Anal. Chem. 1988, 60,372-375
372
Use of a Water-Cooled Low-Flow Torch in Inductively Coupled Plasma Mass Spectrometry Sir: Argon consumption represents the major running cost for inductively coupled plasma (ICP) spectrometers. Hieftje has reviewed the approaches to the development of ICP torches that reduce the argon requirement (I),and de Galan and van der Plas have recently surveyed the extensive literature relating to such torches (2). In broad summary, in low-flow torches the coolant flow is either reduced through optimization of the torch dimensions or made redundant through use of external cooling. Generally, analytical performance equivalent to that obtained with standard ICP torches can be achieved with low-flow torches in optical emission ICP spectrometers. The requirements upon the ICP when it is used as an ion source for an inductively coupled plasma mass spectrometer (ICP-MS) are not necessarily the same as for emission spectrometers. Significant populations of singly-charged ions of analyte species are required a t some point in the plasma accessible to the spectrometer sampling aperture. Ideally the combination of plasma and sampling interface should only present singly charged monoatomic plasma species to the mass spectrometer; in practice there can be nonzero levels of other species. The rapid scanning capability of the quadrupole mass filter means that the spectrometer is effectivelya simultaneous detector in as far as there must be compromise plasma conditions suitable for all analyte species. For the preliminary investigation reported here, a torch externally cooled by water was chosen for its convenience of installation and its established performance in optical spectrometers when using a much lower argon flow than standard torches (3, 4). Standard commercially available ICP-MS equipment (5),based on the work of Gray and Date (6) was used with minimal modification. EXPERIMENTAL SECTION Figure 1 shows the arrangement of the torch, sampling cone, and load coil, as they were mounted in the torch box of a VG Instruments ICP-MS instrument and positioned up against the sampling interface. Details of the torch are given in Table 11. No problems were experienced with horizontal operation of the plasma. The nebulizer used was a V-groove type with a lOO-pm, argon orifice (7). This sprayed well at low nebulizer gas flow rates (Table I). A solution pumping rate of 1.0 mL/min was used. An attempt was made to use a standard Meinhard concentric nebulizer, but the flow rate of nebulizer gas was too great for stable operation of the plasma. The spray chamber used was a Scott dual-pass type with a cooling water jacket. The standard radio frequency (rf) generator and torch box were employed, and the automatic tuning network proved easily capable of matching the load. Ignition was straightfornard over a range of plasma gas flows and power settings, and the torch ran without mishap for the whole day. Other relevant parameters of the ICP-MS instrument are given in Table 111. As is usual for this type of torch only two argon flows are applied, because the ball-shaped plasma becomes unstable upon introduction of an intermediate argon flow. At the low argon flows used, the plasma tends to be ball-shaped rather than have a long tail flame, and this can lead to problems in positioning the sample cone orifice at the most analytically useful position. The torch box positioning was arranged so that the cone tip could be placed as far as possible inside the torch. The position of the injector tube relative to the rest of the torch and thus the load coil was adjustable,but not found to be too critical. Typically it was placed 5-10 mm behind the back of the load coil. Initially a sampling depth (load coil to sample cone tip separation) of around 10 mm was tried, with plasma gas flow typical of that found most useful for optical emission, around 1.0 L/min. Sensitivity was poor however, and the blank spectrum very
Table I. Nebulizer Calibration pressure drop, psi
gas flow, L/min
20 30 40
0.15 0.20 0.29
Table 11. Low Flow Torch Parameters inside diameter injector diameter cooling water
20 mm 0.5 mm 2 L/min
Table 111. Instrumental Parameters ICP generator frequency load coil
sampling cone orifice diameter skimmer cone orifice diameter sample cone-skimmer separation cone material quadrupole frequency
27.12 MHz 3.5 turn 1.00 mm 0.70 mm 5.5 mm
nickel 2.63 MHz
complex. Visually, it was clear that the region being sampled was relatively cool. When the plasma gas flow was increased to around 2.0 L/min, the luminous plasma was extended further downstream from the load coil. Confirmation that hotter plasma regions were thereby sampled was gained by checking the pressure in the first vacuum stage as measured by a Pirani gauge in the pumping line from the rotary pump (Figure 2). Best analytical performance coincided with the lowest preesures, and nebulization of an yttrium solution showed that the region of intense Y I and Y I1 emission was then being sampled. An attempt was made to reduce the argon requirement to 1 L/min by repositioning the load coil close to the torch mouth, with corresponding adjustment of the injector tube position. This produced a sampling depth of 5 mm. Although a low first stage pressure was obtained at lower gas flow, sensitivity was a factor of 10 reduced.
RESULTS A solution containing 1 lrg/mL of the elements cobalt, cerium, bromine and iodine was used to search for analytically useful operating parameters. Cobalt (mass 59) was used for instrument tuning and for checking basic sensitivity. During the optimization, sensitivities of lo6 (counts/s)/(pg/mL) of cobalt were reached, similar to that expected from standard torches and the same ICP-MS configuration. Iodine and bromine have relatively high first ionization potentials (10.95 and 11.81eV, respectively), allowing the ionization efficiency of the plasma to be examined. Cerium has a low second ionization potential (10.85 eV) and a strongly bound oxide, allowing doubly charged ion and oxide levels to be examined. A useful set of plasma operating conditions was found to be 1100 W generator forward power, 2.1 L/min plasma gas and 30 psi nebulizer pressure. These conditions were defined as “standard” for comparison purposes. Table IV shows how relative sensitivities and levels of some potentially interfering species compared with the values obtained when the same solution was run on a PlasmaQuad with a standard torch. The relative levels of various species observed when sampling from the low-flow torch appear quite unusual, and will require further investigation in some detail. Clearly the combination of water-cooled low-flow torch and standard ICP-MS had produced an instrument with notably different characteristics.
0003-2700/88/0360-0372$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60. NO. 4, FEBRUARY 15. 1988 373 Sample cone
I
Logcoil
2,0
Flgun 1. schematic lllustratlm of torch, load coil, and sample cone arrangement.
1
I b l doubly charged ions
0.00
0'02
1
lcl oxide level
18° 4
Generator power, W 10
20
30
Plasma gar f l o w . Llrnl"
Flgue 2. pressure In the Hrst vacuum stage as a fun& gas flow. sampling depth = 10 mm.
of plasma
Table 1V. Comparison of Low-Flow Torch with Standard Tor& parameter lnI+I 6sCo+
(?BBI+ +
measwe of
low-flow standard torch' torch'
degree of ionization
0.45 0.017
0.51 0.11
background
0.043
0.061 0.00037
~'Rr+)/Wo+
+
0.052 0.42
%r2+/'Co*
'wCe"l'We+
doubly charged-ions
0.91
0.65
o.ooo69
'Relative levels of various species observed in a 1 & n L solution of cobalt, bromine, iodine, and cerium. 'Low-flow torch ow mating conditions: 1100 W, 2.1 L/min plasma gas, 30 psi nehulizer pressure = 0.20 L/min. 'Standard torch operating conditions: 1300 W, 13 L/min cool gas, 0.6 L/min auxiliary gas, 0.73 L/min nebulizer gas. The elements with high ionization potentials appear less well ionized in the low-flow torch. Polyatomic species like oxides and the argon dimer were reduced compared to the standard torch, but the doubly charged ion level and contribution from the nickel cones was considerably increased. I t has been suggested that a high doubly charged level reflects the presence of a "pinch" discharge in the sample cone orifice (8,9), but if such a discharge was present in this work, i t was not visible to the eye. The degree of double ionization has also been related to the ion energy (IO),although the relationship is not simple (11). The mean energy of cobalt ions from the low-flow torch was measured as +28 eV hy applying an electrostatic r e t a r d i i potential at the quadrupole (6,12);this is considerably higher than that usually obtained with the standard torch in this ICP-MS system (+5 to +10 eV).
Flgun 3. Effect of generator power variation on senslthrtiy (a).doubb' charged im level (b). ox& level (c), and background species (d). Plasma gas flow = 2.1 Llmin. 30 psi nebulizer pressure. Coban
sensitivity In (a) k normalized to that obtained at standard condnions defined In the text. Table V. Quadrupole Scan Parameters mass range number of sweeps
data channels time per sweep total acquisition time
2-211 amu 735 2048 0.16 s 120 9
Overall, the results appear to indicate a considerable increase in plasma potential when using the low-flow torch. The use of a 3.5-turn as opposed to the usual 2.5-turn load coil may have been a contributing fador, but it was suspected that the much grester proportion of the total plasma than usual being sampled tbrough the orifice may have been significant. With the 1-mm aperture, around 1.9 L/min (at 20 "C) was being taken in (10) from a total flow of 0.2 L/min carrier gas and 2.1 L/min plasma gas, compared to 0.7 L/min and 12 to 15 L/min plasma gas with the standard torch. A 0.5-mm aperture was therefore substituted, reducing the gas intake to 0.5 L/ min. A slight reduction in the cobalt ion energy, to +21 eV, was obtained, hut there was also a factor of 100 loss in sensitivity, so this approach was not persued further. Figures 3, 4, and 5 show how selected parameters varied with generator forward power, plasma gas flow, and nebulizer pressure, with the cobalt sensitivity normalized to that obtained a t the 'standard" conditions defined above. Such data should generally be considered with some caution. Apparent losses of sensitivity can often be recovered through reoptimization of the ion focusing, particularly in the case of carrier gas flow variation. This is illustrated in Figure 5a, where the dotted line shows the improved sensitivity of the cobalt signal if the potentials on the extraction and collection lens elements, that sewe to transfer ions from behind the skimmer cone to the main focusing elements, were reoptimized at each nebulizer pressure. To investigate the a n d y t i d promise of the low-flow torch for ICP-MS the NBS trace elements in water standard SRM
374
ANALYTICAL CHEMISTRY, VOL. 60, NO. 4, FEBRUARY 15, 1988
Table VI. Analysis of NBS SRM 16436O
1
lbi doubly charged Ions
000 L-- ' id1 background species I +
11
15
19
23
21
Plasma gas flow,
element
isotopes used
certified value, ng/g
As Ba Be Bi B Cd Cr co cu Fe Pb Mn Mo Ni Se Ag Sr T1 V Zn
75 138 9 209 11 111, 112, 114 52 59 63, 65 56 206, 207, 208 55 98 60 77, 82 107 + 109 88 203, 205 51 66
(49)* 44 f 2 19 f 2 (Illb (94Y 20 f 1 18.6 f 0.4 26 f 1 21.9 f 0.4 99 f 8 23.7 f 0.7 28 f 2 85 f 3 49 f 3 9.7 f 0.5 9.8 f 0.8 227 f 6 8.0 f 0.2 45.2 f 0.4 66 f 2
analysis, ngjg 1 2 23 38 18 10 78 17 17.1 25 20.2 103 21.3 25 96
23 43 17 11 84 18 18.0 28 21.5 104 25.7 28 104
9.0 225
9.9 235
43.0 62
43.9 58
Ni and Se calibrations were affected by high background levels. T1 was not present in the prepared standard. bParentheses indicate a noncertified value.
25
L/mn
Figure 4. Effect of plasma gas variation on sensitivity (a), doubly charged ion level (b), oxide level (c), and background species (d). rf power = 1100 W, 30 psi nebulizer pressure. Cobalt sensitivity in (a) is normalized to that obtained at standard conditions defined in text.
j
~
(ai sensitivity
I
Ibl doubly charged ions
1
YASS-LEIITS
Figure 6. Part of a wide range mass spectrum showing thallium (masses 203, 205), lead (204, 206, 207, 208),and bismuth (209) in NBS SRM 1643b at 8, 24, and 11 nglmL respectively.
defined by the blank and the 50 ng/mL standard, the concentrations reported in Table VI were calculated for the two acquisitions on the NBS standard. It can be seen that accurate results were obtained for a wide range of elements. Figure 6 shows the region of the spectrum for the NBS standard containing the thallium, lead, and bismuth isotopes.
CONCLUSIONS 03
60NI*/59C0+
0.0
20
30
40
50
Nebuliser p r e s s u r e , P S I
Figure 5. Effect of nebulizer pressure variation on sensitivity (a), doubly charged ion level (b), oxide level (c), and background species (d). rf power = 1100 W, 2.1 L/min plasma gas flow. Cobalt sensitivity in (a) is normalized to that obtained at standard condltbns defined in text. Dotted line shows influence of ion lens refocusing at 20 and 40 psi on sensltivity, as explalned in text.
1643b was determined. A wide mass range scan was defined (Table V). A pair of acquisitions were made in turn on a blank, the NBS standard, and a made up standard containing 50 ng/mL of each of the trace elements. From the calibration
The practicality of the use of a water-cooled torch using only 2.3 L/min of argon for ICP-MS has been demonstrated. Considerable further work is required to investigate the characteristics of the plasma with respect to ICP-MS analysis, notably the reasons for the high ion energy. Stability and matrix effects must also be compared with those found with the standard torch.
ACKNOWLEDGMENT The authors acknowledge the expert assistance of D. Mills in rapidly obtaining reliable operation of the externally cooled ICP torch in the PlasmaQuad ICP-MS system. The original suggestion for this work was made by P. Blair. Registry No. As, 7440-38-2; Ba, 7440-39-3; Be, 7440-41-7; Bi, 7440-69-9; B, 7440-42-8; Cd, 7440-43-9; Cr, 7440-47-3; Co, 744048-4; Cu, 7440-50-8; Fe, 7439-89-6; Pb, 7439-92-1; Mn, 7439-96-5;
375
Anal. Chem. 1988, 6 0 , 375-377
Mo, 1439-98-1; Ni, 7440-02-0; Se, 1182-49-2; Ag, 1440-22-4; Sr, 7440-24-6; T1,1440-28-0;V, 1440-62-2; Zn, 1440-66-6;Brz, 112695-6; 12,1553-56-2;Ce, 7440-45-1;Ar, 1440-31-1;H20, 7132-18-5. LITERATURE CITED Hieftje, G. M. Spectrochim. Acta, Part 6 1983, 386,1465. Galan, L. de; Plas, P. S. C. van der Inductlvely Coupled Plasmas h Ana/ytlcal Atomic Spectrometry; Montaser, A., Gollghtty, D. W., Eds.; VCH Publishers: New York, 1967; Chapter 14. Ripson, P. A. M.; Jansen, L. B. M.:Galan, L. de Anal. Chem. 1984, 56, 2329. Kawaguchl, H.; Tanaka, T.; Miura, S.: Xu, J.; Mizuike, A. Spectrochim. Acta, Part 6 1983, 396, 1319. PlasmaQuad; VG Isotopes, Ltd.: Winsford, Cheshire, England. Gray, A. L.;Date, A. R. Analyst (London) 1983, 108, 1033. Plas, P. S. C. van der: Galan, L. de Spectrochim. Acta, Part 6 1984, 398, 1161. Olhrares, J. A.; Houk, R. S. Anal. Chem. 1985. 5 7 , 2674. Douglas, D. J.; French, J. B. Spectrochim. Acta, Part 6 1986, 4 1 6 , 197. Gray, A. L. Spectrochlm. Acta, Part 6 1986, 416, 151. Gray, A. L. J. Anal. At. Spectrom. 1986, 1 , 247.
(12) Fulford, J. E.; Douglas, D. J. Appl. Spectrosc. 1986, 40, 971.
J. S. Gordon
VG Isotopes Ion Path, Road Three Winsford, Cheshire CW7 3BX, England
P. S. C. van der Plas Leo de Galan* Laboratorium voor Algemene en Analytische Scheikunde de Vries van Heystplantsoen 2 2628 RZ Delft, The Netherlands
RECEIVED for review January 29,1987. Accepted October 1, 1987. This investigation is supported by the Netherlands Technology Foundation (STW), future technical science branch of the Netherlands Organization for the Advancement of Pure Research (ZWO).
Bias in Quantitative Capillary Zone Electrophoresis Caused by Electrokinetic Sample Injection Sir: The use of capillary zone electrophoresis (CZE) has grown dramatically since the papers of Mikkers, Everaerts, and Verheggen ( I ) and Jorgenson and Lukacs (2) first appeared. There are two principal methods for introducing sample into the capillary tube: (1) electrokinetic injection, sometimes referred to as electromigration, and (2) hydrostatic injection, also sometimes called suction, pressure, or gravity injection. The electrokinetic injection method arose from the finding (3) that electroosmosis acts as a pump. It has been tacitly assumed by many that this method delivers a representative sample into the capillary. In fact, under some conditions, it does not. This problem has been mentioned (2, 4 , 5 ) , but, to our knowledge, no one has presented any definitive data illustrating this phenomenon. We found that there are two biases involved. One is brought about by the different mobilities of the species in the sample solution. This effect causes a distortion in the ratio of peak areas for species having different mobilties. The other effect is related to the electrical resistance of the medium in which the species are dissolved. This alters both the electrophoretic and electroosmotic flow rates for different solutions and thus changes the absolute amount injected. Once the origin of this bias is understood, it is possible to correct for it approximately; alternatively, it can be avoided entirely by using hydrostatic injection, provided that the inside diameter of the capillary is not so small that it distorts seriously the injection front. BIAS WITHIN A SINGLE SAMPLE CAUSED BY ELECTROKINETIC INJECTION As previously discussed (6),the amount Q(i) of species i introduced into the capillary by electrokinetic injection is given by Q(i)= l(i)AC(i) (1) where l(i) is the length of the sample zone, A is the crosssectional area of the capillary, and C(i) is the concentration of the species i in the sample solution. The length &) is determined by the electrokinetic injection time t, and the total ion velocity u&), which is equal to the total ion mobility b ( i ) times the electric field strength E
Z(i)
= u,,(i)t = pLtot(i)Et
(2)
0003-2700/88/0360-0375$01.50/0
The total mobility pLtot(i)is the sum of the electrophoretic mobility p(i) and the electroosmotic mobility hoam ~t.ot(i)=
~ (+4poam
(3)
Here we assume that ptot(i)is independent of distance along the capillary. This approximation is expected to hold, in general, to high accuracy when the species i is present in low concentration. Combining eq 1-3 we have the relation
Q(i) = b(i)+ ~ o s m l C ( i ) A E t
(4)
For a sample solution containing the species 1and 2, the ratio of the amounts electrokinetically injected is given by
When b = 1, then Q(1)/Q(2) is directly proportional to C(l)/C(2) and electrokinetic injection is free from bias. Note greatly that Q(l)/Q(2) approaches C(l)/C(2) only when poam ) ~ ( 2 ) Otherwise, . electrokinetic injection exceeds both ~ ( 1and introduces a sampling bias which must be taken into account for accurate quantitation. In order to make this correction, it is useful to introduce the concept of a retention time R(i) for the species i to reach the detector located a distance d from the injection end of the capillary
-
d
[ ~ (+ i )~ o a m I E
(7)
Reference to eq 6 shows that for a two-species system the bias factor is given by
b = R(2)/R(1)
(8)
Thus, by measuring the ratio of the retention times, we can 0 1986 American Chemical Society
