Environmental Analysis Facility

Capabilities

The Environmental Analysis Facility features expertise in the following areas:

Sample Collection

Teflon-membrane filters are used for the measurement of mass and elemental concentrations.

Quartz fiber filters are used for the determination of carbon fractions and inorganic ions in the particulate phase.

Cellulose filters are used to capture acidic gases.

Filter batches are conditioned and acceptance tested prior to use in sampling. Two percent of filters from each batch are acceptance tested by subjecting them to the exact same analysis as sampled filters, to ensure that they are clean before they are used for actual sampling.

Gravimetric Analysis
Unexposed and exposed Teflon-membrane filters are equilibrated at a temperature of 21.5 ± 2 °C and a relative humidity of 35 ± 5% for a minimum of 24 hours prior to weighing.

Weighing is performed on a Metter Toledo MT5 microbalance with ±0.001 mg sensitivity.

The charge on each filter is neutralized by exposure to a 210Po ionizing source for 30 seconds or more prior to the filter being placed on the balance pan. The balance is calibrated with 200, 100 and 50 mg Class 1 weights, and the tare is set prior to weighing each batch of filters.

After every 10 filters are weighed, the 200 mg calibration and tare are re-checked. If the results of these performance tests deviate from specifications by more than ±0.003 mg, the balance is re-calibrated.

Replicate weights are performed on 100% of the filters weighed before sampling (initial weights or pre-weights), and on 30% of the filters weighed after sampling (final weights or post-weights) by an independent technician. Replicate pre-sampling (initial) weights must be within ± 0.010 mg of the original weights. Replicate post-sampling (final) weights on ambient samples must be within ± 0.015 mg.

Post-sampling weights on heavily loaded (i.e., greater than 1 mg) samples must be within 2% of the net weight. Pre- and post- weights, check weights, and re-weights (if required) are recorded on data sheets, as well as being directly entered into a database via an internet connection.

Filter Extraction/Aqueous Solution

Filter Extraction

Water-soluble nitrate, nitrite, sulfate, chloride and ammonium are obtained by extracting the quartz-fiber particle filter (or any other filter used for sample collection) in 15 ml of deionized-distilled water (DDW). The filter is placed in a 16 x 150 mm polystyrene extraction vial with a screw cap (e.g., Falcon #2045).

Each vial is labeled with a bar code sticker containing the filter ID code. The extraction tubes are placed in tube racks, and the extraction solutions are added. The extraction vials are capped and sonicated for 60 minutes, shaken for 60 minutes, then aged overnight to assure complete extraction of the deposited material in the solvent. The ultrasonic bath water is monitored to prevent temperature increases from the dissipation of ultrasonic energy in the water.

After extraction, these solutions are stored under refrigeration prior to analysis.

Aqueous Solution Samples

The aqueous solution samples are analyzed for inorganic ions in the same manner as the filter extracts. If any solid material is present in the samples, they are centrifuged to take the solids to the bottom of the sample tubes to prevent solids from entering our analytical instruments.

X-Ray Fluorescence

XRF analyses is performed on Teflon‑membrane filters with a PANalytical Epsilon 5, EDXRF analyzer using a side‑window, liquid‑cooled, 100 KeV, 24 milliamp dual anode (Sc/W) x‑ray tube and secondary targets, for:

Na, Mg, Al, Si, P, S, Cl, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Br, Rb, Sr, Y, Zr, Nb, Mo, Pd, Ag, Cd, In, Sn, Sb, Ba, La, Ce, Sm, Eu, Tb, Hf, Ta, W, Ir, Au, Hg, Tl, Pb, and U.

In XRF, inner shell electrons are removed from the atoms of the aerosol deposit. An x‑ray photon with a wavelength characteristic of each element is emitted when an outer shell electron occupies the vacant inner shell. The number of these photons is proportional to the number of atoms present.

The characteristic x‑ray peaks for each element are defined by 200 eV‑wide windows in an energy spectrum ranging from 1 to 80 KeV.

Eight separate XRF analyses are conducted on each sample to optimize detection limits for the specified elements.

The EDXRF system is calibrated using Micromatter (Arlington, WA) thin film standards.

Multielement standards are analyzed daily to monitor for any instrument drift.

Carbon Analysis (TOR/TOT)

The thermal/optical reflectance and transmittance (TOR/TOT) method measures organic (OC) and elemental (EC) carbon.

The TOR/TOT method is based on the principle that different types of carbon-containing particles are converted to gases under different temperature and oxidation conditions.

The different carbon fractions from TOR/TOT are useful for comparison with other methods, which are specific to a single definition for organic and elemental carbon. These specific carbon fractions are analyzed following the Interagency Monitoring Protection Visual Environment (IMPROVE) thermal protocol, and also help distinguish among seven carbon fractions reported by TOR/TOT:

The carbon evolved in a helium atmosphere at temperatures between ambient and 140 °C (OC1)
The carbon evolved in a helium atmosphere at temperatures between 140 and 280 °C (OC2)
The carbon evolved in a helium atmosphere at temperatures between 280 and 480 °C (OC3)
The carbon evolved in a helium atmosphere between 480 and 580 °C (OC4)
The carbon evolved in an oxidizing atmosphere at 580 °C (EC1)
The carbon evolved in an oxidizing atmosphere between 580 and 740 °C (EC2)
The carbon evolved in an oxidizing atmosphere between 740 and 840 °C (EC3)

The thermal/optical reflectance carbon analyzer consists of a thermal system and an optical system.

The thermal system consists of a quartz tube placed inside a coiled heater. Current through the heater is controlled to attain and maintain pre-set temperatures for given time periods. A portion of a quartz filter is placed in the heating zone and heated to different temperatures under non-oxidizing and oxidizing atmospheres.

The optical system consists of a He-Ne laser, a fiber optic transmitter and receiver and a photocell. The filter deposit faces a quartz light tube so that the intensity of the reflected laser beam can be monitored throughout the analysis.

As the temperature increases from ambient (~25 °C) to 580 °C, organic compounds are volatilized from the filter in a non-oxidizing (He) atmosphere while elemental carbon is not oxidized. When oxygen is added to the helium at temperatures greater than 580 °C, the elemental carbon burns and enters the sample stream. The evolved gases pass through an oxidizing bed of heated manganese dioxide where they are oxidized to carbon dioxide, then across a heated nickel catalyst, which reduces the carbon dioxide to methane (CH4). The methane is then quantified with a flame ionization detector (FID).

The reflected laser light is continuously monitored throughout the analysis cycle. The negative change in reflectance is proportional to the degree of pyrolytic conversion from organic to elemental carbon, which takes place during organic carbon analysis.

After oxygen is introduced, the reflectance increases rapidly as the light-absorbing carbon is burned off the filter. The carbon measured after the reflectance attains the value it had at the beginning of the analysis cycle is classified as elemental carbon. This adjustment for pyrolysis in the analysis is significant, as high as 25% of organic or elemental carbon, and it cannot be ignored.

The system is calibrated by analyzing samples of known amounts of methane, carbon dioxide, and potassium hydrogen phthalate (KHP). The FID response is ratioed to a reference level of methane injected at the end of each sample analysis. Performance tests of the instrument calibration are conducted at the beginning and end of each day’s operation. Intervening samples are re-analyzed when calibration changes of more than ±10% are found.

Known amounts of American Chemical Society (ACS) certified reagent grade crystal sucrose and KHP are committed to TOR/TOT as a verification of the organic carbon fractions. Fifteen different standards are used for each calibration. Widely accepted primary standards for elemental and/or organic carbon are still lacking.

Results of the TOR/TOT analysis of each filter are entered into the DRI database.

Ion Chromatographic Analysis for Inorganic Ions
Water-soluble chloride, nitrate, and sulfate are measured with the Dionex ICS-3000 (Sunnyvale, CA) ion chromatograph (IC).

In IC, an ion‑exchange column separates the sample ions in time for individual quantification by a conductivity detector.

Prior to detection, the column effluent enters a suppressor column, where the chemical composition of the component is altered, resulting in a matrix of low conductivity.

The ions are identified by their elution/retention times, and are quantified by the conductivity peak area.

Approximately 250 µl of the filter extract are injected into the ion chromatograph. The resulting peaks are integrated, and the peak integrals are converted to concentrations using calibration curves derived from solution standards.

The Dionex system contains a guard column (AG14 column, Cat. No. #37042) and an anion separator column (AS14 column, Cat. No. #37041) with a strong basic anion exchange resin, and an anion micro membrane suppressor column (250 x 6 mm ID) with a strong acid ion exchange resin, and an anion self-regenerating suppressor.

The anion eluent consists of sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) prepared in DDW. The DDW is verified to have a conductivity of less than 1.8 x 10-5 ohm/cm prior to preparation of the eluent. For quantitative determinations, the ion chromatograph is operated at a flow rate of 2.0 ml/min.

Calibration standards are prepared at least once each month by diluting the primary standard solution (Dionex Standard #57590) to concentrations covering the range of concentrations expected in the filter extracts. The calibration concentrations prepared are at 0.1, 0.2, 0.5, 1.0, and 2.0 mg/ml for each of the analysis species.

The standards are stored in a refrigerator.

Calibration curves are performed daily. Chemical compounds are identified by matching the retention time of each peak in the unknown sample with the retention times of peaks in the chromatograms of the standards.

A DDW blank is analyzed after every 20 samples and a calibration standard is analyzed after every 10 samples. These quality control checks verify the baseline and calibration, respectively.

Environmental Research Associates (ERA, Arvada, CO) NIST traceable standards are used daily as an independent quality assurance (QA) check. These standards (ERA Wastewater Nutrient and ERA Mineral WW) are traceable to NIST simulated rainwater standards. If the values obtained for these standards do not coincide within a pre-specified uncertainty level (typically three standard deviations of the baseline level or ±10%), the samples between that standard and the previous calibration standards are re-analyzed.

After analysis, the chromatogram for each sample in the batch is reviewed for the following: 1) proper operational settings, 2) correct peak shapes and integration windows, 3) peak overlaps, 4) correct background subtraction, and 5) quality control sample comparisons.

When values for replicates differ by more than ±10%, or values for standards differ by more than ±10%, samples before and after these quality control checks are designated for re-analysis in a subsequent batch.

Individual samples with unusual peak shapes, background subtractions, or deviations from standard operating parameters are also designated for re-analysis.

Automated Colorimetric Analysis for Ammonium and Ammonia

The Astoria Pacific (Clackamas, OR) Automated Colorimetric System (AC) is used to measure ammonium concentrations by the indolphenol method.

The heart of the automated colorimetric system is a peristaltic pump, which introduces air bubbles into the sample stream. Each sample is mixed with reagents and subjected to appropriate reaction periods before submission to a colorimeter. The liquid’s absorbency is related to the amount of the ion in the sample by Beer’s Law. This absorbency is measured by a photomultiplier tube through an interference filter, which is specific to the species being measured.

Ammonium in the extract is reacted with phenol and alkaline sodium hypochlorite to produce indolphenol, a blue dye. The reaction is catalyzed by the addition of sodium nitroprusside. The absorbency of the solution is measured at 630 nm.

Two milliliters of extract in a sample vial is placed in an autosampler, which is controlled by a computer. Five standard concentrations are prepared from ACS reagent-grade (NH4)2SO4 . Each set of samples consists of 2 distilled water blanks to establish a baseline, 8 calibration standards and a blank, then sets of 10 samples followed by analysis of one of the standards and a replicate from a previous batch.

The system determines carry-over by analysis of a low concentration standard following a high concentration. The percent carry-over is then automatically calculated and can be applied to the samples analyzed during the run.

Astoria Pacific software operating on a Dell Optiplex microcomputer controls the sample throughput, calculates concentrations, and records data on the DRI database.

Formaldehyde has been found to interfere with the measurements when it is present in an amount which exceeds 20% of the ammonium content. Hydrogen sulfide interferes with the measurements when it is present in concentrations which exceed 1 mg/ml. Nitrate and sulfate are also potential interferents when present at levels which exceed 100 times the ammonium concentration. These levels are rarely exceeded in ambient samples. The precipitation of the hydroxides of heavy metals such as calcium and magnesium is prevented by the addition of sodium citrate/sodium potassium tartrate buffer solution to the sample stream.

Soluble sodium and potassium by Atomic Absorption spectrometry

Soluble sodium, and potassium are measured using a Varian Spectra AA-880 atomic absorption spectrophotometer.

Atomic absorption spectroscopy methods rely on the principle that free, uncombined atoms will absorb light at specific wavelengths corresponding to the energy requirements of the specific atom.

Atoms in the ground state absorb light and are exited into a higher energy state.

Each transition between energy states is characterized by a different energy, and therefore a different wavelength of light. The atomic spectrum of each element comprises a number of discrete lines arising from both the ground and exited states. The lines which originate in the ground state atoms, called resonance lines, are the most often of interest in atomic absorption spectrometry, as ground state atoms are most prevalent in practical atomization methods.

The amount of light absorbed is proportional to the concentration of the atoms over a given absorption path length and wavelength. Standards of known concentration are prepared, matched to the sample matrix, and measured.

The unknown sample absorbencies are compared to the absorbencies of the standards. Since the measured absorbance is directly proportional to the concentration of analyte, this gives a simple and accurate method of determining the unknown concentration.

The atomic absorption spectrometer is a system which allows the analyst to measure the absorbance of the analyte, and relate the measured absorbance to the concentration of analyte in the sample.

The instrument consists of: a hollow cathode lamp containing the element of interest, a method of introducing ground state atoms into the light path, a monochromator to isolate the wavelength of interest, a photo detector to measure and amplify the absorbance signal, and a method of displaying the results. The lamp provides the spectral signature of the element to be measured.

Ground state atoms are introduced into the optical path by several methods. The most common of these are: flame, either air-Acetylene or Nitrous Oxide-Acetylene, graphite furnace, metal hydride generation, or cold vapor generation.

The monochromator consists of a grating that diffracts the light, and through the process of mutual interference is dispersed at different angles according to wavelength. The monochromator can be rotated to select and focus the wavelength of interest on the photodetector.

The photodetector is typically a photomultiplier tube, which detects and amplifies the light reaching it to useful levels. This signal is then passed on for further processing.

Results can be displayed using anything from a simple analog meter reading absorbance, to sophisticated computer software offering a wide variety of calculation options. The latter is standard on current instruments.

CONTACT

Judith Chow, Sc.D.
Lab Director
Judith.Chow@dri.edu
775.647.7050

LAB LOCATION

Desert Research Institute
2215 Raggio Parkway
Reno, NV 89512

DIVISION

Atmospheric Sciences