Focus
The Crisis in Instrument Specifications Standards-setting agencies may help us find our way out of the specifications mess, but instrument users must first be concerned enough to demand action Surely one of the most straightforward ways to evaluate competing commercial analytical instruments is to compare specifications, such as resolution, accuracy, linearity, signal-tonoise ratio (S/N), stray-light ratio, and other measurements that are provided by manufacturers. But according to Wilbur Kaye of Beckman Instruments, comparing different instruments based on their specifications is not as simple as it appears. "Unfortunately, the specifications describing the performance of instruments have not kept abreast of changing technology," said Kaye, speaking at a symposium on the history and preservation of chemical instrumentation. "There appears to be an alarming deterioration in the status of specifications. There is a tendency for some manufacturers to conceal real performance by the way specifications are worded." The symposium, organized by J. T. Stock of the University of Connecticut, was held this September at the ACS National Meeting in Chicago, 111. Although Kaye spoke only on spectrophotometer specifications, he feels that similar problems probably exist with many types of scientific instruments. To ensure his objectivity, Kaye restricted his remarks to instruments produced by manufacturers other than Beckman, his employer. Most spectrophotometer specifications were developed in the 1950s and 1960s by instrument industry engineers. Some specifications were described in the original scientific literature; others were completed by committees and documented by organizations such as the American Society for Testing and Materials (ASTM). However, little has been done to redefine specifications in the past 15 years. "Unfortunately, specifications that satisfied needs in the earlier analog world have not always served well in the new digital world," Kaye explained. For an instrumentation specification to be useful, it must be defined in a way that is universally recognized, but many specifications are not so de-
fined. For example, measurement of the resolution of a scanning spectrometer can be based on either of two methods: • Under the assumption of an ideal, perfectly triangular slit function (the signal obtained upon scanning an isolated line source), two merged bands are resolved (two maxima are seen) when the band separation just exceeds one half-bandwidth (HBW, the width of a peak at its half-height point). • Direct measurement of the HBW of the slit function from a line source is the specification that is most often used because it is easier to measure. But Kaye pointed out that both half-bandwidth resolution tests fail when applied to diode array spectrometers, which differ from scanning spectrometers in that they sample a spectrum discretely rather than continuously across the wavelength range. "Difficulties ensue with both measurement methods because an arbitrary monochromatic line can fall anywhere on or between the sampled points on a diode array spectrophotometer," said
Kaye. "Depending on the relative location of array elements and the spectral line used in the test, the observed HBW will fall anywhere between one and two times the 'true' value. At the present time there is no accepted way of defining or measuring resolution in an array instrument." According to Kaye, the problem is even worse in the case of the straylight specification than it is for resolution. Until recently, stray light has been defined as detected radiation from wavelengths more than ± 1 HBW from a primary wavelength (monochromator setting), but there is no practical way to measure this parameter when it is defined this way. The term needs redefinition, said Kaye, and improved methods of measuring stray light are needed. Problems arise with the opaque filter used in stray-light testing because it not only absorbs the primary wavelength (which it is supposed to do) but also absorbs a large fraction of the defined stray radiation. "Application of the opaque filter test," said Kaye,
CO
X(nm)
190
900
Stray radiant power ratio vs. wavelength for a commercial UV-visible spectrophotometer The purple dots are experimental points, which are connected by straight lines to form the stray-light spectrum. The open circles indicate the two stray-light specification points reported by the instrument vendor. (The measurements were slightly inaccurate and hence are off the graph line.) Obviously, these two points provide an unrealistically favorable impression of the stray-light characteristics of the instrument
40 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY
1986
Focus
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"can lead to measurements that are in error by as much as a factor of 500. Stray light is a major source of error in high-absorbance measurements, and a proper knowledge of this parameter is needed." Kaye went on to point out that the common practice of citing performance at only one or two wavelengths ought to be discouraged. "Frequently the specification is-reported for the wavelength of best performance," he explained. "The problems this can pose for the user are obvious. "The parameters particularly sensitive to wavelength variation are dynamic range, stability, and stray light," Kaye continued. "Graphing minimum performance, properly defined, vs. wavelength would provide vastly superior specifications for these parameters." The figure illustrates the improvement in useful consumer information that might result if instrument vendors provided complete graphs of stray light instead of straylight specifications at isolated points (indicated by the open circles). Noise and baseline flatness specifications can be expressed either as maximum (worst-case) peak-to-peak signal excursions or as root-meansquare (RMS) units that average out an entire spectrum, and problems can arise in comparing instruments in which these parameters are expressed differently. An averaged-o,ut RMS baseline flatness number, for example, may obscure the fact that the instrument suffers from an extreme baseline flatness fluctuation at a particular point in the spectrum. Kaye points out that there can be nearly a 10-fold difference in the magnitude of noise or baseline flatness numbers as measured by the peak-to-peak and RMS methods. A similar situation holds for wavelength accuracy specifications. Vendors typically provide a few discrete specification points that represent differences between true and observed wavelengths along the instrument's spectral range. However, if a few particularly bad accuracy excursions are skipped—and they sometimes are, according to Kaye—the consumer gets an incomplete picture of that instrument's wavelength accuracy specifications. Kaye explained that these bad points would show up in a first-derivative plot of the same data. "What is needed," he said, "is a specification giving the maximum first derivative of the wavelength error function. This is not done today." But if some of the specifications currently in use tend to be misleading, what is even worse is that some very
CIRCLE 50 ON READER SERVICE CARD 42 A · ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
important specifications are not provided at all. According to Kaye, major spectrophotometric sources of error arise from sample defocusing and sample scatter, parameters that are strongly influenced by an instrument's sample optics. Optimal spectrophotometric performance, particularly at low absorbance, is realized wjien an optical integrator is inserted between the sample and the detector. However, few instruments are so supplied, probably because there is no specification to alert the user to the need for such an optical configuration. "The problem is particularly acute with diode array instruments," said Kaye, "because the sample-defocused beam has to pass through the narrow entrance slit of the monochromator." Segments of the scientific community that could potentially address the crisis in specifications include the instrument industry (the Scientific Apparatus Makers Association), academia, government agencies such as the National Bureau of Standards and the National Physical Laboratory (U.K.), societies such as the National Committee for Clinical Laboratory Standards and the International Union of Pure and Applied Chemistry, standards organizations such as ASTM, and instrument users. "At first the problem would seem to fall within the responsibility of the instrument industry," said Kaye. "Unfortunately, there are strong forces at work to prevent effective industry cooperation. "A personal experience may illustrate the point. A few years ago I developed what I thought was a superior method of measuring stray light in t h a t it was independent of the absorption properties of the test material. However, it revealed a significantly higher level of stray light than the conventional tests. When Beckman started to use this method, the sales personnel became alarmed because it appeared that our instruments were inferior to the competition. We had to return to the old method. Indeed, the domino effect works to force all manufacturers to the lowest common denominator of specification expression and measurement." Of the six segments of the scientific community mentioned, Kaye feels that ASTM is best suited to the job. But the real key to progress resides with instrument users. "A vocal minority could reverse the present trend," said Kaye. "Users need to prod the instrument industry and standards-writing organizations to improve the situation." S.A.B.
