Editors' Column
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Instant Interfacing Sir: Dr. Dessy (March Report, 1974) has made a valid point that interfacing laboratory equipment and computers is not as awesome a task as one might suppose. However, I believe that he has overstated the ease in which the UART may be used. As the UART is essentially a single ASCII-character parallel/serial convertor, a number of additional circuits are needed to transmit a multicharacter message over any practical data channel. For tunately, the SERDEX modules (available from Analog Devices, Inc., Norwood, Mass.) come much closer to the ideal "instant interface" than the elementary UART integrated cir cuit. Lew Smith, Consultant Sudbury, Mass. Dr. Dessy Replies: Your comments are quite correct. UART chips do require auxiliary cir cuitry to function. The SERial Data EXchange modules provided by Ana log Devices incorporate many of the ancillary functions needed. This is the reason that SERDEX modules were listed in the equipment informa tion section that accompanies the ar ticle. The intent of the article was to educate chemists in the vocabulary and principles involved in INSTANT INTERFACING and provide them with vendors from which they could obtain further data. Obviously, no single commercial enterprise was used as the nucleus of the article. Your perspicacious comments suggest that you are at the frontier of the lab oratory automation area. I would suggest that a "mother board" is necessary for the various Analog Devices' modules if the full potential of the device is to be real ized. The difference between design ing a PC board for a UART and using a 100-pin module represents terrifying terra incognita to the average chem ist. Since the area is changing so rap idly, the chemist is faced with a di
lemma that can be solved only by clear, concise communications in the area.
117/118 = 3 a + 2 i ) + 2 c
Mass Spectrometry Sir:
115/118 = a 3 + 3 ab2 + 3 ac'2 + 6 dlb + 6 a2c + 2 be2 + 2 b2c + 12 abc = 0.00087
As a practicing mass spectrometrist, regularly involved in the analy sis of stable isotopic species by elec tron impact (EI) mass spectrometry, I was particularly interested in the January 1974 Instrumentation article which discussed the application of field ionization (FI) mass spectrome try to stable isotope analysis. However, in two areas this arti cle is misleading: • In the discussion of multilabeled molecular tracers (i.e., measurement of the relative amounts of various sta ble isotopic species), the analysis of a mixture of normal and octadeuterated toluene by FI mass spectrometry was presented. The discussion of this system was followed by the state ment: "With standard electron im pact mass spectrometry the isotopic composition of a given organic com pound could be determined only after its conversion to simple molecules." The implication that such a conver sion is necessary for a highly stable hydrocarbon such as toluene is un founded. In fact, EI mass spectrome try, performed at low ionization volt age, would almost certainly yield more reliable and accurate data than FI since the EI molecular ion sensi tivities are virtually independent of isotopic mass. The majority of organ ic compounds are amenable to EI mass spectrometry analysis—the only compounds which are difficult to an alyze by EI are those which either show no molecular ion or where inter fering fragment ions cannot be re moved by low-voltage techniques. • Further on in the same discus sion, the authors (Anbar and Aberth) describe a hypothetical analysis of a labeled histamine (C5H9N3) in which the imidazole ring consists of three 13 C carbons, two 1 5 N nitrogens and with deuterium substituted for hy drogen at the 2 and 5 positions. It is stated that if this hepta-labeled his tamine were synthesized from 95% pure " Ο , 98% pure 1 5 N, and 99% pure D, then the following ion inten sity ratios would be obtained: 119/118 = 0.026; 117/118 = 0.32; 116/118 = 0.10; and 115/118 = 0.032. Of these ratios only the value as cribed to the 119/118 ratio is correct. The correct values for the other ratios can be easily calculated: Let: 1 2 C/ 1 3 C = 5/95 = 0.053 = a; 1 4 N / 1 5 N = 2/98 = 0.02 = b; H / D = 1/99 = 0.01 = c. Then it is easy to show that a random distribution of these heavy isotopic elements would result in the following ion intensity ratios:
= 0.219
2
116/118 = 3 a + 6 ab + 6 ac + 4 be + b2 + c2 = 0.0193
While I appreciate that numerical calculation was not the major point of the article, the inclusion of incorrect numerical values, implying as it does an invalid calculation procedure, is to be deplored. James E. Morgan Morgan-Schaffer Corp. Montreal, Canada Dr. Anbar Replies: I disagree with Dr. Morgan's state ment about the possible substitution of low-energy electron impact ioniza tion as a tool for multilabeled tracer analysis. The example presented in the article is atypical. It is true that certain aromatic compounds includ ing toluene will produce molecular ions in a relatively high yield under low-energy electron impact, but these are the exceptions rather than the rule. Most natural products, drugs or metabolites, which are the most like ly candidates for tracer and dilution analysis do not fall in this category and cannot be readily handled by electron impact. The comment would thus be misleading. Even toluene produces a very high yield of C7H74" ions, which are totally absent under field ionization. Field ionization does not exhibit any isotopic mass discrimination ef fects, as might be implied from Dr. Morgan's statement. In this respect, both EI and FI are equally adequate methods of ionization. Most organic molecules of interest belong, however, to the "difficult" category referred to by Dr. Morgan. Moreover, most of the organic impurities, which are un avoidable in a practical analysis, pro duce a single peak under FI but many satellite peaks under EI. The back ground of the latter is most likely to interfere with a quantitative dilution analysis, even in cases where the yield of the molecular ion of the ana lyzed compound is high and where no fragments of it occur at the mass numbers of interest. The probability of interference by trace impurities is finite also in the case of FI, but it is minimal in comparison to EI. As far as the computations of the abundances of the isotopic species of histamine are concerned, Dr. Mor gan's calculation is correct, and there is a minor error in the calculation of the example cited in the article. This does not invalidate to any degree the arguments presented there, and the correction is, therefore, trivial.
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 7, JUNE 1974 · 643 A
