A Technique for Measuring Microparticles in Polar Ice Using Micro-Raman Spectroscopy

We describe in detail our method of measuring the chemical forms of microparticles in polar ice samples through micro-Raman spectroscopy. The method is intended for solid ice samples, an important point because melting the ice can result in dissociation, contamination, and chemical reactions prior to or during a measurement. We demonstrate the technique of measuring the chemical forms of these microparticles and show that the reference spectra of those salts expected to be common in polar ice are unambiguously detected. From our measurements, Raman intensity of sulfate salts is relatively higher than insoluble dust due to the speciﬁc Raman scattering cross-section of chemical forms of microparticles in ice.


Introduction
For the past several hundred thousand years, aerosols have been preserved as insoluble impurities (dust) and watersoluble impurities (ions) in the polar ice caps. Dust has been widely discussed as a proxy for terrestrial substances [1][2][3]. The concentrations of dust in melted samples from various ice cores have been measured by the Laser Scattering Method (LSM) [1] and the Coulter counter method [4]. The cited papers agree that the maximum dust input to Antarctica occurred during the last glacial maximum (the coldest event in the glacial period). The dust input was somewhat lower during stadial and interstadial periods, and lowest during interglacials and the Holocene.
The elemental compositions of dust particles in the ice core have been determined using SEM-EDS on sublimated ice samples [5], and by XRD on melted ice samples [6]. Ion concentrations can be measured either by Ion Chromatography as discussed elsewhere [7][8][9][10] or continuous flow analysis of melted samples [11]. However, the chemical forms of ice core inclusions have been a matter of debate for more than twenty years, because the ion concentrations alone do not preserve this information.
Understanding the chemical form of dust and ions present in polar ice is important to clarify the physical mechanisms behind preservation of the climate record. Some progress has been made along these lines, which we now summarize. Wolff and Paren [12] suggested that the DC conductivity of polar ice could be due to the presence of liquid layers at the grain boundaries. In agreement with this idea, SEM-EDS studies have surmised the presence of sulfuric acid at the grain boundaries in Antarctic ice [13][14][15], and Fukazawa et al. [16] have found evidence for sulfuric acid at the triple junction of South Yamato and Nansen ice cores, Antarctica by micro-Raman spectroscopy. Rempel et al. [17,18] suggested that the ions exist as acid solutions (sulfuric and nitric acid) displaced by premelting and anomalous diffusion through the vein network. Lately, study from Dome Fuji ice core, Antarctica have found ions as sulfate and nitrate salts in microparticles measured by micro-Raman spectroscopy [19]. Following this result, a number of other reports [20][21][22][23][24][25] Figure 1: Schematic of the cryostat system. The temperature is controlled by N 2 gas, evaporated by a heater inside a Dewar flask containing liquid N 2 . The heater is controlled by and connected to a thermocouple inside the cryostat. The cryostat container is made of chloroethene, and a Styrofoam setting is inside the container. Heat-absorptive lens and slide glasses are set inside the Styrofoam between the ice sample and the halogen light source, to prevent the latter from warming the sample. measured the chemical forms of microparticles using the same method.
This paper reports our procedure to determine the chemical form of microparticles in polar ice through micro-Raman spectroscopy. Following a brief review of the technique, we will describe our methodology in detail and the importance of settings such as slit width and laser power to how we measure the microparticles. We also provide reference Raman spectra for the most common salts which can be assumed the exist in ice sheets.

Setup of Cryostat
To measure the chemical forms of the microparticles, we use a micro-Raman spectroscope and a cryostat ( Figure 1). The cryostat is set on an x-y-z translation stage. A thermocouple is connected to the cryostat with metal to stabilize and measure the interior temperature. Copper is used for the connection because its thermal conductivity (4.01 W cm −1 K −1 ) is greater than that of ice (0.02 W cm −1 K −1 ) [25]; the metal rapidly equilibrates to the temperature inside the cryostat. In addition to the thermocouple, we place a platinum resistance temperature sensor (100 Ω) by the sample to measure its temperature directly.
To cool the cryostat, we used an N 2 gas flow from liquid nitrogen heated by a sheath coil inside the Dewar. The sheath heater coil emits 400 W and has a resistance of 25.0 Ω. The coil is connected to a PID (proportionalintegral-derivative) temperature controller, which monitors the inside of the cryostat via the thermocouple. Figure 2 shows the temperature of the cryostat as it is cooled from room temperature to −30.0 • C using the PID temperature controller. After a cool-down phase, the cryostat can be maintained at constant temperature for a long period with no significant gradient in the ice sample. Laser light cannot irradiate the microparticles during this phase. To stabilize the temperature at −30.0 • C ± 0.2 • C it took about 30 minutes for our cryostat.

Raman Spectroscopy
We employed an Ar ion laser with a wavelength of 514.5 nm (NEC GLG3260) and a maximum beam power of 1000 mW at the source. A band pass filter (Edmund Optics 47492-J) is placed just after the laser source. The filter blocks 99.995% of wavelengths in the ranges 434-509 nm and 520-670 nm (its optical density in this range is better than 5.0) and brocks only 10% of the central wavelength. We verified that the attenuation of the laser was independent of its power at the source by measuring the laser power at the objective lens for several different intensity settings. The laser power at the objective lens is always about 10% of the source power due to optical loss in the optical system in the equipment.
To detect the Raman spectra, it is necessary to focus the laser on a microparticle through a high-magnification objective lens. However, the focal lengths of such lenses are generally small. In the cryostat, the ice sample has to be kept at a stable temperature so that microparticles and the ice section itself do not move during the measurement. Therefore, managing the heat flux from the objective lens to the ice sample is a significant problem. To avoid creating a temperature gradient between the objective lens and the sample, we chose an objective lens with a super-long working distance (6.0 mm focal length) and a numerical aperture (N.A.) of 0.75 (Mitutoyo, M Plan Apo 100x). Please note that the temperature increases of ice during emitting the laser inside of the ice are not confirmed but some chemical form could be.
We use a triple monochromator (Jobin-Yvon, T64000) and a CCD detector (Jobin-Yvon, Spectraview-2D, 1024 × 256 pixels, cooled by liquid nitrogen) with good quantum efficiency from 500 to 900 nm. Using a grating with 1800 lines/mm and an entrance slit of 100 μm, the spectral resolution between two points is 0.6 cm −1 . The absolute frequency of the monochromator was calibrated using a neon emission line and silicon wafers.
The cross slit is the one of the most important components to measure the microparticles in ice by micro-Raman spectroscopy. It allows the researcher to change the size of the cross slit, from several micrometers down to 1 micrometer. The smallest cross slit results in a low-intensity peak compared to the large cross slits, but all settings greater than 5.0 μm yield nearly the same intensity. The variation in peak intensity will be significant when we discuss the measurement of micron-sized particles in ice.

Reference Samples
To identify the chemical forms of microparticles in ice cores, we need to know the spectra of the chemicals in advance. A database of reference spectra for various minerals, both water-soluble and insoluble, is available at the RRUFF website (http://rruff.info/) among others. However, Raman spectra of some salts are not measured. Thus, to compare the spectra of salt groups, we measured reference spectra for the following specimens: Na 2

Ice Preparation
Ice sections are cut from the Termination I (climate change during LGM to Holocene) of Dome Fuji ice core and planed by microtome to a thickness of several millimeters. To fit inside the cryostat, we usually cut the planed samples down to 50 × 30 × thickness mm 3 . The thickness depends on the grain size of the ice section. If the ice crystals are smaller than the sample thickness, the laser may reflect at a grain boundary and prevent us from clearly seeing the microparticles. Ideally, for the microparticles to be clearly visible, the sample thickness should be approximately equal to the ice grain size. Several minutes after the sample is placed in the cryostat, the temperature has stabilized. The ice is backlit by a metal halide lamp, and we use a low-magnification lens (focal length 33.5 mm and N.A. 0.28; Mitsutoyo M Plan Apo 10x) to search for microparticles. When we find one, we focus the laser on the microparticle with the 100x lens and record its Raman spectrum.

Measuring Microparticles in Polar Ice
To measure microparticles of various diameters in polar ice by micro-Raman spectroscopy, it is necessary to take several parameters into account. The two most important considerations are the width of the cross slit and the power of the laser.
We measured microparticles approximately 3.0 μm in diameter to show how the Raman signal changes with cross slit width and laser power. Figure 3 shows the dependence of peak intensity on slit width for single microparticles in  Figure 3: The plotted intensities are Raman peaks from the S-O symmetric stretching mode of a mirabilite microparticle found in the Dome Fuji ice core, for several cross slit widths. The left-hand plot (a) shows the peak intensities as a function of slit width. The series "Raman spectra of ice and microparticles" is derived from the raw spectral data. "Baseline of ice" is an average of 60 spectra taken from points on the ice without any microparticles. This spectrum is subtracted from the raw spectra of the S-O symmetric stretching mode to obtain the "Microparticle" spectrum. The right-hand plot (b) shows the raw data of each spectra.   Diameter of microparticle/cross slit Figure 6: The signal-to-noise ratios of the Raman spectra are plotted against the ratio of particle diameter to cross slit width. Sulfate salts yield very intense Raman spectra compared to dust, even though the particles are small. that to perfectly distinguish the similar Raman spectra of Na 2 SO 4 ·10H 2 O and MgSO 4 ·11H 2 O, we have to measure the spectrum by higher resolution [19]. An additional factor is the ratio between the width of the slit and the diameter of the microparticle. Figure 6 shows the relation of intensity of microparticles and diameter divided by cross slit. Raman intensity of sulfate salts is relatively higher than insoluble dust due to the specific Raman scattering cross section of chemical forms and insoluble dusts may contain several components including Raman inactive chemicals. Thus, to measure the insoluble dust by this method, we have to identify the chemical form of dusts with the several peaks (more than one peak) of Raman scattering from dusts. For instance, the NaAlSi 3 O 8 (albite) has several peaks that include weaker ones [30]. If these peaks corresponded well, then we surly identified the chemical form of mineral dust. If whole peaks did not correspond well, then the microparticles cannot be identified.
The theoretical cross-sections of microparticles have previously been described in detail by various aerosol studies [31][32][33]. The radiant flux Φ scattered into a solid angle Ω from a single microparticle P with composition A given by Φ P,A = ϕ 0 c A N a V P · Ω0 r A (λ 0 , λ s ) · p(n, x, Θ)dΩ, where ϕ 0 is the radiant flux density of the incident laser beam, c A is the molar concentration of composition A, N a is Avogadro's constant, and V P is the volume of the microparticle. The Raman scattering cross-section r A , included within the integral, depends on the incident wavelength λ 0 and Raman wavelength λ s , but is independent of the microparticle's morphology. The morphology factor p is dependent on the refractive index n, the size x and the scattering angle Θ. To understand the relationship between our quantitative Raman 6 International Journal of Spectroscopy spectroscopic measurements and these parameters, it is necessary to understand the morphology of the microparticles. This is especially true for sulfates, which would not fully understand the refractive index of salts (e.g., Na 2 SO 4 ·10H 2 O, MgSO 4 ·11H 2 O, CaSO 4 ·2H 2 O). Further experimental work on the refractive indices and scattering angle of several salts is needed, not just to estimate the parameter p but also to clarify the most suitable wavelength of laser light to use for measuring specific microparticles (including dust) in ice by Raman spectroscopy. Continued research in this view would aid in future measurements of microparticles and help researchers understand the process of radiative forcing on aerosols in past atmospheric conditions corresponding to the glacial and interglacial cycle.