Application of LA-ICP-MS to Gemmological Field2

4. Detection of Treatment
Detection of heating processes aiming at colour improvement or alteration of a gemstone that have been performed more than several dozens of years (including so-called surface diffusion treatment using external additives of transitiosn metal element contributing directly to colouring) becomes almost possible within the use of common testing instruments. But recently a new diffusion treatment using a light element Be (beryllium) has been applied to corundum, resulting highly saturated orange, pink and yellow sapphire (Fig.5).


Fig.-5 Corundum, diffusion treated with a light element beryllium.

  Detection of such light element as diffusion factor is impossible by traditional gemmological tests, and more sophisticated analytical devices such as secondary ion mass spectrometry (SIMS) is required. The difficulty of the detection is a problem, but authors enabled detection of heat-treated corundum using Be possible by measuring impurity elements including Be with inductively coupled plasma mass spectrometer using laser ablation system (LA-ICP-MS) (Ahmadjan A., H. Kitawaki, J. Shida, 2003; J. Shida et al., 2004).

5. Highly Sensitive Analysis
● Gem Mineral Compositional Analysis
  Compositional analyses of a substance by XRF, SEM-EDX and EPMA do not have enough sensitivity, and for analyses of ultra-trace elements and light elements, the latest and highly sensitive analytical devices, SIMS and LA-ICP-MS, are utilised. To obtain more accurate result of quantitative values, minute area (100μu) of a crystal has to be destructed.
  A new gem mineral variety of beryl group named “Pezzotaite” in 2003 was discovered in granitic pegmatite in the central Madagascar, Mandrosonoro district, and it was once called by a commercial name “Raspberyl” (Fig.6).

Fig.-6

(Left: a new mineral variety of beryl group, Pezzotaite; Right: Morganite)

This mineral, which belongs to beryl group same as emerald, contains Cs element, whose concentration becomes as high as 20wt% compared to morganite that usually contains 5wt% of Cs. Authors tested LA-ICP-MS analysis and compared their quantitative values of main elements and trace elements to show their margin of error. We also analysed trace elements (Mg, P, K, Sc, Cr, Mn, Fe, Ni, Zn, Ga and Tl) and light elements (Li and Be) which cannot be detected by XRF to compare the concentration of main components and trace elements in the both minerals (Table-2), (Ahmadjan A. and H. Kitawaki, 2003).

ペツォッタイト

モルガナイト

単元素

酸化物

微量成分

ppm

主成分

Wt％

1.9～16

～

0.4～5.0

Li2O

1.70％～2.57％

＞

0.53％～1.86％

46.0～51

＜

50.0～83

BeO

4.16％～9.72％

＜

6.11％～14.44％

40.0～80

36.0～86

Na2O

0.81％～1.35％

＜

0.88％～3.23％

13.0～17

～

5.7～100

Al2O3

15.52％～18.70％

＜

19.24％～23.06％

5.0～11

＜

13.0～26

SiO2

58.11％～61.49％

＜

66.28％～74.86％

47.0～150

＞

25.0～92

CaO

0.98％～2.15％

0.81％～1.54％

33.0～75

19.0～91

Rb2O

0.15％～0.62％

＞

0.08％～0.15％

6.3～10

6.4～8.9

Cs2O

16.13％～23.37％

＞

1.61％～3.12％

4.7～14

＜

1.8～200

9.0～15

7.9～18

20.0～50

＞

1.9～11

Table-2

Concentration(wt%-ppm) of main elements and trace elements in Pezzotaite and Morganite by LA-ICP-MS.

● Shell Material Analysis
Pearls are classified broadly into two categories, freshwater pearl and saltwater pearl, and in recent years demand for nucleated freshwater pearl is increasing. A nucleus used for cultured pearl is produced generally from freshwater shell such as Genus Fusconaia from Mississippi River in the States and Anodonta Woodiana from China, and recently saltwater Tridacna Squamosa is also used (Fig.7).

Fig.-7

Nucleated freshwater cultured pearl and a nucleus for cultures pearl with nacre on the cross-section.

However, Tridacna Squamosa has problems in its durability during fashioning process and concerning Washington Convention, and its use for a nucleus of pearl has been concerned. As above, identification of nucleus material for pearls becomes a new issue to be solved for gemmological laboratories. Authors are studying analysis on shell composition using LA-ICP-MS by irradiating the minimum laser beam (30μm) for long period (several dozens of seconds) to examine its application to identification of shell materials (Fig.8).

Fig.-8

Surface photograph of a pearl used for our analysis (right), showing a drilled hole of 700μｍ in diameter and a trace of laser-irradiated spot. Bottom: A spectrum of Mn element, which is contained in a freshwater shell used for the nucleus, detected by LA-ICP-MS.

6. Conclusion
Several examples of our study on LA-ICP-MS analysis were introduced above. Analysis of ultra-trace elements by LA-ICP-MS method is very effective for locality identification of corundum and emerald, and it can be expected to do so for diamond and other gem materials. LA-ICP-MS analysis is also useful to detect diffusion treatment with light elements in corundum and identification of shell materials.
Gemstone identification is supposed to face more complicated circumstances in the future, and the LA-ICP-MS analysis will become a vital analytical method to solve each of forthcoming problems.

References
Ahmadjan.A, H. Kitawaki, J. Shida, 2003. A new Technique for Corundum Analysis Using LA-ICP-MS (Inductively Coupled Plasma Mass Spectrometry) Instrument. Gemmology., Vol.34(11), No.410.,pp.4-7 (in Japanese, with insert of English translation).
Ahmadjan.A, H. Kitawaki, 2003. Analysis on Cs Pink “Beryl” using a Laser Ablation System with Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS). Gemmology., Vol.34(12), No411.,pp.24-26 (in Japanese, with insert of English translation).
Ahmadjan.A and H. Kitawaki, 2004. Study of Origin Determination of Blue Sapphire using IA-ICP-MS Analysis. Gemmology., Vol.35(6), No417.,pp4-7 (in Japanese with insert of English translation).
J. Shida, H. Kitawaki and Ahmadjan.A., 2004. Journal of The Gemmological Society of Japan., Vol.24, No.1-4.,pp.13-23.
M. Resano, F. Vanhaeche, et al., 2003. Possibilities of laser ablation-inductively coupled plasma-mass spectrometry for diamond fingerprinting. J.Anal.At Spectrum., 2003, 18(10), 1238-1242.
Wuyi Wang, Matt et al., 2003. Applications to Diamond Testing. Rapaport Diamond Report. September 5.

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