should also be discussed in any paper using optically dilute measurements: First, the yield data should be calculated using A values estimated from Equation 3b or by a more exact procedure; second, since detailed absorption spectra are not always available for others to assess the magnitude of the errors, the percentage differences between A andA(X,) should be given. In summary, Equation 3b for calculating A is so simple to use and so accurate that there is no longer any excuse for failing to make a first order correction for the finite bandpass error in the usual yield determinations. Only three absorbance values, rather than the usual one, are required, and the arithmetic work involved is trivial even when carried out by hand. It should be stressed that Equation 3b is at best approximate for real systems. Rea-
sonable accuracy can be expected only when it is applied critically and the experimental conditions are such that Equation 2 is nearly satisfied. When significant deviations are expected, when doubt exists concerning the reliability of Equation 3b, or when the greatest accuracy is desired, the exact procedure should always be employed. J a m e s N. Demas Chemistry Department University of Virginia Charlottesville, Va. 22901 Received for review September 25, 1972. Accepted January 5, 1973.
Versatile Technique for Signal Enhancement as Applied to High Resolution Mass Spectrometry Sir: When a mass spectrum is measured using an electron multiplier and a high-speed electrometer coupled to a fast sample-and-hold amplifier with an analog-to-digital converter, the peak profiles will be quite irregular and may drop below base line on smaller peaks. This is due to noise in the electron multiplier and unfavorable ion statistics on small peaks. Thus, small peaks may be so irregular that they will not be recognized and the mathematical deconvolution of partially resolved peaks will be uncertain. Several solutions to this problem have been proposed. One is pulse counting over fixed equal time increments using a pulse amplifier and discriminator coupled to a pulse counter in a small computer (1). The technique essentially integrates the signal over equal time increments and reduces the effect of adverse ion statistics and noise. One such system is commercially available. Use of a time averaging technique to average many scans has been described (2, 3). An elegant method has been proposed using on-line computer control to scan each peak many times during an overall scan of the spectrum and to average these peak profiles (4). In our data acquisition system, we use a sample-andhold amplifier and analog-to-digital converter with a conversion time of 6.3 psec for 14 bits (5). An additional 6.5 psec are required for an IBM 1800 computer to read this output. System logic allows 13.5.isec to complete this process. If, for example, we sample at 10 thousand points per second we have 100 psec between points. Since only 13.5 psec are used for measurements, information in the remaining 86.5 psec is not utilized. More information could be obtained if the signal were integrated during this time. A review of currently available electronic hardware showed that integration for signal-to-noise improvement over this short interval is feasiBulletin 21-094, DuPont Instrument Products Division, Monrovia, Calif. F. J. Biros, Anal. Chem., 42, 537 (1970). S. P. Markey, K . B. Hamond, and J. R. Plattner, Eighteenth Annuai Conference on Mass Spectrometry and Allied Topics, San Francisco, Calif., June 1970, p 870 F. W. Mciafferty, R. Venkataraghaven, J. E. Coutant, and B. G. Giessner, Anal. Chem., 43,967 (1971) R. P. Page, and A. V. Nowak, Nineteenth Annual Conference on Mass Spectrometry and Allied Topics, Atlanta, Ga., May 1971, p 84 Burr-Brown Electronic Switches Cataiog, PDS-222, Burr-Brown Research Corp., Tucson, Ariz., Sept. 1969.
994
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 6, M A Y 1973
I
I I I
I
11
SWITCH AND r3MPLIFIER CURRENT I
A/D CONVERTER
I
-
OPT0 ELECTRONIC
I
CONVERTER LOGIC
Figure 1. Block diagram of analog incremental integration syst em
ble (6). We call this technique “analog incremental integration” and a block diagram is shown in Figure 1. It includes a high-speed operational amplifier integrator circuit reset by a switch with current gain to minimize reset time. This reset is initiated by an opto-electronic isolator operated by a pulse from the analog-to-digital conversion logic. Reset is accomplished during the same 6.5 psec used by the IBM 1800 computer to read the analog-to-digital converter output. Thus, we have peak profiles consisting of the integrals over 86.5-psec increments rather than more random discrete voltage values measured by a sample-and-hold amplifier. This kind of averaging, unlike analog filtering, produces no distortion of the peak envelope so that mathematical deconvolution can be used. This technique can also be used at high pulse rates where pulse counting techniques are not applicable. While this technique has been discussed as a way to improve high resolution mass spectral data, it can be applied to any system using an analog-to-digital converter. R. P.Page A. V. Nowak R. Wertzler Atlantic Richfield Company Harvey Technical Center Harvey, Ill. 60426 Received for review September 14, 1972. Accepted December 26, 1972.
