THERMOFISHER SCIENTIFIC QUANT'X ANALYSIS AND INSTRUMENTATION

The following is the analytical trajectory used in the EDXRF analysis of archaeological and source standard obsidian in the EDXRF lab at Berkeley and now Albuquerque. 

The ThermoFisher Scientific Quant'X EDXRF spectrometer methods:                    

All archaeological samples are analyzed whole. The results presented here are quantitative in that they are derived from "filtered" intensity values ratioed to the appropriate x-ray continuum regions through a least squares fitting formula rather than plotting the proportions of the net intensities in a ternary system (McCarthy and Schamber 1981; Schamber 1977). Or more essentially, these data through the analysis of international rock standards, allow for inter-instrument comparison with a predictable degree of certainty (Hampel 1984).            

The spectrometer is equipped with a peltier air cooled solid-state Si(Li) X-ray detector, with an ultra-high-flux end window bremsstrahlung rhodium (Rh) x-ray target with a 125 micron beryllium (Be) window and an 8.8 mm collimator (3.5 mm for samples < 10mm diameter), an x-ray generator that operates from 4-50 kV/0.02-1.0 mA at 0.02 increments, using an IBM PC based microprocessor and WinTraceTM 7.0 software.  The spectrometer is equipped with a 2001 min-1 Edwards vacuum pump for the analysis of elements below titanium (Ti).  Data is acquired with a pulse processor and analog to digital converter.  This is a significant improvement in analytical speed and efficiency beyond the former Spectrace 5000 and QuanX analog systems (see Davis et al. 2011; Shackley 2005).

TRACE ELEMENT ANALYSES

For Ti-Nb, Pb, Th elements the mid-Zb condition is used operating the x-ray tube at 30 kV, using a 0.05 mm (medium) Pd primary beam filter in an air path at 200 seconds livetime to generate x-ray intensity Ka1-line data for elements titanium (Ti), manganese (Mn), iron (as FeT), cobalt (Co), nickel (Ni), copper, (Cu), zinc, (Zn), gallium (Ga), rubidium (Rb), strontium (Sr), yttrium (Y), zirconium (Zr), niobium (Nb), lead (Pb), and thorium (Th).  Not all these elements are reported since their values in many volcanic rocks is very low. Trace element intensities were converted to concentration estimates by employing a least-squares or quadratic calibration line ratioed to the Compton scatter established for each element from the analysis of international rock standards certified by the National Institute of Standards and Technology (NIST), the US. Geological Survey (USGS), Canadian Centre for Mineral and Energy Technology, and the Centre de Recherches Pétrographiques et Géochimiques in France (Govindaraju 1994). Line fitting is linear (XML) for all elements but Fe where a derivative fitting is used to improve the fit for iron and thus for all the other elements.  When barium (Ba) data are acquired, the Rh tube is operated at 50 kV and .25-1.0 mA in an air path at 200 seconds livetime to generate x-ray intensity Ka1-line data, through a 0.630 mm Cu (thick) filter ratioed to a portion of the bremsstrahlung region (see Davis et al. 2011).  Further details concerning the petrological choice of these elements in Southwest obsidians is available in Shackley (2005; also Mahood and Stimac 1991; and Hughes and Smith 1993). A suite of 19 specific standards used for the best fit regression calibration for elements Ti- Nb, Pb, ,Th, and Ba include G-2 (basalt), AGV-2 (andesite), GSP-2 (granodiorite), SY-2 (syenite), BHVO-2 (hawaiite), STM-1 (syenite), QLO-1 (quartz latite), RGM-1 (obsidian), W-2 (diabase), BIR-1 (basalt), SDC-1 (mica schist), BCR-2 (basalt), TLM-1 (tonalite), SCO-1 (shale), NOD-A-1 (manganese), NOD-P-1 (manganese) all US Geological Survey standards, NIST-278 (obsidian) National Institute of Standards and Technology, BR-N (basalt) from the Centre de Recherches Pétrographiques et Géochimiques in France, and JR-1 and JR-2 (obsidian) from the Geological Survey of Japan (Govindaraju 1994).

            The data from the WinTrace software are translated directly into Excel for Windows software for manipulation and on into SPSS for Windows for statistical analyses when necessary. In order to evaluate these quantitative determinations, machine data were compared to measurements of known standards during each run (Table 1).    RGM-1 is analyzed during each sample run for obsidian artifacts to check machine calibration.  Other appropriate standards from the above list are used for other volcanic rocks.

DETECTION LIMITS

Detection limits for EDXRF have been calculated at 6 Σ here

"Matrix" issues in the analysis of standardsRecently, a number of scholars have questioned the validity of using pressed powder pellets of international standards for empirical calibration and data checking, called a "matrix issue" by some questioning the potential differing analytical results between pressed powder and whole rock samples (Jeff Ferguson, personal communication, 2010; cf. Shackley 2011a).  This potential problem was tested in detail in Kathleen Davis et. al.'s (1998, reprinted 2011:45-64) study of EDXRF on obsidian and found to not be an issue (see also Shackley and Hampel 1992).  Hermes and Ritchie (1997) derived similar conclusions using EDXRF with archaeological felsites. Table 1 exhibits the analysis of one of my pressed powder pellets of USGS RGM-1 and a flake from the same USGS collected boulder sent by Steve Wilson of the USGS.  As one can see, the variability is within 1% or less in most cases, and some of this variability is inherent in the native variability within the single 200 kg boulder collected at Glass Mountain by USGS, not differing matrices.  This issue is well examined by Ron Jenkins and seems to be misunderstood in the discipline (c.f. Jenkins 1999:167-172).  While it is true that his discussion hinges on the assumption of homogeneity, one must assume that for purposes of non-destructive EDXRF that pressed powder standards are heterogeneous while obsidian is homogeneous.  This is, indeed, not the case for obsidian studies in general.  Jenkins explains:

Primary absorption occurs because all atoms of the specimen matrix will absorb photons from the primary source.  Since there is a competition for these primary photons by the atoms making up the specimen [pressed powder versus glass], the intensity/wavelength distribution of these photons available for the excitation of a given analyte element may be modified by other matrix effects (Jenkins 1999:168; emphasis mine).

In the case of pressed powder pellets, the "competition" involves air molecules locked in the powder matrix that is not analyzed in obsidian studies and of course not present in volcanic glass, particularly with XRF and ICP-MS.  While it is true that the borate mix used in the pressed powder can yield some boron, this element is undetectable by XRF and so light (Z=5) as to not cause peak overlap, especially when using any tube filtering.  Perhaps more important is the issue of infinite thickness, particularly when Z≥51 and high tube voltages are required.  In this case, some of the x-rays are radiated through the sample, and ratioing to the Compton or bremsstrahlung regions can yield complex results, as can be seen in the Table below for Ba, and evidently a problem for other analysts as well as seen by the GeoRem values in Table 1.  In this case, careful calibration can mitigate some of this error.  Parenthetically, only recently have PXRF products been able to reach 50 kV in order to adequately move electrons out of orbit for those atoms Z≥51, despite opinion to the contrary (see Jenkins 1999:9-12). 

As I've noted in print and in discussions with many of my colleagues, not using international standards, as is often seen in PXRF studies, severely hinders the potential for establishing validity in obsidian research - it is nearly impossible to compare results across laboratories (Shackley 2010, 2011a; Speakman and Shackley 2013).  Using the "matrix" issue as an excuse is unacceptable and just spitting in the wind.

Table 1. X-ray fluorescence concentrations for selected trace elements of USGS RGM-1 pressed powder pellet (n=49 runs), and whole rock flake from original USGS boulder (n=12). ± values represent first standard deviation computations for the group of measurements. All values are in parts per million (ppm) as reported in Govindaraju (1994), USGS,  and this study. RGM-1 is a U.S. Geological Survey obsidian standard obtained from Glass Mountain, Medicine Lake Highlands Volcanic Field, northern California.  Pressed powder and whole rock elemental analysis of USGS RGM-1 obsidian standard on the ThermoScientific Quant'X and USGS, Govindaraju (1994), and GeoRem recommended values.

 

SAMPLE

Ti

Mn

Fe

Rb

Sr

Y

Zr

Nb

Ba

Pb

Th

RGM-1 (Govindaraju 1994)

1600

279

12998

149

108

25

219

8.9

807

24

15.1

RGM-1 (USGS recommended)1

1619±120

279±50

13010±210

150±8

110±10

252

220±20

8.9±0.6

810±463

24±3

15±1.3

RGM-1, pressed powder standard (this study, n=99)

1523±49

294±13

13723±30

149±2

108±2

25±2

219±2

9±2.0

793±13

21±1.6

17±3.2

                                                                                                                                                                           1 Ti, Mn, Fe calculated to ppm from wt. percent from USGS data.

                                                                                                                                2 information value

                                                                                                                                                                           3 n=20

SAMPLE

Ti

Mn

Fe

Rb

Sr

Y

Zr

Nb

Ba

Pb

Th

RGM-1 (Govindaraju 1994)

1600

279

12998

149

108

25

219

8.9

807

24

15.1

RGM-1 (USGS recommended)1

1618±120

279±50

12998±210

150±8

110±10

252

220±20

8.9±0.6

810±46

24±3

15±1.3

GeoRem values2 1510-1940

282

12788-14263 142-164 96.73-116 21.6-25.1 173-258 8.37-13 791-881 18-28.4 14-16.3
RGM-1, pressed powder (this study, n=49)

1540±44

285±10

13718±30

149±3

108±2

24±2

218±2

9±2

869±61

26±2.1

16±3

RGM-1, flake from original USGS boulder (n=12)

1568±44

311±11

13306±33

153±2

113±2

25±1.5

230±4

9±2

942±14

23±1.5

15±3.4

                                                1 Ti, Mn, Fe calculated to ppm from wt. percent from USGS data.

                                    2 information value

 

Table 2.  X-ray fluorescence concentrations for selected trace elements of NIST-278 pressed powder pellet (n=35 runs, 7 for Ba). ± values represent first standard deviation computations for the group of measurements. All values are in parts per million (ppm) as reported in Govindaraju (1994). NIST,  GeoRem and this study. NIST-278 is a National Institute of Standards and Technology obsidian standard obtained from Clear Lake, Newberry Crater, Oregon. This is a favored standard by Mike Glascok at Missouri University Research Reactor Facility when analyzing obsidian with NAA (Glascock 1991, and personal communication).

SAMPLE

Ti

Mn

Fe

Rb

Sr

Y

Zr

Nb

Ba

Pb

Th

NIST SRM-278 (Govindaraju 1994)

1469

403

14269

127.5

63.5

39

290

18

1140

16.4

12.4

NIST SRM-278 (NIST recommended)1

1469±5

403±2

14269±140

127.5±0.3

63.5±0.1

n.r.

n.r.

n.r.

1140

16.4±0.2

12.4±0.3

GeoRem values2 1170-1469 325.5-401 11710 - 149000 105 - 137 30.2 - 67 35 - 41 211.4 - 287 21.4 - 22 890 - 1072 15.45 - 17.2 11.76 - 13.3
MURR-NAA (Glascock 1991) 1420±70 401±26 14100±16 126±2 67±3 nm 272±31 nm 887±16 17 11.71±0.13
NIST-278, pressed powder (this study, n=35)

1454±39

383±7

14329±37

130±2

67±1

40±2

276±2

15±2

870±29

23±2

15±5

                                                 n.r. = not reported 

                                                nm=not measured                                     

                                                 1 Ti, Mn, Fe calculated to ppm from wt. percent from NIST data.

                                    2 GeoRem values are measurements and compilations from various instruments, NAA, XRF, ICP-MS, PIXE-PIGME

To MURR's oxide to element and element to oxide multiplier table (from Glascock 1991)

MAJOR OXIDE ANALYSES

Analysis of the major oxides of Si, Al, Ca, Fe, K, Mg, Mn, Na, and Ti is performed under the multiple conditions elucidated below.  This fundamental parameter analysis (theoretical with standards), while not as accurate as destructive analyses (pressed powder and fusion disks) is usually within a few percent of actual, based on the analysis of USGS RGM-1 obsidian standard (see also Shackley 2011a).  The fundamental parameters (theoretical) method is run under conditions commensurate with the elements of interest and calibrated with 11 USGS standards (RGM-1, rhyolite; AGV-2, andesite; BHVO-1, hawaiite; BIR-1, basalt; G-2, granite; GSP-2, granodiorite; BCR-2, basalt; W-2, diabase; QLO-1, quartz latite; STM-1, syenite), and one Japanese Geological Survey rhyolite standard (JR-1).   See Lundblad et al. (2011) for another set of conditions and methods for oxide analyses.

CONDITIONS OF FUNDAMENTAL PARAMETER ANALYSIS1

 Low Za (Na, Mg, Al, Si, P)

      Voltage                   6  kV                                     Current                  Auto2

      Livetime                100  seconds                           Counts Limit         0

      Filter                      No Filter                                  Atmosphere           Vacuum

      Maximum Energy 10  keV                                  Count Rate            Low  

Mid Zb (K, Ca, Ti, V, Cr, Mn, Fe)

      Voltage                 32  kV                                    Current                  Auto

      Livetime                100  seconds                           Counts Limit         0

      Filter                      Pd (0.06 mm)                          Atmosphere           Vacuum

      Maximum Energy 40  keV                                  Count Rate            Medium     

High Zb (Sn, Sb, Ba, Ag, Cd)

      Voltage                 50  kV                                    Current                  Auto

      Livetime                100  seconds                           Counts Limit         0

      Filter                      Cu (0.559 mm)                        Atmosphere           Vacuum

      Maximum Energy 40  keV                                  Count Rate            High     

Low Zb (S, Cl, K, Ca)

      Voltage                   8  kV                                     Current                  Auto

      Livetime                100  seconds                           Counts Limit         0

      Filter                      Cellulose (0.06 mm)                Atmosphere           Vacuum

      Maximum Energy 10  keV                                  Count Rate            Low     

1 Multiple conditions designed to ameliorate peak overlap identified with digital filter background removal, least squares empirical peak deconvolution, gross peak intensities and net peak intensities above background. 

2 Current is set automatically based on the mass absorption coefficient.

 

REFERENCES CITED

 Davis, M.K., T.L. Jackson, M.S. Shackley, T. Teague, and J. Hampel (new introduction by M.S. Shackley)

 2011  Factors Affecting the Energy-Dispersive X-Ray Fluorescence (EDXRF) Analysis of Archaeological Obsidian.  In In M.S. Shackley (Ed.) X-Ray Fluorescence Spectrometry (XRF) in Geoarchaeology, pp. 45-64.  Springer Publishing, New York.

 Glascock, M.D.

  1991    Tables for Neutron Activation Analysis.  3rd edition.  Research Reactor Facility, University of Missouri, Columbia.

Govindaraju, K.

  1994  1994 Compilation of Working Values and Sample Description for 383 Geostandards.  Geostandards Newsletter 18 (special issue). 

 Hampel, Joachim H.

1984    Technical Considerations in X-ray Fluorescence Analysis of Obsidian.  In   Obsidian Studies in the Great Basin, edited by R.E. Hughes, pp. 21-25.  Contributions of the University of California Archaeological Research Facility 45.  Berkeley. 

Hermes, O.D., and D. Ritchie

1997    Nondestructive trace element analysis of archaeological felsite by energy-dispersive x-ray fluorescence spectroscopy. Geoarchaeology 12:31-40.

Jenkins, Ron

  1999    X-Ray Fluoescence Spectrometry, 2nd edition.  Wiley-Interscience, New York.

Lundblad, S.P., P.R. Mills, A. Drake-Raue, and S.K. Kikiloi

  2011  Non-destructive EDXRF Analyses of Archaeological Basalts. In In M.S. Shackley (Ed.) X-Ray Fluorescence Spectrometry (XRF) in Geoarchaeology,    pp. 65-80.  Springer Publishing, New York.

McCarthy, J.J., and F.H. Schamber

1981    Least-Squares Fit with Digital Filter: A Status Report.  In Energy Dispersive X-ray Spectrometry, edited by K.F.J. Heinrich, D.E. Newbury, R.L. Myklebust, and C.E. Fiori, pp. 273-296.  National Bureau of Standards Special Publication 604, Washington, D.C. 

Schamber, F.H.

1977    A Modification of the Linear Least-Squares Fitting Method which Provides Continuum Suppression.  In X-ray Fluorescence Analysis of Environmental Samples, edited by T.G. Dzubay, pp. 241-257.  Ann Arbor Science Publishers. 

Shackley, M. Steven

  2005 Obsidian: Geology and Archaeology in the North American Southwest.  University of Arizona Press, Tucson. 

 2010  Is there reliability and validity in portable x-ray fluorescence spectrometry (PXRF)?  The SAA Archaeological Record, Nov. 2010, pp. 17-18&20.

  2011a An introduction to X-ray fluorescence (XRF) analysis in archaeology. In M.S. Shackley (Ed.) X-Ray Fluorescence Spectrometry (XRF) in Geoarchaeology, pp. 7-44.  Springer Publishing, New York .

  2011b  GLOSSARY.  In M.S. Shackley (Ed.) X-Ray Fluorescence Spectrometry (XRF) in Geoarchaeology, pp. 207-226.  Springer Publishing, New York.

Shackley, M.S. and Hampel, J.

  1992  Surface effects in the energy-dispersive X-ray fluorescence (EDXRF) analysis of archaeological obsidian.  Presented at the 28th International Symposium on Archaeometry, Los Angeles.

Speakman, R.J., and M.S. Shackley

  2013  Silo science and portable XRF in archaeology: a response to Frahm.  Journal of Archaeological Science 40:1435-1443.

 

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Revised:15 March 2016 11:20 -0800