IMPROVED AUTOMATIC ATOMIC-ABSORPTION SPECTROMETER

Abstract: An atomic absorption spectrometer in which each sample is analyzed for all desired elements before starting the next sample. New features include the grating drive, lamp-carousel alignment, pulsed atom source, lamp-drift compensation, and dynamic range control. The grating (G) is driven by an arm (A), the arm (A) by a taut band (B) wound on a drum (D), and the drum (D) directly by a motor. The lamp carousel (C) mounts on an ''L''-shaped rocker with one horizontal and one vertical arm. The carousel (C) rotates on a horizontal axis (P1) at the end of the vertical arm. The rocker itself pivots on a horizontal axis (P2) at the corner of the L; it is driven about its axis by a motor (M2) and screw (W) at the other end of the L. Carousel (C) rotation on its axis moves the right lamp into position and adjusts it accurately in vertical direction. Pivoting of the rocker simultaneously on its corner axis positions the lamp accurately in the horizontal direction. The pulsed atom source is a combined angled-gas-jet- and discharge unit. During pulses it yields high absorption with better detectability limits; average power is lower. The lamp-drift compensator makes double duty of the absorber pulsation to obtain a lamp-intensity reading between pulses. Dynamic range control is obtained by even further exploiting the pulsation, namely by taking measurements at a known delay (and decay) time after each pulse.

Description: IMPROVED AUTOMATIC ATOM IC-ABSORPTION SPECTROMETER General backg-nound inforrration. A (XJtTmonly used instrunenal analysis method for the elemental analysis of various materials for trace and minor elements is the atomic absorption method. In the most used manifestation the analytical instrument consists of a means for generating a vapor such as a nebulizer; a burner assembly to disassociate into fr-ee atoms the vapor delivered by the nebulizer: a source of moπoc-hrαnatic light such as a hollow cathode larrp vΛύch light is directed Chrough the atomized vapor, and a device for isolating and measuring the mc-oc-chromatic light after it has passed through the atomized vapor. The quantitative measurement of the elements present in the vapor and thus the liquid saπple from which the vapor was derived is made by comparing the intensity of the monochi ifcttic light characteristic of an element after absorption in the burner flame to the unabsσrbed intensity of the light source. DISCLOSURE. The spectrαi-eter system consists of the following basic parts: 1. A light source that has tp to 16 different hollow cathode lamps mounted in a carousel so that each lamp can be rapidly and accurately -positioned into the optical path when it is needed. 2. A saπpling system that converts a saπple into an atomic vapor so that the absorption of the atoms can be determined and related to the concentration in the original saπple. 3. A mxirochromator that isolates the wavelength -region of interest, converts the light into an electronic signal via a photomultiplier Cube and displays and records the -results. 4. A computer system Chat controls the system and processes and displays the measixnementε . There are 5 ijiprovementε over prior art tάiich we claim as inventions: 1. Grating drive 2. Laπp carousel optical aligπnent method 3. Pulsed atom source 4. Coπpensation method for laπp intensity drift 5. Dynamic range control These will be discussed in order. 1. Grating Drive. The πtonochrαiator has a grating that disperses light of different wavelengths in different directions. The wavelength of light Chat passes through the monocliromator is determined by Che rotational position of the grating. The rotational position of the grating is determined by a unique drive mechanism. Figure 1 is a schematic diagram of Che grating and its unique drive mechanism. The grating is mounted so that iCε front surface G pivots about an axis at point P. An arm A is attached Co Che grating mount Co allow the grating to be rotated as the Caut band or wire B pulls σn the arm. The Caut band or wire is connected to the arm at a hinge point H. A spring S helps to keep Che band or wire Caut. The band is connected Co a drum D aC point C and wraps around drun, less Chan one time or -up Co marry times around Che drun. The drun is rotated about its axis M by a motor (stepping, synchronous, or servo). This mechanism is unique for at least two reasons: (l)because a taut band or wire connects Che arm of the grating drive to a motorized drum and (2)because the particular arrangement of the parts causes the angular position of Che grating, and more iπportantly the wavelength that passes through the monocluunator, Co be related by a unique mathematical function Co Che angle of drun D; no other grating drive mechanism relates the angle of a motor shaft Co the angle of the grating drive by this same mathematical function. This mathematical function depends upon the length of Che arm A, Che length of Che taut member B, the diameter of the drum D, the distance between the pivot points of the drun M and the grating P, and the shape of the drun. The drum may be circular in shape, or it may be * -t-achined Co have a shape that produces a particular desirable mathematical relationship. For example, the drum could be shaped so thaC Che angle of the drum position could be direcCly proportional to Che energy of the light passing through the monochranatσr (that is, inversely proportional Co Che wavelength). Or Che drun could be shaped so that the angle of Che drum position is direcCly pπ-portion Co wavelength. When che functional -relationship beCween motor position and wavelength deviates from a desired relationship, the deviation can be correcCed by Che ccrnputer supplying an offset nurnber for Che position of the motor. In this case the offset -nuribεr can be calculated more quickly Che deviation has Che simplest mathematical form. The versatility of the Caut band system to adjust the functional relationship by selecting dimensions listed in the previous paragraph helps to achieve a deviation with Che εiπplesC maChemaCical form. Prior art consist of Che sine-bar mechanism for driving a grating arm. This produces a drive vΛr se angular rotation of the motor is subsCantially directly proportional Co Che wavelength passing through Che monochromator. This drive requires a worm gear Chat is highly accurate Chroughσut iCs length, and it requires a point of contact between the carriage Chat rides on Che worm gear and Che arm that slides along Che arm. The worm gear and Che sliding contact are boCh subject Co wear. The taut band system is not subject to such wear because there are no gears if Che motor is connected directly Co Che drun shaft M, and there is no sliding contact that -moves along the arm A. The Caut band system also allows Che wavelength Co be changed πuch more rapidly than would be reasonable (because of rapid wear) for Che worm gear system. (The worm gear in this application must have a very fine pitch because Che grating must be positioned with great accuracy Co achieve a reproducible wavelengch seCCing Chat is bet er Chan 0.1 nanometers. The fine pitch -requires exceptionally high speed rotation for rapid scanning.) Prior art also consists of mounting the grating directly on a motor shaft. This does not have the wearing surfaces Chat Che si'e-bar has and it allows rapid scanning of Che wavelength. Connecting the motor shaft directly to the grating shaft produces only one mathematical relationship between Che motor position and the wavelength passing through he mc-ncchrornatσr: the relationship is nonlinear in wavelength and Che inverse of wavelength (frequency or photon energy). A special high precision motor is required to achieve the necessary wavelength precision and accuracy, and Che motor must be driven in such a manner Chat the nonlineariCy is compensated for vhen wavelength scanning is recorded on a strip chart recorder for example. In addition to being more versatile than the direct drive motor syste in the functional relationship between motor shaft position and wavelength, the Caut band system does not require a motor with such high precision ten Che diameter of the drun D is considerably less than the length of the arm A. The demand on precision is decreased by approximately the ratio of these two dimensions. Prior art also consists of a nonlinear drun or cam (mounted on a motor shaft) that is in direct contact with the grating arm. In this case the wavelength range covered is limited by Che extent of Che motion of Che arm as the cam rotates through one full revolution. A compromise is necessary between the range of the wavelength region covered and the accuracy and precision with which the wavelength can be set. The taut band system is able to cover a wide wavelength range with accuracy, especially when a circular drun is used and the diameter of the drum is smaller than the length of Che arm. (In this case the Caut band wraps around the drun more than once during a wide wavelength scan.) Unlike the cam drive, a nonlinear drun is used with Che Caut band system, the functional relationship beCween motor position and vΘvelength is determined not only by the shape of Che drun, but as mentioned above also by Che length of the arm A, the length of Che Caut member B, Che average diameter of the drun D, and Che distance between Che pivot points of the drun M and the grating P. The taut band system has no member chat rubs against the grating arm as does Che cam system. Another advantage is Chat Chiε is more efficient Chan Che acme lead screws and therefore energy -required and drive costs are lower for the same performance. 2. Laπp carousel optical alignment method. Each different chemical element whose absorption is Co be determined requires a different hollow cathode laπp. In a few cases one laπp may suffice for a few different elements. Ujp to 16 lamps are mxnCed on a laπp carousel Chat rapidly rotates each laπp into an accurately deCerr ned position in the optical path when Che laπp is needed. A -unique (and necessary) feature of Che carousel is ics ability to accurately and rapidly position each laπp Chat is present in the carousel. Ic does this rapidly because of the way that the carousel is pivoted about two points PI and P2 In Figure 2. Pivoting about PI moves the pro-pea laσp into position and positions it accurately in a substantially vertical direction. Pivotiηg about P2, -which occurs essentially during the same time that the π vement about PI occurs, positions the laπp accurately in a substantially horizonCal direction. Figure 2 shows the motion about P2 being done with a worm gear W driven by a motor K2. Other driving methods 4tτe possible, such as a direct co ectiσn of the shaft of E2 to a motor shaft. A -unique feature of the carousel is the use of pivoting arσur two pivots to align the laπp in the horizontal and vertical directions. Pivoting can be done more rapidly and with a simpler mechanism than moving Che entire carousel horizontally along a linear translation stage for example. In operation, v * hen a laπp is first mounted in Che carousel, Che computer system that controls the spectrometer scans the laπp alternately about PI and P2 two or three times in each direction Co determine ι» en Che laπp is centered on the optical axiε.The two motor poεitions (for the motors Chat drive the carousel about PI and P2) are then stored in memory for later use to position the laπp the laπp is needed during a chemical analysis. Prior art Includes a carousel Chat is moved around a pivot PI, and where the accuracy of alignment iε done by manual adjustments on the mount for each individual laπp. 3. Pulsed atom source One claim here is for a pulsed means of removing atoms from the surface of a solid saπple (and keeping the atoms in a substantially free state long enough for them to be observed) that combines Che -removal effects of (a) at least one jet of gas disposed adjacent such saπple surface and pointing at a substantially nonzero angle toward such surface and (b) a low pressure electrical discharge. There are Chree ways Co pulse this special Cype of discharge (1) pulse Che electrical discharge, (2) pulse Che gas jeC, and (3) pulse both siπulCaneously. Another claim iε that pulsing Che atom source has the advantage of getting high absorption generated by high current pulses without the instability associated with high current DC operation. This also permits better limits of detectability for less power costs. Prior art includes pulsing a hollo ' cathode laπp, which iε a glow discharge. It is not obvious however fnjn this prior art that this special combined discharge can be pulsed. For example, it would be reasonable to expect that a gas jet trfiβn suddenly turned on would blow out the electrical discharge. IC would also be reasonable Co expect that the presence of a jet prior to turning on the electrical discharge would make it impossible Co rapidly eεCablish during Che electrical pulse an electrical discharge in Che rapidly moving jet. Even if i were noC possible Co establish a pulsed mode of operation (where the jet or electrical discharge are suddenly Curned on) i iε still possible to establish Che previously known continuous mode of operation of the combined discharge by the different means of slowly turning cn the jet. 4. Compensation method for laπp intensity drift Prior art. An absorption measurement requires knowledge of Che light source signal both in the presence of and in the absence of the absorber. This iε usually done by physically placing a cell containing a nσnabsorbing substance in the light path and -recording the signal, and then physically removing the nonabsorber and physically placing a cell containing Che absorber in he light path and recording Che signal. This is done rather frequently Co compensate for changes in the light source intensity with time. This meChod is also used Co compensate for changes in the light source intensiCy with wavelength during a spectral scan. It iε uniquely possible when using a pulsed atom source to supply the absorbing atoms from the sample material, to obtain Chese Cwo signals by recording Che signal before each pulse and then recording Che signal after the pulsed atom source has been turned on and the absorbing εaπple atoms are in the optical path. The general claim here is either (a) Chiε method of obtaining Chese Cwo signal before and during each pulse or/and (b) the use of a pulsed absorber to allow this method to be used without using an external force to physically manipulate the nonabsorber and absorber between these measurements. By making these tv-o measurements for each pulse, it iε poεεible to compensate for relatively rapid changes with time in Che light source intensity. If the measurement of the nonabsorber signal iε made jαst prior to (within microseconds of) turning on the sampling pulse, then the compensation can be achieved on a time scale that is considerably shorter than can be done with Che methods that involve physical manipulation (even with mechanically vibrating systems) ; more rapid changes in the lamp intensity can thereby be tolerated. This pulεed method alεo has Che advantage of observing Che same part of the beam from the laπp for both signals, and with the same optical cocrponents in the beam. Double beam systems either observe different parts of the beam at the same time, or the same part of the beam wiCh different optical components in each beam. When different parts of the beam are observed, each part may behave somewhat differently, and the compensation will not, Chen, be completely accurate. When Che same part is observed by different optical coπponents, then the optical components may not be identical, causing the cαnpensatiσn to be somewhat inaccurate. The claims presented here are not restricted to solid saπple atomization but apply to flame ato ization as well. Another embodiment of thiε claim iε the use of a pulsed li^rt source Chat is syr hrϋiiized with the pulsed absorbing source so Chat a nonabsorber light measurement pulse occurs just prior to the -pulse of the absorbing source, and another occurs after the absorbing source is pulsed on. l 0 Also he separation of the atomization process from the absorption measurement temporally, also eliminates the e isεion noiεe from the absorption measurement thus iπproving detection limits. In addition high intensity light source pulεeε reduce front end noiεe improving detection limits. 5. Dynamic range control. As mentioned in the previous patent application, an ύiportant limitation of absorption -methods is the limiCed dynamic range (of absorbance values and Cherefore concentrations) over -which accurate meaεunementε can be πede. The use of a pulsed source of absorbing atoms allows the dynamic range to be easily extended without changing wavelength and wiChout a significanC loss of time or saπple. This uniqe method involves making measurements at a known delay time after the absorber pulse is turned off (and the absorption has had a chance Co decay) -when the absorption is too high during the pulse. When a continuous light source is used, then many measurements can be made at fixed delay-time intervals after each pulse. The data for the best delay-time inCerval are then selected for use after the measurements have been completed. When a pulsed light source is used, then a measurement made during a previous absorber pulse can be used to select the delay time to pulse the laπp for the present absorber pulse. Alternatively, the delay time can be set before a saπple is run if some previous knowledge of Che approximate range of concenCratiσn iε available for Che sample. In practice, Che relationship between delay time and absorbance relative to Che abεorbance Chat occurs during the absorber pulse iε determined by measurement. Thiε relationship is used to esCabliεh a l i large dynamic range over vΛich concentrations can be accurately compared; Chat is, the delay time and Che measured absorbance are both entered into Chiε relationship to obtain an effective absorbance (σr parameter with some other name) for comparison purposes over a large dynamic range of concentrations. - l -T- The Analyte 16 differs in two fundamental ways from currently existing multielement AA -instruments. First, it analyzes each sample completely for all the desired elements, before moving on to the next sample. Conventional multielement AA's analyze a group of, say, 50 samples by determining a single element in all of them, then going to a second element, and so on.] Second, the Analyte 16 is intended for use with the Analyte Atomsource 1211 nan-tfaeπi B l Atomizer, which is designed for the analysis of solid, undissolved samples. [Note to Ed: Conventional AA's use either a flame or a furnace atomizer, both of which are most suitable for the analysis of liquids.] Equipped with the Atomsource Atomizer, the Analyte 16 preserves the flame AA advantages of specificity and ease of use, but provides even better precision (0.12 RSD) , and the ability to determine a wide range of concentrations, from ppm to consti- - tuent levels, with little or no sample preparation. The Analyte 16 SUBSTITUTE SHEET Analyte 16 AA - 2 also has good sensitivity for refractory elements such as B, Si, Ti, and W, which give problems by conventional AA. A labora¬ tory might therefore consider the use of an Analyte 16 AA Spectro- photo eter instead of, or as a supplement to, an X-ray fluorescence or an arc/spark emission spectrometer. The Analyte 16 is equipped with a turret that can hold sixteen hollow-cathode lamps, some of which can be multielement sources. If the first element to be determined in a given sample is, say, silver, and the second is iron, the automated operation is as follows: The source optics of the instrument set themselves to accept the light from the silver hollow cathode lamp, while the drives to the preselected silver wavelength. Immediately after the silver has been determined, the optics move to select the emissiun from the iron lamp, while the monochromator moves to the iron wavelength, and so on until the sample has been completely analyzed. The time for each determination can be as low as 5 seconds per element. A sample can be analyzed for ten elements in 60 seconds, and for 20 elements in under two minutes. Physically, the Analyte 16 consists of three modules. The optical module includes the lamp turret, monochromator, power supplies, and microprocessor controllers. The second module is the Atomsource Atomizer. The third module is an IBM-compatible personal computer, which provides all the required control and intelligence for instrument and atomizer. The Analyte 16 can also be provided with a conventional atomic absorption flame atomizer. In the flame mode, the Ana- Analyte 16 AA - 3 lyte 16 has performance that " is approximately equivalent to that of a good sequential ICP (inductively coupled plasma) spectro¬ meter. The Analyte 16 has, however, far fewer spectral inter¬ ferences, and can use all the well-established atomic absorption methods that have been developed over the past 25 years. The Analyte 16 is therefore worth considering as an alternative to sequential ICP instruments.

Claims: WE CLAIM : All of the novel apparatus, methods / and other subject matter hereinabove disclosed.

Inventors: BERNHARD AL E, SCUITTO THEODORE JAMES, SCUITTO THOMAS JOHN, BERNHARD, AL, E., SCUITTO, THEODORE, JAMES, SCUITTO, THOMAS, JOHN
Applicants: BERNHARD AL E, SCUITTO THEODORE JAMES, SCUITTO THOMAS JOHN, BERNHARD, AL, E., SCUITTO, THEODORE, JAMES, SCUITTO, THOMAS, JOHN
Application year: 1988
Main classification: G01J3
IPC codes: G01J3, G01N21