Table 11. Analysis of n-Paraffins Phosphor Silica gel technique, vol GC, wt 1.08 1.25 0.29 0.298 0.28 0.311 0.09 0.12 0.047 0,052 0,020 0,025 0,015 0.018
+
0,011
0.015
0.004
0.005 0.006 0.005
0.003
0,002
Table 111. Analysis of n-Paraffins Phosphor technique, Carbon range VOl 75 FIA, vol cs - c 1 0 0.65 0.62 c 1 o c 1 4 0.52 0.54 cll-Cl3 0.16 0.14 C11-C13 0.10 0.12 ~~
synthetic blend of aromatics. Accordingly, an equal volume blend of toluene and a-methylnaphthalene in isooctane was chosen for the calibration standard. The band lengths obtained using six 1-ml charges of 0.1 vol % of the calibration mixture were found to be 22.5, 22.0, 23.5, 21.0, 22.5, and 22.0 mm. The average value is 22.3 mm with a standard deviation of 0.82. With this information, a calibration curve was constructed using different percentages of the standard solution.
Eleven normal paraffin samples were analyzed in accordance with this procedure and the results compared with those obtained using the silica gel-gas chromatographic method. These data are shown in Table 11. Because the aromatics have greater densities than the paraffins, the volume per cent results are less than weight per cent results. Taking this into consideration, the results obtained with the faster phosphor technique agree favorably with the results from the more timeconsuming silica gel-gas chromatography method. Four samples were also analyzed in accordance with the modified FIA true bore method ( I ) in which the results were reported in volume per cent. There was good agreement with the volume per cent results obtained with the phosphor method as shown in Table 111. Conjugated diolefins were found to interfere in the aromatic determination by similarly quenching the phosphor’s fluorescence and, thus, they are calculated as aromatics. Monoolefins can be tolerated to the extent of ten times the aromatic content; if present in amounts larger than this, they cause the aromatic values to be high. If nonaromatic sulfur and oxygen-containing compounds are present in the paraffins in amounts larger than the aromatics, similarly high aromatic values result. Duplicate results, for the phosphor method, should not be considered suspect unless they differ from each other by more than 10% of the mean or 0.002 vol %, whichever is larger. The limit of detectability is 0.002 vol with the calibration mixture. This method may be of value to the petroleum industry for the determination of trace aromatics in high purity paraffins; it is both precise and rapid.
RECEIVED for review May 23, 1967. Accepted July 18, 1967.
Ratio Recording of Mass Spectra in Combined Gas Chromatography-Mass Spectrometry Bruce H. Kennett Commonwealth Scientijic and Industrial Research Organization, Dicision of Food Presercation, P. 0. Box 43! Ryde, N.S. W., Australia
THE MASS SPECTRUM of a compound recorded while it is emerging from a gas chromatographic column will be distorted if a change in concentration occurs during the scanning period. To minimize this distortion, it is common practice to scan rapidly close to the top of the gas chromatographic peak. This is difficult with narrow peaks such as those obtained from high resolution columns, and generally requires the use of the fastest scan rates of which mass spectrometers are now capable (0.1-1 second/mass octave). When such fast scan rates are employed, two factors adversely affect the quality of the recorded spectrum. First, the statistical fluctuations associated with small ion currents can cause serious errors in the recorded magnitude of these currents, the spectrum becoming less accurate and less reproducible as the scan rate is increased [BrunnCe, Jenckel, and Kronenberger ( I ) ] . Second, a n increase in the scan rate must be accompanied by a corresponding increase in the bandwidth of the detecting and recording system to avoid reduction in both resolution and magnitude of the recorded spectrum; this increase in bandwidth has the undesirable effect of decreasing the signal-to-noise ratio.
Thus the best spectrum is obtained by using the slowest permissible scan rate consistent with keeping within the defines of the gas chromatographic peak, and adjusting the bandwidth accordingly so as to obtain the optimum signal-tonoise ratio. BrunnCe, Jenckel, and Kronenberger ( 2 ) have further shown that spectral distortion caused by concentration changes in the ion source during scanning can be avoided if the individual ion currents are recorded as ratios of the total ion current rather than as their absolute values. To obtain the desired ratios they used a measuring bridge-an expensive accessory normally needed only when the mass spectrometer is utilized for the determination of isotope ratios. Because other feasible systems utilizing analog computer techniques are also expensive, the simple system described below was developed. In this system (Figure 1) the ratios are obtained by attenuating the photomultiplier output with a Potentiometer mechanically coupled to the recorder monitoring the total ion current, this current being derived from a double ion source (2). The potentiometer (10 KQ, 10-turn) is geared to the main slidewire shaft of this recorder so that the zero resistance
(1) C. BrunnCe, L. Jenckel, and K. Kronenberger, Z . Aizal. Chem., 189, 50 (1962).
(2) Zbid.,197, 42 (1963).
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ANALYTICAL CHEMISTRY
RECORDER DEFLECTION
---+-
10%
r--1 - ~ - TSPECTRVM M AIIS S , I
, , L
-1
RECORDER
I
L-,,J
Figure 1. Ratio recording circuit
Figure 2. Diagram for current amplifier using Philbrick P45A Amplifier Photomultiplier amplifier. (b) Philbrick P45A amplifier. (c) Microswitch. (d)Ratio-recording potentiometer (a)
( a ) Photomultiplier amplifier. ( b ) Microswitch actuated by total-ion recorder. (c) Potentiometer wiper mechanically coupled to total-ion
recorder Ri = R2/9 RP = Effective resistance of potentiometer at 100% recorder deflection ARz = Effective resistance to potentiometer wiper Ei, = Input voltage to potentiometer E,,, = Output voltage from potentiometer position of the potentiometer corresponds t o a recorder deflection of 10 % of full scale, and a t 100 % recorder deflection the effective potentiometer resistance (R2) is in the range of 9-10 Kn. Mechanical modification of the potentiometer enables the wiper to travel below the electrical zero to allow f o r recorder deflections below 10 %. The resistor Ri (Figure 1) is selected t o be precisely 1/9 of the resistance of the potentiometer a t a recorder deflection of 100 %. A microswitch actuated by a cam attached to the recorder slidewire shaft disconnects the input signal to the potentiometer when the recorder is below the 10% position. From Figure 1:
+
Because R 1 is constant and (R, AR,) is proportional to recorder deflection, which in turn is proportional t o total ion current, and Ei,(the photomultiplier amplifier output voltage) is proportional to the individual ion current, then: E o u t CY
Individual ion current Total ion current
(2)
Hence, the recorded spectra will be independent of the molecular concentration in the ion source provided the deflection of the total ion current recorder is between 10% and 100% of full scale. The spectra so produced will be identical with those obtained, had the total ion current remained constant a t a level equivalent to a recorder deflection of 10 %. Because the limited output current of the photomultiplier amplifier of our Atlas CH4 mass spectrometer precluded direct connection to the ratio-recording potentiometer, a current amplifier was incorporated in the output of the photomultiplier amplifier. Figure 2 shows the circuit requirements for a Philbrick Model P45A amplifier (Philbrick Researches Inc., Boston, U.S.A.) employed as this current amplifier. The above ratio-recording system and a magnetic scan rate of 6 seconds/mass octave gave results far superior t o those obtained a t the fastest available magnetic scan rate of 0.36 second/mass octave. A further improvement in spectral quality was effected by installing after the ratio-recording potentiometer a noise filter (frequency attenuator) that allowed the bandwidth of the recording system t o be adjusted t o yield the optimum signalto-noise ratio. The filter (Figure 3) has a cut-off rate of 12 db/octave and a dc gain of 0.5, and the 10-H inductor used
Figure 3. Circuit of noise filter Switch position 1 2 3
4 5 6 L1=5H L2 = 2.5 H L3 = 1.25 H L4 = 1.25 H
Circuit function Filter bypassed, d.c. gain = 1 Filter bypassed, d.c. gain = 0.5 Cut-off frequency, 250 Hz Cut-off frequency, 120 Hz Cut-off frequency, 60 Hz Cut-off frequency, 30 Hz Nominal Nominal Nominal Nominal
C1 C2 C3 C4
= = = =
6.8 p F 3.3 pF 1.6 pF 0.8 p F
comprised a "Vinkor" No. LA2004 (Mullard Ltd., London, U.K.) wound with 2460 turns of 36 S.W.G. wire. Tappings were made a t 870, 1230, 1740, and 2460 turns t o correspond with the inductances required for cut-off frequencies of 250, 120, 60, and 30 Hz, respectively. When ratio recording a t a scan rate of 6 secimass octave, the signal-to-noise ratio was improved by a factor of 10 after incorporating the filter set to a cut-off frequency of 30 Hz. This setting was selected because, under the operating conditions prevailing when the mass spectrometer is directly connected to the gas chromatograph, the shape of the recorded mass ion peaks approximates to a half sine wave of 15 Hz. The improvement in signal-to-noise ratio permits acceptable spectra t o be obtained a t higher photomultipler gains, and this allows the use of narrower slits, thereby increasing resolution. To compensate for the signal amplitude reduction caused by the filter, a second Philbrick P45A amplifier was installed between the filter and the mass-spectrum recorder. Ratio recording of spectra as described above has been employed in this laboratory over the past 18 months in food flavor investigations. I n addition to the general high quality of the spectra obtained, ratio recording has enabled directly comparable spectra t o be recorded for any part of a gas chromatographic peak. This has proved of great value in dealing with overlapping peaks and in quickly matching unidentified spectra against reference spectra obtained by means of the combined gas chromatograph-mass spectrometer. Ratio recording of spectra allows the scanning time t o extend over most of the period during which a compound is emerging from the gas chromatographic column. AccordingVOL. 39, NO. 12, OCTOBER 1967
* 1507
ly, mass spectrometers with minimum scan rates of 20-30 seconds/mass octave, previously considered too slow for the purpose, can now be coupled directly to a gas chromatograph -provided, however, that the peak duration permits a sufficient mass range t o be scanned. In addition to the advantages obtained from its application to a directly coupled gas chromatographmass spectrometer, the ratio-recording system can be used to advantage in other circumstances where fast scan rates are not available or desirable and where relatively rapid changes of concentration in the ion source are encountered. Such concentration changes occur when compounds are fed into the ion source
from small reservoirs, or when they are evaporated into the ion source from a direct insertion probe. Ratio recording also has potential application in correcting spectra obtained when scanning gas chromatographic effluents with infrared or ultraviolet spectrophotometers. ACKNOWLEDGMENT
The author is indebted to E. J. Bourn for technical assistance RECEIVED for review April 10, 1967. Accepted June 12, 1967
A Simple Gelation Procedure for Liquid Scintillation Counting James N. Bollinger, William A. Mallow, James W. Register, Jr., and Donald E. Johnson Southwest Research Institute, San Antonio, Texas 78206 THIXOTROPIC GELS are commonly used to suspend insoluble materials for gel-scintillation analysis of 8-ray emitting isotopes (I, 2). The gels prepared from Cab-0-Si1 (Cabot, Inc., Boston, Mass.), although relatively efficient, have not always been entirely satisfactory in our hands for the simple reason that they are difficult to prepare and awkward to handle. Preliminary experiments in this laboratory have shown that a gel formed from a urea-type inclusion complex of a toluene cocktail serves as a n efficient system for the determination of &emissions. Employing very small concentrations of toluene diisocyanate and a branched aliphatic primary amine, the preparation of the toluene cocktail gel is simple and exceedingly rapid, Counting efficiencies are similar to those prepared with Cab-0-Sil. EXPERIMENTAL
Reagents. The scintillation cocktail used in this study contained 5 grams of premix “P” ( 9 8 Z PPO and 2 x POPOP, Packard Instrument Co., Inc.) per liter of toluene. Armeen L-11 (Armour Industrial Chemical Co.) was used as the source of the branched aliphatic primary amine. The molecular weight determinations of several samples of this amine yielded variable results (185 to 195 grams/mole). Consequently, a n average molecular weight of 190 grams/ mole was used to prepare the various molar concentrations used in this investigation. When toluene diisocyanate is added to a toluene cocktail containing the branched aliphatic amines, a gel forms as the result of the addition reaction and the urea-type inclusion of toluene. The toluene diisocyanate (TDI) used in this study was Hylene TM-65 (Dupont). Hylene TM-65 is a mixture toluene 2,4-diisocyanate and 35 Z of approximately 65 toluene 2,6-diisocyanate. Individual Sample Gelation. A toluene cocktail can be gelled by the direct addition of 3.3 volumes of the Armeen L-11 and one volume of the toluene diisocyanate. The volume values were derived from density and molecular weight measurements of the reactants and the reaction requirement of 2 moles of the amine per 1 mole of TDI. Molecular weights of 190 and 174 and densities of 0.8 and 1.22 for the Armeen L-11 and TDI, respectively, were used in these (1) I). G. Nathan, J. D. Davidson, J. G. Waggoner, and N. J Berlin,J. Lob. Clin. Med., 52,915 (1958). (2) H. J. Cluley, Amdysr, 87, 170 (1962).
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ANALYTICAL CHEMISTRY
Figure 1.
Gelled liquid scintillation counting solutions
1. Ungelled toluene cocktail 2. 5 % Cab-0-Si1 toluene coektail 3. TDI-amine gelled toluene cocktail
TDI-amine gelled fire brick
4.
toluene
cocktail with 40-64 mesh suspended
calculations. Experimentally, 0.17 to 0.2 ml of the Armeen L-11 is added to a glass counting vial containing 10 ml of the toluene cocktail. After mixing, 0.05 ml of T D I is added and the solution swirled again until the gel forms. The time required for gelation can be varied from a few seconds to a few minutes and the viscosity or rigidity of the gel varied by proper selection of amine and TDI concentrations. For example, a 10-ml toluene cocktail will set to a rigid gel within approximately 30 seconds with the addition of 0.20 ml of Armeen L-11 and 0.05 ml TDI. Since excess TDI will lower the viscosity and also quench, it is advisable to add a slight excess of the amine. Gelation of Multiple Samples. The second method that may be employed in the formation of gels is to prepare a toluene scintillation solution which contains 0.1 to 0.2M Armeen G11. Thereafter 10- to 20-ml toluene cocktails can be gelled as required by the additirn of 0.05 t o 0.075 ml of the toluene diisocyanate. Preparation of Gelled Suspensions. A suspension is prepared by adding particulate matter any time prior to the addition of the toluene diisocyanate. Once the toluene diisocyanate is added, sharp swirling of the solution will start the gelation and simultaneously suspend the particulate matter. Material with densities equal to glass beads are permanently and uniformly suspended.
