Anal. Chem. 1981, 53,538439 Braun, W.; Peterson, N. C.; Base, A. M.; Kurylo, M. J. J . Chromatogr. 1971, 55, 237. Dagnall, R. M.; West, T. S.; Whitehead, P. Anal. Chim. Acta 1972, 60, 25-35. Daanall. R. M.; West. T. S.; Whitehead, P. Anal. Chem. 1972, 44, 2074-2076. McLean, W. R.; Stanton, D. L.; Penketh, G. E. Analyst (London) 1973, 98, 432-442. Skogerboe, R. K.; Coleman, G. N. Anal. Chem. 1976, 48, 611A622A. Beenakker, C. I. M. Spectrochim. Acta, Part 8 1976, 318,483-486.
(22) Beenakker, C. I. M. Spectrochlm. Acts, Part 8 1977, 328, 173-187. (23) VanDalen, J. P. J.; de Lezenne Coulander, P. A.; de Galan, L. Anal. Chlm. Acta 1977, 94, 1-19.
RECEIVED for review September 24,1980. Accepted December 29,1980. This paper was presented in part a t the 1980 EXPOCHEM, Houston, TX. This work was supported by the Kansas Agricultural Experiment Station, project no. 143.
CORRESPONDENCE Improvement of Algorithm for Peak Detection in Automatic Gas Chromatography-Mass Spectrometry Data Processing Sir: There is currently a great deal of general interest in the automatic processing of complex environmental samples by use of gas chromatography/mass spectrometry/computer methods. The aspect of this process which is most important to success is the first stage, in which individual components present in the mixture are identified and then subjected to eluent deconvolution and noise reduction. A widely used algorithm available for this purpose is due to Dromey et al. (1). A program which uses the Dromey algorithm (CLEANUP) has been implemented in our laboratory and subjected to extensive testing. Introduction of CLEANUP into the data processing scheme has resulted in very much improved unknown identification. A recurring problem observed with CLEANUP has been its failure to identify components clearly visible as singlets in a total ion current (TIC) plot of the data. In addition, the system has ignored numerous weak peaks which can readily be identified and correctly interpreted by human analysts. This note identifies the source of this problem and presents an alternative to eliminate it. The approach used by CLEANUP is to examine the profiles for each individual ion in a time window surrounding the suspected eluent. One profile is then chosen which most closely represents an ideal singlet eluent signal (model peak). The intensities of all the other ions in the spectrum are then determined according to the similarity of their profile with that of the model peak. The crucial point is the way in which the model peak is selected. The profile chosen must contain from five to seven points. The subsequent description will be for the seven-point case. Other cases are treated similarly. The peak to be chosen as a model can be represented schematically for one mass as in Figure 1. The intensity of the center scan must be at a relative maximum, with each scan intensity progressively lower than the previous one proceeding outward from the center scan. The original CLEANUP program chose the model peak as the one with the numerically greatest “sharpness” function ( S )which is defined, for this case, by the expression
J-3
r‘
The “sharpness” or rate function is calculated for each nonsaturated unimodal profile in the window, and the profile with the highest value of S is selected as the model. Then the sharpness of every other peak in the spectrum is examined, and the intensities of those peaks whose sharpness is a fixed precent of that of the model are summed. Unknown components are then identified on the basis of whether this sum exceeds a fixed threshold. Working with this algorithm with real data, we observed that components in mixtures of known compounds were frequently missed for the following reason: The measured intensities of weak peaks in a mass spectrum are subject to larger relative variation than strong peaks because they are affected to a greater extent by random system noise, of whatever origin. As a result it occurred quite frequently that the calculated sharpness integral (2) for a weak peak was abnormally high, and by comparison, all the other ions in the spectrum having normal sharpness were deemed unimportant by the algorithm, and discarded. As a result, the component was missed. The calculated sharpness function should, in principle, be independent of peak intensity, and in an ideal system expression 1would fulfill this condition. However, because of the reason just stated, weak ions frequently are given far greater weight in the algorithm than their presence implies, and the system is unbalanced in their favor. To correct this bias, we chose a different function to define a peak’s sharpness, namely 3
s = t=l c [(Yt-l- Yt) + W-(t-l)- YdI
(3)
This expression is a numerical approximation of the integral
s=
r+3
J
-3
Idyl
(4)
When eq 3 is evaluated it reduces to the simple expression = 2Y, - Y3 - Y-3 (5)
s
where t is the relative scan number shown in Figure 1. This expression is in fact a numerical approximation of the logarithmic integral given by 0003-2700/81/0353-0536801 .OO/O
This criterion reduces, therefore, to the selection as the sharpest peak that peak with the greatest intensity difference between the maximum and the points on either side near its base. This avoids the risk of creating abnormally high 0 1981 American Chemical Society
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Anal. Chem. 1981, 53, 539-540
-3
-2
-1
0
1
2
3
R E L A T I V E SCAN NUMBER Ill
Although the function described may not be optimal, since frequently only a very limited amount of raw data are available to the system, it is preferable to employ a rather simple estimate of peak sharpness rather than more elaborate procedures suitable for more well-defined profiles. After this change was implemented, the number of detected eluents increased by as much as 3070,and none of the maxima which could be seen manually using a TIC plot were rejected by the system. The suggested new sharpness criterion is thus not only more effective in selecting eluents but it is conceptually easier to understand and computationally simpler to implement.
LITERATURE CITED
Flgure 1. Schematic representation of model peak.
sharpness values for weak peaks associated with the previous criterion. It may be argued that this criterion, which emphasizes the importance of absolute peak intensities, could give erroneous results when applied to very strong peaks or to poorly resolved doublets. This turns out not to be a problem, since such peaks generally are excluded by other criteria applied prior to the sharpness calculations. Thus, all saturated peaks and peaks which do not reduce their intensity unimodally from the center are excluded. Closely spaced doublets are treated subsequently in the CLEANUP procedure and would presumably be detected later in the processing. The new algorithm effectively weighs each peak in proportion to its intensity. This bias is precisely that which is recognized by practicing mass spectrometrists as the practical way to approach data analysis. Regardless of theoretical considerations, strong peaks in a spectrum contain more information than weak ones.
(1) Dromey, R. G.; Steflk, Mark J.; Rindfleisch, T. C.; Duffield, A. M. Anal. Chem. 1076, 48, 1368.
'
Address correspondence to: OCA Corp., Burlington Divlslon, Bedford, MA 01730.
William F. Hargrove David Rosenthal*' Chemistry and Life Sciences Group Research Triangle Institute Research Triangle Park, North Carolina 27709
Phillip C. Cooley Energy and Environmental Research Division Research Triangle Institute Research Triangle Park, North Carolina 27709
RECEIVED for review June 5, 1980. Accepted November
24,
1980.
Double-Beam Laser-Induced Photoacoustic Spectrometer Sic Photoacoustic spectroscopy (PAS) has been the subject of much interest as a technique especially suited to the measurement of absorption spectra of solids in contact with gases (1,2).Moreover, recent innovations in this technique have involved the use of piezoelectric transducer (PZT) in place of a microphone as a detector in the measurement of absorption spectra of liquids and solids. Particularly in the area of ultratrace analysis of liquids, a laser-induced PZT-PAS technique has been demonstrated by the authors to be very useful (3-5). However, as the concentration of a solute decreases, a big background photoacoustic (PA) signal, caused by the absorption of solvent itself or impurities in the solvent, cannot be ignored. Therefore, it is necessary to adopt a double-beam system. The authors have already applied a double-beam photoacoustic spectrometer to the measurement of PA spectra of some rare earth ions in liquid (6). As the laser beam was divided into two with a beam splitter in this system, the sensitivity decreased less than half in principle. In the present paper, a new double-beam system is proposed to eliminate a background PA signal due to a solvent. The block diagram of the system is shown in Figure 1. The amplitude of a laser beam was modulated at a given frequency with an acoustooptic light modulator (Intra-Action Corp., Bensenville, IL, Model AOM-40). The phase difference between first- and zeroth-order diffracted laser beam is exactly 180'. The first-order beam was incident upon a sample cell and the other one, upon a reference cell. Therefore, the phase
difference between PA signals, generated in sample and reference cells, must also be 180O. When the PA signals are added with a T connector and measured with a lock-in amplifier, the measured PA signal amplitude coincides with the difference of PA signals arising from a sample and reference cells. The principle of operation is shown schematically in Figure 2. An ac component of a PA signal, measured with a lock-in amplifier, should correspond to a net PA signal from a solute in a solvent, whereas a dc component, which is not measured with a lock-in amplifier, is ascribed to a PA signal from a solvent. The PZT acts simultaneously as a component of the cell and also as a pressure sensor. The middle part of the cell was of a cylindrical PZT (length 15 mm, inner diameter 15 mm), as was described in ow previous papelg (3-6). A fully polished platinum foil (0.1 mm thickness) was attached inside the PZT in order to prevent it from being in direct contact with the liquid. This modification effectively decreased the absorption of Ftayleigh and Mie scattering light by the PZT. Moreover, the new PZT cell had an increased detection sensitivity several times larger in comparison with the one immersed directly in liquid, with the corresponding noise level decreased enormously. Since the inner surface of a PZT transducer is usually uneven, small bubbles, generated a t the early stage of the experiment possibly resulting from sample solution injected and trapped there, made the PA signal lower and caused an intolerable noise during the measurement. The cells
0003-2700/81/0353-0539$01.00/0 0 1981 American Chemical Society
