Then give to Caesar what is Caesar’s, but give to God what is God’s. Luke 20:22
Continuous sis for the segmented version of the technique, and flow injection analysis
Muller’s “Komplementar Kolorimeter”
Continuous-flow procedures constione analytical answer to the increasing load being imposed on the practicing chemical analyst. The answer applies mainly to the problems posed when a large number of samples of similar nature have to be processed for the determination of a single to species; such problems are common clinical chemistry, environmental studies and control, and industrial processes and quality control. Continuous-flow procedures meet the need, in those cases, for shorter times and decreased operating costs per determination. By their use: tute
Figure
1.
• a large number of samples can be processed with acceptable (many
times highly competitive) precision and accuracy; • a meaningful statistical treatment of the data is possible; • human participation can be eliminated in many routine manipulations; and •
better utilization of reagents
is pos-
sible.
The term analysis is frequently misunderstood in the vocabulary of analytical chemists. Its use in the title of this REPORT is a reluctant recognition of usage: continuous flow analy-
Muller’s '‘Komplementar Kolorimeter”
(a) Glass cylinder in which the solution to be examined was placed; (b) concentric tube capable of vertical movement; (c) millimeter scale; (d) glass filter of color complementary to that of the solution. Observation was made from above against white light reflected by the adjustable mirror (e). If the inner tube was raised, the color of the solution predominated. If It was lowered to virtual contact at point (f),
the color of the filter would be seen. A balance would be obtained at a certain intermediate point in which an approximately white color would be observed. Readout was obtained from scale (c) and determination based on comparison with standards. Reproduced from Reference 2 with permission
1312 A
•
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
for the unsegmented one. The term analysis refers to operations on the material or, as proposed recently, the system under study (I). As such, it involves steps ranging from the planning of strategy to attacking a given analytical problem, to reporting and helping with the interpretation of final results. Considering that most continuous flow systems lack several of those steps, the analysis in the title is open to criticism. It should be recognized that what we are actually dealing with are sample processing systems that may incorporate more than one (never all) steps of analysis. Continuous-flow procedures often provide advantageous alternatives to wet chemical methods. The practice of wet chemical analysis has undergone several changes since the times when classical gravimetric and titrimetric methods monopolized the arsenal of practicing analytical chemists. Colorimetric instrumentation can be traced back to Muller’s “Komplementar Kolorimeter” (Figure I), and to Vierodt’s description of an apparatus for obtaining absorption spectra for quantitative determinations (2). These contributions date back to 1853 and 1873, respectively. The introduction of the Beckman Model DU quartz photoelectric spectrometer, in the early 1940s, represented a quantum jump in instrumental development. About 15 years later Skeggs was responsible for another drastic change in the approach by which colorimetric determinations, and by extension practically all instrumental variations of wet chemical analysis, were performed. Skeggs’s radical contribution was to introduce dynamic measurements instead of the classical “static” measurements using
a
cell
or
cuvette (3).
The statement, “Scientific research consists in seeing what everyone else has seen, but thinking what
thought” is attributed to Albert Szent-Gyorgyi, the Hungarino one else has
0003-2700/81 / A351 -1312$01.00/0 American Chemical Society
©1981
Report Horacio A. Mottola Department of Chemistry Oklahoma State University Stillwater, Okla. 74078
Flow Analyses Revisited biochemist who received the Nobel Prize for physiology and medicine in 1937; he was the first to isolate and characterize vitamin C as ascorbic acid. This statement seems to apply to every corner that we turn in tracing the development of analytical continuous flow procedures. The elements of Skeggs’s modular, continuous-flow concept that resulted in the workhorse of practically every clinical laboratory and most industrial analytical facilities, the AutoAnalyzer developed and marketed by Technicon, existed in process control (4) before Skeggs adapted them to the analytical laboratory. An example of this is the Beckman Model 77 continuous-flow colorimeter, recommended for monitoring the color of water, beverages, liquid chemicals, suspended solids (turbidimetry), and chlorine in water (4). Skeggs thought of what nobody else thought before. He proposed a novel manner of sample-reagent mixing and transport to detection, and emphasized the utility inherent in all modular set-ups. His first full-length paper, however, established what became a condition, almost unchallenged for years, for the development of continuous-flow analyzers; the need for air segmentation. Demonstration of the fact that such procedures do not necessarily need air segmentation constitutes the latest landmark in the sequence of developments that led to wet chemical analysis as practiced today. Although analytical chemists have been injecting samples into unsegmented flows (chromatography being a typical example) for a long time, the real impact of unsegmented continuous-flow procedures was first felt in the middle 1970s. Szent-Gyorgyi’s statement on scientific research again comes to an
The material covered in this REPORT was presented as part of the opening remarks at the symposium “Flow Injection and Other Unsegmented Continuous Flow Sample Processing Systems,” I82nd National Meeting of the American Chemical Society, New York, N.Y., Aug. 24, 1981.
Figure 2, Experimental set-up of Pungor et al. for continuous-flow analyses with graphite electrodes C1 and C2
= stopcocks; K = stirrer; £1 = indicating electrode; point. Reproduced from Pungor et al. (7) with permission
E2
=
reference electrode;
I =
injection
Figure 3. Flow system used by Frantz and Flare for silica determination (a) Reagent reservoir; (b) 32-gauge Teflon tubing; (c) flow meter; (d) three-way valves; (e) sample injector;
