Anal. Chem. 1991, 63, 1497-1498
1497
CORRESPONDENCE Signal Enhancement of Elements Due to the Presence of Carbon-Containing Compounds in Inductively Coupled Plasma Mass Spectrometry Sir: The use of an inductively coupled plasma (ICP) as an ionization source for mass spectrometry (MS) has gained general acceptance. The advantages of ICPMS are wellknown: low limits of detection and quasi simultaneous multielement determination with the possibility of measuring isotopic abundance. The main limitation is the observation of isobaric interferences, mostly due to the formation of molecular compounds (I), in particular, oxides (2). The mass of the matrix has also been reported (3) as a possible source of analyte interferences, probably caused by ion transport in the mass spectrometer. We surmised the possible role of organic compounds as signal enhancers for some elements during analysis of biological samples by ICPMS. In this work, we will report some preliminary investigations on this enhancement effect using glycerol and glucose as the organic matrix. The results will be compared to those previously published for optical spectrometry. EXPERIMENTAL SECTION The system has been described in detail in a previous work (4). It consisted of a 56-MHz tuned line ICP generator and a modified Nermag R 1010 C mass spectrometer. A Meinhard nebulizer associated with a double-pass spray chamber was used. The operating parameters were power of 1.2 kW, outer gas flow rate of 12 L m i d , and carrier gas flow rate of 0.85 L mi&. Mass flow controllers (Brooks Models 5878/5850) were used to measure both flow rates. The detection was performed by means of an electron multiplier working in analog mode.
RESULTS The effect of increasing glycerol concentration on some ion signals is summarized in Figure 1. It can be seen that the enhancement for Hg can reach 600% for a glycerol concentration of 1 mol L-l. Similarly, As and Au showed enhancements of 240% and 325%, respectively. Other elements exhibiting the same effect were Se (250%) and Te (190%). This behavior was in contrast to other elements such as Bi, Co, Eu, Ho, I, In, La, Mo, Ni, Pb, Pt, Sn, Sr, T1, and U where the change in the signal was between 90% and 110%. A similar effect on the ion signals was observed when working with glucose. A t this stage, it was thought that the change in the signal was not due to any variation in the nebulizer efficiency but to a change in the ion formation in the plasma due to the presence of carbon. To confirm this assumption, a similar experiment was carried out with methane in the argon carrier gas. Concentrations of methane up to 6% (v/v) were added to the carrier gas flow. The variation in the responses for As, Se, Te, Eu, and Pb is shown in Figure 2. It can be seen that the enhancement was observed for the same elements as for the addition of glucose or glycerol. The effect is therefore linked to the presence of carbon in the matrix. Moreover, this effect seemed to be relatively independent of the concentration of the analyte because the calibration curves for As, for example (Figure 3), are linear for deionized water and for two concentrations of glycerol. 0003-2700/91/0363-1497$02.50/0
DISCUSSION Additions of methane and propane have already been used in ICP atomic emission spectrometry (AES) (5,6)and atomic fluorescence spectrometry (AFS) (7,8).The purpose of the addition of these gases was to reduce the formation of the refractory oxide by producing CO, with the following proposed reaction:
MO
+ C -,M + CO
Alder and Mermet (5) used a 6-kW ICP. At this high power, no La0 emission was observed with pure argon and the addition of CHI did not modify the La signal. Long and Winefordner (7) used a torch configuration modified for AFS with an extended tail. They used a low power (500-700 W), so the results were very poor for elements forming refractory oxides or monohydroxides such as Mo and Ca, respectively. However, the addition of propane significantly enhanced the fluorescence signal of Ca and Mo. Demers (8)used the same torch configuration and found that a moderate addition of propane (10 mL min-') reduced the formation of oxides with a dissociation energy between 4 and 6 eV. A larger concentration of propane (50 mL min-') was needed for the dissociation of oxides with an energy greater than 6 eV. Elements such as Mo, Ta, Ti, and V could only be determined in the presence of propane. Walters et al. (9), using a saturated absorption method for measuring the spatial distribution of absorbing species, indicated that a change of the Sr population was observed at the edge of the central channel when propane was added. This was attributed to the reaction of carbon radicals with the Sr oxide. In a recent paper (6),Long and Bolton studied the effects of the addition of propane in ICPAES. They also found that the addition of propane reduced the formation of Lao. This reduction in oxide formation can also be applied to elements such as A1 and Cr. However, they observed a different result for Ca, where the decrease in the possible formation of CaO or CaOH did not correspond to an increase in the Ca signal. This is in marked contrast to the results obtained in AFS. It should be noted that AES and AFS do not produce the same type of information, as AES deals with excited states and AFS with ground states. It does not seem that the results mentioned above can be applied to those obtained in this work. Elements such as La, Eu, and Ho exhibit the most refractory oxides and should be strongly affected by the presence of methane, if the oxide reduction is the main process. In fact, the signals from these ions were not markedly modified. An alternative explanation of the depression observed for some elements, proposed by Long and Bolton (6),was the formation of stable metal carbides. However, no depression effects were observed for elements such as Mo. They also suggested that a change in the population of excited states was obtained by collisions with C or carbon radicals. However, this should not drastically affect the concentration of the ions in the ground state. As it does not seem that the enhancement effect is linked to the chemical properties of the elements, we have investi0 1991 American Chemical Society
0
ANALYTICAL CHEMISTRY, VOL. 63,NO. 14, JULY 15, 1991
1498 700
0
al
600 -
0
5
4-
500
C
w
e
2
-
4001
n
f
C
al
E0
c 200 m c 3100, w 1
m c C
joo: 400
jool 200
0 loot
0,2
0
0,4
0.6
0,s
1,2
1
glycerol concentration (mol/l) Figure 1. Enhancement percentage for selected elements as a function of the glycerol concentration: (0)Hg: m Au; (A)As: (e) La. 600
0
v
~
4
5
6
7
6
9
10
1 ~~
11
12
ionization energy (eV) Figure 4. Enhancement percentage as a functlon of the lonlzatlon energy. pounds modifies the ionization equilibrium over a limited range of energy. It must be noted that the ionization energy of C (11.20 eV) is slightly above the upper limit. A consequence of this difference in response of each element, is the difficulty in selecting an internal standard. In previous work (4,Eu was selected as an internal standard. The use of Eu is adequate with most elements but not with those which exhibit an enhancement effect. The use of MS as the detection method for an ICP allows access to information on the population of the ion ground state in contrast with AES, which concerns ion excited states. This probably explains why such a matrix effect has not been reported for ICPAES.
0
1
2
3
4
6
5
7
methane concentration (% v / v ) Figure 2. Enhancement percentage for selected elements as a functbn of the methene concentratbn in the c a w gas fbw: (0)As; (+) Te; (W) Se; (A)Eu: (A)Pb.
LITERATURE CITED Tan, S. H.; Horlick, G. Appl. Spectrosc. 1988, 40, 445-460. Vaughn, M. A.; M l c k . G. Appl. Spectrosc. 1986, 40, 434-445. Tan. S. H.: Horllck. G. J. Anal. At. Specirom. 1987. 2 , 765-772. Allah, P.; Mauras, Y.; Doug& C.; Jaunault. L.; Delaporte. T.; Beaugrand, C. Anawst 1990. 775. 813-815. Alder, J. F.; Mermet, J. M. Spectrmhim. ACt8, P8ft 6 1973, 23. 42 1-433. Long, 0. L.; Bolton, J. S. Spectroehim. Acttr, Perf 6 1987, 42, 581-589. Long. G. L.; Winefordner, J. D. Appl. Spectrosc. 1984, 38, 563-567. Demers, D. R. Spectrochim. Acttr, eft 6 1985, 40, 93-105. Welters, P. E.; Long, 0. L.; Wlmfordner, J. D. m h .Act8, eft 8 1984, 39, 69-76.
Pierre Allah* Laurent Jaunault Yves Mauras Laboratoire de Pharmacologie Centre Hospitalier Universitaire 49033 Angers Cedex, France 0
100
200
300
400
Jean-Michel Mermet
500
Arsenic concentration (ng/ml) Flgure 3. Calibration cwves of As for several glycerol concentratbns: (0)deionized water: (B)glycerol 0.25 mol L-l; (+) glycerol 0.5 mol L-l. gated the influence of the ionization energy. A plot of the enhancement percentage for glycerol at 1mol L-' as a function of the ionization energy for each element is given in Figure 4. There is obviously an effect between 9 and 11eV. Iodine (10.44 eV) is the only noticeable exception in this range. It should be noted that Br (11.30 eV) also does not exhibit an enhancement effect. Clearly, the addition of organic com-
Laboratoire des Sciences Analytiques Bat. 308, Universit6 Claude Bernard-Lyon I 69622 Villeurbanne Cedex, France
Thierry Delaporte Delsi-Nermag 98ter, Boulevard Heloise 95100 Argenteuil, France
RECEIVED for review November 21, 1990. Accepted March 12, 1991.
