Automatic A computer-controlled data acquisition and analysis … · 2019. 8. 1. · j. P. ShieldsandE. H.PiepmeierAcomputercontrolled data acquisition andanalysis system determined

Journal of Automatic Chemistry, Vol. 13, No. 4 (July-August 1991), pp. 129-137

A computer-controlled data acquisition andanalysis system for use in thecharacterization of plasma atomic emissionsources

James P. Shields* and Edward H. PiepmeierDepartment of Chemistry, Oregon State University, Corvallis, Oregon 97331,USA.

The characterization ofnew spectroscopicplasma sourcesfor use inatomic emission spectroscopy often presents the needfor informationabout the spatial emission characteristics ofthe source. In addition,information about the variation of the emission over time is oftenbeneficial. It is also useful to be able to graphically see, in real-time,the effects of varying a particular experimental parameter. Thispaper describes a flexible data acquisition system in which anechelle spectrometer is interfaced to an IBM-PC compatiblemicrocomputer. The FORTH-based ASYST software, whichproves real-time display of acquired data, allows any point in theemission source to be imaged on the entrance slit ofthe spectrometerby using the cursor keys of the PC and a computer-controlledmirror. The different acquisition modes ofthe system are illustratedwith examples of two-dimensional spatial profiles of the plasmaand monitoring of plasma stability over time. In addition, anillustration of the optimization of the echelle’s aperture plate andphotomultiplier tube positions in the focal plane is presented.

Introduction

A variety of information is useful in attempting toinvestigate and improve the analytical capabilities of anew spectroscopic plasma emission source. For instance,common analytical figures ofmerit include determinationofdetection limits, linear dynamic range, short- and long-term precision, and susceptibility to chemical inter-ference effects. In addition to these common analyticalfigures of merit, several researchers [1-5] have empha-sized the need for spatial information about a plasmaunder varying experimental conditions, such as gas flowrates and input power. Spatial information is also usefulfor evaluating the effects of matrix components in asample on the analytical capabilities of a plasma [6-9].

While spatial and precision studies are useful in theevaluation of a new plasma emission source, it is oftendifficult or cumbersome to obtain this type ofinformationwith a commercially available spectrometer. Commercialsystems are often designed more for the purpose ofanalysing samples on a routine to semi-routine basisrather than for more specialized, diagnostic studies. Aswith other researchers [10], the authors have also found

Present address: Hewlett-Packard Company, Ink-Jet ComponentsDivision, 1020 NE Circle Blvd, Corvallis, Oregon 97330, USA.]- Author to whom correspondence should be sent.

that the data analysis and hard-copy output options ofcommercial spectrometers are often very limited.

After the completion of an experiment, it is very commonfor the analyst to plot the results. The authors’ early workutilizing post-experiment plotting often revealed a flaw oran unexpected result in the experiment that perhapscould have been corrected during the experiment if real-time plotting could have been done. Therefore, anotherdesirable capability to aid in the characterization ofa newplasma source is a means of real-time display of acquireddata in the form of plots to a graphics screen.

This paper describes the interfacing of an IBM-PCcompatible microcomputer to a high-resolution echellespectrometer to facilitate a variety of data acquisitionfunctions. An integrated, menu-driven program is pre-sented for the purposes of instrument control, dataacquisition, real-time plotting, post-experiment analysis,and hard-copy output. This program was developedusing ASYST, a commercially available data acquisitionand analysis programming environment. The applicationof this system to the characterization of a new plasmaemission source will be described.

Instrumentation

A schematic representation of the instrumentation isshown in figure 1. The spectrometer, the computer and itsperipherals, and the ASYST software will each bedescribed separately. The plasma emission source hasbeen described previously [11-13], and the instrumen-tation aspects of the source will not be covered here.

Spectrometer

A PLASMA-SPEC (Revision C Board, Leeman Labs,Lowell, Massachusetts, USA) high-resolution echellespectrometer was used to isolate the analytical spectralline of interest. The merits of echelle spectrometers forhigh temperature, line-rich emission sources, such asplasmas, have been described [14]. The most frequentlycited of these merits is the high resolution attainable. Aparticularly unique feature of the PLASMA-SPEC spec-trometer is the fixed nature of the two dispersion devices(a grating and a prism). Conventional spectrometersgenerally move a single dispersion device (a grating) toselect which wavelength will fall on a fixed detector.Indeed, previous echelle spectrometers involved the

0142-0453/91 $3.00 ( 1991 Taylor & Francis Ltd.129

J. P. Shields and E. H. Piepmeier A computer controlled data acquisition and analysis system

ECHELLEGRATING

COMPUTERCONTROLLEDMIRROR

PHOTOMULTIPLIERTUBE --IHOUSING APERTURE

PLATE

PRISM/LENS

PLANEMIRROR

COLLIMATINGMIRROR

PLASMASOURCE

Figure 1. Schematic representation of instrumentation. Image ofplasma source is focused onto entrance slit of echelle spectrometer by acomputer-controlled source mirror. Data from the spectrometer are sent to an IBM-PC compatible microcomputer via an RS-232 line.Hardcopy plots are output to a plotter and numeric data to a printer. (Figure adapted with permissionfrom ICP/ECHELLE brochure,Leeman Labs, Inc., Lowell, Massachusetts.)

movement of their dispersion devices to select lines ofinterest 15, 16].

The PLASMA-SPEC system utilizes fixed dispersiondevices. To achieve line selection, an aperture platelocated in the focal plane is moved in conjunction with aphotomultiplier tube. This aperture plate, shown infigure 1, contains hundreds of exit slits. To select ananalytical line, the aperture plate must be positioned sothat one of the exit slits is positioned over the point in thefocal plane where the wavelength and order of light ofinterest appears. In addition, the photomultiplier tube(PMT) must be positioned directly above this exit slit.

The PLASMA-SPEC spectrometer contains a Motorolla68000-based microprocessor which positions the PMTand aperture plate in the focal plane. However, for thepresent applications, complete control of the movementofthe aperture plate and PMT was required. The reasonsfor this requirement are twofold. First, the ROM of thespectrometer contains the PMT and plate co-ordinatesfor 230 lines, which includes 66 elements. It wasnecessary to access other lines which were not known tothe spectrometer, that is, whose plate and PMT co-

ordinates were not in the ROM of the spectrometer.Secondly, although the PLASMA-SPEC provided ameans of peaking the position of the aperture plateperiodically, no such means was readily available for thePMT co-ordinates. When the PMT co-ordinates in theROM of the spectrometer are used, an optimal PMTposition is sometimes not achieved with this early versionof the spectrometer. Therefore, a method of optimizingthe PMT and plate positions in the focal place wasneeded.

The PLASMA-SPEC contains one RS-232 communica-tions port through which ASCII coded commands can besent to the spectrometer. In addition, data from thespectrometer can be sent to an external device via thisRS-232 port. Many ofthe commands for the spectrometerare carried out using the spectrometer’s own code calledP-Code, which will be described in the section onsoftware. Inherent to the design of the PLASMA-SPEChardware is the collection of 32 individual data points peracquisition request. In other words, each time a request ismade for the spectrometer to begin an acquisition, theacquisition set always consists of 32 samplings which areevenly spaced in time. The duration of each data point is

130

j. P. Shields and E. H. Piepmeier A computer controlled data acquisition and analysis system

determined by the integration time selected, which canvary from 0"31 s/point to 3 s/point.

As shown in figure 1, the PLASMA-SPEC contains acomputer-controllable mirror which focuses an image ofthe emission source on the entrance slit of the spec-trometer. By moving this mirror horizontally and verti-cally, spatial mapping of the emission source can beachieved. The step resolution of this source mirror is0"21 mm for horizontal motion and 0"34 mm for verticalmotion.

Computer and peripheral devices

The PLASMA-SPEC was interfaced to a Corona PPC-2(Cordata, Thousand Oaks, California, USA), which is anIBM-PC compatible microcomputer. Communicationbetween the spectrometer and the computer was achievedvia an RS-232 port on a JRAM-3 multifunction board(Tall Tree Systems, Palo Alto, California, USA) installedin the Corona. In addition to the RS-232 port, themultifunction board also provided an additional parallelport, clock/calendar functions, and additional memory.

A Star Model SD-10 printer (Star Micronics, Irvine,California, USA) was used to print out data both duringand after experiments. An HP7475A plotter (Hewlett-Packard, San Diego, California, USA) was used as theoutput device for plotting purposes.

P-Code routines

Before describing the ASYST software, some understand-ing of the routines written in the native language of thespectrometer is necessary. P-Code is the programmingcode used to control the operation ofthe PLASMA-SPECechelle spectrometer. The P-Code instruction set consistsof 44 operation codes or ’op-codes’. These op-codes areused to control low-level functions such as loadingcounters and masking bits, as well as higher-levelfunctions such as initiating data collection and serial portdata transmission. A complete list of these op-codes, aswell as other details of the P-Code system, can be found inthe echelle spectrometer operation manual [17].

The execution of these op-codes is totally independent ofany external computer system. The op-codes are pro-cessed internally by the spectrometer’s own microproces-sor. The op-codes can be entered into sequential locationsin the spectrometer’s memory and executed starting atany location using the menu displayed on the screen ofthe spectrometer. Since any menu selection offered by thespectrometer can be remotely chosen by ASCII com-mands sent via the RS-232 line, P-Code routines can beexecuted as though they were special sub-routines ofsome external program. Indeed, the P-Code routineswhich were written were generally executed as sub-routines from the ASYST program running on the PC.

A total of nine routines were written. The first fourroutines are used to perform aperture plate and PMThigh-resolution spectral scans in the X and Y directions ofthe focal plane. These routines are mainly used inconjunction with externally running software to optimize

the aperture plate and PMT positions in the focal plane.This external software will be described in the nextsection. When a given scan is completed, the 32-pointdata set from the spectrometer is sent via the spec-trometer serial port to the external computer system.

A fifth routine collects data at a specific wavelength andorder. In other words, no spectral scanning is done. Theaperture plate and PMT are stationary during theexecution of this routine. As before, when the 32-pointdata set has been collected, transmission via the spec-trometer serial port is initiated. This routine is generallyused in conjunction with external software to collect dataonce an optimal set of aperture plate and PMT co-ordinates has been determined.

A sixth and seventh routine perform aperture plate X andY scans, respectively, as do routines and 2. However,these routines cover a wider spectral range than routine 1,and, therefore, are useful in investigating the backgroundaround an analytical line and in searching for lines not inthe ROM of the spectrometer.

An eighth routine spatially scans the emission source inthe horizontal direction by moving the source mirror ofthe spectrometer. The range of this spatial scan iscontrolled by selecting the step size of the source mirrormotor.

Finally, routine 9 spatially scans the emission source inthe vertical direction. The range of the scan is controlledby varying the step size of the motor operation. Theprimary use of this routine is to determine the location ofthe top of the outer quartz tube of the plasma source. Asharp minimum in the vertical spatial profile generallyoccurs at the top of the quartz tube. This allows allsubsequent measurements to be referenced relative to thetop of the outer quartz tube.

ASYST softwareASYST (Version 1.51, MacMillan Software Co., NewYork, USA) is a data-acquisition and analysis program-ming environment based on the FORTH language[18, 19]. The ASYST software package has beenreviewed elsewhere [20-23] and a variety of applicationsin the area of chemsitry have been published [24-26].

All numeric and string manipulations in ASYST arestack oriented. The ASYST environment is a crossbetween an interpretive environment, where commandscan be given and immediately processed, and a compiledenvironment. Many predefined, system commands,called ’words’, exist for numeric, array, string, plotting,I/O, and other manipulations. These words can beexecuted in the ASYST environment and interpretedimmediately, simply by typing the name of the word.Other, user-defined words can be defined based on thesystem words. These words can then be compiled andsaved as an executable file, and this is how a program isconstructed.

Two programs were written in ASYST. The majorfunction of the first program, called ACQDATA, is for

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data acquisition and real-time graphics plotting of data.The second program, called DATAANAL, was used forpost-experimental data analysis and hard-copy plot-ting. Only ACQDATA will be described here. SinceACQDATA is a menu-driven, integrated program, it isnatural to approach the discussion of this program byway of the different menu options. The main menu andeach main menu selection will be described in a separatesection.

Main Menu

The main menu of ACQDATA is shown in figure 2. Allmenu options are assigned to function keys F1-F10.These menu selections appear in a specially definedwindow on the screen. The location of this menu windowis shown in figure 3, which also shows the rest of thescreen setup during a typical acquisition run. In additionto reserving a special portion of the screen for the variousmenus, a special window is devoted to a text screen,where system prompts and other information appear.The entire middle section of the screen, and the upper leftportion of the screen, are reserved for real-time plotting ofdata. The upper right portion of the screen is another textscreen which displays a variety ofinformation, such as themean value, standard deviation, and relative standarddeviation of a data set immediately after it is collected.

Finally, the uppermost line of the screen is divided in halfand each side serves as a title line ofwhat appears below.The bottom line of the screen is also divided in half. Theleft side is reserved for displaying error messages and theright side displays the status of various system variables.

With the exception of the first menu selection, all of themain menu choices shown in figure 2 offer submenus. Thevarious submenus are shown in figure 4. Note that mainmenu selections F2, F8, and F9 are not currently assignedto any operation. This is indicated by ’NOP’ for NoOPeration. The one main menu selection which does notoffer a submenu is the Zero Motor Coords option, whichis selected via the F key. This selection allows one to zerothe horizontal, vertical, or both of the spatial co-ordinates. These spatial co-ordinates refer to the positionof the emission source relative to the entrance slit of theechelle spectrometer. When the source mirror of thePLASMA-SPEC is moved under program control, thehorizontal and vertical spatial co-ordinates change.ACQDATA keeps track of these changes. The systemstatus line, shown in figure 3, displays these co-ordinatesand updates them as required.

Manual control

Selecting F3 from the main menu presents the ManualControl Menus shown in figure 4. This mode allows themost freedom and flexibility in acquiring data. When in

Main MenuFI Zero Motor Coords F6 Echelle UtilF2 NOP F7 Echelle Set-UpF3 Manual Control F8 NOPF4 Data Acquisition F9 NOPF5 Optimize Plate/PMT FI0- This Menu

Figure 2. Main menu ofACODATA.

the manual mode, the cursor keys on the PC keyboardcan be used to move the source mirror of the PLASMA-SPEC. Moving the mirror in this way has the effect ofchanging the portion of the emission source which falls onthe entrance slit of the spectrometer. When the sourcemirror is moved in this way, the horizontal and verticalspatial co-ordinates are kept track of by ACQDATA andupdated on the system status line.

The F1 and F2 selections from the Manual Control Menuare the selections which actually initiate data acquisition.They are both completely general modes of acquisition,that is, a data set will be collected irrespective ofwhat thespatial co-ordinates are. A typical use ofthe manual modeis to move the image of the emission source to the desiredposition using the cursor keys. A data set is then acquiredat that position by depressing F1 or F2. Movement of theemission source image and subsequent data collectioncan proceed in this manner indefinitely. When each 32-point acquisition in this manner is finished, the user isprompted for information regarding the optional writingof data to a disk file.

The two manual acquisition keys, F and F2, differ in thefollowing manner. When F1 is used, both the apertureplate and PMT are stationary during the acquisition.Therefore, all of the data are at a single wavelength (andgrating order). When F2 is used, the aperture plateactually moves in the wavelength direction (dimension)of the focal plane. In this way, a spectral scan is achieved,limited by the aperture slit in the PMT housing.

Selecting F3 provides the ability to move the sourcemirror by a large amount in the horizontal direction. Thehorizontal spatial co-ordinate is adjusted accordingly. F4allows editing of the current background correctionmode. This background correction parameter will bediscussed later. F5 causes the execution of a P-Coderoutine which scans the plasma spatially in the horizontaldirection and moves to the peak emission point whendone. F8 allows changing of the step size of the sourcemirror, that is, the distance which the source mirror willmove each time a cursor key is depressed. The horizontaland vertical step sizes can be different. The F9 selectionallows one or both ofthe spatial co-ordinates to be zeroed.Note that this does not have any effect on the position ofthe source mirror; only the numbers assigned to thenumerical co-ordinates are set to zero. This is useful, forinstance, when one has found the peak horizontal positionat a particular vertical position in the plasma. Zeroing thehorizontal co-ordinate then provides a convenient refer-ence point in the plasma. Finally, F10 returns to the mainmenu.

Data Acquisition Menu

Selecting F4 from the Main Menu brings up the DataAcquisition Menu, shown in figure 4. Apart from the F10selection, which returns the user to the Main Menu, thereare two options. F1 puts the system in the timedacquisition mode. In this mode, data sets, consisting of32individual samplings/data set, are collected at periodicintervals. The lengths of these intervals depends on theintegration time which is being used.

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YimeO Acquisition Mode # Mean Std. Dev.

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3 68

3 62

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36457 5273636436314 52636712 59736216 388

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Data Collection CompletedMake a selection from theData Acquisition Menu"

Error Hindow

Data Acquisition MenuFi- Signal vs TimeF2- Spatial AcquisitionF3- Move To Another LineFIO- Main Menu

Hot iz VertiO 4

HStep VStep BGStep BGMode-2 2 32 -1

Figure 3. Example ofscreen during a timed acquisition experiment. Top left graphics screen shows most recent 32-point data set. Middlegraphics screen shows cumulative 32-point average values over time. Lowerportion ofscreen shows menu window, prompt window,status line, and error window. Top right of screen displays information about the five most recent acquisition sets.

Before actually beginning an acquisition session in thetimed mode, the user is prompted for information, such aswhether to enable real-time plotting. Each raw 32-pointdata set can be optionally plotted immediately after it iscollected. In addition, the average value of each 32-pointdata set can be plotted versus the time of acquisition ofthe data set. By default, each 32-point data set is averagedand the standard deviation and relative standard devi-ation is computed and displayed in the upper rightportion of the screen. Figure 3 gives an example of thescreen during a timed acquisition session. As can be seen,the raw 32-point values are plotted in the upper leftportion of the screen, the average value of each 32-pointdata set is plotted in the middle, and the statistical valuesare plotted in the upper right.

During acquisition in the timed mode, the user canoptionally halt acquisition and then either continue orexit the acquisition mode. Upon exiting, prompts appearin the text window which allow the data to be written to adisk file. This timed mode of acquisition is particularlyuseful for investigating such phenomena as the short- andlong-term precision of the emission source. This mode isalso useful for observing the response ofthe plasma sourceto various changes to the system such as sample change-

over or a change in a gas flow rate. In general, it is not ofinterest to move the source mirrors during the timedmode ofacquisition. Each 32-point data set is collected byviewing the same portion of the plasma.

The other acquisition mode accessible from the DataAcquisition Menu is geared towards the collection ofspatial information and is entered by selecting F2. Thefunction of this mode of acquisition is to successivelycollect horizontal profiles of the plasma at progressivelyhigher points in the plasma. When the Spatial Acqui-sition Mode is selected, a variety of user prompts aredisplayed in the text window. These prompts ask forinformation such as the desired spatial width of eachhorizontal profile, the distance to step up vertically aftereach horizontal profile has been completed, and the totalnumber of horizontal profiles to collect.

As with the Signal versus Time Mode of acquisition,plotting of each 32-point raw data set can be enabled ordisabled. However, the spatial mode ofacquisition differsfrom the timed mode in that during the collection of each32-point data set, the source mirror is stepping in thehorizontal direction. Therefore, each one of the 32 datapoints corresponds to a different horizontal position in the

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Manual Control Menu,Use Cursor Keys To Move Mirror OR

FI Acq Data Point F6 NOPF2 Acq Wave Scan F7 NOPF3 Gross X Move F8 Edit Step SizeF4 Edit Bkgd. Mode F9 Zero CoordsF5 Peak Horiz. Pos. FI0- Main Menu

--Data Acquisition Menu--FI Signal vs TimeF2 Spatial AcquisitionFI0- Main Menu

--Optimization Menu--FI Plate XF2 Plate YF3 PMT XF4 PMT YFI0- Main Menu

Echelle Utilities MenuF! Ed/Create Coord Set F6 NOPF2 Change # Coord Sets F7 Print Coord SetF3 Coord Set To File F8 NOPF4 Coord Set From File F9 NOPF5 Lst Coord Set Files FI0- Main Menu

Echelle Set-Up MenuFI Down Load P-CodeF2 Load Hg Mirror Motor OpF3 Set Integration TimeF4 Edit Background OffsetF5 Load Hg PMT CoordinatesF6 Quartz Finder Fl0-Main Menu

Figure 4. Submenus ofACQDATA.

plasma. So if the plotting of each 32-point data set isenabled, one can view the horizontal profile at aparticular vertical position in the plasma immediatelyafter the data are collected.

Optimize plate/PMT

The fifth choice from the Main Menu facilitates theoptimization of the position of the aperture plate andPMT in the focal plane of the echelle. Selecting F5presents the Optimization Menu, shown in figure 4.Selecting F1 through F4 from the Optimization Menuinitiates a peaking routine which scans the aperture plate(or PMT) over a very small distance in the horizontal orvertical dimension of the focal plane. Then, an algorithmin ACQDATA determines a correction factor. Thiscorrection factor represents the amount by which theaperture plate (or PMT) must be moved from its currentposition to achieve maximum light throughput. The usercan enable or disable the display of the aperture plate andPMT scans. In addition, the user has the option, afterseeing what the scan looks like, of applying the correctionfactor. If the correction factor is applied, a library ofaperture plate and PMT co-ordinates is updated byACQDATA for the particular line in use. This librarysystem is described below.

Echelle Utilities Menu

Selecting F6 from the main menu presents the EchelleUtilities Menu shown in figure 4. These menu selectionsallow the user to manipulate a library of aperture plateand PMT co-ordinates. To set the spectrometer to awavelength in a particular order of the grating, a uniqueset of four co-ordinates is required: the horizontal andvertical aperture plate co-ordinates and the horizontaland vertical PMT co-ordinates. From the Echelle Utili-ties Menu, the user can create or edit a co-ordinate set(F1) and save it to a file (F3). At a later time, the desiredco-ordinate set can be retrieved (F4) and made known tothe system. In this way, the co-ordinate sets need not bemanualy re-entered each time an experiment is per-formed. The desired co-ordinate set can simply beretrieved from the library file. F5 lists, to the monitor, theelements whose co-ordinates are in the library. F7 printsthe actual co-ordinates for a particular wavelength andorder to either the monitor or the printer. Finally, F2allows the user to change the number of co-ordinate setswhich the system will keep track of.

Another pre-acquisition prompt requests the type ofbackground correction to be used. One or two-point off-line background corrections can be selected or thebackground correction can be done by introducing ablank solution to the plasma and repeating the spatialacquisition under the same conditions. If the off-line typeof background correction is selected, the backgroundinformation is collected immediately after each horizontalprofile. However, the analyte emission data and thebackground data consist of independent 32-point datasets.

Finally, the user can optionally interrupt acquisition aftereach horizontal profile has been completed. At this point,acquisition can be restarted or the user can exit. Whenthe user exits in this manner, or when all of the requestedprofiles have been completed, prompts appear whichrequest the necessary information for saving the data to adisk file.

Echelle Set-Up Menu

The final choice offered by the Main Menu (F7) presentsthe Echelle Set-Up Menu shown in figure 4. Theseselections allow the user to perform a variety of taskswhich are required before an experiment can begin. Forinstance, the first choice, F1, automatically down-loadsthe P-Code routines to the spectrometer. These P-Coderoutines, which were discussed previously, need to bereloaded occasionally due to the occurrence of a poweroutage, a complete power-down of the spectrometer formaintenance purposes, or if another user down-loadsdifferent P-Code routines to the spectrometer. The F2and F5 selections relate to an internal Hg lamp which canbe used for wavelength calibration purposes. F3 allowsthe integration time to be set to a value other than thedefault value. F4 allows the value ofthe background offsetto be edited. This background offset determines how faroff-line the background correction will be made. Finally,F6 initiates a routine which ’finds’ the top of the

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J. P. Shields and E. H. Piepmeier A computer controlled data acquisition and analysis system

quartz tube by looking for the sharp discontinuity in thevertical emission profile caused by the quartz tube. Thetop of the quartz tube can then be used as a referencepoint for all subsequent vertical positions. As is the casethroughout all of the menus, F10 will again present theMain Menu.

Application

Optimization ofaperture plate and PMTpositions infocal planeAs was mentioned previously, there is a need to optimizethe position of the aperture plate and the PMT in thefocal plane. Both the aperture plate and the PMT canvary in two directions. An example of how a PMToptimization proceeds is as follows. When F3 is selectedfrom the Optimization Menu, the PMT is moved over avery narrow distance in the wavelength domain andintensity readings are taken throughout the scan. Atypical profile before optimization is shown in figure 5(a).The optimization algorithm uses the width at half peakmaximum to determine the centre ofthe profile. After thisinitial profile is obtained, ACQDATA produces a correc-tion factor to properly centre the profile, applies it afteruser confirmation, reruns the scan, and displays acorrected profile, which is shown in figure 5(b). A similarprocedure occurs for the other optimization selections inthe Optimization Menu.

The consequences of a non-optimal PMT position aretwofold. First, when the PMT co-ordinates for a givenwavelength and order in the ROM of the spectrometerare used, a profile of the type shown in figure 5(a) mayresult. During a typical experiment, the PMT would befixed at the position marked P throughout the dataacquisition period. Therefore, since the Y axis of theprofile in figure 5(a) represents emission intensity, it isevident that the maximum possible amount of light is notbeing collected by the PMT. This can degrade detectionlimits and sensitivity.

Secondly, if the PMT is at the position marked P in figure5(a), as it is when the ROM values ofthe spectrometer areused to position the PMT, increased noise will result dueto slight changes in the PMT position. This is due to thepositioning ofthe PMT on the side ofthe profile instead ofon the plateau region of the profile. However, afteroptimization of the PMT position (figure 5[b]), the PMTis positioned at the plateau of the profile. Therefore, aslight change in the PMT position will not significantlychange the signal intensity. Slight changes in the PMTposition can occur over time due to such factors asfluctuation in the room temperature.

Monitoring ofplasma stabilityThe Signal versus Time Mode, which is selected from theData Acquisition Menu (figure 4), can be useful formonitoring the stability of the plasma over time. Emis-sion resulting from the introduction of 100 btg/ml of Zninto the plasma was monitored for a period of 360 s.Figure 6 shows the results of this Signal versus Timeacquisition session. The small dots (’) represent theindividual 32 points in each data set while the symbols

CU 3 2’. 4.. ..,B2 B 3 B4

t.I t..I t..I--2, ---"

ii’:’0-"

ii

3 L. _a,,2,

E:4N

0

b

Figure 5. (a) Aperture plate scan at the Cu1324"75-nm line beforeoptimization. The profile should be centred about the ’P’ positionfor maximum light throughput. (b) Aperture plate scan at the CuI324"75-nm line after the ACQDATA plate optimization routinehas been performed. Profile is now centred about the ’P’ positionwhere maximum light throughput and minimum noise occur.

represent the average value of each 32-point data settaken over the time period.

This graph is displayed on the computer screen andupdated with each additional 32-point data set as it iscollected. In addition, as shown in figure 3, the mean,standard deviation, and relative standard deviation ofeach 32-point data set are displayed.

This mode of acquisition provides a useful means ofmonitoring drift in the system. Since the time intervalbetween data sets can easily be changed, short or long-term drift can be monitored. In addition, this timed modeof acquisition provides an excellent means of watchingthe effect ofa change in an experimental parameter on the

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2000

1800-

1600

1400

0-1

ZnI 2!3.86 nm

40.0 120 200 280 360Time (sec)

Figure 6. ZnI emission at 213"86 nm versus time. Monitoring ofplasma stability using the timed acquisition mode ofACQDA TA.The small dots (’) represent the individual points ofeach 32-pointdata set. The solid line and symbol () represent the average valueof each 32-point data set.

Figure 8. Horizontal emission profiles ofplasma at seven differentobservation heights above outer quartz tube. Monitoring the MgH280"27 nm line while aspirating a 10 gg/m solution.

signal. In the present application, this mode of acqui-sition was frequently used to observe the effect ofa changein a plasma gas flow on the plasma emission intensity.

Monitoring of nebulizer chamber wash-out timeFigure 7 illustrates how the emission intensity changeswith time after a blank solution is introduced into thesystem. The original sample concentration was 100 tg/ml of Ca. As in the previous example, the Signal versusTime Mode was used. This provides an excellent way ofmeasuring the time required for sample change-over inthe nebulizer system.

Spatial profiles of the plasmaThe Spatial Acquisition Mode, which is selected from theData Acquisition Menu, facilitates the acquisition ofspatial information about the emission from the plasma

Call 393.37 nm

80

4,0

0

i00 Ng/mL Ca

0 300 600 900

Time (see)Figure 7. Monitoring of nebulizer wash-out period upon change-over from lO0[xg/ml Ca to blank. Timed acquisition ofACQDATA was used. Monitoring Call emission at 393"37 nm.Eachpoint on the plot represents the average of32-datapoints takenover a time period of 1.5 s.

source. Figure 8 shows a set of horizontal emissionintensity profiles, each taken successively at a differentvertical height in the plasma. All vertical positions arereferenced from the top of the outer quartz tube of theplasma assembly. This particular set of horizontalprofiles was obtained while a solution of 10 gg/ml of Mgwas introduced to the plasma. The ion line of Mg at280’27 nm is being monitored.

Spatial information of the type provided by the spatialacquisition mode is useful in the investigation of a varietyof experimental variables. Figure 8 shows that theemission profile is highly symmetrical in the horizontaldirection and that there is a height above the quartz tubethat is optimal for viewing the emission intensity. Thisspatial emission profile, however, is highly sensitive to avariety of experimental parameters such as nebulizer gasflow, height of the sample introduction tube, plasmaelectrode heights, and current to the plasma [11]. Thus,this mode of acquisition provides an easy way of visuallyobserving, in real-time, the effects of these experimentalvariables on the spatial emission profiles.

Conclusions

The software presented here provides a means ofacquiring data of the type that is often very difficult toobtain with the capabilities of many commerciallyavailable spectrometer systems. In addition, the softwareprovides the analyst with real-time feedback of data as itis obtained. This feedback is in the form of plots to agraphics screen of the computer monitor and is useful inmonitoring the course of an experiment in progress. Thisreal-time feedback offers an advantage over the commonpost-experiment feedback in that it allows the analyst toalter the experiment in progress or modify an experimen-tal variable to improve subsequent results.

The aperture plate and PMT optimization features of thesoftware allow the analyst to fine tune the position of the

136


plate and PMT in the focal plane of the spectrometer.This results in an increase in the amount of light to thedetector thus potentially improving detection limits andsensitivity.

Perhaps most importantly, fundamental studies of aplasma source, which often require a knowledge of thespatial emission characteristics of the plasma, are facili-tated by the flexible spatial acquisition mode. This modeautomates the acquisition and display of real-timehorizontal and vertical emission profiles of the plasmasource.

Acknowledgements

This work was supported in part by the National ScienceFoundation, Grant Number CHE-8206692. The authorsalso gratefully acknowledge the support ofLeeman Labs,Inc., Lowell, Massachusetts, USA. The reception of anN. L. Tartar Fellowship by one of the authors (J. P. S.) isalso gratefully acknowledged.

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