the computer bulletin board 98.6 mVls and the area and radius of the hanging mercury drop electrode (HMDE)used were A = 0.0225 cm2 and r = 0.0423 cm, respectively. AU other conditions and apparatus used are described in detail elsewhere (9).Under these experimental conditions, monoethyl fumarate gives an LSV reduction peak that is typical of a fully irreversible process taking into account that no oxidation peak was obtained. Figure 1 shows the numerical fitting performedMathCAD uses the Marquardt-Laveuger algorithm (1,2). Boxes denote experimental data and the solid line the value of eq 5 for the parameters obtained in the fitting, uiz. an = 1.48 f 0.06, D = 1.01 0.04 x lo4 cm2/sa n d E =~-684 f 1mV. The first two were obtained by DC polarography under the same conditions and turned out to be an = 1.5 and D = 1.1x lo4 cm2/s,respectively (9). This procedure can he applied to other electrode reduction mechanisms with tabulated solutions (8) and, in gen-
eral, to any non-analytical function whose calculation is rather laborious.
Implementation of Fourier Transform Rapid Scan NMR S~ectrosco~v Techniaues in Continuous Wave NMR ~~ectiometers G. Moyna, J. Cernadas, L. Mussio ,and G. Hernandez NMR Laboratory, Facultad de Quimica
Avda. General Flores 2124 Montevideo, Uruguay
*
Introduction Old continuous wave N M R spectrometers (i.e., Varian T-60, Varian A-60, etc.) are usually used only for very simple NMR analysis or student training. Connection to an IBM PC compatible computer through an ADDAconverter and simpie programming can make these machines useful in a ORIGIN := 1 wide range of experiments.
* CONSTANTS: T ---------------
:= 298.15
v := 98.6
R :=
8.314
n := 2
** INITIAL F m I N G PARmmERS: ** .................................. FIT: ------------------+ NUMERICAL
Given
:= fi-(D,m,W)
rm
:= 1.5
D
Journal of Chemical Education
> 0
D :=
CA := 0.5
r := 0.0423
1.10
m
:= -690
s s ~ ( ~ , r m , n*) 0
-5
D =
1.01.10
Figure 1. MathCAD document for the numerical finingin LSV. A314
96487
A := 0.0225
a! < -500
[i]
F :=
m = 1.48
W = -683.7
Hardware and Software Considerations In our particular case, a Varian T-60 (10) was upgraded to Fourier transform rapid scan capabilities. A Data Translation DT2801 ADDA converter board (11) and a simple home-made voltage-controlled current source were used to scan and co-add the spectra with the PC. In order to scan the spectra, the voltage-controlled current source was driven with one of the ADIDA converter analog outputs (12). This current source supplies a current proportional to the output voltage of the ADDA analog output and is used to sweep the magnetic field of the T60 spectrometer. At the same time, NMR data are obtained with one of the AD/DA analoe inputs, and kept in the PC memory for later co-addine with data from successive scans. To correct magnetic field drift, spectra are accumulated using the field cure technique, in which the spectra to be co-added are shifted left or right in the computer memory using one of the spectrum peaks as a reference (13). Continuous wave spectra with improved sensitivity a r e obtained in this way They are still affected by well-known continuous wave artifacts, such a s strong ringing present after sharp peaks. These artifacts can be removed by processing the spectra in the computer with Fourier transform and cross-correlation algorithms (13, 14, 15).
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(Continued on page A3l6)
the computer bulletin board
a
b
Figure 2. Comparison between a single scan spectrum of ethyl acetate (a)and a 64 scan cross-correlatedspectrum of the same product (b) The equation that rules ringing in the time domain has the following form:
where i is the square root of -1,b is the sweep rate in radians per second, and t is the time in seconds. To cross-correlate this function with the spectrum, the latter must be taken to the time domain. This is done by an inverse Fourier transform of the spectrum in the frequency domain. The spectrum in the time domain can be filtered with well-known FT NMR apodization functions, thus improving sensitivity or resolution (16).ARer crosscorrelation (complex multiplication of the two functions in the time domain), data are taken again to the frequency domain by performing a direct Fourier transform. A single scan CW spectrum and a multiple scan cross-correlated spectrum are compared in the Figure 2. Conclusions With a small investment, simple electronic circuits and some programming, an old CW NMR machine was transformed into a powerful instrument for simple routine analysis. Since all the data are in digital form, spectra can be stored in the computer disk to be recalled when needed. Simple program routines make integration, J calculations and ~ e a assimments k vew easv in com~arisonwith the origihal machlhees h he work described here can be easilv adaoted to other CW NMR machines. as well a s d i f f e r e n t " ~ ~ /hoards r j ~ and computer languages other than Turbo Pascal 6.0. 'Author to whom correspondence should be addressed. (Note to editor: this author's address has changed and will change again. A longterm address will be provided at an appropriate later time.) 'Current address: Department of Chemistry,Corneli University. Ithaca, New York 14853 A316
Journal of Chemical Education
Acknowledgment The authors wish to acknowledze the s u ~ ~ oofrthe t CEC through Avant C11.0317.U. We &o thank'the invaluable advice of Hector Casal from British Petroleum Research (US).
Interpretation of Mass Spectra: A Direct Learning Approach through Software Thomas ~ e h m a n 'and Grigory vagenin2 Bethel College North Newton, KS 67117 Interpretation of mass spectra is best learned by working with many spectra rather than with a text that offers numerous generalizations illustrated by a limited set of spectra. Software available from the National Institute of Standards and Technology (NIST) makes this possible in an attractive format at no cost. Students can peruse 735 carefully chosen spectra on a demonstration disk that uses the same menu-driven display commands as the library of 62.000 mectra develooed for eeneral scientific use. Both graphical and tabular displays are available for each compound, which is identified on screen by formula and name. The structure also appears for all but the largest molecules. Synonymous names are given for each compound; for aspirin the list exceeds 100 entries because of its commercialization. The instructional utility of the software derives primarily from its flexibility in calling up specific spectra, or groups of spectra for simultaneous display. Asearch can be made by formula, by molecular weight, by reference number, by a particular peak in the spectrum, or by a sequence of specifications of molecular weight, name fragment, allowed elements, numbers of atoms of these elements, and up to ten specified peaks. Up to five spectra can be dis-
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