03.076
403 307
128 POINTS
b l L
I
ZERO FILLED TO 512
-
Figure 2. Fourier transform of the spectrum shown in Figure l b illustrating Fourier domain interpolation (zero filling)
L
Figure 1. Spectra of the Mn 403 nm triplet: ( a ) 100-Mrn slit width, ( b ) 15-pm slit width, and ( c ) 15-prn slit width after Fourier domain interpolation. Wavelengths are given in nm
of the zero filling (Le. interpolation), there are three times as many points. The peak maxima in Figure ICare now located at points 375, 386, and 404. Using the same method as before, there are now 0.0129 nm per point and the calculated wavelength of the central peak is 403.308 nm which is in excellent agreement with the listed value of 403.307 nm. No apodizing function was applied before zero filling and retransformation. This results in interpolation of the spectrum with the monochromator line shape function. If desired, apodization techniques could be used to control the nature of the interpolation function. This is necessary in some cases to avoid generation of excessive side lobes ( 5 ) . The spectra shown in Figure 1indicate that Fourier domain
interpolation is quite useful in optimizing the sampling capability of discrete array image sensors. The technique is general and, hence, applicable to any sampled spectral signal. As a final comment, interpolation does not mean that we can successfully violate the sampling criteria presented in Table I. Interpolation by its very nature means assumption of some type of line shape function with which to interpolate; either the instrumental line shape function or, via apodization, some imposed interpolation function. The criteria presented in Table I are based on the premise that there is no prior knowledge of the signal shape and, if it is desired to ascertain within a given accuracy that a peak signal is a particular shape, then these sampling criteria must still be followed.
LITERATURE CITED (1) (2) (3) (4) (5)
Yair Talmi, Anal. Chem., 47, 658A (1975). P. C. Kelly and G. Horlick, Anal. Chem., 45, 518 (1973). G. Horlick and E. G. Codding, Anal. Chem., 45, 1490 (1973) G. Horlick, Appl. Spectrosc., 30, 113 (1976). P. R . Griffiths, Appl. Spectrosc., 29, 11 (1975).
RECEIVEDfor review December 29,1975. Accepted April 30, 1976.
Method to Reduce Noise in Silver Nitrate-Benzyl Cyanide Columns Fred B. Wampler University of California, Los Alamos Scientific Laboratory, Los Alamos, N.M. 87545
A recent photochemical kinetic study in this laboratory required the determination by gas chromatography of minute quantities, 6 X mol, of cis-2-pentene in the presence of 1.2 X mol trans-2-pentene. The literature available for the separation of the isomeric 2-pentenes (1-3) suggested a column prepared by dissolving silver nitrate in some nonvolatile polar solvent. Ethylene glycol, propylene glycol, and benzyl cyanide are most commonly used. Columns prepared using benzyl cyanide as a solvent were found to be superior to the other suggested solvents. A 12-ft column, %-inch o.d., consisting of 10% by weight of silver nitrate on 60/80 mesh Firebrick, operated at 22 O C and at a He flow rate of 30 cm3/min, was found to give a very good separation of 30 min between the two isomers when large quantities, mol, of each isomer were present. However, even when matching reference and sensing columns were used in the Perkin-Elmer Model 810 FID gas chromatograph, a noise level from column bleed hindered the operation of the gas chromatograph at the sensitivity level necessary to detect 6 X mol of the isomers. The signal from the eluting solvent, benzyl cyanide, caused the recorder to go off scale at the high 1644
ANALYTICAL CHEMISTRY, VOL. 48,
NO. 11,
sensitivity levels required. Conditioning of AgN03 columns at elevated temperatures cannot be done very well since solutions of AgN03 decompose above 50 T.Also, i t was desirable to have the benzyl cyanide present on the column support since columns prepared from it were far superior to those using propylene or ethylene glycol in resolving the two isomers. It was found that by inserting a 12-ft length of a 8%DC-550 silicone oil column between the detector and the AgN03 column that the benzyl cyanide was completely retarded in the silicone oil column, but that the separation achieved for the isomers in the AgN03 column was not diminished since the order of elution, trans first, was the same for both columns. However, the use of this length of silicone oil column by itself could not resolve the isomers. Analyses performed with and without the silicone oil column indicated that the resolution of isomers was the same in both cases but that, for high sensitivity runs, the silicone DC-550 column was necessary to retard the benzyl cyanide from entering the detector. Under the operating conditions, the benzyl cyanide took about 4 weeks to start eluting from the silicone oil and a t this time a replacement silicone oil column was inserted. This technique
SEPTEMBER 1976
should have general applicability in those cases where it is desired to have a certain amount of a high boiling solvent residing on the column and still be able t o operate at high sensitivity levels where, under ordinary conditions, the solvent excessive noise inbleed would enterthe detector and terference. In particular, this method should allow isomeric separations on AgN03 columns containing benzyl cyanide to have very good resolutions at high sensitivities since enough benzyl cyanide can be added to aid in the separation and yet not interfere with the signal reaching the detector unit.
LITERATURE CITED F, Armitage, J, chfomatogf,, *, 655 (1959), (2) B. W. Bradford, D. Harvey, and D. E. Chalkley, J. Inst. Pet., London, 41,80 (1955).
(3) M. E.
Bednas and D. s. Russell, Can. J. Chem., 3% 1272 (1958).
RECEIVEDfor review January 15, 1976. Accepted May 10, 1976. This work was supported by the Energy Research and Development Administration.
Instrument Calibration for the Mass Chromatograph Robert J. Lloyd, David E. Henderson, and Peter C. Uden” Department of Chemistry, University of Massachusetts, Amherst, Mass. 0 1002
The mass chromatograph (I-3), a dual gas density detector gas chromatograph designed for on-the-fly molecular weight determination, is based on the fundamental concepts of numerous workers (4-9). Samples are split into two approximately equal fractions after volatilization in the injection port, are trapped on identical traps, and then simultaneously backflushed onto two matched columns, two different carrier gases being employed. Theoretical discussions (3,I O ) indicate that the value of the molecular weight (M,) of a n unknown compound is given by Equation 1
where A1/A2 is the ratio of the compound’s response in de~ McG:!are the molecular weights of tectors l and 2, M C Gand the carrier gases and K is an instrumental calibration factor. K must be determined initially from known compounds using Equation 2.
There are three distinct regions of the calibration curve of the K factor vs. molecular weight of the standard compound ( M W s t ) ;Le., where M W s t < M W C G I ,M W C G < ~ MWst < M W C G ~and , M W C G
