802
Anal. Chem. 1983, 55, 802-805
The dramatic reduction of the hydrogen content of GF/C filters by heating a t 500 "C shows that heat treatment is a useful way of reducing problems due to the presence of hydrogen in glass. Most of the water lost by heating probably results from two silanol groups which are sufficiently close for hydrogen bonding rather than from isolated silanol groups. Isolated silanol groups cannot be removed by heating silica at 500 "C (13). The results in Figure 3 also show that heat treatment may be useful in reducing the reactivity of glass with water. The freeze-drying method described here would be useful in conjunction with surface area measurements, since the apparent surface density of hydrogens exchangeable with water in various insoluble solid materials could then be obtained. Registry No. Water, 7732-18-5; hydrogen, 1333-74-0.
LITERATURE CITED (1) Glasoe, P. K.; Bush, C. N. Anal. Chem. 1972, 4 4 , 833-834. (2) Byrne, R. H.; Bryan, W. P. Anal. Biochem. 1970, 33, 414-428. (3) "Glass Microflber Fllters"; Bull. No. 400, Whatman, Inc.: Clifton, NJ. 1976. (4) Bryan, W. P.; Rao, P. B. Anal. Chim. Acta 1976, 8 4 , 149-155. (5) Doremus, R. H. "Glass Sclence"; W h y : New York, 1973; pp 242-248. (6)Todd, 8. J. J . Appi. Phys. 1955, 2 6 , 1238-1243. (7) Todd, B. J. J . Appl. Phys. 1956, 2 7 , 1209-1210. (8) Boulos, E. N.; Kreldl, N. J. J . Can. Ceram. SOC. 1972, 4 7 , 83-90. (9) Davydov, V. Y.; Klselev, A. V.; Zhuravlev, L. T. Trans. Faraday SOC. 1964, 6 0 , 2254-2264. (10) Burn, I.; Drury, T.; Roberts, J. P. Silic. Ind. 1965, 30, 403-407. (11) Lanford, W. A.; et al. J. Non-Cryst. Solids 1979, 33, 249-266. (12) Wllllams, J. P.; et al. Am. Ceram. SOC. Bull. 1976, 55, 524-527. (13) Armistead, C. G.; et 81. J . Phys. Chem. 1969, 73, 3947-3953.
RECEIVEDfor review September 27,1982. Accepted January 17, 1983.
Thermal Vaporization for One-Drop Sample Introductlon into the Inductively Coupled Plasma Ellchl Kltazume Central Research Laboratoty, Hitachi Ltd., Kokubunj;, Tokyo 185, Japan
Inductively coupled plasma (ICP) atomic emission spectrometry has become one of the most useful techniques for determining multielement traces. However, the nebulizer commonly used for sample introduction is inefficient and requires a solution of at least 0.1 mL. More efficient sample introduction systems are required that can provide an analysis using less solution. If only one drop of sample could be effectively introduced into the ICP, absolute detection limits and analytical capability would be greatly improved. Recently, electrothermal vaporization techniques that use a tantalum filament (1)and graphite rod (2, 3) were investigated. However, their evaporation chambers were above 100 mL in volume because of the structure of the large electric current devices used for the thermal vaporization systems. Large chamber dead volumes will lead to temporal peak broadening due to vapor diffusion mechanisms. For improved detection limits, a smaller evaporation chamber volume is better for preventing sample aerosol dilution by the carrier gas. In addition, it is necessary to provide a sufficiently high heating rate to the filament to ensure a rapid rate of removal of analyte from the surface (2). This paper describes the application of a wire filament electrothermal vaporization technique for introducing samples into the ICP. In this technique, a microliter sample solution is vaporized on a filament heated by momentary condenser discharge (4)in a small evaporation chamber. Detection limits for six elements were measured and the effects of sodium, potassium, and lithium on analyte emissions were studied.
EXPERIMENTAL SECTION Apparatus. A block diagram of the overall apparatus is shown in Figure 1. The ICP atomic emission spectrometer system is a Jarrell-Ash Model 975 plasma spectrometer which has 40 fixed-wavelength channels, and an additional variable-wavelength channel. For observation of the signal profile, a 1-m grating monochromator (Nippon Jarrell-Ash, M-l,1200 grooves/mm) and a storage oscilloscope (Hitachi, Model V-038) were mainly used. The wire filament vaporization apparatus employed is shown in Figure 2. The filament was platinum or tungsten wire, 0.25-0.30 mm in diameter, and was placed in a small quartz evaporation chamber (about 4.5 mL in volume). The regular operating con-
Table I. Operating Conditions coolant gas flow rate plasma gas flow rate carrier gas flow rate generator forward power generator reflected power observation height desolvation current condenser charging voltage sample volume integration time
18 L of Ar/min 1.0 L of Ar/min 1.0 L of Ar/min 1.1kW 5W 16 mm above load coil 3.0-4.0 A 9.75 V for Pt filament 8.00 V for W filament 10 ML 3s
ditions are described in Table I. The filament temperature was measured by an optical pyrometer. Procedure. After the plasma was generated and stabilized, the argon flowing through the filament chamber was adjusted to the value established as optimal. The monochromator was set at the desired wavelength for the emission profile that was observed. A 10-pLsample was deposited at the top of the wire filament with an Eppendorf microliter pipet. The sample was dried slowly by passing a 3-4 A current through the filament and vaporized by pulse heating from a high capacity condenser (0.22 F, 16 V). The condenser had been charged to an appropriate voltage that was determined experimentally, and the filament temperature quickly increased to approximately 1400 "C for the platinum filament and to more than 1500 "C for the tungsten filament. The vaporized specimen was introduced into the ICP torch through polypropylene tubing and a three-way stopcock. The emission intensities of elements of interest were then integrated in the polychromator system and printed. (The condenser was discharged immediately after integration was started.) At the same time, the emission signal of the element of interest could be observed by the monochromator and storage oscilloscope. After the measurement, the condenser was discharged twice with no sample on the filament to clean the filament. Standard Solutions. A boron standard solution was prepared by dissolving reagent grade potassium tetrafluoroborate in water, because boron in samples prepared from boric acid was apt to evaporate when the solution was dried on the filament. A phosphorus standard solution was prepared by dissolving reagent grade potassium dihydrogenphosphatein water. Other standard
0003-2700/83/0355-0802$01.50/00 1983 Arnerlcan Chemlcal Soclety
ANALYTICAL CHEMISTRY. VOL 55. NO 4, APRIL 1983
803
ICP otomic emission
V~mrilolion S'O"
npUn 1. B W dlagram of apparab.
d Tirn.
Figure 4. Emlasuion sbnals of boron 249.88 nm: (a) 1 pglmL E. in 5 0 M chamber; (b) 0.1 flg/mL 6. In 4.5-mL chamber: (c) blank signal. in 4.5-mL chamber.
Jb
~
npUn 5. Oscnlograms 01 sbnal 01 boron 249.88 nm for solutlcn containing 1 pglmL B: carrlef gas Row rate. 1.0 Llmin: time base. (a) 200 msldivisbn; (b) 50 msldivision.
-
Fbuv 2. W e fllemmn vapabalbn apDBratu(I: (a)sample in]ecaar port; (b) sample orme. connected thcum pdypopm tubing (20 cm) w M the tach; (c) sample; (d) platinum a tungsten il!amsnt: (e) a m k resh: (1)brass m e w ; (g) sillcon rubber stopper. (h) c a d to condenser; (i) argon gas inlet.
1500 1'00
1300
Y
900
L
'
80
CMrginp
9 0 "oltag.,
10.0
v
npue S.
R e l a l b ~ h i pbetwwn charm Vomtge and Rlammt tern psratue: (a) plannum filament. 0.30 mm in dlameter: (b) tungsten fllament. 0.25 mm in diameter.
solutions were prepared from commercially available standard aolutions for atomic absorption spectrometry. RESULTS A N D DISCUSSION T e m p e r a t u r e Profile of Filament. Figure 3 shows the relationship between the charging voltage of the condenser and the temperature of the filament The platinum filament
was quite firm below 1400 "C. and the platinum emission intensity at 269.95 nm was not detected until platinum fusion occurred (above 10.5 V charging voltage). The tungsten filament temperature could be raised above 1500 OC, however, tungsten began to vaporize from 1300 O C . The following experiments were mainly made using a platinum filament. A tungsten f h e n t was used when a rectilinear calibration curve was not obtained with a platinum filament. Vaporization Chamber Volume. Figure 4 shows boron emission signals measured while using a platinum filament. The signal was obtained through the profile mode of a computer program. Figure 4a was obtained when a 500-mL chamber for a conventional nebulizer was used. With a large volume sample evaporation chamber, the signal appeared slowly, and the peak width was about 1min. Figure 4b was obtained with a 4.5-mL chamber as shown in Figure 2. Figure 4c shows a blank signal of filament "firing" using a 4.5-mL chamber. The peak intensity increased remarkably, and the peak width was about 1 8. For more exact measurement of the signal profile, a storage oscilloscope connected with the monochromator was used. Figure 5 shows the oscillograms. The signal appeared about 250 ms after the discharge and finished before 1 8 had elapsed. Sample vapor condensation losses on chamber walls were not studied in this paper. Although it is possible that such losses may be more significant with an unusually small chamber where the cold wall is in closer proximity to the hot expanding vapor cloud, further studies are needed to establish whether or not this ia a significant problem. If it is a problem, Figure 4 clearly shows that any such condensation disadvantage is a t least heavily counterbalanced by elimination of dead volume, peak hroadening, vapor diffusion, and gas di-
804
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4. APRIL 1983
Table 11. Detection Limits detection limit graphite rod atomizer-ICP,d element
line," nm 249.68 199.82 214.91b 220.35 189.99 206.20
B Ge P Ph Sn Zn a
filament Pt Pt Pt
W W Pt
Wavelengths used for present work.
relmL 0.0004 0.006 0.01 0.002 0.002 0.003
Second-order line.
E
From ref 1.
PB
4 60 100 20 20 30
TFV-ICP: pg 10
PB
2000
200
300
100
2000 20
From ref 2.
t
-
600
-
I
; I 21 l0
L . 10
12
14
16
ObWC C0d.
in
20
e s
, , , \
,
H.,Qh,
E 400
- 200
22
. P
mm
Figure 8. Relationship between obsefvntbn height and S/B ram boron concsnlralbn. 0.1 pglmL: carrbr gas Row rate. 0.8 Llmin.
lution effecta with the present smaller chamber. In fact. the smaller chamber is seen to eive a much narrower. more intense signal profile. Optimization of Observation Height and Carrier Gas Flow Rate. Figure 6 shows the relationship between the observation height above the load coil and the ratio of signal to background. The maximum signal to background was obtained between 15 and 16 mm above the load coil. The following measurement was made a t a height of 16 mm. This height was also optimum for a conventional nebulizer introduction system. Figure 7 shows the relationship between the carrier gas flow rate, and the ratio of signal to background. T h e relationship between the carrier gas flow rate and the emission intensity is also shown. T h e emission intensity proved to be maximum a t a flow rate of 0.4 Lfmin. However. as the carrier gas flow rate decreased. the background level increased. Consequently, the maximum signal to background ratio was obtained at a flow rate of 1.0 Lfmin. The measurements that follow were made a t a flow rate of 1.0 L/min. Figure 8 shows the oscillograms observed when the carrier gas flow rate was changed from 0.75 Lfmin to 0.85 Lfmin. In these regions. double peaks appeared. Below 0.75 Lfmin, the second peak was the larger of the two, but above 0.85 Lfmin. the reverse was true. If the sample changes to two or more different compounds when vaporized from the filament, boron arrival time to the plasma may differ because of the difference in the evaporation r a t a for these cumpounds Presumably, at high carrier gas flow rate, all boron will be removed quickly enough to prevent any secondary compound formation. In all cases, as the emission signal is ordinarily i n t e m a d . the intemated sinnal would not chanae - even if the double peak appeared. Effect of Coexisting Elements. The effect of alkali metals on the emission intensities of boron, germanium, and phosphorus (each 1 &mL) was measured. Generally, signals were suppressed with an increase in alkali metal concentration. When 0.5 mg/mL of potassium or sodium was present in the solution, 10-15% decreases in germanium and 30-5070 decreases in boron and phosphorus intensities were observed. The presence of potassium and sodium did not change the maximum emission intensity position in each oscillogram.
2E E
Y
~
-
lb r
FIgm 0. Osallogams 01 signal 01 Mron 249.88 nm for sohmon mntahkrg 1 p g l m l E. Canler gas Row rate was as l o b w (a) 0.75 Llmin: (b) 0.80 Llmh: (c) 0.85 Llmln.
However, the preaence of 0.5 mg/mL of lithium delayed the attainment of maximum boron emission intensity. Detection Limits. Table II shows the detection limits for six elements when a 10-pLsample was injected. The detection limit was defined as the concentration required to produce a net emission intensity equivalent to twice the standard deviation of the background noise. Literature values with use of a tantalum filament vaporization system (I) and graphite
805
Anal. Chem. 1983, 55, 805-807
rod vaporization system (2) are also shown in Table 11. A comparison of the absolute detection limits expressed as analyte mass shows that in most cases a superior power of detection was obtained with the present method. The relative standard deviation was 2-4% for boron, lead, phosphorus, and tin, and 5-10% for germanium and zinc with a 1 pg/mL concentration. A rectilinear calibration curve was obtained from 0.001 pg/mL over 4 orders of magnitude for boron, from 0.01 pg/mL over 3 orders of magnitude for phosphorus, lead, tin, and zinc, and from 0.01 pg/mL over 2 orders of magnitude for germanium. The author acknowledges that the present study is limited to relatively volatile elements. It is not known whether the same advantages of this small chamber system would also occur with less volatile elements. This will be the subject of future investigation in this laboratory.
ACKNOWLEDGMENT The author is grateful to Atsushi Mizuike, Horishi Kawaguchi, Fumio Nagata, and Hisao Kojima for their helpful suggestions. Registry No. B, 7440-42-8; Ge, 7440-56-4; P, 7723-14-0;Pb, 7439-92-1; Sn, 7440-31-5;Zn, 7440-66-6. LITERATURE CITED (1) Nixon, D. E.; Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974, 46, 2 10-2 13. (2) Gunn, A. M.; Millard, D. L.; Kirkbright, G. F. Analyst (London) 1978, 103, 1066-1073. (3) Millard, D. L.; Shan, H. C.; Klrkbright, G. F. Analyst (London) 1980, 105, 502-508. (4) Kawaguchl, H.; Vallee, B. L. Anal. Chem. 1975, 4 7 , 1029-1034.
RECEIVED for review April 13, 1982. Accepted December 9, 1982.
Breath Analysis by Atmospheric Pressure Ionization Mass Spectrometry Frank M. Benoit” Environmental Health Dlrectorate, Health and Welfare Canada, Tunney’s Pasture, Ottawa, Ontario KIA OL2, Canada
W. R. Davldson, A. M. Lovett, Sabatlno Nacson, and Angle Ngo Sciex Inc., 55 Glencameron Road, Thornhill, Ontario L3T 7P2, Canada
The value of human breath analysis in medical, forensic, and environmental applications has long been recognized. Sampling techniques have consisted of the collection of large volumes of breath in Teflon ( I ) , Mylar (2-5), or Plastigas (6) bags, in glass tubes ( 4 , 5 , 7-9),or in cryogenic traps ( 1 0 , I I ) followed by concentration of the volatiles onto Tenax (1,111, charcoal (3), silica gel (6),or cryogenic ( 2 ) traps prior to analysis by infrared spectrometry, gas chromatography, or gas chromatography/mass spectrometry. A recent report (12) suggested that direct introduction of expired breath into the ion chamber of API/MS system would allow the extraction of the organics directly from the gaseous mixture by ionmolecule ionization. However, direct breath introduction into the ion source caused difficulties due to interferences from ammonia (12). In the present study a novel breath inlet system which allows the direct sampling of exhaled human breath for analysis by API/MS is described.
EXPERIMENTAL SECTION Breath Inlet. The breath inlet shown in schematic form in Figure 1 consisted of a mouthpiece, filter tube, manifold, and mixing chamber mounted on a metal frame. The disposable polyethylene mouthpiece (Inspiron,Rancho Cucamonga, CA) was separated from the glass fiiter tube (22 mm 0.d. X 270 mm, Sovirel, France) by a diaphragm one way valve (Inspiron, Rancho Cucamonga, CA). The filter tube could be filled with filter material contained between two loose fitting glass wool plugs, if required. The filter tube was fitted with a port (Swagelok fittings) though which carrier gas could be directed, if required, to simulate an exhalation for calibration purposes. The filter tube assembly was attached to the stainless steel manifold via a tee connection of which one arm, fitted with an adjustable valve, led to the atmosphere and the second arm, containing a replaceable glass capillary, led to a stainless steel manifold. The manifold was connected to a mixing chamber containing perforated macor disks to ensure mixing of the breath sample with the carrier gas prior to introduction into the ion chamber. The manifold and filter tube assembly were fitted with outlets for the pressure transducers such that the pressure differential across the capillary was measured by two manometers connected in parallel. One ma0003-2700/83/0355-0805$01.50/0
nometer (Magnehelic,range 0-2 in. of water, Dwyer Instruments, Michigan City, IN) provided a visual reading to the subject who was required to maintain a prescribed constant pressure differential across the capillary during exhalation. The second manometer (Robinson Halpern, range 0-10 in. of water, Plymouth Meeting, PA) controlled the scan of the TAGA ion analyzer and provided a reading of the pressure differential during each scan period. The carrier gas, zero grade air (Matheson) was passed through a filter (molecular sieves Linde 13X and Linde 4A) to remove impurities in the air prior to introduction into the manifold. The carrier gas flow was controlled by a Lee Jet orifice such that the flow could be set by adjusting the regulator pressure.
RESULTS AND DISCUSSION Atmospheric Pressure Ionization. In the positive mode of operation a series of protonated water clusters H+(H20), are generated from the moisture in the ambient air following a series of ion-molecule reactions (13)initiated by the ionization of molecular nitrogen in a point to plane corona discharge. Ionization of a sample molecule (T) produces protonated moleculewater clusters of varying size depending on n (reaction 1).
T + H+(H,O),
+
TH+(HZO),
Upon passage of a clustered target ion through a gas (N,) curtain located between the ion chamber and the mass analyzer (14) collisions with the neutral nitrogen molecules activate the ion sufficiently to strip the loosely held water molecules from the ion and a protonated molecule, (eq 2) is transmitted to the analyzer. One consequence of the “declustering” is that the distribution of ions collected at the detector does not necessarily correspond to the original distribution of ions generated in the ion source a t the moment of ionization. In the negative mode of operation the reagent ion 02-is generated by point to plane corona discharge ionization of oxygen in ambient air. Some of the observed reactions with 0 1983 American Chernlcal Soclety
