1145
Anal, Chem. 1981, 53, 1145-1146
Table I. DeterminaiiGof Toluene and MEK in Wax and Lubricating Oil Stocks toluene amt amt wt % in added, determined, % wt % in sample distillate ppm PPm error distillate lubricating oil
slack wax
1.37 0.84 1.9 1.35 0.28 1.05 0.9 0.7 0.29
150 105 196 139 40 150 120 105 38
137 94 185 128 37 138 109 97 35
Figure 1. Schematic dlragram of distillation assembly: (1) to condenser, (2) thermometer pocket, (3) cannon packed column, (4) to buret.
in Figure 1. The contmts were slowly heated and homogenized. The unit was run undler total reflux for 1 h before the collection of the distillate at about 63-90 "C. About 7-11 mL of the sample was collected, weighed, and analyzed by GLC. The quantities of toluene and MEK were calculated in the sample.
RESULTS AND DISCUSSION The synthetic samples of toluene, MEK, and wax or lube oil were distilled with methanol in the presence of cyclohexanone. The quantities of toluene and MEK added and
1.3 0.8 1.2 0.98 0.21 0.97 0.67 0.8 0.18
8.6 10.4 5.6 7.9 7.0 8.0 9.1 7.6 7.9
MEK amt amt added, determined, ppm PPm
137 99 126 99 31 140 90 125 29
129 92 118 93 29 127 82 111 26
%
error
5.8 7.0 6.3 6.0 6.0 9.2 8.9 11.2 10.3
determined by the method are presented in Table I. Although the concentration of MEK and toluene in the synthetic samples (Table I) was varied from about 30 to 200 ppm, there is no limitation in determining reasonably lower or higher concentrations of these solvents in the samples. The technique of azeotropic distillation is reported (3) to recover as low as 0.01-1.0ppm of volatile polar organic solvents like acrolein acrylonitrile and alcohols from water in a microdistillation apparatus. The data given in Table I show that the technique used is quite accurate as the errors are less than about 11% by weight. This error is quite low particularly when such small concentrations are involved. There is no other method available for this purpose in the knowledge of the authors except the GLC method ( I ) described in the beginning. This method is quite cumbersome and needs a freeze-out trap at -50 "C and reports errors up to 13.3% by weight. Determination of these solvents in wax or lube oil is important particularly to control the parameters in a solvent recovery process for complete recovery of the solvents and to get products of required specifications. The solvents presesnt in concentrations as low as 0.1% affect the properties ( 4 ) of these products, for example, flash point. The method is being used in the laboratory for determining solvents in waxes and lubricating oils obtained in a solvent dewaxing step. Other chemicals can also be tried which will dissolve azeotroping components like methanol and wax or lube oil.
LITERATURE CITED (1) (2) (3) (4)
Durrett, L. R. Anal. Chem. 1959, 37, 1824-1825. Horsly, L. H., Ed. Advan. Chem. Ser. 1952, No. 6 , Table I. Peters, T. L. Anal. Chem. 1980, 52, 211-213. Kalichevsky, V. A.; Kobe, K. A. "Petroleum Refining with Chemlcals", 1st ed.; Elsevier: London, 1956; Chapter 7.
RECEIVED for review June 12, 1980. Accepted February 6, 1981.
Multisampling of Microgram Quantities for Infrared Spectrometric Analysis P. J. Zanzucchi" and W. R. Frenchu RCA Laboratories, Princeton,
New Jersey 08540
Small amounts of contaminants, typically in microgram quantities, when found on the surface of electronic devices need to be identified to determine the origin of the unwanted material (I). Devices for supporting small quantities of sample for infrared analysis are available, e.g., the Perkin-Elmer microsampling accesriories (Perkin-Elmer Corp., Nonvalk, CT, Catalog No. 0186-03lO),but they often require use of separate supports for each sample. Preparing one sample a t a time or preparing samples on a number of separate salt disks, as 0003-2700/81/0353-1145$01.25/0
is done with commercially available devices, is a slow process. Moreover, since the sampling procedure is spread over a considerable time period, the possibility of contaminating the sample and sample transfer equipment is increased. We have found that a single device which will support a number of samples of microgram quantity is an aid to infrared analysis. For example, all sample transfer can be done in the shortest time period on the same sample holder reducing the possibility of error in the identity of the samples. 0 1981 American Chemlcal Society
114B
ANALYTICAL CHEMISTRY, VOL. 53. NO. 7, JUNE 1981
I
-
-
I
Flpun 2. Scale h w h g of the dsvlca la mUsm@e Mrared a n a w of solvent-soluble materlal.
Fbxa 1. Ratcgaphofthe assemMed devia, famumSample Mrared analysis of soIVent-sOIublB materlal.
The construction of a device for supporting up to 12 microsamples is described here. After more than l year of use, the sample holder has proved to be flexible and convenient to use. When used with a Fourier transform infrared spectrometer, the spectra of microgram amounts of solvent soluble organic and inorganic materials can be obtained (2).
EXPERIMENTAL SECTION Devica Construction. The assembled device shown in the photograph, Figure 1,consists of a thin silver chloride disk held in a rotatable Teflon holder attached to a modified base of a Perkin-Elmer microsampling device (Catalog No. 137-1585). The side and front view of the components of the device are shown in the scale drawing, Figure 2. These components are (A) a modified Perkin-Elmer microcondensor cell plate (Catalog No. 137-1585), (B) a brasa outer ring (nonrotntable), (C) a rotatable Teflon ring insert to (B),and (D)a Teflon piece which fits onto the rotatable Teflon ring (C). When asembled, Figure 1,(C) and (D)hold a 25 mm o.d., 1 mm thick AgCl disk (Harshaw, Solon, OH; Catalog No. 02501), and the two Teflon pieces rotate around the brass ring. The position of the sample with respect to the aperture in the plate is determined hy the position of the rotatable Teflon piece relative to marks, at 30" intervals, on the brass ring. By alignment of one of the Allen-head screws holding the Teflon assembly together with the marks on the brass ring, the position of the sample is known. The particular screw is selected to have a different color or shape from the other two. Necessarily, the AgCl plate is always rotated in one direction, Le., clockwise. The direction of rotation and history of each sample placed on the device is always listed on a form which is prepared each time the device is used. In addition to fabricating the brass and Teflon rings, the standard Perkin-Elmer microcondensor plate (Catalog No. 13715851, (A) in Figure 2,is modified by pressing an aluminum disk into the plate, i.e., the piece shaded in (A), Figure 2. A hole, nominally '/lain. 0.d. drilled into this disk is the aperture for the device. The aperture dimension is chosen to be sufficiently large to provide a reasonable signal-to-noise ratio but small enough to ensure that only light which has passed through the sample is transferred. Overall. for the cell fabrication,the dimensions given in Figure 2 are not critid and can he varied. The use of Teflon. or a similar material, is important hecause AgCl will react with mmmon metals such as aluminum or stainless steel. Sampling Procedure. The sample is dissolved in organic solvent and transferred to the device with a IO-rL syringe (e.g., Hamilton Co., Reno, NV, Catalog No. 701-RN). This is done by
placing small drops of solvent containing the sample onto the AgCl plate in an area over the device aperture. As successive drops of the solvent evaporate, the nonvolatile sample is left on the AgCl plate. This must be done slowly to avoid spreading the sample on the AgCl plate beyond the I d region defined hy the aperture; see Figure 1. Tbree samples of organic material, s p a 4 30" apart obtained by this method are shown in the photograph, Figure 1. These samples were obtained by dissolving a sample in 10 p L of methylene chloride. As can be expected, this procedure works best when a very volatile solvent, e.g.. methylene chloride, acetone, or heptane, is used. The rate of solvent evaporation can he increased hy directing a jet of dry nitrogen gas toward the sample deposition region. For the less volatile solvents heat may be applied to speedup the evaporation by using either an infrared lamp or a hot plate. VisibleW light should be filtered out, eg., by using a silicon wafer as a filter, to avoid excessive darkening of the AgCl disk. As the sample is deposited on the AgCl disk the syringe needle should not scratch the disk. Because of its relatively high refractive index, an excessive number of scratches reduces light transmittance.
RESULTS AND DISCUSSION The sampling method is, at best, semiquantitative. The samples shown in Figure 1 are not always uniform in thichess which limits the use of the device for quantitative analysis. As noted, solvents with a low volatility or a large heat of vaporization, such as water, are difficult to use because they do not evaporate quickly. Ideally, all componenta of a sample contacted by the solvent of choice should be equally soluble or the solvent will preferentially extract select components. This preferential solubility can lead to errors in determining the relative composition of small samples. Nevertheless, by the use of appropriate solvents, this problem can be minimized. Although the need to dissolve material is a limitation on the use of the device, the method allows for a great deal of flexibility in sampling. For example, this procedure is particularly useful for sampling small amounts of material ordinarily intractable for infrared analysis such as material, typically solder flux, a t the connections to a transistor.
ACKNOWLEDGMENT The assistance of John ODroniec in fabricating the parts for the sampling device is greatly appreciated.
LITERATURE CITED (1) Szsdon. J. R.: Steknak. J. P. EEE Tram. ELstr. Imd. 1970. €I-5. 3-10. (2) Couroyer. R.: Shsara, J. C.; Andaaon. D. M. AMI. Own. 1077. 49. 2275-2277.
RECEIVED for review January 19,1981. Accepted March 20, 1981.
