808
Anal. Chem. 1083, 55,808-809
1
4 -F
Flgure 1. Modified centrlfugal filtration apparatus for direct filtration into an autosampler vial: (A) receiving tube and cap: (B) 0-rlng, filter membrane, and filter support disk: (C) glass microfunnel; (D) filter support compartment: (E) autosampler vial cap; (F) microvial Insert assembly. the chromatographic system. However, an extra sample transfer step is required when samples are injected with an autosampler. This extra transfer step (from receiver tube to autosampler microvial) results in loss of sample and increased processing time.
An approach to minimizing this loss in sample volume is to filter the sample directly into the autosampler microvial (Waters Associates, No. 73030). Figure 1 illustrates the necessary modifications required to allow direct filtration from a centrifugal microfilter into an autosampler microvial. The filter support compartment is first press-fitted into the cap of a sample vial (Waters Associates). The microfunnel which is fitted into the filtration device (BioanalyticalSystems Inc., Model MF-1) is a borosilicate ground glass joint (Lab Glass Co. No. LG0015-inner member) that has been cut and ground to fit into the filter support compartment. The device is then assembled as illustrated in Figure 1. Samples are placed in the sample compartment and the sample is centrifuged at high speed. After replacement of the filter assembly with an autosampler cap, the sample can be directly injected into the HPLC. The modified filtration device is easily disassembled for cleaning. We have used the above modified centrifugal filter for several months and found it to be helpful in methods development and bioavailability studies where final filtration of the sample is required. The modified centrifugal filter was designed to be used with one type of autosampler vial (Waters Associates No. 73030). The modified filter assembly can be adapted to other types of vials by using a 13 mm X 8.0 mm i.d. Nalgene tube (Scientific Products, No. R5365-90) to connect the filter support compartment directly to the autosampler vial.
LITERATURE CITED (1) Rosten, D. A.; Kissinger, P. T. Anal. Cbem. 1981, 53, 1695-1699.
RECEIVEDfor review October 12,1982. Accepted December 17, 1982.
Purification of Nitric Acid at Trace Metal Levels Richard P. Maas" and Steven A. Dressing Department of Environmental Sciences and Engineering, School of Public Health, Universi?y of North Carollna, Chapel HIII, North Carollna 27514
Concentrated nitric acid has been used increasingly in recent years in the field of trace metal analysis for digestion or preservation of both organic and inorganic environmental samples. The presence of significant levels of trace metals in both commercial reagent grade and expensive ultrapurity nitric acid creates problems such as large reagent blanks in digested samples. This paper describes a rapid, inexpensive method for purifying commercial reagent grade nitric acid. Several investigators have reported methods for purifying nitric acid and other acids, but' unfortunately all of these have proved to be very time-consumingor expensive. Coppola and Hughes (1) first used a polyethylene subboiling still for preparing pure hydrofluoric acid. Mattinson (2) refined the procedure by constructing an apparatus which consisted of two Teflon bottles threaded to a Teflon block. The apparatus is kept nearly airtight, and heat is applied to the feed bottle. The collecting bottle is immersed in cold water and acid gradually condenses in the collection bottle. High-purity nitric, hydrochloric,and hydrofluoric acids were obtained from *To whom correspondence should be addressed at: Department
of Biological and Agricultural Engineering, National Water Quality Evaluation Project, 1300 St. Mary's St., Raleigh, NC 27605.
0003-2700/83/0355-0808$01.50/0
Table I. Metal Concentrations in 70% Nitric Acid in g of Metal/g of Acid) Parts per Billion method boiling glass still (this paper) starting material (reagent ACS grade nitric acid) two-bottle subboilingTeflon still ( 2 ) subboiling auartz still in cleanair chamber (3)
Zn
Cd
0.70 0.01 10.0
1.60
Cr 0.10 106.7
Pb 0.20 7.2
0.049 0.04 0.01
0.05
0.01
reagent grade acids, although the maximum rate with roomtemperature cooling water is about 100 mL/day. The highest purity acids have been obtained by Kuehner et al. (3). They used a pure quartz subboiling still with a Teflon collector bottle. The distillation was performed in a clean air chamber with commercial reagent grade acids.
DESCRIPTION AND OPERATION The still consisted of a 500-mL triple-necked Pyrex distilling flask which was connected to a glass condenser (Figure 1). The condenser was connected to a linear polyethylene collection bottle 0 1983 American Chemical Society
Anal. Chem. 1083, 55, 809-810
A
G l a s g Condenser
\ Feed Flask
\
&lass Beads
Heating Coil
LPE
Collection Bottle
Figure 1. Dlagram of boiling glass dlstlllation apparatus.
by polyethylene tubing which was inserted through a hole in the top of the collection bottle. The connection was left unsealed to prevent pressure buildup in the apparatus. Following distillation the collection bottle was sealed and used for storage of the distilled acid. Reagent grade nitric acid (metal content shown in Table I) was added to the distilling flask, and a heating coil was used to boil the acid. A slow, steady boiling rate was attained by using several small glass beads and by carefully adjusting the current to the heating coil. Prior to use the apparatus was thoroughly cleaned and soaked in concentrated nitric acid. The still was given a final cleaning by operating the apparatus continuously for 1 week. Metal concentrations of the distilled acid were monitored daily during this time and, although initially quite high, reached a constant low level after 5 days.
809
RESULTS AND DISCUSSION The metal concentrations reached a constant level after about 1500 mL of nitric acid had been distilled through the apparatus. The distilled nitric acid and the reagent grade nitric acid used as starting material were analyzed for zinc, cadmium, chromium, and lead by heated graphite furnace AAS. The results show that highly pure acid can be prepared by this simple and rapid method (Table I). Silicate was determined in the distilled and reagent grade acid to be 3.98 and 4.12 mg/L, respectively, indicating that glass matrix materials such as silicate are not significantlyremoved during distillation. The still can produce up to 800 mL/day although we found that a slower boiling which produced about 500 mL/day gave acid of slightly higher purity. Also it should be noted that the metal concentrations reported here are from the collection bottle. Use of polyethylene or Teflon collecting bottles might be preferable to the linear polyethylene (LPE) collecting bottle used here since the production of LPE uses catalysts containing metals. Since the collecting bottle was not airtight, the laboratory air may also be a source of at least some of the metals remaining in the distilled acid, especially lead, in which case a clean air chamber such as that used by Kuehner et al. might further improve results. Registry No. Nitric acid, 7697-37-2. LITERATURE CITED (1) Coppola, P. P.; Hughes, R. C. Anal. Chem. 1952, 2 4 , 768. (2) Mattinson, J. M. Anal. Chem. 1972, 4 4 , 1715-1716. (3) Kuehner, E. C.; Alverez, R.; Paulsen, P. J.; Murphy, T. J. Anal. Chem. 1972, 4 4 , 2050.
RECEXVEDfor review May 12,1980. Resubmitted and accepted January 14,1983.
Room-Temperature Gas Chromatography/Mass Spectrometry of Volatile Solids and of Volatiles Trapped in Solids John R. Sutter," James W. Wheeler, Arnold S. Collier, Willlam M. Jackson, and Thomas Rush Department of Chemistry, Howard University, Washington, D.C. 20059
Recently we required a knowledge of the compounds in coal that were volatile at room temperature. This requires an analysis that does not involve extraction of the sample or other technique that would enhance the yield, such as heating the sample. The following should be applicable to any analysis of this sort. The electrodes of self-sealing Parr oxygen bomb (Parr 1108 oxygen combustion bomb) were removed and a one-piece sample of fresh coal weighing 25-50 g was introduced with two 20-g brass weights. The bomb was sealed, flushed with helium (or nitrogen), and finally pressurized to slightly less than 1atm gauge. The bomb and contents were then shaken vigorously by hand in order to pulverize the sample, the brass weights acting as pestles, thus releasing whatever volatiles were trapped in the coal. At the end of the experiment, the bomb was opened and the coal sieved. It was found that most of the coal would pass through a 120 mesh sieve (A.S.T.M. E-11 specification), ensuring maximum yield of volatiles. With a short length of rubber tubing fitted with a hypodermic needle, the entire volatile contents of the bomb in the helium stream were led through the septum of a Finnigan 3200 GC/MS onto the room-temperature GC column, the vacuum diverter being
held open. By use of a 1.6-m column packed with 10% SP-1000 or A-71 Polypak I and a heating rate of 10 OC/min from 40 to 200 "C, several GC peaks were observed. Through the use of comparison spectra ( I ) , the mass spectral assignments were made. Two of the compounds found were butane and pentane. In order to show that this technique is applicable to any mixture of solids containing volatile components, synthetic mixtures of compounds of various room-temperature vapor pressures were analyzed in the above fashion. Naphthalene and anthracene were chosen to represent a reasonable range of vapor pressures. These solids were placed in the bomb in approximately equal amounts and analyzed as above. The results were as expected, we found what we put in. The only limitation seems to be whether a compound has sufficient vapor pressure so that enough sample will be generated in the 340-mL bomb volume. If one uses the Parr bomb, although this is not required, the bomb can be heated to moderately high temperatures, thus extending the method. This method, then, offers a way of simplifying the analysis of complex solid mixtures by analyzing the volatiles first and
0003-2700/83/0355-0809$01.50/00 1983 American Chemical Soclety
