ARTICLES A Continuous Analysis Technique for Trace Species in I c e Cores Andreas Slgg,'*t Katrln Fuhrer,? Martin Ankiln,? Thomas Staffelbach,* and Daniel Zurmuhlet Physics Institute, University of Bern, Sidlerstrasse 5, CH-30 12 Bern, Switzerland, and National Center for Atmospheric Research, Boulder, Colorado 80307
A continuous melting technique, combined with continuous flow analysis, has been developed for in situ measurements of chemical trace species in ice cores. A crosssection of 1.8 X 1.8 cm2 of the core is needed for the simultaneous analysis of at least four species. The subcore is melted continuously from one side, and only the inner, uncontaminated part of the melted sample is used for the analysis. The main advantage of this method as compared to conventional sampling and analysis procedures is given by a very high spatial resolution, combined with a significant reduction of sample handling work. The method can be applied for any species for which a sensitive continuous flow analysis method exists. This technique has been applied successfullyfor the parallel measurement of HzOZ,HCHO, NH4+, and Ca2+during the Greenland Ice Core Project (GRIP) deep drilling project at Summit, central Greenland (72'34'N, 37O38'W, 3200 m above mean sea level). Introduction Ice cores from Greenland and Antarcticaprovide a great variety of informations about the history of the climate and the composition of the ancient atmosphere. Air bubbles, which are built into polar ice during firnification, are samples of ancient atmospheric air, and the analysis of the bubble air yields direct information on the concentration and isotopic composition of gases like Nz, 0 2 , COz, NzO, and others. Also, the ice matrix itself contains information about the ancient atmosphere. Oxygen and hydrogen isotopes of water are related to paleotemperatures, and measurements of particulate and dissolved impurities in the ice reveal natural events like volcanic eruptions as well as anthropogenic changes of the global atmosphere, Examples for this category are ionic impurities (e.g., H+, NH4+, Na+, Ca2+, sod2-, N o s , Cl-), dissolved gases (e.g., HzOz, HCHO), trace metals, radioisotopes, and dust particles. For most studies on ice impurities, detailed and continuous measurements along the entire ice core are needed due to the high variability of their concentrations. While the inclusion process limits spatial variations of the air bubble composition along an ice core to wavelengths in the order of meters (I),the impurities located in the ice matrix can exhibit large concentration gradients within ~~~~
+ University of Bern.
* National Center for Atmospheric Research.
204 Environ. Scl. Technol., VoI. 28, No. 2, 1994
centimeters or even millimeters (2). One of the basic informations needed for any ice core research is the dating of the core. Like the stable isotopes of water, some of the ice impurities exhibit seasonalvariations (e.g.,Hz02, NH4+, etc.) and can be used for accurate ice core dating by counting annual layers ( 3 , 4 ) . Using conventional sampling techniques, the number of samples required for this purpose is considerable. For the dating of a deep core like the GRIP core (5), drilled in central Greenland (ice thickness 3028 m), this would be prohibitive. Already for the Holocene part of the core (the last 10 000 years), at least 50 000 samples would be required. Considering the tedious work connected with sample cutting and decontamination procedures, it is very desirable to find an alternative. Continuous measurement methods for the dust content (6)and the solid-state electrical conductivity (7, 8) are well-established in ice core research. In this paper, we present a method for continuous, high-resolution measurements of impurities in ice that can be applied for any species for which a sensitive and simple continuous analysis is possible. This method drastically reduces sample handling work and includes a simple, but very efficient, sample decontamination step. The method cannot be applied in the firn part of the core. It has been applied successfully during the GRIP deep drilling operations in 1990-1992 in Greenland for continuous in situ measurements of HzOz, HCHO, NH4+, and Ca2+. Up to now, more than 2000 m of the ice core has been analyzed with a spatial resolution in the order of 1 cm. Continuous Melting Technique The principle of the automatic melting technique is shown in Figure 1. A cross-section of 1.8 X 1.8 cm2is cut from the core using a bandsaw. If the core quality is good, Le., if there are no cracks, it is possible to cut very long sections with a good bandsaw. The surprising stability of the ice allows the handling of up to 250-cm-long core sections. This subcore is mounted into a sample holder, which holds the sample in a vertical position centered over the melting head. The sample is pressed down to the melting head by its own weight and an additional load of about 100 g. The melting head is made of aluminium and is surface-coated with PTFE. It is thermostated to 48 OC by a 200-W heat source. The melting speed was found to be independent of the sample weight, i.e., the length of the remaining ice section. The geometry of the melting surface allows a separation of the meltwater into an inner and an outer fraction. The meltwater of a central circle area of 11-mm diameter is pumped away separately with 0013-936X/94/0928-0204$04.50/0
0 1994 Amerlcan Chemical Society
Flgure 1. Melting head and sample holder.
a flow rate that is 25% smaller than the melting rate of the inner circle area. The remaining fraction of the inner meltwater flushes the gap between ice and melting head and prevents a possible contamination of the inner fraction with surface meltwater. The outer meltwater fraction drains into a cylindrical channel with a tilted ground area. The outer sample is pumped away from the deepest point of this channel with a flow rate smaller than the total melting rate minus the inner sample flow rate. The remaining meltwater is removed from the channel at an angle of 90" from the deepest point with a flow rate exceeding the remaining meltwater flow rate. This guarantees a constant meltwater level in the channel during the melting process. For the analysis of species which are susceptible to contamination, e.g., HCHO, NH4+, and Ca2+,the inner fraction has to be used for the analysis. For species which are not sensitive to contamination, e.g., H202, the outer fraction of the meltwater can be used. A glass wool filter removes coarse particles and fibers, which could otherwise block the flow system. At Summit, the melting speed was 5.0 cm/min and showed only small variations. The corresponding melting rates are 4.8 mL/min for the inner fraction and 11.8 mL/min for the outer fraction. The three meltwater fractions are pumped away by a multichannel peristaltic pump. The flow rates, as indicated in Figure 2 are chosen according to the above-mentioned melt rate/ pump rate conditions. The sample fractions are segmented by air bubbles which are included in the ice. These air bubbles have to be removed prior to the analysis, which is achieved by a mechanical 'debubbler' (see Figure 2). The debubbler volume of about 0.2 mL was chosen as small as possible to minimize sample dispersion.
Continuous Flow Analysis of H 2 0 2 , HCHO, NH4+, and Ca2+
The temperature in the science trench at Summit, where the measurements were performed, was between -10 and -20 "C, and obviously all chemical analysis had to be performed in a heated environment. Therefore, the whole analytical apparatus except the melting system was placed into insulated boxes that were heated to a temperature of about 10 "C. The following properties are common to all four analytical systems:
All systems are continuous flow systems, i.e., the sample stream is analyzed continuously. Valves are only necessary to switch between sample stream and calibration solutions, and reagents are added continuously to the sample stream using peristaltic pumps. The flow charts of the individual systems are displayed in Figure 3, panels a-d. Before and after every subcore sample, blank solutions are measured to define the baseline. All methods are linear in the relevant concentration ranges, and one-point calibrations are sufficient. Switching from blank solutions to the sample stream and back is done automatically. This is achieved by bubble detectors consisting of a focusing LED (light emitting diode) and a phototransistor, acting as a light gate. The Teflon sample tubing (0.5-mm i.d., 1.5mm 0.d.) between LED and phototransistor scatters the light less when it is filled with water than when it is filled with air, and the resulting phototransistor signal differs by as much as a factor of 10. When the melting process has been started, the meltwater front passes a first bubble detector (LED l),a 6-port sample injection valve, and then a second bubble detector (LED 2). When the meltwater reaches LED 2, the valve is switched to connect the sample stream to the detector line. When the melting process is finished and the end of the meltwater stream passes LED 1,the valve is switched back and blank water enters the detector line. This configuration prevents air from entering the detector line. The tubing volumes between bubble detectors and valve can be adjusted to discard the possibly contaminated portions of both ends of the subcore. H202. Hydrogen peroxide is analyzed using the enzymatic fluorometric technique described by Lazrus et. al. (9).Instead of (p-hydroxypheny1)aceticacid, 4-ethylpheno1 is used as the substrate. Although the peroxidasecatalyzed reaction of H2Oz with 4-ethylphenol is favored at a neutral or acidic pH, whereas the fluorometric detection is optimal at pH > 9, both reaction and detection are conducted in the same borate buffer (pH = 6). This reduces the sensitivity but simplifies the system, and the sensitivity of the method is still sufficient for the concentrations found in polar ice. Bidistilled water is used as a blank solution and used for the preparation of calibration standards. A self-built filter fluorimeter (326 nm/410 nm) with a cadmium lamp (Hamamatsu L2264) and a 25-pL fluorescence cell (Hellma 178.010-08-40) is used as the detector. The reaction coil is a 50-cm-long piece of PTFE tubing (0.5-mm i.d.) with knots to reduce sample dispersion (10). A short piece of microporous polypropylene membrane tubing (Accurel Q3/2, Enka AG Wuppertal, Germany) after the sample pump tubing removes air bubbles that may be formed in the low pressure section between the debubbler and the pump. HCHO. Formaldehyde is analyzed using the fluorometric method adopted from Dong and Dasgupta (11). Fluorescence is measured by a fluorescence spectrophotometer (412 nm/510 nm, Shimadzu RF350). CO2-free mineral water is used as a blank solution, and millipore water is used for the preparation of calibration standards. The flow system is similar to the H202 system, except that the reaction coil has to be heated to 85 "C in a thermostated bath. Air bubbles formed in the hot water have to be removed by a mechanical debubbler. NH4+. Ammonia is analyzed using the reaction with o-phthaldialdehyde (OPA). The fluorometric method is described in Genfa and Dasgupta (12). The same type of Envlron. Scl. Technol., Vol. 28, No. 2, 1994 205
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melting head debubbler 2 debubbler 1 Figure 2. Flow chart of the melting system. The numbers in the arrow boxes indicate flow rates of peristaltic pump channels (In mL/min). The inner and outer sample pump rates are 25% smaller than the correspondlng melting rates (see text).
detector (365 nm/425 nm) as for H202 is used. For the preparation of blank and standard solutions, millipore water was used. The flow system is very similar to the HCHO system, except that buffer and OPA are added separately to improve the sensitivity. Instead of a mechanical debubbler, a microporous membrane debubbler is used. Cooling the sample after the reaction coil improves the lifetime of the membrane. Ca2+. Calcium is analyzed by an absorption technique, which is described in Kagenow and Jensen (13). A commercial absorption spectrophotometer (Shimadzu UV120-01)is used as detector. Because of the relatively low sensitivity, a large 300-pLabsorption cell (Hellma 176.052QS) has to be used. Blanks and standards are prepared in millipore water. For the Summit measurements, fresh reagent solutions were prepared daily. The composition of the solutions is summarized in Table 1.
Examples Figure 4 shows an example for continuous H202 measurements at Summit. Two parallel subcores of 90-cm length have been analyzed to demonstrate the reproducability of the method. The characteristic features of the HzOzvariations are very similar. The differences between the two profiles are not necessarily due to measuring errors, they can also indicate horizontal concentration variations. In Figure 5, a 4-m section of the Summit record is displayed. All four components exhibit seasonal variations. Especially NHd+, followed by H202 and Ca2+,can be used for a reliable dating by counting annual layers. The section in Figure 5 covers 41 f 1years, and based on a preliminary dating of the core (5), the absolute age is roughly 9000 years. An interpretation of the data, e.g., a discussion of the seasonality of the individual species, or of the anticorrelation of H2Oz with Ca2+is given elsewhere (14).
Spatial Resolution and Signal Disconvolution Figure 5 demonstrates that the spatial resolution of the method is sufficient to resolve seasonal variations in a core with an annual layer thickness of 10 cm. However, in the deeper part of the core, the annual layer thickness will decrease continuously, and therefore it is desirable to keep the resolution as high as possible. The spatial resolution of the method is limited by sample dispersion 206
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taking place in the melting head, the debubblers, the tubings, and the detector cells. The relative importance of these contributions are discussed in the followingsection. The effect of sample dispersion can be described mathematically by a low pass filter. As a simplification of the real situation, the flow system of each species can be described by a series of several ideal mixing chambers (melting head, debubbler, tubing, and detector cell). The volumes of the different mixing chambers determine the degree of sample dispersion. Obviously,the largest mixing volume has the strongest effect. As a further simplification, we assume that there is only one dominating mixing chamber. An ideal mixing volume corresponds to a first-order low pass filter. The filtered functiong(x) can be expressed as a convolution of the original function f ( x ) with the pulse answer of the first-order low pass filter: g(x) = Jomf(x - t)X-'e-t/x dt The mixing length X (cm) and the mixing volume V (cm3) are related by
X = VuIF where u is the melting speed (cmlmin) and F is the flow rate (cm31min) through volume V , In principle, it is possible to reconstruct the original signal f(x) from the filtered signal g(x) by a deconvolution, if X is known. This inversed filtering is called restitution. However, a complete reconstruction of the high frequencies is not possible, since wavelengths small as compared to X are strongly attenuated by the low pass filter and a deconvolution would only amplify signal noise. An alternative to the complete deconvolution is a partial restitution: the low pass filter is not removed but is replaced by a low pass filter with a smaller A. This can be done numerically by a simple recursive algorithm (15). With gi (i = 1 ... n),measured signa1;fi (i = 1...n),restituted signal; dx, length increment (cm); XI (old) mixing length (cm); X2, (new) mixing length (cm); and the definitions:
a
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Flgure 3. (Panel a) Contlnuous flow analyzer for hydrogen peroxide: d, microporous membrane debubbler: cal., Calibration standard; w, waste. (Panel b) Contlnuous flow analyzer for formaldehyde: D, mechanical debubbler. (Panel c) Continuous flow analyzer for ammonia. (Panel d) Contlnuous flow analyzer for calclum.
The recursive restitution is done by fl fj
= a&
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This algorithm represents a very economic but effective method of improving the resolution to some extent. The ratio X 1 J X p determines the degree of restitution. A series of tests yielded that AI/ A2 = 4 is a reasonable value. Larger ratios do not significantly improve the resolution, but merely lead to an amplification of signal noise. An example for the restitution of a calcium record is given in Figure 6. It shows that a significant improvement of the resolution can be obtained by this numerical restitution. Nevertheless, a reduction of the dispersion in the system is still favorable. In the following section, it will be shown how
the sources of dispersion can be localized and what components of the system should be improved. For the restitution of the signal, it is necessary to know the overall dispersion of the system, i.e., the mixing length AI. The best way to determine A1 is to record the system response to a concentration step and to adjust A1 until the restituted signal reaches the new concentration as fast as possible, but without overshooting. Two such concentration steps are provided during every calibration, when the sample stream is switched from the blank to astandard solution and vice versa. However, the response on these calibration steps contains only the dispersion of the continuous flow analyzer but not the contributions of the melting system and the debubbler. The relative importance of the dispersion before and after the line switching valves can therefore be investigated by evaluating AI for Envlron. Scl. Technol., Vol. 28, No. 2, I994 207
Table 1. Composition of Reagent Solutionsa
H2Oz reagent 4-ethylphenol peroxidase (type 11)
$
0.005 M 1unit/mL 0.1 M 0.1 M 5 mM
2 0
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KCl NaOH HCHO reagent ammonium acetate 2M acetic acid 0.25 M 2,4-pentanedione 0.1 M NH4+ reagent o-phthaldialdehyde 0.01 M buffer Na2HP04.12HzO 0.1 M 6 mM NaOH formaldehyde 13 mM Na2SOd 6 mM Caz+reagent o-cresolphthalein complex 0.01 mM NaOH 15 mM H3B03 15 mM 8-hydroxyquinoline 2.2 mM All chemicals were purchased from Merck, except peroxidase (Sigma).
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Flgure 5. A 4-m section of the continuous Summit deep core record. Concentrationsare given in pmol/L. 8
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Figure 6. Example for the restitution of a Ca record. The filter parameters are as follows: I, = 3.5 cm, /2 = 0.9 cm. The length increment is 0.33 cm.
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Table 2. Mixing Lengths of Four Systems (See Text) X1"dm*
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H2Oz HCHO NH4+ Ca2+
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anormal calibration (Xlmalyzer) and for a calibration where the standards are inserted at the melting head (Alba).An independent test for Xltoa is given by the condition that the restitution should never lead to negative concentrations. Table 2 compiles the determined mixing lengths of the four systems. The mixing lengths of the NH4+ system show that the overall dispersion is dominated by the melting and debubbler system. For the inner melting fractions, the dispersion in the melting head can probably be neglected compared to the dispersion in the debubbler. The dispersion of the analyzer is smaller than in the case of H202 and HCHO due to the higher flow rate (see Figure 3, panels a-c). A further improvement of the resolution 208
Envlron. Scl. Technol., Vol. 28, No. 2, 1994
(cm)
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of the NH4+ system can therefore be achieved mainly by reducing the dispersion in the debubbler, either by reducing its volume or increasing its input flow rate. On the other hand, the dispersion in the Ca2+ system is obviously dominated by the large dispersion in the analyzer, which is caused by the high-volume absorption cell. It is clear that the best way to improve the resolution of the Ca2+system would be to reduce the cell volume, but this would reduce the sensitivity of the absorption method. A better solution would be to find a sensitive fluorescence method for Ca2+. The dispersion in the H202 system is caused by both the melting system and the analyzer. The higher dispersion of the H202 analyzer compared to the NH4+ analyzer is caused by the lower total flow rate through the H202 detector cell. Additionally, the H202 sample is taken from the outer melting fraction, and the meltwater collecting channel represents a fairly large mixing volume. It would be favorable to take the sample also from the inner meltwater fraction, but then the sample flow rates of the analyzers would have to be reduced, which in turn would deteriorate the resolution of the analyzers. A solution of this problem would be either to increase the sample cross-section or to build fluorescence cells with
smaller dispersion. Finally, the resolution of the HCHO system is limited by the dispersion caused by the analyzer. A higher sample flow rate or a better fluorescence cell would improve the resolution.
3H, Mineral Dust and P O in Greenland Snow. Atmos. Environ. 1991,25A, 899-904. Sigg, A.; Neftel, A. Seasonal variations of hydrogen peroxide in polar ice cores. Ann. Glaciol. 1980, 10, 157-162. Sigg, A.; Neftel, A. Evidence for a 50% increase in HzOz over the past 200 years from a Greenland ice core. Nature
Conclusions
1991, 351, 557-559. Johnsen, S. J.; Clausen, H. B.; Dansgaard, W.; Fuhrer, K.; Gundestrup, N.; Hammer, C. U.; Iversen, P.; Jouzel, J.; Stauffer, B.; Steffensen, J. P. Irregular glacial interstadials recorded in a new Greenland ice core. Nature 1992,359, 311-313. Hammer, C. U.; Clausen, H. B.; Dansgaard, W.; Neftel, A.; Kristinsdottir, P.;Johnson, E. Continuous impurity analysis along the Dye3 deep core. Geophys. Monogr. 1985,33,9094. Hammer, C. U. Acidity of polar ice cores in relation to absolute dating, past volcanism, and radioechoes. J.Glaciol. 1980,25, 359-371. Neftel, A.; AndrBe, M.; Schwander, J.; Stauffer, B.; Hammer, C. U. Measurements of a kind of DC-conductivity on cores from Dye 3. In Greenland ice core: geophysics, geochemistry and the environment; Langway, C. C., Jr., Oeschger, H., Dansgaard, W., Eds.; American Geophysical Union: Washington, DC, 1985; Vol. 33, pp 32-38. Lazrus, A. L.; Kok, G. L.; Gitlin, S. N.; Lind, J. A. Automated fluorometric method for hydrogen peroxide in atmospheric precipitation. Anal. Chem. 1985, 57, 917-922. Dasgupta, P. K.; Hwang, H. Application of a nested loop system for the flow injection analysis of trace aqueous peroxides. Anal. Chem. 1985,57, 1009-1012. Dong, S.; Dasgupta, P. K. Fast fluorometric flow injection analysis of formaldehyde in atmospheric water. Enuiron. Sci. Technol. 1987,21, 581-588. Genfa, Z.; Dasgupta, P. K. Fluorometric measurement of aqueous ammonium ion in a flow injection system. Anal. Chem. 1989,61,408-412. Kagenow, J. Kinetic determination of magnesium and calcium by stopped flow injection analysis. Anal. Chim. Acta 1982,514, 125-127. Fuhrer, K.; Neftel, A.; Anklin, M.; Maggi, V. Continuous in situ measurements of HzOz, HCHO, CaZ+ and NH4+ concentrations along the new GRIP ice core from Summit, Central Greenland. Atmos. Environ. 1993,27A (12). Robinson, E. A.; Treitel, S. Geophysical Signal Analysis; Prentice-Hall: Old Tappan, NJ, 1980; Chapter 4.
A method for high-resolution measurements of trace species in polar ice cores has been developed. The new melting technique combined with continuous flow analysis offers fast, contamination-free, fully automated parallel measurements of several species with a spatial resolution in the order of 1 cm. It has been applied successfully during the GRIP deep drilling operation in central Greenland, where continuous records of H202, HCHO, NH4+, and Ca2+along the entire 3-km-long ice core have been obtained. These high-resolution records offer informations on the long-term changes and small-scale behavior of these species in polar ice. One of the applications, the dating of the core by counting annual layers, becomes critical with decreasing annual layer thickness. The spatial resolution of the records can be improved to some degree by numerical deconvolution. A direct improvement of the resolution can be achieved by optimizing the dispersion in the flow system or by increasing the cross-section of the ice sample. Acknowledgments The deep ice core at Summit was obtained by the Greenland Ice Core Project (GRIP), an ESF associate program, with Belgium, Denmark, France, Germany, Great Britain, Iceland, Italy, and Switzerland participating. The laboratory work was supported by the Swiss National Science Foundation and through COST 611. We thank H. Oeschger, B. Stauffer, and A. Neftel for continuous support and valuable discussions. Literature Cited (1) Schwander, J.; Stauffer, B. Age difference between polar ice and the air trapped in its bubbles. Nature 1984, 311, 45-47. (2) Beer, J.; Finkel, R. C.; Bonani, G.; Gaggeler, H.; Gorlach, U.; Jacob, P.; Klockow, D.; Langway, C. C., Jr.; Neftel, A.; Oeschger, H.; Schotterer, U.; Schwander, J.; Siegenthaler, U.; Suter, M.; Wagenbach, D.; Wolfli, W. Seasonalvariations HzOz, 210Pb, in the concentration of 'OBe, C1-, Nos-, S042-,
Received for review October 29, 1992. Revised manuscript received March 1, 1993. Accepted October 20, 1993.'
* Abstract published in Advance ACS Abstracts, December 1, 1993.
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