Introduction to Signals and Noise in an Instrumental Methods Course Richard J. Stolzberg University of Alaska, Fairbanks, AK 99701
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Some of t h e inherent capabilities and limitations of an instrument can be described in terms of signal to noise ratio (SIN). Choice of instrumental operating conditions is often
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by real time or post-experiment manipulation of data in such a way as t o improve SIN ( I , 2). Students ohserve limitations t o instrument performance in an informal fashion in their laboratory experiments when measurements are made a t low analyte concentrations, e.g. when an NMR spectrum is excessively noisy hecause the sample size is too small or when zero net ahsorhance is observed when measuring cadmium in t a p water by flame atomic absorption spectrometry (AAS). One of the goals of our instrumental methods course is t o have the students gain a n appreciation of the importance of both signal strength and noise level in instrument performance and how SIN can be increased hy choice of conditions and data manipulation. T o this end we have designed two 3-hour experiments. The first is a study of the importance of the interaction of signal strength and noise level on detectahility in flame AAS. Analytical wavelength and spectral bandpass are varied, signal strength and noise level are determined, and a choice of "hest"conditions is made. The second experiment shows the use of ensemble averaging as a technique for SIN enhancement in mass spectrometry (MS). T h e mass spectrum of an unknown liquid with a weak molecular ion is scanned repeatedly until the (M l)+/M+ ratio can he determined. The empirical formula of the liquid is then calculated.
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Experimental Atomic Absorption A 1000 ppm Pb stock solution is supplied. Working solutions of 1 ppm and 10 ppm Pb are prepared by the students. A Perkin-Rimer Model 107 atomic absorption spectrometer is used with a 3-sec integration period. A Ph, Ag, Cd. Zn hollow cathode (HC) is used. Four combinations of wavelength (217.0 nm, 283.3 nm) and slit width (0.2 nm, 0.7 nm) are used. For a particular experiment, slit width is chosen, the monochromater is adjusted to the proper wavelength, and the gain is adjusted in the normal fashion. The scale enpansion setting is held constant for all experiments. At each wavelength and slit width, the following is done: a) Zero the output. b) Make six replicate measurements of net signal with the flame off.
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Ignite the flame and observe the output from the phutomulti~ plier tube (PMT). Readjust gain and zero output. Make six replicate measurements of net signal with the flame on, while aspirating water. Make 10 replicate measurements of net signal while aspirating 1ppm Pb. Make six replicate measurements of net signal while aspirating 10 ppm Ph. Extinguish the flame.
is measured again so that data in arbitrary units can be converted to mA to calculate sensitivity.
Ensemble Averaging Ethyl benzoate is used as the unknown organic liquid. A Varian EM600 mass spectrometer is used in its normal operating mode. This instrument is ideal for this experiment hecauseit is easy lo use and its SIN characteristics are poor compared to a research quality instrument. Even so, some detuningrnay be necessary to get a sufficiently noisy signal: Data are collected and manipulated with a Bascom-Turner 8110 microprocessor controlled recorder. Acquisition is triggered on the rising portion of the large mie 122 peak. With a scan rate of 10 amuI min and a data acquisition rate of 1 point per 400 msee, a 500 point record will allow scanning from mie 122 to mle 155. The M+ peak is at mIe 150. Spectra are stored, added, and scaled for presentation. Peak-to-peak noise level is measured. Nine spectra are enough [or sufficient SIN enhancement to measure the (M + 1)+ peak height accurately. Results and Discussion St N in AAS Results of this experiment prove to the student that sirnal strength alone doeinot determine detectahility. The se'nsitivity a t 217.0 nm is significantly greater than at 283.3 nm, but the noise level is even greater a t the low wavelength due t o strong flame absorption. T h e net result is that the detection limit is sienificantlv better a t the hirher wavelength. Indeed. the 283.3nm line is-the one suggestez for routine zkalys&(3): Student results are shown in Table 1. Individual results seem to depend on how carefully the monochromator is adjusted. a t the hieher Choice of slit width is not verv. imuortant . ~ ~ v r l c ~ i c. It ththe . I.AU I n . t \ v I 1 1 ~thv h Iiam j v *IN 1s 11,
