Guest Editorial Infrared Solves Some Unusual Problems for the Film Industry

Paul A. Wilks

aving recounted in in a while there is a mix-up We happened to have a my previous guest in the shop and we lose Thunder Dome ATR attachment editorials some of the track of the film orienta- supplied by Thermo Spectra- past history of the tion, which could lead to a Tech (Madison, WI), which was HHevolution of infrared disaster if film with the being evaluated for another instrumentation and how it has wrong orientation is used purpose. We found that there begun to move out of the labo- in further processing. was such a concentration of IR ratory into the real world of ma- “Can you provide us energy at the upper surface of terials analysis (1, 2), I thought with some sort of equip- the spherical element in the it might be interesting to de- ment that will quickly tell Thunder Dome that one needed scribe three analytical problems us which side is which?” only to press a piece of the film

” . . . Our extruder was functioning erratically, and we have produced Paul A. Wilks several hundred rolls of film that may or may not have the correct is president of Wilks Enterprise (140 Water Street, South Norwalk, thickness moisture barrier. How can we check these rolls to determine CT 06854) and a member of which has the correct barrier thickness and which doesn’t?” Spectroscopy’s editorial advisory board. He can be contacted by phone that came to us from the plastic Although each material had on the spherical surface with at (203) 855-9136 and film industry and discuss how characteristic infrared absorp- one’s thumb and there would be by e-mail at pwilks@ infrared techniques solved tion bands, taking transmission a strong positive reading from wilksir.com. them. spectra would obviously not the infrared filtometer if the Wilks Enterprise was ap- help in film orientation. Attenu- sealant surface was down, and a proached several months ago by ated total reflectance (ATR) much weaker reading from the a manufacturer of plastic films sampling would, however. The other side. A reading takes less for packaging with the following trouble was that the one usable than 30 s. The film orientation problem: band in the sealant spectrum problem was solved and the in- was rather weak and conven- frared filtometer, with spherical “We manufacture a film tional horizontal ATR sampling ATR surface on the “thumb that is basically polyethyl- methods, even with the clamp sampler” is now in use in the ene with a sealant layer on set at fairly high pressures, did film processing plant. one side. Trouble is, you not show significantly different A short time later, another can’t tell which side is absorption at the wavelength of film maker approached us with which by just looking at the the characteristic sealant band. another problem: film. Although we take (We managed to break a ZnSe great care in our processing ATR plate in the evaluation to make sure we know process!) “Guest Editorial” which side is up, every once continued on page 120

116 Spectroscopy 17(12) December 2002 www.spectroscopyonline.com TUTORIAL

Walther Gerlach and the Foundations of Modern Spectrochemical Analysis

Volker Thomsen

The author discusses the life and sometimes-overlooked work of German scientist Walther Gerlach.

Volker Thomsen many compilations of scientific thereof are generally conceded is senior biographies. to begin with the work of Bun- applications For most scientists, the name sen and Kirchhof (1). Their 1860 scientist at Gerlach brings to mind only the publication (3) showed that NITON classic Stern–Gerlach experiment qualitative chemical analysis was Corporation, 900 Middlesex of atomic physics. It is not gener- feasible. One year later, the au- Turnpike, ally appreciated that Gerlach, a thors proved this rather conclu- Building 8, first-rate experimental physicist, sively by discovering two new el- Billerica, MA transformed the spectrographic ements — cesium and rubidium 01821, and an technique from a qualitative or — by spectroscopic analysis. adjunct faculty member in semiquantitative tool into a quan- Attempts to produce quanti- physics at Mount titative method of instrumental tative analyses using photo- Wachusett chemical analysis. He did this by graphic film (spectrograph) Community introducing the internal standard rather than the human eye College, Gardner, method, which remains central to (spectroscope) ran into grave MA 01440. He can be reached modern spectrometric analysis, problems, caused by the number via e-mail at whether the excitation is by of variables associated with the vthomsen@ niton.com. lmost without excep- tion, all precision It is not generally appreciated that Gerlach, quantitative analysis AA using emission spectra a first-rate experimental physicist, transformed is based on some variant of the internal-standard principle the spectrographic technique from a qualitative first enunciated by Walther Gerlach (1). or semiquantitative tool into a quantitative It seems strange, therefore, that so little is known today method of instrumental chemical analysis. about the founder of modern optical emission spectrochemi- cal (OES) analysis. Aside from arc/spark or the more recent in- intensity of the spectral lines an excellent biography in Ger- ductively coupled plasma (ICP). produced by a given material. man (2), little else is available; These included excitation his name appears in a 110-word Historical Development source instability and variable entry in Encyclopedia Britannica, Although spectroscopy has a photographic film sensitivity. Vol. 5, 15th Edition (1994), but long history going back to New- Several researchers achieved he is not noted in any of the ton, the chemical applications semiquantitative results in the

December 2002 17(12) Spectroscopy 117 Tutorial

Table I. Improvement in precision using internal standardization. homologous and fixation spectral line pairs. Gerlach was the first to call spec- Spectral line pair Intensity analyte Intensity ratio tral line pairs homologous if they (%RSD) (%RSD) worked well together to compensate for Si(I) 251.6/Fe(I) 281.3 0.82% 0.18% changes in excitation conditions. It is 0.85% 0.22% important to note that not all line pairs Cr(II) 289.8/Fe(II) 273.0 0.44% 0.17% of an analyte and matrix are homolo- 0.95% 0.24% gous. For example, an ionic line of the Mn(II) 293.3/Fe(II) 273.0 0.72% 0.28% matrix element does not couple well 0.98% 0.28% with an atomic analytical spectral line. What defines a pair of homologous late 19th and early 20th centuries. The against the concentration of the stan- spectral lines? No set of necessary and work of W.N. Hartley in Dublin, Ire- dards to produce a working (calibra- sufficient conditions exists, as more re- land, is noted, as his 1882 study appears tion) curve. search in this area is required. We can to have been the first systematic study Two types of internal standard lines state, however, that their excitation po- of the change in spectral line intensity can be used: tentials must be similar; or, in the case with concentration. He also made the ● A weak (nonsensitive) spectral line of of ionic lines, their combined excitation first quantitative spectrographic analy- the major component of the sample. and ionization energies must be similar. sis (determination of beryllium in This approach is most common with Beyond this, we must turn to experi- cerium compounds [1]). arc/spark spectrometry. mentation. Several types of tests may be De Gramont in Paris, France, was a ● A strong (sensitive) line of an ele- performed. driving force for the development of ment such as strontium or yttrium The basic definition of homologous spectrographic methods as a quantita- that is not initially present, but added spectral line pairs, due to Gerlach, is tive technique for chemical analysis (1). to the sample. This approach is that they compensate for changes in the In the United States, Meggers, Kiess, generally used today in ICP excitation energy. The simplest test to and Stimson attempted to bridge the spectrometry. determine if a line pair meets this crite- gap to quantitative analysis in their How does this technique work? Let a rion is to make measurements at differ- 1922 paper (4). However, Meggers and measurement of a spectral line of an ent power levels. co-workers relied on constant excita- analyte, A, be IA 1000 counts. The in- Homologous spectral line pairs im- tion conditions, extremely difficult to ternal standard spectral line, measured prove the precision of analysis, gener- produce experimentally at that at the same time, yields ISTD 10,000 ally by a factor of about three or four. time (5). counts. The intensity ratio is IA/ ISTD Table I shows some examples. A counter In Germany, Gerlach made the real 0.10. Now a second measurement is example is W(I) 400.8/Fe(I) 360.8 in breakthrough to quantitative analysis. made at some time later: IA 950 tool steel where the %RSD of the ana- In 1929 Gerlach and co-worker Eugen counts and ISTD 9500 counts. Note lytical spectral line alone was found to Schweitzer introduced the method of that despite the change in measured be 0.75% and that of the intensity ratio internal standards along with the con- values, the intensity ratio remains the 0.70%. This test is simple to perform. cepts of homologous spectral lines and same. This simple yet powerful tech- It has also been found that homolo- fixation pairs (6). Although his name nique compensates for both short and gous line pairs improve the calibration seldom surfaces in today’s spectro- long-term changes in the measuring in- and show less interelement (matrix) ef- chemical literature, it was very promi- strument, the spectrometer. fects. Some examples from arc/spark nent in the spectroscopy texts of the The improvement in precision OES are noted here (8): 1940s (1, 5, 7). through the use of internal standards is ● The use of the Al(I) 305.9-nm spec- shown in Table I with arc/spark OES tral line rather than Al(I) 308.2-nm Internal Standard Method measurements made on low alloy and line improves the calibration for high The early spectrographic attempts at stainless steel samples (8). The center aluminum concentrations in zinc al- quantitative analysis were plagued by column gives the precision of the ana- loys by a factor of three as measured multiple problems, running the gamut lytical spectral line alone, while the next by the standard error of estimate of from unstable excitation sources to the column shows the precision of the in- the calibration curve. The internal response of the photographic plate or tensity ratio. standard line in both cases is Zn(I) film itself. Gerlach solved these prob- 267.0 nm. lems by introducing the internal stan- Homologous and ● Consider the spectral line pairs Ni(I) dard method: Ratio the intensity of the Fixation Line Pairs 380.7/Cu(I) 296.1 and Ni(II) analytical spectral line to the intensity Two allied concepts arise naturally from 376.9/Cu(I) 296.1. Using the first of a spectral line of another component the method of internal standards, both pair, the nickel silvers (Cu–Zn–Ni) of the material. Then plot this ratio introduced by Gerlach. They are and cupro nickels (Cu–Ni) plot to-

118 Spectroscopy 17(12) December 2002 www.spectroscopyonline.com Tutorial

Table II. Change in fixation pair metal pieces and also at low pressures. Amidst all this spectroscopy, Gerlach intensity ratio with excitation. Their results confirmed Einstein’s the- found the time to write a textbook on ory of the photoelectric effect over an magnetism, Magnetismus, published in Peak current (A) Intensity alternate proposed by Sommerfeld and 1931. A decade earlier (1921), Gerlach ratio Debye. had published his text on quantum me- [Fe(II) 273.0/ Gerlach completed his “habilitation,” chanics, Grundlagen der Quantentheorie Fe(I) 360.8] a requirement to teach at German uni- (Foundations of Quantum Theory). 130 4.029 versities, in 1916 and became a lecturer After World War II, Gerlach was in- 110 3.885 at Tübingen University. After service in terned with fellow German scientists 57 2.902 World War I, Gerlach spent a year Werner Heisenberg, Carl von (1919–1920) in industry, working in the Weizsaecker, Otto Hahn, and six others gether nicely. With the second pair, Physics Laboratory of Farbenfabriken at Farm Hill in England. Details of this an interelement correction for zinc, Elberfeld. internment and insights into the Nazi or the use of two separate calibration In 1920 Gerlach became professor at atomic bomb effort are only now com- curves, is necessary. the Institute for Experimental Physics ing to light. (Gerlach’s name surfaces in We can summarize the characteristics in Frankfurt. Max Born, head of the in- several publications as one of the 10 of homologous spectral line pair ratios: stitute, wrote Einstein to express his de- German physicists interned at Farm ● They correct for changes in source light in acquiring Gerlach: “We now Hill in England at the end of World War excitation (the basic definition) have the famous Gerlach here. He is en- II. See, for example, A.D. Beyerchen, ● Their excitation potentials (and ion- ergetic, knowledgeable, ingenious, and Scientists Under Hitler [Yale University ization energies) are similar helpful” (2). Press, New Haven, CT, 1977]; M. ● Their precision is better That same year, his colleague Otto Walker, Nazi Science: Myth, Truth, and ● They improve the calibration Stern approached him for assistance in the German Atomic Bomb [Plenum ● They show less interelement (matrix) an experiment he had designed. “To- Press, New York, 1995]; and K. effects. gether they succeeded in proving the re- Hentschel, ed., Physics and National So- A fixation pair is the opposite of ho- ality of space quantization and in meas- cialism: An Anthology of Primary Sources mologous spectral line pairs. Here we uring the magnetic moment of the [Birkhauser, Boston, 1996].) choose an atomic and ionic spectral line silver atom” (9). Five papers were pub- In 1950 Gerlach published a philo- of the matrix or base element. The ratio lished during the years 1921–1924, four sophical work, Humaniora und Natur of the two will provide a sensitive indi- of them jointly, on what came to be (Humanity and Nature). Gerlach retired cator of any changes in excitation known as the Stern–Gerlach experi- from the University of Munich in 1957. source conditions. ment. Stern later received the Nobel In 1973 he published an historical A fixation pair may be monitored to Prize in Physics in the year 1943 for the study, Kepler und die Copernicanische ensure that the excitation remains con- detection of the magnetic moment of Wende (Kepler and the Copernican stant. If the excitation is stable, then the protons. Revolution). ratio of the fixation pair will also In 1924, when Alfred Landé asked Gerlach died in Munich on 10 Au- remain constant. Table II shows the who should be considered for the open- gust 1979, only days after his 90th variation in the Fe(II) 273.0 nm/Fe(I) ing in experimental physics at the Uni- birthday. 360.8 nm intensity ratio with change in versity of Tübingen, Albert Einstein the peak current of the spark-like exci- replied, “In my opinion, Mr. Gerlach Conclusions tation (8). would be the correct successor to The internal standard principle of Paschen. Among the younger German Walther Gerlach lies at the heart of Biographical Notes experimental physicists who might be modern spectrometric analysis. It was Walther Gerlach was born 1 August considered, he is the best” (2). Gerlach most important in elevating the spark 1889, in Biebrich am Rhein, Germany. returned to the University of Tübingen optical emission method of spectro- He studied physics at the University of in 1925 as professor of physics. chemical analysis from semiquantitative Tübingen and received his PhD in 1912 His last move came in 1929 when he to quantitative analysis status. Today, under Friedrich Paschen. His disserta- joined the University of Munich. He re- this principle is a standard part of the tion concerned blackbody radiation mained there until his retirement in arsenal of the modern chemist using in- and a means to determine the Stefan- 1957. strumental methods for quantitative Boltzmann constant. It is not generally known that Ger- chemical analysis. From 1913–1915 Gerlach, together lach’s work on spectrochemical analysis However, the twin concepts of ho- with colleague Edgar Meyer, produced a resulted in three volumes of Die chemis- mologous spectral lines and fixation series of publications on the photoelec- che Emissionsspektralanalyse, published pairs introduced by Gerlach seem to tric effect. They described their obser- in English translation in 1929, 1934, have been forgotten by most or simply vations of this effect on microscopic and 1936 (6, 10, 11). lost over time. Perhaps their reexamina- tion by today’s practicing spectro-

December 2002 17(12) Spectroscopy 119 Tutorial

chemists may provoke theoretical stud- (Prentice-Hall, New York, 1948). Applications of Spectrum Analysis ies and promote instrumental advances. 6. W. Gerlach and E. Schweitzer, Founda- (With Notes on Spectrography in tions and Methods of Chemical Analy- Chemistry and Mineralogy and Tables sis by the Emission Spectrum (Hilger, for Qualitative Analysis) [Adam Hilger, References London, 1929). (This is the English London, 1934].) 1. R.A. Sawyer, Experimental Spectroscopy translation of Die chemische Emission- 11. W. Gerlach and E. Riedl, Die chemische (Prentice-Hall, New York, 1944). sspektralanalyse, Volume One [Voss, Emissionsspektralanalyse, Volume III, 2. H. Rechenberg, ”Walther Gerlach,“ Leipzig, Germany, 1929].) Tabellen Zur Qualitativen Analyse www.th.physik.unifrankfurt.de/ 7. W.R. Brode, Chemical Spectroscopy, (Leopold Voss, Leipzig, Germany, ~wwwphys/paf/paf63.html (accessed Second Edition (John Wiley and Sons, 1936). (Published in English translation Nov. 2001). New York, 1943). as Chemical Emission Spectrum Analy- 3. R. Bunsen and G. Kirchhof, ”Chemical 8. V. Thomsen, Modern Spectrochemical sis [C. Zeiss, London, 1938].) ■ Analysis by Observation of Spectra,” in Analysis of Metals (ASM International, Annalen der Physik und der Chemie, Materials Park, OH, 1996). Vol. 110 (1860). Reprinted in English 9. I. Estermann, “Otto Stern,” in Dictionary translation in Milestones of Modern of Scientific Biography, C.C. Gillispie, Chemistry, E. Farber, Ed. (Basic Books, Ed. (Scribner's and Sons, New York, New York, 1966). 1980). 4. W.F. Meggers, C.C. Kiess, and F.S. Stim- 10. W. Gerlach and We. Gerlach, Die son, “Quantitative Spectroscopic Analy- chemische Emissionsspektralanalyse, sis of Materials,” National Bureau of Volume Two (Voss, Leipzig, Germany, Standards Scientific Paper 444 (1922). 1933). (F. Twyman provided the author- 5. G.R. Harrison, R.C. Lord, and J.R. Loof- ized English translation of Volume Two, bourow, Practical Spectroscopy published as Clinical and Pathological

“Guest Editorial” so well that, after the previously ex- were solved by infrared analysis. The continued from page 116 truded rolls were checked, the manufac- original infrared evaluation was done turer decided to mount the filtometer with conventional laboratory spectrom- “We make a film with a moisture permanently after the extruder to mon- eters but the ultimate plant procedures barrier with a layer of polyethylene itor barrier-layer thickness of newly ex- were carried out by dedicated infrared on either side. We recently discov- truded film. filtometers. ered that our extruder was func- And finally, a third film problem All three problems were solved using tioning erratically, and we have came to us — this one also to provide a absorption bands in the mid-IR region, produced several hundred rolls of thickness measurement of an inner where they are generally isolated and film that may or may not have the layer — in this case, nylon — in a film free from interference. This is usually correct thickness moisture barrier. that eventually became bubble wrap. not the case with NIR absorption How can we check these rolls to de- Because nylon has a very strong band at bands, which frequently overlap and in- termine which has the correct bar- 3 m (3333 cm1), it was easy to obtain terfere with each other. Furthermore, rier thickness and which doesn’t?” a precise measurement of its thickness those three solutions illustrate the fact in an infrared filtometer equipped with that, if measurement of an infrared ab- It turned out that the barrier layer a 3-m detector. The problem was that sorption band can be used for quality material had a strong carbonyl absorp- the plant procedure was to take a piece control or process management, then tion band — the polyethylene didn’t — of film, weigh it, then strip off the poly- simple, reliable, inexpensive filter in- so it was easy to determine the barrier ethylene and weigh the residue. In this struments can be used successfully in layer thickness by transmission. How- way, the thickness of both components plant environments. ever, it wasn’t very practical to cut a lot could be determined. The procedure of pieces from the rolls and put them was time consuming and, furthermore, References individually into the infrared filtometer required the use of hazardous reagents. 1. P. Wilks, Spectroscopy 16(12), 14–15 to check the barrier layer thickness. By combining the infrared measure- (2001). 2. P. Wilks, Spectroscopy 17(3), 14 Our solution was to take the source ment for the nylon thickness and meas- (2002). ■ and detector out of the body of the fil- uring the overall film thickness with a tometer and mount them on either side micrometer, the time-per-analysis was of a U-shaped holder, so that the film greatly reduced and the need for a could be passed continuously through reagent was eliminated. them and the layer thickness measured Hence, these are three real-world continuously. The arrangement worked process quality control problems that

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