AUTOMATION IN ATOMIC SPECTROSCOPY: THE CHALLENGE OF REPLACING HUMAN JUDGMENT WITH COMPUTER LOGIC Thomas W. Barnard The Perkin-Elmer Corporation Norwalk, Conn. 06856
Activity in the field of analytical chemistry has been growing rapidly for many years. New techniques facilitate better analyses, which create the demand for more analyses, which in turn motivate new developments. This positive feedback causes exponential growth. Demands on the analytical chemist have grown proportionately to greater numbers of more sophisticated analyses. In response to this need, a new class of analytical instruments is emerging. They are characterized by versatility and the capacity for total automation. Utilizing their versatility, the chemist can develop unique analytical methods to solve specific measurement problems. For routine work, the instruments are programmed to perform these analyses automatically. Consistent results thus are obtained rapidly with reduced demands on laboratory personnel. A recent addition to the class of flexible yet automatic instrument systems is the Perkin-Elmer Model 5000 atomic absorption spectrometer. This report discusses the technology and activities involved in the development of the Model 5000 system. Specifications Defining functional specifications is the starting point of any develop-
ment program. This process took place in early 1975 for the Model 5000 and involved the contributions of many people from several disciplines. The fundamental concept, of course, was to automate the operation of an atomic absorption/emission spectrometer. The recent availability of 8-bit microprocessors was an essential factor in making this goal practical within the cost constraints of a commercial instrument. Experience at PerkinElmer with the first generation of microcomputer-based instruments tended to confirm the feasibility of an automated system. The Model 283 infrared and the Model 460 atomic absorption spectrometers introduced in early 1975 utilized microcomputers for wavelength control and analytical data processing, respectively. Several competitive instrument offerings also helped to motivate the development of the Model 5000. In particular, the Jarrell-Ash Model 850, which featured microprocessor control of several instrumental parameters, as well as data processing, was introduced at about this time. The 850 could be set up for a single element at a time by the operator using keyboard commands. Also available from several manufacturers, but at much higher prices, were simultaneous multiele-
1172 A • ANALYTICAL CHEMISTRY, VOL. 5 1 , NO. 12, OCTOBER 1979
ment analyzers based on emission from a plasma source and direct reading polychromators. In this category were the ICP/Q from ARL, the Atomcomp from Jarrell-Ash, and the Spectraspan from Spectrametrics. Programmable calculators also influenced the product definition through the use of special function keyboards and program storage on magnetic cards. The degree of automation was an important issue for the Model 5000. The ability to analyze many samples for many elements without operator attention became the criterion for establishing design requirements. Unattended operation was deemed essential for a breakthrough in laboratory productivity. Highest quality performance also was needed to go along with this top-of-the-line instrument. The following requirements were developed for the Model 5000 system: • automation of spectrometer physical parameters including wavelength, spectral bandwidth, lamp turret position, lamp current, and data processing conditions; • automation of the atomizer systems including gas flows for the burner, and heating program for the graphite furnace; • automation of sequential multielement sampling devices; 0003-2700/79/A351-1172$01.00/0 © 1979 American Chemical Society
Report
• provision for permanent storage and retrieval of complete analytical programs; • correction for background ab sorption throughout the entire ultravi olet and visible spectrum using a dou ble-beam technique; • improvements in optical through p u t and spectral resolution relative to previous atomic absorption instru mentation; • compatibility with the major ana lytical techniques in atomic spectros copy, i.e., atomic absorption and emis sion with the flame, furnace, and hy dride generator and atomic emission with plasmas.
Multisample, Multielement Analyses T h e most distinguishing feature of the Model 5000 is the concept of se quential multielement analysis. Com pared to simultaneous multielement systems, this approach has the advan tages of hardware simplicity, cost, and flexibility, b u t with some sacrifice in speed. Unique analytical conditions can be established for each element. T h e r e are two possibilities for se quencing multisample, multielement analyses. Many samples can be ana lyzed for one element, then re-ana lyzed for the next, etc. This procedure will be referred to as sample consecu tive. Alternatively, each sample can be analyzed for all elements consecu tively, i.e., element consecutive. For conventional atomic absorption or emission analyses with the burner or furnace atomizer, sample consecutive is more efficient. Atomizer stabiliza tion and instrument calibration occur once per element and the data rate is rapid, as many as 600 determinations per hour with the flame. Also, data memory requirements and software complexity are minimized. Multiele ment plasma emission analyses, how ever, are best done element consecu tively because of the long nebulizer stabilization time for each sample. In general, different types of atom izers have different sampling and se quencing requirements. For most ver satility, specialized autosamplers con
trol the multielement, multisample se quencing process. Complete analytical programs for each element are stored in the spectrometer and, if necessary, in the atomizer controller. T h e autosampler presents samples to the in strument and commands it to set u p programs, perform calibrations, ana lyze unknowns, and print results. T h e instrument in turn signals the autosampler when it starts and completes each operation. This handshake rou tine continues until all samples have been analyzed for all elements.
Project Organization T h e broad scope of specifications for the Model 5000 implied t h a t a completely new instrument system in cluding atomizer controllers and sam pling accessories had to be designed. A large development program was re quired which involved u p to 25 engi neers, designers, instrument makers, and technicians. Many other profes sionals also contributed to specifying, testing, and evaluating the several breadboards and prototypes which led to the final designs. Three and one half years of elapsed time and about 40 man-years of development effort were required to complete the total 5000 program. Effective organizational dynamics are necessary to coordinate such a large activity. In general, responsibili ties are divided among small groups, each with its own authorities and ac countabilities. For example, one group designs a product and builds a proto type, another tests it analytically and for compliance with specifications, an other tests it for reliability, another evaluates its manufacturing feasibili ty, etc. T h e process is perforce slow and sometimes painful but the results are dependable. Surprisingly small problems can jeopardize the effective ness of a complex product so the veri fication process has to be thorough.
Automation Concept Designing automation into the Model 5000 was the most challenging part of the engineering program. There were two aspects to this prob lem. One was to control each physical
degree of freedom via electronic sig nals. T h e other was to develop a logic structure which coordinated these de grees of freedom into an effective overall analytical system. Physical parameters requiring con trol included: the monochromator wavelength, spectral bandwidth, selec tion of the atomic line source and its intensity, selection of the continuum background corrector source and its intensity, the burner gas flow rates, the graphite furnace heating cycle, and various actions of the autosamplers. Most of the technology for auto mating these functions already existed and involved commercially available stepper motors, servo motors, and op tical and pneumatic transducers. However, a completely new instru ment configuration was needed to meet all the instrument specifications. T h e design goal for the system con trol logic was complete automation of sequential analyses without loss of in s t r u m e n t flexibility. T h e essential as pects of the control system philosophy were to provide: • a control structure hierarchy t h a t progresses from individual parameters to automatic analytical sequences; • operator interaction with the in strument via special function keys and digital displays; • distributed processing of digital data. T h e control hierarchy is divided into three levels. T h e first deals with specific instrumental functions such as wavelength, lamp turret position, atomizer conditions, signal mode, cali bration factors, and several others. T h e instrument is programmed for each parameter separately by specify ing the required numerical value and pressing a special function key. For in stance, 324.7 followed by the key λ-slew causes the wavelength to be set a t the 324.7-nm copper line. T h e next level consists of storing for later use, or recalling for current use, complete programs developed for particular analyses. For example, program No. 3 might be manganese in steel deter mined by flame atomic absorption with three calibration standards. Two keystrokes and typically 10 to 15 sec onds would be required to set u p the
ANALYTICAL CHEMISTRY, VOL. 5 1 , NO. 12, OCTOBER 1979 • 1173 A
Timesharing Mainframe RS232C
Autosampler
Intelligent Terminal RS232C
GPIO
UART
RAM
Microcomputer Otis
GPIO
GPKD
SERVO
Keyboard and Display
CPU ROM
Figure 1. Model 5000 Microcomputer; partial block diagram instrument and atomizer for this analysis and to be ready for calibration. In the highest level of control, an external device triggers the recall of programs and coordinates the automatic sequential analysis of many samples for many elements. T h e external device may be one of the autosamplers designed specifically for the Model 5000, in which case it utilizes a dedicated autosampler electrical interface or an intelligent terminal operating over the two-way RS232C interface. Using the flame autosampler and atomizer, 50 samples can be analyzed for six elements in as little as one-half hour. Dividing the control logic into functionally distinct levels makes the ins t r u m e n t easier to use, easier to design, and generally more flexible. Once the integrity of lower level procedures has been assured, they can be combined into complex sequences at higher levels. Experience has shown t h a t special function keyboards and digital displays are an efficient means for interacting with complex instruments. Sophisticated pocket calculators probably led the way in this area. Operational simplicity is achieved by minimizing the number of keystrokes required for each function. One key per function is ideal but results in many keys and the appearance of complexity. Juxtaposing functionally related keys, separating distinct key groups, and selectively defining second key functions help to make keyboard operation self-evident and convenient. For example, calibration keys are separated from physical parameter keys by numerical keys on the spectrometer while the atomizer and autosampler
keyboards are on separate chassis. Several of the function keys are used either to enter or display numerical values depending on the "check" mode status. In keeping with the multilevel control philosophy of the Model 5000 and the nature of microcomputer components, a distributed processing configuration was developed for the digital logic. T h e microcomputer hardware was divided into many functional blocks which communicate via a common signal path, called the microcomputer bus. Figure 1 shows a partial block diagram of the system. T h e central processing unit (CPU) performs logic operations defined by fixed program data stored in Read Only Memory (ROM) which typically involves modifying variable data in Random Access Memory (RAM) and exchanging data with I n p u t / O u t p u t (I/O) devices. T h e ROM program is the machine language version of the assembly language program developed especially for the instrument. T h e I/O devices perform complex logic operations in their own right and reduce the real time data processing burden on the CPU. For example, the General Purpose Keyboard and Display (GPKD) on the 5000 continuously updates 26 display digits, and 32 display LED's, and checks 74 different keys for inputs. T h e CPU periodically interrogates the G P K D for new key entries and gives it new display information. Likewise, the C P U distributes many other detailed logic operations to other sophisticated I/O devices such as the Printer Controller, the Universal Asynchronous Receiver/Transmitter (UART) for RS-232C communication, and the General Purpose I/O
1174 A • ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
(GPIO) device. Without help from these satellite microcomputer I/O devices, the CPU could not keep up with the digital data processing requirements, Another facet of the distributed processing concept involves interaction with higher level control systems. In this case the 5000 receives commands from an autosampler or an intelligent terminal and responds to them as if they came from the keyboard. T h e operator interface moves from the spectrometer to the autosampler or intelligent terminal. T h e ins t r u m e n t itself becomes a node in the distributed processing configuration. T h e intelligent terminal in t u r n may become another intermediate node t h a t is driven, for example, by a large mainframe time-sharing system. T h e advantages of distributed processing, like those of hierarchical control, include flexibility, adaptability to change, and simplification of the overall problem. Microprocessor Design Microprocessor hardware and software represent a major part of the Model 5000 system. T h e design process consisted of the following major steps: • Define the overall system design concept. • Define the hardware configuration including microprocessor components, I/O signals, and external circuitry. • Design the software program. • Write the assembly language code. • Assemble the program and correct the syntax. • Test and debug the software. • Document the software. • Convert the software to ROM firmware. A fundamental system concept was t h a t all control signals originate from the microprocessor based on spectrometric, instrument status, and external command information. T h e photometric signal is periodically fed to the microprocessor by an analog to digital (A to D) converter. T h e microprocessor issues digital commands which control photomultiplier voltage, spectral slit width, lamp turret position, lamp current, and background corrector current. T h e microprocessor controls wavelength directly by sending electrical pulses to a stepper motor which fixes the grating position with a precision of about one arc second. Analytical measurements result from digitized photometric data processed by various arithmetic operations depending on the instrument calibration status. Microprocessor program interrupts facilitated many of the data process-
ing operations. This feature permits the program sequence to be interrupt ed at any time in response to external control signals. A special sequence of instructions, the interrupt routine, is performed, and then the program re turns to where it left off. Interrupt routines are used by the Model 5000 to gather photometric data after each chopping cycle (60 times per second) and to synchronize stepper motor pulses for wavelength scanning opera tions. Asynchronously with these real time requirements, the microprocessor performs analytical data processing, other I/O operations, and control logic processing. An important class of control func tions involves physical adjustments of the instrument based on the photo metric signal amplitude. Examples in clude peaking the wavelength for max imum energy, setting optimum elec tronic gain via the photomultiplier voltage, and matching intensities of the line and continuum source lamps. In each case the control logic issues an I/O command to change incrementally the physical parameter, waits for new photometric data, and, depending on its value, either makes another change or concludes the task. The result is a feedback loop involving the photomet ric A to D converter, the microproces sor and the rest of the instrument. In
automating these adjustments, the mi croprocessor plays the role assumed by the human operator in a manual instrument. After evaluation of the overall sys tem requirements, a second generation microcomputer, the Rockwell PPS-8, was selected. Some of its attributes in clude 8-bit parallel operation, three levels of program interrupts, 16K bytes of direct access memory space, and compatibility with distributed processing. Since 1975 many advances have been made in microcomputer technology; however, the functional capabilities of the PPS-8 are still quite competitive. Perhaps the most benefi cial recent advance in microcomputer technology for analytical instrument makers has been the development of higher level language compilers for improving programming efficiency. Once the system design concept was clear, the microprocessor hardware was designed and the software pro gram was planned in detail. The hard ware design was straightforward. The CPU, ROM, and RAM parts were in terconnected on one printed circuit (PC) board via the common micropro cessor bus. This bus is connected to other PC boards containing distribut ed I/O devices which interacted with specialized instrument circuitry. Programming began with the func
tional definition of the overall pro gram which was broken down into functional modules and submodules. Each of the lowest level modules re quired only a few hundred lines of code and typically was written as a subroutine. Minimizing logical inter faces among modules is an important programming discipline which we tried to follow. This practice leads to a well-structured program which is easier to test, modify, document, and maintain. The next stage in creating the soft ware was writing the assembly lan guage mnemonic code. Each line of code represents an elementary in struction to the CPU such as adding two data bytes or moving data from one register to another. The assembly language code is processed by a macro assembler which identifies syntax er rors, generates a program listing, and converts the program to binary form. Testing and debugging the software was done next, using a program devel opment system based on the PPS-8 microprocessor. This tool permitted following the program sequence, ex amining CPU and RAM data, and testing electrical signals as the pro gram executes. The program develop ment system was coupled to the rest of the instrument electronics via the microcomputer bus and was essential
MICROMASS
THE
MASS
SPECTROMETRY
COMPANY
THE MICROMASS ZAB FEMTOGRAM DETECTION LEVELS AT 10,000 RESOLUTION W i t h a basic sensitivity of 2 χ l O ^ C / p g for the methyl stéarate M + at 10,000 resolution, the Z A B is ideally suited for quantitative analysis d o w n t o f e m t o g r a m levels. A g o o d example of its outstanding sensitivity is in the analysis of the toxic pollutant T C D D . Chromatograms indicating the detector response at mass 321.8936 are for a 200 f g T C D D injection o n t o a W . C . O . T . capillary c o l u m n directly coupled t o the Z A B and this is f o l l o w e d immediately by a solvent blank. Signal t o noise ratio is approximately 18 t o 1 for the 200fg injection, indicating a detection limit of better than 2 0 f g . The a m o u n t calculated t o be in the blank itself is less than 15fg. For more details of this, and other applications of the Z A B contact: Blank U.S.A. T h e Kearns G r o u p , 58 B u c k i n g h a m Drive, Stamford, C o n n e c t i c u t 06902.
CANADA D a t a c o m p Electronics L t d . 2 Silver A v e n u e , Toronto, Ontario M6R 3 A 2 .
U.K. V . G . Organic L t d . , Tudor Road, Altrincham, Cheshire W A 1 4 5R2
CIRCLE 219 ON READER SERVICE CARD 1 1 7 6 A • A N A L Y T I C A L CHEMISTRY, V O L . 5 1 , NO. 12, OCTOBER
1979
