Inorganic Chemical Analysis of Environmental 

Materials—A Lecture Series

Open-File Report 2011–1193

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia 2011

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Suggested citation:

Crock, J.G., and Lamothe, P.J., 2011, Inorganic chemical analysis of environmental materials—A lecture series: U.S. 

Geological Survey Open-File Report 2011-1193, 117 p. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. 

Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted material contained within this report.
iii

Contents

Abstract ......................................................................................................................................................................... 1 

Introduction.................................................................................................................................................................... 1 

Standard Reference Materials ....................................................................................................................................... 3 

Development of QC materials matrix matched to a project study area - an example................................................. 4 

Slides and Lecture Notes............................................................................................................................................... 7 

Acknowledgments ......................................................................................................................................................111 

Selected References ..................................................................................................................................................111 

Figures

1. Major element comparisons in QC material versus median element concentrations in Pebble Deposit study........ 5 

2. Minor and trace element comparison in QC material versus median element concentrations in Pebble Deposit 

study. ................................................................................................................................................................ 5 

3. Minor and trace element comparison in QC material versus median element concentrations in Pebble Deposit 

study. ................................................................................................................................................................ 6 

4. Minor and trace element comparison in QC material versus median element concentrations in Pebble Deposit 

study.................................................................................................................................................................6 

Table

1. Atomic Spectroscopy detection limits (ppb or micrograms/liter)...........................................................................108
1

Inorganic Chemical Analysis of Environmental 

Materials—A Lecture Series

By J.G. Crock and P.J. Lamothe

Abstract

At the request of the faculty of the Colorado School of Mines, Golden, Colorado, the 

authors prepared and presented a lecture series to the students of a graduate level advanced 

instrumental analysis class. The slides and text presented in this report are a compilation and 

condensation of this series of lectures. The purpose of this report is to present the slides and notes 

and to emphasize the thought processes that should be used by a scientist submitting samples for 

analyses in order to procure analytical data to answer a research question. First and foremost, the 

analytical data generated can be no better than the samples submitted. The questions to be 

answered must first be well defined and the appropriate samples collected from the population that 

will answer the question. The proper methods of analysis, including proper sample preparation and 

digestion techniques, must then be applied. Care must be taken to achieve the required limits of 

detection of the critical analytes to yield detectable analyte concentration (above “action” levels) 

for the majority of the study’s samples and to address what portion of those analytes answer the 

research question—total or partial concentrations. To guarantee a robust analytical result that 

answers the research question(s), a well-defined quality assurance and quality control (QA/QC) 

plan must be employed. This QA/QC plan must include the collection and analysis of field and 

laboratory blanks, sample duplicates, and matrix-matched standard reference materials (SRMs). 

The proper SRMs may include in-house materials and/or a selection of widely available 

commercial materials. A discussion of the preparation and applicability of in-house reference 

materials is also presented. Only when all these analytical issues are sufficiently addressed can the 

research questions be answered with known certainty. 

Introduction

The precise and accurate analyses of environmental materials, including geological, 

biological, and man-made matrices, form the basis for most environmental studies. The analysis of 

environmental materials, including fresh and weathered metal-mining waste products, includes both 

laboratory-based determinations of the stable or stabilized analytes, as well as the in-field 

determination of the non-stable analytes. Difficulties in providing such analyses include the large 

range of analyte and matrix element concentrations, phase associations of the analyte elements, 

sample size (too small or too large), sample homogeneity, analyte volatility or stability, and 

contamination. Current trends in analytical chemistry focus on: lower detection limits (at least in 

the low μg/kg range) and smaller sample sizes for simultaneous multi-element determinations;

automation of the sample digestion/preparation methods; automation of data handling; and species 

characterization (phase association and/or valence) state. The growth in environmental analytical 
2

chemistry has also given rise to increased awareness of the need for a wide range of appropriate 

standard reference materials, both for total analyses and for operationally defined extraction 

procedures or speciated analyses.

There are many complete references for the analysis of environmental and geological 

materials available. Some of the more noteworthy are Carter (1993), Jeffery and Hutchison (1983), 

Ingamells and Pitard (1986), Keith (1992), Potts (1987), Smith (1994), Smoley (1992), Sparks 

(1996), Stoch (1986), and Westerman (1990). 

One of the most important aspects to the analysis of environmental materials is the 

digestion/decomposition of the material prior to presenting the sample to the individual 

instrumental method. Excellent references for the total dissolution of environmental materials 

include Bock (1979) and Sulcek and Povondra (1989). Chao (1984) addresses in detail the 

application of partial and sequential dissolution schemes. Chao and Sanzolone (1992) discuss both 

total and partial dissolution techniques, as well as their application. Often, a method of 

determination is not sensitive enough, either because the original concentration of the analyte is too 

low or the required dilution of the resulting digestate is too high. Often, the separation and (or) 

preconcentration of the analyte from the matrix is required. Minczewski and others (1982) present 

a comprehensive overview of these useful techniques. 

Sample dissolution is usually the most tedious, time-consuming, and limiting factor in 

chemical analyses. A multi-acid digestion, combining hydrofluoric, hydrochloric, nitric, and 

perchloric acids at low temperatures and pressures (Crock and others, 1983; Taggart, 2002), is a 

common dissolution method. Many of the common rock-forming alumino-silicate minerals can be 

dissolved by this method. The advantages of acid digestion are the ease of the method, use of large 

samples (as much as 2 g, although 0.2 g is more common), low reagent blanks, and low total 

dissolved salts in the analytical solution. With an acid digestion, the final dilution factor commonly 

is less than 100, allowing many elements to be determined at or near their crustal abundance. One 

disadvantage of acid digestion using hydrofluoric acid is the volatilization of some elements, such 

as silicon and boron, as fluoride compounds. Commonly, a higher pressure, closed-vessel “bomb” 

digestion used in conjunction with a complexing agent (for the excess fluoride) is used to avoid 

volatilization of silicon and boron. 

Some minerals are resistant to routine acid digestions and require a more rigorous digestion. 

These minerals include spinels, beryl, tourmalines, chromite, zircon, monazite, niobates, tungstates, 

topaz, and cassiterite. These minerals can be completely dissolved by the proper choice of a sinter 

or fusion digestion procedure. A sodium peroxide sinter will dissolve most resistant minerals (for 

example, Sulcek and Povondra, 1989). For example, boron and silicon are routinely determined in 

tourmaline by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) following a 

sodium peroxide sinter in a zirconium crucible at 445°C. Lithium metaborate, sodium and (or) 

potassium hydroxide, sodium carbonate, and the alkali persulfates are commonly used as fusion 

reagents. However, there are drawbacks to the use of fusions or sinters. They introduce a much 

higher total salt content into the analytical solution, which can clog the nebulizer and torch 

assembly in ICP-AES and the interface cones in inductively coupled plasma–mass spectrometry 

(ICP-MS), leading to significant signal drift. These fusions and sinters also tend to have long-term 

memory effects and higher reagent blanks. A larger dilution factor is used because of the smaller 

sample size (10 to 100 mg is common) in a larger final solution volume. The final dilution factor is 

commonly 200 to 400, making the determination of some trace elements impossible by ICP-AES 

direct aspiration without prior separation and preconcentration. Also, at least one element common 

to the reagent is not determinable, such as lithium and boron from a lithium metaborate fusion. 
3

With advancements in analytical instrumentation over the past three decades, the analyst 

has a choice of several precise and accurate methods with sufficient sensitivity for most 

environmental study requirements. Some of the more popular methods of spectrographic 

instrumental analysis are: flame atomic absorption spectroscopy (F-AAS), graphite furnace-atomic 

absorption spectroscopy (GF-AAS), inductively coupled plasma-atomic emission spectroscopy 

(ICP-AES), and inductively coupled plasma-mass spectrometry (ICP-MS). Table 1 presents a 

comparison of the detection limits for these spectrographic techniques. Few laboratories rely on 

only one of these analytical methods, but often use complimentary combinations of these 

techniques. Other methods of analysis commonly used in the laboratory for environmentally 

important analytes include X-ray fluorescence, instrumental neutron activation analysis, 

electrochemical methods (especially specific ion electrode methods), infrared detection of 

combustion products, ion chromatography, and colorimetry. Sandell (1959) offers a classic 

overview of many of the colorimetric methods and many applications. Skoog and others (2006) 

present a discussion of the other techniques. 

Standard Reference Materials

The use of well-characterized, matrix-matched standard reference materials (SRMs) in any 

geological, geochemical, or environmental study involving any analytical data is critical for the 

study to withstand scientific scrutiny. The appropriate use of SRMs cannot be over emphasized. 

Issues of precision, the reproducibility and stability (both short-term and long-term), and accuracy 

(how close to the “true, absolute value” your measurement approximates) must be addressed and 

assessed. Issues of quality assurance and quality control (QA/QC) must be quantified. The prudent 

use of SRMs, laboratory and field duplicates, and laboratory and field blanks addresses these 

critical issues for the scientist. The use of SRMs is discussed at length by Taylor (1993). An 

excellent compilation of geochemical SRMs and their compositions is presented by Potts and 

others (2000). 

The issue of how well analytical laboratories perform on long-term or regional studies is a 

concern to many people. Incorporating QC materials into the sample submittal process is one way 

to address this issue but there are certain limitations that should be recognized. One of the main 

issues is the importance of matrix matching samples and QC materials. The effect of mineralogy 

and matrix can be significant in terms of chemical analysis, and if the QC material and samples 

differ significantly, a false sense of laboratory performance can be realized, particularly for the 

efficacy of sample preparation methods. This problem is further complicated by analysts not 

having an extensive supply of QC materials that cover a wide range of sample types from which

they can draw. This means that often a reduced number of QC samples are submitted and 

something off the shelf is selected which has the same general description (rock, soil, sediment, and 

so forth).

The USGS is actively involved in the world community of producing and certifying 

geological SRMs. Consult http://minerals.cr.usgs.gov/geo_chem_stand/ for details of the USGS 

SRM project. The USGS Geochemical Reference Materials project provides geochemical 

reference materials for use by scientists in all areas of earth science throughout the world. The 

majority of USGS reference materials are based on silicate rocks that were collected from the 

continental United States and Hawaii and range in composition from basalt to shale. The 

composition of these reference materials has been determined through rigorous testing procedures 

by multiple laboratories using a wide variety of analytical techniques. The reference materials are 

suitable for use in calibration of analytical instrumentation, testing analytical methodologies, and 
4

for use as quality control samples. For more information on this program please see Wolf and 

Wilson (2007). 

Development of matrix-matched QC materials to project study area—an 

example

Recently, a different approach was taken in the development of QC material for the Pebble 

copper deposit study in Alaska (Anderson and others, 2011). Rather than rely on stock QC 

material, one was developed for the project using the splits of samples collected. We believe that 

this approach offers several scientific advantages, as well as a cost effective way to produce QC 

material that provides the best possible test of laboratory performance. 

One of the challenges associated with developing a QC program for long term or large scale 

geochemical studies is to find high quality reference materials in sufficient quantities that match the 

lithology of the area under investigation that will be available for the duration of a multi-year 

study. Issues pertaining to sample decomposition and inter-element interferences often have a 

profound impact on the quality of analytical results. This is why matrix-matched QC materials are 

highly desirable in order to monitor laboratory performance. Traditionally, established reference 

materials are selected as QC materials that have the same general identifier (for example, soil or 

sediment) with little regard to their mineralogical composition, especially as it compares to the 

study area under investigation. This can lead to an erroneous evaluation of laboratory performance, 

especially if the QC materials are more or less easily digested/analyzed as compared to the typical 

study area sample.

For this Pebble copper mine study, after discussion with project staff, a plan was developed 

that utilized the samples already collected as the feedstock for the desired QC material. The 

collected samples were split and a portion set aside for QC preparation. Approximately 45 kg were 

obtained in this manner for an in-house QA/QC material. The material was then ground, blended, 

and split into containers using the procedures developed for normal USGS geochemical reference 

materials preparation. Consult http://minerals.cr.usgs.gov/geo_chem_stand/ for details of the 

sample preparation methods. Aliquots were submitted to the USGS contract laboratory as part of 

the normal sample stream and analytical results compiled. A series of total element and partial 

extraction analyses were performed using techniques from a variety of laboratories. Target element 

concentrations were determined from these tabulated values and used to evaluate laboratory 

performance.

Using information provided by study geologists, a comparison was made of average QC 

element concentrations values with median values from the study. Only the USGS contract 

laboratory’s 55 element analytical package was used in this comparison (details of this method 

given at: http://minerals.cr.usgs.gov/intranet/chem/labmethods.html#m22). If the prepared QC 

material was representative of the “average” study area soil, then for any/all element(s) the ratio of 

QC concentration versus median study value should yield a value of one. A graphical presentation 

of major and minor elements quantified is presented below in Figures 1–4. 
5

Pebble Deposit study ratio QC vs. Median values

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

Al % Ca % Fe % K % Mg % P % Ti %

Element, %

Ratio QC value/Median

QC/Median 10% -10%

Figure 1. Major element comparisons in QC material versus median element concentrations in Pebble 

Deposit study. 

Figure 2. Minor and trace element comparison in QC material versus median element concentrations in 

Pebble Deposit study. 

Pebble Deposit study ratio QC/Median values

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

Ba Bi Ce Co Cr Cs Cu Dy Er Eu Ga Gd Ge Hf

Element

Ratio

QC/Median 10% -10%
6

Pebble Deposit study QC values vs. Medians

0.7

0.8

0.9

1

1.1

1.2

1.3

Ho La Lu Mn Mo Nb Nd Ni Pr Rb Sb Sc Sm

Elements

Ratio (QC/Median)

QC/Median 10% -10%

Figure 3. Minor and trace element comparison in QC material versus median element concentrations in 

Pebble Deposit study. 

Pebble Deposit study ratio QC vs. Median values

0.8

0.85

0.9

0.95

1

1.05

1.1

1.15

Sr Ta Tb Th Tm U V W Y Yb Zn Zr

Element

values)

QC/Median 10% -10%

Ratio (QC/Median 

Figure 4. Minor and trace element comparison in QC material versus median element concentrations in 

Pebble Deposit study. 
7

Based on historical results from USGS contract laboratories, it is anticipated that a ±10

percent level of uncertainty is the upper limit expected for this type of comparison. Element ratios 

outside this value suggest that either a bias exists between the two sample types or that there are

analytical issues associated with the analyses. An examination of Figures 1 through 4 suggests that 

the QC material aligns itself well with the median concentrations observed in the study. 

It is apparent that, for the majority of elements studied, the composite QC approach 

provides a suitable mechanism for the development of a study QC material. In addition to 

providing a material with a reasonable matrix composition, the cost associated with this approach 

can be reasonable if the optimal amount of material is prepared and the container costs are 

minimized. Improvements to this approach can be expected if the following improvements are 

considered: (1) Better initial planning in terms of sample collection so that QC material(s) is/are 

prepared before the major sample load arrives at the laboratory. This may require an initial sample 

reconnaissance effort to collect typical sample types. (2) Collection of sample amounts are in 

excess of anticipated laboratory needs. (3) Analysis of composite QC material is made alongside 

certified reference materials so that traceability is established to primary reference materials. 

Doing so improves the reliability factor for QC data. (4) Earlier coordination with research 

chemistry group for preparation of QC material. 

Slides and Lecture Notes

The following series of slides and associated lecture notes is a condensation of several 

lectures presented by the authors at the Colorado School of Mines (CSM), Golden, Colorado. 

Additional material was also presented by the authors at various lectures in the past; it is included 

here to supplement the original CSM lecture material.
8

Slide 1

Inorganic Chemical Analysis 

of Environmental Materials

J. G. Crock and P.J. Lamothe

Crustal Geophysics and Geochemistry Science Center

Denver, Colorado

U. S. Geological Survey

The accurate analysis of environmental materials—including biological, geological, and 

man-made matrices—form the basis for most environmental studies. Although the title says this 

report and presentation are on the chemical analysis of environmental materials, we will include 

information on sampling, quality assurance, and control. Soil will be our primary example of an 

environmental material. This discussion can be extended to include many man-made materials, 

especially those that are silicate based, ceramics, concrete, or other road building materials. 

Difficulties in providing unbiased determinations include the large range of analyte and matrix 

element concentrations, phase associations of the analyte element, sample size (too small or too 

large), sample homogeneity, analyte volatility, and sample contamination. Current trends in 

analytical chemistry relate to lower detection limits (at least in the low μg/Kg range) simultaneous 

multi-element determinations, automation of the sample digestion/preparation, automation of data 

handling and storage, and species characterization (phase association and/or valence). The growth 

in environmental analytical chemistry has also given rise to increased awareness of the need for a 

wide range of appropriate standard reference materials, for both total and partial analyses.
9

Slide 2

Analytical Results can be

NO Better

than the sample

submitted.

First and foremost, analytical results can be NO BETTER than the sample submitted to the 

laboratory. Also remember that you will never determine the absolute true answer, but you can 

attempt to make a reasonable estimate of it. Another way to look at this statement is “Garbage in is 

garbage out!” The final scientific interpretation is only as good as the analytical data and the 

analytical data can only be as good as the sample taken and analyzed. Sample size can range from 

an entire outcrop of rock down to microscopic inclusions in a solid, and everything in between. 

The sample taken must be representative of the population that answers the question being asked. 

Samples can range from that single inclusion to a large composited sample made from tens of 

subsamples. Sample mass required for a representative sample increases as the particle size of the 

material increases, as the sample exhibits increasing heterogeneity as the concentration of the 

desired analyte decreases, and as the desired degree of confidence increases.
10

Slide 3

What Really Is An 

Environmental Material?

• Naturally occurring materials including 

flora, fauna, aqueous materials, soils, 

rocks, weathering products, ores, coal, 

peat, ashes, and sediments

• Man-made materials including slags, 

manufactured products and by-products, 

effluents, and waste stream

Environmental sciences incorporate most of the physical world around us. If it can be 

collected, preserved, and stored, chances are it will eventually be analyzed for an environmental 

study. The accurate analysis of environmental materials for their total or partial elemental content is 

anything but a simple or routine task. There are numerous problems that must be addressed to 

ensure a high quality analysis. The first and foremost problem to address is what question does the 

environmental scientist need answered. Does the scientist need an average concentration of an 

analyte for a volume or area, or is a distribution map required? Is the scientist asking for elemental 

speciation, phase association, or would a total analysis answer the correct question? The scope of 

the phrase “Environmental Materials” is where the semantic problems continue. Most naturally 

occurring materials and some man-made materials will fall into this classification. The 

concentration of an element of interest commonly ranges from less than one part per billion 

(microgram per kilogram or μg/Kg) to the tens of weight percent. Furthermore, there are several 

potential sampling problems in the analysis of environmental samples. 
11

Slide 4

What element or group of elements will answer the study’s question or questions? 

Environmental studies usually require data about a large portion of the elements in the periodic 

table to understand and answer the posed questions. Also, many environmental studies require the 

analyses of many samples, often in multiple matrices or on different fractions of a given matrix. 

This would include the analyses of water, stream bed sediments, rocks, soil, different soil horizons, 

and various parts of plants. 
12

Slide 5

What Really is the Question?

• Average concentration of a given layer or 

volume or is a distribution map required?

• Elemental speciation or phase association?

• Total analysis?

• Required regulations to meet?

• What is the population to be described?

• What question does the customer or end 

users of the data need to have answered?

The very first hurdle to clear for meaningful analytical data to be obtained is, “What is the 

question to be answered with the analytical data?” The question to be answered by the study and 

the sampling must be clearly defined first. Only after the question is defined can a sampling and 

analytical protocol be defined and implemented. 
13

Slide 6

What? Why? When? How?

•What medium to sample – must define the 

target population and how sampled

•Reasons for sampling

•Question(s) to be answered

•Desired degree of statistical confidence in 

the answer(s)

•When to sample – temporal issues

You’ll never know the “true” answer, but you 

can make a reasonable estimate of the truth!!

These questions are critical to be answered prior to the beginning of any sampling 

campaign. Only when these are addressed sufficiently prior to sampling will the samples presented 

to the analytical chemist be of use to answer the appropriate questions with any degree of 

confidence. 
14

Slide 7

This is an example of a stratified outcrop in Utah of mixed lithologies. Does the question 

ask the composition of the entire outcrop, a given layer, or a group of layers? Although this is an 

extreme visual example of outcrop variability and heterogeneity, similar features can occur at all 

scales of an outcrop to the thin section and must be addressed. This stratification also applies to 

soils—what horizon or composited depth sample will answer the defined question? 
15

Slide 8

Although this massive sandstone outcrop from Utah could appear to be homogeneous from 

a distance, scale and sampling issues must be addressed prior to the collection of samples. 
16

Slide 9

For vegetation, one must define which part of the plant is to be sampled—leaves, twigs, 

bark, woody parts, fruit, roots, or a combination? All parts react differently when exposed to given 

elements and will tell a different story. Essential versus non-essential elements of plants respond 

differently in the part samples. Non-essential elements tend to concentrate in the leaves, fruit, and 

seeds since they are usually shed annually. Also, one must: (1) address whether or not to composite 

multiple plants in a given area or multiple plants from multiple areas; (2) consider the prevalence of 

a given species across the study area—the plant (same species, genus) must be present throughout 

the study area for usage; (3) determine whether to sample a native or invasive species or an 

agricultural/domestic species; and (4) be aware of the historical usage of land. In this orchard, for 

example, what type of herbicides and pesticides historically used will affect the soil’s elemental 

composition; for example, As, Hg, or Pb? 
17

Slide 10

Temporal issues must also be addressed. This is especially true with water, soil, and 

vegetation samples. Not only does one need to be aware and consistent of the season the sample 

was collected, but even the time of day will affect the concentration of some analytes in water, as is 

shown in the next slide. 
18

Slide 11

•

•

Seven-day record of flow (A), temperature (B) and Zn (C) on the upper 

Snake River. Periodic injections of a blank and 2000 μg L−1 Zn standard 

provided in situ calibration (Chapin and Wanty, 2005)
19

Slide 12

Is the sample collected at the base of Devil’s Tower (Wyoming) the same material further 

up the tower? One must be very cautious of collecting float (material not attached to the original 

outcrop) and using that to infer the chemistry of the entire formation. The scientist must consider 

the amount of weathering present in a rock sample. Certain elements and mineral phases will 

differentially leach from the original material, causing either concentration or depletion of certain 

elements as weathering occurs.
20

Slide 13

There are many different soils across the USA, as shown in this generic, low resolution soils 

map of the USA from the Natural Resources Conservation Service’s (NRCS) STATSGO2 

Database (http://soils.usda.gov/survey/geography/statsgo/description.html—Soil Survey Staff, 

1999, Soil Taxonomy, A basic system of soil classification for making and interpreting soil 

surveys: United States Department of Agriculture, Natural Resources Conservation Service, 

Agriculture Handbook Number 436). 

For any soil sampling scheme, an investigator is advised to consult with local soil maps and 

local agricultural experts. The type of soil that is predominant in an area will help determine the 

sampling scheme. The type of soil is a direct reflection of the combined effects of local climate 

and soil parent material. 

There are many questions that must be addressed prior to sampling. These questions include 

the following: 

• Is there an average concentration of a given layer, horizon, or volume? Or is a distribution map 

required? 

• Elemental speciation or phase association? 

• Total analysis?

• Required regulations to meet? 

• Reasons for sampling? 

• Desired degree of confidence in the answer(s)?
21

Slide 14

Soil sampling with a 

bucket auger for an 

agricultural soil

What is the type of soil to be analyzed? This is an agricultural soil in eastern Colorado 

being sampled to determine an average elemental content of the field’s plow zone. For this study, 

the question required only one sample for analyses, but this one sample needed to be a composite of 

at least 30 subsamples through the plow zone (top 12 inches) taken from an evenly spaced grid 

placed over the field’s planar dimensions. 
22

Slide 15

A composite sample attained by 

mixing the horizons gives 

different elemental 

concentrations than the 

individual horizons. 

When compositing through a soil profile, take care to equally represent the entire profile 

and collect an appropriate amount from each profile. For example, if the B horizon is half of the 

exposed profile, the final composite sample must contain half the B horizon. Sample bias would 

occur if the texture or composition of a given layer would lead to over or under representation in 

the final composited material. 
23

Slide 16

Are the soils alpine or forest sites? Here is a boreal soil from the Fortymile Mining District 

in east-central Alaska. Questions of what to include in the sample must be addressed. For 

example, defining horizons in these types of soils is often very challenging. Also, how much of the 

organic material should be incorporated into the final sample must be addressed. Commonly, loose 

roots, leaves, twigs, and other surface material are removed prior to compositing for the final field 

sample. Other material may be removed in the laboratory preparation, including rocks and more 

organic material, usually by sieving to a given mesh size (commonly -10 mesh or 

2 mm).
24

Slide 17

Inorganic Trace, Minor, and 

Major Element Composition

• Total

• Extractable (operationally defined)

• Bioavailable/Accessibility

The question of what is to be determined must be addressed prior to any analytical effort. 

Any partial digestion procedure will give different results when compared to a true total digestion. 

A true total digestion implies that the entire sample is dissolved with no material either precipitated, 

suspended, or undigested in the analytical solution. 
25

Slide 18

Variables to Consider When Choosing 

a Method of Analysis

• Qualitative or quantitative information – Must be 

robust!

• Detection limit and precision required?

• Multi-element or single-element determinations?

• What methodology and facilities are available?

• Budget and resources - number of samples in 

what time frame?

• What is the operators’ required skill level for 

operating instrumentation and performing the 

digestions?

There are many variables to consider when choosing an analytical scheme to answer the 

prenominate questions. Again, these questions must be addressed prior to any analytical work. 

Careful forethought prevents unnecessary confusion and work for both the analytical chemist and 

the scientist using the analytical data. A chosen method of analysis should provide data for a set of 

samples above that method’s detection or reporting limit. It usually does no good if the data set has 

a majority of its values for a given analyte as “ND – not detected” or “<X – less than the detection 

limit.”
26

Slide 19

Hurdles to Consider for Quality Analytical Results

• There may be a large and varied concentration range of 

the analytes and concomitant elements of concern.

• Is the analyte a major component of a trace phase 

(arsenic in arsenopyrite) or a trace component in a major 

phase (arsenic absorbed onto clays, oxy-hydroxides, or 

organic material)?

• Digestion method required – partial extraction or total.

• Precision and accuracy required (confidence level).

• Homogeneity of the sample – What sample size is 

required to give the required confidence in the data?

• Pretreatment required – in the field or laboratory.

There are many questions that must be addressed prior to sample collection and analysis. It 

is strongly advised that the scientist consult with the analytical staff prior to sampling and 

submission of samples for analytical work.
27

Slide 20

More Hurdles • Contamination is always a possibility, especially 

for trace analytes

• Interferences for the chosen method – both 

chemical and spectral

• Analyte speciation concerns

• Preservation of the sample; that is, freezing, 

type of sampling container (paper, cloth, or 

plastic bag, amber glass?, Teflon®), acidification 

with which acid, drying in the field

• Transportation of the sample to the laboratory 

and the associated shipping regulations

• Representative sample of the population to be 

studied
28

Slide 21

Yet Even More Hurdles to Consider

• Volatilization of the analyte, such as Hg, As, or Se

• Loss of analyte to the container walls

• Stability of the sample once collected – required 

(legal) holding times

• Collection of more than one sample split in different 

containers for different preservation method at the 

site for different analytical procedures 

• Precipitation of the analyte during the digestion of 

the sample
29

Slide 22

Soil Parameter Measurements – Not 

Discussed, but Important for Some Studies

Salinity Organic Matter Physical 

Properties

Soil gases Organic Species Mineralogy

Soil pH Nitrogen Forms Gypsum

Halogens Cation Exchange 

Capacity

Soil Acidity

Sulfur Forms Redox Potential Isotopes

Carbon Forms Nutrients Saturation Index

Life Forms Radionuclides Alkalinity

There are many analytical measurements pertinent to soil studies that are beyond the scope 

of this report. Many of these measurements are unique to soils for agricultural studies indicating 

soil fertility and viability. Many of these measurements are also used in various environmental 

studies, especially soil pH, isotopic composition (both stable and radioactive), and organic 

constituents. 
30

Slide 23

Sample Collection Questions

• Representative sample

• Sample size collected

• Homogeneity

• Drying, sieving 

• Regolith - Composite or 

individual horizon

• Contamination

(equipment and 

sample)

• ANOVA design

• Containers – Paper, 

glass, plastic?

• Sample splits -

Bulk, Archive, Active

• Preservation 

• Transportation/

- Regulations for DOT? 

- Dept. of Agriculture?

• Customs

Ideally, these issues are addressed before the sampling begins in a collaborative arena 

between the analytical staff and the scientists performing the study.
31

Slide 24

Sampling Concerns (continued)

• Sampling and analytical error 

• Precision requirements (field and 

laboratory)

• Improper collection, including defining 

the target population, sampling 

location, spatial or temporal variation, 

sampling media, sampling tools and 

equipment, and calibration of 

instruments
32

Slide 25

This tongue-in-cheek cartoon is from Rose (1979) and depicts the need for an appropriate 

sample size submitted to the laboratory for analyses. The analyst will always want sufficient 

material to run laboratory duplicates and, at times, spiked samples to test difficult or novel 

matrices. Too large (within reason) is always far better than too small.
33

Slide 26

Sample Preparation Questions

• Are all procedures standardized to allow comparison 

of data between studies of the past, present, and 

future? 

• Drying temperature – heated or ambient?

• Forced air or static conditions?

• Grind, disaggregate, or both?

• Sieving to what size fraction for total analyses and 

partial determinations – same or different?

• Grinding surface of the equipment used –steel, 

agate, tungsten carbide, or ceramic?

The final analytical results can never be better than the preparation of the samples. This 

part of any analytical procedure is critical for the successful analysis and final interpretation of the 

data. A study is quickly devastated by sample contamination, mislabeling, sample mix up, or 

mistreatment, especially by an incorrect choice of grinding media. Agate or ceramic grinding 

surfaces will contaminate with Si, Al, Ca, Mg, and other major elements, but are the preferred 

grinding surfaces when transition and trace metals are the major focus of the study. Many 

agricultural studies will use tungsten carbide or hardened steel when metals are not important to the 

study.
34

Slide 27

The “Soil Juicer,” or more 

formally the mechanical 

soil disaggregator

(disaggregates and then 

sieves at 2 mm).

Agate shatter box is used for very fine grinding. 

Vertical ceramic grinder and Jones Splitter 

The choice of appropriate sample preparation equipment is the first critical step in the 

successful analytical scheme. The appropriate care, maintenance, and cleaning of all equipment is 

critical to the integrity of the analytical sample. For most trace element studies, one should use 

only high quality agate or ceramic grinding surfaces, especially when trace metals are the focus of 

the study. This is critical to avoid the possible contamination of the sample with metals. Common 

contaminants from the use of steel surfaces include Cr, Fe, Ni, Mo, REE and Co. Tungsten carbide 

will contaminate the sample with W, C, and many transition metals. Agate will contaminate with 

Si, but usually the contamination is lost in the samples’ original Si content. This is also true of 

most ceramic material where the Si, Ca, Al, and Na contamination is overpowered by the samples’ 

original content. 
35

Slide 28

Sample Archival Issues

Current working USGS sample 

storage in Building 20, DFC. There 

are also over 1 million samples in 

storage in the USGS long-term 

storage archive.

Issues of sample storage for both current and future use of the samples must be addressed. 

Although cardboard containers are convenient, glass jars with plastic lined metal lids should be 

used if volatile elements are of a concern, especially for Hg sample preservation. Cardboard 

containers do not offer any protection from moisture in humid climates. Common home canning 

jars of various sizes, ranging from 4 oz. up to 64 oz., are usually most suitable, such as Mason® or 

Ball® jars, for long term storage. These types of jars are readily available, lower-cost alternatives 

to more costly laboratory jars.
36

Slide 29

Sample Archival Issues

• Quantity and availability of material stored.

• Temperature of storage – is freezing required?

• Sample preservation – air-dried only or 

treatment (sterilization, autoclaving, freezedried, or irradiation).

• Humidity control

• Container material (Hg, volatile analytes, N, S, 

As, Se forms, organics, biota) – glass with a 

Teflon® – lined lid is optimal for metal species.

• Where? Who pays? How long (holding times)?
37

Slide 30

Many analytical schemes require that solid, and even most liquid, samples undergo some 

type of digestion/pretreatment prior to elemental determination after suitable particle size reduction. 

For most silicate samples, a digestion usually requires the use of strong mineral acids or alkali 

fluxes. Most digestions entail elevated temperatures and sometimes elevated pressures.
38

Slide 31

Total Digestions

• Four-acid digestion under reflux conditions

- Nitric, hydrofluoric, perchloric, and hydrochloric acids 

(may still not be total), under various temperature and 

pressure conditions.

• Alkaline sinter, for example, sodium peroxide 

• High temperature fusion, including sodium carbonate and 

lithium metaborate

• Microwave digestions, with or without hydrofluoric acid

• Aqua regia—especially for sulfide-rich materials—but will 

not always completely attack silicates. Commonly used 

by the USEPA and in European labs.

There are several good references for choosing the proper method of digestion, which are 

given in the reference section of this report. Those shown here is a representative selection of the 

total methods available to the analytical chemist. Each method has its advantages and 

disadvantages. These include specialty equipment, for example, the required perchloric acid fume 

hood for the use of perchloric acid or Teflon® digestion vessel when using hydrofluoric acid; 

analyte volatility; blank contamination; ineffectiveness in dissolving some resistant phases, such as 

rutile, chromite, cassiterite, corundum, and tourmaline; and exclusion of some analytes, for 

example, Li from a lithium metaborate fusion or Na from a sodium carbonate fusion. 
39

Slide 32

Microwave digestion system used 

routinely for biological and 

organic-rich samples.

Multi-acid open vessel digestion on a 

hot plate with ambient pressure at 

elevated temperatures limited to the 

melting temperature of Teflon®. 
40

Slide 33

Extractable Elemental Content • Operationally defined! – usually phase associated

• More aggressive extractions may not be better

• Based on soil characteristic?

- Organic Matter Content, Texture, Color

- Saturation Index, CEC, Soil pH 

- Horizon, Order, Mineralogy, Grain Size

• All or a Subset of the Samples – Topical Studies?

• Exploration or Agricultural Extraction? 

• Traditionally used for Anomaly Enhancement

• Analyze for Total Content First?

• Tend to be costly and time consuming, generating 

multiple solutions for analysis

There are many operational variables that affect the amount of the analyte dissolved in a 

sample. These include the size, shape, and makeup of the digestion vessels; speed and direction of 

mixing; and the combination and concentration of the extracting solution. Great care must be 

observed by the analytical chemist to maintain consistency in all aspects of the analytical scheme.
41

Slide 34

USGS scientist performs a field leach of 

soil and a leach test in the laboratory.

There are both in-field extractions and laboratory extractions. To determine the appropriate 

extraction, all lies in “What is the Question?” Details of the USGS Field Leach test are given in 

Hageman (2007b). 
42

Slide 35

Common Partial Digestions

• EPA 3050B – HNO3-H2O2 for total recoverable 

metals

• EPA 3052 – Microwave equivalent of 3050

• EPA 3005A, 3010A, 3020A – Acid digestion of 

waters and extracts

• TCLP – Leach with acetic acid (pH 5)

• 3 –30% H2O2 - Organic matter

• ABC-DTPA – Plant-available

• Custom-designed to address specific topical study 

– Prolific literature available

• Simulated gastric or lung fluid

There are a wide variety of partial extractions available to today’s scientists. They range 

from very mild, for example, distilled water extraction, to very aggressive, strongly oxidative acidic 

(HNO3 and H2O2). Again, knowing the question will determine which extraction or extraction 

scheme will provide the best information to the scientist.

EPA 3050B: http://www.epa.gov/wastes/hazard/testmethods/sw846/pdfs/3050b.pdf 

EPA 3052: http://www.epa.gov/waste/hazard/testmethods/sw846/pdfs/3052.pdf 

EPA 3005A: http://www.epa.gov/waste/hazard/testmethods/sw846/pdfs/3005a.pdf 

EPA 3010A: http://www.epa.gov/wastes/hazard/testmethods/sw846/pdfs/3010a.pdf 

EPA 3020A: http://www.epa.gov/waste/hazard/testmethods/sw846/pdfs/3020a.pdf 

USEPA TCLP (SW-846): "Test Methods for Evaluating Solid Waste, Physical/Chemical Methods", 

http://www.epa.gov/waste/hazard/testmethods/sw846/index.htm 

ABC-DTPA: Crock and Severson (1980). 
43

Slide 36

An example: Various Selective and 

Sequential Extractions for Selenium • 0.25 M KCl: Soluble forms

• 0.1 M Phosphate: Ligand exchangeable

• 1 M NaOAc, pH 5: Carbonates

• NaOCl, pH 9.5 (boil): Oxidizable (organic matter)

• 0.1 M Hydroxyl amine HCl: Easily reducible oxides (Mn 

predominantly)

• 0.25 M Hydroxyl amine HCl, 0.25 M HCl: Amorphous 

oxides

• 4 M HCl, boil: Crystalline oxides

• 0.5 M NaOH, boil: Alkali soluble Al/Si phases

• Nitric, perchloric, HF acids: Residual 

• Will not usually do all of these digestions on a given 

sample

Commonly, extractions are a single digestion, but there are available extraction sequences 

that will partition analytes into operationally defined phases and occurrences. Chao (1984) 

describes many partial and sequential extraction schemes commonly used.
44

Slide 37

Bioavailability

• Definition? What is to be approximated?

• A good indication of bioavailability is to grow 

the species in question in the material in 

question, and then determine the 

concentration of the element(s) in question.

• Which biota? Human, mammals, 

invertebrates, vegetation?

• Method of introduction of the soil to the biota 

in question? 

- Inhalation, contact, or ingestion?

An increasing number of environmental studies are addressing the effects of geological 

materials on biota. These are typical questions that should be addressed prior to sampling and 

analysis. An excellent overview summary of bioavailability is presented in Smith and Huyck 

(1999). 
45

Slide 38

Bioavailability Measurements

• Complicated and abundant literature, 

especially in the medical literature

• Large range of reagent strength, 

composition, temperature

- Biota whole tissue

- Simple water extraction

- Weak to strong acids

- Simulated lung fluid or gastric juice
46

Slide 39

Concentrations and ranges (mg/Kg) for selected 

trace elements in U.S. soils

Average

U.S. Soils, 

Range

Western 

U.S., grand 

mean

Eastern 

U.S., grand 

mean

As 5 <0.1 - 97 5.5 4.8

Cu 30 <1 - 700 21 13

Hg 0.03 <0.01 – 4.6 0.046 0.081

Pb 10 <10 - 700 17 14

Mo 2 <1 - 15 0.85 0.32

Se 0.3 <0.1 – 4.3 0.23 0.30

Zn 50 <5 - 2900 55 40

This chart is taken from Smith, K.S. and Huyck, H.L. (1999). This chart demonstrates the 

natural wide-concentration range for several important elements in soil studies. The analytical 

method of detection must be robust enough to provide a sufficiently large enough analytical 

reporting range to accommodate this large natural variation. When soils are affected by 

anthropogenic sources, this robust nature becomes even more important. 
47

Slide 40

Popular Instrumental Techniques

• Atomic Absorption Spectroscopy (AAS)

• Inductively Coupled Plasma-Atomic Emission 

Spectroscopy (ICP-AES)

• ICP-Mass Spectroscopy (ICP-MS)

• Atomic Fluorescence Spectroscopy (AFS)

• X-ray Fluorescence Spectroscopy, both 

energy dispersive and wavelength dispersive 

(EDXRF-S AND WDXRF-S)

• Instrumental Neutron Activation Analysis 

(INAA)

These are some of the common analytical spectroscopic methods available to the analytical 

chemist for both routine and research methods. The bottom line in any study is for the method of 

analysis to provide high quality, quantifiable data. If a technique provides data at the “less than” or 

“not detectable” level for the majority—or, in some cases, even one sample—the data set loses 

value as a scientific tool. The analytical technique chosen must provide both accurate and precise 

data at or preferably below the “action level” of a study. So if one were to choose AAS and get 

only “ND” or < data, an alternative technique must be used. A classical reference for instrumental 

analysis is Skoog and others (2006). 
48

Slide 41

I want to hear 

more “Eureka’s” 

and fewer “Ah 

Darn’s” out of 

here!!

Original cartoon by David L. Fey (USGS, Denver, CO). 

Modern instrumentation can be very complex and technically challenging to operate and 

keep at peak performance. This is especially true when dealing with complex matrices common to 

the analyses of geological and environmental materials. The trends in instrumental methods are to 

be multi-elemental, either sequential or simultaneous in determination of a diverse group of 

elements. The prevalent demands are for more and more elements quantified at lower and lower 

concentration levels, often ranging into the sub-μg/Kg level in the sample and even lower in the 

sample digestate—often more than 100 fold less due to dilution. 
49

Slide 42

Advantages and Disadvantages of Common 

Spectroscopic Methods.

Benefit ICP-MS ICP-AES GF-AAS F-AAS

Multi-element + + - -

Qualitative Analysis + + - -

Low detection limits + -/+ +/- -/+ 

Analytical Speed + + - +/-

Precision -/+ + +/- + 

Dynamic Range + + - -/+

Matrix Interferences -/+ + +/- +

Spectral Interferences - - + +

Ease of use +/- + +/- + 

Small volumes -/+ - + -

Purchase Price - -/+ + +

Cost per analysis + + - +

A + would mean this benefit would be a relative advantage for this method, a – would be a 

disadvantage, and a +/- could mean an advantage or disadvantage, depending on the sample and 

digestion solution matrix and the analyte of concern. A technique must be chosen that will provide 

enough sensitivity to obtain quantifiable data for the analytes of interest for most, if not all, samples 

in a study. 
50

Slide 43

Atomic Spectroscopy detection limits (μg/L in 

solution) – single element

Flame 

AAS

HGAAASICPAESICPMSHydride

-AAS

As 150 0.5 30 0.13 0.03

Cu 1.5 0.25 1.5 0.003 --

Hg 300 1.5 30 0.004 0.009

Pb 15 0.15 30 0.001 --

Mo 45 0.2 7.5 0.003 --

Se 100 0.7 90 0.08 0.03

Zn 15 0.3 1.5 0.003 --

The determination of Hg is usually performed using cold vapor (CV) AAS or atomic 

fluorescence spectroscopy (AFS) methodologies, usually with stannous chloride as the reductant. 

Sodium borohydride has been reported in the literature for the reductant, but is not commonly used 

because the background is very noisy. When Hg is determined by AFS, picogram/gram 

concentrations are routinely quantified. Crock (1996) gives an overview of Hg analysis in soils. 

Hageman (2007a) presents a CV-AFS method for Hg in soil. 
51

Slide 44

Atomic Absorption Spectroscopy

• Flame AAS, different flames 

• Graphite furnace AAS, single and multi-channel, 

platform or tube vaporization

• Hydride generation, continuous flow and flow 

injection

• Different background correction methods

• Cold-Vapor AAS for Hg determination

• Abundant application literature and is time-tested 

and proven for more than 50 years

Atomic absorption spectrometry (AAS) became the backbone of many of the environmental 

and geochemical laboratories by the mid-1970s and continues today to be a very important 

technique for the determination of many environmentally important elements. Among the reasons 

for AAS popularity are its relative low purchase price and maintenance cost, abundant literature 

and applications, acceptance by many regulatory agencies, capability of determining about 70 

elements, sensitivity and detection limits that satisfy many studies, speed of analysis, relative 

freedom from interferences, reasonable precision and accuracy, simplicity, and its field portability. 

But AAS also has its limitations. AAS is not useful for determining nonmetals, such as sulfur, or 

refractory elements at useful levels. AAS also tends to be a sequential method of analysis. 

Simultaneous multielement AAS determinations have not yet proven to be practical on a routine 

basis for more than a few elements. 

AAS has been subdivided into four main categories. These categories are based on the 

method of sample introduction and the absorption cell. These include flame - AAS (F-AAS), 

graphite furnace - AAS (GF-AAS), hydride generation - AAS (HG-AAS), both continuous flow 

and flow injection HG-AAS, and cold vapor-AAS (CV-AAS). F-AAS has seen great application 

over the years as the method of choice for trace element analysis, replacing many of the 

colorimetric methods. A sample is first dissolved, diluted to an appropriate concentration level, 

and then aspirated into the flame of the AAS instrument. F-AAS is simple to perform and has few 

spectral interferences and minimal chemical interferences. However, F-AAS is not always 

sensitive enough for the levels of analytes found in environmental samples. 
52

Slide 45

Continuous Flow AAS for Se Cold Vapor AAS for Hg

AAS remains most useful for single element determinations and, 

even after 50 years, it is still a viable method of determining some 

environmentally important elements. 

AAS notes continued: 

For a comparison of detection limits between the four common spectroscopic techniques, 

consult table 1 in the text of this report. To increase sensitivity and to make the analysis of small 

volumes possible, GF-AAS was developed. Here the flame has been replaced with a heated 

graphite tube, and the sample is injected into this tube for a more complete and efficient 

atomization of the sample. GF-AAS, however, tends to be a tedious, slow method. It also suffers 

from more chemical and spectral interferences, often giving the illusion that GF-AAS is to be 

considered more art than science.

For the hydride-forming elements—such as As, Sb, Se, Bi, Sn, and Pb—the introduction of 

continuous flow HG-AAS offered a reduction in both chemical and spectral interferences while 

offering increased automation with the sensitivities of the GF-ASS methods (for example, 

Crock,1986; Crock and Lichte, 1982). In general, for HG-AAS methods an acidic digest of the 

sample is mixed with pre-reducing and/or complexing reagents, it is then reduced to form the 

gaseous hydride of the analyte of concern (usually with sodium borohydride), the gaseous phases 

are subsequently separated from the liquid phases, and, finally, the gases are decomposed in a 

heated quartz cell in the path of an AAS spectrometer. HG-AAS methods tend to be the favored 

method of analysis for As, Sb, and Se in environmental samples due to their lower detection limits 

and relative freedom from interferences. 
53

Slide 46

AAS Advantages and Limitations

• Low purchase and maintenance costs

• Widespread use with abundant time-tested 

methods and well-defined interferences

• Acceptance by regulatory agencies

• Reasonable sensitivity and accuracy/precision

• Simplicity and field portable

• Sequential determination, speed of analysis

AAS notes continued: CV-AAS remains the chosen method of analysis for mercury in most 

matrices. Over the years, many modifications have been made to this method, but it still remains 

the method of choice for environmental samples. The latest innovations in the determination of 

mercury in environmental samples have focused on using atomic fluorescence as the method of 

detection after standard digestion and separation procedures. Mercury both absorbs and fluoresces 

at 254 nm. Atomic fluorescence inherently has a much larger dynamic analytical range and tends 

to be at least two orders of magnitude more sensitive than the CV-AAS method. Cold vaporatomic fluorescence spectrometry (CV-AFS) offers the ability to determine mercury at or below the 

part per trillion levels (ng/L) in water. CV-AFS tends to be relatively interference free when 

compared to the CV-AAS method, but the analyst must be very careful of sample contamination 

and reagent purity. CV-AFS is steadily gaining popularity as the method of choice for Hg 

determinations. Also, gaining acceptance is thermal sample decomposition and subsequent 

determination by AAS or AFS. This method has the distinct advantage of no sample preparation 

and thus no Hg loss due to the digestion procedure. 
54

Slide 47

Inductively Coupled Plasma – Atomic 

Emission Spectroscopy

• Cornerstone technique for most laboratories

• Linear response of 4 –5 orders of magnitude

• About 75% of the elements can be determined

• Spectral interferences well documented; usually 

more than one adequate spectral line for most 

elements is available

• Well documented and mature, established 

technique

ICP-AES has also become a very popular and trusted multi-element method complimentary 

to AAS, especially graphite furnace and hydride generation methods. Since its introduction in the 

early 1970s, inductively coupled plasma-atomic emission spectroscopy (ICP-AES) has become an 

important technique for the analysis of environmental and geochemical materials for their trace, 

minor, and major elemental content. About three-fourths of the common elements can be 

determined by this technique, with lower limits of detection in the range of 0.05 to 50 μg/L. This 

technique is noted for linearity of response, often covering four to five orders of magnitude, and 

relative freedom from matrix effects that often plague other spectroscopic and classical methods. 

Another advantage is that the analyte solution can contain high total dissolved salts. The technique 

offers excellent measurement precision, usually from 1 to 3 percent relative standard deviation, and 

has good accuracy. ICP-AES is a rapid multi-element technique, by which 30 to 40 elements can 

be determined routinely within two to three minutes.

However, ICP-AES is subject to spectral interferences, background shifts, and matrix 

effects. An internal standard, for example, lutetium, can be used to help minimize these problems. 

Inter-element correction factors and background corrections are applied routinely. Further 

corrections are made when an element influences other elements beyond the “normal correction.” 

It is common to not report an element due to the extraordinary interference of an interfering 

element. Proper matching of standard and sample matrices can generally negate matrix effects.

Modern ICP–AES systems generally come in one of two types: radially-viewed plasma or 

axially-viewed plasma. Radially-viewed systems have detection limits ranging from 0.2 to 100 

μg/L (depending on the element), and upper linear ranges as high as 1000 mg/L. Axially-viewed 

systems provide lower detection limits by roughly a factor of ten (0.03 – 10 μg/L); however, there 

is also a ten-fold reduction in the upper linear range to around 100 mg/L for most elements. Some 

systems allow both radial and axial viewing on the same system, and this has some advantages 

where the analytical goal is to improve detection limits, while maintaining the high upper linear 

range ICP–AES is known for. A survey of major ICP–AES instrumentation suppliers, including 

Thermo, Varian (recently purchased by Agilent), and PerkinElmer shows that only Thermo (ICAP 
55

6000 Series) and PerkinElmer (Optima 7000 DV series) offer ICP–AES instruments capable of 

performing axial or radial viewing in the same instrument. The Varian instruments are provided in 

a fixed axial-only or radial-only configuration. During instrument evaluation, laboratory 

management should decide which elements, and the required detection limits and linear ranges of 

said elements, they want to determine by ICP–AES. This would help to narrow the choice of 

instrument and ensure that an instrument is obtained that will fulfill their needs. Based the USGS’s 

experience with the relatively complex nature of geochemical samples and the resulting solutions 

from total rock digestions, it is recommended that laboratory management consider ICP–AES 

instruments that provide some type of spectral deconvolution software to correct for spectral 

overlaps and interferences, rather than an instrument that uses the traditional Interfering Element 

Correction (IEC) method. Both the Varian (720/730-ES) system and the PerkinElmer (Optima 

7000 DV Series) offer spectral deconvolution capabilities. The Varian Fast Automated CurveFitting (FACT) software provides spectral deconvolution capabilities as does the Multi-component 

Spectral Fitting (MSF) software on the PerkinElmer systems. The systems currently offered by 

Thermo (ICAP 6000 Series) only offer traditional IEC corrections where each interfering element 

is run to determine the apparent signal on the analyte of interest, and then a ratio of analyte to 

interfering element signal intensities is used to subtract the interference away from the analyte 

signal. The limitation is that this method cannot easily correct for the possibility that more than one 

interfering element is acting upon an element of interest at the same time or that the interference 

might not be linear with an increase in the concentration of the interfering element. At the USGS, 

we have found the spectral deconvolution software provides much more accurate results in 

complex geochemical matrices. 

ICP–AES instruments can be obtained in a simultaneous or sequential optical configuration. 

It is recommended that laboratory management consider simultaneous ICP–AES systems that can 

determine a large number of elements simultaneously using a single solid-state “chip” detector or 

multiple photomultiplier detectors rather than a sequential configuration where each element is 

determined separately using a single photomultiplier detector. Since most modern ICP–AES 

systems have solid-state (“chip”) detectors of some type, an evaluation should be done that all of 

the elements of interest are covered on the detector chip for the instrument selected or that more 

than one line exists for elements that are often found in a wide range of concentrations. For 

example, one limitation of the PerkinElmer Optima ICP–AES is that there are no lines for iridium 

on the chip. This is an important limitation for labs where iridium is an element of interest. 

Evaluations should also be performed such that signals from elements typically high in a 

laboratory's samples do not “bloom” over into adjacent pixels on the chip for other elements of 

interest or that the detector chip provides some type of anti-blooming protection circuitry. 

Blooming has become less of an issue as this type of detector technology has evolved. 

Finally, applications and service support is a critical feature in the selection of an ICP–AES 

system that should not be overlooked. Laboratory management should evaluate the number and 

experience of the instrument vendor applications and service personnel that support their 

geographic area. The availability of parts and service should the instrument need to be repaired is 

of utmost importance, particularly if there is no backup analytical method or instrumentation in 

place. 
56

Slide 48

ICP-AES Instrumentation

Perkin Elmer Optima—ICP-OES System

ICP-OES 

Instrumentation in 

the laboratory

USGS reporting limits/range for 40 elements by ICP-AES after an acid digestion 

Al 0.005 50% Ga 4 50,000 mg/kg 

Ca 0.005 50% Ho 4 5,000 mg/kg 

Fe 0.02 25% La 2 50,000 mg/kg 

K 0.01 50% Li 2 50,000 mg/kg 

Mg 0.005 5% Mn 4 50,000 mg/kg 

Na 0.006 50% Mo 2 50,000 mg/kg 

P 0.005 50% Nb 4 50,000 mg/kg 

Ti 0.005 25% Nd 9 50,000 mg/kg 

Ag 2 10,000 mg/kg Ni 3 50,000 mg/kg 

As 10 50,000 mg/kg Pb 4 50,000 mg/kg 

Au 8 50,000 mg/kg Sc 2 50,000 mg/kg 

Ba 1 35,000 mg/kg Sn 5 50,000 mg/kg 

Be 1 5,000 mg/kg Sr 2 15,000 mg/kg 

Bi 10 50,000 mg/kg Ta 40 50,000 mg/kg 

Cd 2 25,000 mg/kg Th 6 50,000 mg/kg 

Ce 5 50,000 mg/kg U 100 100,000 mg/kg 

Co 2 25,000 mg/kg V 2 30,000 mg/kg 

Cr 2 50,000 mg/kg Y 2 25,000 mg/kg 

Cu 2 15,000 mg/kg Yb 1 5,000 mg/kg 

Eu 2 5,000 mg/kg Zn 2 15,000 mg/kg 
57

Slide 49

ICP-AES

• Interelement and background 

mathematical corrections applied 

routinely

• RSD’s commonly 1 –3%, accurate results

• Broad-based literature and methods

• General acceptance by many regulatory 

agencies

• Commonly interfaced with hydride 

generation and chromatography systems

Relative standard deviation (RSD) is a measure of the precision of the method. Good 

precision does not imply good accuracy. One can measure the wrong value quite precisely and still 

be wrong. Accuracy is a measure of how close the reported value is to the hypothetical “true” 

value. Many of the problems with ICP-AES determinations have been defined and can be 

corrected for, thus contributing to a more accurate determination. 
58

Slide 50

ICP-AES

• Internal standards are very useful; for 

example, Lu for background, matrix, and 

drift problems

• Axial and radial views of the plasma for 

optimum performance 

• Multielement, simultaneous, or 

sequential analyses

• Very cost-effective technique with high 

sample throughout
59

Slide 51

ICP-MS Instrumentation

ELAN 6000 Quadrupole ICP-MS Nu Instruments MC-HR-ICP-MS

ICP-MS instrumentation is also complimentary to other spectroscopic methods, offering a 

wide detection range for much of the periodic table at sufficiently small limits of determination to 

be useful for many investigations.
60

Slide 52

Inductively Coupled Plasma –

Mass Spectrometry

• Ability to measure isotope abundances of most 

elements between mass 6 (Li) and mass 238 (U).

• Many elements have at least one isotope free of 

isobaric interferences.

• Few molecular interferences and these are well 

known.

• Commonly 7 –8 orders of magnitude analytical 

range (sub ppb – x00 ppm).
61

Slide 53

ICP-MS Quantitative Analysis

• Direct aspiration of digested material

• Gas introduction; for example, hydride 

generation

or cold vapor

• Direct solid analysis by slurry nebulization, 

laser ablation, arc or spark ablation, or direct 

sample insertion into the plasma

• Chromatographic speciation analysis

• Isotope analysis
62

Slide 54

Plasma-Spectrometer Interface

• Cones (orifices) will collect a deposit of Al, 

Ca, and Mg oxides when rock/soil digests 

are analyzed. Need to be cleaned daily.

• ICP-MS will show intensity drift as cones 

become dirty. Need to precondition cones 

at start of day by running the sample 

matrix for a period of time before

calibration.

As with most instruments, there are unique problems with digested geological materials, 

usually from concomitant elements and high dissolved contents of the analytical solution. 
63

Slide 55

Detectors • Large changes in ion counts require use of 

dual detectors (pulse and analog).

• Dual detector response needs to be 

calibrated weekly (we do it daily and is 

manufacturer specific). Must check each 

mass to be measured in samples (do not 

forget to do interfering elements as well).

• Detectors will wear out and will need to be 

replaced every 6 – 18 months, depending 

on usage and sample concentration
64

Slide 56

Benefits of ICP-MS • Rapid multi-element analysis

• Very low detection limits

• Isotopic analysis ( isotope dilution quantitation is 

accurate and cheap )

• Spectral simplicity for mass >84 (must be careful 

at masses <84 due to molecular interferences)

• The REEs have many interference issues and 

many can be overcome with proper isotope 

selection and correction for molecular, usually 

oxide, interferences
65

Slide 57

Nebulizers • Clogging is a concern, so filter samples if 

precipitates are observed (just like ICPAES)

• For routine operation, flow rate must be 

optimized to keep CeO/Ce ratio below 

0.03 (high flow rate is good for sensitivity 

but bad for oxide interferences) or lower 

for REE analyses. With special aerosol 

desolvation systems, the CeO/Ce can be 

reduced to <0.5%.
66

Slide 58

Peristaltic Pump

• Ensures constant flow rate to nebulizer

• Corrects for differences in viscosity of 

samples and standards

• Can be used to add internal standard 

solution to each sample on-line
67

Slide 59

Internal Standards

• Must be an element NOT in the samples

• MUST use internal standards to correct for drift

• Need several to cover mass range. Ratio element to 

internal standard that is +/- 50 mass units away

• Must be similar in the 1st ionization potential

• Can be added to each sample or can be added on-line 

by combining 2 flows 

• We use a solution of 500 ppb 6Li, 20 ppb Rh, 10 ppb Ir

• In, Lu, Tb, Ho, and Bi are commonly used 

• Sc is acceptable for water, but not for rocks or soils
68

Slide 60

Element Mass 

Recommendations

• The most abundant isotope is not always 

the best choice

• Be aware of molecular ion interferences –

combinations of Ar, O, H, Cl, C, REE 

oxides

An in depth study of polyatomic interferences in ICP-MS is presented by May and 

Wiedmeyer (1998).
69

Slide 61

Calibration

• Can use either solution standards or digests of 

rock standards for rock/soil work. We prefer rock 

digests because of matrix match; however, this 

can introduce sample preparation bias into the 

calibration curve. We are currently investigating 

changing to aqueous standards for calibration.

• For water analyses, best to use a combination of 

solution standards and natural water standards.

• For vegetation, we calibrate with solution 

standards and ashed, digested NIST plant 

standards.
70

Slide 62

Daily Performance Check

• After calibration, good idea to run the same 

instrument check solution each day. Helps 

to spot instrument problems if changes are 

seen from one day to the next.

• We use a solution containing 10 ppb of: 

Ba, Cd, Ce, Cu, Mg, Rh, Pb, U in 2% HNO3

• Use the solution as recommended by the 

instrument manufacturer
71

Slide 63

Interferences - Isobaric

• 35Cl and 37Cl form molecular ions that interfere with 

many elements

• ArCl on 75As 40 + 35 = 75

As = As75 – (3.127*(Se77-0.874*Se82))

= As75 –3.127*Se77 + 2.733*Se82

where As75 = signal at mass 75 in counts/sec

Se77 = signal at mass 77 in counts/sec

Se82 = signal at mass 82 in counts/sec

• ClO on 51V 35 + 16 = 51

Avoid using HCl for ICP-MS work whenever 

possible, especially if As, Se, or Cr are analytes 

of interest. Use HNO3.
72

Slide 64

Interferences

High sulfur matrices:

SO on 48Ti 32 + 16 = 48

(we use 49Ti to avoid this interference)
73

Slide 65

Interferences

High carbon matrices (organics):

CO on 28Si 12 + 16 = 28

( we use 29Si )

ArC on 52Cr 40 + 12 = 52

( we measure 52Cr and 53Cr )
74

Slide 66

Interferences

Oxide forming matrices:

• CaO on 56Fe 40 + 16 = 56

(we use 54Fe for waters and 57Fe for rocks/soils)

• MoO on 114Cd 98 + 16 = 114

( not a common problem )
75

Slide 67

Memory Effects

• Some elements have long washout times 

because they adsorb onto the 

nebulizer/spray chamber.

• Elements that form oxy-anions have 

memory effects; for example, W, Mo, Ta, 

and Nb

• Au and Ag also have memory effects. 

Can use 5% HCl between samples to 

rinse out Ag ( forms AgCl2

- complex)
76

Slide 68

Contamination • Because ICP-MS is so sensitive, contamination 

is always a concern.

• We distill (sub-boiling) the HNO3 acid used to 

make standards and calibration solutions and for 

our digestions. Can purchase ULTREX grade 

HNO3

• We use 18 megohm deionized water and plastic 

or TEFLON ® labware.

• Natural water samples are easily contaminated 

with Al, Fe, Zn.
77

Slide 69

ICP-AES vs. ICP-MS • For rock/soil/plant/water analysis, we use ICP-AES for 

major elements (Na, K, Ca, Mg, Ti, Mn, P, Fe, Al ) and 

ICP-MS for trace elements. In water P, Fe, Al are done 

by ICP-MS.

• ICP-MS is especially good for heavy trace elements 

(Cd, Pb, Bi, Th, U, Mo, Rb, Sr, Sn, REE) providing 

interferences are properly accounted for or reduced by 

sample preparation and introduction

• Same dissolution problems as ICP-AES: FeCr2O4, 

ZrSiO4, BaSO4, rutile, corundum, beryl do not dissolve 

completely in acids.

• Must do fusion or sinter for total REE measurements for 

both methods and usually need to separate and 

preconcentrate for ICP-AES

Both ICP methods yield high quality multi-elemental data. The two methods are highly 

compatible, each offering unique strengths and weaknesses. Both methods should be available in 

quality analytical laboratories.
78

Slide 70

Laser-Ablation ICP-MS • In situ analysis using a combination of laser 

technology and ICP-MS.

• Sample is ablated using an UV laser (193, 266, or 

213 nm are the common lasers used)

- Laser diameter can be as small as 10 micron

- Leaves a round flat-bottomed crater in sample, 

depending on the laser and the conditions used

- Some fractionation based on volatility

- Ablated sample is solid particles mostly <1

micron

Must have matrix-matched standards for accurate 

quantification

Complimentary to solution based ICP-MS methods is the laser ablation accessory. LA-ICPMS offers the determination of trace levels of many elements on the solid sample. It too has its 

unique problems and advantages, most in the sampling of the matrix, usually resulting from matrix 

inhomogeneity. 
79

Slide 71

Current USGS Applications of LA-ICP-MS

Geologic

Siderophile trace 

elements in sulfides

Trace elements in 

glasses

Trace elements in 

garnets

Standards development

Methods development

Trace metals in coals

Biological

Trace element contents of:

tree rings

fish otoliths

diatoms

corals

tortoise shells

bones

One of the newest tools in the toolbox of the analytical chemist, LA-ICP-MS has been 

applied to a myriad of novel analytical problems. It has proven to be invaluable for many diverse 

matrices and investigations.
80

Slide 72

X-ray Fluorescence Spectrometry

• WD-XRF and ED-XRF

• ED-XRF is the method of choice if atomic 

number > 26 (Fe)

• Non-destructive, qualitative and quantitative 

determinations for major to trace levels

• Field portable, even hand-held units available

• Highly matrix dependent – fusion of samples 

reduces matrix problems

Another workhorse instrumental method is x-ray fluorescence spectrometry (XRF). There 

are two major methods of detection: wavelength and energy dispersive. Although commonly 

plagued with matrix problems, XRF methods do play an important role in the modern laboratory. 

X-ray fluorescence spectrometry is commonly applied to the nondestructive analysis of 

environmental samples, both in the laboratory and in the field for many major, minor, and trace 

elements. There are basically two different types of X-ray fluorescence analysis: wavelength 

dispersive X-ray fluorescence (WDXRF) and energy dispersive X-ray fluorescence (EDXRF). 

Both techniques entail the excitation of characteristic X-rays within a sample followed by their 

subsequent detection and measurement. WDXRF is very precise and accurate for major and 

selected trace constituents in a silicate matrix. EDXRF finds applications for minor and trace 

element determinations and can be field portable. EDXRF is also a very fast and nondestructive 

qualitative and quantitative tool for measuring samples of unknown composition. It is, however, 

not as precise as some of the other trace element methods. In general, if an element has a lower 

atomic number than Fe (26), WDXRF is the preferred method for its determination; whereas if an 

element has an atomic number larger than Fe, EDXRF is the method of choice. Both methods are 

matrix dependent for their respective sensitivities and detection limits.
81

Slide 73

Lower limit of detection and upper limit of 

calibration for selected elements by EDXRF

Element Lower limit, mg/kg* Upper limit, mg/kg

==============================================

Cr 20 – 4200

Ni 10 – 3000

Cu 10 – 1000

Zn 10 – 1300

Rb 10 – 2000

Sr 10 – 2000

Y 10 – 200

Zr 10 – 2000

Nb 10 – 500

Ba 30 4700

La 30 – 1300

Ce 30 – 500

*Represents the highest LOD observed; limits may vary.

===============================================

EDXRF is a method for the qualitative and quantitative analysis of elemental composition 

in solid or liquid samples. It is based on the instantaneous generation, detection, and measurement 

of characteristic X-rays emitted by the elements in a sample when the sample is bombarded with 

high energy X-rays. This is a nondestructive analytical process that requires little or no sample 

preparation. Spectral-line interferences include line overlap and absorption/enhancement effects 

(matrix effects). The problem of spectral-line overlap is shared by all emission and fluorescence 

methods. Spectral-line overlap results from the incomplete resolution of two or more spectral lines 

or peaks. There are two types of spectral-line overlaps in routine EDXRF analysis. The first is a 

Kβ line from one element overlapping the Kα line from the adjacent, heavier element in the 

periodic table (for example, Ni Kβ overlaps Cu Kα). The second is L-series lines from one element 

interfering with K lines from another element (for example, Ba Lg1 with Cr Kα). All these 

interferences can be removed by peak stripping or peak deconvolution techniques using computer 

algorithms. Matrix effects (absorption/enhancement) occur when radiation emitted by the analyte 

interacts with other components in the sample before it reaches the detector. The effects are 

corrected by a scattered radiation method that has been widely used for routine trace-element 

analysis of various geologic materials. This correction method is based on the fact that the analyteline intensity and Compton-scatter intensity are affected in the same way by differences in mass 

absorption coefficients from one sample to another. Although the scatter line and the analyte line 

intensities vary with the matrix, their ratio is constant over a wide range of matrix compositions. 

Furthermore, if the energy of the Compton-scatter peak lies close to the analyte line, the absorption, 

particle size, packing density, and instrumental effects are more effectively corrected. The matrix 

effect of secondary enhancement is not corrected by this method, but is usually negligible for 

elements with an atomic number greater than 26 (Fe). Because this method is nondestructive, the 

sample can be used for other chemical and instrumental analyses after EDXRF analysis, providing 

a binder has not been used for the preparation of sample disks. 
82

Slide 74

Concentration range for the 10 common rock-forming 

oxides determined by WDXRF after a lithium 

metaborate fusion and LOI (loss on ignition) .

Constituent Range (percent)

SiO2 0.10 – 99.0

Al2O3 0.10 – 28.0

Fe2O3 0.04 – 28.0

MgO 0.10 – 60.0

CaO 0.02 – 60.0

Na2O 0.15 – 30.0

K2O 0.02 – 30.0

TiO2 0.02 – 10.0

P2O5 0.05 – 50.0

MnO 0.01 – 15.0

LOI (925°C) 0.01 – 100
83

Slide 75

Niton* Field Portable X-ray 

Fluorescence Unit 

This is an example of a hand-held XRF device found to be very versatile and useful in field 

assessments.
84

Slide 76

EPA Method 6200 Detection limits for Field Portable 

XRF (FPXRF) for the determination of elemental 

concentrations in soil and sediment, ppm in quartz sand

Sb 40 Fe 60

As 40 Pb 20

Ba 20 Mn 70

Cd 100 Hg 30

Ca 70 Mo 10

Cr 150 Ni 50

Co 60 Cu 50

K 200 Rb 10

Se 40 Ti 50

Ag 70 V 50

Sr 10 Zn 50

Tl 20 Zr 10

Th 10

Sn 60

These are interference free detection limits.

http://www.epa.gov/epawaste/hazard/testmethods/sw846/pdfs/6200.pdf is the web page for 

this method. From this reference: “This method is applicable to the in situ and intrusive analysis 

of the 26 analytes listed below for soil and sediment samples. Some common elements are not 

listed in this method because they are considered ‘light’ elements that cannot be detected by field 

portable x-ray fluorescence (FPXRF). These light elements are: lithium, beryllium, sodium, 

magnesium, aluminum, silicon, and phosphorus. Most of the analytes listed below are of 

environmental concern, while a few others have interference effects or change the elemental 

composition of the matrix, affecting quantitation of the analytes of interest. Generally elements of 

atomic number 16 or greater can be detected and quantitated by FPXRF.” 
85

Slide 77

Instrumental Neutron Activation 

Analysis (INAA)

• Selectively measures radioactive nuclide 

activity from reactions on naturally occurring 

isotopes (gamma ray detection)

• Non-destructive – no dissolution required

• Very matrix dependent

• Multiple analyses required for accurate work

• Requires a nuclear reactor

Instrumental neutron activation analysis (INAA) selectively measures radioactive nuclide 

activity produced by nuclear reactions on naturally occurring isotopes. The activity of the indicator 

radionuclide produced during irradiation is directly proportional to the amount of the element in the 

sample. The analytical determination is made by comparing the induced activity in the sample with 

well-characterized standards activated under identical conditions of neutron flux. The activities of 

the samples and standards are measured using gamma-ray spectroscopy. Gamma-ray radiation 

emitted by a radionuclide is converted into an electrical signal by a semi-conductor detector. A 

multichannel analyzer analyzes the electrical signal. Semiconductor detectors, such as high-purity 

and lithium-drifted germanium detectors are used to exploit their high resolution. Spectra produced 

are analyzed by software that locates peaks, identifies peaks, and calculates the area of each peak. 

Not all elements will be detectable for all matrixes. Samples having unusual matrixes will require 

adjustments to the counting protocol. A minimum of 4 months is required for completion of the 

analysis. The technique is “nondestructive” and the sample may, with some restrictions, be 

analyzed by other methods if the amount of a sample is limited. A lower limit on the concentration 

of an element is calculated by estimating the minimum detectable peak area above the observed 

background using a peak detection criterion used in a peak-fitting algorithm. The minimum 

detectable peak is determined by 3 sigma of the blank. Reporting limits are matrix dependent and 

may be higher for routine analysis. Lower reporting limits are highly matrix dependent. 

Element Lower reporting limit

==========================================

Ba 100 ppm 

Na, Fe, Ni, Sr 10 ppm 

Zn, Rb, Nd 1 ppm 

Cr, Co, Sb, Cs, Ce, Sm, Yb, Th, U 0.1 ppm 

Sc, La, Eu, Tb, Lu, Hf, Ta 0.01 ppm 

==========================================
86

Slide 78

Now that you have a data set, 

how comfortable are you with it?

Just because you have an 

answer, it does not mean that it 

is the correct one!
87

Slide 79

Minimization of Analytical Bias

• Robust, documented, established methods

• Temporally consistent analytical 

methodologies and data archiving

• Limited number of participating analytical 

facilities – greatly prefer only one

• Inter-laboratory comparisons

• Generous usage of SRMs and duplicate real 

samples

The goal of all analytical procedures is to produce data of the highest quality with the least 

amount of error for any given study. 
88

Slide 80

Quality Assurance/Quality Control Issues

• Temporally stable methodologies!

- Critical for a long term program requiring 

a consistent data base

• Precision and accuracy

- In-house SRMs

- Certified SRMs

- Real sample duplicates

- Analyte – Spiked materials

• Frequency of quality assessment materials

• Quantified values of critical analytes in 

SRMs
89

Slide 81

Precise, unbiased,

and accurate

Precise, biased,

and inaccurate

Imprecise, biased,

and inaccurate

Imprecise, unbiased,

and inaccurate

Precision versus Accuracy

The common goal for all chemical analyses is to have data that are precise, accurate, and 

unbiased. The bull’s eye concept depicts this above. Having good precision does not ensure good 

accuracy and vice versa.
90

Slide 82

Quality Assurance / Quality Control

• Standard Reference Materials (SRMs)

– Matrix matched in-house reference materials

• Standard Operating Procedures (SOPs)

• Replicates (laboratory and field)

• Blanks (laboratory and field)

• Spiked samples

• Documentation
91

Slide 83

Significant Figures

• Never represent the measured data to be 

more than what they really are – do not 

promise precision and/or accuracy better 

than what was measured.

• In general, many spectroscopic detection 

methods are at best 2% relative. As a 

result, never report more than three 

significant figures in the raw data or any 

perturbations of that data.

This is a subject that has been discussed at length at numerous USGS Research Chemistry 

Project meetings. The bottom line is that when using analytical data, do not represent data to be 

more than what they really are. For example, the reality is that ICP-AES data are usually only 

precise to +/- 2 percent relative error under normal circumstances. This limit of precision in ICPAES is determined by the noise of the pneumatic nebulization process which is a subject beyond 

the scope of this discussion. Suffice it to say that ICP-AES determinations cannot be more precise 

than the noisiest component in the instrument, and in this case it is the nebulizer. In our laboratory 

we have strongly advocated reporting three significant figures for ICP-AES determinations that are 

above 10 times the detection limit for a given element because rounding all ICP-AES numbers to 

two significant figures can introduce large rounding errors into the data. Consider the following 

example (keep in mind that ICP-AES data are precise to +/- 2 percent relative):

A value of 104 ppm (three significant figures) gets rounded to a value of 100 ppm if one 

rounds to two significant figures. A value of 106 ppm gets rounded to 110 ppm if one rounds to 

two significant figures. The relative difference between 100 ppm and 110 ppm is 10 percent, which 

is 5x more error than was contained in the original data (do not forget that ICP-AES data is precise 

to 2 percent). Therefore, rounding to two significant figures can introduce unnecessary errors into 

the data. It used to be popular to do so because it is a very conservative approach. The best way to 

report the data in the above example would be 104 +/- 2 ppm. However, based on the authors’ 

collective experience of more than 70 years at the chemistry bench, if the analysts were to do this, 

most sample submitters would merely drop the +/- data and use the numerical value alone in a 

spreadsheet. Therefore, everything considered, it is better to report 3 significant figures than it is to 

over round the data. 

Another problem associated with rounding to two significant figures is that is tends to 

"quantize" the data. Have you ever plotted data and noticed that sometimes they look stair-stepped 

at certain concentration levels? Over rounding the data causes this and it causes problems (biases) 

when one performs a linear regression plot of the data. To further illustrate this point, when we 

participate in International round robin laboratory proficiency analyses (for example, GeoPT), the 
92

people who send our USGS laboratories the test samples ask us to report one more significant 

figure than we normally report. Their rationale for this request is precisely for the reason outlined 

above. Therefore, when we report ICP-AES data to sample submitters, we always report three 

significant figures at concentrations above 10x the detection limit.
93

Slide 84

Standard Reference Materials

• Define and monitor accuracy and precision 

– quantitate the quality of the results

• Establish effective working range

• Must be analyzed routinely

• Extensive list of SRMs available for a wide 

range of analyte concentrations in a 

variety of matrices, commonly from the low 

ppb to percent. 

Not enough emphasis can be placed on the liberal usage of SRMs in any given study. Not 

only do they help assess the precision and accuracy of the data, but also elucidate any interference, 

bias, or matrix problems to the analyst. The analysis of homogeneous, well-characterized 

geochemical reference materials provides the backbone for the data quality of the modern 

environmental laboratory. Numerous agencies from around the world have prepared, characterized, 

and issued reference materials in a multitude of rock, ore, soil, sediment, and coal matrix types. 

Despite the more than 380 reference materials that have been generated, the number of 

geochemical reference materials of a specific sample class is limited, as is the number of highquality analyses. Nevertheless, the analysis of reference materials in method validation, instrument 

calibration, and quality control is essential to ensure the reliability of geochemical analyses—this 

fact cannot be emphasized too greatly. Accuracy of an analytical method is both a difficult and 

required measurement in the establishment of an analytical method. To measure accuracy, SRMs 

whose elemental content has been established by several independent methods are essential. 

Standard reference materials are intended as a control to ensure that different laboratories are 

analyzing the same homogeneous, well-characterized material. Several SRMs need to be analyzed 

to establish the complete working range of a method, both in sample matrix composition and level 

of the analyte. Accuracy assessments are required to establish both quality control and quality 

assurance for the data produced. Consult Taylor (1993) for a thorough discussion of the use of 

SRMs.
94

Slide 85

Standard Reference Materials

• Homogeneous, well-characterized 

material, matrix appropriate

• Method and laboratory validation

• Instrument Calibration and Normalization

• Quality Control and Quality Assurance

• Method limitations

Please consult the text portion of this report for additional information and discussion of 

SRMs and their use in environmental studies. 
95

Slide 86

There are many soil SRMs 

available for purchase—both 

from the USGS and many other 

national, international, and 

private sources.
96

Slide 87

Examples of Sources of Certified SRMs

• CCRMP: Canadian Certified Reference Materials 

Project, Canada Center for Mineral and Energy 

Technology, Mines and Resources

• NRCC: National Research Council of Canada, 

Chemistry Division, National Research 

Council

• NIST: National Institute of Standards and 

Technology, Office of Standard Reference 

Materials 

• USGS: United States Geological Survey (Contact 

S.A. Wilson @303-236-2454 or swilson@usgs.gov)

The USGS is a world leader in the development of certified reference materials for the 

determination of many elements in many environmental matrices. Please see the current USGS 

Fact Sheet for the USGS Reference Materials at: http://pubs.usgs.gov/fs/2007/3056/pdf/fs07-

3056_508.pdf. 
97

Slide 88

Selected SRMs for Selenium

(and associated concomitant elements, all 

concentrations as ppm)*

Se As Pb Zn

GSS-7 Soil 0.32 4.8 13.6 142 

MESS-2 Sed 0.72 20.7 21.9 177 

GSS-5 Soil 1.56 412 552 494 

SY-2 Rock 20 17.3 85 248 

SY-3 Rock 30 18.8 133 244

*Govindaraju (1994)

SRMs can be chosen for a study in several ways:

• For a given element over a desired concentration range.

• For a given matrix, such as soils. 

• For a given suite of elements.

• Combination of the above. 
98

Slide 89

Selected Soil SRMs from CCRMP For 

Selected Elements*

As Se Cu Mo Pb Zn Hg

SO-1 2 0.09 61 - 20 144 0.0 21

SO-2 1.2 0.4 8 - 20 115 0.082 

SO-3 2.5 -- 17 - 13 48 0.017

SO-4 7.4 0.49 21 1 14 94 0.032 

*Govindaraju (1994)
99

Slide 90

ICP-MS is the Suggested Approach

• Suitable total digestion must be chosen

-usually multi-acid

-must be highly oxidative to destroy organic material 

quickly to minimize volatility problems

• Multi-element, especially for the trace element 

content

• Applicable for large variation in analyte 

concentrations and matrices

• Usually most applicable for partial digestions 

because of required lower limit of determinations

• Using XRF would compliment ICP-MS for the 

determination of the major elements
100

Slide 91

If ICP-MS is not available • ICP-AES is usually very applicable for 

most elements. ICP-AES may be more 

suitable for the major and minor 

elements.

• Will not have limits of determination at 

crustal abundance or less for some 

critical elements; that is, Mo, Cd, As, 

Pb, REE.
101

Slide 92

Single Element Methods

• Single element determinations may be 

the most appropriate technique; that is, 

Hg by CV-AAS or CV-AFS, Hydride 

Generation-AAS for Se, As, Bi, or Sb.

• Forms of Carbon and Total Sulfur by 

combustion/IR.

• Specific elements for topical studies; 

that is, nitrogen, forms of sulfur, REEs, 

precious metals.
102

Slide 93

Qualitative Information

• ED-XRF, both laboratory-based and 

field methods for elemental content

- Mylar film 

- Pressed pellets, with or without 

binders

• X-ray Diffraction - Mineralogy

• INAA

• ICP-MS elemental scans 

-Laser Ablation

-Solution aspiration
103

Slide 94

An example of a large-scale USGS 

environmental program - NAWQA 

(National Water Quality Assessment)

• Sample analyzed: Wet-sieved in the field, 

<63μ stream-bed sediment from the higher 

order streams of a drainage basin

• Samples are shipped moist and refrigerated 

to a central laboratory

• Samples are dried at ambient temperature 

under forced air

• Dry samples are pulverized with a ceramic 

plate grinder to homogenize
104

Slide 95

NAWQA Analytical Protocol

• 42 elements are determined at or below 

crustal abundance by ICP-MS after a 4-acid 

digestion in an open vessel

• Hg determined by CV-AAS

• Se determined by HG-AAS

• Total S by IR-combustion

• Forms of C (IR combustion and titration)

• QA/QC: For every 40 sample job – 1 

duplicate real sample, 1 stream sediment 

SRM (to match matrix), and 1 stream 

sediment SRM enriched in critical elements to 

ensure detection
105

Slide 96

Conclusions • The sample analyzed should represent the 

question being asked.

• Sensitive, accurate, and precise instrumental 

methods are available for a broad range of 

elemental determinations for use in 

geochemical surveys, including soil and 

environmental. A method of analysis should 

be chosen that will yield quantifiable data.

• The use of appropriate QA/QC assessment 

materials is essential for durable quality 

analyses.
106

Slide 97

Conclusions (continued)

• Total analyses tend to be the least 

expensive and are the most reliable data 

available.

• Total and Speciation/Phase association 

methods are available for environmental 

studies of soils, but the costs may be 

prohibitive for regional soil surveys.

• Resources should be defined up front for 

this study. How much is available to get 

what accomplished in a given time frame? 

How do we prioritize the analytical portion 

of this program?
107

Slide 98

Cartoon from Rose (1979). 
108

Table 1. Atomic Spectroscopy detection limits (ppb or micrograms/liter).

Element F-AAS GF-AAS ICP-AES ICP-MS

Aluminum 45 0.1 1 0.005 

Antimony 45 0.05 2 0.0009 

Arsenic 150 0.05 2 0.0006 

Barium 15 0.35 0.03 0.0002 

Beryllium 1.5 0.008 0.09 0.003 

Bismuth 30 0.05 1 0.0006 

Boron 1000 20 1 0.003 

Bromine 0.2 

Cadmium 0.8 0.02 1.5 0.003 

Calcium 1.5 0.002 0.1 0.00009 

Cerium 1.5 0.0002 

Cesium 15 3200 0.0003 

Chlorine 12 

Cobalt 9 0.15 0.2 0.0009 

Chromium 3 0.004 0.2 0.002 

Copper 1.5 0.014 0.4 0.002 

Dysprosium 50 0.5 0.001 

Erbium 60 0.5 0.0001 

Europium 30 0.2 0.00009 

Gadolinium 1800 0.9 0.0008 

Gallium 75 1.5 0.0002 

Germanium 300 1 0.001 

Gold 9 0.15 1 0.0009 

Hafnium 300 0.5 0.0008 

Holmium 60 0.04 0.00006 

Indium 30 1 0.0007 

Iodine 0.002 

Iridium 900 3 1 0.001 

Iron 5 0.06 0.1 0.0003 

Lanthanum 3000 0.4 0.0009 

Lead 15 0.05 1 0.00004 

Lithium 0.8 0.06 0.3 0.001 

Lutetium 1000 0.1 0.00005 

Magnesium 0.15 0.004 0.04 0.0003 

Manganese 1.5 0.005 0.1 0.00007 

Mercury 300 0.6 1 0.016 

Molybdenum 45 0.03 0.5 0.001 

Neodymium 1500 2 0.0004 

Nickel 6 0.07 0.5 0.0004 

Niobium 1500 5 0.0009 

Osmium 6 

Palladium 30 0.09 2 0.0005 

Phosphorous 75,000 130 4 0.1 
109

Table 1. Atomic Spectroscopy detection limits (ppb or micrograms/liter).—continued

Element F-AAS GF-AAS ICP-AES ICP-MS

Platinum 60 2.0 1 0.002 

Potassium 3 0.005 1 1 

Praseodymium 7500 2 0.00009 

Rhenium 750 0.5 0.0003 

Rhodium 6 5 0.0002 

Rubidium 3 0.03 5 0.0004 

Ruthenium 100 1 1 0.0002 

Samarium 3000 7 0.001 

Scandium 30 2 0.0002 

Selenium 100 0.05 3 0.0007 

Silicon 90 1 10 0.03 

Silver 1.5 0.005 0.6 0.002 

Sodium 0.3 0.005 0.5 0.0003 

Strontium 3 0.025 0.05 0.00002 

Tantalum 1500 1 0.0005 

Tellurium 30 0.1 2 0.0008 

Terbium 900 2 0.00004 

Thallium 15 0.1 2 0.0002 

Thorium 2 0.0004 

Thulium 15 0.6 0.00006 

Tin 150 0.1 2 0.0005 

Titanium 75 0.35 0.4 0.003 

Tungsten 1500 1 0.005 

Uranium 15,000 10 0.0001 

Vanadium 60 0.1 0.5 0.0005 

Ytterbium 8 0.1 0.0002 

Yttrium 75 0.2 0.0002 

Zinc 1.5 0.02 0.2 0.0003 

Zirconium 450 0.5 0.0003 

All detection limits were determined using elemental standards in dilute aqueous solutions 

under optimized conditions for each individual element. All detection limits are based on 

a 98 percent confidence level (3 standard deviations). The actual detection limits observed when 

analyzing environmental samples would be higher due to concomitant elements and various 

interferences, depending on the method of choice. All data from Perkin Elmer, Inc., Waltham, MA, 

“Atomic Spectroscopy: A Guide to selecting the appropriate technique and system” on-line at: 

http://www.perkinelmer.com/Content/Manuals/GDE_InorganicAnalysis.pdf

 F-AAS: Flame Atomic Absorption Spectrometry

GF-AAS: Graphite Furnace Atomic Absorption Spectrometry 

ICP-AES: Inductively Coupled Plasma-Atomic Emission Spectrometry

ICP-MS: Inductively Coupled Plasma-Mass Spectrometry
110

All detection limits are given in micrograms per liter (μg/L) and were determined using elemental 

standards in dilute aqueous solution. All detection limits are based on a 98 percent confidence level 

(3 standard deviations). 

All atomic absorption detection limits were determined using instrumental parameters optimized 

for the individual element, including the use of System 2 electrodeless discharge lamps where 

available. Data shown were determined on a Perkin Elmer AAnalystTM 800. 

All ICP-OES (Perkin Elmer OptimaTM 7000/7300) detection limits were obtained under 

simultaneous multi-element conditions with the axial view of a dual-view plasma using a cyclonic 

spray chamber and a concentric nebulizer.

Cold-vapor mercury detection limits were determined with a Perkin Elmer FIAS-100TM or a Perkin 

Elmer FIAS-400TM flow injection system with amalgamation accessory. The detection limit 

without an amalgamation accessory is 0.2 μg/L with a hollow cathode lamp, and 0.05 μg/L with a 

System 2 electrodeless discharge lamp. (The Hg detection limit with the dedicated FIMS-100 or 

FIMS-400 mercury analyzers is < 0.005 μg/L without an amalgamation accessory and < 0.0002 

μg/L with an amalgamation accessory.) Hydride detection limits shown were determined using an 

MHS-15 Mercury/HydrideTM system. 

GFAA detection limits were determined on a Perkin Elmer AAnalyst 800TM using 50 μL sample 

volumes, an integrated platform and full STPF conditions. Graphite furnace detection limits can be 

further enhanced by the use of replicate injections.

Unless otherwise noted, ICP-MS detection limits were determined using a Perkin Elmer ELAN 

9000TM equipped with RytonTM spray chamber, Type II Cross-Flow nebulizer and nickel cones. All 

detection limits were determined using 3-second integration times and a minimum of 8 

measurements.
111

Acknowledgments

The authors wish to thank the faculty of the Colorado School of Mines for the opportunity 

to present this material to the next generation of geoscientists. The authors also wish to thank Dr. 

Stephan A. Wilson for his insights and words for the SRM section of this report and for his 

guidance of the SRM Program of the USGS. We also thank Drs. Ruth E. Wolf and Todor Todorov 

of the USGS for their insightful reviews of this report. 

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