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Chemistry LibreTexts

II. Introduction

Spectrophotometry is covered in every introductory text on quantitative analysis or instrumental analysis. Beer's Law can be quoted by nearly every student who has taken chemistry in the last half century. Yet it is our experience that students have only a weak understanding of the relationship between light intensity, transmittance, and absorbance, that they report absurd numbers of significant figures, and that they fail to grasp topics such as stray light, noise, dynamic range, linearity, saturation, and order overlap. Until recently, having students dissect photometric instrumentation was prohibitively time consuming and expensive. In the days of photographic photometry, film calibration and quantification of emission (let alone absorption) took endless hours of experimentation, dark room work, densitometry, and interpretation. Photomultiplier tubes were delicate, required high voltage power supplies, and could only measure one wavelength at a time. Although semiconductor photodiodes were not so delicate and required only low voltage power, they required a current‐to‐voltage converting amplifier and readout electronics, and were difficult for students to understand. In recent years, diode array or CCD array spectrometers have provided full spectral coverage, but all of the optics and electronics are buried behind a computer screen. Student interaction with the measurement was reduced to a press of a (virtual) button, and the instrumentation for everything from computer‐controlled chromatographs to cybernetic potentiostats became indistinguishable from a parametrically‐intensive computer game. Direct sensing of measured quantities has effectively ceased.

Sometime during 2008, it occurred to the corresponding author that cell phone cameras had become common among students. "They're only 8 bit CMOS chips," was the initial thought. "The signal‐to‐noise ratio will be terrible. Dark current is likely to be a problem. No one in their right mind would use such a poor detector for doing quality spectrophotometry." In November, 2008, the author had a double‐take. If all these measurement problems were so blatant, wouldn't that make the concepts behind the problems easier to sense and learn than if one used a high quality detector and system? The result is reported in this paper: an inexpensive array detector spectrometer useful for teaching the basics of visible absorption spectrophotometry and the concepts general to spectroscopy at all wavelengths. The students supply the detector, while the instructor provides all other parts, costing no more than $3 per group (2009 prices). After watching individuals, pairs, and triples of students use the parts, the authors believe that a team size of 3 is optimal, being big enough to have discussions, but small enough that all students get a turn to influence the engineering. This paper reports work in progress. Comparative, statistically‐validated student evaluation has not yet occurred. Bugs continue to appear in the software. And yet, the enthusiasm with which the cell phone spectrometer has been met by nearly everyone who has heard of it suggests that an early "roll‐out" of the concept is warranted. As an Open Access/Creative Commons publication, the author solicits participation by the community in implementation, debugging, and evaluation of this pedagogical instrument.

The use of cameras in cellular telephones for optical spectroscopy has been previously patented.1