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Proper Laboratory Protocols for Analytical Instrumentation
The proper operation of analytical instruments requires hands-on experience, even though studying the various components of instrument and understanding the theory is important. Experiential learning is a must for most students, especially when dealing with analytical instruments. This chapter is divided into two sections: Section 1.1 contains preliminary information that is not included in most textbooks but is necessary to truly understand the difficulty in measuring analytes in the parts-per-billion (ppb) and parts-per-trillion (ppt) concentrations; Section 1.2 deals with trace analysis and concerns the proper setup and testing of instruments to ensure that accurate and reproducible numbers are obtained and a brief discussion on quality assurance/quality control is provided.
1.1 Laboratory Preliminaries
The analysis of metals in the ppb and ppt concentrations is a difficult and tedious process that requires optimum laboratory conditions and analytical skills. Many laboratories even have their sample handling and preparation sections of the lab physically separated from their instrument rooms in order to avoid contaminating instruments and cross contamination of samples. Error associated with laboratory work can be divided into three general categories: sampling, sample preparation, and analysis. Entire books and statistics courses have been dedicated to proper sampling design and implementation in order to avoid errors (referred to as bias) in the collection of samples, such as in the proper sampling of lake water. Laboratory error has been significantly reduced by standardizing sample extraction procedures for a particular sample, element, compound, industry, and even a specific technician.
This beginning section of this chapter is dedicated to a few of the more important factors associated with the instrumental analysis of samples. Before actual experimental procedures and results are presented, it is necessary to comment on sample preparation, special chemicals, and the calibration of an instrument. These topics are not normally covered in textbooks but are important for first-time instrument users. The types of samples that are commonly analyzed by analytical instruments will be discussed, including proper procedures that are critical to avoid common laboratory errors.
1.1.1 Types of Samples and Sample Preparation
There are various sample types that need to be analyzed for their analyte concentration. For example, a geochemist may be interested in determining the concentration of a particular metal, such as lead (Pb), in a terrestrial or oceanic rock, or a fisheries biologist may be interested in determining the concentration of mercury in a particular fish species. In addition, a wide range of instrumentation is available for the analysis of organic compounds such as hydrocarbons and pesticides. However, the various instruments that could perform this task at a trace level (ppm or lower) are unable to accommodate the introduction of a rock, fish, or soil matrix. As a result, sample preparation, specifically sample digestion, is needed before an instrument can be utilized. The chemical process to transform the solid sample into an aqueous sample must be undertaken carefully with the proper reagents and glassware. After the concentration of the digested sample is determined, simple calculations can be performed to accurately determine the concentrations of Pb in a rock, mercury in a fish, or pesticide in a soil or sediment sample. If sample preparation is not performed properly, no degree or expense of instrumental components or laboratory technique can correct for a sampling or digestion error. Despite the obviousness of some of these techniques, this component of analytical chemistry is still a considerable portion of error associated with the final analyte concentration.
1.1.1.1 Samples
Before a procedure of sample preparation can be created (commonly referred to as a standard operating procedure, or SOP), the analyte(s) of interest must be identified as well as the matrix. The most common type of sample matrix for metals and organic compounds is aqueous. The first step when analyzing water samples is to determine whether the total or dissolved metal concentration is of interest. All natural water samples contain particles; some are extremely small, while others are visible to the naked eye. Frequently, particle size is used to differentiate between dissolved and suspended solids. For example, in environmental chemistry and other disciplines, most scientists accept that constituents of a sample are dissolved if they pass through a 0.45-µm glass-fiber filter; anything not passing through the filter is considered particulate. This will be the distinction between the dissolved and particulate phases used in this text. If the total analyte concentration needs to be determined, the water sample may require strong acidification and heating for metals or solvent extraction for organic compounds prior to analysis depending on the concentration in the solid phase. If the dissolved solid concentration is desired, the water is first filtered and then acidified or extracted, usually with high-purity nitric acid (HNO3) (1-3% acid) in a plastic container or with high-purity (chromatography-grade) organic solvents. The purpose of the extraction is to permanently dissolve any analyte that is adsorbed to colloidal (very small) particles or the container walls. Some procedures require that samples be stored at 4 °C for less than one month prior to analysis. Each industry, governmental agency, or company will have an SOP that must be rigidly followed.
Other sample matrices, such as gaseous, solid, and biological tissues, are usually chemically converted to aqueous samples through digestions or extractions. Metals and organic analytes contained in the atmosphere from geological processes, dust from wind, and high-temperature industrial processes are frequently measured to model the fate and transport of pollutants. Analytes may be in the atomic gaseous state, such as mercury vapor, or associated with suspended particles, such as cadmium (Cd) or Pb ions adsorbed to clay particles. One common sampling method is to utilize a vacuum to pull large volumes of air (tens to hundreds of cubic meters) through a filter. The filter is then removed and the metals or organics are extracted into an aqueous or organic solution that is then analyzed. This type of sampling and analysis is commonly performed in urban areas to monitor atmospheric metal emissions and organic pollution. Standardized methods for these types of measurements have been developed by the US Environmental Protection Agency (EPA) and other governmental agencies.
The metal concentration in a solid sample or geological material is also frequently analyzed by flame atomic absorption spectrometry (FAAS) and inductively coupled plasma-mass spectrometry (ICP-MS). Organic analytes are commonly measured by gas chromatography (GC) with flame ionization detection (FID) or MS detection, or high-pressure liquid chromatography-mass spectrometry (HPLC-MS). Sometimes analytes are analyzed directly by special attachments to FAAS and ICP units (as described in Chapters 4 and 5) but are more commonly digested or extracted with acids followed by analysis. Similarly, organic compounds in soil, sediment, and biological tissue are extracted with organic solvents and analyzed by GC or HPLC. The resulting solution is then analyzed as described in Section 1.1.3. However, the specific type of digestion/extraction depends on the ultimate goal of the analysis. If adsorbed metal concentrations are of interest, soil/sediment samples are simply placed in acid (usually 1-5% HNO3), heated for a specified time, diluted, filtered, and then the filtrate is analyzed as if it were a liquid sample. If the total metal concentration is desired, as in many geological applications, the sample is completely digested with a combination of hydrofluoric (HF), HNO3, and HClO4; again, the samples are then diluted, filtered, and analyzed as an aqueous sample. The extraction of organic analytes can be far more complicated and may involve liquid-liquid extraction for water samples, digestion of biological tissues followed by extraction, or extensive Soxhlet extraction for soil and sediment samples. Procedures are available from the US EPA and the US Geological Survey (USGS).
Biological tissues present considerable problems as do soil and sediment samples. In order to create an aqueous sample, the tissue sample is first digested in acid to oxidize all biological matter and "free" up any present metal or organic analyte. This can be accomplished with a combination of sulfuric acid (H2SO4) and HNO3 (1-5% each) and 30% hydrogen peroxide (H2O2), after digesting for 24 hours, and when necessary, heat. After the tissue has been dissolved, the sample is diluted, filtered, and analyzed as an aqueous sample. Each of the digestion procedures described above dilutes the original metal concentration in the sample. Thus, a careful accounting of all dilutions of a sample is required. After a concentration is obtained from an instrument, it must be adjusted for the dilution(s) to determine the analyte concentration in the original sample. Calculations concerning these dilutions will be discussed in the final paragraphs of this section.
1.1.1.2 Chemical Reagents
A variety of chemicals are used in the trace analysis. Mostly, these include reagents used in the...
