1500
000
500 sm‘
Figure 1. Emission spectra of sodium oxalate (A) vibromill-ground powder, (B) pure salt prepared by our technique, (C)mixture with KBr prepared by our technique: (0) absorption spectrum
absorption spectra (compare for example the 525 and 1630 cm-’ bands in Figure 1) can be eliminated either by reducing the thickness of the emitting layer (not always feasible because for intense bands very thin layers are required in emission) or by diluting the compound with another one having no absorptions in the spectral range considered. Some experiments have shown that potassium bromide can be successfully employed as diluent (in the range above
400 cm-’), but any salt giving no chemical or spectral interferences can be used. Sample preparation is carried out, working as before, from solutions containing the two salts. The concentration ratio between the compound and potassium bromide in the solution determines the ratio between the components in the spectroscopic sample. In fact, we observed that the percentage of one component was higher in the solid sample than in the solution. However, these facts, which will be treated in a subsequent note, have little importance for qualitative purposes. The emission spectrum of a sample of mixed sodium oxalate and potassium bromide (3% of oxalate in the solution) obtained by the technique here described is reported in Figure 1C. Since wider slits had to be used for the recording of the emission spectra, the doublet found in absorption a t 1330-1340 cm-I was not observed in all spectra. Nevertheless, as can be seen, the intensity ratios among the bands in the emission spectra obtained on mixtures with KBr are closer to those recorded in absorption than those obtained on the pure salt. The intensity ratio among the 525 cm-’ band and the others (compare Figure l , B and C) is a clear example of this.
LITERATURE CITED (1) (2) (3) (4)
G. Fabbri and P. Baraldi, Appl. Spectrosc.,28,593 (1972). G. Fabbri and P. Baraidi, Atti SOC.Nat. Mat. Modem, 103, 255 (1972). G. Duyckaerts. Analyst(LondonJ, 84, 201 (1959). J. M. Hunt, M. P. Wisherd. and L. C. Bonham, Anal. Chem., 22, 1478 (1950).
RECEIVEDfor review July 28,1975. Accepted December 15, 1975.
Determination of Mercury in Fish Mariiyn R. Hendzel” and Duncan M. Jamieson’ Department of the Environment, Fisheries and Marine Service, Inspection Branch, 50 1 University Crescent, Winnipeg, Manitoba, Canada
“Cold vapor” flameless atomic absorption spectrophotometry is widely used for the routine analysis of mercury in biological materials (1-6). The majority of these methods employ a wet digestion followed by reduction of the oxidized mercury which is then partitioned with air and determined spectrophotometrically. The digestions are rather complicated, requiring careful attention through a number of manipulations (1-3), thereby reducing the efficiency of these methods. The method described here is very simple and permits one person to analyze 50 samples daily. The samples are oxidized with a mixture of concentrated sulfuric and nitric acids a t a temperature of 180 O C , volumed with distilled water, and analyzed according to a previously published semiautomated method (1).Output is increased through the use of graduated digestion tubes for the sample container, an aluminum “hot block” for the digestion, and a pneumatic sampler for the analysis.
EXPERIMENTAL Special Apparatus. Aluminum Hot Block. This block was custom made according to specifications by Pritchard Engineering Co. Ltd., Winnipeg, for an approximate cost of $250.00 ( S e e Figure 1). Pneumatic Sampler. A standard sampler (Carlo Erba SD3) was modified to permit sampling directly from sample containers. A four-way valve operated a t 10 psi (lab compressed air line) is connected electrically to the sampler and actuates a pneumatic cylinFalcon Motor Hotel, Falcon Lake, Manitoba, Canada. 926
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
der according to the timer controls on the sampler. An automatic oiler is connected to the input of the four-way valve to ensure proper lubrication (See Figure 2). At the start of a sampling cycle, air passes through the hose connecte’d to the bottom of the pneumatic cylinder forcing the sample arm up. The sample wheel advances to bring a sample into position under the sampler arm. The air flow is now reversed by the four-way valve and enters the top of the cylinder forcing the piston down and the contained air out of the exhaust. The sample is then drawn up through the Teflon tube and on to the proportioning pump. The cycle repeats. Sample Wheels. These were fabricated from poly(viny1 chloride). Each wheel can hold up to 40 test tubes (See Figure 3). Both the sampler and the wheels were custom made a t an approximate cost of $600.00 by Micro Tool & Machine Ltd., Winnipeg. Reagents. Standard Mercury Solutions. 1 mg/ml: Dissolve 0.1354 g of HgClz in water, add 1 ml concentrated H2S04, and make to 100 ml with distilled water. 10 wg/ml: Dilute 1 ml of 1 mg/ml solution to 100 ml with 1 N HzS04. Digestion Acid Solution. Mix 1 part nitric acid (S.G. 1.42) with 4 parts sulfuric acid (S.G. 1.84). Reductant Solution T o approximately 2400 ml of distilled water, add 400 ml H2S04 (S.G. 1.84), 120 ml 20% (w/v) NaCl solution, 40 g ( N H Z O H ) ~ ” Z S Oand ~ , 80 g SnS04. Cool, make to 4000 ml with distilled water and mix. Procedure. A 0.1- to 0.5-g sample is weighed into a graduated digestion tube (Corning ‘2-7952) and 5 ml of the digestion acid is added, It is important that no sample is above the level of the acid. One or 2 ml (i.e,, > 20%) of fuming nitric may be added to aid digestion of high fat samples. Working standards (100, 200, 300, and 400 ng of mercury) and two reagent blanks are prepared a t this
140- I"@
Holes
PLAN
2-
I"d 1
ELEVATION
Figure 3. Sample wheel
E N D ELEVATION
Figure 1. Hot block dimensions
-----
AIR FLOW (upstroke)
.....-.- AIR FLOW
T
(downstroke)
AUTOMATIC OILER
\ A
/
1
,
\
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PUMP
41R
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ELECTRICAL CONNECTlOhi TO SAMPLER
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Figure 2. Pneumatic sampler
time and are taken through the entire procedure with the samples. Each test tube is placed in the "hot block" which in turn is placed on an oscillating hot plate (Fisher 11-492) and heated. Oscillation is continued for a few minutes after brown fumes appear, to break the layering which occurs in the sample-acid solution. T h e digestion proceeds a t 180 O C until white fumes appear and the samples are clear and almost colorless (approximately 4 h). The hot plate is cooled, the samples removed from the block, brought to a constant volume (25 ml) with distilled water, and mixed. An atomic absorption spectrophotometer (Le., Perkin-Elmer 403) is set up with a mercury lamp and flow-through cuvette in place of a burner ( I ) . The wavelength is adjusted for maximum energy, the cuvette is aligned for minimum absorption (suitable oper-
ating parameters: wavelength 253.7 n m ; slit, 3; response, 3; recorder full scale, 0.5 A). The analysis manifold is connected via a "debubbler" ( 1 ) and the pump is started. The zero is adjusted on the spectrophotometer and chart recorder with only the reductant passing through the manifold. A sample wheel is loaded with blanks, samples, and standards. The sampler is started. An aliquot of the sample (approximately 2 ml) is combined with a permanent flow of the reductant and air. Reductant and air volumes are chosen so as to achieve maximum sensitivity with complete reduction of the mercury. The system passes through a mixing coil and is split in the debubbler into two phases. The liquid is drained and the gaseous phase flows into the cuvette. As each sample wheel is completed, peak heights recorded on the chart are measured and calculations made.
RESULTS AND DISCUSSION Precision of Analysis. Precision of routine determinations was caiculated from 50 replicate determinations on 1 sample. The mean of the 50 trials was 0.478 ppm with a standard deviation of 0.025 ppm. Detection Limit. With recorder a t 10-mV full scale representing 0.5 absorbance, the detection limit is 0.01 ppm assuming a sample weight of 0.250 g. Accuracy. The method of analysis formerly used by our laboratory was that of Armstrong and Uthe ( I ) . Comparing 41 pairs of results obtained using both the above named method and the proposed one gave an average difference of 3.8 ppb which is not significant a t the 95% level of confidence.
Table I. Mercury Determinations on Interlaboratory Sample Exchange Sample No.
1
2 3 4 5 6 7 8 9 10 11 12 13 14
15
Other methods
This method,
ppm
Mean =k Std dev, ppm
Range, ppm
0.26 0.65 0.49 0.47 0.79 0.55 0.56 2.62 3.49 1.41 1.48 0.57 0.39 0.57 1.00
0.28 f 0.05 0.63 f 0.09 0.52 f 0.08 0.45 f 0.06 0.77 f 0.11 0.54 f 0.08 0.55 f 0.21 2.52 f 0.31 3.70 0.60 1.35 f 0.14 1.53 f 0.26 0.60 f 0.10 0.40 f 0.05 0.59 zk 0.06 0.93 f 0.11
0.19-0.41 0.46-0.81 0.33-0.67 0.30-0.55 0.51-0.98 0.34-0.74 0.40-1.29 1.87-2.84 2.68-4.53 1.15-1.76 1.15-2.24 0.49-0.95 0.34-0.50 0.51-0.74 0.73-1.18
*
~
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
~~~
927
A number of samples which were part of collaborative studies were also analyzed using this method. One of these programs is ongoing, instituted by the Inspection Branch four years ago. Approximately 20 laboratories from Canada and the United States using a variety of methods participate in this study. Results are shown in Table I and good agreement is evident. Sample Types. Although the method has been primarily used for the analysis of fish tissue, other types of samples such as fish meal, blood, hair, and sediments have been analyzed with no special manipulations. This method is particularly suitable for routine work where a simple, rapid method is required. I t is appealing to the small laboratory since it allows for the digestion of a large number of samples in a small working area (in this case 140 in an area 18 X 14 inches.). Very little attention is required during the digestion process, thereby freeing the analyst. Moreover, the digestion process is simple in comparison to others ( I , 2 ) . Chemicals such as potassium permanganate and hydrogen peroxide are no longer required, resulting in a savings of money and time. The use of digestion tubes permits voluming the sample directly in the sample container, thereby eliminating the possibility of loss or contamination from transfers, together with the
time required for such manipulations. In addition, the cost of a graduated digestion tube is considerably less than that of a volumetric flask. The modified sampler permits sampling directly from the sample container and eliminates the need to transfer the sample a t any point throughout the entire procedure. Thus, in addition to the benefits mentioned above, errors due to misnumbering a sample are reduced to a minimum.
ACKNOWLEDGMENT We thank J. Harding and B. Berger for technical assistance and F. A. J. Armstrong for his advice in the writing of this paper. LITERATURE CITED (1) F. A. J. Armstrong and F. J. Uthe, At. Absorp. News/., 10, 101 (1971). (2) M. P. Stainton. Anal. Chem., 43, 625 (1971). (3) R. K. Munns and D. C. Holland, J. Assoc. Off. Anal. Chem., 54, 202 (1971). (4) R. Tkachuk and F. D. Kuzina, J. Sci. Fd. Agric., 23, 1183 (1972). (5) F. D. Deitz, J. L. Sell, and D. Bristol, J. Assoc. OM.Anal. Chem.,56, 378 (1973). (6) F. M. Teeny, J. Agric. Food Chem., 23, 668 (1975).
RECEIVEDfor review September 11, 1975. Accepted December 23,1975.
Heart Cutting Technique in High Resolution Gas Chromatography Applied to Sulfur Compounds in Cigarette Smoke W. Bertsch" University of Alabama, University, Alabama 35486
F. Hsu and A. Zlatkis University of Houston, Houston, Texas 77004
Complete resolution of trace organic compounds in complex mixtures has been a challenge since the invention of gas chromatography. The current trend in high resolution gas chromatography goes towards refinement of open tubular column technology. In spite of the impressive resolving power of such columns, the technique, unfortunately, has not found its way into the average laboratory. In many instances, inadequate packed columns are still employed for separations which demand the use of high resolution columns. The application of such columns is still considered a specialty by many potential users. The reluctance to switch from packed columns to capillary columns has to be attributed partially to difficulties in their preparation. On the other hand, even good capillaries often give disappointing results unless the instrument is properly adapted to such columns. Options to Effect Ultimate Resolution. In cases where a detailed characterization of all compounds in a mixture is required, the use of capillary columns is clearly advantageous. As complexity increases, a point will be reached a t which even the best capillary column with an optimally chosen stationary phase will not be capable of completely resolving all components and overlap will occur. Products from hydrocracking operations in the petroleum industry and tobacco smoke are examples of samples with extreme complexity. The resolving power of a column is a function of phase selectivity, capacity ratio, and efficiency. Keeping the first two parameters within reasonable limits, resolution can be 928
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
improved most dramatically by an increase of the column efficiency. Total efficiency, in turn, can be improved by a decrease of column diameter or an increase in column length, the maximum tolerable pressure drop being the principal limitation. A t the same time, however, other problems are generated arising from a decrease of loading capacity and need for longer analysis times. In many cases, complete characterization of complex mixtures may not be necessary and only a few compounds within a complex mixture may be of interest. The use of heart cutting techniques and multidimensional chromatography can, in principle, effect the resolution of even the most complex mixtures. In-line valves have been used for this purpose (1-3), but the valveless system proposed by Deans ( 4 ) has significant advantages. Several semiautomated versions have been described (5, 6), but no commercial instrument is available. Heart cutting also has advantages if ultimate sensitivity is required for only a few selected compounds, i.e., detection of pesticides (7) or electron capturing derivatives of biologically active materials (8) by ECD. In heart cutting, a small fraction of partially resolved compounds is isolated and diverted into another column of different selectivity where further separation takes place and overlap with coeluting substances is minimized. In principle, the process can be repeated making use of different phase selectivities until complete resolution is obtained. Practical arrangements usually require intermediate trapping to counteract spreading effects during the first
