Table I. Values Obtained by Cyclic and Manual Processes LF X
a
values
Sample No
No proof
Average X c y c l i c
I appar.'
11 appar. b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
4 6 7 5 4 4 5 5 7 5 4 3 5 6 5 6 5 3
1697 1737 1834 1894 1961 2024 2102 2175 2270 2302 2399 2475 2589 2682 2822 2883 2975 3308
1690 1740 1830 1895 1960 2030 21 00 2175 2265 2300 2395 2475 2595 2685 2825 2885 2980 3305
1702 1737 1835 1897 1964 2021. 2100 2178 2270 2306 2400 2478 2590 2679 2816 2881 2975 3311
Std dev
Variation coefficient
12,5 13,5 9,6 9,2 13,l 6,3 11,6 11,2 8,2 6,8 5,4 7,6 15,5 8,7 9,1 12,3 12,l 23,O
0,737 0,777 0,523 0,486 0,668 0,311 0,552 0,515 0,361 0,295 0,225 0,307 0,599 0,324 0,322 0,427 0,407 0.695
Halosis conductometric bridge. Jones L & N conductometric bridge.
instrumental method is suitable for checking the constancy of the composition of flowing liquids in order to control the salinity of nutrition tanks of culture and checking of pollution (PCP). This system of monitors informs simultaneously at a distance about the conductivity variations occurring in a liquid system, amplitude and duration of these variations, and the return to the standard conditions. In continuous daily controls, the recorded information in a cyclic analysis is undoubtedly to be preferred to the subjective interpretation of the operator.
CONCLUSIONS The described apparatus, adaptable to more channels, is suitable for remote monitoring of the ionic pollution in water (PCP). A good reproducibility and agreement with manual methods are obtained but, with respect to the latter ones, the time and personnel required is less. The broadcast frequency used by us for transmission is legal in our country although it is in the middle of the F M broadcast band. Anyway, the broadcast frequency may be selected according to the frequency allowed. We hope, by means of this method, to make a useful contribution to automatic research on signalizing the pollution for the continuous transmission of the conductivity measurements. The method presented to measure the conductivity of
water is suitable to illustrate one of the many applications of the H F technique in the branch of electrochemical automatic analytical chemistry.
ACKNOWLEDGMENT We thank C. D'Arrigo for his helpful collaboration.
LITERATURE CITED (1) K. Cruse and R. Huber, "Hochfrequenztitration", Monographien, Angewandte Chemie No 69, Verlag Chemie, Weinheim, 1957. (2) E. Pungor, "Oscillometry and Conductometry", Pergamon Press, Elmsford, NY, 1965. (3) D. Dobos. "Electronic Electrochemical Measurements". Terra Ed., Budapest, 1966. (4) A. Bellomo and G. D'Amore, Affi Soc. Peloritana Sci. Fis. Mat. Mat., 5 , 119 (1959). (5) A. De Robertis, A. Casale, D. De Marco. and A. Bellomo, Ani Soc. Peloritana Sci. Fis. Mat. Mat., 18, 65 (1972). (6)A. De Robertis, A. Casale, D. De Marco. and A. Bellomo. Affi Soc. Peloritana Sci. Fis. Mat. Mat., 18, 81 (1972). (7) F. Oehme, J. Necfroanal. Chern., 1, 181 (1959). (8) C. N. Reilley and N. H. McCurdy, Jr., Anal. Chem., 25, 86 (1953). (9) G. B. Blake, Analyst(London),75, 689 (1950). (10) J. Millman and C. Halkias. "Electronic Devices and Circuits", McGrawHill Book Co., Tokyo, 1967. (11) B. Ridge, Electronic Circuit Design Handbook, €€€Magazine, 1970.
Received for review April 17, 1973. Accepted February 19, 1974. The present paper was presented a t the 4th Congresso della SocietA Italiana di Biologia Marina, Lipari, 18-20 May 1972. Supported by the Consiglio Nazionale delle Ricerche-Roma.
Freeze-Dry Method for Coating Capillary Columns Ian T. Harrison Institute of Organic Chemistry, Syntex Research, Stanford lndusfrial Park, Palo Alto, CA 94304
Capillary columns for gas chromatography are usually coated by the dynamic method ( 1 ) in which the stationary phase is deposited, rather irreproducibly, from a slug of solution passing slowly through the column. We find t h a t
more even coatings of the desired thickness can be produced by filling the column with a solution of the calculated amount of stationary phase in benzene, freezing the solution, and evaporating the solvent in vacuo. Uniform coatANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
*
1211
ings are obtained from non-polar stationary phases without the use of wetting agents. A related method based on evaporation of liquid solvent has been described ( 2 ) previously, but requires carefully controlled conditions.
EXPERIMENTAL A 30-ft stainless steel honeycomb column ( 3 ) (0.02-in. i.d.1 was filled with a benzene solution of methyl silicone (OV-101, Applied Science Labs., 6 mg/ml) using a syringe connected by a short piece of silicone tubing Phz-in. i.d. X 5h2-in. 0.d.). T h e column was immersed in a dry ice-acetone bath for 10 min and then transferred t o a n ice bath. Both ends of the column were connected t o an oil p u m p using short lengths of silicone tubing. Completion of t h e evaporation (6 hr) was checked by disconnecting t h e column and applying slight pressure t o one end of t h e column with a syringe. A bubbler connected t o t h e other end gives an indication of t h e passage of gas. Pumping was continued for a further 12 hr during which time t h e ice bath was allowed t o slowly warm u p to ambient temperature. T h e resulting coated column had a n H E T P of 0.52 m m (estimated with hexadecane, partition ratio 2 5 ) . Larger values were obtained by t h e dynamic method.
RESULTS AND DISCUSSION The new method was less effective with very viscous stationary phases. A 200-ft polyamide coated column (0.03-in. i.d.; Poly-A 103, Applied Science Labs) required 6 days for removal of solvent and had an HETP of 1.20 m m . Carbowax solutions could not be satisfactorily freeze dried in capillary tubing. The method could presumably be modified for the case of these higher melting stationary phases by evaporation of higher melting solvents a t elevated temperatures from capillary columns enclosed in a vacuum oven.
LITERATURE CITED (1) A. D. Littlewood, “Gas Chromatography”, Academic Press, New York,
73).
RECEIVEDfor review December 11, 1974. Accepted January 30, 1975. Contribution No. 447 from the Institute of Organic Chemistry.
Hygroscopic Properties of Potassium Bromide in Infrared Spectrophotometry Laszlo Borka National Centre for Medicinal Products Control, Sven Oftedals vei 8, Oslo 9, Norway
In the process of preparing monographs for the “Pharmacopoeia Nordica”, the author is in the position to be able to compare the infrared (IR) spectra of drug samples from the same batch when examined in three different pharmacopoeia laboratories. We have found that presumptively identical IR spectra may show marked differences in the 3400 cm-I region, yielding the well known, large or small OH (“water”)-bands, when potassium bromide is used for making IR discs. The samples being identical, these differences were usually attributed to different climatic conditions, varying relative or absolute humidity of the laboratory air, or poor drying of the potassium bromide. We have now studied the hygroscopicity of ground potassium bromide and found that the water uptake can be directly related to the method and duration of grinding. There are, or course, other ways to avoid disturbing OH bands, e.g., by running a mull (Nujol or Fluorolube) or by using an evacuable bakeable die. However, several pharmacopoeias or other official prescriptions recommend the KBr pelleting for several substances. Our aim was to optimize the KBr spectrum in the 3400 cm-I region in case this region is to be interpreted when the KBr pelleting is prescribed.
EXPERIMENTAL Apparatus. A Beckman IR 10 infrared spectrophotometer was used. For t h e grinding of KBr powder a Grindex GR 200 ball mill and a Grindex M K I1 vibration mill were used. Pressing of the KBr discs was carried out in a 13-mm vacuum die with a 30-ton hydraulic press, all these items being supplied by Research and Industrial Instruments Co. Evacuation of the die was by means of a Speedivac 25C20A rotary vacuum pump. Chemicals. Potassium bromide, UVASOL grade from Merck was used in these experiments. 1212
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
Procedure. 1) T h e initial water content of the KBr powder was determined by drying the commercial KBr a t 120 “C for 3 hr and measuring the weight loss. 2) T h e 250-mg samples of KBr were ground for periods u p t o 300 sec with the different mills and the reduction of particle size was measured with the aid of a microscope micrometer scale with 10-pm division units. From t h e ground samples, discs were pressed under vacuum. (-0.1 mmHg) with 650 MNm-’ compaction pressure, followed by the recording of their IR spectra. 3)The hygroscopic properties of ground KBr powder were studied by keeping the powder samples in hygrostats for 24 hr a t room temperature and measuring t h e weight increase due to water adsorption. 4) T h e spectra of different drug samples were recorded with KBr discs prepared under different conditions.
RESULTS AND DISCUSSION 1) The initial water content of the commercial KBr found by heating and measuring the weight loss was only 0.08% w/w. Re-exposure to laboratory air (-50% relative humidity) for 24 hr gave a weight increase below 0.08%, demonstrating the very low hygroscopicity of potassium bromide of this brand. Correspondingly, in the later experiments, there was no difference between the tests carried out with dried or with untreated KBr. 2) Data on the reduction of particle size of KBr during grinding are summarized in Table I. 3) Data on the hygroscopicity of ground KBr, expressed by weight increase in the hygrostats, are included in Table 11. One may note the absence of hygroscopicity of KBr at “normal” humidity ranges. Only above 90% relative humidity has the powder adsorbed water when kept in hygrostats. This is surprising since practical IR analysts are bothered by the presence of unwanted “water” bands working a t much lower humidity as Figure 1 shows.
