung der Linienintensität bei den Wellenlängen, die Fluoreszenz auslösen können verbunden.
• 3. Durch eine gepulste HKL als Hilfsstrahlungsquelle (nach Anspruch 5) Die gepulste HKL als Hilfsstrahlungsquelle wird ähnlich wie die gepulste Bogenentladung betrieben. Die Lampe brennt, solange keine Fluoreszenzanregung benötigt wird, mit einem niedrigen Lampenstrom (einige mA) wird Fluoreszenzanregung gewünscht, wird der Lampenstrom auf den maximal zulässigen Wert (meist in der Größenordnung 100 mA) erhöht. Alternativ kann der Lampenstrom aber auch im Kiloherzbereich ein- und komplett ausgeschaltet werden. Dann stehen für die einem Funken nachgeschaltete Fluoreszenzphase mehrere Lampenpulse zur Verfügung. Bei der gepulsten HKL kann es zu den gleichen Problemen wie beim elektrischen Bogen als Hilfsanregungsquelle kommen. Auch hier hat eine Stromüberhöhung möglicherweise eine Linienverbreiterung und Selbstabsorption zur Folge.
• 4. Durch einen Farbstofflaser nach Anspruch 6 Ein Farbstofflaser kann direkt die zur Erzeugung der Fluoreszenz erforderliche Wellenlänge erzeugen. Sollen mehrere Analyten gemessen werden ist es sinnvoll, den Lichtleiter (11) durch ein Lichtleiterbündel zu ersetzen, dessen Einzellichtleiter zu je einem Farbstofflaser führt.
• 5. Durch ein laserinduziertes Plasma nach Anspruch 7 Das laserinduzierte Plasma als Hilfsstrahlungsquelle ist in seinen Eigenschaften dem Funkenplasma ähnlich. Von der Hilfsprobe wird durch einen Laserimpuls Material verdampft, welches ein Plasma bildet, das zur Fluoreszenzauslösung verwendbare Strahlung aussendet.

This auxiliary radiation source can be realized as follows:

• 1. By a spark plasma according to claim 3 In this case, the auxiliary radiation source consists of a second spark, a second excitation generator and a metal sample, which is fixedly mounted on the second spark stand. The metal sample contains only elements intended for fluorescence. The spark of the auxiliary generator is triggered delayed with respect to the main spark, namely, when there is a cooled-off atomic cloud after a discharge.
• 2. By a bow plasma according to claim 4 Alternatively, instead of a spark generator, a bow generator can be used. It is recommended to use a generator with variable arc current. The arc then burns continuously with low current when no fluorescence is needed. As soon as the atomic cloud in the main spark level is to be excited to fluoresce, an excess of current ensures sufficient signal intensity. It should be noted that the current can not be increased arbitrarily. Strong currents cause broadening of the analyte lines and self-absorption effects. This is associated with a reduction in line intensity at the wavelengths that can trigger fluorescence.
• 3. By a pulsed HVAC as an auxiliary radiation source (according to claim 5) The pulsed HVAC as an auxiliary radiation source is operated similarly to the pulsed arc discharge. The lamp burns, as long as no fluorescence excitation is needed, with a low lamp current (a few mA), fluorescence excitation is desired, the lamp current is increased to the maximum allowable value (usually in the order of 100 mA). Alternatively, however, the lamp current can also be switched on and off completely in the kilohertz range. Then there are several lamp pulses available for the fluorescence phase connected downstream of a spark. The pulsed HVAC can have the same problems as the electric arc as an auxiliary excitation source. Again, an excess current may result in line broadening and self-absorption.
• 4. By a Dye Laser According to Claim 6 A dye laser can directly generate the wavelength required to produce the fluorescence. If several analytes are to be measured, it makes sense to connect the light guide ( 11 ) To replace by a fiber optic bundle whose individual light guide leads to a respective dye laser.
• 5. By a laser-induced plasma according to claim 7 The laser-induced plasma as an auxiliary radiation source is similar in its properties to the spark plasma. From the auxiliary sample material is evaporated by a laser pulse, which forms a plasma that emits usable radiation for fluorescence release.

The auxiliary light sources mentioned under 1-4 emit a complex spectrum which, in addition to the spectrum of the analyte or analytes, can also contain lines of Ar, O or N. A pre-decomposition optics ( 19 ) allows the radiation emitted by the auxiliary radiation source to be limited to a defined frequency range, usually an area in which only lines of the analyte or analytes are to be found. For most applications, a bandpass color filter is sufficient, as will be discussed in the discussion of a concrete invention implementation. However, it is also possible to use narrowband interference filters, grating or prism optics for pre-decomposition.

A modulation device ( 20 ) allows chopping the fluorescence excitation light. As chopper according to claim 8 rotating diaphragms or according to claim 9 electro-optical diaphragms such as Pockels or Kerr cells can be used. The chopping of the fluorescence excitation light causes a modulation of the fluorescence emission. This is desirable for the following reason: fluorescence excitation occurs after the end of the current-carrying phase of a spark. Although the radiation emission sounds exponential, residual radiation is due to resonance fluorescence (light quanta are repeatedly absorbed and re-emitted time-delayed) and radiation from forbidden transitions after more than 300 μs calculated from the end of the current flow. If the fluorescent light is now modulated, can according to claim 10 using a lock-in amplifier ( 24 ) in front of the integrator ( 25 ) the modulated signal component is separated from the unmodulated residual emission signal.

In order to use the proven in the spark OES spark stands, it is recommended to measure according to claim 2 exclusively the Stokes fluorescence (shifted fluorescence). Although resonance fluorescence is prinzpiell usable, in In this case, however, precautions must be taken that no light of the auxiliary radiation source the light outlet ( 32 ) reached. Light inlet opening ( 33 ) and light outlet are to be aligned at right angles to each other. Scattering at the tip of the electrode ( 3 ), the surface of the sample ( 2 ) and coagulated condensate particles must be prevented. That's difficult to put into practice.

In 5 the construction is reproduced for the exclusive use of Stokes fluorescence. The spark tripod ( 1 ) with counter electrode ( 3 ) is then largely constructed as usual in the spark AES. The only extension consists of a light guide or direct light inlet ( 11 ), after the plasma has cooled, the remaining atomic cloud ( 10 ) is illuminated.

The optical system is similar to the one in 3 featured classic setup with photomultiplier detection. However, the spectral gap widths can be chosen to be much larger than for the spark OES because i. A. no spectral interference are to be feared. Further gaps allow smaller designs, a higher optical conductance and make the gap adjustment less critical.

It is advantageous according to claim 11, the power supply of the photomultiplier ( 23 ) and according to claim 12, the output of the photomultiplier via an analog switch ( 26 ) from the integrator ( 25 ) to separate. During the emission process, the photomultiplier remains switched off and disconnected from the integrator input. Only when the fluorescent light source is switched on is the connection to the integrator established and the PMT voltage switched on. This largely prevents the photocurrent emanating from the emission signal from reaching the integrator. It is useful, in addition, an optical shutter ( 22 ) either according to claim 14 in front of the exit slits or according to claim 13 to provide in front of the entrance slit. This prevents the emission radiation from reaching the photocathode of the photomultiplier. If this precaution is not taken, a parasitic current pulse results at the beginning of the fluorescence measurement after the spark. This current pulse stems from the fact that photons of the emission radiation have released electrons from the PMT cathode, which are not completely recombined. When switching on the PMT power supply ( 23 ) and switching the analogue switch ( 25 ) to the integrator these electrons are amplified in the PMT.

A variant that does not require modulated auxiliary radiation source is in 6 shown. Here is according to claim 16 via an additional analog switch ( 34 ) after odd radio numbers the fluorescence signal to the first integrator ( 36 ), after even radio numbers on the second ( 35 ). Only with odd radio numbers but the auxiliary radiation source is turned on. If now the charge of the first integrator is reduced by that of the second (this can be done by software after analog-to-digital conversion in the ADC ( 27 ) in the microcomputer ( 29 )), this forms the difference:
(Fluorescence signal + residual emission_uneven_fused) - residual emission_even_fused

So it remains exactly the fluorescence signal back. The equipment after 6 is less expensive than that of 5 because neither electro-optical switches nor a lock-in amplifier is needed. Because of the exponential emission intensity drop after a spark event, it is essential that all sparks have the same emissions profile from the ignition point. If the emission curve of individual sparks is stretched or compressed in time, the residual emission of the odd sparks no longer coincides with that of the even sparks and the measured charge difference no longer corresponds to the fluorescence signal.

In principle, it is also possible to use multi-channel detectors as radiation sensors. However, this type of sensor has its own characteristic that its pixels convert incident radiation into charge and at the same time integrate this charge. A lock-in amplifier can therefore not be connected between the sensor and the integrator. As a result, a separation of residual emission and fluorescence useful signal is not readily possible, which leads to a deterioration of the detection sensitivity by at least two orders of magnitude. However, detection limits are still achievable that are more than an order of magnitude better than with the spark AES. A system equipped with a multi-channel sensor optics is in 7 outlined. The closures ( 22 ) in front of the sensor or in front of the entrance slit again serve to block the emission radiation. In contrast to optical systems with PMT, the emission signal must be blocked either in front of the sensor or before the entrance splitting because the much stronger emission signal otherwise saturates the sensors and makes detection of the fluorescence signal impossible.

In order to eliminate the disturbing emission residual light, according to claim 15, a lock-in amplifier function can be modeled using two optics. 8th shows how such a double optics can be interpreted. Over a Y-shaped bundle consisting of two light guides ( 18 ) the radiation from the light outlet ( 33 ) to two optics ( 39 ) ( 40 ) guided. Alternatively, the radiation may also be transmitted via the direct light path in conjunction with a semitransparent mirror or two outlet openings (FIG. 33 ) into the optics become. In front of both entrance gaps there is an electro-optical or electromechanical switch which can close the entrance gap. During the emission, ie up to approximately 300 μs after the end of the current-carrying spark phase, both closures ( 37 ) and ( 38 ) closed. During the fluorescence phase, the light of the auxiliary radiation source ( 21 ) and in the duty cycle 1: 1 from the modulator ( 20 ) interrupted or let through. The closure ( 37 ) of the first optics are always open, even if the modulator ( 20 ) Lets light through. As a result, the sensors of optic 1 receive the sum of fluorescence and emission residual radiation. The closure ( 38 ) of the second optics is always open when the modulator ( 20 ) closed is. The sensors of optics 2 therefore only see emission residual radiation. The desired fluorescence signal of a certain wavelength is obtained by subtracting the signal intensities of the associated pixels of optics 2 from those of the optics 1. Some optical designs allow two or more sensors to be positioned so that both capture a same spectral range. In this case, it is sufficient according to claim 17, in front of the sensors to attach an electro-optical shutter or a microchannel plate. Here, too, the closure or microchannel plate are switched so that one of the sensors registers the sum of fluorescence signal and residual emission, while the other only registers the residual emission. The fluorescence signal sought is again obtained by subtracting corresponding pixel intensities.

description an execution the invention

To conclude a concrete execution of the Invention described for the measurement of the element Pb in cast iron is designed.

With the spark AES, a determination of Pb contents below 2 ppm is problematic. Even low levels of element Pb can cause considerable damage to foundry furnaces. It is therefore desirable to be able to reliably determine Pb from a content of 0.1 ppm. This analytical task can be solved with a structure that is basically according to the arrangement 6 equivalent.

It will discuss details of the design of individual assemblies.

Anreaungsgenerator ( 4 )

It becomes a commercial one Spark generator (Spectro Source 3000) used with the discharge currents of various current courses can be generated. The spark repetition frequency is 200 Hz.

Spark level ( 1 )

It uses a spark stand of a standard spark OES device (SpectroLab of the company Spectro), which has as standard several light taps for the extraction of radiation via a light guide. 9 shows the principle. A window ( 42 ) ensures the tightness of the spark chamber ( 7 ). The radiation ( 30 ) of the excited atom cloud ( 10 ) is a biconvex lens ( 43 ) on the end of a light guide ( 11 ) focused. Spark stands with up to four such light taps are used as standard. One of the light taps can be used without hardware changes to couple the radiation from the auxiliary radiation source. This is done by the light coming from the auxiliary radiation source from the fiber optic ( 11 ) and through the biconvex lens ( 43 ) on the atomic cloud ( 10 ) is focused.

Auxiliary radiation source ( 21 ) and filters ( 19 )

The construction of the auxiliary radiation source ( 21 ) 10 refer to. It consists of another spark stand ( 57 ) on which a pure lead sample ( 54 ) is attached. The activation takes place via a second spark generator ( 55 ). The sparks of the second generator ( 55 ) are controlled as follows: Once in the main radio booth ( 1 ) stops the flow of current, the voltage drop across the current measuring resistor ( 41 ) back to zero. This is done at the output of the negative edge-triggered delay circuit ( 56 ) generates a positive voltage pulse of duration 300 μs. This time is the delay time between emission and onset of fluorescence measurement. At the end of the delay time, the positive flank-triggered pulse generator ( 58 ) triggered. In parallel, the output is used to generate a 1-bit counter ( 61 ) weiterzuschalten. The auxiliary spark generator ( 55 ) for odd-numbered radio numbers and activated pulse generator for fluorescence generation ( 58 ) generate a spark. This is achieved by switching the output of the pulse generator ( 58 ) with the output of the 1-bit counter via an AND gate ( 63 ) linked together. Both the output of the pulse generator ( 59 ) as well as the counter output ( 60 ) are also used to control the integration process.

In the auxiliary spark level ( 57 ) creates radiation for a variety of Pb lines. One of the strongest Pb spectral lines has the wavelength Pb 283,307 nm. It starts from the ground state and has an excitation energy of 4.37 eV. When light of this wavelength irradiates Pb atoms in the ground state, they are excited. A part of the excited atoms does not fall back to the ground state, but only to a low-energetic excited state and radiates thereby in Stokes fluorescence the line Pb 405.782 nm from.

A bandpass filter ( 53 ) ensures that only radiation between 250 and 390 nm is transmitted and not radiation of the wavelength 405.7 nm from the auxiliary spark level ( 57 ) in the main radio booth ( 1 ). The attenuation of the wavelength Pb 405.7 nm is at least 10 ^-6 . This ensures that everything in the main radio booth ( 1 ) light of Pb atoms of the sample to be analyzed during the fluorescence phase ( 2 ).

optical system

The optical system has the following key data;
Holographic concave grating with 2400 lines per mm and 750 mm focal length
An exit slit with photomultiplier, intended for measuring wavelength Pb 405.78 nm
Incidence angle 43 °, exit angle 16.95 °
The gap widths of entrance slit and exit slit are each 0.5 mm and are therefore much wider than usual in the spark OES (typically 10 μm for entrance slit and 25 μm for the exit slit).

Switchable PMT power supply ( 23 ), Analogue switch ( 26 ) Switching between the integrators ( 34 ) and integrators ( 35 and 36 )

11 shows the analog electronics of the system. A high voltage of approx. - 1000V is connected to the cathode and via ten series-connected resistors ( 48 ) connected to ground. The dynodes ( 49 ) are connected to the voltage divider so that the n + 1st dynode has a potential of 100 V with respect to the nth dynode. The first dynode has a potential of 100 V with respect to the cathode ( 50 ). These potential differences accelerate the photocathode escaping electrons from dynode to dynode, with each electron striking a dynode several electrons are knocked out of this. Is via an analog switch ( 47 ) put the first dynode on cathode potential, this avalanche effect does not come about. From the anode ( 51 ) flows a greatly reduced photocurrent. To further weaken the photocurrent, an analogue switch ( 26 ) the PMT output during the emission phase of the main spark level ( 1 ) connected to ground and only during the time frame provided for fluorescence measurement to the integrators ( 35 and 36 ) through. The switch ( 34 ), controlled by the 1-bit counter ( 61 ) of the auxiliary radiation source (s. 10 ), connects the output of the analogue switch in case of odd radio numbers ( 26 ) with the odd radio number integrator ( 36 ). For even radio numbers, the connection to a second integrator ( 35 ) produced. Since only with odd radio numbers during the fluorescence time window and the auxiliary radiation source generates a spark, accumulates in integrator ( 35 ) only photocurrents, which consists of residual radiation of the largely decayed emission. In the integrator for odd photocurrents ( 36 ) photocurrents are integrated, which additionally contain the actually interesting fluorescence signal. Forming the voltage difference at the integrator outputs ( 52 ), we obtain the sought fluorescence signal. For the integrators, the circuit principle of analogue switches ( 62 ) resettable miller integrators.

In 12 is the complete time sequence of an odd spark, so a spark, followed by a fluorescence measurement with activated auxiliary radiation source reproduced. The spark current ( 64 ) in the main radio booth ( 1 ) sounds out and hears at the moment ( 67 ) completely. Now the waiting time, which is necessary, so that the residual emission radiation ( 65 ) drops to a value close to zero (realized in the example by delay circuit ( 56 )). The current from the anode of the photomultiplier ( 66 ) is reduced because the first dynode is still controlled by analogue switches ( 47 ) is held at cathode potential. After expiration of the delay circuit ( 56 ) predetermined delay time is at the time ( 68 ) the analogue switch ( 47 ) and the fluorescence discharge in the auxiliary spark level ( 57 ) started. It is convenient to separate the read line from ground potential via analogue switches ( 26 ) only a few microseconds later at the time ( 69 ) because turning on the PMT is associated with a small glitch on the read line. The implementation of this time delay is for clarity in 11 and 12 not included. At the end of the fluorescence phase, at the time ( 70 ) the read line is grounded and at the time ( 71 ) the voltage of the first PMT dynode switched off again. In the Miller Integrator for odd radio numbers ( 36 ), only the portion of the PMT anode current ( 66 ), which is used between the times ( 69 ) and ( 70 ) was. The timing diagram for even radio numbers looks similar. But then the radiation curve is between the times ( 69 ) and ( 70 ) flat and near zero, since no fluorescence excitation occurs.

Analog multiplexer ( 28 ), A / D converter part ( 27 ) and microcomputers ( 29 )

The outputs of the integrators are via analog multiplexer with an analog-to-digital converter ( 27 ) connected. A microcomputer ( 29 ) controls the measurement process and further processes the measured raw intensities. Analog multiplexer ( 28 ), A / D converter part ( 27 ) and microcomputers ( 29 ) correspond to those of a standard spark OES device.

With The described arrangement can be measured Pb contents from 100 ppb.

Cited non-patent literature

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• [7] K. A. Slickers: Automatic Atomic Emission Spectral Analysis, Bruehl University Press, To water, 1992
• [8] V. Thomsen: Modern Spectrochemical Analysis of Metals, ASM International, Materials Park, OH, 1996
• [9] C. Lührs, G. Kudermann: Spark Spectrometry, Chemists Committee of the GDMB Society for Mining, Metallurgy, Raw Materials and Environmental Technology, Clausthal-Zellerfeld, 1996

1

spark stand (Tripod)

2

sample

3

counter electrode

4

excitation generator

5

optical system

6

focal curve

7

Argon flushed spark chamber

8th

ionized route

9

Spark chamber opening

10

Mushroom cloud

11

optical fiber or fiber optic bundles

12

Fluorescence excitation source

13

entrance slit

14

concave grating

15

exit slit

16

Photomultiplier tubes (PMT)

17

Multi-channel sensors

18

direct Light path, optical fiber or fiber optic bundles

19

filter or pre-decomposition optics

20

electro-optical or electromechanical modulator

21

Auxiliary radiation source for fluorescence excitation

22

electro-optical or electromechanical closure

23

disconnectable Power supply for photomultiplier

24

Lock-in amplifier

25

integrator

26

analog switches

27

Analog to digital converter

28

analog multiplexer for switching the PMT outputs

29

microcomputer

30

fluorescence radiation

31

Integrator / integrators for one PMT channel

32

analog multiplexer for switching the multi-channel sensor outputs

33

Lichtauslassöffnung

34

analog switches for even / odd radio numbers

35

integrator for straight radio numbers

36

integrator for odd radio numbers

37

electro-optical or electromechanical shutter, with interrupted auxiliary radiation source closed

38

electro-optical or electromechanical shutter, with interrupted auxiliary radiation source open

39

optics for measuring the sum signal from fluorescence and residual emission

40

optics for measuring the residual emission

41

Low impedance Resistor for current measurement

42

quartz window

43

biconvex

44

angle of reflection

45

angle of incidence

46

changeover switch for switching between integrators for even / odd radio numbers

47

Acceleration voltage switch first photomultiplier dynode

48

Dynode voltage divider chain

49

dynodes

50

photocathode

51

anode (Read line)

52

Integrator output to the multiplexer ( 28 )

53

Bandpass color filter

54

Pure lead sample

55

Auxiliary spark generator

56

Delay circuit negative edge triggered

57

Auxiliary spark stand

58

Pulse generator Positive edge triggered

59

Cable for controlling the analogue switch ( 20 )

60

Line to the integrator changeover switch ( 46 )

61

1 bit counter

62

Integrator reset switch

63

And gate

64

Current flow in the main spark level ( 1 )

65

Radiation in the main radio booth ( 1 )

66

Photocurrent from the anode ( 51 ) of the PMT ( 16 )

67

The End the current flow in the main spark gap

68

Switch-on time high voltage supply PMT ( 16 ) via analogue switch ( 47 )

69

Switch-on time of read line PMT ( 16 ) on integrator for odd radio numbers ( 36 )

70

Disconnection timing of read line PMT ( 16 ) of integrator for odd radio numbers ( 36 )

71

Switch-off time high-voltage supply PMT ( 16 ) via analogue switch ( 47 )

72

timeline

Claims (20)

Atomic fluorescence spectrometer comprising - a spark generator for generating a plasma in a region between an electrode and a sample surface, - a spectrometer optics having a first entrance slit, a dispersive element and a number of sensors, and having - an electronic control and evaluation, characterized in that the spectrometer has an auxiliary radiation source which is optically set up to irradiate the area between the electrode and the sample surface and is provided for generating fluorescence radiation, and the spectrometer has at least one further sensor for detecting the fluorescence radiation, the radiation having either a second spectrometer optics Falls a second entrance slit or through an entrance slit with variable gap width on the other sensor. Spectrometer according to claim 1, characterized that the sensor for detecting the fluorescence radiation, a second Entrance gap is assigned, whose entrance gap is larger as the entrance slit width of the first entrance slit. Spectrometer according to one of the preceding claims, characterized characterized in that as an auxiliary radiation source for generating the Fluorescence is provided an electrical spark. Spectrometer according to one of the preceding claims, characterized characterized in that as an auxiliary radiation source for generating the Fluorescence is provided an electric arc. Spectrometer according to one of the preceding claims, characterized characterized in that as an auxiliary radiation source for generating the Fluorescence is a pulsed hollow cathode lamp is provided. Spectrometer according to one of the preceding claims, characterized characterized in that as an auxiliary radiation source for generating the Fluorescence at least one dye laser is provided. Spectrometer according to one of the preceding claims, characterized characterized in that as an auxiliary radiation source for generating the Fluorescence a laser-induced plasma is provided. Spectrometer according to one of the preceding claims, characterized characterized in that the detection of the fluorescence radiation in the wavelength range a shifted fluorescence (Stokes fluorescence) takes place whose Wavelength is greater as that of the auxiliary radiation source. Spectrometer according to one of the preceding claims, characterized characterized in that in the beam path between the auxiliary radiation source and the area provided a shutter for modulation of the auxiliary radiation is, in particular a mechanical shutter, a Kerr cell, a Pockels cell or other electro-optical shutter. Spectrometer according to claim 8, characterized that the at least one further sensor is a photomultiplier and that between photomultiplier and integrator on the Mo dulation, in particular on the modulation frequency tuned lock-in amplifier for Filtering the emission residual radiation is connected. Spectrometer according to one of the preceding claims 8 or 9, characterized in that the controller is set up is while the Emission of the plasma generated by the spark generator in the area turn off the voltage on one or more dynodes of the photomultiplier tubes or reduce. Spectrometer according to one of the preceding claims 8 or 9, characterized in that the controller is set up is while the Emission of the plasma generated by the spark generator in the area with the help of an electronic switch the photomultiplier output separate from the signal integrator. Spectrometer according to one of the preceding claims 8 or 9, characterized in that the controller is adapted, during the emission of the plasma generated by the spark generator in the region, the beam path between the plasma and at least one sensor to break with a mechanical shutter. Spectrometer according to one of the preceding claims, characterized characterized in that two further sensors are provided, which have two spectrometer optics can be acted upon by the fluorescent radiation. Method for analyzing metallic samples by means of Atomic fluorescence spectrometry with spark atomization, characterized that after completion of the emission processes fluorescence by a Auxiliary radiation source generates and before condensation of the atomic cloud is measured. Method according to claim 14, characterized in that that for measurement the shifted fluorescence (Stokes fluorescence) is being used. Method according to one of the preceding claims, characterized in that the auxiliary radiation source is amplitude-modulated becomes. Method according to one of the preceding claims, characterized characterized in that the fluorescence signal by means of two or more equipped with multi-channel sensors Spectrometer optics is determined, with a first optics the sum from fluorescence signal and residual emission records, a second Optics only the residual emission records, and the sought fluorescence signal for one Wavelength λ itself Difference of the measured values for λ in the first Optics and the second optics yields. Method according to one of the preceding claims, characterized characterized in that the fluorescence signals are odd and even spark of a spark sequence on two different integrators switched and only with odd sparks, the fluorescence auxiliary radiation is turned on, so that the emission residual radiation during the Fluorescence measurement eliminated by subtraction of the two integrals become. Method according to one of the preceding claims, characterized characterized in that an optics with at least two multi-channel sensors for detecting a wavelength equipped is whose fluorescence is to be measured, with a first sensor the sum of fluorescence signal and residual emission and a second Sensor records only the residual emission and where the fluorescence signal for one Wavelength λ as difference the readings for λ in the first Sensor and the second sensor is determined.