IMPROVEMENTS IN PROCEDURAL BLANKS AT NOSAMS: REFLECTIONS OF IMPROVEMENTS IN SAMPLE PREPARATION AND ACCELERATOR OPERATION

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1 IMPROVEMENTS IN PROCEDURAL BLANKS AT NOSAMS: REFLECTIONS OF IMPROVEMENTS IN SAMPLE PREPARATION AND ACCELERATOR OPERATION A. P. McNICHOL A. R. GAGNON E. A. OSBORNE D. L. HU?TON K. F. VON REDEN and R. J. SCHNEIDER National Ocean Sciences Acceleato Mass Spectomete Facility Depatment of Maine Geology and Geophysics Woods Hole Oceanogaphic Institution Woods Hole Massachusetts 2543 USA ABSTRACT. Duing fou yeas Sample Pepaation Laboatoy (SPL) at National Ocean Sciences Acceleato Mass Spectomete (NOSAMS) Facilty has been in opeation we have accumulated much data fom which we can assess ou pogess. We evaluate ou pocedual blanks hee and descibe modifications in ou pocedues that have impoved ou anal- yses of olde samples. In SPL we convet e distinct types of samples-seawate CaCO3 and oganic cabon-to CO2 pio to pepaing gaphite fo acceleato and have distinct pocedual blanks fo each pocedue. Dissolved inoganic ca- bon (ECO2) is extacted fom acidified seawate samples by spaging with a nitogen caie gas. We outinely analyze "line blanks" by pocessing CO2 fom a 14C-dead souce though entie stipping pocedue. Ou hydolysis blank IAEA C-1 is pepaed by acidifying in vacuo with 1% H3PO4 at 6 C ovenight identical to ou sample pepaation. We use a dead gaphite NBS-21 o a commecially available cabon powde fo ou oganic combustion blank; oganic samples ae com- busted at 85 C fo S h using CuO to povide oxidant. Analysis of ou wate stipping data suggests that one step in pocedue contibutes majo potion of line blank. At pesent contibution fom line blank has no effect on ou seawate analyses (faction moden (fm) between.7 and 1.2). Ou hydolysis blanks can have an fm value as low as.6 but ae moe outinely between.2 and.25. The fm of ou best oganic combustion blanks is highe than those ou- tinely achieved in o laboatoies and we ae cuently alteing ou methods to educe it. INTRODUCTION Studies of ocean cabon cycle wate-mass ciculation and paleoceanogaphy povide in Sample Pepaation Laboatoy (SPL) at National Ocean Sciences Acceleato Mass Spectomety (NOSAMS) Facility with e geneal types of samples-dissolved in oganic cabon seawate (CO2) CaCO3 as foaminifea and o shells and oganic cabon in many diffeent wide foms. vaiety The of pojects and eseach goals in oceanogaphic community also esults in of vaying samples sizes with diffeent pecision equiements. Each of main sample types we equies analyze a unique chemical cabon pepaation duing pocessing to convet cabon diffes to fo CO2 and each method v potential fo intoducin g g e past five yeas we have developed efined methods and fo chemical pepaation of samples and fo isotopic analysis of a hite acceleato in mass spectomete g p (AMS). We descibe h ee ou methods fo pepaing samples discuss and we pocedual blanks we measue on spectomete. We define a pocedual blank amount as of cabon intoduced duing g convesion of a sample to gaphite. METHODS Convesion to CO2 Inoganic cabon samples ae hydolyzed to CO2 in a side-am eaction vessel. When enough e is sample mateial (>5 mg CaCO3) samples ae weighed into a beake teated fo 3 sec with a 1% HCl solution insed e times with cabon-fee wate died and ewei hed. The acid teatment eemoves fom 12-36% of sample. Fo small samples (<5 mg CaCO3) we omit peteatment acid step. The samples ae tansfeed to eaction vessels 1% H3PO4 is added to sideam and vessel is evacuated and sealed. Afte sealing acid is tipped onto sample and eaction vessel is maintained at 6 C ovenight. We use IAEA C - 1(Caaa mable) as ou hydolysis pocedual blank efeed to as hydolysis blank. We pepae C-i identically to ou samples Poceedings of 15th Intenational 14C Confeence edited by G. T. Coop D. D. Hakness B. F. Mille and E. M. Scott. RADIOCARBON Vol. 37 No P

2 684 A. P. McNichol et al. and pepae one C-1 with each batch of 1 samples. Until ecently we used C-1 to detemine AMS machine backgound and thus have ou lagest data set fo this substate. Oganic samples ae oxidized in a closed-tube combustion. Samples eady fo combustion ae weighed into 9-mm OD x 2-cm long Vyco ound-bottomed combustion tubes. We have used two pocedues to pepae ou combustion tubes. Fo both pocedues Vyco combustion tubes ae ecombusted at 85 C fo 5 h. In fist pocedue (Method I) batches of oxidant (CuO in P wie fom) wee pepaed by baking at 85 C fo 3 h and stoed in a desiccato until added to an individual combustion tube. Batches wee used ove a 2-4 month peiod befoe pepaation of a new batch. In second pocedue (Method II) we pepae combustion tubes by adding 2 g CuO (wie fom) and placing tubes in a muffle funace at 85 C fo 5 h. The tube is filled with a sample and 1 mg Ag powde as soon as possible afte tube has cooled n it is evacuated and flame-sealed. If it is not possible to use pepaed tubes immediately y ae stoed evacuated on a vacuum line. When possible we use enough sample to poduce ca. 81umol cabon. The sample is placed in a muffle funace at 85 C fo 5 h. We have used two cabon powdes (NBS-21 gaphite and Johnson-Maty % pue cabon powde) as ou combustion pocedual blanks efeed to as oganic combustion blanks. The cabon powde is fist weighed into a 6-mm OD x 5-cm-long pecleaned Vyco tube tansfeed to a combustion tube and n teated identically to ou samples. Seawate samples ae stipped in bottle in which y ae collected; method is discussed in detail in McNichol et al. (1994). In a nitogen-filled glove bag a stipping pobe is attached to a sample bottle. The bottle is n attached to ou seawate stipping vacuum line sample is acidified with 85% H3PO4 acid line is filled to.8 atm with nitogen and CO2 is stipped fom each sample using nitogen as a caie gas. Development of an appopiate pocedual blank which we efe to as a line blank has been difficult. We discuss hee esults of two diffeent methods. In fist method a stipping pobe is attached to an empty sample bottle. The bottle is placed on vacuum line and evacuated. An aliquot of 14C-dead CO2 simila to quantity we ecove fom seawate samples is tansfeed to evacuated bottle. The bottle and stipping line ae bought to.8 atm with nitogen while CO2 is still fozen down. The teatment of line blank is n identical to that of samples. In second method a measued aliquot of 14C-dead CO2 in a valved vessel is attached to a stipping pobe. The pobe is n attached to an empty bottle in same glove bag in which samples ae pepaed and bottle is attached to vacuum line. At this point gas is eleased into bottle and line blank is n teated identically to ou samples. Convesion to Gaphite All CO2 samples ae educed to gaphite ove Fe powde using H2 as educing agent (Vogel et al. 1987; McNichol et al. 1992). Ou line blanks anged in size fom 6-124umol CO2. Fo ou hydolysis and oganic combustion blanks we poduced CO2 fom paent mateial in quantities anging fom 2 to 8 umol. Fo vey lage samples we ei split gas into smalle potions fom which we made small individual batches of gaphite o we made one lage batch of gaphite which was late pessed into many tagets. We epot hee an individual esult fo each sepaate taget un on AMS as well as aveage of all esults fo one individual hydolysis o combustion eaction. We assume that most of pocedual blank is added duing eaction to CO2 and that gaphite eduction makes a mino contibution to oveall blank (Vogel et al. 1987).

3 Impovements in Pocedual Blanks at NOSAMS 685 AMS Analysis Appoximately 2 mg of gaphite/catalyst mixtue fo each sample is poued into a dilled aluminum catidge whee it is pessed against a gound flat suface with a pessue of 4.6 kba by an automated pess. The diamete of each taget is 1.5 mm. We have ecently stated using Johnson- Maty % pue cabon powde (heeafte efeed to as JME powde) to evaluate ou machine backgound. It is used diectly fom bottle and pessed with no additional ingedients. The samples standads and blanks ae loaded into a caousel with 59 load positions. One position is left empty and a solid aluminum catidge is loaded into ano position. The caousel is placed into ion souce ai lock whee it is pumped down and outgassed fo seveal hous. Afte a cleaning cycle unde cesium sputte beam data acquisition commences with all tagets being sputteed in tun. Up to nine exposues ae made to each taget ove a peiod of two days. The faction moden (fm) values epoted hee ae aveages of data acquied duing all taget exposues. Duing data analysis time-intepolated values of machine backgound ae used fo subtaction fom standads and some of samples (Schneide et al. 1994). Fo this eason JME powde tagets ae measued egulaly thoughout data acquisition. Residual gases in ion souces can lead to highe machine backgounds so data acquisition is not begun until pessue in ion souce is below 3 x 1-6 mba. Peiodic cleaning of ion souces is also necessay. RESULTS AND DISCUSSION We pesent data collected on acceleato fom June 1992 to June Duing this peiod we established outine potocols fo opeations in SPL and AMS. We have not subtacted machine backgound fom any of data epoted in following sections except whee noted. Inoganic Cabon Data fom analyses of hydolysis blanks and JME cabon powde ae pesented in Figue 1. Until we switched to JME cabon powde we used hydolysis blank to detemine AMS machine backgound. Results ae plotted as a function of seial taget pess (TP) numbe and quantity Y of CaCO3 eacted. The TP numbe inceases sequentially f o each taget pessed fo AMS analysis. Thee is a geneal decease in fm values ove time with highest values obseved below TP numbe 135 and lowest values above TP numbe 381. Most of fluctuations in hydolysis blank data ae pobably due to vaiations in ion souce pessue with outgassing and gettein of poducts sputteed g fom vaious tagets. The ange of fm values obseved fo hydolysis blank is between.6 (596 y) and.932 (376 y). The aveage of ou analyses since TP numbe 381 is.28 ±.45 (496 y). Thee appeas to be a significant eduction in hydolysis blank and a concuent impovement in pecision above TP numbe 381 which occued afte epai of a mino leak in ion souce. Befoe this impovement we believe that most of scatte obseved in hydolysis blank data was elated to acceleato opeation i.e. machine backgound because we continued to see impovement in measued blank values although we have used same hydolysis pocedue fo >5 y (Gagnon and Jones 1993). Thee appeas to be no elation between quantity of CaCO3 hydolyzed and measued fm (Fig. 1B) in ange of sample sizes analyzed indicating that we ae not adding goss amounts of contamination duing hydolysis. If we wee intoducing a measuable amount of moden contaminant to ou hydolysis blanks we would expect fm to incease as sample size deceased.

4 686 A. P. McNichol et al. A N. B.15 I'ii iii 111th 1 I I do d O AAh%/ iuiliui ttI TP Numbe I 1 i-i--- 1 n M p IAEA Gl JMB.1.5. O cp O O' B O $ o cpr$apo c u I _ C2 Micomol 1_i t 6 7 Fig. 1. Faction moden data fo IAEA Ci (Caaa mable) hydolysis blanks and JME cabon powde. The JME powde was pessed diectly into a catidge without any sample pocessing. Recent analyses of JME cabon powde suggest that it poduces a consistently lowe machine backgound than hydolysis blank; fo five most ecent analyses aveage fm is.2334 ±.61 (487 y) with a ange of 453 to 535 y. Fu analyses of both hydolysis blank and JME cabon powde will enable us to estimate pocedual blank intoduced duing hydolysis. As we analyze lage quantities of olde samples we will focus moe attention on souces of machine backgound and wok to educe m even fu. Oganic Cabon Figue 2 shows data fom analyses of combusted NBS-21 and JME cabon powde. The fm values epoted in Figue 2A wee coected fo intoduction of oganic combustion blank using following mass balance:

5 Impovements in Pocedual Blanks at NOSAMS 687 whee f s = (f mcm - f ccc /C5 (1) fm f5 f = measued sample and oganic combustion blank faction moden espectively and Cm CS C = amount of measued sample and oganic combustion blank cabon espectively. B A F o o I Ii.l.l..lli...lll.l.l Taget Numbe i 4 5 NBS21 Method I NBS21 Method II JME Method II M.2.1 NBS-21 Method I NBS-21 Method II JME Method II.!+1 t III..t1i...Ii <<I..ii1 iiiiii Cabon combusted µmol Fig. 2. fm data fo oceanic combustion blanks; data ae pesented fo combustions using Method I and Method II.

6 688 A. P. McNichol et al. We assume that oganic combustion blank is moden (i.e. fm =1) and that majoity of cabon is added duing combustion of sample. We quantified oganic combustion blank fom each method by combusting empty combustion tubes and measuing CO2 geneated (Table 1) in a small calibated volume using a sensitive accuate pessue tansduce. The values epoted in Table 1 ae significantly geate than those epoted elsewhee (Vogel et al. 1987; Donahue et al We believe this may be elated to elatively lage size of ou combustion tubes. Vogel et al. (1987) suggested that much of cabon contamination contibuted duing sample combustion aises fom gas adsobed to Vyco glass. Inceasing Vyco suface aea by using a lage tube should incease contibution fom this souce. We use lage tubes than most eseaches because many of oganic samples we pocess contain only.5-2% oganic cabon by weight and we must place up to 1 mg mateial in a tube. It is appaent fom Table 1 and Figue 2 that ou oganic pepaation has been impoved by changing technique fo pepaing combustion tubes. Consideing fm data fom old method it appeas that we have not constained contibution fom oganic combustion blank well; we have undeestimated it in some cases and oveestimated it in os paticulaly in ealie samples. Using old method we calculate an aveage fm of _.769 (415 y). With new method we calculate aveages of.254 ±.37 (48 y) fo NBS-21 and.198 ±.6 (5 y) fo JME cabon. Consideing uncetainty se values ae simila to those obseved fo machine backgound confiming ou assumption that oganic combustion blank is moe impotant than gaphite pepaation blank o machine backgound. In Figue 2B we have plotted uncoected values of fm against amount of cabon combusted. Figue 2B fu emphasizes how changing ou analysis pocedue has educed size of blank as well as demonstates that fo ange of sample sizes we have analyzed that e is no elation between quantity of oganic matte combusted and measued fm. As with hydolysis blanks this is patly because we have not analyzed many small samples. Dissolved Inoganic Cabon TABLE 1. Measuement of Combustion Pocess Blank: Values Repoted in Micomol Cabon Method I Method II (Batch pepaation of CuO) (Individual pepaation of CuO) Mean sd Figue 3A shows data fom wate stipping line blanks. We plotted se values with espect to TP numbe and did not analyze elationship to quantity of CO2 extacted because we analyze such a lage sample (ca.1 mmol of CO2) that sample size elations ae not meaningful. Thee ae e distinct egions of gaph. The values below TP numbe 21 ae elatively scatteed and eflect esults fom seveal diffeent methods that we wee investigating to pepae ou line blanks.

7 Impovements in Pocedual Blanks at NOSAMS 689 A TP Numbe 4 B Valves Stipping pobe Sample bottle Fig. 3. A. fm data fo wate stipping line blanks; B. schematic of wate-sample bottle with stipping pobe attached Duing this time we used line blank pimaily to veify that no contaminated stipped samples on wate line. The values between TP numbe 21 and 32 coespond pepaed using to line fist blanks method descibed. The aveage faction moden is which.286 is simila ±. to ' ±.165 value w we have measued by analyzing diectly. The values tank CO2 shown fo TP numbes >32 have an aveage fm of.136 ±.259 geate than that obtained with fist method. The incease coesponds CO2 added to to amount sample of when attaching stipping pobe to sample As bottle shown in in Figue 3B both stipping pobe and sample bottle have significant pepaing volumes ou line that in blanks ae exposed to glove bag atmosphee. We attempted bag with to fil a pue NZ atmosphee but have appaently entained some CO2 fom glove laboatoy bag. Using ai in a mass balance and assuming that CO entained in glove bag is mod-

8 69 A. P. McNichol et al. em we calculate that we add ca. 1 µmol CO2 to line blank. Pepaation of line blank isolates ost 8 ml of love bag atmosphee in empty sample bottle and stipping pobe volume. alm g With actual seawate samples we isolate only 3 ml in stipping pobe volume. Thus only 4 µmol of CO2 ae added to each sample. The addition of this cabon to even most depleted seawill alte measued fm by <2%o within eo we measue; We wate sample (fm -.7) believe that most of line blank is added to wate samples duing glove bag opeations. One o step in wate stipping pocess whose blank contibution is being quantified is addition of acid though a septum in stipping pobe. SUMMARY A summay of data fom pocess blanks is shown in Figue 4 and Table F.1.Py u JME Gl NBS-21 JMB I oganic AMS combustion Fig. 4. Summay of coected pocess blanks; eo bas epesent 2... H2O Stipping TABLE 2. Summay of cuent measued pocess blanks afte coection (see text). The amount of cabon added duing pocessing is listed fo each pocess. Fo fm values 1 Q eo is epoted and numbe of analyses included in aveage is epoted in paenses. Pocedue µmol C fm AMS machine IAEA C-1 JME cabon powde Hydolysis Oganic combustion NBS-21 JME cabon powde DIC.2334 ±.61(12).266 ±.56 (5) Not detemined.7.254±.37 (9).7.198±.6 (5) 1*.257 ±.26 (1) *This value efes only to cabon added to line blank; see text fo amount added to wate samples.

9 Impovements in Pocedual Blanks at NOSAMS 691 The fm values pesented hee ae those calculated by subtacting dual conti blank intoduced g button fom any poceduing sample e pepaation. With this coection guish it is any not pocess possible blank to distinfom AMS machine backgound veifying quantified that amount of cabon added duing sample handling. CONCLUSION We analyzed pocedual blanks fo e most common sample NOSAMS. pepaation methods We outinely we measue use at fm values between.2 and samples..25 fo Afte 14C-fee coection hydolysis fo oganic combustion blank we measue simila 14Cfee oganic samples. values At fo pesent contibution fom line blank analyses. has no effect on ou seawate ACKNOWLEDGMENTS We thank Geg Cohen fo had wok in engineeing and AMS labs eview g and of John Vogel manuscipt. fo citical This wok was suppoted by ants OCE-8825 fom National Science ' g 9 and OCE Foundation. This is WHOI contibution numbe 94. REFERENCES Donahue D. J. Linick T. W. and Jull A. J. T. 199 Isotope-atio and backgound coections fo acceleato mass spectomety adiocabon measuements. Radiocabon 32(2): Gagnon A. R. and Jones G. A AMS-gaphite taget poduction methods at Woods Hole Oceanogaphic Institution duing Radiocabon 35(2): McNichol A. P. Gagnon A. R. Jones G. A. and Osbone E. A.1992 Illumination of a black box: Analysis of gas composition duing gaphite taget pepaation. In Long A. and Ka R.S. eds. Poceedings of 14th Intenational 14C Confeence. Radiocabon 34(3): McNichol A. P. Jones G. A. Hutton D. L. and Gagnon A. R The apid pepaation of seawate 1C2 fo adiocabon analysis at National Ocean Sciences AMS Facility. Radiocabon 36(2): Schneide R. J. Jones G. A. McNichol A. P. von Reden K. F. Elde K. L. Huang K. and Kessel E. D Methods fo data sceening flagging and eo analysis at National Ocean Sciences AMS Facility. Nuclea Instuments and Methods in Physics Reseach B92: Vogel J. S. Nelson D. E. and Southon J. R C backgound levels in an acceleato mass spectomety system. Radiocabon 29(3):