Thermo Fisher Scientific Inc., the world’s largest maker of lab instruments, agreed to acquire Dionex Corp. for about $2.1 billion in cash to expand its water analysis business in China.

The $118.50-a-share price represents a premium of 21 percent over the closing price on Dec. 10, the last trading day before the announcement. The deal will be completed in the first quarter of 2011, the companies said in a statement today.

Dionex, based in Sunnyvale, California, makes laboratory products used to identify chemical mixtures and made the first ion chromatography system for water analysis. Dionex will increase Thermo Fisher’s business in China, where there is demand for measuring the quality of water, the companies said.

“This is a natural fit with our range of products and services, and Dionex has a great presence in China,” said Marc Casper, the chief executive officer of Waltham, Massachusetts- based Thermo Fisher. “This is very much a complementary fit for the two companies.”

The deal will add 13 cents to 15 cents a share to Thermo Fisher’s earnings this year, Casper said.

Dionex’s chromatography instruments are used to detect what’s in a sample of water and will expand the range of products offered by Thermo Fisher, which makes instruments for mass spectrometry, another way of screening liquids and foods for contaminants. The instruments also have applications in discovering new drugs.

Thermo Fisher rose 77 cents, or 1.5 percent, to $53.81 as of 8:30 a.m. in trading before the opening of the New York Stock Exchange. Dionex jumped 20 percent, or $19.63, to $117.80 in early Nasdaq Stock Market trading.

Thermo Fisher has announced 29 pending or completed acquisitions in the past five years, with an average premium of 18 percent and an average value of $15.4 billion. The biggest deal was the purchase of Fisher Scientific for about $11 billion in 2006, according to Bloomberg data. Thermo had about $930 million in cash and short term investments as of Sept. 30.

Barclays Capital and J.P. Morgan advised Thermo Fisher, and Goldman Sachs advised Dionex.