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The measurement of rubidium and strontium blanks in the geochronological laboratory
Chemical Geology, 32 (1981) 303--315
303
Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
THE MEASUREMENT OF RUBIDIUM AND STRONTIUM BLANKS IN THE GEOCHRONOLOGICAL LABORATORY
P. STILLE
Mineralogisch--Petrographisches Institut, Universitiit Bern, Bern (Switzerland) (Received October 14, 1980; accepted for publication February 16, 1981)
ABSTRACT Stille, P., 1981. The measurement of rubidium and strontium blanks in the geochronological laboratory. Chem. Geol., 32: 303--315. More than 100 analyses have been performed on chemicals and on air to examine the effect of working conditions upon Sr and Rb blanks. The investigations show the different behaviour of the Rb and Sr blanks, the Sr blank depending very much on the air conditions and the Rb blank on "handling". The effect of "handling" has a strong influence on the degree of Rb contamination. With the exception of perchloric acid, the other chemicals used for the analysis of mineral and rock samples by standard isotope dilution procedures do not appreciably affect the Sr and Rb blanks.
INTRODUCTION
The new arrangement of the chemistry laboratories and the new installation of a special t y p e of air-filtering unit in the Department of Isotope Geology, Bern, made it important to examine very carefully the working conditions with respect to Sr and Rb blanks. Investigations on chemicals, air and distillations have been performed over a period of t w o years. The aim of this note is to report the Sr--Rb contaminations obtained from air and chemicals during the analysis of mineral and rock samples by standard isotope dilution procedures. FILTERING UNIT
The special filtering unit is designed f o r the circulation of 3000 m 3 of air per hour and serves for the renewal of the air in the hoods and the laboratories. The commercial filtering block contains, in addition to a ventilator and electric heating, a coarse mechanical filter, an electrostatic filter operated at 14 kV, and an " a b s o l u t e " mechanical filter. Since March 1980 the electrostatic filter system is cleaned automatically every night with water which is purified by passing through an ion~exchange cartridge. The purification of the water
0009-2541/81/0000--0000!$02.50 © 1981 Elsevier Scientific Publishing Company
304 for cleaning is important because of the high Sr content (see Table II, p. 307) of the Bern tap water, creating a visible residue on the filter surface. The air for the filtering unit is taken from the anteroom and is returned to this room after having circulated in the laboratories. Since this filtering unit is always running, including the time when no chemical work is being done, the air is continuously recycled and the different filters do n o t become strongly charged with dust. The acid ~umes from the hoods are directly expelled to the outside. The filtered air is blown into the hoods and into two laboratories, which are kept under small overpressure against the other rooms. Thus the cleanest air is supplied to the hoods, forming an air curtain against the laboratory. CHEMICALBLANKS A spike solution of 871~b or S4Sr was added to the chemicals in platinum dishes, the a m o u n t varying between 1 and 10 ng. The a m o u n t of chemicals added was ~ 50 g, several times more than the weights normally used. All chemical and total blanks are treated as samples and evaporated in the open air in the hoods. No air contamination correction has been made on the blank values. AIR CONTAMINATION As a primary check, Rb and Sr blanks have been performed during two years under different air conditions. Platinum dishes containing only spike and ~ 10 g nitric acid Merck ® suprapur were exposed to the open air at different places under different air conditions for seven days. One place was the lounge were people smoked, another the h o o d of the mineral separation laboratory under various rock dust conditions: in the laboratory with unfiltered air, in the clean laboratories with filtered air, and in the hoods of clean laboratories. The measured Sr and Rb contaminations of this experiment are compiled in Table I and are expressed as ng/cm2; the area exposed in each platinum dish was ~ 25 cm 2. In the air the two elements behave differently: the highest Rb contamination is f o u n d in the seminar room where smoking is allowed. The highest Sr contamination is f o u n d in air which contains some mineral dust. The effect of filtering is much better for Sr than for Rb. The Sr contamination between the most dirty and the cleanest air is reduced by a factor of more than 60 in the initial stage (February--March 1979) of the new laboratories. Rb is reduced only by a factor of ~ 2. The time- and work
305 TABLE I Sr and Rb air contaminations (ng): sedimentation measured in ng/cm ~ during 7-day exposure to open air, dissolved in HN03, corrected for HNO 3 blank
Strontium
February--March (1979)
February--March (1980)
June (1980)
August (1980)
0.009
0.010
n.d.
n.d.
0.019
n.d.
n.d.
n.d.
0.008
0.002
n.d.
n.d.
0.005
0.002
0.0008
n.d.
n.d.
0.001
0.0002
n.d.
0.0003
0.0002
--
n.d.
0.0009 0.0006 0.0006
0.0007 n.d. 0.0005
n.d. n.d. n.d.
n.d. n.d. n.d.
0.0005
0.0002
0.0009
n.d.
n.d.
--
0.0002
0.00004
0.0004
0.0002
0.0010
0.0003
:
Seminar room (smoking!) Mineral separation (rock dust) Chemical laboratory, unfiltered air Chemical laboratory, filtered air Chemical laboratory, hood I Chemical laboratory, h o o d II Rubidium:
Seminar room Mineral separation Chemical laboratory, unfiltered air Chemical laboratory, filtered air Chemical laboratory, hood I Chemical laboratory, h o o d II n.d. = not determined.
surements were made; people were very often coming into one of the cleanest laboratories for loading, and more than 100 R b samples were passed through the hoods. After these activities it was observed that the R b air contaminations fell back again to their expected values. These observations demonstrate the sensitivity of R b air contamination to activities in a laboratory and to "handling".
WATER BLANKS
The water blanks are compiled in Table II. The Bern tap water has a Rb c o n t e n t of 0.55 ppb, which is half the average c o n t e n t of river water (Turekian, 1969), and a Sr c o n t e n t of 435 ppb. In contrast to Rb, the Sr c o n t e n t is, according to Brass (1976), much higher than the average river
306
ng
Sr
@
0.0!9 0.018 0016 _ 0.01,:, 0,012 0,0!
G e m i ~ -
. _,____----------
0.008 0.0@6 0,004 0.002 !
<~
ng Rb
® ~ ' o 5 - ~ ~ ,
, ,
/, , .J"~"'~1
°°dl
Fig. 1. T h e time- a n d w o r k - d e p e n d e n t R b a n d Sr air c o n t a m i n a t i o n s . N o t e : t h e " u n f i l t e r e d a i r " is t h e air in a l a b o r a t o r y w h i c h results f r o m a n overpressure o f filtered air in an adj a c e n t clean l a b o r a t o r y .
content of ~ 55 ppb in limestone regions. After the first distillation in a quartz-glass still the depletion factor for Rb is ~ 400 and for Sr ~ 20,000. The second and third distillations are in a quartz still. The essential component of this still is a 1 m long column filled with Rasching ® dull quartz rings, having a rough surface but a very good cleaning effect. The water vapor is led through a 1.5 m long quartz tube, upon which the vapour condenses, draining into a storage tank. The triple~listilled water has only 0.00036 ppb Rb and 0.00088 ppb St. The effect of these three distillations is shown in Fig. 2. Fig. 3A and B shows the time-dependent self-cleaning process of the quartz stills for double- and triple-distilled water with respect to Rb and Sr, respectively. In February--March 1980 the quartz still for the double-distilled water was broken. After repair the Sr contamination of the double~tistilled water rose to a value of 0.043 ppb and the contamination of the triple-
307 TABLE H Sr and Rb contaminations (ppb) of water (A) Strontium
Tap 1x 2 × 2 × 3 × 3 x
water distilled distilled distilled distilled distilled
water water water, after accident water water, after accident
February--March (1979)
February--March (1980)
April (1980)
June (1980)
435 0.02 0.0042 n.d. 0.00088 n.d.
n.d. n.d. n.d. 0.043 n.d. 0.013
n.d. n.d. n.d. n.d. n.d. 0.0025
n.d. n.d. n.d. 0.0054 n.d. 0.0026
February--March (1979)
February--March (1980)
June (1980)
August (1980)
0.55 0.0014 0.00049 n.d. 0.00036 n.d.
n.d. n.d. n.d. 0.0028 n.d. 0.00064
n.d. n.d. n.d. 0.0081 n.d. 0.0094
n.d. n.d. n.d. 0.002 n.d. 0.0007
(B) Rubidium
Tap 1 × 2 × 2 x 3 × 3 ×
water distilled distilled distilled distilled distilled
water water water, after accident water water, after accident
n.d. = not determined.
distilled water to a value of 0.013 ppb. Six weeks after this repair the Sr contamination of the triple-distilled water had fallen to 0.0025 ppb. In June 1980 the Sr contamination of the double-distilled water was 0.0054 ppb, a b o u t the same as f o u n d before the accident. The Sr contamination value of the triple
308 ppb ~000
100
10
Dist. H20 0.1
OD 1
0,001
---ORb
0.0001
,
f
,
l x dist.
2 x dist.
3 x dist,
Fig. 2. The effect of the water triple-distillation process.
deterioraton of the quartz stills. To check whether this was true, further Rb contamination measurements were performed in August. At that time the Rb air contaminations of both hoods were decreasing, approaching the values before their strong increase and this improvement is also observable in the double- and triple~listilled water. The triple~tistilled water shows a Rb contamination similar to the value in February--March 1980, directly after the accident with the quartz still. The d o t t e d lines in Fig. 3B, joining the
309
ppb Sr
0.04-
®
o 2xdist.woter • 3xdtstwoter
®
I
', \
0.030.02-
i /
1.
,If\;" \
o.o
~- . . . . . . . . . . . • ........
9. . . . . .
__.
JIl\
:
? . . . . . . . .
• - - o
h ppb Rb
0'0091 ®
0.0071 0.005°
0.003-
®.
0.001, ~:~-:=:~7
..........
i
~O
•
u
~ m~
2~ R
Fig. 3. The time-dependent self-cleaning process of the quartz stills for double- and tripledistilled water with respect to Sr and Rb.
measured values of February and August 1980 and leaving out the values of June 1980, reflect the self.leaning process in the quartz stills after the accident. The blank peaks lying over the dotted lines represent the time of contamination by air to which the water is very sensitive, being of high purity in respect to Rb. These investigations show the high sensitivity of Rb contaminations to "handling" and of Sr contaminations to accidents such as a broken quartz still.
310 ACIDS
The blank values of the Merck ® Suprapure grade acids used in this laboratory are compiled in Table III. These acids are expensive and can vary in quality. Consequently we distil our own hydrochloric acid, using 37% Merck ® analytical grade with a Sr c o n t e n t of 0.14 ppb. It is diluted to constant-boiling 6 N hydrochloric acid with triple-distilled water. The quartz still, used for the hydrochloric acid, is similar to that used for the triple-distilled water. The Rb c o n t e n t is ~ 0.005 ppb and the Sr c o n t e n t is ~ 0.006 ppb after the second distillation. This purity of the hydrochloric acid is as good as the commercial suprapur acids from Merck v . With a self-constructed, very sensitive pycnometer the hydrochloric acid was adjusted to a constant concentration of ~ 2.5 N. The commercial perchloric acid shows high values of St, the worst value of 0.3 ppb Sr having a strong influence upon the total blank of chemicals. The Sr contents of different bottles vary by a factor of 30. On the other hand, the impurity of Rb in the perchloric acid is very low because Rb chloride is less soluble. The hydrofluoric and nitric acids do not contribute much to the Sr and Rb contaminations. Concerning the variations of the Rb and Sr contents of the acids, especially of the perchloric acid, the purity of each new chemical should be tested; since each chemical is diluted with very pure water only a small rise in the total blank is visible. T A B L E III Sr a n d R b c o n t a m i n a t i o n s o f c h e m i c a l s
HC1, M e r c k @ s u p r a p u r , 30% HC1, M e r c k @ p.A., 37% HC1, M e r c k ® p.A., 6 N, 1 X distilled HC1, M e r c k @ p.A., 6 N, 2 x distilled HClO4, M e r c k @ s u p r a p u r HNO3, M e r c k ® s u p r a p u r HF, Merck @ suprapur
Sr (ppb)
Rb (ppb)
0.0054--0.0063 0.14 0.0084 0.005 --0.0064 0.3 -0.010 0.011 0.05 --0.0068
0.0014--0.0041 n.d. n.d. 0.002 -0.0052 0.003 0.002 -0.0052 0.013
n.d. = n o t d e t e r m i n e d . TOTAL BLANKS
For the t r e a t m e n t of 1 g sample, we use the following a m o u n t of acids, the total blank of which represents an absolute m a x i m u m figure including "handling", dish washing and columns:
311 110 g triple-distilled water 12 g hydrofluoric acid, 40%
17 g perchloric acid, 70% 20 g hydrochloric acid, 6 N
T h e t o t a l blanks f o r the p e r i o d F e b r u a r y - - M a r c h 1 9 7 9 t o J u n e - - A u g u s t 1 9 8 0 are c o m p i l e d in T a b l e IV. In F e b r u a r y - - M a r c h 1 9 7 9 the t o t a l Sr b l a n k was 3.0 ng/g sample. In F e b r u a r y - - M a r c h 1 9 8 0 the t o t a l Sr blank, f o r the double-distilled w a t e r m e a s u r e d d i r e c t l y a f t e r the a c c i d e n t with t h e q u a r t z still, increased t o a value o f 7.0 ng/g sample. With the t i m e - d e p e n d e n t i m p r o v e m e n t o f the tripledistilled w a t e r (see Fig. 3A), and also o f the filtered air (see Fig. 1A), an imp r o v e m e n t o f the t o t a l Sr blanks is also observable. In April 1 9 8 0 the Sr t o t a l b l a n k was 6 ng/g sample and in J u n e 1 9 8 0 , 4.5 ng/g sample. T h e b e h a v i o u r o f t h e t o t a l Sr b l a n k in t h e t i m e b e t w e e n 1 9 7 7 a n d J u n e 1 9 8 0 is r e p r e s e n t e d in TABLE IV Sr and Rb total blanks (A) Strontium contents (ng Sr/g sample) February--March (1979) Total blank Total blank, after accident 20 g, 6 N HCI, 2 x distilled 110 g, 3 × distilled water 12 g, HF 17 g, HCIO4
3.0
Synthetic blank
2.86
February--March (1980)
April June (1980) (1980)
7.0
6.0
4.5
0.13 0.10 0.08 2.55
(B) Rubidium contents (ng Rb/g sample) February--March (1979) Total blank Total blank, after accident 20 g, 6 N HC1, 2 x distilled 110 g, 3 x distilled water 12 g, HF 17 g, HCIO4 Synthetic blank
February--March June August (1980) (1980) (1980)
0.5 0.2 0.08 0.06 0.16 0.05 0.35
0.8
0.3
312
Fig. 4. It shows the strong improvement of the Sr total blanks after the new arrangement of the chemistry laboratories and the installation of the new airfiltering unit, reaching a minimum value of 3.0 ng/g sample in February-March 1979. Even after the accident with the quartz still the Sr total blank did not rise up to the values of that before the installation of the new laboratories. In February--March 1979 the total Rb blank was 0.5 ng/g sample. In February--March 1980 the total Rb blank reached the value of 0.2 ng/g sam~le. Thus contrary to the Sr blank, the Rb blank was less influenced by the accident with the quartz still than by the improvement of air in respect to Rb. This air~lependent behaviour of the Rb total blank is clearly shown by the measurement made in June 1980, rising to a value of 0.8 ng/g sample. In August 1980, the total Rb blank fell to a value of 0.3 ng/g sample, nearly equal to the best Rb blank value obtained in February--March 1980. This blank improvement parallels the improvement of the air similar to the water blanks. The behaviour of the Rb total blanks in the time between 1977 and August 1 9 8 0 is also demonstrated in Fig. 4. In addition to the strong ira-
ng/g
mineral
13 12 11 10 g 8 7 6 5 4 3
I I I
2 I 0
i
I
I
~ L
i
i
~ L
i
i
i
Fig. 4. T h e behaviour o f the total Sr and Rb blanks in the period b e t w e e n 1 9 7 7 and A u g u s t 1980.
313 provement of the Sr total blanks after the new arrangement of the chemistry laboratories, there is also an improvement of the R b total blanks. However, this improvement is small because of the limited effect of filtering for Rb as already discussed in an earlier section (p. 304). EFFECT OF "HANDLING" During the blank investigations it could be observed that "handling" has a strong influence on the degree of R b contamination in a sample, b u t a much smaller effect on Sr contamination. The data, compiled in Table V, demonstrate this effect. Samples of the 6 N hydrochloric acid were measured directly from the quartz still, from the reservoir, and from a spray bottle. The Sr blanks seem to be unaffected by "handling". However, the Rb contaminations increase significantly from the least to the most treated sample. This is also observable in the 2.5 N HCI, which was made b y mixing 6 N HC1 with triple-distilled water. Because of the high purity of the water, the 2.5 N HC1 might be expected to show lower Rb and Sr contamination values than the 6 N HC1. However, this is n o t the case, the Sr and Rb contaminations of the 2.5 N HC1 are little higher than those of the 6 N HC1. Between the ~ 2.5 N HC1 and the calibrated 2.5 N HC1 in a spray bottle, no "handling" effect is observable for Sr, in contrast to the Rb contaminations which increase significantly from the least to the most treated sample, as already described for the 6 N HC1. The same effects are also observable in the triple~istilled water. R b c o n t ~ n i n a t i o n b y "handling" is also perceptible comparing the synthetic blanks (by adding the blanks of chemicals) with the total measured blanks. Table IV shows that the difference between total measured blanks and synthetic blanks is more critical for R b than for St. This again indicates R b contamination by "handling". TABLE V The effect of "handling"
6 N HCI, 2 × distilled, directly f r o m quartz still 6 N HCI, 2 × distilled, tank 6 N HCI, 2 x distilled, spray b o t t l e 2.5 N HC1, e x a c t 2.5 N HCI, spray b o t t l e 3 × distilled water, directly f r o m quartz still 3 x distilled water o f a spray b o t t l e 3 x distilled water of a nearly empty spray b o t t l e n.d. = n o t d e t e r m i n e d .
Sr (ppb)
Rb
0.005 n.d. 0.004 0.007" 0.006 n.d. n.d. n.d.
0.002 0.005 0.013 0.007 0.013 0.0004 0.0005 0.0015
(ppb)
314 CONTAMINATION FROM GLASSWARE In this section the Sr contamination from laboratory glassware will be discussed briefly. This contamination could be particularly serious as already reported by Wasserburg et al. (1964) because in blank investigations the reagents do n o t come into contact with glassware with one exception -- the ionCxchange columns. This study should be considered as the first step of an investigation into Sr contamination by glassware. This contamination effect was determined very roughly. Two Pyrex ® glass flasks (100 ml) for the Sr samples were filled with 20 ml of hydrofluoric acid Merck ® suprapur conc. During one week the acid reacted on the glass. After this time a Sr spike was added to the contaminated hydrofluoric acid in platinum dishes, and evaporated and prepared for measurement. The results are compiled in Table VI. TABLE VI Contamination from glassware Sr (ppb) HF, Merck® suprapur, without glassware HF, Merck® suprapur, in flask 1 HF, Merck® suprapur, in flask 2
0.007 160 120
They show the strong contamination of the hydrofluoric acid from dissolving Sr from the glass. These contamination values are extreme figures which are clearly never reached during the analysis of mineral samples by standard isotope dilution procedures. Nevertheless this effect of Sr contamination from laboratory glassware must be taken into consideration. CONCLUSIONS
The blank investigations have shown that Rb and Sr behave differently in air. The effect of the filtering unit is much better for Sr than for Rb. The investigation of the water sample with respect to Rb and Sr also demonstrates the different behaviour of both elements. This is clearly revealed by the contamination values measured directly after the accident, showing a much stronger increase in Sr than in Rb concentration. The quartz stills needed more than half a year to produce water of nearly the same quality as measured before the accident. During the blank investigations it could be observed that "handling" had a strong influence on the degree of Rb contamination in a sample, but that it is much less for St. This effect of "handling" is reflected by measurements on Rb contamination levels derived from the air, in the chemicals and also
315 by making comparisons between synthetic blanks and total measured blanks. Distillation of water is good enough to make the Sr contamination negligible. The distilled hydrochloric acid is in good agreement with commercial varieties. Contamination from perchloric acid is critical and therefore routine checks of perchloric acid must be made. Commercial hydrofluoric acid and nitric acids do n o t affect significantly the Sr and R b blanks. Contamination from laboratory glassware could be particularly serious because normally it is n o t measured in blank investigations. Thus, one must carefully consider the limits that chemical contamination imposes. Only in consideration of all the points discussed in this paper is it possible to date samples, e.g., y o u n g micas, p o o r in either Rb or radiogenic Sr. The concentration of radiogenic STSr in y o u n g alpine micas is ~ 10--80 ppb. Therefore the contribution through chemical treatment of c o m m o n Sr should be much smaller than ~ 1 ppb. In the case of higher contributions a serious error in age determination could be produced. If such a contaminated system is plotted on a strontium evolution diagram, contamination will cause the mineral to deviate below the isochron, in the direction of c o m m o n Sr as described by J~iger (1979). ACKNOWLEDGEMENTS
The author would like to thank Prof. E. J~iger for his help as well as for many stimulating discussions. The author also wishes to express his appreciation to Dr. T. Hurford for correcting many English passages and to Mrs. U. Mischler for working very carefully in the chemistry laboratories. The assistance in measurements of R. Brunner, Dr. K. Hammerschmidt, Dr. J.C. Hunziker, Dr. W. Morauf and R. Siegenthaler as well as the support of this work b y the "Schweizerischer Nationalfonds zur FSrderung der wissenschaftlichen Forschung" is gratefully acknowledged.
REFERENCES
Brass, G.W., 1976. The variation of the marine s~sr/s~Sr ratio during Phanerozoic time: interpretation using a flux model. Geochim. Cosmochim. Acta, 40: 721--730. J~/ger, E., 1979. The Rb--Sr method. In: Isotope Geology. Springer, Heidelberg, pp. 13--26. Riley, G.H. and Compston, W., 1962. Theoretical and technical aspects of Rb--Sr geochronology. Geochim. Cosmochim. Acta, 26: 1255--1281. Turekian, K.K., 1969. The oceans, streams and atmosphere. In: Handbook of Geochemistry, I. Springer, Heidelberg, pp. 297--323. Wasserburg, G.J., Wen, T. and Aronson, J., 1964. Strontium contamination in mineral analyses. Geochim. Cosmochim. Acta, 28: 407--410.
303
Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
THE MEASUREMENT OF RUBIDIUM AND STRONTIUM BLANKS IN THE GEOCHRONOLOGICAL LABORATORY
P. STILLE
Mineralogisch--Petrographisches Institut, Universitiit Bern, Bern (Switzerland) (Received October 14, 1980; accepted for publication February 16, 1981)
ABSTRACT Stille, P., 1981. The measurement of rubidium and strontium blanks in the geochronological laboratory. Chem. Geol., 32: 303--315. More than 100 analyses have been performed on chemicals and on air to examine the effect of working conditions upon Sr and Rb blanks. The investigations show the different behaviour of the Rb and Sr blanks, the Sr blank depending very much on the air conditions and the Rb blank on "handling". The effect of "handling" has a strong influence on the degree of Rb contamination. With the exception of perchloric acid, the other chemicals used for the analysis of mineral and rock samples by standard isotope dilution procedures do not appreciably affect the Sr and Rb blanks.
INTRODUCTION
The new arrangement of the chemistry laboratories and the new installation of a special t y p e of air-filtering unit in the Department of Isotope Geology, Bern, made it important to examine very carefully the working conditions with respect to Sr and Rb blanks. Investigations on chemicals, air and distillations have been performed over a period of t w o years. The aim of this note is to report the Sr--Rb contaminations obtained from air and chemicals during the analysis of mineral and rock samples by standard isotope dilution procedures. FILTERING UNIT
The special filtering unit is designed f o r the circulation of 3000 m 3 of air per hour and serves for the renewal of the air in the hoods and the laboratories. The commercial filtering block contains, in addition to a ventilator and electric heating, a coarse mechanical filter, an electrostatic filter operated at 14 kV, and an " a b s o l u t e " mechanical filter. Since March 1980 the electrostatic filter system is cleaned automatically every night with water which is purified by passing through an ion~exchange cartridge. The purification of the water
0009-2541/81/0000--0000!$02.50 © 1981 Elsevier Scientific Publishing Company
304 for cleaning is important because of the high Sr content (see Table II, p. 307) of the Bern tap water, creating a visible residue on the filter surface. The air for the filtering unit is taken from the anteroom and is returned to this room after having circulated in the laboratories. Since this filtering unit is always running, including the time when no chemical work is being done, the air is continuously recycled and the different filters do n o t become strongly charged with dust. The acid ~umes from the hoods are directly expelled to the outside. The filtered air is blown into the hoods and into two laboratories, which are kept under small overpressure against the other rooms. Thus the cleanest air is supplied to the hoods, forming an air curtain against the laboratory. CHEMICALBLANKS A spike solution of 871~b or S4Sr was added to the chemicals in platinum dishes, the a m o u n t varying between 1 and 10 ng. The a m o u n t of chemicals added was ~ 50 g, several times more than the weights normally used. All chemical and total blanks are treated as samples and evaporated in the open air in the hoods. No air contamination correction has been made on the blank values. AIR CONTAMINATION As a primary check, Rb and Sr blanks have been performed during two years under different air conditions. Platinum dishes containing only spike and ~ 10 g nitric acid Merck ® suprapur were exposed to the open air at different places under different air conditions for seven days. One place was the lounge were people smoked, another the h o o d of the mineral separation laboratory under various rock dust conditions: in the laboratory with unfiltered air, in the clean laboratories with filtered air, and in the hoods of clean laboratories. The measured Sr and Rb contaminations of this experiment are compiled in Table I and are expressed as ng/cm2; the area exposed in each platinum dish was ~ 25 cm 2. In the air the two elements behave differently: the highest Rb contamination is f o u n d in the seminar room where smoking is allowed. The highest Sr contamination is f o u n d in air which contains some mineral dust. The effect of filtering is much better for Sr than for Rb. The Sr contamination between the most dirty and the cleanest air is reduced by a factor of more than 60 in the initial stage (February--March 1979) of the new laboratories. Rb is reduced only by a factor of ~ 2. The time- and work
305 TABLE I Sr and Rb air contaminations (ng): sedimentation measured in ng/cm ~ during 7-day exposure to open air, dissolved in HN03, corrected for HNO 3 blank
Strontium
February--March (1979)
February--March (1980)
June (1980)
August (1980)
0.009
0.010
n.d.
n.d.
0.019
n.d.
n.d.
n.d.
0.008
0.002
n.d.
n.d.
0.005
0.002
0.0008
n.d.
n.d.
0.001
0.0002
n.d.
0.0003
0.0002
--
n.d.
0.0009 0.0006 0.0006
0.0007 n.d. 0.0005
n.d. n.d. n.d.
n.d. n.d. n.d.
0.0005
0.0002
0.0009
n.d.
n.d.
--
0.0002
0.00004
0.0004
0.0002
0.0010
0.0003
:
Seminar room (smoking!) Mineral separation (rock dust) Chemical laboratory, unfiltered air Chemical laboratory, filtered air Chemical laboratory, hood I Chemical laboratory, h o o d II Rubidium:
Seminar room Mineral separation Chemical laboratory, unfiltered air Chemical laboratory, filtered air Chemical laboratory, hood I Chemical laboratory, h o o d II n.d. = not determined.
surements were made; people were very often coming into one of the cleanest laboratories for loading, and more than 100 R b samples were passed through the hoods. After these activities it was observed that the R b air contaminations fell back again to their expected values. These observations demonstrate the sensitivity of R b air contamination to activities in a laboratory and to "handling".
WATER BLANKS
The water blanks are compiled in Table II. The Bern tap water has a Rb c o n t e n t of 0.55 ppb, which is half the average c o n t e n t of river water (Turekian, 1969), and a Sr c o n t e n t of 435 ppb. In contrast to Rb, the Sr c o n t e n t is, according to Brass (1976), much higher than the average river
306
ng
Sr
@
0.0!9 0.018 0016 _ 0.01,:, 0,012 0,0!
G e m i ~ -
. _,____----------
0.008 0.0@6 0,004 0.002 !
<~
ng Rb
® ~ ' o 5 - ~ ~ ,
, ,
/, , .J"~"'~1
°°dl
Fig. 1. T h e time- a n d w o r k - d e p e n d e n t R b a n d Sr air c o n t a m i n a t i o n s . N o t e : t h e " u n f i l t e r e d a i r " is t h e air in a l a b o r a t o r y w h i c h results f r o m a n overpressure o f filtered air in an adj a c e n t clean l a b o r a t o r y .
content of ~ 55 ppb in limestone regions. After the first distillation in a quartz-glass still the depletion factor for Rb is ~ 400 and for Sr ~ 20,000. The second and third distillations are in a quartz still. The essential component of this still is a 1 m long column filled with Rasching ® dull quartz rings, having a rough surface but a very good cleaning effect. The water vapor is led through a 1.5 m long quartz tube, upon which the vapour condenses, draining into a storage tank. The triple~listilled water has only 0.00036 ppb Rb and 0.00088 ppb St. The effect of these three distillations is shown in Fig. 2. Fig. 3A and B shows the time-dependent self-cleaning process of the quartz stills for double- and triple-distilled water with respect to Rb and Sr, respectively. In February--March 1980 the quartz still for the double-distilled water was broken. After repair the Sr contamination of the double~tistilled water rose to a value of 0.043 ppb and the contamination of the triple-
307 TABLE H Sr and Rb contaminations (ppb) of water (A) Strontium
Tap 1x 2 × 2 × 3 × 3 x
water distilled distilled distilled distilled distilled
water water water, after accident water water, after accident
February--March (1979)
February--March (1980)
April (1980)
June (1980)
435 0.02 0.0042 n.d. 0.00088 n.d.
n.d. n.d. n.d. 0.043 n.d. 0.013
n.d. n.d. n.d. n.d. n.d. 0.0025
n.d. n.d. n.d. 0.0054 n.d. 0.0026
February--March (1979)
February--March (1980)
June (1980)
August (1980)
0.55 0.0014 0.00049 n.d. 0.00036 n.d.
n.d. n.d. n.d. 0.0028 n.d. 0.00064
n.d. n.d. n.d. 0.0081 n.d. 0.0094
n.d. n.d. n.d. 0.002 n.d. 0.0007
(B) Rubidium
Tap 1 × 2 × 2 x 3 × 3 ×
water distilled distilled distilled distilled distilled
water water water, after accident water water, after accident
n.d. = not determined.
distilled water to a value of 0.013 ppb. Six weeks after this repair the Sr contamination of the triple-distilled water had fallen to 0.0025 ppb. In June 1980 the Sr contamination of the double-distilled water was 0.0054 ppb, a b o u t the same as f o u n d before the accident. The Sr contamination value of the triple
308 ppb ~000
100
10
Dist. H20 0.1
OD 1
0,001
---ORb
0.0001
,
f
,
l x dist.
2 x dist.
3 x dist,
Fig. 2. The effect of the water triple-distillation process.
deterioraton of the quartz stills. To check whether this was true, further Rb contamination measurements were performed in August. At that time the Rb air contaminations of both hoods were decreasing, approaching the values before their strong increase and this improvement is also observable in the double- and triple~listilled water. The triple~tistilled water shows a Rb contamination similar to the value in February--March 1980, directly after the accident with the quartz still. The d o t t e d lines in Fig. 3B, joining the
309
ppb Sr
0.04-
®
o 2xdist.woter • 3xdtstwoter
®
I
', \
0.030.02-
i /
1.
,If\;" \
o.o
~- . . . . . . . . . . . • ........
9. . . . . .
__.
JIl\
:
? . . . . . . . .
• - - o
h ppb Rb
0'0091 ®
0.0071 0.005°
0.003-
®.
0.001, ~:~-:=:~7
..........
i
~O
•
u
~ m~
2~ R
Fig. 3. The time-dependent self-cleaning process of the quartz stills for double- and tripledistilled water with respect to Sr and Rb.
measured values of February and August 1980 and leaving out the values of June 1980, reflect the self.leaning process in the quartz stills after the accident. The blank peaks lying over the dotted lines represent the time of contamination by air to which the water is very sensitive, being of high purity in respect to Rb. These investigations show the high sensitivity of Rb contaminations to "handling" and of Sr contaminations to accidents such as a broken quartz still.
310 ACIDS
The blank values of the Merck ® Suprapure grade acids used in this laboratory are compiled in Table III. These acids are expensive and can vary in quality. Consequently we distil our own hydrochloric acid, using 37% Merck ® analytical grade with a Sr c o n t e n t of 0.14 ppb. It is diluted to constant-boiling 6 N hydrochloric acid with triple-distilled water. The quartz still, used for the hydrochloric acid, is similar to that used for the triple-distilled water. The Rb c o n t e n t is ~ 0.005 ppb and the Sr c o n t e n t is ~ 0.006 ppb after the second distillation. This purity of the hydrochloric acid is as good as the commercial suprapur acids from Merck v . With a self-constructed, very sensitive pycnometer the hydrochloric acid was adjusted to a constant concentration of ~ 2.5 N. The commercial perchloric acid shows high values of St, the worst value of 0.3 ppb Sr having a strong influence upon the total blank of chemicals. The Sr contents of different bottles vary by a factor of 30. On the other hand, the impurity of Rb in the perchloric acid is very low because Rb chloride is less soluble. The hydrofluoric and nitric acids do not contribute much to the Sr and Rb contaminations. Concerning the variations of the Rb and Sr contents of the acids, especially of the perchloric acid, the purity of each new chemical should be tested; since each chemical is diluted with very pure water only a small rise in the total blank is visible. T A B L E III Sr a n d R b c o n t a m i n a t i o n s o f c h e m i c a l s
HC1, M e r c k @ s u p r a p u r , 30% HC1, M e r c k @ p.A., 37% HC1, M e r c k ® p.A., 6 N, 1 X distilled HC1, M e r c k @ p.A., 6 N, 2 x distilled HClO4, M e r c k @ s u p r a p u r HNO3, M e r c k ® s u p r a p u r HF, Merck @ suprapur
Sr (ppb)
Rb (ppb)
0.0054--0.0063 0.14 0.0084 0.005 --0.0064 0.3 -0.010 0.011 0.05 --0.0068
0.0014--0.0041 n.d. n.d. 0.002 -0.0052 0.003 0.002 -0.0052 0.013
n.d. = n o t d e t e r m i n e d . TOTAL BLANKS
For the t r e a t m e n t of 1 g sample, we use the following a m o u n t of acids, the total blank of which represents an absolute m a x i m u m figure including "handling", dish washing and columns:
311 110 g triple-distilled water 12 g hydrofluoric acid, 40%
17 g perchloric acid, 70% 20 g hydrochloric acid, 6 N
T h e t o t a l blanks f o r the p e r i o d F e b r u a r y - - M a r c h 1 9 7 9 t o J u n e - - A u g u s t 1 9 8 0 are c o m p i l e d in T a b l e IV. In F e b r u a r y - - M a r c h 1 9 7 9 the t o t a l Sr b l a n k was 3.0 ng/g sample. In F e b r u a r y - - M a r c h 1 9 8 0 the t o t a l Sr blank, f o r the double-distilled w a t e r m e a s u r e d d i r e c t l y a f t e r the a c c i d e n t with t h e q u a r t z still, increased t o a value o f 7.0 ng/g sample. With the t i m e - d e p e n d e n t i m p r o v e m e n t o f the tripledistilled w a t e r (see Fig. 3A), and also o f the filtered air (see Fig. 1A), an imp r o v e m e n t o f the t o t a l Sr blanks is also observable. In April 1 9 8 0 the Sr t o t a l b l a n k was 6 ng/g sample and in J u n e 1 9 8 0 , 4.5 ng/g sample. T h e b e h a v i o u r o f t h e t o t a l Sr b l a n k in t h e t i m e b e t w e e n 1 9 7 7 a n d J u n e 1 9 8 0 is r e p r e s e n t e d in TABLE IV Sr and Rb total blanks (A) Strontium contents (ng Sr/g sample) February--March (1979) Total blank Total blank, after accident 20 g, 6 N HCI, 2 x distilled 110 g, 3 × distilled water 12 g, HF 17 g, HCIO4
3.0
Synthetic blank
2.86
February--March (1980)
April June (1980) (1980)
7.0
6.0
4.5
0.13 0.10 0.08 2.55
(B) Rubidium contents (ng Rb/g sample) February--March (1979) Total blank Total blank, after accident 20 g, 6 N HC1, 2 x distilled 110 g, 3 x distilled water 12 g, HF 17 g, HCIO4 Synthetic blank
February--March June August (1980) (1980) (1980)
0.5 0.2 0.08 0.06 0.16 0.05 0.35
0.8
0.3
312
Fig. 4. It shows the strong improvement of the Sr total blanks after the new arrangement of the chemistry laboratories and the installation of the new airfiltering unit, reaching a minimum value of 3.0 ng/g sample in February-March 1979. Even after the accident with the quartz still the Sr total blank did not rise up to the values of that before the installation of the new laboratories. In February--March 1979 the total Rb blank was 0.5 ng/g sample. In February--March 1980 the total Rb blank reached the value of 0.2 ng/g sam~le. Thus contrary to the Sr blank, the Rb blank was less influenced by the accident with the quartz still than by the improvement of air in respect to Rb. This air~lependent behaviour of the Rb total blank is clearly shown by the measurement made in June 1980, rising to a value of 0.8 ng/g sample. In August 1980, the total Rb blank fell to a value of 0.3 ng/g sample, nearly equal to the best Rb blank value obtained in February--March 1980. This blank improvement parallels the improvement of the air similar to the water blanks. The behaviour of the Rb total blanks in the time between 1977 and August 1 9 8 0 is also demonstrated in Fig. 4. In addition to the strong ira-
ng/g
mineral
13 12 11 10 g 8 7 6 5 4 3
I I I
2 I 0
i
I
I
~ L
i
i
~ L
i
i
i
Fig. 4. T h e behaviour o f the total Sr and Rb blanks in the period b e t w e e n 1 9 7 7 and A u g u s t 1980.
313 provement of the Sr total blanks after the new arrangement of the chemistry laboratories, there is also an improvement of the R b total blanks. However, this improvement is small because of the limited effect of filtering for Rb as already discussed in an earlier section (p. 304). EFFECT OF "HANDLING" During the blank investigations it could be observed that "handling" has a strong influence on the degree of R b contamination in a sample, b u t a much smaller effect on Sr contamination. The data, compiled in Table V, demonstrate this effect. Samples of the 6 N hydrochloric acid were measured directly from the quartz still, from the reservoir, and from a spray bottle. The Sr blanks seem to be unaffected by "handling". However, the Rb contaminations increase significantly from the least to the most treated sample. This is also observable in the 2.5 N HCI, which was made b y mixing 6 N HC1 with triple-distilled water. Because of the high purity of the water, the 2.5 N HC1 might be expected to show lower Rb and Sr contamination values than the 6 N HC1. However, this is n o t the case, the Sr and Rb contaminations of the 2.5 N HC1 are little higher than those of the 6 N HC1. Between the ~ 2.5 N HC1 and the calibrated 2.5 N HC1 in a spray bottle, no "handling" effect is observable for Sr, in contrast to the Rb contaminations which increase significantly from the least to the most treated sample, as already described for the 6 N HC1. The same effects are also observable in the triple~istilled water. R b c o n t ~ n i n a t i o n b y "handling" is also perceptible comparing the synthetic blanks (by adding the blanks of chemicals) with the total measured blanks. Table IV shows that the difference between total measured blanks and synthetic blanks is more critical for R b than for St. This again indicates R b contamination by "handling". TABLE V The effect of "handling"
6 N HCI, 2 × distilled, directly f r o m quartz still 6 N HCI, 2 × distilled, tank 6 N HCI, 2 x distilled, spray b o t t l e 2.5 N HC1, e x a c t 2.5 N HCI, spray b o t t l e 3 × distilled water, directly f r o m quartz still 3 x distilled water o f a spray b o t t l e 3 x distilled water of a nearly empty spray b o t t l e n.d. = n o t d e t e r m i n e d .
Sr (ppb)
Rb
0.005 n.d. 0.004 0.007" 0.006 n.d. n.d. n.d.
0.002 0.005 0.013 0.007 0.013 0.0004 0.0005 0.0015
(ppb)
314 CONTAMINATION FROM GLASSWARE In this section the Sr contamination from laboratory glassware will be discussed briefly. This contamination could be particularly serious as already reported by Wasserburg et al. (1964) because in blank investigations the reagents do n o t come into contact with glassware with one exception -- the ionCxchange columns. This study should be considered as the first step of an investigation into Sr contamination by glassware. This contamination effect was determined very roughly. Two Pyrex ® glass flasks (100 ml) for the Sr samples were filled with 20 ml of hydrofluoric acid Merck ® suprapur conc. During one week the acid reacted on the glass. After this time a Sr spike was added to the contaminated hydrofluoric acid in platinum dishes, and evaporated and prepared for measurement. The results are compiled in Table VI. TABLE VI Contamination from glassware Sr (ppb) HF, Merck® suprapur, without glassware HF, Merck® suprapur, in flask 1 HF, Merck® suprapur, in flask 2
0.007 160 120
They show the strong contamination of the hydrofluoric acid from dissolving Sr from the glass. These contamination values are extreme figures which are clearly never reached during the analysis of mineral samples by standard isotope dilution procedures. Nevertheless this effect of Sr contamination from laboratory glassware must be taken into consideration. CONCLUSIONS
The blank investigations have shown that Rb and Sr behave differently in air. The effect of the filtering unit is much better for Sr than for Rb. The investigation of the water sample with respect to Rb and Sr also demonstrates the different behaviour of both elements. This is clearly revealed by the contamination values measured directly after the accident, showing a much stronger increase in Sr than in Rb concentration. The quartz stills needed more than half a year to produce water of nearly the same quality as measured before the accident. During the blank investigations it could be observed that "handling" had a strong influence on the degree of Rb contamination in a sample, but that it is much less for St. This effect of "handling" is reflected by measurements on Rb contamination levels derived from the air, in the chemicals and also
315 by making comparisons between synthetic blanks and total measured blanks. Distillation of water is good enough to make the Sr contamination negligible. The distilled hydrochloric acid is in good agreement with commercial varieties. Contamination from perchloric acid is critical and therefore routine checks of perchloric acid must be made. Commercial hydrofluoric acid and nitric acids do n o t affect significantly the Sr and R b blanks. Contamination from laboratory glassware could be particularly serious because normally it is n o t measured in blank investigations. Thus, one must carefully consider the limits that chemical contamination imposes. Only in consideration of all the points discussed in this paper is it possible to date samples, e.g., y o u n g micas, p o o r in either Rb or radiogenic Sr. The concentration of radiogenic STSr in y o u n g alpine micas is ~ 10--80 ppb. Therefore the contribution through chemical treatment of c o m m o n Sr should be much smaller than ~ 1 ppb. In the case of higher contributions a serious error in age determination could be produced. If such a contaminated system is plotted on a strontium evolution diagram, contamination will cause the mineral to deviate below the isochron, in the direction of c o m m o n Sr as described by J~iger (1979). ACKNOWLEDGEMENTS
The author would like to thank Prof. E. J~iger for his help as well as for many stimulating discussions. The author also wishes to express his appreciation to Dr. T. Hurford for correcting many English passages and to Mrs. U. Mischler for working very carefully in the chemistry laboratories. The assistance in measurements of R. Brunner, Dr. K. Hammerschmidt, Dr. J.C. Hunziker, Dr. W. Morauf and R. Siegenthaler as well as the support of this work b y the "Schweizerischer Nationalfonds zur FSrderung der wissenschaftlichen Forschung" is gratefully acknowledged.
REFERENCES
Brass, G.W., 1976. The variation of the marine s~sr/s~Sr ratio during Phanerozoic time: interpretation using a flux model. Geochim. Cosmochim. Acta, 40: 721--730. J~/ger, E., 1979. The Rb--Sr method. In: Isotope Geology. Springer, Heidelberg, pp. 13--26. Riley, G.H. and Compston, W., 1962. Theoretical and technical aspects of Rb--Sr geochronology. Geochim. Cosmochim. Acta, 26: 1255--1281. Turekian, K.K., 1969. The oceans, streams and atmosphere. In: Handbook of Geochemistry, I. Springer, Heidelberg, pp. 297--323. Wasserburg, G.J., Wen, T. and Aronson, J., 1964. Strontium contamination in mineral analyses. Geochim. Cosmochim. Acta, 28: 407--410.