ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
and a 15-point Hamming window. Figure 3A shows the frequency response of this nonrecursive digital filter (clock rate = 33 kHz). While it is effective, it is time consuming-too time consuming for on-line use in the data processor of a gas chromatograph/mass spectrometer. Using a tight assembly language routine, integer arithmetic, and the extended arithmetic unit of our PDP-11/20, it took 61 s to process the whole n-tetracontane spectrum; a FORTRAN routine required a sevenfold larger time interval. Effect of Hardware Filtering with a Tapped Analog Delay Device, The heart of the filter is a Reticon TAD-32 tapped analog delay (Reticon Co., Sunnyvale, Calif.). This unit provides up to 32 successive delay taps, each separated from its nearest neighbor by a single sampling interval. The sampling frequency is controlled by a clock and can range from 1 kHz to 5 MHz. The dynamic range of the unit is 60 db, a range well within the requirements of ion multipliers used in mass spectrometry. T h e output of the various taps can be weighted individually by resistor networks prior to summing to yield the device output. The unit can therefore be made a direct hardware replica of the digital filter described above. The circuitry of the filter we use in shown in Figure 4. The portion that was provided by the manufacturer as part of board TC-32 (or its recent update, board TC-32A) is enclosed with dotted lines. From the board we removed the signal extraction network and all dc-blocking capacitors (as purchased, board TC-32 is designed to process ac signals) to be left with the TAD-32 chip itself and a unit which processes an externally provided clock signal such that it can control the sampling of the TAD-32. The remaining circuitry, devices D1 through D6, is required to: bias the input to the TAD-32 positive and remove spikes from the input (D1, see reference IO); remove that bias from the output (D4); generate an external clock signal (D2 and D3); and filter out a noise spike introduced into the output signal by the TAD-32 at the frequency of the controlling clock (D5 and D6). Only 31 of the 32 available channels are used so that a 31-point running weighted average can be produced. In the unit, the clock is run a t 40 kHz, i.e., a t a frequency slightly above that of the analog-to-digital converter of the data system (33 kHz). The weighting coefficients, and therefore the reciprocal of the output resistor size on each tapped line, were calculated by the same method as had been used for the digital filter. A cut-off frequency 2.0% of the 40-kHz sampling frequency and a 31-point Hamming window, were selected. The frequency transmission of the resulting filter is shown in Figure 3B and the effect on the same range of the n-tetracontane mass spectrum, albeit in a different run, appears in Figure 2B. The
1021
result is satisfactory both from the signal smoothness and from the fact that this extent of filtering was accomplished in real time.
CONCLUSION In the tapped analog delay device, there is made available to analytical chemists a device that can perform effective digital-quality smoothing in real time, Such is the versatility of these devices (they can be concatenated, for instance) that nonrecursive filters of just about any size can be designed, built, and made operational in a few hours. Low-pass, band-pass, high-pass, and signal matching needs can be met with equal ease. Specifically, for the mass spectrometrist needing to smooth the ion-multiplier output signal from a fast scanning mass spectrometer, the low-pass, nonrecursive filter described above is an extremely valuable addition to his armory of hardware. To some extent, we were fortunate in having to deal with an LKB 9000 where the scan mode of the magnetic field is close to linear throughout the mass/charge range. Obviously, our design can be applied directly to other linear or semilinear scanning mass spectrometers, i.e., all the quadrupole units. For magnetic sector spectrometers where the magnetic field scan mode is logarithmic, care would need to be taken to set the cut-off frequency high enough to permit quantitative passage of the high mass/charge end of the spectrum. Alternatively, a variable clock frequency could be used. ACKNOWLEDGMENT The authors acknowledge the technical assistance of Lauren Ernst and John Naworal. LITERATURE CITED (1) M. H. Ackroyd, "Digital Filter" in "Computers in Medicine", D. W. Hill, Ed., Butterworths, London, 1973. (2) A. Savitzky and M. J. E. Golay, Anal. Chem., 36, 1627 (1964). (3) C. G. Enke and T. A. Nieman. Anal. Chem., 48, 705A (1976). (4) D. Lancaster, "Active-Filter Cookbook", Howard W. Sams & Co.. Inc., Indianapolis, Ind., 1976. (5) T. Jupille, Am. Lab., 9 (IO). 109 (1977). (6) S. C. Tanaka, R. R. Buss, and G. P. Weckler, I€€€ Trans. Parts, wbrids, Packag., php-12, 118 (1976). (7) G. Horlick, Anal. Chem., 48, 783A (1976). (8) K. R. Betty and G. Horlick, Anal. Chem., 48, 2248 (1976). (9) I.M. Campbell, D. L. Doerfler, S. A. Donahey, R. Kadlec, E. L. McGandy, J. D. Naworal, C. P. Nulton, M. Venza-Raczka, and F. Wirnberly. Anal. Chem., 49, 1726 (1977). (10) B. Gilbert, Electronics, April 1, 1976, p 82.
RECEIVED for review December 14, 1977. Accepted February 27,1978. This work was supported by the U S . Public Health Service (RR-00273 and AI-11819).
Microwave Oven-Based Wet Digestion Technique Peter Barrett' and Leon J. Davidowski, Jr.' Lead Exposure Laboratory, Childrens Hospital Medical Center, Boston, Massachusetts 02 1 15
Kenneth W. Penaro Food and Drug Administration, Boston District Laboratory, Boston, Massachusetts
Thomas R. Copeland" Department of Chemistry, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02 1 75
The determination of trace metals in biological samples by most procedures first requires the destruction of the organic P r e s e n t address, D e p a r t m e n t U n i v e r s i t y , Boston, Mass.
of C h e m i s t r y , N o r t h e a s t e r n 0003-2700/78/0350-1021$01 .OO/O
matrix. Dry ashing is one method used to accomplish this ( I ) . More commonly used are methods of wet ashing which involve the oxidation of organic matter by heating with strong acids (2). These methods demand Periodic supervision for 10% times and also a special perchloric acid hood. 0 1978 American
Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
Table I. Comparison of Sample Preparation Methods (cold vapor atomic absorption analysis) Results, Av Results, Av ppmHg Method PPm Hg PPm Hg Product Methoda PPm Hg 1.6311.66 1.65 Swordfish (1) 1.4311.43 1.43 (2) 1.18/1.14 1.16 (1) Swordfish 0.9711.02 1.00 (2) 2.8212.78 2.80 Swordfish (1) 2.3512.46 2.40 (2) 1.9811.90 1.94 Swordfish (1) (2) 1.4511.46 1.46 0.8510.85 0.85 (1) 0.86l0.87 0.87 (2) Swordfish 1.3411.36 1.35 Swordfish (3) 1.2511.28 1.27 (2) 1.01/1.01 1.01 Swordfish (3) 0.9710.97 0.97 (2) 2.60/2.60 2.60 Swordfish (3) 2.5012.56 2.53 (2) 1.9211.92 1.92 Swordfish (3) 1.8711.93 1.90 (2) 0.9210.95 0.94 (2) 0.9310.87 0.90 (3) Swordfish 0.2310.23 0.23 (2) 0.2410.24 0.24 Tuna Fish (1) a (1)A.O.A.C. 12th ed., 25.CO1-25.CO2. ( 2 ) Microwave oven digest-determination-A.O.A.C. 1 2 t h ed., 25.105. ( 3 ) A.O.A.C. 12th Ed., 25.103-25.105. Koirtyohann et al. (3)have overcome some of the problems associated with t h e wet ashing methods by use of a commercially available microwave oven. This method employs an oven specially fitted with a Plexiglas box interior which is vented to a fume scrubber. We have adopted this procedure in our laboratories. Drawbacks encountered with this system involve t h e rate of acid fume removal from the cavity, subsequent treatment of the acid fumes, deterioration (warping and blistering) of the interior Plexiglas box and resultant leakage of acid fumes into the microwave oven’s interior. I n this paper we describe some modifications of the Koirtyohann method and present data from real samples to illustrate t h e practical utilization of this type of digestion system. EXPERIMENTAL Apparatus. A commercially available microwave oven rated at 600 W is used. Oven cavity capacity is 35 L. A 3-cm diameter hole was drilled in the side to vent the cavity and was checked for microwave radiation leakage with a Narda Microline Model No. 8200. The recorded levels were less than the government safety standards. The interior of the oven and electronics were sprayed with silicon spray. A Pyrex rectangular chromatography jar (14 X 16 x 27 cm) is used as an interior cavity to protect the oven. A Nalgene No. 6140 aspirator is used to remove fumes from the cavity and pull the vapors through a trap and KOH scrubber. Reagents. All acids used are reagent grade, doubly-distilled nitric, perchloric, and sulfuric acids, available from G. Frederick Smith Chemical Co. Procedure. The exact procedure followed depends on the nature and amount of sample to be digested. Described below are the methods used for human teeth at Children’s Hospital Medical Center of Boston and for swordfish at the Food and Drug Administration’s Boston laboratories. Teeth. A dried section of tooth weighing between 5 and 15 mg is placed into an Anodic Stripping Voltammetry (ASV) cell (Pyrex, 20 X 85 mm test tube) and 300 MLof an acid mix is added to it. The acid mix is 79.5% nitric, 20% perchloric, and 0.5% (v/v) sulfuric acids. The interior cavity has a capacity of up to 30 ASV cells. The samples must be taken t o dryness to ensure sufficient matrix elimination for ASV. The time for completion is dependent on oven load but is typically 10 min. Samples are then brought up to 5-mL volume with supporting electrolyte. The procedure for blanks is identical. Suordfish. A composite sample weighing between 5 and 10 g is added to a 125-mL Erlenmeyer flask and 20 mL of a 1:l nitric and sulfuric acid mixture is then added. To provide increased refluxing, a small funnel is placed in the flask. The sample is placed in the oven and digested for 3 min. It is then brought up to volume for analysis.
RESULTS A N D DISCUSSION At the Children’s Hospital Lead Exposure Study Laboratory, we originally employed a conventional digestion technique utilizing a hotplate. The hotplate was housed in
-~
-~
- -
--
i -
a
Figure 1. Multiple sample configuration (a) KOH bubbler, (b)trap, (c) Pyrex interior cavity
Figure 2. Single sample configuration a special perchloric acid hood and exposed the samples to the air for long periods of time. Atmospheric contamination and large sample load led us to incorporate t h e microwave oven method. A Plexiglas interior cavity and fume scrubber were constructed and fitted similar to those described (3). Several shortcomings became apparent shortly after t h e system was p u t into daily use. T h e first problem noticed was the long length of time required for fume removal from the oven cavity. Soon afterward the 1/2-in. thick Plexiglas box warped excessively, presumably caused by t h e combination of acid fumes and microwave heating. This allowed leaking of vapors into the microwave oven itself. Because of these inadequacies, we investigated other means of fume containment and elimination while still exploiting the rapid heating ability of the microwave oven. The properties of the interior cavity must include resistance to acid attack and be capable of withstanding rapid heating and cooling. The material we chose was Pyrex. Also the volume of the Plexiglas box was larger than needed, which hindered the removal of vapors. Therefore the cavity should have floor area large enough to accommodate normal sample loads with minimum dead space. T h e box now employed is a rectangular Pyrex chromatography jar with a ground glass Pyrex cover, with a total volume less than a third of that of the original Plexiglas box. The cavity is vented via a right angle 10/30ground glass joint (Figure 1). For evacuation of the smaller volume Pyrex box, a water aspirator was sufficient. This was coupled to a simple trap and KOH bubbler system which successfully removes corrosive fumes. This system can be constructed for under $50 as compared to over $200 for the originally employed method. At Northeastern University where the lower sample load requires less than daily use of digestion, we have designed a single sample digestion apparatus. This arrangement consists
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
of a 19/22 ground-glass 10- to 25-mL round bottom flask (size dependent on amount of sample) connected directly to an exit port which is coupled to an aspirator (Figure 2). Since employing t h e microwave digestion technique a t CHMC, a significant decrease in the blank level has been observed. With the conventional hotplate method, the average amount of P b in the blank was 9 ng. This level has been reduced to 5 ng using microwave digestion. This effect was shown not t o be due t o P b loss during digestion by sample recovery studies, but instead reflects the decrease in exposure time of the sample to ambient air. T o show that the microwave radiation has no other effect on the sample but to decrease the digestion time, we offer a comparison of sample preparation methods used at the Boston FDA laboratory in Table I. The sample preparation time for t h e microwave method is 3 min as compared to 30 min and 4 h for A.O.A.C. methods 25.C01 and 12.103, respectively. These data reveal no Hg loss during microwave digestion. T h e design introduced here solves the problems of acid corrosion and removal and is very inexpensive to implement.
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We are currently in contact with a commercial trace metal laboratory that is using a commercial scrubber unit for fume removal. This approach is much more efficient, but is roughly 20 times the cost of the presented method. The data in Table I show that the method decreases the time of practical analysis and minimizes airborne contamination while not affecting analysis results. The digestion proceeds far enough to be useful in ASV where the presence of large organics interfere.
LITERATURE CITED C. E. Gieit and W.D. Holland, Anal. Chem., 34, 1454 (1962). T. T. Gorsuch, "The Destruction of Organic Matter", Pergamon Press, New York, N.Y., 1970. A. Abu-Samra, J. S . Morris, and S . R. Koirtyohann, Anal. Chem. 47, 1475 (1975).
RECEIL-ED for review January 9,1978. Accepted February 21, 1978. Two of the authors, Peter Barrett and Leon J. Davidowski, Jr., wish to acknowledge financial assistance from the Department of Health, Education and Welfare, grant numbers HD08945 and HD06276.
Scatter Eliminated Spectrometry by Fourier Transform Infrared Spectrometry Tomas Hirschfeld Block Engineering, Cambridge, Massachusetts 02 139
In infrared spectrometry, it is often desirable to separate t h e scattered light signal from that due to the normally or collinearly transmitted light. This is desirable not only to eliminate the sloping baseline produced by scattering, but also to reduce the intensity errors in the bands themselves produced by the Christiansen effect ( I ) . This can be accomplished by mathematical combination of a pair of spectra provided form criteria are met: (1)A large, uniform beam in the sample compartment. (2) Very high spectral SNR.(3) A wavelength scale reproducible to 10-2-10-3 of the narrowest band encountered. (4) The spectra must be available in digital form in a computer. These criteria are met in FT-IR systems. Consider a sample a t a focal plane in between two aperture planes as shown in Figure 1. If symmetrical apertures giving the same vignetting are inserted a t both aperture planes, the collinearly transmitted beam will be attenuated only a t the first. However, the scattered beam, which changes direction within the sample, will suffer attenuation a t both apertures. This spatial aperturing a t the aperture plane, equivalent to angular aperturing a t the focal or sample plane, can then be used to obtain two measured values from which the scattered and collinearly transmitted light can be calculated ( 2 ) . In a not too strongly scattering sample, the transmission is given by:
transmission is again measured after aperturing (relative to the apertured empty beam), we have
T'
=
TATS
0003-2700/78/0350-1023$01 .OO/O
(2)
where V i s the extent of vignetting. From these two equations we can then calculate the intermediate variables a=
T - T' f ( 1 - V ) = (1 - T,)
(3)
and
b=
TI- VT
1-v
=
TST,
(4)
from which we can calculate
Error analysis of these equations shows the error in T.4 to be controlled by that of a for not too small values of Ts,since f < < 1.
AT,=where T Ais the transmission due to pure absorption, Ts that due to scattering alone, and f the fraction of the scattered light t h a t falls within the field of view of the optics. If the
+ (1- T s ) d T x f V
TAL,'dl + l i p 1 Ts3I2 f(1 - V ) SNR
where SNR is the signal noise ratio of'the 100% line in the absence of vignetting. Clearly, f should be maximized here, by using microsampling optics. These can be reasonably C 1978 American Chemlcal Society
