US3898501A

US3898501A - Light source lamp for atomic light absorption analysis

Info

Publication number: US3898501A
Application number: US468528A
Authority: US
Inventors: Akira Hosoya; Makoto Tadokoro; Sadami Tomita; Yoji Arai; Shinji Mayama
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1973-05-11
Filing date: 1974-05-09
Publication date: 1975-08-05
Anticipated expiration: 1992-08-05
Also published as: GB1468606A; JPS503390A

Abstract

In a lamp used as light source for atomic light absorption analysis, comprising a cathode having a hollow therein, an anode disposed in the vicinity thereof, a hermetical envelope to enclose the cathode and the anode, and gaseous atmosphere contained in envelope, the cathode is formed of an alloy composed of silver and at least one metal having a melting point equal to or lower than 500*C and emitting the same spectral line as the metal to be analyzed so that the cathode is prevented from being deformed and the luminous intensity and the analytic accuracy are improved.

Description

United States Patent Hosoya et a1. Aug. 5, 1975 [54] LIGHT SOURCE LAMP FOR ATOMIC 3,422,301 l/l969 Sebens et a1. 313/346 R X LIGHT ABSORPTION ANALYSIS 3,623,136 1 H1971 TOmlta et a1. 313/178 3,725,716 4/1973 Yamasaki v. 313/218 X [75] In n Ak r ym Mskoto Tadokoro, 3,820,487 1 1974 Tomita et a1. 1. 313/311 x both of Hitachi; adami Tomita, Katsuta; Yoji Arai, Katsuta; Shinji Mayama, Katsum, all of Japan Primary E.iammerP almer C. Demeo [73] A H} h Ltd J Attorney, Agent, or FzrmCra1g & Antonelll ss1gnee: lac 1, apan [22] Filed: May 9, 1974 57 ABSTRACT [21] Appl. No.: 468,528 1 In a lamp used as light source for atomic light absorption analysis, comprising a cathode having a hollow [3O] Forelgn Appthcatlon pnonty Data therein, an anode disposed in the vicinity thereof, a May 11.1973 Japan 48-51644 hermetical envelope to enclose the Cathode and the anode, and gaseous atmosphere contained in enve- [52] 313/178 313/2095 lope, the cathode is formed of an alloy composed of 51 I Cl 61 0 8 silver and at least one metal having a melting point f 346 equal to or lower than 500C and emitting the same 1 0 I spectral line as the metal to be analyzed so that the cathode is prevented from being deformed and the lu- References Cited minous intensity and the analytic accuracy are improved. UNITED STATES PATENTS I 3,089,054 5/1963 Walsh ct al. 313/209 X 10 Claims, 15 Drawing Figures Si-TEE? PATENTEU AUG 5|975 SHEET 2 EMmEZEW I MQW FIG. 7

PATENIEUIUB 51% 3,898,501

F|G.8 I50 IIIII LUMINOUS INTENSITY (RELATIVE VALUE) l I I I II I STABILITY TIME (MINUTE) 0 IO 203040506070809OIU) CONTENT OF CADMIUM IN ALLOY (ATOMIC PAIENTED 5I975 3.898.501

SHEET 7 FIG. 9

IIIIII STABILITY TIME (MINUTE) LUMINOUS INTENSITY (RELATIVE VALUE) l O m 4 m 00 6 K; 3 0 5 8 lb o3'o4 05 06bToao9omo CONTENT OF TIN IN ALLOY (ATOMIC pmgmmus 51915 3,898,501

SHEET 8 FIG. I0

I lllIl I x -IO LUMINOUS INTENSITY (RELATIVE VALUE) 5 3 STABILITY TIME (MINUTE) I. lllllllllj CONTENT OF LEAD IN ALLOY (ATOMIC%I PATENTEU AUG 5 I975 FIG.

T 1 N E On R U C n w 2 E M 0/0 l A o o w n u 0/ O/ 1 0 2 m v 0 O 2 PATENTED 5M5 3,898,501

' SHEET 10 5 FIG. l2

0 Zn i E a z 0 E g (I) 0 8m; Z g

22 I llllllllo l lllllliz DISCHARGE CURRENT (mA) SHEET FIG.

I llllllll I02 DISCHARGE CURRENT (mA) mi H mo o P O l H ///.I I w W m LIGHT SOURCE LAMP FOR ATOMIC LIGHT ABSORPTION ANALYSIS The present invention relates to a lamp used as light source for atomic light absorption analysis, and more particularly to the constitution of the cathode of such a lamp.

The atomic light absorption analysis based on the principle of atomic light absorption is used for the quantitative analysis of the metal salts contained in a sample solution.

The atomic light absorption analysis is adapted to the case where the quantity of the sample is very small and very frequently used in the field of medicine, industrial chemistry, food chemistry or oil chemistry. In a typical example of this analysis, a metal salt in a sample solution is decomposed into atomic vapor of metal, by means of thermal energy due to flame and a resonance spectral line from an external light source is passed through the metal vapors to be absorbed in the vapors and the degree of absorption of the spectral line gives the result of analysis. The basic matters concerning the analysis are disclosed in the US. Pat. No. 2,847,899 specification.

In the method of atomic light absorption analysis, only the spectral line having specific wavelength is absorbed in the atomic vapor and the analysis is effected on the basis of the predetermined relationship existing between the quantity of absorbed light and the concentration of the atom to be analyzed. Accordingly, even if other kinds of atoms coexist in the material forming the cathode of the light source lamp used in the analysis, there is no nuisance unless there is any overlap of spectral lines. Therefore, the cathode may ,be formed of any suitable alloy or metal composite.

In practice, the cathode is formed of an alloy of several metals fused, or to prepare a material for use as cathode the powder of metals or alloys are mingled, pressed into shape and sintered or a porous sintered material is impregnated with a fused metal or alloy having a melting point lower than that of the porous sintered material.

The alloy or composite cathode is widely used in an atomic light absorption analyzer adapted especially for the analysis of metal having a melting point lower than 500C. This is because of the restrictions imposed upon the fabrication of a lamp used as light source for the analyzer. Namely, such a light source lamp is fabricated as follows. Electrodes are arranged in place in a glass envelope, inert gas such as argon or neon gas is introduced within the envelope and the open end of the envelope is hermetically sealed. At the time of sealing the envelope, the electrodes are heated up to near 500C. Accordingly, in the case of analyzing a metal having a melting point lower than 500C, if the cathode is made of the same metal as that to be analyzed so as to emit a resonance spectral line of the metal to be analyzed, the cathode is deformed due to the heating for sealing the glass envelope so that the proper function of the cathode is damaged. For this reason, the cathode is formed of alloy or composite metal to have a higher melting point and therefore a higher resistance to thermal deformation.

In the case of a cathode formed of alloy or composite metal (this term means throughout this specification such a sintered body of shaped metal-powder or a sintered porous body impregnated with fused metal or alloy as mentioned above), the metal (hereafter referred to as the other metal) combined with a metal which emits the same resonance spectral lines as the metal to be analyzed, needs to have the following properties.

I. Where formed of alloy,

la the other metal must form an alloy having a melting point higher than 500C,

lb the metal must be able to be shaped into a cathode, and

1c the other metal must not emit light having wavelengths equal to or very near those of light emitted from the metal to be analyzed.

2. Where formed of composite metal,

2a the composite cathode must have a melting point higher than 500C,

2b the powders are easy to be pressed in the shape of cathode, and

2c the wavelengths must not overlap those of the metal to be analyzed.

In such aspects as mentioned above, many investigations have been made and it has now been proved preferable to use copper as the other metal. In the case of copper used as the other metal, however, there is a first drawback that the kinds of metals (to be analyzed) are strictly limited. The concrete description will be given below. The wavelengths corresponding to the spectral lines emitted from metals having melting points lower than 500C, are within a range of 2,000 A to 3,000 A. Table 1 given below shows the wavelengths of the spectral lines of the metals having melting points lower than 500C and it is seen that all the metals except thallium satisfy the condition mentioned above.

On the other hand, copper has wavelengths of spectral lines 2,136 A near that of zinc, 2,293 A near that of cadmium, and 3,072 A near that of bismuth. With a combination of copper with zinc, cadmium and/or bismuth, therefore, some of the spectral lines partially overlap with one another so that high accuracy in analysis becomes impossible.

A second drawback of copper used as the other metal is that the luminous intensity is diminished since the concentration quenching phenomenon is caused owing to the self-absorption due to too much vapor of luminous metal being generated under saturated condition because of the small sputtering rate of copper. The phenomenon takes place as follows. At sputtering, copper and metal having a low melting point, in the form of small particles or atoms are emitted from the surface of the cathode. Since copper has a small sputtering rate, the metal having a low melting point occupies the major part of the sputtered particles. Accordingly, unexcited atoms or particles exit in excess of electrons to be collided with during excitation so that the selfabsorption of light, i.e., concentration quenching phenomenon, takes place to diminish the luminous intensity. The luminous intensity can be increased to a certain extent by increasing the discharge current, but the increase in the discharge current will be accompanied by the degradation in the life time of the cathode. Moreover, the analysis is started after the sputtered particles have reached saturation and the spectral lines have been stabilized. And when such an element like copper as having a small sputtering rate is included, the time when the sputtered atoms or particles reach a predetermined concentration is so slowly reached that the warming-up period, i.e., the period during which the emitted spectral lines converge into stability, is too long.

It is, therefore, one object of the present invention to provide a cathode which has a high accuracy in analysis even if the other metal is combined with each or some of many kinds of metals having melting points lower than 500C.

Another object of the present invention is to provide a cathode in which the other metal has so large a sputtering rate that the self-absorption of light may hardly take place.

An additional object of the present invention is to provide a cathode which has a greater luminous intensity and a shorter warming-up period than a conventional cathode using copper as the other metal.

According to the present invention, which has been made to attain the above objects, the cathode is formed of an alloy of silver and at least one of the metals having melting points lower than 500C and emitting resonance spectral lines. Silver does not emit light having a wavelength lying between 2,000 A and 3,000 A, nor has a spectral line overlapping the spectral line of thallium, i.e., a wavelength of 4,096 A. Therefore, silver can be combined with any of the metals given in the Table l and also can attain a high accuracy in analysis. Moreover, according to the present invention, the hollowed cathode must be made of an alloy prepared through melting. Other methods besides alloying through melting can not attain satisfactory luminous intensity and accuracy in analysis. In the case where the cathode is not formed of an alloy made through melting but of composite metal, the powder metallurgy techniques must be usually used. The composite metal prepared through the powder metallurgy contains a considerable amount of gas, especially oxygen and the oxygen can not be completely removed even through degassing operation in the lamp fabrication procedure, so that the lamp quality is very much degraded, that is, the luminous intensity is decreased and the accuracy in analysis is also degraded. On the other hand, through alloying techniques, the melting of metals in vacuum or non-oxidizing atmosphere can shut out oxygen so that the drawback inevitable with the powder metallurgy can be eliminated. Thus, high luminous intensity and high accuracy in analysis can be attained.

Silver has a much larger sputtering rate than copper. Therefore, by the use of silver, the concentration of the sputtered atoms of the metal under its unexcited state can be kept small, which metal has a low melting point and emits the resonance spectral lines. Accordingly, the self-absorption of light can be avoided so that the concentration quenching phenomenon never takes place. Moreover, the time during which the sputtered particles reach saturation in the lamp envelope is shorter as compared with the case where copper is used in place of silver and the time of starting analysis can be quickly reached.

The cathode provided according to the present invention can also be applied to the analysis of plural metals having melting points lower than 500C. For this purpose, the cathode is formed of a multi-element alloy of silver and plural metals which can emit the same spectral lines as the metals to be analyzed. In the formation of such a multi-element alloy, however, it is necessary to except a combination of zinc and tellurium whose spectral lines are proximate to each other and a-combination of zinc and lead which are separated from each other and can not form an alloy. An alloy of cadmium, zinc and silver or an alloy of cadmium, lead and silver is suitable for a cathode used in a so-called multi-element lamp employed in the analysis of plural elements.

The preferable composition of a zinc-silver alloy cathode is 20 to 80 at. percent (atomic percent), especially 40 to 60 at. percent of Zinc and silver as the rest. If zinc is less than 20 at. percent, the luminous intensity becomes very poor, and if zinc is more than 80 at. percent, the resultant alloy becomes brittle so that the cutting of the alloy into shape is almost impossible. The preferable composition of a bismuth-silver alloy cathode is 40 to 80 at. percent of bismuth and silver as the rest. If bismuth is less than 40 at. percent, the luminous intensity is poor, and if it is more than 80 at. percent, the period during which the spectral lines are stabilized, i.e., the warming-up period, rapidly increases. Especially, 50 to percent of bismuth gives an optimum condition.

The cadmium-silver alloy cathode has a preferable composition of cadmium 15 to 70 at. percent and silver as the rest and the luminous intensity of the cathode takes the maximum value when cadmium is 50 at. percent and decreases if the atomic percentage of cadmium is less or more than 50 at. percent. And the warming-up period increases with the increase in the content of cadmium.

The tin-silver alloy cathode has a preferable composition of time 30 to 70 at. percent and silver as the rest.

The lead-silver alloy cathode has a preferable composition of lead 30 to at. percent, especially 50 to 70 at. percent, and silver as the rest.

The selenium-silver alloy cathode has a preferable composition of selenium 33 to 40 at. percent and silver as the rest.

The cadmium-lead-silver alloy cathode has an optimum composition of cadmium 15 to 30 at. percent, lead 40 to 60 at. percent and silver as the rest. If the amount of cadmium is excessive or the amount of lead is decreased, then the sputtered particles of cadmium abnormally increase so that the concentration quenching phenomenon takes place.

In the case of the cadmium-zinc-silver alloy cathode,

the concentration quenching phenomenon tends to be caused since the vapor pressures of cadmium and zinc are both high, so that the amount of silver in the alloy should be increased to control the relative concentrations of Cd, Zn and Ag. A preferable composition consists of cadmium 5 to 30 at. percent, zinc l0 to 30 at. percent and silver as the rest.

Each of the alloys given above may be prepared through melting in inert gas if each constituent is sufficiently deoxidized and has a purity of higher than 99.9

percent. If, however, each constituent is not sufficiently deoxidized, the melting of the constituents should be performed in vacuum and the forced deoxidization of the constituents should be carried out in vacuum for a sufficient time so that the purity of the resultant alloy may be higher than 99.9 percent. The alloy in the molten state is then poured into a mold having the shape of cathode to be used and finally finished after necessary cutting. The completed cathode is placed in a glass envelope, the glass envelope is degassed, inert gas such as argon, neon or helium gas is introduced within the envelope, and the end of the envelope is hermetically sealed. The degassing operation is usually performed to such an extent that the pressure in the envelope is about 5 X mmHg. Neon gas at pressures of 5 to 9 mmHg is usually contained in the envelope.

Other features and advantages of the present invention will be apparent when one reads the following description of the preferred embodiments with the aid of the attached drawings in which:

FIG. 1 is a longitudinal cross section of a lamp used as light source in an atomic light absorption analyzer;

FIG. 2 shows the structure of the electrodes of a lamp as shown in FIG. 1;

FIG. 3 shows in graphical representation two curves, one indicating the luminous intensity of the zinc spectral line emitted from a light source lamp using a zincsilver alloy cathode and the other representing the time required for the zinc spectral line to be stabilized;

FIG. 4 shows the spectrum of light emitted from silver;

FIG. 5 shows the spectrum of light emitted from copper;

FIG. 6 shows in graphical representation two curves, one indicating the luminous intensity of the bismuth spectral line emitted from a light source lamp using a bismuth-silver alloy cathode and the other representing the time required for the zinc spectral line to be stabilized;

FIG. 7 shows in graphical representation two curves, one indicating the luminous intensity of the selenium spectral line emitted from a light source lamp using a selenium-silver alloy cathode and the other representing the time required for the selenium spectral line to be stabilized;

FIG. 8 shows in graphical representation two curves, one indicating the luminous intensity of the cadmium spectral line emitted from a light source lamp using a cadmium-silver alloy cathode and the other representing the time required for the cadmium spectral line to be stabilized;

FIG. 9 shows in graphical representation two curves, one indicating the luminous intensity of the time spectral line emitted from a light source lamp using a tinsilver alloy cathode and the other representing the time required for the tin spectral line to be stabilized;

FIG. 10 shows in graphical representation two curves, one indicating the luminous intensity of the lead spectral line emitted from a light source lamp using a lead-silver alloy cathode and the other representing the time required for the lead spectral line to be stabilized;

FIGS. 11 and 12 show in graphical representation the relationships between the luminous intensity of a light source lamp using a cadmium-zinc-silver alloy cathode and the discharge current; and

FIGS. 13 to 15 show in graphical representation the relationships between the luminous intensity of a light source lamp using a cadmium-lead-silver alloy cathode and the discharge current.

A lamp used as light source in an atomic light absorption analyzer usually has such a structure as shown in FIG. 1. In FIG. 1, a cathode l is formed of an alloy of silver and other elements which emit the same resonance spectral lines as the metals to be analyzed. The cathode 1 has a cylindrical form with a hollow 2 therein to increase the luminous intensity. The resonance spectral lines are created in the hollow 2. An anode 3 has a ring-like form and the discharge between the cathode 1 and the anode 3 gives rise to the resonance spectral lines. The cathode 1 and the anode 3 are connected respectively with a cathode lead 4 and an anode lead 5. A discharge protection plate 6 is made of, for example, mica. An insulating tube 7 is usually made of steatite. An insulating tube 8 is provided for the anode lead 5. A metal tube 9 made of, for example, nickel covers the surface of the cathode 1 except that of the hollow 2 to create the resonance spectral lines therein. The metal tube 9 serves to prevent the discharge between the anode 3 and the surface of the cathode 1 except that of the hollow 2. A hermetical envelope 10 is usually made of transparent glass. The hermetic envelope 10 contains therein inert gas such as argon, neon or helium gas. The envelope 10 is coupled to a base socket 11 by means ofa metal tube 12 made of, for example, nickel. A window 13 through which the resonance spectral lines are emitted is usually made of quartz glass.

When an appropriate voltage is applied between the cathode 1 and the anode 3 of the lamp having the structure described above, the discharge current flows between the

electrodes

1 and 3. Accordingly, the inert gas in the envelope 10 is ionized to produce positive ions, which bombard the surface of the hollow 2. As a result, the atoms of the metals forming the cathode 1 are evaporated due to sputtering effect and Joules heating, and the vaporized atoms are excited in the hollow 2 to emit the resonance spectral lines.

FIG. 2 shows the rough structure of the electrodes of the lamp shown in FIG. 1. In FIG. 2, the arrows indicate the direction of travel of light having the resonance spectral lines. The discharge between the cathode l and the anode 3 is controlled to occur in the normal glow region in order to suppress the consumption of the cathode body through sputtering, and especially in that normal glow region which lies near the abnormal glow region.

EMBODIMENT 1 Silver having a purity of 99.9 percent and zinc having a purity of 99.9 percent were fused in argon gas and nine kinds of alloys having different compositions were formed. The alloys in the molten state were poured into molds and cast into cathodes. The cathodes were finished after necessary cutting. Each of the finished cathodes has an outer diameter 8 mm, a height 20 mm, an inner diameter of cylindrical hollow 4 mm and a depth of the hollow 15 mm. Each of the cathodes was placed together with the mated anode and other parts in a glass envelope to fabricate a lamp as shown in FIG. 1. Neon gas at pressure of 9 mmHg was contained in the envelope. With the lamps equipped with the cathodes, the luminous intensity of the zinc spectral line and the time (hereafter referred to as stability time for brevity) during which the spectral line is stabilized were'measured. The discharge current was mA. The result of the measurement is shown in FIG. 3. The luminous intensity is plotted in the relative values to a lamp equipped with a cathode formed of an alloy consisting of zinc 10 at. percent and silver as the rest, the luminous intensity of which is assumed to be 10. The luminous intensity is represented in solid curve 21 while the stability time is represented in dashed curve 20. The luminous intensity is maximum when the content of zinc in the alloy is 50 at. percent and it is lowered when the content is more or less than 50 at. percent. The stability time tends to be shorter with the increase in the content of silver in the alloy. With a cathode of pure zinc, the relative value of the luminous intensity is 20 and the luminous intensity of the light source lamp can be increased by using a cathode formed of zinc-silver alloy containing zinc of more than at. percent.

As a conventional cathode used in a light source lamp for the analysis of zinc is known a sintered body consisting of zinc and copper powders mixed in a ratio of l l. The luminous intensity of this sintered cathode is about 30 if the intensity of the cathode of an alloy consisting of zinc 10 at. percent and silver as the rest is assumed to be 10. And the stability time of the sintered cathode is about 30 minutes. The cathode of zincsilver alloy has a stability time by far shorter than that of the sintered cathode and a cathode of an alloy consisting of zinc to 80 at. percent and silver as the rest has a high luminous intensity, too.

Thus, a zinc-silver alloy cathode has a higher luminous intensity and a shorter stability time than zinccopper sintered cathode. This is due to the fact that the sputtering rate of silver is greater than that of copper and therefore the concentration quenching phenomenon hardly takes place.

FIGS. 4 and 5 respectively show the spectral lines of silver and copper. It is seen from the inspection of FIGS. 4 and 5 that silver has no spectral line in the vicinity of the spectral line at 2,138.6 A of zinc while copper has a spectral line at 2,136 A. Therefore, with a zinc-copper sintered cathode, the spectral lines of zinc and copper overlap each other so that the accuracy in analysis tends to be lowered.

EMBODIMENT 2 Bismuth-silver alloy cathodes were formed the dimensions and the fabricating method of which were the same as the cathode in the embodiment 1. FIG. 6 shows the luminous intensity and the stability time as the result of measurement with discharge current of 10 mA. The luminous intensity is represented in relative values to a cathode of alloy consisting of bismuth 10 at. percent and silver as the rest, the luminous intensity of which is assumed to be 10. The luminous intensity is especially large when the content of zinc in the alloy is 50 to 80 at. percent. The stability time is very short, that is, shorter than 14 minutes, when the content of silver in the alloy is more than 30 at. percent.

EMBODIMENT 3 Light source lamps equipped with selenium-silver alloy cathodes were fabricated according to the same process as in the embodiment l and the luminous intensity and the stability time of the selenium spectral line were measured with the discharge current of 10 mA. The result of the measurement is shown in FIG. 7. The

ity.

EMBODIMENT 4 Cathodes of cadmiumsilver alloys and a cathode of cadmium only were formed according to the same proce'ss as in the embodiment 1 and the luminous intensity and the stability time of the cadmium spectral line were measured with discharge current of 10 mA. The result of the measurement is shown in FIG. 8. The luminous intensity is represented in relative values to a cathode of alloy composed of cadmium 10 at. percent and silver as the rest, the luminous intensity of which is assumed to be 10. The cadmium-silver alloy cathode gives the maximum luminous intensity when the content of zinc in the alloy is 50 at. percent. The stability time is shorter with the increase in the content of silver in the alloy and especially short for silver content of more than 30 at. percent.

EMBODIMENT 5 Lamps having tin-silver alloy cathodes and a lamp having a cathode of tin only were fabricated according to the same process as used in the embodiment I and the luminous intensity and the stability time of the tin spectral line were measured with discharge current of 10 mA. FIG. 9 shows the result of the measurement. The luminous intensity is represented in relative values to a cathode of alloy composed of tin 10 at. percent and silver as the rest, the luminous intensity of which is assumed to be 10. The luminous intensity is very high when the content of tin in the alloy is 50 to at. percent.

EMBODIMENT 6 Lamps having lead-silver alloy cathodes and a lamp having a cathode of lead only were fabricated according to the same process as in the embodiment l and the luminous intensity and the stability time were measured with the discharge current of 10 mA. FIG. 10 shows the result of the measurement. The luminous intensity is represented in relative values to a cathode of alloy composed of lead 10 at. percent and silver as the rest, the luminous intensity of which is assumed to be 10. The lead-silver alloy cathode gives the maximum luminous intensity when the content of lead in the alloy is 60 at. percent and the stability time is very short when the content of silver in the alloy is more than 20 at. percent. Since the stability time of a conventional cathode which was fabricated by impregnating a porous body of sintered copper with lead and has the composition of copper at. percent and lead 25 at. percent, is about 40 minutes, then the cathode according to the present invention can be claimed to have an excellent quality.

EMBODIMENT 7 Lamps having cathodes of alloys composed respectively of cadmium 10 at. percent, zinc 10 at. percent and silver as the rest and of cadmium I0 at. percent,

zinc l5 at. percent and silver as the rest, were so fabricated as to have such a structure as shown in FIG. 1. The dimensions and the fabricating process of each cathode were the same as those of the cathode in the embodiment l and the luminous intensities of the cathodes with respect to the zinc spectral line at a wavelength 2138.6 A and the cadmium spectral line at a wavelength 2,288 A were measured. FIG. 11 shows the plotting of the luminous intensities of the cadmium and zinc spectral lines of a cathode of alloy composed of cadmium 10 at. percent, zinc 10 at. percent and silver as the rest, against the discharge current. The luminous intensities are represented in relative values which was defined by assuming the intensity of the cadmium spectral line to be 50 when the discharge current is 8 mA. FIG. 12 shows the plotting of the luminous intensities of the cadmium and zinc spectral lines of a cathode of alloy composed of cadmium 10 at. percent, zinc 15 at. percent and silver as the rest, against the discharge current. In this case, the luminous intensities are represented in relative values defined by assuming the intensity of the cadmium spectral line to be 70 when the discharge current is 10 mA. It is seen from FIGS. 11 and 12 that a higher luminous intensity can be obtained for the same discharge current in FIG. 12 than in FIG. 11. The stability times of the cadmium and zinc spectral lines of the cathode of alloy composed of cadmium 10 at. percent, zinc 10 at. percent and silver as the rest are about l minutes and about 6 minute, respectively and those of the cadmium and zinc spectral lines of the cathode of alloy composed of cadmium at. percent, zinc 15 at. percent and silver as the rest are about 17 minutes and about 10 minutes, respectively.

EMBODIMENT 8 Lamps having cathodes of cadmium-lead-silver alloys were so fabricated as to have such a structure as shown in FIG. 1. The dimensions and the fabricating process of each cathode were the same as those of the cathode in the embodiment l and the luminous intensities of the cadmium and lead spectral lines were measured. FIGS. l3, l4 and 15 respectively show the plottings of the luminous intensities of the cadmium and lead spectral lines of the cathodes of alloys composed of cadmium 30 at. percent, lead 50 at. percent and silver as the rest, of cadmium 20 at. percent, lead 60 at. percent and silver as the rest, and of cadmium 20 at. percent, lead 40 at. percent and silver as the rest, against the discharge currents. The luminous intensity of the lead spectral line varies almost in the same rate independent of the composition of the alloy, as seen from FIGS. l3, l4 and 15, while that of the cadmium spectral line varies in different manners depending upon the composition of the alloy. The cathode of alloy composed of cadmium 20 at. percent, lead 40 at. percent and silver as the rest, gives the maximum intensity of cadmium spectral line for a given discharge current. The stability times of the cadmium and lead spectral lines of the cathode of alloy composed of cadmium 20 at. percent, lead 60 at. percent and silver as the rest, were about 10 minutes and about l5 minutes, respectively.

What we claim is:

1. A lamp used as light source for atomic light absorption analysis, comprising a cathode having a hollow, an anode arranged in the vicinity of said cathode, a hermetical envelope to enclose said cathode and anode, and gaseous atmosphere contained in said envelope, wherein said cathode is formed of a molten alloy of silver and at least one metal selected from the group consisting of zinc, bismuth, cadmium, tin and lead and which can emit the same resonance spectral line as the metal to be analyzed, and said alloy has a purity equal to or higher than 99.9 percent.

2. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein said cathode is formed of an alloy composed of zinc 20 to 80 at. percent and silver as the rest.

3. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein said cathode is formed of an alloy composed of bismuth 40 to 80 at. percent and silver as the rest.

4. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein said cathode is formed of an alloy composed of cadmium 15 to at. percent and silver as the rest.

5. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein said cathode is formed of an alloy composed of tin 30 to 70 at. percent and silver as the rest.

6. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein said cathode is formed of an alloy composed of lead 30 to at. percent and silver as the rest.

7. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein said cathode is formed of an alloy composed of cadmium 15 to 30 at. percent, lead 40 to 60 at. percent and silver as the rest.

8. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein said cathode is formed of an alloy composed of cadmium 5 to 30 at. percent, zinc 10 to 30 at. percent and silver as the rest.

9. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein said cathode is formed of an alloy composed of silver having a purity equal to or higher than 99.9 percent and metal having a purity equal to or higher than 99.9 percent.

10. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein each of said at least one metal has a melting point lower than 500C.

Claims

1. A LAMP USED AS LIGHT SOURCE FOR ATOMIC LIGHT ABSORPTION ANALYSIS COMPRISING A CATHODE HAVING A HOLLOW, AN ANODE ARRANGED IN THE VICINITY OF SAID CATHODE, A HERMETICAL ENVELOPE TO ENCLOSE SAID CATHOE AND ANODE, AAND GASEOUS ATMOSPHERE CONTAINED IN SAID ENVELOPE, WHEREIN SAID CATHODE IS FORMED OF A MOLTEN ALLOY OF SILVER AND AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF ZINC, BISMUTH, CADMIUM, TIN AND LEAD AND WHICH CAN EMIT THE SAME RESONANCE SPECTRAL LINE AS THE METAL TO BE ANALYZED AND SAID ALLOY HAS A PURITY EQUAL TO OR HIGHER THAN 99.9 PERCENT.

4. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein said cathode is formed of an alloy composed of cadmium 15 to 70 at. percent and silver as the rest.

6. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein said cathode is formed of an alloy composed of lead 30 to 80 at. percent and silver as the rest.

10. A lamp used as light source for atomic light absorption analysis, as claimed in claim 1, wherein each of said at least one metal has a melting point lower than 500*C.