Add-on Computing lntegrator/Recorder for Improved NMR Integration Alexander Hatzis and Robert Rothchildl The City University of New York, Toxicology Research and Training Center, John Jay College of Criminal Justice. Department of Science, 445 West 59th Street, New York, NY 10019
The great power of NMR spectroscopy lies both in the qualitative information that it can provide and in the quantitative analytical data that i t offers. Typical inexpensive NMR spectrometers available in most instructional laboratories piovide the quantitative aspect by means of the familiar integral trace, in which the step heights provide information such as relative numbers of nuclei. The integrations of the absorption spectra can also be used in determinations of proximity by means of the nuclear Overhauser effect. For analysis of mixtures, the integral traces are essential to determine amounts of various components in a sample. Quantitative precision is often limited in basic NMR spectrometers bv the crudeness of the integral step technique, with reiativeerrursof3-29~t~eingcommon.In pnrt,poor precision lirs with the suhiective iudgemrnt of the analvst in deciding where a particuiar stepd.a;ts or ends. In connection with some analytical studies involving chiral lanthanide shift reagents for determining optical purities of samples ( I ) we became acutely aware of the limitations of a simple step-height integration technique. The problem is aggravated for measurements of absorption signals that partly overlap because of incomplete separation with the spectrometer being used. For broad peaks that are ooorlv resolved. it can even he difficult to distineuish the mlltvt~unpmnt i n rhe Integral trace hy u,hich the chem~st d~itinauishesthe end of une step and rhe srsrt of another. In such cases, simple peak height is often easier to measure but can lead to errors if the peaks being measured differ in their intrinsic widths. Selective lanthanide-induced broadening of signal peaks for one enantiomer of a pair during studies with chiral shift reagents would be an example in which peak height measurements are less desirable. In the last few years, microprocessor-controlled electronic integrators have become available that employ digital electronic measurement techniques. Such units can provide numerical printouts of peak areas with improvements in precision and accuracv compared to alternative manual methods or to integral step heights. However, these instruments appear to have been used primarily in conjunction with cbromatographic detectors despite their potential utility for other applications, such as for NMR. The of NMR inteerations has been addressed - nrohlem ~ ~ earlier with the suggestion of the use of an accessory digital voltmeter (2. . , 3):.. some technioues for ootimizine-step. integrals have been discussed (4). We were not aware of reports of the interfacing of one of the currently availablecomputing integrators that offer memory and reprocessing capacity. We have upgraded a Varian EM360A NMR spectrometer, a lowcost, 60-MHz, continuous-wave unit that is representative of NMR instruments available in many undergraduate labora-
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Presented, in part, as paper no. 205. Division of Chemical Education. 191st American Chemical Society National Meeting. New York City. April 13-18, 1986. Author to whom correspondence should be addressed.
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Journal of Chemical Education
tories, by connection of a Shimadzu C-R3A Chromatopac computing integrator and chart recorder. The latter unit offers the capability of memorizing and storing (in volatile memory) about 6 hof analog signal data a t a sampling rate of twice every second. Students can gain computer literacy by exploiting some of the potential power of this type of instrument, since stored data can be recalled and reprocessed, replotted, or recalculated using changed parameters a t the students' leisure outside of formal laboratory hours. Advanced students can also perform BASIC programming on this instrument. Our connection of the C-R3A to the EM360A NMR took advantage of signal points located conveniently on the rear of the EM360A electronics console, a t pins 7 and 8 of jack J4. Pins 7 and 8 provide the positive and negative signals, respectively, to the analog input terminals of the C-R3A. In the Varian Accessory Interconnect puhl. no. 276317-1, point 7 is identified as "Recorder Y Monitor"; in the Varian Electronic Console Interconnect publ. no. 276361 Rev. B 176, points 7 and 8 are labeled "Recorder Input". As supplied in the basic spectrometer, pins 7 and 8 are missing from the RS-232C plug P-4, so that pins must he inserted into the plug. From here, leads can be brought to a barrier terminal strip that we cemented to the rear chassis of the NMR electronics console. This terminal strip permitted easy connection and disconnection of spade lugs from the C-R3A input terminals. Use of pins 7 and 8 as take-off points provides the advantage of accessibility and still permits use of the NMR flat-bed recorder for recording of the absorption spectrum and step integral traces. (Extended details of the interconnection procedure and methods for use have been submitted by us for publication elsewhere (5).)We found that even in the absence of a signal peak, there is a dc "offset" voltage at our signal points, independent of the setting of the Baseline Position control, but which varies with the setting of the Fine Spectrum Amplitude control of the NMR. Thus, depending on the position of the Fine Spectrum Amplitude control, we observed a range of this offset voltage from about +40 to -60 mV. The C-R3A requires an input signal between -1 and +5 mV to establish a baseline zero, so a convenient means of adjusting the offset voltage to lie within this range is desirable. We found that a separate digital multimeter (200-mV range) was very helpful in making this adjustment. Adjustment of the NMR timeconstant response ("Filter") control smooths out the shortterm variations resulting from electronic noise and facilitates obtaining a stable voltage reading. Normal positions of fast response (0.05 to 0.1 s) are used for recording spectra. The stepped power-of-ten Coarse Spectrum Amplitude can he used to bring actual signal peaks into a range compatible with the external integrator, and the integrator's power-oftwo attenuator selects an optimum amplitude if spectral recording on the integrator is desired. Note that once a spectrum is acquired and stored in C-R3A memory, the amplitude of the spectrum printed out can be readjusted on
