Spectrographic Analysis 

For Selected Minor 

Elements in Pier re Shale

GEOLOGICAL SURVEY PROFESSIONAL PAPER 391-B
Spectrographic Analysis 

For Selected Minor 

Elements in Pierre Shale

By PAUL R. BARNETT

ANALYTICAL METHODS IN GEOCHEMICAL 

INVESTIGATIONS OF THE PIERRE SHALE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 391-B

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1961
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington 25, D.C.
CONTENTS

Page

Abstract - .. _. ...... ..... .. ..... .... .... . ... . . B- 1

Introduction -- --- ._ ________ __. 1

General remarks - __ _ __ _ ._ __ 1

Material analyzed _ ... .. , -_ __ . __ 1

Methods of analysis .. _ .. __._..-_ _ __ .. 1

Standards _____ ____ _._. _._ __ . 2

Measurement of precision _ _._ 3

Analytical results . ... . . _ __ .. 3

Boron - ---_ .. _- . . __. __ _ ._ _... 3

Barium __-.._. ._........_. ...__.____ _.. ._..._ 4

Cobalt .___..._ _..__ _..........._ _.. 5

Chromium . ._ . .-..... .... .. _. .. .............. 6

Page

Gallium ...__..... ._ _.. .........._-_.___.___.-...__ B- 7

Nickel _ . _ - -.. -

Scandium . ,._. ............

Strontium ..._ _............ . .

Titanium .... .... ...._.. .._..........

Zirconium . _ 

Summary and conclusions .. .._ 

Precision ___ _.._-. 

Comparison with chemical analysis 

Accuracy - .. _ -

References cited . . . ..... .- . .-

7

8

8

8

9

9

9

10

10

10

ILLUSTRATIONS

Page

FIGURE 1. Comparison of determinations of boron in 10 check samples by two spectrographic laboratories ... ....... B- 4

2. Comparison of determinations of barium oxide (converted from barium) in 10 check samples by two

	spectrographic laboratories .._ ..... _._. .. . .-....-... _ _.. -._-.....--_...--...-.--..-.._....-----.----.--.----_--------------------.-- 5

3. Comparison of spectrographic with chemical determinations of barium oxide in 80 samples. . .- 5

4. Comparison of spectrographic with chemical determinations of cobalt in 80 samples .._ ,- 6

5. Comparison of determinations of cobalt in 10 check samples by two spectrographic laboratories _.... . 6

6. Comparison of spectrographic and chemical determinations of chromium in 80 samples _ . - 7

7. Comparison of chromium determinations in 10 check samples by two spectrographic laboratories .... - - 7

8. Comparison of gallium determinations in 10 check samples by two spectrographic laboratories .... ...._ 7

9. Comparison of spectrographic and chemical determinations of nickel in 80 samples ._ . . 8

10. Comparison of determinations of scandium in 10 check samples by two spectrographic laboratories... . 8

11. Comparison of strontium determinations in 10 check samples by two spectrographic laboratories.. .. ... 9

12. Comparison of spectrographic and chemical (hydrogen peroxide method) determinations of titanium

	in 80 samples._ ___. ._.__..____ _________________-__ _ ___ _._ ~_ .._ 9

13. Comparison of determinations of titanium in 10 check samples by two spectrographic laboratories _ 9

TABLES

Page

TABLE 1. Spectrographic determination by laboratory F of selected minor elements in hidden splits .__. . . . B- 3 

2. Spectrographic determinations by laboratories F and G of selected minor elements in check samples _. .. 4

III

ANALYTICAL METHODS IN GEOCHEMICAL INVESTIGATIONS OF THE PIERRE SHALE

SPEGTROGRAPHIG ANALYSIS FOR SELECTED MINOR ELEMENTS IN PIERRE SHALE

By PAUL R. BARNETT

ABSTRACT

Spectrographic determinations were made by one speetrographie laboratory on 80 samples of Pierre shale (Late 

Cretaceous) for 10 elements. The precision of single measurements of these elements, in terms of the coefficient of variation, is: boron, 10 percent; barium, 11; cobalt, 7; chromium, 

9; gallium, 20; nickel, 7; scandium, 10; strontium, 13; titanium, 6; and zirconium, 10 percent. Eight of these elements in 

10 check samples were determined by a second speetrographie 

laboratory. The precision, in percent, between laboratories for 

each indicated element is: boron, 10; barium, 12; cobalt, 16; 

chromium, 11; gallium, 16; scandium, 12; strontium, 10; and 

titanium, 10. As computed from data on all 80 samples, the 

coefficients of variation between determinations by chemical 

and by speetrographie methods are: barium, 34 percent; cobalt, 12; chromium, 10; nickel, 7; and titanium, 10 percent. 

The precision of the first speetrographie laboratory on disguised replicate samples is: boron, 6 percent; barium, 8; cobalt, 10; chromium, 5; gallium, 12; nickel, 5; scandium, 12; 

strontium, 20; titanium, 4; and zirconium, 14 percent.

INTRODUCTION

GENERAL REMARKS

Geochemical investigations of the Pierre shale of 

Late Cretaceous age in the western interior of the 

United States were begun in 1956. The investigations 

seek to relate the chemical and mineralogical composition of a large body of typical marine shale with 

the lithological characteristics and geographic and 

stratigraphic distribution of rock units within the 

shale. The nature of the problem and the results of 

preliminary investigations are described by Tourtelot (written communication).

The data on chemical composition are provided 

by standard and rapid rock analyses, by chemical 

analyses for selected elements occurring in trace 

amounts, and by speetrographie analyses for boron, 

barium, cobalt, chromium, gallium, nickel, scandium, 

strontium, titanium, and zirconium. In order that 

the significance of differences in content of the elements in various samples could be judged, the precision of the different analytical methods was determined. The methods of chemical analyses for selected 

minor elements and their precision have been described by Rader and Grimaldi (1961). The purpose 

of this report is to examine the precision of speetrographie determinations for minor elements in 

samples of Pierre shale and to compare the 

speetrographie determinations with chemical determinations.

MATERIAL ANALYZED

Seventy samples of shale and claystone, bentonite, 

and marlstone from the Pierre shale were collected 

by Harry A. Tourtelot, James R. Gill, and Leonard 

G. Schultz. After drying and preliminary crushing 

(Rader and Grimaldi, 1961), seven of the samples were divided by Tourtelot and Gill into two 

portions, yielding seven additional samples; and one 

sample was divided into four separate portions, 

yielding three additional samples. Each of the 10 

new samples thus obtained was assigned a new and 

different field number, a fictitious field location, and 

a different serial number. The disguised samples are 

referred to as hidden splits and bring the total number of samples analyzed to 80. After the 80 samples 

were analyzed, Tourtelot selected 10 samples believed 

to be representative of the shale samples with regard 

to element concentration and type of material. These 

10 check samples were then analyzed spectrographically by a different spectrographer in a different laboratory for most of the elements that had been 

determined in the 80 samples. The hidden splits and 

10 check samples were also used to obtain data on 

precision of chemical analyses as described by Rader 

and Grimaldi. The speetrographie analyses of the 

hidden splits are shown in table 1 and the speetrographie analyses by two different laboratories of 

the 10 check samples are shown in table 2.

METHODS OF ANALYSIS

The methods used by the U.S. Geological Survey 

for spectrographic analysis are described in detail 

by Bastron, Barnett, and Murata (1960). These 

methods as applied to the analysis of samples of 

Pierre shale are briefly reviewed below.

A 7-gram split, reduced to approximately 200 

mesh, of each of the 80 samples was received for 

analysis (Rader and Grimaldi, 1961). Each sample was prepared for arcing by mixing a 100-mg

B-l
B-2 ANALYTICAL METHODS IN GEOCHEMICAL INVESTIGATIONS OF THE PIERRE SHALE

portion of the sample with 95 mg of pure quartz, 

5 mg of pure sodium carbonate, and 50 mg of pure 

graphite powder. The quartz was added to raise the 

silica content of the material to be analyzed to the 

approximate level of that in the synthetic standards 

used. The sodium carbonate stabilizes the arc and 

the graphite powder tends to reduce selective volatilization during arcing. After thorough grinding 

and mixing in an agate mortar, 25-mg quantities of 

the mixture were weighed into cupped graphite 

electrodes for arcing.

Apparatus and operating conditions

Spectrograph _.._..._..

Wavelength regionPower source __.. 

Slit 

Optics._ .

Emulsion.- . 

Development-_._ 

Arc gap .. .

TransmissionArcing time 

Anode_________

Cathode 

Emulsion calibration

Microphotometer _

....Wadsworth mounted grating, dispersion 5.2 A/mm in first order.

 2250-4750 A, first order.

 Varisource, approx 16 amp d-c arc.

 25 microns.

 Center part of arc focused on collimating mirror.

 Eastman type III-O.

 4 min at 20°C in D-19.

 .4 to 5 mm maintained throughout arcing.

 12, 16, or 20 percent, depending upon 

the sensitivity of the particular set 

of plates being used. Adjusted with 

neutral quartz filters.

 .Sample burned to completion.

 Sample electrode; United Carbon type 

1590.

 High-purity graphite rod, one-eighth 

inch diameter.

 Selected iron lines after method of 

Dieke and Crosswhite (1943) and 

Crosswhite (1950).

._.Nonrecording projection comparator 

microphotometer.

STANDARDS

When the quantitative spectrographic analysis of 

the Pierre shale was started in 1957 it was not the 

practice in laboratory F to photograph the spectra 

of standards on each plate along with the samples 

being determined. Although the advantages of including the spectra of standards were recognized, 

no single set of standards was available that contained more than a minority of the elements usually 

determined in the unknowns. If sets of standards 

containing all the elements to be determined in the 

unknowns were recorded on each plate, little or no 

space was left for the unknowns. Alternatively, when 

each new set of plates (all with the same emulsion 

number) was received from the manufacturer, spectra of standards for all elements ordinarily determined were recorded in duplicate or triplicate on 

separate plates. Working curves for each element 

were prepared by drawing on log-log paper the 

regression line for the intensity of the given element 

line versus the concentration. Spectra of unknowns 

were then recorded on separate plates from the same 

set as that from which the standard plates were 

taken. Even though plate calibrations tend to minimize plate differences, all conditions leading up to the 

finished plate were carefully controlled and made as 

nearly the same as possible for all plates. Unknowns 

were duplicated on separate plates so that if something should go awry with one plate the deviation 

would be detected by comparison with the duplicate 

plate. The 10 elements in all 80 samples of Pierre 

shale were initially determined in this way.

The standards used were prepared by thoroughly 

mixing the carefully weighed pure oxides of the 

elements with a matrix consisting of 60 parts pure 

quartz, 40 parts microcline or perthite, and 1 part 

ferric oxide. In this study such a matrix is called 

pegmatite base. Each constituent of the matrix was 

carefully chosen to insure the absence of the element 

whose standard was being prepared. This requirement made necessary the several sets of standards 

referred to previously.

In 1958 Sol Herman (written communication) of 

the U.S. Geological Survey prepared a single set of 

standards containing all the elements determined in 

the Pierre shale, except barium and strontium. The 

analysis of the 80 samples was repeated for cobalt, 

chromium, nickel, and titanium, but this time the 

spectra of this new set of standards were recorded 

on each plate along with those of the samples. About 

half the samples were duplicated on separate plates 

and half on the same plate. This procedure affords 

an opportunity to study the precision within plates 

and between plates and the comparison of the average of two spectrographic results by each method 

of duplication to results by chemical methods.

By the use of a little logic, the outcome of this 

study might be predicted. Duplicates on the same 

plate should have greater precision because both 

determinations are made from one working curve, 

which is constructed from the standard on the plate. 

This precision does not depend upon correctness of 

the working curve. However, the accuracy of the 

average of the two determinations does depend upon 

the correctness of the working curve and could be 

inaccurate even with good precision. Duplicates on 

separate plates should have the poorer precision, 

since each is determined from the working curve
SPECTROGRAPHIC ANALYSIS FOR SELECTED MINOR ELEMENTS B-3

constructed from its own plate. However, the accuracy of the average of the two determinations should 

be improved, because one of two working curves 

has a greater probability of being constructed correctly than one alone. This difference in accuracy 

should be reflected in the comparison to the chemical 

determinations if the latter are sufficiently close to 

the unknown true value.

Of course, determinations made with standards on 

each plate should be more accurate than those made 

with standards on separate plates, if the standards 

themselves are equally accurate.

MEASUREMENT OF PRECISION

Precision is measured by comparison of duplicate 

determinations of a single sample, comparison of 

the analyses of the hidden splits, and comparison 

with results reported by a second spectrographic 

laboratory on the 10 check samples. In addition, some 

comparisons are made with chemical analyses for 

some elements. The precision and comparisons are 

expressed, in percent, in terms of the coefficient of 

variation, v, which is computed by the formula :

where SD is the standard deviation and x is the 

arithmetic average of the numbers from which the 

standard deviation was computed. The standard 

deviation is a convenient way of expressing the 

precision of measurements of a single quantity, or 

of two or more quantities that are relatively close together. However, the precision of replicate measurements of two or more quantities differing in value by 

even a factor of two, can be better expressed by the 

coefficient of variations, sometimes aptly termed the 

"relative standard deviation."

The laboratories of the U.S. Geological Survey 

that provided data for the study of precision of both 

chemical and spectrographic methods of analysis 

were identified by Rader and Grimaldi (1961) by 

the letter, A to G.

ANALYTICAL RESULTS

The analytical results obtained by the several laboratories are discussed element by element in the 

following sections. The data are presented in the 

tables and figures.

BORON

For the determination of boron a special series of 

standards was prepared by successive dilutions of 

the National Bureau of Standards standard sample 

No. 92 with boron-free pegmatite base. This procedure was believed necessary, because standards 

prepared from boric acid have sometimes been found 

to be unreliable. Standard sample No. 92 is a glass 

containing 0.70 percent boric acid. The standards 

were arced on each plate of samples. All samples 

were arced in duplicate, on one plate or on separate 

plates with approximately equal frequency. Those 

samples whose duplicates did not check closely 

enough were arced again; and if a sample had to 

be repeated for analyzing some other element, the 

boron was redetermined. The reported values for 

boron, and for the other elements, are the arithmetic 

averages of 2 to 6 determinations.

The coefficient of variation for a single determination as computed from the first two boron determinations on each sample is 7.5 percent for duplicates 

on the same plate (49 pairs), 12.5 percent for duplicates on separate plates (32 pairs), and 10.2 percent 

for both methods taken together. The coefficient of 

variation for an average of two determinations can 

be determined by dividing the coefficient of variation 

for a single determination by -\/2, and it is 7.2 percent 

for the 10.2 percent given above. The coefficient of 

variation as computed from the results reported on 

the 10 samples and their hidden splits (table 1) 

is 6.1 percent. This greater precision reflects the 

greater number of determinations used in reporting 

the average.

TABLE 1. Spectrographic determination, in percent, by 

laboratory F of selected minor elements in hidden splits

Sample

259533.... 

259597. . . .

259549.... 

259596.... 

259599.... 

259603....

259553.... 

259604....

259565.... 

259598....

259568.... 

259601....

259580. . . . 

259600. . . .

259582.... 

259602. . . .

259586.... 

259605. . . .

Coefficient 

of variation. ....

B

0.014 

.012

.013 

.012 

.013 

.012

.006 

.007

.008 

.009

.012 

.013

.011 

.010

.012 

.012

.012 

.012

6.1

Co

0.0020 

.0020

.0017 

.0018 

.0019 

.0016

.0016 

.0012

.0014 

.0012

.0020 

.0018

.0017 

.0014

.0034 

.0028

.0015 

.0014

10.3

Cr

0.0050 

.0058

.013 

.012 

.013 

.013

.0098 

.0100

.0090 

.0090

.013 

.012

.011 

.011

.0091 

.0100

.0096 

.011

5.2

Ga

0.0007 

.0008

.0009 

.0012 

.0012 

.0011

.0012 

.0011

.0019 

.0016

.0019 

.0015

.0016 

.0016

.0014 

.0017

.0017 

.0014

12.0

Ni

0.0094 

.0100

.0065 

.0068 

.0070 

.0063

.0057 

.0063

.0036 

.0039

.0042 

.0049

.0038 

.0041

.017 

.016

.0044 

.0042

5.4

Sc

0.0012 

.0015

.0011 

.0018 

.0015 

.0015

.0022 

.0028

.0023 

.0022

.0022 

.0019

.0024 

.0024

.0025 

.0023

.0025 

.0023

12.3

Sr

0.011 

.013

.013 

.017 

.017 

.018

.12 

.10

.012 

.018

.015 

.020

.019 

.018

.0082 

.0087

.014 

.013

20.3

Ti

0.20 

.20

.23 

.23 

.24 

.22

.23 

.22

.33 

.36

.36 

.36

.34 

.35

.34 

.31

.38 

.38

4.2

Zr

0.009 

.011

.014 

.021 

.015 

.015

.015 

.015

.020 

.019

.023 

.019

.018 

.019

.015 

.016

.019 

.017

14.4
B-4 ANALYTICAL METHODS IN GEOCHEMICAL INVESTIGATIONS OP THE PIERRE SHALE

Laboratory G determined boron spectrographically in the 10 check samples. The coefficient of 

variation between the values reported by the two 

laboratories is 9.6 percent as computed from the 

paired results given in table 2. These reported results 

are also compared graphically in figure 1.

0.04-

0.001 0.01

LABORATORY F ANALYSIS, IN PERCENT

FIGUBB 1. Comparison of determinations of boron in 10 check samples by 

two spectrographic laboratories.

The various comparisons are summarized in the 

following table. Boron was not determined chemically.

BARIUM

Barium was analyzed spectrographically by the 

use of the line at 4554 A. At the concentrations in 

the Pierre shale, even with the 1:1 dilution, this 

line is too heavy to measure with a densitometer.

Summary of precision of spectrographic determination of 

boron

Duplicates on 

same plate.....

Duplicates on 

different plates.

Hidden splits, 

laboratory F. . .

Check samples, 

laboratories 

F and G. ......

Number 

of 

comparisons

49

32

13

10

Range of 

concentration 

(percent)

0.0050-0.021

.0030- .024

.0060- .014

.0020- .024

Average 

concentration 

(percent)

0.0114

.0136

.0111

.0138

Standard 

deviation

0.00085

.0017

.00069

.0013

Coefficient 

of 

variatiou

7.5

12.5

6.1

9.6

Analytical curves were prepared by plotting on a 

linear scale the measured width of the lines against 

the concentration of the prepared standard on a logarithmic scale. These curves are straight lines, except 

near their ends. It is imperative that the standards 

be arced on each plate of samples, because the plate 

is not calibrated for this type of measurement.

The precision of a single spectrographic determination was 11.2 percent for samples diluted 1:1 and 

arced on separate plates and was 14.5 percent for 

samples with higher barium content diluted 1:4 

with barium-free pegmatite base and arced on one 

plate. The poorer precision of determinations for the 

samples of greater dilution, even with the greater 

dilution on the same plate and the lesser dilution on 

separate plates, may indicate the greater hazards 

of weighing and thorough mixing for the greater 

dilutions.

The coefficient of variation between the results 

of laboratories F and G in the 10 check samples is 

11.7 percent. (See following table.) These analyses 

are compared graphically in figure 2. The coefficient 

of variation between the spectrochemical analyses

TABLE 2. Spectrographic determinations, in percent, by laboratories F and G of selected minor elements in check samples

Sample

259528...........

259533....... ....

259537...........

259539...........

259546...........

259548.. .........

259549.. .........

259563...........

259592...........

259594...... .....

Coefficient of

E

F

0.004

.014

.009

.014

.018

.023

.013

.014

.013

.016

9.

G

0.002

.010

.009

.014

.020

.024

.013

.015

.013

.019

6

c

F

0.0011

.0020

.0004

.0012

.0024

.0016

.0017

.0004

.0014

.0012

15

0

G

0.0010

.0015

.0

.0010

.0020

.0012

.0013

.0

.0012

.0011

9

Ci

F

0.0060

.0050

.0005

.0060

,0095

.0014

.013

.013

.011

.0095

10

r

G

0.0058

.0047

.0003

.0047

.011

.0010

.013

.016

.010

.0095

6

G

F

0.0006

.0007

.0020

.0016

.0018

.0018

.0009

.0019

.0020

.0018

16

a

G

0.0006

.0

.0024

.0014

.0024

.0012

.0010

.0015

.0023

.0020

1

Si

F

0.0016

.0012

.0008

.0012

.0018

.0005

.0011

.0016

.0025

.0020

12.

z

G

0.0012

.0010

.0009

.0013

.0018

.0007

.0010

.0018

.0020

.0022

3

s

F

0.23

.011

.022

.017

.011

.011

.013

.0073

.010

.017

!9.7

r

G

0.20

.0080

.028

.0092

.011

.010

.010

.0090

.010

.016

F

0.14

.20

.15

.24

.35

.13

.23

.38

.35

.40

9.

Ti

G

0.16

.18

.14

.22

.32

.12

.22

.46

.40

.40

9

i19.6 percent for the determinations that ranee in concentration from 0.0073 to 0.028 percent.
SPECTEOGRAPHIC ANALYSIS FOR SELECTED MINOR ELEMENTS B-5

of laboratory F and the chemical analyses of laboratories C and D in these samples is 33.9 percent.

o.oi

0.01 0.1

LABORATORY F ANALYSIS, IN PERCENT

FIGURE 2. Comparison of determinations of barium oxide (converted from 

barium) in 10 check samples by two spectographic laboratories.

Determination of barium oxide, in percent, in check samples

Sample

259528. ..........

259533. ..........

259537. ..........

259539...........

259546. ..........

259548. ..........

259549...........

259563...........

259592. ..........

259594. ..........

Spectrographic

Laboratory F

0.027 

.038 

.078 

.069 

.075 

.025 

.063 

.046 

.067 

.089

Laboratory G

0.025 

.038 

.083 

.063 

.069 

.022 

.049 

.045 

.054 

.109

Chemical

Laboratory E

0.03

.08 

.07 

.05

.02

.07

Laboratory C

0.00

.00 

.08 

.03

Chemical barium oxide was reported on 25 samples by laboratory C and on 55 samples by laboratory 

E. These 80 analyses are compared graphically in 

figure 3 with the results reported by laboratory F. 

The coefficient of variation between the results of 

laboratory C and those of laboratory F is 32.7 percent, and between the results of laboratory E and 

those of laboratory F is 23.0 percent; spectrographic 

values were converted from barium.

The precision of laboratory F on the hidden splits 

(see the following table) is 8.1 percent. The precision of chemical determination of barium oxide on 

these samples is 33.0 percent.

SPECTROGRAPHIC ANALYSIS (LABORATORY F). IN PERCENT

FIGURE 3. Comparison of spectrographic with chemical determinations of 

barium in 80 samples.

Determination of barium oxide, in percent, in hidden splits 

by laboratories F, C, and E

Sample

259533. .........................

259597..........................

259549..........................

259596..........................

259599..........................

259603..........................

259553. .........................

259604. .........................

259565..........................

259598. .........................

259568..........................

259601..........................

259580..........................

259600. .........................

259582..........................

259602..........................

259586..........................

259605..........................

Spectrographic

F

0.038 

.038

.062 

.054 

.058 

.069

.045 

.047

.090 

.087

.10 

.099

.068 

.078

.054 

.060

.059 

.055

Chemical

C

0.00

.08 

.03

.04

.03

.08

.05

.08

.07

E

0.02

.05 

.05

.04

.05

.09

.07

.05

0.09

COBALT

The precision of the spectrographic determinations for cobalt and a comparison of the spectrographic results of laboratory F to the chemical 

results of laboratory B are given in the following 

table.

Examination of the data shows that precision 

was improved from 9.9 percent to 8.3 percent when 

the standard was on the plate with the samples. 

However, there was not a corresponding improvement in the comparison of spectrographic to chemi-
B-6 ANALYTICAL METHODS IN GEOCHEMICAL INVESTIGATIONS OF THE PIERRE SHALE

Coefficient of variation of cobalt determinations

[Letters in parentheses indicate laboratory furnishing data]

Duplicates on same plate: 

Precision of a single spectrographic

Spectrographic (F) versus chemical (B)

Duplicates on separate plates: 

Precision of a single spectrographic

Spectrographic (F) versus chemical (B)

Standard not 

on plate

9.9

11.4

Standard 

on plate

5.2

11.9

8.3

11.2

cal determinations. Perhaps this is in part due to 

the relatively poor precision of the chemical determinations as shown by Rader and Grimaldi (1961). 

Spectrographic precision is better when the duplicates are on the same plate but the comparison of 

the average of two spectrographic determinations to 

the chemical determination is better when the duplicates are on separate plates.

The spectrographic values reported are the averages of two or more determinations. Approximately 

40 samples were duplicated on the same plate and 

40 samples on separate plates with the standard 

on each plate. The coefficient of variation between 

these values and the reported chemical results is 

11.7 percent. This comparison is made graphically 

in figure 4.

0.003

o.ooi -

0.0004

0.0004 0.001 0.003

SPECTROGRAPHIC ANALYSIS (LABORATORY F), IN PERCENT

FIGURE 4. Comparison of spectrographic with chemical determinations of 

cobalt in SO samples.

On the eight check samples in which the concentration of cobalt was above the limit of sensitivity

for laboratory G (table 2), the coefficient of variation between the results of laboratories G and F 

is 15.9 percent. Figure 5 compares these paired 

values graphically.

o.oi0.001

0001 0.01 

LABORATORY F ANALYSIS, IN PERCENT

FIGURE 6. Comparison of determinations of cobalt in 10 check samples by 

two spectrographic laboratories.

The results by laboratory F on the hidden splits 

are given in table 1. The coefficient of variation for 

these numbers is 10.3 percent.

CHROMIUM

The precision of a single spectrographic determination of chromium computed from duplicates on the 

same plate is 5.9 percent and from duplicates on 

separate plates, 11.6 percent. The precision as computed from the reported results given in table 1 

for the hidden splits is 5.2 percent. Each reported 

value is an average of two or more individual determinations. The coefficient of variation between the 

values given for spectrographic laboratories F and G 

in table 2 is 10.6 percent. These results are graphed 

in figure 6. The comparisons between the results 

reported by laboratories F and B are given in the 

following table and in figure 7.

Comparison of chromium values determined spectrographically 

by laboratory F and chemically by laboratory B

Number of 

comparisons

36..................

33..................

5...................

74..................

Eange of 

concentration 

(percent)

0.010 -0.020

.001 - .010

.0004- .001

.0004- .020

Average 

concentration 

(percent)

0.012

.0073

.00076

.0096

Coefficient of 

variation

9.0

12.1

45.9

10.3
SPECTROGRAPHIC ANALYSIS FOR SELECTED MINOR ELEMENTS B-7

0.02 -

0.01

tn I 

0.003

0.003 0.01 0.02

SPECTROGRAPHIC ANALYSIS (LABORATORY F), IN PERCENT

FIGURE 6. Comparison of spectrographic and chemical determinations of 

chromium in 80 samples.

0.02

0.01 -

0.001

0.001 0.01 

LABORATORY F ANALYSIS, IN PERCENT

0.02

FIGURE 7. Comparison of chromium determinations in 10 check samples 

by two spectrographic laboratories.

National Bureau of Standards standard sample 

No. 98, a plastic clay with a chemical composition 

similar to the Pierre shale, was prepared for arcing 

in the same way as were the shales, and its spectrum 

was recorded in duplicate on each plate of samples. 

The chromium determinations for this sample are 

146, 150, 146, 124, 153, 168, 157, 152, 162, 142, 157,

176, 135 ppm. These determinations indicate a precision of 6.0 percent and, when compared to the certified chromium determination of 144 ppm, a deviation of 7.4 percent for a single determination or 

5.2 percent for an average of two determinations.

GALLIUM

The precision of a single measurement, computed 

from duplicates on separate plates and with standards not on these plates, is 19.6 percent. The results 

of the gallium determinations on the hidden splits 

are given in table 1. The coefficient of variation for 

these values is 12.0 percent. The results reported 

by laboratories F and G on the check samples are 

given in table 2 and compared in figure 8. The coefficient of variation for these determinations is 16.1 

percent. Gallium was not determined chemically.

0.003 -

0.0004

0.0004 0.001 0.003 

LABORATORY F ANALYSIS, IN PERCENT

FIGURE 8. Comparison of gallium determinations in 10 check samples by 

two spectrographic laboratories.

NICKEL

The precision of the spectrographic determinations for nickel and a comparison of the spectrographic results of laboratory F with the chemical 

results of laboratory A are given in the following 

table. Although precision is poorer when duplicates 

are arced on separate plates, the comparison with 

chemical results is better than when duplicates are 

on the same plate. Also having the standard on the 

plate improves the precision of determinations and 

improves the comparison with chemical results even 

more.
B-8 ANALYTICAL METHODS IN GEOCHEMICAL INVESTIGATIONS OF THE PIERRE SHALE

Coefficient of variation of nickel determinations 

[Letters in parentheses indicate laboratory furnishing data]

Duplicates on same plate: 

Precision of a single speetrographic

Spectrographic (F) versos chemical (A)

Duplicates on different plates: 

Precision of a single spectrographic

Spectrographic (F) versus chemical (A)

Standard not 

on plate

11.5

10.5

Standard 

on plate

4.0

8.4

9.3

6.6

The coefficient of variation between the reported 

spectrographic and chemical results for all 80 samples (fig. 9) is 6.7 percent. The average concentration of nickel is 0.0053 percent and ranges from 

0.0010 to 0.016 percent.

SPECTROGRAPHIC ANALYSIS (LABORATORY F), IN PERCENT

FIGURE: 9. Comparison of spectrographic and chemical determinations of 

nickel in 80 samples.

The spectrographic results on the hidden splits 

are given in table 1. The coefficient of variation for 

these determinations is 5.4 percent. Nickel was not 

determined by laboratory G.

SCANDIUM

The precision of a single scandium measurement, 

computed from duplicates on separate plates and 

with standards not on these plates, is 10.0 percent. 

The reported values for the hidden splits are given 

in table 1. The coefficient of variation between the 

results of the two spectrographic laboratories, computed from the data in table 2, is 12.3 percent. These

0.003

CO 

CO

a

a: 

o

0.001

0.0004

0.0004 0.001 0.003 

LABORATORY F ANALYSIS, IN PERCENT

FIGURE 10. Comparison of determinations of scandium in 10 check samples 

by two spectrographic laboratories.

paired values are graphed in figure 10. Scandium 

was not determined chemically.

STRONTIUM

The precision of a single measurement of strontium as determined from duplicate arcings on separate plates is 13.1 percent for 68 samples with an 

average concentration of 0.0137 percent and a range 

of 0.003 to 0.02 percent. The coefficient of variation 

is 24.8 percent for 11 samples with an average concentration of 0.048 percent and a range of 0.02 to 

0.13 percent.

The precision of the determinations on the hidden 

splits (table 1) is 20.3 percent.

The reported results of the two spectrographic 

laboratories on the check samples are given in table 

2. The coefficient of variation for all the paired determinations, graphed in figure 11, is 9.7 percent; but 

it is 19.6 percent for the determinations that range 

in concentration from 0.0073 to 0.028 percent. Strontium was not determined chemically.

TITANIUM

The following table gives the precision of the 

spectrographic determinations for titanium. It is 

significant that precision is better when the standard is on each plate. The poorer precision for duplicates on the same plate is not explained.

The coefficient of variation for laboratory F compared to laboratory C on 25 samples is 10.6 percent;
SPECTROGRAPHIC ANALYSIS FOR SELECTED MINOR ELEMENTS B-9

0.007

0.007 0.01 0.03 

LABORATORY F ANALYSIS, IN PERCENT

FIGURE 11. Comparison of strontium determinations in 10 check 

by two spectrographic laboratories.

Coefficient of variation of spectrographic determinations for 

titanium

Duplicates on sai 

Duplicates on dii

Standard not 

on plate

9.9

Standard 

on plate

5.9 

5.0

for laboratory F compared to laboratory D on 55 

samples, 10.1 percent; for laboratory F compared 

to laboratories C and D combined (the peroxide 

method), 10.2 percent; and for laboratory F compared to laboratory B (the tiron method), 13.9 percent. Comparisons of determinations of laboratory F 

to those of C and D combined are shown graphically 

in figure 12.

The results of laboratories F and G on the check 

samples are given in table 2 and compared in figure 

13. The coefficient of variation for these results is

9.9 percent.

The precision of titanium determinations on the 

hidden splits (table 1) is 4.2 percent.

ZIRCONIUM

The precision of a single zirconium determination, 

computed from duplicates on the same plate and 

with the zirconium standard not on the plate, is

10.0 percent. Zirconium was not replicated in any 

other way. The coefficient of variation on the hidden 

splits (table 1) is 14.4 percent. Zirconium was not 

determined chemically or by laboratory G.

SPECTROGRAPHIC ANALYSIS (LABORATORY F), IN PERCENT

FIGURE 12. Comparison of spectrographic and chemical (hydrogen peroxide) 

method of determination of titanium in 80 samples.

0.1

LABORATORY F ANALYSIS, IN PERCENT

FIGURE 13. Comparison of determinations of titanium in 10 cheek samples 

by two spectrographic laboratories.

SUMMARY AND CONCLUSIONS

PRECISION

The precision of a single measurement of each of 

the elements by the various methods of duplication 

are summarized in the following table. In general, 

the best precision results when the duplicates and 

standards are on the same plate. The poorest preci-
B-10 ANALYTICAL METHODS IN GEOCHEMICAL INVESTIGATIONS OF THE PIERRE SHALE

Precision, in terms of the coefficient of variation, of a single 

spectrographic measurement in the Pierre shale and in 

miscellaneous igneous rocks

Duplicates on separate plates,

Duplicates on same plate,

Duplicates on separate plates, 

standards on plate. ........

In igneous rocks, from 

Bastron and others (1960) . .

B

71?

12.5

Ba

11.2 

15.6

Co

9 9

1?«

8.3 

9.5

Cr

1? 91

11.6 

16.8

Ga

19.6

12.4

Ni

11.5 

4 0

9.3 

10.6

Sc

10.0

14.4

Sr

13.1

17.4

Ti

9.9

5 9

5.0 

6.9

Zr

10.0

10.2

sion results when duplicates are on separate plates 

and standards are on a third plate.

This table also gives the precision of single measurements of elements in igneous rocks computed 

from the precision of the average of two determinations (Bastron, Barnett, and Murata, 1960). These 

data were obtained in large part from duplicates on 

separate plates and standards on a third plate. The 

precision of the measurement of the elements in 

the Pierre shale, when computed from duplicates 

on separate plates and with standards not on these 

plates, is comparable to that for igneous rocks. The 

poorer precision for gallium is probably due to the 

lower range of concentration of gallium in the 

Pierre shale (near limit of sensitivity in the diluted 

sample) than in the igneous rocks analyzed.

The precision, in percent, as determined from 

the results reported on the hidden splits is: boron, 

6.1; barium, 8.1; cobalt, 10.3; chromium, 5.2; gallium, 12.0; nickel, 5.4; scandium, 12.3; strontium, 

20.3; titanium, 4.2; and zirconium, 14.4.

COMPARISON WITH CHEMICAL ANALYSIS

Computed from the data on all 80 samples and 

given in percent, the coefficients of variation between 

determinations by chemical and by spectrographic 

methods are: barium, 34; cobalt, 12; chromium, 10; 

nickel, 7; and titanium, 10. Only the data for cobalt 

and nickel are complete enough to make comparisons 

with the chemical results with respect to the three 

variables in the spectrographic method. These data 

are given in the following table. The improvement 

in the comparison of nickel with each successive 

change in the spectrographic method might also be

Comparison of spectrographic results with chemical results, 

in terms of the coefficient of variation

Cobalt

9.9

11.9

11.2

Nickel

10.5

8.4

6.6

expected to hold true for cobalt, because these two 

elements can be analyzed spectrographically with 

equal facility. The fact that there is no improvement 

in the comparison of cobalt may be a reflection of 

the relatively poorer precision of the chemical determination of cobalt as contrasted to that of nickel 

(Rader and Grimaldi, 1961).

ACCURACY

Because no shale has been certified as a standard, 

it is not possible to draw conclusions about the accuracy of the spectrographic determination of minor 

elements in the Pierre. However, the comparison of 

the nickel determinations with the chemical determinations as set forth above probably is not an 

unreasonable estimate of the accuracy expected for 

most of the elements.

One certified standard, National Bureau of Standards sample No. 98, with a chemical composition 

comparable to that of shale, was repeatedly analyzed 

for chromium. The deviation from the certified value 

of an average of two determinations is 5.2 percent.

REFERENCES CITED

Bastron, Harry, Barnett, P. R., and Murata, K. J., I960, 

Method for the quantitative spectrochemical analysis of 

rocks, minerals, ores, and related materials by a powder 

d-c arc technique: U.S. Geol. Survey Bull. 1084-G, p. 

165-182.

Crosswhite, H. M., 1950, Photoelectric intensity measurements 

in the iron arc: Spectrochimica Acta, v. 4, p. 122.

Dieke, H. G., and Crosswhite, H. M., 1943, Use of iron lines 

as intensity standards: Optical Soc. America Jour., v. 33, 

p. 425-434.

Rader, L. F., and Grimaldi, F. S., 1961, Methods of chemical 

analysis for selected minor elements in Pierre shale: U.S. 

Geol. Survey Prof. Paper 391-A.

 & U. 8. GOVERNMENT PRINTING OFFICE: 1961 596897