~~
Table 11. Measured and Calculated Retention Volumes (GEAC: rz-pentane-ether, eluent flowrate 65 ml/hour. Column length, 36 inches, weight W = 13.8 g) Alumina neutral Solute v, vo 1 (A 2 3 1 Naphthalene 13.2 14.7 11.4 20.7 18.5 Biphenyl 18.0 19.0 5.6 24.3 22.0 Anthracene 46.5 48.0 3.2 52.2 54.8 p-Terphenyl 49.5 54.0 9.1 59.0 59.2 Vo = measured retention volume; Vor = calculated retention volume using Simpson’s d e ; (A%),
=L
v
- Vo
vo
x
-10.6 -9.5 5.0 0.3
100
of the integral where a remarkable effect is faced with, for example, a subjective error encountered when reading off the data in the graph. F o r this reason retention volume for benzene was not calculated. On the other hand, for compounds with higher retention volumes, such as anthracene and p-terphenyl, the relative deviation is remarkably smaller. When integrating a n analytically expressed dependence of Eabus. V,, the fact should be taken into account that the gradient curve is only approximated by a mathematical function. Another reason for difference between measured V , values and those obtained by calculation may be in a worse precision of measuring the retention volume or of charging the stronger eluent. Those differences are markedly manifested in the region of higher elution volumes where a very high value of In Eaboccurs. Thus, the whole procedure shows requirements for a high precision a t all stages, for continuous control of the eluent flow-rate and for accurate calculations. The graphical integration based o n Simpson’s rule is relatively rapid. It is easy to check the possibility to approximate the course of the gradient by one or more curves. Whenever this is practicable, the whole calculation can be facilitated and made essentially more rapid.
SYMBOLS USED
solvents A and B, where B is the stronger :olvent adsorbed solute effective area (units 8.5 A*) lOaao.bda, see Equation 6 adsorbed solvent B effective molecular area mole fraction of solvent B in a binary solvent mixture solute linear isotherm equivalent retention volume (mlig) value of R”for elution with pentane solute adsorption energy, pentane solvent adsorbent surface volume (ml/g) retention volume for elution with mixture of solvents A , B retention volume for elution pentane measured retention volume for gradient elution calculated retention volume for gradient elution total weight of the adsorbent in the bed (g) adsorbent activity function solvent strength parameter values of e o for solvents A , B, A-B RECEIVED for review December 6, 1971. Accepted May 22, 1972.
A Thermal Analysis-Mass Spectrometric Technique for lderitifying Trace Impurities in Gas Samples Rudolf Schubert Bell Telephone Laboratories, Incorporated, Columbus, Ohio 43213
OFTENIT IS of interest to determine the trace amount and chemical composition of various unknown impurities in a gas sample. Many techniques with varying sensitivity exist for a large range of compounds ( I ) . Recently, utilization of the physical properties of the gases themselves (2) and improvements in gas chromatography (3) have extended detection limits for certain impurities. Gas chromatographs (GC) and mass spectrometers (MS) singularly or in combination detect and identify the broadest range of compounds. (1) “Air Pollution,” A. C. Stern, Ed., Academic Press, New York, N.Y.. 1968. Vol. I. 11. and 111. (2) L. B. Kreuzer, J.’App/. Phys.. 42, 2934 (1971). (3) R. K. Stevens and A. E. O’Keeffe, ANAL.CHEM.,42(2), 143A (1970). 2084
When the composition of the pollutants is unknown or one is interested in a variety of impurities, the usage of GC and/or MS presents problems. GC analysis of trace amounts of pollutants requires different traps, columns, packings, and detectors for different classes of impurities. Interpretation of MS analysis is difficult because of overlapping mass spectra. Possible reactions in a concentrated sample, or with traps and containers, can present additional complications. This paper describes a freeze-out (4)-thermal analysis-mass spectrometric technique which is particularly useful for de(4) M. Shepherd, S. M. Rock, R. Howard, and J. Stormes, ANAL. CHEM.,23, 1431 (1951).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1 9 7 2
tecting unknown pollutants. Freeze-out is performed at the time of analysis rather than at the collection point (5); this reduces the possibilities of chemical reactions during the time interval between collection and analysis of the sample. By recording both a n unconcentrated and concentrated sample mass spectrum, sensitivities o n the order of 10 ppb are achieved. Separation of 2C and higher hydrocarbons, as well as various halogenated compounds, by thermal analysis is shown in the example presented. Relative vapor pressure data aid identification of compounds. Complete identification of impurity quantities present is made by mass spectrometry. The separation of mass spectra by sample temperature eliminates difficulties of interpretation due to overlapping spectra.
I
A
I
EXPERIMENTAL Apparatus. A stainless steel (SS) vacuum system with Cu seals which has a 110 liter/sec ion pump, Ti sublimation pump, Bayard-Alpert ionization gauge, and various ports comprised the pumping station. A Varian leak valve (LV) was attached to a standard 11/2-inch tee opposite a General Electric Monopole 300 MS having a dynamic range >5 X l o 5 (ix.,the ratio of the mass signal to the background signal) and a mass range of 2-300 amu. The electron multiplier output of the MS was amplified by an EA1 Model ESA 75 logarithmic amplifier and recorded on a Brush Mark 280 recorder. Figure 1 schematically illustrates the above system and the all SS sample collection cylinder. These cylinders were connected to the LV with Cajon Cu sealed fittings through a Nupro all SS, bellows sealed valve. A 1/4-inch diameter SS tube extended into the cylinder for 90% of its length, was Heli-arced to the cylinder at the valve end, and the external end was sealable with a Cajon Blank fitting. This second opening allowed one to draw a n air sample through the cylinder, whose volume was 500 ml. The use of all 304 SS and OFHC Cu seals eliminated potential system contaminants from stopcock and seal lubricants. Further system cleanliness was maintained by using oil-free pumps. Procedure. Before sample collection, the cylinders were cleaned by being baked at 300 "C while at pressures
