Letters
Flow Injection Analysis Revisited Sir: Professor Mottola's article in the November issue (1 ) attempted to restore some balance to what has been written on flow-injection analysis (FIA). His historical treatment is especially good. Mottola concluded that "unsegmented continuous flow systems invite, more than many other approaches, the ingenuity of researchers." Inevitably, what seems to be new has often been done before, but the earlier work may not always be widely known. I mentioned some previous unsegmented-flow methods soon after the "first" paper on unsegmented continuous flow analysis (2), and others have since reported earlier precedents. Perhaps the documentation by one who is known as an advocate of FIA will focus more attention on the question of novelty. Mottola's article also brings up some important questions of terminology. In particular, he points out that an analysis "involves steps ranging from the planning of strategy, to attacking a given analytical problem, to reporting and helping with the interpretation of the results." There are no instruments that automate chemical analysis in this sense. The term "automated analysis" has referred to automation of the chemical manipulations and the final measurement, with or without conversion of the transducer output to analyte concentrations. I suggest that we retain this meaning until it becomes feasible to automate the other steps of an analysis. Attention should also be given to how the throughput rate of an analyzer is described. By convention, occasional calibration runs are counted as if they are samples. For example, an instrument that can make measurements on one solution per minute may be calibrated with two standards at intervals of 20 min. The instrument will then process six calibrant solutions and 54 samples each hour, but it is customary and accepted to speak of a
throughput of 60/h. A justification for this usage is that the frequency of calibration is determined by the user and it is not inherent to the method or the apparatus. Also, occasional calibration does not materially affect the throughput; there is only a 10% difference in this example. It is sometimes necessary to run a single calibrant solution immediately before or after each sample; this calibration process may be built into the instrument. By custom, an instrument of this type is said to have a throughput of 60/h when it processes 60 samples and 60 calibrant solutions each hour. As in the first example, occasional calibrants or quality control materials are counted in the throughput when introduced to the instrument as if they were samples. This customary distinction is not always followed. For example, a paper on the determination of lactic dehydrogenase in serum (3) mentions three times that the method does 260 determinations per hour, and only once that the analyses are made by the method of additions. At least one addition, and often two, is required for each sample (4). While the method allows for 260 injections per hour, the number of analyses is one-half or onethird of that rate. The authors of this paper did not claim that the method is automated, presumably because the additions are made manually and a reagent is also added manually to each solution before it is injected. The sample size is another important parameter in describing an analytical method. Many FIA instruments introduce the sample by means of a valve similar to those that are used in liquid chromatography. In reading the papers on FIA, I have noticed that most authors specify only the amount of sample that enters the flowing stream. A much larger amount of sample, typically two or three times the volume that enters the stream, is
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needed to flush clean the sample valve and its associated tubing. The description of any method should specify the total minimum quantity of sample that is needed, including the amount that is necessarily wasted. A REPORT on FIA by Ranger (5) mentions "single point titrations," referring to an earlier publication by Astrom (6). I have also heard this unusual phrase from the representative of a firm other than Ranger's that is selling FIA apparatus. In the usual sense, a titration involves systematically varying the relative amounts of analyte and titrant to determine the equivalence point, and "single point titration" is a contradiction in terms. Àstrom's paper describes a method to determine acids and bases, in which the sample is injected into a flowing stream of buffer and the change of pH is recorded at the peak. This method is not unlike some segmented continuous flow analyses. Cool and Annokkee in 1970 (7) gave a theory for this technique, which they termed, the "bufferindicator system." (They used an indicator to measure the pH, while Astrom used an electrode.) Âstrom's choice of terminology is inappropriate. Nearly 200 years ago, Lavoisier said (8), "It is impossible to dissociate language from science or science from language, because every natural science always involves three things: the sequence of phenomena on which the science is based; the abstract concepts which call these phenomena to mind; and the words in which the concepts are expressed." In his weekly column, "On Language," William Safire (9) recently said, "Words not only mean what you want them to mean, words mean what they mean to most people who understand them." Our obligation in scientific writing is to convey information as precisely as possible to our readers. Choosing words that are inappropriate, or giving information that is only partly correct,
Letters will mislead or confuse. W h e n we d o t h a t , we a r e n o t m e e t i n g o u r obliga tions. Marvin Margoshes Technicon Instruments Corporation T a r r y t o w n , N . Y . 10591 References (1) Mottola, H. A. Anal. Chem. 1981,53, 1312 A. (2) Margoshes, M. Anal. Chem. 1977, 49, 17. (3) Nikolelis, D. P.; Painton, C-D. C ; Mottola, H. A. Anal. Biochem. 1979, 97, 255. (4) Mottola, Η. Α., personal communica tion, Oct. 17, 1979. (5) Ranger, C. Anal. Chem. 1981, 53, 20 A. (6) Àstrom, Ο. Anal. Chim. Acta 1979, 705,67. (7) Cool, W.; Annokkee, Y. In "Advances in Automated Analysis; Technicon Inter national Congress 1970"; Barton, E. C. et al., Eds.; Thurman Associates: Miami, 1971; Vol. 1, pp 607-18. (8) Lavoisier, A. L. In "Traité Élémentaire de Chimie," 1789; from "Bartlett's Familiar Quotations," 14th éd.; Beck, Ε. M., Ed.; Little, Brown: Boston, 1968; ρ 474. (9) Satire, W. Ν. Υ. Times Magazine 1982, Feb. 7,13.
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"Angle of Incidence" Sir: T h e t e r m " a n g l e of i n c i d e n c e " h a s a well-defined m e a n i n g in science a n d is widely u s e d in d e s c r i b i n g t h e i n t e r action of r a d i a t i o n or p a r t i c l e b e a m s w i t h surfaces (see, for e x a m p l e , J . Thewlis, " C o n c i s e D i c t i o n a r y of P h y s ics a n d R e l a t e d S u b j e c t s , " P e r g a m o n P r e s s , 1973). T h e angle of incidence is defined as t h e angle b e t w e e n t h e d i rection of incidence a n d t h e n o r m a l t o t h e surface a t t h e p o i n t of incidence. T e c h n i q u e s in m a s s s p e c t r o m e t r y in which b e a m s of ions or n e u t r a l p a r t i cles strike a surface (in s e c o n d a r y ion m a s s s p e c t r o m e t r y or fast a t o m b o m b a r d m e n t , for e x a m p l e ) u s e t h e t e r m angle of incidence. I t is i m p o r t a n t t h a t t h i s t e r m b e used only in t h e m a n n e r defined above a n d n o t t o refer t o t h e angle b e t w e e n t h e direction of inci d e n c e a n d t h e surface. J . H. B e y n o n P a s t - c h a i r m a n of A S M S Nomenclature Committee and IUPAC Mass Spectroscopy Subcommittee D. Cameron C h a i r m a n of A S M S N o m e n c l a t u r e Committee a n d C h a i r m a n of A S T M S u b c o m m i t t e e E14.10 J. F. J. Todd C h a i r m a n of I U P A C M a s s Spectroscopy Subcommittee
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CIRCLE 24 ON READER SERVICE CARD ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982 · 679 A
