《分析化学样品前处理手册》（英文版）Sample Preparation Techniques in Analytical Chemistry

SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE Table 1.3. Sample Preservation Techniques Preservation Method Container Type Holding Time Immediately on Temperature Immediately on nide chloride None Plastic or glass 28 days fluoride None Plastic or glass Analyze imme- diately lodide Cool to4°C Plastic or glass 24 hours Nitrate. nitrite Cool to4°C Plastic or glass 48 hours Cool to4°C,add Plastic or glass 7 days te and oH to pH 9 Metals Dissolved Filter on site, acidify Plastic 6 to pH 2 with HNO, Total Acidify to pH 2 with Plastic HNO, Cr(VI) 4° Plastic 24 hours g to pH 2 with Plastic Orga arbon Plastic or brown 28 days H2SO, to pH 2 Purgeable hydro- Cool to 4C, add h Teflon 14 days 0.008%Na2S2O3 Purgeable Cool to4°C,add Glass with Teflon 14 days 0.008%Na2S2O3 PCBS Cool to4°C Glass or Teflon 7 days to 40 days aft Organics in soil Cool to 4C Glass or Teflon ossible Fish tissues Freeze Aluminum foi As soon as Cool to4°C gen demand Chemical oxygen Cool to 4C Plastic or glass 28 days

Table 1.3. Sample Preservation Techniques Sample Preservation Method Container Type Holding Time pH — — Immediately on site Temperature — — Immediately on site Inorganic anions Bromide, chloride fluoride None Plastic or glass 28 days Chlorine None Plastic or glass Analyze imme￾diately Iodide Cool to 4C Plastic or glass 24 hours Nitrate, nitrite Cool to 4C Plastic or glass 48 hours Sulfide Cool to 4C, add zinc acetate and NaOH to pH 9 Plastic or glass 7 days Metals Dissolved Filter on site, acidify to pH 2 with HNO2 Plastic 6 months Total Acidify to pH 2 with HNO2 Plastic 6 month Cr(VI ) Cool to 4C Plastic 24 hours Hg Acidify to pH 2 with HNO2 Plastic 28 days Organics Organic carbon Cool to 4C, add H2SO2 to pH 2 Plastic or brown glass 28 days Purgeable hydro￾carbons Cool to 4C, add 0.008% Na2S2O3 Glass with Teflon septum cap 14 days Purgeable aromatics Cool to 4C, add 0.008% Na2S2O3 and HCl to pH 2 Glass with Teflon septum cap 14 days PCBs Cool to 4C Glass or Teflon 7 days to extraction, 40 days after Organics in soil Cool to 4C Glass or Teflon As soon as possible Fish tissues Freeze Aluminum foil As soon as possible Biochemical oxy￾gen demand Cool to 4C Plastic or glass 48 hours Chemical oxygen demand Cool to 4C Plastic or glass 28 days (Continued) 18 sample preparation: an analytical perspective

PRESERVATION OF SAMPLES Table 1.3.(Continued) Preservation Method Container Type Holding Time Store in TE (pH 8) Years 20C: freeze RNA Deionized formamide Years at-80°C Solids unstable in Store in argon-filled air for surface and spectroscopic hydrocarbon oil characterization stable for months, whereas Cr(VI) is stable for only 24 hours. Holding time can be determined experimentally by making up a spiked sample(or storing an actual sample)and analyzing it at fixed intervals to determine when it begins to degrade 1. 4.1. Volatilization Analytes with high vapor pressures, such as volatile organics and dissolved gases(e. g, HCN, SO2) can easily be lost by evaporation. Filling sample containers to the brim so that they contain no empty space(headspace)is the most common method of minimizing volatilization. Solid samples can be topped with a liquid to eliminate headspace. The volatiles cannot equilibrate between the sample and the vapor phase (air) at the top of the container. The samples are often held at low temperature (4C) to lower the vapor pressure. Agitation during sample handling should also be avoided. Freezing liquid samples causes phase separation and is not recommended 1. 4.2. Choice of Proper Containers The surface of the sample container may interact with the analyte. The sur- faces can provide catalysts (e.g., metals) for reactions or just sites for irre- versible adsorption. For example, metals can adsorb irreversibly on glass surfaces, so plastic containers are chosen for holding water samples to be nalyzed for their metal content. These samples are also acidified with HNO3 to help keep the metal ions in solution. Organic molecules may also interact with polymeric container materials. Plasticizers such as phthalate esters can diffuse from the plastic into the sample, and the plastic can serve as a sorbent(or a membrane)for the organic molecules. Consequently, glas containers are suitable for organic analytes. Bottle caps should have Teflon liners to preclude contamination from the plastic caps

stable for months, whereas Cr(VI ) is stable for only 24 hours. Holding time can be determined experimentally by making up a spiked sample (or storing an actual sample) and analyzing it at fixed intervals to determine when it begins to degrade. 1.4.1. Volatilization Analytes with high vapor pressures, such as volatile organics and dissolved gases (e.g., HCN, SO2) can easily be lost by evaporation. Filling sample containers to the brim so that they contain no empty space (headspace) is the most common method of minimizing volatilization. Solid samples can be topped with a liquid to eliminate headspace. The volatiles cannot equilibrate between the sample and the vapor phase (air) at the top of the container. The samples are often held at low temperature (4C) to lower the vapor pressure. Agitation during sample handling should also be avoided. Freezing liquid samples causes phase separation and is not recommended. 1.4.2. Choice of Proper Containers The surface of the sample container may interact with the analyte. The sur￾faces can provide catalysts (e.g., metals) for reactions or just sites for irre￾versible adsorption. For example, metals can adsorb irreversibly on glass surfaces, so plastic containers are chosen for holding water samples to be analyzed for their metal content. These samples are also acidified with HNO3 to help keep the metal ions in solution. Organic molecules may also interact with polymeric container materials. Plasticizers such as phthalate esters can di¤use from the plastic into the sample, and the plastic can serve as a sorbent (or a membrane) for the organic molecules. Consequently, glass containers are suitable for organic analytes. Bottle caps should have Teflon liners to preclude contamination from the plastic caps. Table 1.3. (Continued) Sample Preservation Method Container Type Holding Time DNA Store in TE (pH 8) under ethanol at
20C; freeze at
20 or
80C Years RNA Deionized formamide at
80C Years Solids unstable in air for surface and spectroscopic characterization Store in argon-filled box; mix with hydrocarbon oil preservation of samples 19

SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE Oily materials may adsorb strongly on plastic surfaces, and such samples are usually collected in glass bottles. Oil that remains on the bottle walls should be removed by rinsing with a solvent and be returned to the sample A sonic probe can be used to emulsify oily samples to form a uniform sus- pension before removal for analysis 1.4.3. Absorption of Gases from the Atmosphere Gases from the atmosphere can be absorbed by the sample during handling, for example, when liquids are being poured into containers. Gases such as O2, CO2, and volatile organics may dissolve in the samples. Oxygen may oxidize species, such as sulfite or sulfide to sulfate. Absorption of CO2 may change conductance or pH. This is why pH measurements are always made at the site. CO2 can also bring about precipitation of some metals. Dissolu tion of organics may lead to false positives for compounds that were actually absent. Blanks are used to check for contamination during sampling, port, and laboratory handling. 1. 4. 4. Chemical changes A wide range of chemical changes are possible For inorganic samples, con- olling the pH can be useful in preventing chemical reactions. For example metal ions may oxidize to form insoluble oxides or hydroxides. The sample is often acidified with HNO, to a pH below 2, as most nitrates are soluble and excess nitrate prevents precipitation. Other ions, such as sulfides and cyanides, are also preserved by pH control. Samples collected for NH3 analysis are acidified with sulfuric acid to stabilize the NH3 as Nhso ons. Organic species can also undergo changes due to chemical reacti Storing the sample in amber bottles prevents photooxidation of organics (e. g, polynuclear aromatic hydrocarbons. Organics can also react with dis- solved gases; for example, organics can react with trace chlorine to form halogenated compounds in treated drinking water samples. In this case, the addition of sodium thiosulfate can remove the chlorine Samples may also contain microorganisms, which may degrade the sam ple biologically. Extreme pH (high or low) and low temperature can mini- mize microbial degradation. Adding biocides such as mercuric chloride or pentachlorophenol can also kill the microbes 1. 4.5. Preservation of unstable solids Many samples are unstable in air. Examples of air-sensitive compounds are alkali metal intercalated C60, carbon nanotubes, and graphite, which are

Oily materials may adsorb strongly on plastic surfaces, and such samples are usually collected in glass bottles. Oil that remains on the bottle walls should be removed by rinsing with a solvent and be returned to the sample. A sonic probe can be used to emulsify oily samples to form a uniform sus￾pension before removal for analysis. 1.4.3. Absorption of Gases from the Atmosphere Gases from the atmosphere can be absorbed by the sample during handling, for example, when liquids are being poured into containers. Gases such as O2, CO2, and volatile organics may dissolve in the samples. Oxygen may oxidize species, such as sulfite or sulfide to sulfate. Absorption of CO2 may change conductance or pH. This is why pH measurements are always made at the site. CO2 can also bring about precipitation of some metals. Dissolu￾tion of organics may lead to false positives for compounds that were actually absent. Blanks are used to check for contamination during sampling, trans￾port, and laboratory handling. 1.4.4. Chemical Changes A wide range of chemical changes are possible. For inorganic samples, con￾trolling the pH can be useful in preventing chemical reactions. For example, metal ions may oxidize to form insoluble oxides or hydroxides. The sample is often acidified with HNO3 to a pH below 2, as most nitrates are soluble, and excess nitrate prevents precipitation. Other ions, such as sulfides and cyanides, are also preserved by pH control. Samples collected for NH3 analysis are acidified with sulfuric acid to stabilize the NH3 as NH4SO4. Organic species can also undergo changes due to chemical reactions. Storing the sample in amber bottles prevents photooxidation of organics (e.g., polynuclear aromatic hydrocarbons). Organics can also react with dis￾solved gases; for example, organics can react with trace chlorine to form halogenated compounds in treated drinking water samples. In this case, the addition of sodium thiosulfate can remove the chlorine. Samples may also contain microorganisms, which may degrade the sam￾ple biologically. Extreme pH (high or low) and low temperature can mini￾mize microbial degradation. Adding biocides such as mercuric chloride or pentachlorophenol can also kill the microbes. 1.4.5. Preservation of Unstable Solids Many samples are unstable in air. Examples of air-sensitive compounds are alkali metal intercalated C60, carbon nanotubes, and graphite, which are 20 sample preparation: an analytical perspective

POSTEXTRACTION PROCEDURES 2 usually prepared in vacuum-sealed tubes. After completion of the intercala tion reaction in a furnace, the sealed tubes may be transferred directly to a Raman spectrometer for measurement. Since these compounds are photo- sensitive, spectra need to be measured using relatively low laser power den sities. For x-ray diffraction, infrared, and x-ray photoelectron spectroscopy (XPS), the sealed tubes are transferred to an argon-filled dry box with less than 10 parts per million(ppm) of oxygen. The vacuum tubes are cut open in the dry box and transferred to x-ray sampling capillaries. The open end of the capillaries are carefully sealed with soft wax to prevent air contami- nation after removal from the dry box. Samples for infrared spectroscopy are prepared by mixing the solid with hydrocarbon oil and sandwiching a mall amount of this suspension between two K Br or NaCl plates. The edges of the plates are then sealed with soft wax. For the XPs measurements, the powder is spread on a tape attached to the sample holder and inserted into a transfer tube of the XPS spectrometer, which had previously been introduced into the dry box. Transfer of unstable compounds into the sampling cham- ber of transmission and scanning electron microscopes are difficult. The best approaches involve preparing the samples in situ for examination 1.5. POSTEXTRACTION PROCEDURES 1.5.1. Concentration of Sample Extracts The analytes are often diluted in the presence of a large volume of solvents sed in the extraction. This is particularly true when the analysis is being done at the trace level. An additional concentration step is necessary to increase the concentration in the extract. if the amount of solvent to be removed is not very large and the analyte is nonvolatile, the solvent can be vaporized by a gentle stream of nitrogen gas flowing either across the surface or through the solution. This is shown in Figure 1.6. Care should be taken that the solvent is lost only by evaporation. If small solution droplets are lost as aerosol, there is the possibility of losing analytes along with it. If large volume reduction is needed, this method is not efficient, and a rotary vac- lum evaporator is used instead. In this case, the sample is placed in a ror bottomed flask in a heated water bath. a water-cooled condenser is attached at the top, and the flask is rotated continually to expose maximum liquid surface to evaporation. USing a small pump or a water aspirator, the pres ure inside the flask is reduced. The mild warming, along with the lowered pressure, removes the solvent efficiently, and the condensed solvent distills into a separate flask. Evaporation should stop before the sample reaches

usually prepared in vacuum-sealed tubes. After completion of the intercala￾tion reaction in a furnace, the sealed tubes may be transferred directly to a Raman spectrometer for measurement. Since these compounds are photo￾sensitive, spectra need to be measured using relatively low laser power den￾sities. For x-ray di¤raction, infrared, and x-ray photoelectron spectroscopy (XPS), the sealed tubes are transferred to an argon-filled dry box with less than 10 parts per million (ppm) of oxygen. The vacuum tubes are cut open in the dry box and transferred to x-ray sampling capillaries. The open ends of the capillaries are carefully sealed with soft wax to prevent air contami￾nation after removal from the dry box. Samples for infrared spectroscopy are prepared by mixing the solid with hydrocarbon oil and sandwiching a small amount of this suspension between two KBr or NaCl plates. The edges of the plates are then sealed with soft wax. For the XPS measurements, the powder is spread on a tape attached to the sample holder and inserted into a transfer tube of the XPS spectrometer, which had previously been introduced into the dry box. Transfer of unstable compounds into the sampling cham￾ber of transmission and scanning electron microscopes are di‰cult. The best approaches involve preparing the samples in situ for examination. 1.5. POSTEXTRACTION PROCEDURES 1.5.1. Concentration of Sample Extracts The analytes are often diluted in the presence of a large volume of solvents used in the extraction. This is particularly true when the analysis is being done at the trace level. An additional concentration step is necessary to increase the concentration in the extract. If the amount of solvent to be removed is not very large and the analyte is nonvolatile, the solvent can be vaporized by a gentle stream of nitrogen gas flowing either across the surface or through the solution. This is shown in Figure 1.6. Care should be taken that the solvent is lost only by evaporation. If small solution droplets are lost as aerosol, there is the possibility of losing analytes along with it. If large volume reduction is needed, this method is not e‰cient, and a rotary vac￾uum evaporator is used instead. In this case, the sample is placed in a round￾bottomed flask in a heated water bath. A water-cooled condenser is attached at the top, and the flask is rotated continually to expose maximum liquid surface to evaporation. Using a small pump or a water aspirator, the pres￾sure inside the flask is reduced. The mild warming, along with the lowered pressure, removes the solvent e‰ciently, and the condensed solvent distills into a separate flask. Evaporation should stop before the sample reaches dryness. postextraction procedures 21

SAMPLE PREPARATION: AN ANALYTICAL PERSPECTIVE Figure 1.6. Evaporation of solvent by nitrogen For smaller volumes that must be reduced to less than 1 mL a kuderna- Danish concentrator(Figure 1.7)is used. The sample is gently heated in a water bath until the needed volume is reached. An air-cooled condenser provides reflux. The volume of the sample can readily be measured in the narrow tube at the bottom 1.5.2. Sample Cleanup Sample cleanup is particularly important for analytical separations such as GC, HPLC, and electrophoresis. Many solid matrices, such as soil,can contain hundreds of compounds. These produce complex chromatograms where the identification of analytes of interest becomes difficult. This is especially true if the analyte is present at a much lower concentration than he interfering species. So a cleanup step is necessary prior to the analytical measurements. Another important issue is the removal of high-boiling materials that can cause a variety of problems. These include analyte adsorption in the injection port or in front of a GC-HPLC column, false positives from interferences that fall within the retention window of the alyte, and false negatives because of a shift in the retention time window

For smaller volumes that must be reduced to less than 1 mL, a Kuderna– Danish concentrator (Figure 1.7) is used. The sample is gently heated in a water bath until the needed volume is reached. An air-cooled condenser provides reflux. The volume of the sample can readily be measured in the narrow tube at the bottom. 1.5.2. Sample Cleanup Sample cleanup is particularly important for analytical separations such as GC, HPLC, and electrophoresis. Many solid matrices, such as soil, can contain hundreds of compounds. These produce complex chromatograms, where the identification of analytes of interest becomes di‰cult. This is especially true if the analyte is present at a much lower concentration than the interfering species. So a cleanup step is necessary prior to the analytical measurements. Another important issue is the removal of high-boiling materials that can cause a variety of problems. These include analyte adsorption in the injection port or in front of a GC-HPLC column, false positives from interferences that fall within the retention window of the analyte, and false negatives because of a shift in the retention time window. dispersed small bubbles N2 Figure 1.6. Evaporation of solvent by nitrogen. 22 sample preparation: an analytical perspective