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.
wavelength microns
35
4 55
2.5
6 65 7
',ooj_
u A B
-C
80
1
1 4000
wavelength microns 3.0 35 4.0
0 3500
3000 2500 180Ot 1600 wavenumber cm-
4000
1400
3500
3600
2500
wavenum ber cm-'
Figure 1. IR spectra of potassium bromide discs with increasing OHbands.
Figure 2. IR spectra of the local anesthetic drug lidocaine in the 4000-2500 cm-' region
( A ) Untreated KBr. ( B ) Hand-ground or 5-min grinding with MK II. (C) Ground 10 sec with GR 200. ( D )Ground 60 sec with GR 200
(A) Hand-ground. (8) Ground 35 sec with GR 200: KBr pellets. The arrow marks the water peak present in spectrum 8
Table I. Comparison of Grinding Effects of Two Vibration Mills
Table 11. Hygroscopicity of Ground Potassium Bromide ',L'eig!it increase in :c/w c i a t
Model of m i l l
RIIC MK I1
RIIC G R 200
room t e m p e r a t u r e , 24 hr
Duration of
Duration of
Mean par-
Mean par-
grinding ,with
a t 20 to 85:;
grinding, seta
t i c l e s i z e , urn
t i c l e size, urn
RIIC G R 200 m i l l ,
rei. !>midit?
50-80 50-70 45-65 3 5-5 5 35-50 30-50 2 5-5 0 20-40 15-3 0 Agate capsule, 2 agate balls, 250 mg KBr. 0 10 20 30 40 50 60 120 3 00
50-80 20-30 10-15 8-1 2 5-1 0 3-8 5 2 1
T h e KBr discs for Figure 1 were manufactured partly from milled powders, partly from hand-ground powder a t 50% relative humidity in the laboratory air. This Figure demonstrates the increasing water uptake as the milling time increased. The heavy agitation of the mill seems to favor water adsorption which fails to take place in the hygrostats a t similar humidity contents. Pressing the powder into discs gives only a slight, if any, protection, although one might expect a significantly higher hygroscopicity for a loosely packed powder. 4) Organic compounds and drug substances of varying hardness were ground with KBr and the above findings were tested for validity. Representative examples of these experiments are the spectra in the 4000-2500 cm-I region of the local anesthetic drug lidocaine in KBr pellet pressed after handmilling and after grinding with the Grindex GR 200, see Figure 2. Several local anesthetics such as lidocaine, tetracaine, and benzocaine have slight differences in the NH, NHz and CH region and a "dry" KBr spectrum is a great advantage for interpreting. Also, the dryness of the substance can be simultaneously estimated from spectrum A (hand-ground). From spectrum B (ball-milled KBr) on the contrary, we cannot judge whether the water-peak has its origin in the humidity of the sample or of the KBr.
a t 93': r e l . h u m i d i t y
SeC
powder
Powder
Disc
0 10 20 30 40 50 60 120 300
0 0 0 0 0 0 0 0 0
8.3 8.8 8.6 8.3 7.9 8.0 7.5 7.1 7.0
6.2 6.8 7.5 7.6 7.4 7.4 7.4 7.0 7.0
From the tables and the Figures, we may conclude that: 1) the Grindex GR 200 mill which is capable of milling hard geological samples, seems to be far too effective for grinding potassium bromide for the preparation of IR discs. Reduction of the milling time is of course possible, but usually to the detriment of the mixing effect. 2) According to our experience with different samples of organic substances of varying hardness, grinding with the MK I1 mill for 1 min is insufficient but 5 rnin yields good results, both as to the grinding and the mixing effect without resulting in annoying water uptakes. 3) Hand grinding, with the simultaneous mixing of the organic substance with KBr, when carried out in an agate mortar, seems to be better than or equivalent to the method described under 2) and is more satisfactory than the use of a too effective ball mill. 4) Excessive drying of the KBr powder and careful protection from airhumidity seems to be unnecessary, since water uptake will take place during grinding and not during storage.
ACKNOWLEDGMENT T h e author is grateful to Mrs. Lisbeth Mentveit for her skillful technical assistance. RECEIVEDfor review December 26, 1974. Accepted February 18,1975. ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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